Symposium QN05—Emerging Thermal Materials---From Nanoscale to Multiscale Thermal Transport, Energy Conversion, Storage and Thermal Management

Nanomaterials introduce new opportunities in tuning the thermal transport for various applications. Among various nanostructured materials, phonon transport within periodicnanoporous materials has been intensively studied for its potential applications in thermoelectrics, heat waveguides, thermal diodes, and heat imaging.^1,2It is now acknowledged that wave effects of lattice vibrations, i.e., phononic effects, are only important for ultrafine nanoporous structures and at cryogenic temperatures. Experimental evidence is usually found by comparing the thermal conductivities of periodic and aperiodic nanoporous Si films,^3,4or by measuring the specific heat to justify the phonon dispersion variation.⁵
In this work, a new approach is proposed to validate the wave effects, simply by comparing the thermal conductivity of the same Si thin film with increased number of nanopores. When phononic effects exist, it is anticipated that the thermal resistance can be largely increased from single to multiple rows of nanopores. Without phononic effects, the thermal resistance of a patterned Si film should linearly increase with the number of nanopore rows. The measured thermal conductivities are compared to frequency-dependent phonon Monte Carlo simulations that assumes incoherent phonon transport and diffusive pore-edge and film-surface phonon scattering.

Reference:
1. Maldovan, M. Narrow, Low-Frequency Spectrum and Heat Management by Thermocrystals. Phys. Rev. Lett.110, 025902 (2013).
2. Anufriev, R., Ramiere, A., Maire, J. & Nomura, M., Heat guiding and focusing using ballistic phonon transport in phononic nanostructures. Nature communications8, 15505 (2017).
3. Maire, J., Anufriev, R., Yanagisawa, R., Ramiere, A., Volz, S. & Nomura, M. Heat conduction tuning by wave nature of phonons. Science advances3, e1700027 (2017).
4. Lee, J., Lee, W., Wehmeyer, G., Dhuey, S., Olynick, D. L., Cabrini, S., Dames, C., Urban, J. J. & Yang, P. Investigation of phonon coherence and backscattering using silicon nanomeshes. Nature communications8, 14054 (2017).
5. Hao, Q., Xu, D., Zhao, H., Xiao, Y. & Medina, F. J., Thermal Studies of Nanoporous Si Films with Pitches on the Order of 100 nm —Comparison between Different Pore-Drilling Techniques. Scientific Reports8, 9056 (2018).

Thermal storage offers the potential to reduce the amount of waste heat generated by thermal processes and improve overall energy efficiency. Thermochemical storage using reversible reactions has shown considerable potential for high energy density. However, most current thermochemical systems involve the use of at least one gas phase, thus requiring large volumes and reducing energy density. Condensed-phase, particularly all-liquid, thermochemical storage systems are thus highly desirable in order to improve energy density. In this study, we present a computational search for all-liquid thermochemical storage materials based on the Diels-Alder [4+2] cycloaddition reactions. Using high-throughput density functional theory (DFT) calculations, we determined the enthalpy change △H_rxn, entropy change △S_rxn, and turning temperature (T* = △H_rxn / △S_rxn) of 54 Diels-Alder reactions that had been performed in an aqueous solvent in the literature. We screened this test set for turning temperature, identifying nine reactions with a turning temperature close to the working temperature range of water (-50 - 150 degrees Celsius). Several of these reactions (the exo-reaction between furan and maleimide and the reaction between furan and acrylonitrile) were selectively modified using functional group substitution to generate additional reactions for study. These modified reactions were predicted to display exceptional thermal properties, increasing the heat capacity of water by as much as 58.7% and improving the thermal energy density of water by as much as 33.8%. Gravimetric energy densities as high as 0.5598 MJ/kg were predicted. Experimental work to verify the predicted properties of Diels-Alder reactions are ongoing.

Coherent wave effects of thermal phonons hold promise of transformative opportunities in thermal transport control, but remain largely unexplored due to the small wavelength of thermal phonons, typically below a few nanometers. This small length scale indicates that, instead of artificial phononic crystals, a more promising direction is to examine the coherent phonon effects in natural materials with hierarchical superstructures matching the thermal phonon wavelength. In this work, we characterize the thermal properties of dodecagraphene (D-[BL1] graphene), a previously unstudied two-dimensional carbon allotrope based upon the traditional graphene structure but containing a secondary, in-plane periodicity. We use density functional theory (DFT) to calculate harmonic and anharmonic interatomic force constants (IFCs), which were then used to calculate the phonon dispersion, scattering rates, group velocities, and lattice thermal conductivity via an iterative solution to the linearized Boltzmann Transport Equation (BTE). We find that despite a very similar atomic structure, D-graphene possesses significantly different thermal properties than that of pristine graphene. At room temperature the calculated thermal conductivity of D-graphene is 600 Wm^-1K^-1 compared to 3000 Wm^-1K^-1 for graphene. The out of plane acoustic (ZA) mode contribution decreases from 84% in graphene to 47% in D-graphene. We also reportattribute these distinct properties to the presence of three naturally occurring, low frequency optical phonon modes that possess characteristics of phonon coherence and arise from a folding of the acoustic modes and the associated frequency gap opening, a phenomenon also found in superlattices where an out of plane periodicity is introduced. The optical modes make a significant contribution to the thermal conductivity due to enhanced dispersion, comprising over 18% of the thermal conductivity, while the three coherent branches contribute 9% of the total conductivity. The construction of the D-graphene unit cell presents a new method with which the thermal conductivity of 2D materials can be reduced without making drastic changes to its fundamental composition and demonstrates the potential of using coherent phonon effects to significantly modify thermal transport. This work is supported by a Department of Energy Early Career Research Program under award number DE-SC0019244.

The heterogeneities such as defects and dopants can give rise to exotic electronic properties in 2D transition metal dichalcogenides (TMDs), but to this date, there is no detailed study to illustrate how heterogeneities can be engineered to tailor their thermal properties. Here, through combined experimental and theoretical approaches, we have explored the effect of defects, doping, and metal isotopes on the thermal transport of monolayer 2D TMDs grown by chemical vapor deposition (CVD). We found that doping and defects in a CVD-grown monolayers 2D crystals can significantly affect their thermal conductivity due to the mass induced kinetic energy and potential energy difference. Furthermore, we find the isotopically pure monolayer 2D crystals synthesized by CVD can significantly boost their in-plane thermal conductivity resulting from combined effects of the reduced isotope disorder and a reduction in defect-related scattering. Our work demonstrates that heterogeneity engineering can effectively tune the thermal conductivity of 2D TMDs, which is important for thermal properties development and thermal management in 2D electronic and optoelectronic building blocks.

Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Characterizations were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Efficient thermal insulation using materials with a small environmental footprint is essential for sustainable thermal management in buildings. Nanocellulose produced from wood cellulose fibers is a nanosized, renewable, light weight cellulose particle of high aspect ratio which exhibits tunable surface properties and low thermal conductivity¹. We have previously shown that anisotropic nanocellulose foams prepared by directional freezing exhibit lower thermal conductivity than air (=25 mW/mK) perpendicular to the fibers direction at room temperature².
Here, we will present the heat transfer of anisotropic nanocellulose foams as a function of relative humidity (RH) and temperature (T). Hence, we measure the axial and radial thermal conductivities of anisotropic freeze-casted nanocellulose foams by using a customized hot disk thermal constant analyzer at controlled RH and T. Thermal conductivity measurements have been combined with X-ray diffraction and modelling. The strong dependence of the thermal conductivity on the RH will be related to the interaction of the hygroscopic nanocellulose fibrils with water and humidity-dependence of the directional interfacial thermal resistance. We will also address the effect of the crystallinity and alignment of nanocellulose on the thermal conductivity of anisotropic nanocellulose foams.

References

1. Lavoine, N. & Bergström, L. Nanocellulose-based foams and aerogels: processing, properties, and applications. J. Mater. Chem. A 5, 16105–16117 (2017).
2. Wicklein, B. et al. Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide. Nat. Nanotechnol. 10, 277–283 (2014).

When materials are nanostructured in the 1-100 nm range, fundamental length scales related to transport are often crossed. In the case of heat transport by phonons, the dominant wavelength at room temperature is typically in the 1-10 nm range. Hence, by nanostructuring to this length scale, thermal transport can be modulated in unprecedented ways. Here I will discuss our observations to reduce thermal conductivity below the “alloy limit” as well as reaching the upper limits of phonon conductance. What has eluded the community is the lower limit, which is likely to occur due to Anderson localization. That is the focus of our current work.

The van der Waals, electrostatic or steric forces between liquid molecules and between liquid-solid interfaces fall in the range of 1-10 nm. When liquids are confined to these length scales, they undergo a variety of transitions that control the liquid, ionic and macromolecular transport, as well as liquid-vapor phase transitions. This talk will discuss what we discovered in nanofluidics, which is forming the basis for new research to probe ions, solvation shells and macromolecules in nanofluidic channels.

Finally, I will discuss our current research to use a new class of oxide material for catalysis of a few redox reactions that are important in energy science. This research will also underscore the need for new experimental probes to study catalysis and surface reactions at nanoscales.

The performance of line-focus concentrating solar power (CSP) systems, operating at concentrations <100 suns, is limited by receiver thermal losses. Existing linear CSP receivers rely on spectrally selective surfaces enclosed within cylindrical vacuum tubes to minimize heat loss due to radiation and convection. However, using spectrally selective coatings and maintaining a high-quality vacuum at high temperatures (up to 400 °C) increases cost, reduces durability, and restricts receiver geometry to a cylindrical shape. We have developed a solar thermal aerogel receiver comprising of custom-fabricated highly-transparent thermally-insulating silica aerogels that allow transmission of concentrated sunlight, minimization of thermal losses and enables operation in air. We demonstrate the on-sun performance of the prototype aerogel receiver with a 1 m×10 cm aperture area. The prototype receiver consists of seven monolithic solar-transparent aerogel tiles that cover the solar-absorber consisting of a piping loop connected in series and coated with black Pyromark paint. The receiver is paired with a 6 m×6 m linear Fresnel reflector (LFR) array capable of achieving concentration up to 30 suns. Pressurized Dowtherm A is used as the heat transfer fluid which is heated to temperatures up to 350 °C before entering the receiver section. We performed on-sun measurements for different receiver inlet temperatures and incident flux conditions, and report a peak receiver thermal efficiency >65%.

Concentrated solar power (CSP), also known as high-temperature solar-thermal energy conversion, is a promising solar energy harvesting technology due to its efficient sunlight utilization, and high availability in energy storage. For the sake of higher Carnot efficiency and greater cost reduction, next-generation CSP plants are expected to operate at higher temperatures (≥900 K) than those conventional systems. Selective solar absorbers, as the key components in CSP systems, are required to offer high stability and great selectivity at such high temperatures. Selective solar absorbers based on multilayer metal/ceramic thin films are a kind of low-cost and scalable absorbers fabricated by facile processes such as sputtering deposition. However, their thermal stability and spectral selectivity fall behind those state-of-the-art cermets and photonic absorbers. Previously works demonstrated that multilayer absorbers suffered from a variety of high-temperature degradation cases associated with the metal/ceramic interfaces, including delamination, surface oxidation, and atom diffusion.
In this work, we introduced a TiN IR reflector into the multilayer absorbers to replace refractory metals (W, Ta, Mo, etc.) used in conventional designs; meanwhile, another ceramic material, titanium oxy-nitride (TiNO), was utilized as the absorptive layer due to its tunable absorption properties and excellent compatibility with TiN. Two ceramic anti-reflection layers, ZrO₂ and SiO₂, with gradient refractive indexes were adopted to reduce the surface reflection, and enhance the sunlight absorption. The fabricated all-ceramic absorber displayed highly selective absorption with a high solar absorptance of 91.2% and an ultralow IR emittance of 15.7% at 1000 K. Consequently, a high solar-thermal conversion efficiency of 82.1% was achieved under the irradiation of 100 suns. This value is on par with or exceeds the record of state-of-the-art selective absorbers. As expected, the all-ceramic absorber was able to sustain its superior performance at high temperatures up to 800 °C. Both SEM-EDX and depth profiles of elements results reveal that the diffusion and oxidation were effectively suppressed in the all-ceramic structure. Moreover, the delamination was also avoided in the ceramic/ceramic interfaces. Compared with previous multilayer absorbers, this all-ceramic absorber boosted the operating temperature of such low-cost multilayer absorbers by ~300 °C, rendering it suitable for the high-temperature applications in next-generation CSP plants.

Thermal processes and materials involving phase change, are practically omnipresent in nature and technology. Viewed in the direction of decreasing temperature, the encountered phase transitions are condensation, freezing and de-sublimation. All share significant scientific challenges in terms of material/surface engineering to control nucleation and growth of the generated phase on a surface, and assure its continuous and facile, passive removal from the surface, to maintain surface functionality and robust performance at a high level, depending on the application of interest. Here, I will focus on the rational, physics-derived, surface nanoengineering, for condensation applications, ranging from power generation targeting high efficiency, to sunlight-driven fogging retardation and rapid defogging of transparent materials.

Enhancing the thermal efficiency of a broad range of condenser devices requires means of achieving sustainable dropwise condensation on metallic surfaces, where heat transfer can be further enhanced by facilitating a reduction in the droplet departure diameter. I will present a rationally driven, hierarchical texturing process of metallic surfaces, guided by the collaborative action of wettability and coalescence, which achieves controlled droplet departure, also under challenging vapor flow conditions significantly enhancing heat transport. The textures are attained by both etching processes generating a random but hierarchical re-entrant landscape, as well as by fabricating an array of 3D laser-structured truncated microcones on the surface, covered with papillae-like nanostructures and a hydrolytically stable, low surface energy self-assembled-monolayer coating. Passive droplet departure on this surface is achieved through progressive coalescence of droplets generated and continuously arising from microcavities, resulting in robust depinning and subsequent departure of the condensate, through vapor shear or gravity, Refs. 1,2.

Condensation is also responsible for fogging, a common phenomenon that can have detrimental effects on visibility through otherwise tramnsparent surfaqes. Fogging affects the performance of a wide range of everyday applications including windshields, visors, displays, cameras, and eyeglasses. I will present a novel approach, based on sunlight absorbing metasurfaces, which goes well-beyond state-of-the-art anti-fogging methods such as superhydrophilic coatings. We rationally nanoengineer such transparent metasurfaces, by varying the concentration of embedded plasmonically enhanced light absorbing nanoparticles in an ultra-thin titania film to achieve broadband absorption with tunable transparency. Such surfaces upon illumination induce significant heating at the air-substrate interface where fog is most likely to form and can rapidly de-fog or completely inhibit fog nucleation altogether, Ref. 3. For the same environmental conditions, we demonstrate that such metasurfaces are able to reduce defogging time by up to four-fold compared to reference samples and markedly outperform the most widely implemented solution in anti-fogging, namely, superhydrophilic surfaces. This approach work paves the way for large-scale, low-cost manufacturing that can be applied to a versatile range of materials, including polymers and other flexible substrates, which can further be combined with state-of-the-art technology to overcome remaining impracticalities, safety, and energy-related costs related to fogging.

References:
1. C. S. Sharma, J. Combe, M. Giger, T. Emmerich and D. Poulikakos. ACS Nano, 11 (2): 1673-1682, DOI: 10.1021/acsnano.6b07471
2. C. S. Sharma , C Stamatopoulos, R. Suter, P. Rudolf von Rohr and D. Poulikakos. ACS Appl. Mater. Interfaces, 2018, 10 (34), pp 29127–29135. DOI: 10.1021/acsami.8b09067
3. C. Walker, E. Mitridis, T. Kreiner, H. Eghlidi, T. Schutzius and D. Poulikakos, Transparent Metasurfaces Counteracting Fogging by Harnessing Sunlight, 2018, in review.

I will review several collaborative efforts at developing new non-contact methods for heating and thermometry at the nanometer scale. Examples include techniques based on SEM (e-beam as a point heater; secondary electron yield as a thermometer), TEM (thermometry using the Debye-Waller effect), and confocal microscopy (luminescence thermometry of individual nanoparticles).

Conventionally, the two-temperature model has been widely used for electron-phonon coupled non-equilibrium thermal transport. However, many recent applications have shown that different phonon branches can be in strong thermal non-equilibrium. Therefore, assuming a local equilibrium lattice can lead to misleading or wrong results. Here, we present a multi-temperature model to capture the non-equilibrium among different phonon branches, and demonstrate its advantages over the conventional two-temperature model for bulk materials and across interfaces.

The thermal conductivity of materials is not a fixed physical property but can be manipulated by controlling the transport properties of thermal phonons. Recently, a large number of experiments have been introduced where thermal conduction is reduced by orders of magnitude via phonon mean free path reduction through diffuse surface scattering. In contrast to established work that use the diffuse surface scattering of phonons as the physical mechanism to reduce the thermal conductivities, in this talk we show that the largest reduction of thin film heat conduction is achieved via specular scattering. Our results thus create a new paradigm for heat conduction manipulation since smooth surfaces – in contrast to rough surfaces – can be more effective on suppressing thin film phonon heat conduction.

Effect of structural disorder on phonon thermal conduction remains an open question with a large spectrum of physical effects, as the plane-wave description of atomic vibrations is expected to become irrelevant. In a first stage, atomic scale disorder will be investigated in various systems –silicon [1], SiGe nanowires [2], partial-crystal partial-liquid [3], 2D [4,5] - with atomic scale simulations. Secondly, thermal properties of nanoscale random materials [6,7] will be presented to raise the question of the eventual impact of localization on heat conduction.

[1] K. Sääskilahti, J. Oksanen, J. Tulkki, A. J. H. McGaughey, and S. Volz, Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations, AIP Advances.
[2] Honggang Zhang, Haoxue Han, Shiyun Xiong, Hongyan Wang, Sebastian Volz, and Yuxiang Ni, Impeded thermal transport in composition graded SiGe nanowires, App. Phys. Lett., 111, 121907, (2017).
[3] Y. Zhou, S. Xiong, X.L. Zhang, S. Volz, M. Hu, Thermal Transport Crossover from Crystalline to Partial-crystalline Partial-liquid State, Nat. Comm., accepted.
[4] Van-Truong Tran, Jérôme Saint-Martin, Philippe Dollfus & Sebastian Volz, Optimizing the thermoelectric performance of graphene nano- ribbons without degrading the electronic properties, Sc. Rep., 7: 2313 | DOI:10.1038/s41598-017-02230-0.
[5] Van-Truong Tran, Jérôme Saint-Martin, Philippe Dollfus and Sebastian Volz, High thermoelectric performance of graphite nanofibers, Nanoscale, DOI: 10.1039/C7NR07817J
[6] J. Maire, R. Anufriev, R. Yanagisawa, A. Ramiere, S. Volz, and M. Nomura, Heat conduction tuning by wave nature of phonons, Science Advances, 3(8), e1700027 (2017).
[7] S Hu, Z Zhang, P Jiang, J Chen, S Volz, M Nomura, B Li, Randomness-Induced Phonon Localization in Graphene Heat Conduction, The journal of physical chemistry letters 9 (14), 3959-3968

Polymers have entered almost every aspect of modern life, from packaging and soft robotics to aerospace sector and 3D printing. Traditional polymers are both electrically and thermally insulating. The discovery and development of electrically conductive polymers has led to innovative electronic applications such as flexible displays, wearable biosensors and lightweight photovoltaics. As in the case of electrically conductive polymers, the development of thermally conductive polymers would open up a range of advanced thermal applications including an emerging self-cooling system to existing electronics casings. This talk will summarize our recent work on improving thermal conductivity of polymers. We developed scalable polyethylene films with thermal conductivity ~62 W/m-K by extrusion and roll-to-roll drawing processes . Structural studies and thermal modeling reveal that the film consists of nanofibers with crystalline and amorphous regions, and the amorphous region has a remarkably high thermal conductivity of ~16 W/m-K. We grew thermally conductive conjugated polymer by bottom-up oxidative chemical vapor deposition technique, and realized high thermal conductivity ~2.2 W/m-K in conjugated polymer (P3HT) thin film by engineering both inter- and intramolecular interactions and taking advantage of both strong C=C covalent bonding along the extended polymer chain and strong π-π stacking noncovalent interactions between chains. We will also discuss our current work on enhancing heat conduction in hydrogels. This work is supported by Department of Energy–Basic Energy Sciences under award number DE-FG02-02ER45977.

According to conventional theories of heat conduction in semiconductors and insulators, only crystals composed of strongly-bonded light elements can have high lattice thermal conductivity (k_L), and intrinsic thermal resistance comes only from lowest-order anharmonic three-phonon interactions. Recently, we proposed a new paradigm for achieving high k_L in which the vibrational properties are engineered to reduce the phase space for three-phonon scattering. Some binary compounds with a heavy-light atom combination in the unit cell can achieve the requirements, and first principles calculations for one material, cubic Boron Arsenide (BAs), gave three-phonon limited k_L > 2000 W/m-K at room temperature [2], comparable to that of best heat conductor, diamond. Subsequent calculations [2] showed the importance of higher-order four-phonon scattering in also limiting the k_L in compounds such as BAs but still predicted room temperature values > 1000 W/m-K for BAs, far higher than any other semiconductor, and recent synthesis and measurement efforts [3-5] have confirmed that BAs does indeed have the predicted high k_L. In this talk, I will review the theoretical predictions and the challenging material constraints that must be satisfied in order to achieve the unconventional high k_L, as in BAs. I will discuss one set of candidate compounds we have examined – group V transition metal carbides – that have ideal vibrational properties similar to BAs but are metals with nested Fermi surfaces. This gives rise to an unusual combination of the desired weak phonon-phonon scattering but strong phonon-electron scattering and results in low k_L that is nearly temperature independent, contrary to the typical behavior in metals.

D.B. acknowledges support from the Office of Naval Research MURI, Grant No. N00014-16-1-2436.

[1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013).
[2] T. Feng, L. Lindsay, and X. Ruan, Phys. Rev. B, 96, 161201 (2017).
[3] J. S. Kang M. Li, H. Wu, H Nguyen, and Y. Hu., Science 361, 575 (2018).
[4] S. Li, Q. Zheng, Y. Lv, X. Liu, X. Wang, P. Y. Huang, D. G. Cahill, and B. Lv, Science 361, 579 (2018).
[5] F. Tian et al., Science 361, 582 (2018).
[6] C. Li, N. K. Ravichandran, L. Lindsay, and D. Broido, Phys. Rev. Lett. 121, 175901 (2018).

Optical phonon linewidth is crucial to infrared dielectric function of polar materials and thermal conductivity of certain materials that have a large number of optical phonon branches. The current understanding of optical phonon linewidth is associated with three-phonon scattering. In this work, however, we show that four-phonon can dominate the optical phonon linewidth and infrared optical properties at elevated temperatures for a range of important materials, including cubic boron arsenide (BAs), cubic silicon carbide (3C-SiC), and α-quartz. Strikingly, in large band gap-materials, e.g., BAs and AlSb, four-phonon scattering rates are found to be orders of magnitude higher than three-phonon scattering rates even at room temperature. The predicted infrared optical properties of α-quartz after including four-phonon scattering can well explain experimental measurements.

Heat dissipation has become an increasingly important technological challenge in modern electronics. Discovering new high thermal conductivity materials that can efficiently dissipate heat from hot spots and improve device performance are urgently needed. In this talk, I will describe our recent progress in developing emerging high thermal conductivity semiconductors including boron arsenide (BAs) and boron phosphide (BP). We synthesized BAs and BP single crystals without detectable defects, and measured a room temperature thermal conductivity of 1300 W/mK [1] and 500 W/mK [2] respectively. Our ultrafast spectroscopy study in conjunction with atomistic theory reveals that the unique band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. The single-crystal BAs has the highest thermal properties among all common metals and semiconductors. In addition, I will briefly discuss our efforts in characterizing thermal boundary resistance and integrating BAs and BP with high-power electronics for near-junction thermal management applications through the combination of measurement and atomistic simulations. Our study establishes BAs and BP as new benchmark materials for thermal management applications, and exemplifies the power of combining experiments and ab initio theory in new materials discovery.

References:
1. Joon Sang Kang, Man Li, Huan Wu, Huuduy Nguyen, and Yongjie Hu, “Experimental observation of high thermal conductivity in boron arsenide,” Science 361, 575-578 (2018).
2. Joon Sang Kang, Huan Wu, and Yongjie Hu, “Thermal Properties and Phonon Spectral Characterization of Synthetic Boron Phosphide for High Thermal Conductivity Applications,” Nano Letters 17, 7507 (2017).

Thermal management is critical for electronic systems ranging from servers and smartphones to radar HEMTs and hybrid vehicle conversion devices. Much progress is being achieved by leveraging nanofabrication strategies from the electronics, photonics, and materials science communities for heat transfer structures and systems, which have been limited by more traditional methods of synthesis. One example is the template-fabrication of metal inverse opals for application as porous media for phase change heat transfer. This fabrication methodology offers very systematic pore shapes and distributions, down to a few micrometers, and facilitates breakthrough combinations of high fluidic permeability and high solid thermal conductivity (1,2). We have shown that inverse opals can be conformally coated onto microchannel walls in silicon and diamond microfluidic heat sinks, thereby facilitating solid-state heat conduction spreading with liquid and vapor phase routing and management in the heat sink (3). We have also shown recently that controlled sintering of the templates before electrodeposition can be used to modify the dimensions of the “necks” or hydraulic vias between adjacent pores and augment the permeability with modest reductions in the effective thermal conductivity, which is an important result for wick engineering in capillary-based devices (4). Our simulations and experiments have explored the impacts of wick dimensions and structure on the suppression and ultimate onset of boiling crisis, as well as the reduction of superheat values for heat fluxes well above 1 kW/cm2 (5). This talk will highlight both our fundamental simulations and experiments on inverse-opal based wicks and microfluidic heat sinks, as well as our efforts to bring these technologies to our collaborations with the semiconductor industry, with US defense companies, and within the NSF center on power electronics (POETS).

(1) Zhang, Palko, Barako, Asheghi, Santiago, Goodson, 2018, "Enhanced Capillary Fed Boiling in Copper Inverse Opals via Template Sintering,” Advanced Functional Materials 1803689.

(2) Barako, Sood, Zhang, Wang, Kodama, Asheghi, Zheng, Braun, Goodson, 2016, “Quasi Ballistic Electronic Thermal Conduction in Metal Inverse Opals,” Nano Letters, Vol. 16, pp. 2754-2761.

(3) Palko, Won, Agonafer, Resler, Altman, Asheghi, Santiago, Goodson, et al., 2017, “Extreme Two Phase Cooling from Laser Etched Diamond and Conformal, Template Fabricated Microporous Copper,” Advanced Functional Materials, Vol. 27, 1703265.

(4) Zhang, Palko, Pringle, Barako, Asheghi, Santiago, Goodson, et al., 2018. "Tailoring permeability of microporous copper structures through template sintering,” ACS Applied Materials & Interfaces.

(5) Palko, Zhang, Asheghi, M., Goodson, Santiago, et al., 2015, "Approaching the Limits of Two Phase Boiling Heat Transfer: High Heat Flux and Low Superheat," Applied Physics Letters, Vol. 107, 253903.

Condensation and boiling are integral processes in many industrial applications including power generation, HVAC, and thermal management of electronic systems. Current efforts aimed at increasing heat transfer during these processes predominantly focus on altering either the chemistry or the texture of the solid-fluid interface. In this talk we will expand beyond these two design variables and will discuss how softening of these surfaces impacts dropwise condensation [1] and nucleate boiling [2]. An intriguing component of these phase change processes on soft surfaces is that both microscale droplets and bubbles can significantly deform such materials through Laplace pressure and capillary force at the triple phase contact line. We first discuss how these elastocapillary processes impact heat transfer during condensation across individual droplets as well as their nucleation, growth, and shedding. By integrating these trends over droplet population, we show that softening of the surfaces below shear modulus of about 500 kPa is detrimental to dropwise condensation. In contrast, we use theoretical arguments to show that softening of the surface could significantly facilitate onset of nucleate boiling (ONB). Specifically, we use classical kinetic theory to show that softening of a smooth-surface could mildly decrease superheat needed for ONB. We also discuss a close-form model of vapor trapping and bubble seeding from soft surfaces with conical cavities to show that superheat needed for ONB decreases linearly with shear modulus of the surface. Thus, our work shows that boiling could be manipulated using mechanical properties of the surface. We will also briefly discuss practicalities involved in implementing soft surfaces in boilers and some of the remaining outstanding fundamental questions.

[1] A. Phadnis and K. Rykaczewski, Langmuir, 33, (43), 12095-12101, 2017.
[2] K. Rykaczewski and A. Phadnis, Nanoscale and Microscale Thermophysical Engineering, 22, 3, 2018.

A novel solar heating (SH) and radiative cooling (RC) system based on black surface with selective cover (BS-S) is proposed. This system can provide heat via photothermal conversion and obtain cooling energy by radiative cooling. Photonic approach is applied to design selective cover for SH-RC system. The selective rigid cover consists of a 500 μm-thick zinc sulfide (ZnS) substrate and alternating layers of ZnS and ytterbium fluoride (YbF₃) with varying thicknesses. It has strong and remarkably spectral selective characteristics. The transmissivity in SH band (i.e., 0.3–2.5 μm) and RC band (i.e., 8–13 μm) is approximately 0.85, the reflectivity in wavelength band 3–8 μm is approximately 0.70. With the selective rigid cover, air pressure between the cover and black surface could be low (maybe even vacuum) to suppress heat conduction and heat conduction. Numerical analysis demonstrates that in SH process, when surface temperature is 80 °C, the vacuum BS-S case shows a thermal efficiency of 45.9%, approximately 11.5% higher than that of the typical air pocket BS-S case. In RC process, the equilibrium temperature of the vacuum BS-S case and the typical air pocket BS-S case are lower than those of ambient air by approximately 28.9 °C and 22.5 °C, respectively. This selective rigid cover provides an alternative choice for integrating combined SH-RC system and has potential to enhance the performance of the diurnal solar heating and nocturnal radiative cooling processes.

Liquid-vapor phase change is essential in many applications including water purification, power generation, and thermal management. However, effectively utilizing these processes requires detailed understanding and manipulation of interfacial transport. In the first part of the talk, we discuss evaporation from ultra-thin nanoporous membranes. Fundamental understanding during evaporation remains limited to date as it is generally challenging to characterize the heat and mass transfer at the interface, particularly when the heat flux is high (>100 W/cm²). We fabricated ultra-thin (≈ 200 nm thickness) nanoporous (≈ 130 nm pore diameter) membrane devices which reduced the thermal-fluidic transport resistance and accurately monitored the temperature of the liquid-vapor interface. We showed that kinetically limited evaporation, when normalized properly, is solely determined by the pressure ratio between the ambient and the interface. We modeled the gas kinetics and demonstrated good agreement between experiments and theory. Our work provides a general figure of merit for evaporative heat transfer as well as design guidelines for achieving efficient evaporation. In the second part of the talk, we show a nanostructured surface that can repel liquids even during condensation. This surface consists of isolated reentrant cavities with a pitch on the order of 100 nanometers to prevent droplets from nucleating and spreading within all structures. We developed a model to guide surface design, and subsequently fabricated and tested these surfaces with various liquids. We demonstrated repellency to various liquids up to 10 °C below the dew point and showed durability over three weeks. These works provide insights into nanostructured designs for enhanced phase change heat transfer.

Solar thermophotovoltaic (STPV) systems, which leverage the benefits of both solar-thermal and photovoltaic (PV) technologies, involve efficient and dispatchable approaches to generating electricity. The key components in a STPV system include a selective solar absorber, a selective emitter, and a solar cell at least. The total input power of a SPTV system is determined by the sunlight absorption efficiency of the selective absorber. Considering the high operating temperatures of STPV systems, selective absorbers are required to maintain high absorption for solar radiation and low emission beyond a cut-off wavelength in the infrared region to avoid thermal re-radiation even at elevated temperatures. To date, almost all the state-of-the-art selective absorbers/emitters such as cermets and photonic crystals with superior performance (i.e., high selectivity and great thermal stability) were generally manufactured with complicated and expensive micro-fabrication or nano-fabrication techniques, leading to high cost and challenges for large-scale production. Moreover, the obtained absorbers/emitters often encounter problems when being integrated with solution-processed solar cells. Therefore, developing high-performance solution-processed absorbers/emitters is urgently demanded.
Titanium nitride (TiN), as an emerging plasmonic ceramic material, offers tunable plasmonic properties, as well as great thermal and chemical stability. TiN nanoparticles have been proven to show stronger optical losses in the visible range compared to traditional metallic plasmonic materials. In this work, we took advantage of well-dispersed colloidal TiN nanoparticles (20-80 nm) to serve as the absorptive medium for sunlight. The absorption bandwidth, i.e., the cut-off wavelength, can be tuned through rationally controlling the concentration of the TiN colloid and the coating speed. The excited plasmonic resonance of TiN nanoparticles resulted in highly selective absorption of sunlight near their resonance wavelengths. Due to the in-plane plasmonic coupling of adjacent TiN nanoparticles, the resonance wavelength was able to red-shift to near-infrared (NIR) range. As a result, selective sunlight absorption can be attained in the UV-visible-NIR range below the cut-off wavelength. An amorphous SiO_x layer, prepared by spin coating a perhydropolysilazane (PHPS) solution, acted as the protection and anti-reflection coating. Both full-spectrum sunlight absorption and strong IR reflection were achieved simultaneously when coated on various IR reflectors, including Au, stainless steel, Al, and TiN. Particularly, the absorbers with Au reflectors exhibited a high solar absorptance of 92.1% and an ultra-low IR emittance of 10.5% at an elevated temperature of 1000 K, producing a solar-thermal energy conversion efficiency of 86.1% under 100 suns. To the best of our knowledge, this performance surpasses or equals those of state-of-the-art selective absorbers fabricated by micro and nano-fabrication techniques. The superior performance of such low-cost solution-processed solar absorbers will potentially put the steps of cost-effective and large-scale STPV systems forward.

The dynamic control of thermal transport properties in solids must contend with the fact that phonons are inherently broad-band. Thus, efforts to create reversible thermal conductivity switches have resulted in only modest on/off ratios, since only a relatively narrow portion of the phononic spectrum is impacted. Here, we report on the ability to modulate the thermal conductivity of topologically networked materials by nearly a factor of four following hydration, through manipulation of the displacement amplitude of atomic vibrations. By varying the network topology, or crosslinked structure, of squid ring teeth-based bio-polymers through tandem-repetition of DNA sequences, we show that this thermal switching ratio can be directly programmed. This on/off ratio in thermal conductivity switching is over a factor of three larger than the current state-of-the-art thermal switch, offering the possibility of engineering thermally conductive biological materials with dynamic responsivity to heat.

Manipulating heat conduction is an appealing thermophysical problem with enormous practical implications, which requires insight into the lattice dynamics. Strain engineering is one of the most promising and effective routes towards continuously tuning the thermal transport properties of materials due to the flexibility and robustness. However, previous studies mainly focused on quantifying how the thermal conductivity is affected by strain, while the fundamental understanding on the electronic origin of why the thermal conductivity can be modulated by mechanical strain has yet to be explored. In this talk, I would like to present our comparative study of thermal transport in two-dimensional group III-nitrides (h-BN, h-AlN, h-GaN) and graphene. Although the monolayer group III-nitrides possess similar planar honeycomb structure with graphene, their thermal conductivity is substantially lower and the root reason cannot be intuitively attributed to the mass difference. We then establish a microscopic picture to connect phonon anharmonicity and lone-pair electrons. Direct evidence is provided for the interaction between lone-pair electrons and bonding electrons of adjacent atoms based on the analysis of orbital-projected electronic structures, which demonstrates how nonlinear restoring forces arise from atomic motions and lead to strong phonon anharmonicity. The microscopic picture of lone-pair electrons driving strong phonon anharmonicity provides coherent understanding of the diverse thermal transport properties of the monolayer group III-nitrides compared to graphene. Furthermore, the thermal conductivity (κ) of planar monolayer group III-nitrides is unexpectedly enlarged by up to one order of magnitude with bilateral tensile strain applied, which is in sharp contrast to the strain induced κ reduction in graphene despite their similar planar honeycomb structure. The anomalous positive response of κ to tensile strain is attributed to the attenuated interaction between the lone-pair s electrons around N atoms and the bonding electrons of neighboring (B/Al/Ga) atoms, which reduces phonon anharmonicity. The microscopic picture for the lone-pair electrons driving phonon anharmonicity established from the fundamental level of electronic structure deepens our understanding of phonon transport in 2D materials and would also have great impact on future research in micro-/nanoscale thermal transport such as materials design with targeted thermal transport properties.

We report the electrothermal properties of AZO/Ag/AZO (AAA) trilayer on PET, which helped us develop a low cost high performance transparent flexible electrode and heater (TFEH) using RF magnetron sputtering. The thickness of the Ag interlayer (1.6 nm to 5 nm) in the AAA trilayer was varied, and its electrical, optical and structural properties was thoroughly investigated. At optimized Ag interlayer thickness of 5 nm, the AAA trilayer based TFEH yielded saturation temperatures beyond 100 °C at 10 V which makes it a suitable energy efficient device. The time dependent temperature profile along with its highly stable and reversible thermal behavior demonstrated that it is a promising low cost TFEH device that can be developed industrially on large areas using RF magnetron sputtering. Most importantly the AAA trilayer based TFEH provides a high-performance alternative to the conventional ITO electrodes at a much lower cost.

We present new composite with significantly improved Thermal Conductivity (TC) that comprises a continuous network of thermally conductive fillers and a natural polyol. The network of thermally conductive filler, in this case boron nitride nanosheets (BNNS), was an aerogel that was fabricated with the ice template method. Subsequently, the aerogel was carbonized and filled with xylitol with help of vacuum infiltration of liquefied xylitol. The aerogel provided a 3D thermal conductive pathway and also acted as a scaffold to promote the oriented crystallization of xylitol. Our results demonstrate that the BNNS scaffold and packing of oriented xylitol crystals play a key role in improving TC of the composite. With BNNS content of 18.16 wt% (6.14 vol %) TC of above 4.5 W/ (m.K), which is higher than that reported in recent literatures, was obtained.

We use molecular dynamics simulations to determine the pressure-induced structural changes in several oxide glasses including permanent densification and the effect of these changes on thermal properties. We show that densified glass structures exhibit increased thermal conductivity, which may be attributed to the increased density and elastic modulus. The local properties analysis reveals that the higher-pressure treatment reduces elastic heterogeneities, which might also contribute to higher thermal conductivity of densified glasses.

Gallium doped zinc oxide (GZO) and Aluminum doped zinc oxide (AZO) thin films were often grown on substrates such as glass, flexible polyethylene terephthalate substrate through using physical vapor deposition technique. The thin films were ultra-smooth in nature and showed outstanding optical, and electrical properties. Highly compact and dense PLD target was made in the laboratory fromAl2O3, Ga2O3 and ZnO powder in an appropriate proportion with a less than 10 atomic weight percentages of Ga and Al followed by isotactic press as well as high temperature annealing (1200 oC) for 12 hours. AZO thin films show stable and reproducible joule heating effect of more than 100oC by the application of low (<10 V) voltages. GZO transparent heater also showed a stable and reproducible Joule heating effect and the temperature can reach easily close to the 100oC by the application of low input 〈8V voltages. The samples showed low resistivity of about 2.6×10⁻⁴ Ω cm and 3.7×10⁻⁴Ω cm and also exhibited high optical transparency value in the visible region of the electromagnetic spectrum. The temperature dependent resistivity behavior of the films were investigated using four-point probe technique. This exciting results encourages the use of GZO and AZO transparent oxide in different optoelectronics device application.

Advances in nanotechnology have contributed to dramatically increasing the thermal conductivity of fluids. Nanoparticles with high thermal conductivity have a high surface area relative to their mass and thus increase the thermal conductivity of the fluid more efficiently with a small amount of addition. However, unlike a pure solution such as DI water, complex fluids such as drilling mud have a large influence of other components so not too much increase the thermal conductivity through the addition of nanoparticles. Therefore, we do not simply add nanoparticles but use a method to increase the thermal conductivity by growing ceramics with high thermal conductivity directly on the solid particle surface added to the complex fluid. Synthesis of nanoparticles using the conventional bottom-up method was inferior in terms of cost and yield compared to synthesizing nanoparticles in a top-down method. But it is expected that only small amounts of nanoparticles will have a great effect, if the particles are synthesized only on the surface of the other particles by the bottom-up method.
We made drilling mud after functionalization of ceramics (BN, SiC, MoS₂, WS₂) with high thermal conductivity on bentonite surface which is the most important additive substance to drilling mud. Thermal conductivity of the drilling mud was compared with the conventional method in which the same amounts of nanoparticles were simply mixed in the conventional method in which the same amounts of nanoparticles were simply mixed in the drilling mud. We also measured the FE-SEM, RAMAN, and XPS to check the growth of the particles on the bentonite surface and measured the viscosity according to the flow rate to confirm the performance as a drilling mud. We report how to increase the thermal conductivity of a complex fluid in a different way than previously.

The exceptionally high thermal conductivity of graphene has driven interest toward its applications in thermal interface materials (TIMs) [1-3]. In addition to its unique heat conduction properties, graphene is also a strong conductor of electricity, which is problematic for certain TIM applications where electrical insulation is paramount. A common strategy for the optimization of composite materials is to combine two or more different thermally conductive fillers. Typically, work along this vein employs fillers of disparate size, shape, and aspect ratios with the larger-sized fillers providing the greatest contribution of overall thermal transport and the smaller fillers improving the interstitial thermal coupling between the larger fillers. Unlike much of the previous work into binary fillers, we report on polymer composites filled with few-layer graphene and hexagonal boron nitride (h-BN) of similar lateral dimensions, thicknesses, and aspect ratios. In each filler material, phonons are the primary heat carrier, necessitating the selection of lateral dimensions larger than the “gray” phonon mean free path (MFP) in the micrometer distance range. The composition and structure of the resulting epoxy-based composites were verified using scanning electron microscopy (SEM), Raman, and Brillouin spectroscopy. Thermal measurements were conducted using the “laser flash” techniques. It was found that the use of electrically conductive graphene and electrically insulating h-BN fillers of similar physical dimensions can be complementarily leveraged to achieve an independent control of the thermal and electrical conductivity of the TIM. Varying the constituent fraction of graphene in composites with ~44% total filler loading can tune the thermal conductivity enhancement from a factor of ×15 to ×34 while changing the electrical resistivity from 3×10⁸ Ω-mm to 10² Ω-mm, i.e. spanning the resistivity range from an insulator to a conductor. We offer an analytical model that describes the experimental thermal conductivity data of the binary filler composites. The obtained results are illustrative of a promising strategy for the development of next-generation thermal interface materials with specific control of electrical properties, allowing for the expression of electrically insulating or electrically grounding behaviors.
This work was supported, in part, by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE award 1433395, and by the UC – National Laboratory Collaborative Research and Training Program LFR-17-477237.
[1] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials. Nature Materials, 10, 581 (2011).
[2] D. L. Nika and A. A. Balandin, Phonons and thermal transport in graphene and graphene-based materials, Reports on Progress in Physics, 80, 36502 (2017).
[3] F. Kargar, Z. Barani, R. Salgado, B. Debnath, J.S. Lewis, E. Aytan, R.K. Lake, A.A. Balandin, Thermal percolation threshold and thermal properties of composites with high loading of graphene and boron nitride fillers, ACS Applied Materials and Interfaces, (2018) doi:10.1021/acsami.8b16616.

Since the discovery of the Quantum Spin Hall Effect, electronic and photonic topological insulators have made substantial progress, but phononic topological insulators in solids have received relatively little attention due to challenges in realizing topological states without spin-like degrees of freedom and with transverse phonon polarizations. Here we present a holey silicon-based phononic topological insulator design, in which simple geometric control enables topologically protected in-plane elastic wave propagation up to GHz ranges when the unit cell reaches submicron scales. By integrating a hexagonal lattice of six small holes with one large hole in the center and by creating a hexagonal lattice by themselves, the six-petal holey silicon, which has C₆ symmetry, induces zone folding to form a double Dirac cone. Based on the hole dimensions, breaking the discrete translational symmetry allows the six-petal holey silicon to achieve the topological phase transition, yielding two topologically distinct phononic crystals. Based on the unit cell periodicity, the transition readily shifts from low- to high-frequency ranges. Our numerical simulations confirm inverted band structures and show backscattering-immune elastic wave transmission up to 90 % at 14.83 GHz through defects including a cavity, a disorder, and sharp bends when the unit cell periodicity is 500√3 nm. The six-petal holey silicon design also offers robustness against geometric errors and potential fabrication issues such as over- or under-etching. The simulations of the six-petal holey silicon with the same periodicity show up to 90 % transmission of elastic waves at 13.8 and 15.37 GHz even when the holes are under-sized by 5 % or over-sized by 2.5 %, respectively, in which the shift of bandgap is led by the change of porosity. These findings provide a detailed understanding of the relationship between geometry and topological properties and pave the way for developing high-frequency phononic topological insulators and future phononic circuits.

Continuum-scale simulations of phase change processes are essential for assisting the design of engineering systems, such as heat pipes and combustion chambers. To quantitatively predict the rate of the phase change in micro-scale systems it is imperative to properly account for the boundary conditions at the interface between phases. For example, in liquid-vapor phase change simulations, the typical continuum assumptions are, a) temperature is continuous at the interface, and b) vapor density near the interface is equal to the saturated density. Results of molecular dynamics (MD) simulations have indicated that in many situations such assumptions are not satisfied. To address these issues, we present a framework for continuum simulations capable of relaxing the aforementioned assumptions. We use finite elements to solve the Navier-Stokes equations while correctly accounting for the flux jump conditions at the interface. Expressions for phase change rates and temperature jumps are obtained from theoretical considerations and augmented by MD simulations. Problems including the burning of solid propellants and evaporation/condensation of liquids are discussed.

Spectral emissivity control is critical for optical and thermal management in the ambient environment because solar irradiance and atmospheric transmissions occur at distinct wavelength regions. For instance, selective emitters with low emissivity in the solar spectrum but high emissivity in the mid-infrared can lead to significant radiative cooling. Ambient variations require not only spectral control but also a mechanism to adjust the emissivity. However, most selective emitters are fixed to specific wavelength ranges and lack dynamic control mechanisms. Here we show ultraviolet to mid-infrared emissivity control by mechanically reconfiguring graphene, in which stretching and releasing induce dynamic topographic changes. We fabricate crumpled graphene with pitches ranging from 40 nm to 10 µm using deformable substrates. Our measurements and computations show that 290-nm-pitch crumpled graphene offers ultraviolet emissivity control in 200-300 nm wavelengths whereas 10-µm-pitch crumpled graphene offers mid-infrared emissivity control in 7-19 µm wavelengths. Significant emissivity changes arise from interference induced by the periodic topography and selective transmissivity reductions. Dynamic stretching and releasing of 290-nm- and 10-µm-pitch crumpled graphene show reversible emissivity peak changes at 250 nm and at 9.9 µm wavelengths, respectively. This work demonstrates the unique potential of crumpled graphene as a reconfigurable optical and thermal management platform.

Plasmonic nanostructures generate heat via the photothermal effect when excited with a specific wavelength of light, where optically excited surface plasmons decay to produce hot electrons that subsequently couple to the lattice vibrations. Photothermal cancer therapy shows promise as an application using this effect, where local hyperthermia leads to cell death. However, noble metal nanoparticles traditionally used in plasmonic applications exhibit surface plasmon resonance in the lower-wavelength visible range, where human tissue is not transparent. As a result, recent research efforts have pushed new plasmonic nanostructures and materials into the near- and mid-infrared ranges by using anisotropic geometries. Gold nanorods are extensively used due to their wavelength tunability of the plasmon resonance and their ability to be modified and functionalized for specific binding, enhanced detection, and drug delivery. The thermal transport of this system at the cellular level is not heavily studied, therefore we experimentally measure the thermal transport in a system of cancer cells infiltrated with gold nanorods in vitro at various concentrations, to understand the minimum illumination intensities and durations required for cancer cell death. We report, for the first time, the interfacial thermal conductance between gold nanorods and the ovarian cancer cellular environment as measured by ultrafast pump-probe laser techniques. We also report the thermal conductivity and heat capacity of the cancer cells. By understanding the thermal properties of this system, more defined and tailored photothermal therapy regimens are made possible. This study also provides an experimental framework for measuring the thermal properties of other photothermal therapy agents in cellular environments in the future.

Aerospace applications in which high heat fluxes are reached necessitate the use of unique materials to withstand these harsh environments. The need for environmental barrier coatings (EBCs) are of particular importance in limiting the oxidation of aerospace components and the volatility of the SiO₂ scale commonly found in silicon-based ceramic coatings. Rare earth mono- and disilicates have been identified as promising EBCs due to their ease of application via various plasma spray processes. However, a robust examination of the insulating thermal properties of these systems has yet to be performed. To better understand the thermal processes and anisotropy associated with these disilicates, we examine the commonly used polymorph of yttrium disilicate, γ-Y₂Si₂O₇. Using a combination of time- and frequency-domain thermoreflectance, we compare the extracted thermal conductivity maps to associated electron backscatter diffraction micrographs. The thermal conductivity maps exhibit strong anisotropy in the monoclinic structure, in good agreement with other anisotropic features of the system. The results are significant in that they are the first experimental measurements of the anisotropic thermal conductivity in a thermal barrier coating, and should provide a basis for plasma-based applications in which the potential for texturing is strong.

Materials science is challenged with developing new materials in order to meet the demands of technological innovation. Consequently, this opens the door to novel or complex properties awaiting exploration. High entropy carbides (HECs) continue to demonstrate the viability of materials engineering using configurational entropy to aid in phase development. Using two compositions, Hf-Nb-Ta-Ti-Zr-C (HEC3) and Hf-Mo-Ta-W-Zr-C (HEC6), we explore the thermal transport properties of these systems as carbon stoichiometry is varied. Using time domain thermoreflectance (TDTR), we measure thermal conductivity of both the HEC3 and HEC6 thin film series and then relate trends to other observed characteristics of each system such as electrical conductivity, crystallinity, and modulus. Total thermal conductivity systematically varies with increasing deposition flow rate of methane, while the electrical contribution to the thermal conductivity decreases. Results are discussed in terms of various scattering mechanisms from different types of defects, emphasizing understanding of thermal properties from both a local and a global structural perspective. In this talk we focus on the experimental process of elimination using several metrology techniques in conjunction with TDTR to gain meaningful perspective on configurationally disordered, highly crystalline systems.

Wavelike thermal transport in solids, referred to as second sound, has until now been an exotic phenomenon limited to a handful of materials at low temperatures. This has restricted interest in its occurrence and in its potential applications. Through time-resolved optical measurements of thermal transport on 5-20 μm length scales in graphite, we have made direct observations of second sound at temperatures above 100 K. The results are in qualitative agreement with ab initio calculations that predict wavelike phonon hydrodynamics on ~ 1-μm length scale up to almost room temperature. The results suggest an important role of second sound in microscale transient heat transport in two-dimensional and layered materials in a wide temperature range.

As increasingly powerful microelectronic chips shrink in size and scale, inadequate waste heat dissipation ultimately results in poor computational performance or early device failure. Therefore, advances in thermal interface materials (TIMs) are necessary in combating detrimental heating issues by providing improved avenues for thermal transport. Many types of polymer-filler combinations for TIMs have been explored to maximize the composite thermal conductivity, while only leading to thermal conductivities of up to 2 W m^-1K^-1. One of the primary limitations to efficient thermal transport in TIMs lies in the boundary resistance generated by the point-contact of filler particles. Additionally, the thermally conductive inclusions are often rigid particles which can lower the elasticity of the composite at high fill factors. Increasing the effective contact area between filler materials could potentially result in much higher composite thermal conductivities [1,2]. Previous work done by Ralphs et al. has shown a substantial increase in thermal conductivity by up to 17 W m^-1K^-1with copper particles bridged by liquid metals in a PDMS matrix [3].
In this work, multi-phase fillers comprised of a solid metal core with a liquid metal shell are proposed as a promising solution to reducing point-contact boundary resistance. Liquid metals form a self-limiting oxide layer that encapsulates the liquid metal and can rupture under applied pressure, therefore allowing liquid metal coated particles to connect. Due to the tendency of liquid metals to alloy with and/or embrittle many metals, tungsten is selected as the core material for its relative inertness towards liquid metals. In addition, tungsten particles coated with liquid metals can also be mechanically processed to increase the effective contact area between adjacent particles. The novel combination of deformable liquid shells and highly thermally conductive solid particles can potentially lead to composite materials that facilitate more efficient heat transport and are still stretchable. This research seeks to demonstrate a reduction in thermal boundary resistance that improves TIMs through novel materials processing methods.
References:
[1] Mamunya et al. Eur. Polym. J. 2002. 8(9), 1887-1897.
[2] Seshadri et al. Adv. Mater. Interfaces. 2015. 27(17), 175601.
[3] Ralphs et al. ACS App. Mater Inter., 2018.10(2), 2083-2092.

Rechagrable Li-ion batteries (LIBs) are very promising candidates for electrochemical energy storage in electrical vechiles. Electrical conduction propeties in these systems are strongly affected by thermal transport properties during electrochemical cycling. Therefore, thermal management in these systems is very crucial for the control of electrical properties of these storage devices [1]. Si-based anode is a very attractive material because of potentially large achievable capacity of 3700 mAhg^-1 [2] that is ten times more than that for graphitic carbon, which is mainly used as an anode material in LIBs. However it is well known that Si electrodes suffer from volumetric change during the electrochemical cycling/recycling processes, which in turn leads to change in mechanical and electrical properties. But little research has been done on thermal conductivity variation in Si films during the lithiation. In this work we aim to study nanoscale thermal transport in this nanoscale-thick material using ex situ picosecond time-domain thermoreflectance (TDTR) approach [3]. RF magnetron sputtering was used to deposit a ~300 nm thick Si films on glass subatrate. A 50 nm thick Al was deposited on top of Si serving as an optical absorber and efficient heat transducer in TDTR experiments. The lithiation of films has been performed in the beaker cell with lithium hexafluorophosphate (LPF) in the solution of ethylene, diethyl carbonate and ethyl methyl carbonates (EC:DC:EMC = 1:1:1, v/v) using a constant current of 25 µA up to 0.05 V. Thermal transport measurements were done for non-lithiated and lithiated samples. The results show that for non-lithiated amorphous Si films the thermal conductivity value is ~ 1.4 W/m*K, which is very close to literature value [4]. After electrochemical lithation process, the thermal conductivity of lithiated amorphous Si was in the range between 1.3 to 2.2 W/m*K. This sizeable thermal transport fluctuation was likely due to inhomogeneous lateral Li ion distribution on the near-surface region. We also have done measurements on Young’s elastic modulus of these thin film materials using nanosecond laser pulse-induced surface acoustic waves. The results showed the decrease in Young’s modulus after lithiation as it is expected because of volumetric expansion of Si crystal.
Funding from MES RK state-targeted programs BR05236454, BR05236524 and grant AP05130446 is acknowledged.
References:
[1] J. Cho et al., Nature Communication, 5 (2014) 4035.
[2] C. J. Wen, R.A. Huggins, J. Solid State Chem. 37 (1981) 271.
[3] D.G. Cahill, Rev. Sci. Instrum. 75(12), 5119 (2004).
[4] S. Moon et al, Inter. J. Heat and Mass Transfer, 45 (2002) 2439.

Two fundamentally different avenues for controlling a materials thermal conductivity are phonon scattering and lattice softening. In the phonon scattering picture, the phonon dispersion relation and group velocities are assumed to be fixed and lattice defects reduce lattice thermal conductivity (κ_L) by introducing phonon scattering centers. Lattice softening recognizes that lattice defects alter the phonon dispersion relation and thus reduce κ_L by reducing phonon frequencies and group velocities. Here, I will present experimental data on several systems (Si, PbTe, and SnTe) which demonstrate that microstructural defects such as grain boundaries, dislocations, and vacancies can significantly softening a materials lattice, reducing the materials speed of sound and elastic moduli. By analyzing the data on elasticity and thermal conductivity through standard transport modeling, it is shown that lattice softening is a dominate mechanism for the reduction of κ_L in these systems. Additionally, it will be shown that lattice softening is theoretically expected to be more effective than phonon scattering effects in anharmonic materials and at high temperatures. This work demonstrates how lattice softening is emerging as an important mechanism for controlling a materials thermal conductivity, and provides new avenues to engineer a materials κ_L, beyond phonon scattering.

Gallium-based room temperature liquid metals have become a hot topic in the thermal management of traditional semiconductor packaging and stretchable electronics. However, the native gallium oxide shell on these liquids both facilitates many unique processing techniques as well as hinders others, making it a force that needs to be reckoned with. Some studies have been done on chemically and mechanically altering the oxide layer, but very few have focused on the thermal ramifications of the oxide and its rupture. In this work, we present a comprehensive set of experiments on parameters controlling fracturing of the oxide shell on micro and nanoscale liquid metal droplets and the resulting thermal conductivity of the droplet ensemble. Specifically, we fabricate liquid metal droplets with diameters ranging from hundreds of nanometers to ten micrometers and pack them into macroscopic films. We then compress the films while measuring thermal resistance, pressure, and electrical resistance in situthrough and beyond oxide rupture. This allows us to interrogate the mechanical role of the oxide layer and the thermal ramifications of the oxide shells when dealing with liquid metal droplets. Various sizes of liquid metal droplets are investigated as well as mixtures of liquid metal droplets and solid metal particles. The inclusion of solid and liquid metal particles together gives insight into the role of particle packing and how it relates to thermal resistance and oxide failure. The measured thermal properties and how they relate to the mechanics of the rupturing oxide will be explained. Additionally, this work will lead to insights on using liquid metal coated solid metal particles in a polymer composite to reduce the thermal boundary resistance between particles and greatly enhance the thermal conductivity of such composites.

Thermal energy storage devices (i.e. “thermal batteries) are utilized in scenarios where thermal energy is available at certain times but required for use at a later time. The amount of energy that can be stored within a thermal energy storage device depends on both the storage material and the mode of storage. Systems that incorporate thermochemical processes are often able to store significantly more energy than sensible and latent heat thermal energy storage systems.

This presentation focuses on the use of N-isopropylacrylamide (NIPAAm) for sorption thermal energy storage. NIPAAm is a thermoresponsive polymer that transitions from hydrophilic to hydrophobic upon crossing its critical temperature (typically 32 °C). In addition, NIPAAm has the ability to absorb water vapor (i.e. humidity) and expel liquid water. This creates huge energy savings potential during sorbent regeneration that is equivalent to the enthalpy of vaporization of water. The energy density of NIPAAm is also particularly attractive because it can be used in conjunction with hydrogels that can store up to 40 times the gel’s dehydrated mass in water.

To explore the potential of NIPAAm for cooling applications, we conduct system-level modeling of sorption systems utilizing this novel material. This system charges up by absorbing water from humid air that is passed through the device. When cooling is desired, the NIPAAm is heated above to 32 °C to expel liquid water that is then evaporated to cool the system. This sorption process can potentially exceed the energy density of traditional latent heat systems (e.g. cold thermal storage with ice) because the enthalpy of vaporization significantly exceeds the enthalpy of melting. The presentation on sorption system modeling with NIPAAm will highlight the exciting opportunities for this novel material in thermal energy storage applications.

The role of the computational material modeling and simulations is indispensable in the design and development of advanced materials with superior material properties. Here we present a coarse-grained molecular dynamics (CGMD) simulations-based investigation to understand a melting behavior of the mesoscopic electro-spun silica-nanofiber based insulator materials, and subsequently elucidate the silica nanofiber size-melting temperature relationship. A coarse-grain bead, which is equivalent to the spherical (SiO2)6 silica atomic structure, was used to construct the several mesoscopic silica nanofiber structures with sizes > 100 nm that were consequently simulated for several hundred nanoseconds. The temporal bead trajectory information was used to decipher the melting behavior of the silica nanofibers. The silica nanofiber size-melting temperature dependency evaluated through CGMD simulations could provide some valuable guidelines for the further optimization of the silica nanofiber manufacturing processes.

In the United States, nearly 68% of the primary energy produced each year is wasted as heat. A couple tangible examples of this wasted energy is the heat that radiates from laptops and air conditioning units that remove heat from air and release it into the environment. It would be efficient to convert this waste heat into a better source of energy such as current. Using the ferroelectric properties of BaTiO₃, this energy conversion can be done.

BaTiO₃ undergoes structural transitions when heat is applied at its curie temperature, approximately 120-130^ο C. During this structural transition, BaTiO₃ goes from paraelectric to ferroelectric. While in its ferroelectric state, BaTiO₃ behaves like a very good capacitor and during its paraelectric state, it behaves like a poor capacitor. Consequently, we can attach BaTiO₃ to a circuit with a reference capacitor (ordinary capacitor), and we can charge this circuit to equilibrium and remove all energy sources. As BaTiO₃ is heated and cooled, current flows from the reference capacitor to the activate BaTiO₃ capacitor. Experimentally, we have observed the current flows consistently as the heat oscillates in the curie temperature range.

Converting waste heat to current is an efficient energy conversion method.

In this work, we present an experimental study on magneto-thermal transport and spin-Seebeck effect behavior in ferromagnetic and semiconductor materials. The magneto-thermal transport measurements are carried out using in-plane self-heating three-omega method. The in-plane three-omega measurement requires a freestanding specimen. We address this challenge using micro-electro-mechanical systems (MEMS) fabrication methods. The measurements are carried out on Py/Au and Py/Si. We also demonstrate giant spin-Seebeck effect in Py/Si bilayer thin films. The mageto-thermal transport measurements on these thin films are essential for the design and development of energy efficient spintronics devices. We employ four point probe configuration for our MEMS device structure to probe mainly the second and third harmonic voltages as well as resistance with respect to changing temperature and applied magnetic field. The results can be related to thermal transport properties of the system using the three-omega method as mentioned above.

Disordered media are of special interest due to low thermal conductivity. Amorphous silicon (a-Si) has been widely used due to their low-cost potential. Due to the lack of long-range order, identifying thermal transport in a-Si has been challenging. For example, mean-free paths (MFPs) distribution of propagon, a propagating delocalized atomic vibrational state, and its contribution to the thermal conductivity have long been contentious. Here we report measurements of in-plane quasi-ballistic thermal transport from suspended a-Si thin film using transient grating spectroscopy. Our observations demonstrate that the phonons with grating period greater than 1 (mm) can contribute more than half to the thermal conductivity. Nearly temperature independent diffusivity further shows that anharmonic scattering may not be dominant over the temperature range that we studied: 30 (K) to 300 (K). Our non-contact measurement will provide useful insights into studying disordered materials that have been challenging with conventional thermal characterization methods.

Cubic boron arsenide (BAs) bulk crystal has been identified to have an ultrahigh thermal conductivity (~1300Wm^-1K^-1) both theoretically¹ and experimentally^2-4. On the other hand, two-dimensional (2D) stable monolayers of BAs have also been reported to have promising electronic, transport, optical and thermoelectric properties⁵. However, the thermal conductivity of BAs monolayer has not yet been investigated. Does thermal transport in BAs monolayers parallel that of cubic BAs bulk crystals? We present our first-principles study on the thermal conductivity of BAs monolayer.
The vibrational spectra of BAs monolayers have been calculated and are quite different from that of cubic BAs bulk crystals⁵. Two transversal acoustic and optical branches, which correspond to out-of-plane atomic vibrations in BAs monolayers, carry lower energy due to the fact that atoms can more easily vibrate perpendicular to the plane. The so-called “acoustic bunching” behavior in cubic BAs is also not seen in the vibrational spectra of BAs monolayer due to the presence of these out-of-plane branches. Nonetheless, the giant phononic band gap between the acoustic branches and two in-plane optical branches still exists in the vibrational spectra of BAs monolayers. At this point, qualitative analysis of thermal conductivity based on the vibration spectra is inconclusive. More rigorous computational calculations need to be performed to derive the lattice thermal conductivity of BAs monolayers.
For our computations, we employ density-functional perturbation theory to generate second-order (harmonic) and third-order (anharmonic) interatomic force constants (IFC) for BAs monolayer using the VASP package. The harmonic IFC allows us to derive the vibrational properties, while the anharmonic IFC allows us to determine the three-phonon scattering rates using Fermi’s golden rule. With the only inputs of harmonic and anharmonic IFC, we can derive the lattice thermal conductivity by solving the exact phonon Boltzmann Transport Equation using the ShengBTE package⁶.
Reference:
1. L. Lindsay, D. A. Broido and T. L. Reinecke, Phys Rev Lett, 2013, 111, 025901.
2. F. Tian, B. Song, X. Chen, N. K. Ravichandran, Y. C. Lv, K. Chen, S. Sullivan, J. Kim, Y. Y. Zhou, T. H. Liu, M. Goni, Z. W. Ding, J. Y. Sun, G. A. G. U. Gamage, H. R. Sun, H. Ziyaee, S. Y. Huyan, L. Z. Deng, J. S. Zhou, A. J. Schmidt, S. Chen, C. W. Chu, P. S. E. Y. Huang, D. Broido, L. Shi, G. Chen and Z. F. Ren, Science, 2018, 361, 582.
3. J. S. Kang, M. Li, H. A. Wu, H. Nguyen and Y. J. Hu, Science, 2018, 361, 575-578.
4. S. Li, Q. Y. Zheng, Y. C. Lv, X. Y. Liu, X. Q. Wang, P. S. E. Y. Huang, D. G. Cahill and B. Lv, Science, 2018, 361, 579-581.
5. K. Manoharan and V. Subramanian, Acs Omega, 2018, 3, 9533-9543.
6. W. Li, J. Carrete, N. A. Katcho and N. Mingo, Comput Phys Commun, 2014, 185, 1747-1758.

In aerospace applications operating at very high temperatures (> 3000 K), such as hypersonic flight or rocket propulsion systems, there is a critical need for the development of materials which can survive such extreme temperatures. To meet this need, ultra-high temperature ceramics (UHTCs) have proven to be at the forefront of research. While early development of UHTCs were primarily focused on SiC and Si₃N₄, currently there is a strong interest in transition metal diborides such as hafnium diboride (HfB₂) and zirconium diboride (ZrB₂) as well as carbides such as hafnium carbide (HfC) and tantalum carbide (TaC). Looking beyond binary carbides and diborides, materials synthesis of high-entropy ceramics consisting of five or more elements has enabled a new research direction to explore the engineering of thermophysical properties to enable sustainable high-temperature application. One of the most critical properties to understand in these systems is thermal conductivity. While bulk thermal conductivity measurements provide a metric for determining how heat flows on average in the material, these techniques disregard any insight into microstructure on thermal transport. Because failure can occur due to local weak points, it is therefore necessary to probe local thermal properties to understand the microstructural impacts on thermal transport.

To this end, here we use time-domain thermoreflectance (TDTR) to conduct thermal mapping over a large area to resolve localized thermal conductivity as a function of real space. Paired with Scanning Electron Microscope (SEM) and Electron Backscatter Diffraction (EBSD) images, we relate local properties to crystal orientation and microstructure. We measure the thermal conductivity of a high entropy diboride (HfZrTaNbTi)B₁₀ and high entropy carbide (HfNbTaTiZr)C₅. Using TDTR, we map the thermal conductivity of these bulk samples, which have ~30 micron grains. Comparison of the thermal conductivity map to SEM and EBSD images reveals the role of anisotropy and grain boundary scattering of electrons and phonons on reducing the overall thermal conductivities of these samples.

Theoretical studies have shown that solar thermophotovoltaic (STPV) systems have the potential to produce efficiencies up to 85%, but current technology has only reported actual efficiencies around 6.8%. Studies have clearly shown that the major drawbacks in the components of a STPV system are the solar absorber and emitter. Current solar absorbers have shown to have high thermal emission causing heat loss and emitters display a lack in thermal emission above the bandgap of TPV cells. It is desired to have a solar absorber with unit absorptance in the solar spectrum, while having zero emittance in the longer wavelengths. In addition to this, emitters need to have unity emittance only within a narrow band that matches the high quantum efficiency of a specific TPV cell to minimize non-useful photons. Our experimental study will mainly address the issue of high thermal emission of solar absorbers by applying the highly-efficient selective metafilm coating recently developed in our lab. We propose to use the metafilm which has demonstrated high temperature thermal stability, as a solar absorber to investigate STPV system energy conversion. We will also address the potential to use such selective coatings as a selective emitter with further structure optimization to further improve STPV systems. The result of our experimental study will produce a better understanding of loss mechanism in STPV systems and bridge the gap between theoretical studies and real application. Our theoretical calculations predict a potential to achieve an STPV efficiency of 10% or higher with the aforementioned metafilm selective solar absorber and TPV emitter. In conclusion, this project, by closely examining the inefficiencies of solar absorbers in STPV systems, will bring to light the potential of highly-selective metafilm enhanced STPV systems.

A basic thermophotovoltaic (TPV) system consists of a thermal emitter and a photovoltaic cell that converts heat to electricity via infrared photons or thermal radiation. While great progress has been reported recently to achieve spectrally-selective thermal emitters made of photonic crystals, metamaterials and multilayers, a wavelength selective optical filter is another possible route to increase the TPV efficiency by reflecting the photons with energy below the bandgap at longer wavelengths. Optical filters with narrow transmission band right above the bandgap of TPV cells are not restrained by the rigorous thermal reliability as required for the emitters. In this work, we fabricate and characterize a novel metasurface filter made of an aluminum (Al) nanodisc array to achieve spectrally selective transmission above the bandgap of TPV cell. The filter is fabricated by depositing thin Al onto the quartz substrate which is covered by anodized aluminum oxide (AAO) templates. Optical simulations will be carefully performed to determine the appropriate AAO interpore distance, AAO pore diameter and the Al film thickness such that the resulting Al nanodisc array will show narrowband transmission at a wavelength of 1.6 micron, which is the bandgap of a commercial Gallium Antimonide (GaSb) TPV cell. Preliminary simulations indicate that the narrow-band transmission is due to the articular magnetic resonance between neighboring Al nanodiscs, which channels energy through at wavelengths near the resonance. The optical and radiative properties of the Al nanodisc metasurfaces will be characterized by an FTIR spectrometer and a monochromator with an integrating sphere. The external quantum efficiency (EQE) of the commercial GaSb TPV cell will be measured with and without the fabricated metasurface filter covered, aiming to show the narrow-band EQE spectrum right above the cell bandgap due to the spectral filtering. By pairing the filter with a tungsten thin film emitter at 600°C or higher, the I-V curve from the GaSb TPV cell will be measured and the heat-to-power TPV efficiency will be calculated. Enhanced TPV efficiency is sought to be experimentally demonstrated with the metasurface filter. Theoretical energy analysis for the TPV system will be performed to validate and explain the experimental data.

With the ever shrinking devices sizes following Moore’s law, heat dissipation becomes a bottle-neck issue for semiconductor industry. Discovering new materials with ultrahigh thermal conductivity can lead to efficient thermal management and has been intensively investigated by recent theories. In particular, a new semiconductor, boron arsenide (BAs) has been predicted to have ultrahigh thermal conductivity by ab initio calculations. However, experimental development has been scarce and challenged by crystal quality. Here, we report our recent experimental development in synthesizing single-crystal BAs with minimum defects and measured a record-high thermal conductivity of 1300 W/mK beyond all bulk semiconductors and metals. To better understand the fundamental origin of such high thermal conductivity, we performed advanced phonon spectroscopy. Our measurements reveal that, unlike the commonly accepted rule for most materials that the three-phonon process is the only dominant scattering mechanism, high-order anharmonicity through the four-phonon process is significant in BAs because of its unique band structure. In addition, we probed the phonon spectra through ballistic transport and show that phonons with long mean free paths are the major contributors to the BAs and BP thermal conductivity. Our study established BAs as a new benchmark thermal management material and underscores the promise of discovering advanced thermal materials through experimental-theory synergy.

References
1. Joon Sang Kang, Man Li, Huan Wu, Huuduy Nguyen, and Yongjie Hu, “Experimental observation of high thermal conductivity in boron arsenide,” Science 361, 575-578 (2018).
2.Joon Sang Kang, Huan Wu, and Yongjie Hu, “Thermal Properties and Phonon Spectral Characterization of Synthetic Boron Phosphide for High Thermal Conductivity Applications,” Nano Letters 17, 7507 (2017).
3. Joon Sang Kang, Ming Ke, and Yongjie Hu, "Ionic Intercalation in Two-Dimensional van der Waals Materials: In Situ Characterization and Electrochemical Control of the Anisotropic Thermal Conductivity of Black Phosphorus," Nano Letters 17, 1431–1438 (2017).
4. Zhaoyang Lin*, Courtney Hollar*, Joon Sang Kang* (co-first author) et al., "A Solution Processable High Performance Thermoelectric Copper Selenide Thin Film," Advanced Materials 29, 1606662 (2017).
5. Man Li, Joon Sang Kang, and Yongjie Hu, "Anisotropic thermal conductivity measurement using a new Asymmetric-Beam Time-Domain Thermoreflectance (AB-TDTR) method," Review of Scientific Instruments 89, 084901 (2018).

One of the most challenging obstacles industry faces as device sizes become smaller is effective thermal mitigation. Fundamental theories for phonon scattering such as the Klemens model predict reductions in thermal conductivity from point defects, extended defects, and strain. To experimentally measure the perturbation of individual phonon scattering mechanisms, we employ ion irradiation as a controlled means of introducing strain and assorted defects into a crystalline lattice. In particular, we observe the reduction in thermal conductivity as a result of varying the mass of a phonon scattering source. To achieve this mass perturbation, we irradiate diamond wafers with three different ions: C+, N+, and O+, exposing the samples to ion fluences ranging from 4(10¹⁴) to 4(10¹⁶) cm^-2. To measure the thermal conductivity of the samples, we employ time domain thermoreflectance (TDTR), a laser based technique which correlates the change in reflectivity of the sample, in response to a heating event, to a change in temperature. For the highest-dosed diamond samples, we record a reduction in thermal conductivity of two orders of magnitude. Furthermore, we find that thermal conductivity decreases more with increasing ion mass. To gain further insight into these reductions in thermal conductivity, a Klemens model for phonon scattering is applied which takes into consideration ion mass, atomic volume, and change in local strain.

Thermal regulation in solid-state devices has received extensive attention over the years due to its potential applications in thermal management, thermal circuits and thermal computing. Near-field thermal radiation with super-Planckian heat flux beyond Blackbody limit inspired a new route for thermal control with thermal photons, particularly with recent achievements in near-field radiative heat transfer measurements. In this talk, we will first review the theoretical studies on tunable near-field thermal radiation, in particular from our group, such as near-field thermal rectification, thermal switching and thermal modulation based on thermochromic VO₂ and tunable graphene. VO₂, which changes phase between an insulator with strong infrared phonons and a metal with optical phonons around 68degC. Therefore, the near-field coupling due to surface phonon polaritons between VO₂ and another dielectric could be weakened by changing the VO₂ to metallic phase at higher temperatures. As a result, the near-field radiative heat flux can be tuned thermally with the VO₂ temperature. On the other hand, graphene plasmon and its radiative properties can be modulated by chemical doping or electrical gating, such that the near-field coupling strength and resulting radiative heat transfer between graphene surface plasmons can be tuned dynamically. Fabrication and characterization of VO₂ thin films and graphene will be then discussed in detail. By using a plate-plate thermal metrology with nanometer gaps created by sparse polymer posts, we experimentally demonstrate near-field radiative thermal regulation between graphene covered SiO₂/Si plates with 1cm² area for the first time by modulating graphene conductivity through external gating potential. Besides, a near-field radiative thermal rectifier and switch based on VO₂ phase change will be experimentally demonstrated as well. Our work will facilitate the active control of near-field thermal radiation in solid-state applications.

During the last two decades, tremendous progress has been made in quantitative understanding of several major phonon scattering mechanisms, including phonon-phonon, phonon-boundary, phonon-defects, as they are the determinant factors in lattice thermal transport that is critical for the proper functioning of various electronic and energy conversion devices. However, the roles of another major scattering mechanism, electron-phonon (e-ph) interactions, remain elusive. This is largely due to the lack of solid experimental data for the effects of e-ph scattering in the lattice thermal conductivity for the material systems studied thus far. Through systematic measurements, we show distinct signatures in the lattice thermal conductivity observed below the charge density wave (CDW) transition temperatures in NbSe₃ nanowires, a type of quasi-1D van der Waals (vdW) crystals with covalently bonded molecular chains assembled together through vdW interactions. Owing to the restricted dimensionality, the density of free electrons in NbSe₃ spontaneously develops a wave-like variation due to CDW phase transitions. The variation of charge carrier concentration upon the onset of CDW modulates the e-ph scattering strength, resulting in unique features in the lattice thermal conductivity.

In addition, while the measured lattice thermal conductivity demonstrates a decreasing trend as size reduces for wires with diameter >26 nm, an unexpected increasing trend was observed as the lateral dimension further reduces. In fact, the room-temperature thermal conductivity of NbSe₃ nanowires could be enhanced by more than 20 fold as the diameter reduces, which is attributed to the dimensional crossover from 3D to 1D for phonon transport in thin NbSe₃ nanowires. The excited 1D phonons propagate along the molecular chains and contribute to the observed much enhanced thermal conductivity. These studies provide new insights into thermal transport through vdW crystals, which helps to lay the physical foundation of quasi-1D vdW materials in engineering applications.

A bridging layer with matching properties can faciliate phonon transport across interfaces. In this work, we study the role of anharmonicity in such systems. Our results demonstrate the existence of a minimum interfacial thermal conductance G versus layer thickness L with weak anharmonicity and the existence of a maximum G vs. L with strong anharmonicity. The minimum thermal conductance appears at a thin layer thinkness and is due to the competing roles of quantum effect and anharmonicity. The maximum thermal conductance is a result of two contradictary effects of anharmonicity: "re-thermalization" and "umklapp scattering". Moreover, we present that the effect of aharmonicity on the conductance can be tuned by varying temperature or bridging layer thickness, as both parameters can change the strength of anharmonicity. Additionally, we propose a critical length λ_s as a criteria at which the resistance can be separated as resistances in series in the presence of both weak and strong anharmonicity. Additionally, we compared them to the bulk mean free path λ_b, and demonstrated that λ_s is much smaller than λ_b, hinting that the summation over resistance rule can be used at much smaller length than the bulk mean free path, and a comparison between λ_b with L cannot be used to treat boundary resistance or classical size effect for thin films between two materials.

Wearable applications with multi-function integration afford exciting research opportunities. Among all the functions, effectively regulating the heat flow between human body and environment not only increases thermal comfort but also presents a novel and cost-effective approach for reducing the building energy consumption. Here I will present my group work on thermal consideration and materials design for new generation of textile and wearables. Particularly with regulation of human body infrared radiation, we demonstrated the best heating and cooling textiles. We also develop new concept of bifunctional textile which can switch between heating and cooling mode. In addition, we also demonstrate the cooling textile under the sun.

The flow of heat through materials is a topic of scientific interest and technological importance in fields of microelectronics, power generation, heat management, and thermoelectrics. For example, advancement in microelectronic technologies (e.g. microprocessors, and high-power electronics) demands ever more efficient removal of the heat generated in these devices. In contrast, technologies such as thermal barrier coatings and thermoelectric materials are designed to stop the flow of heat. Thus, progress in these fields and many others relies on understanding how microstructural defects such as interfaces, dislocations, and point defects influence a materials thermal conductivity.

In this presentation, I will highlight several experimental and theoretical results which aim to establish a fundamental understanding of heat transport in defective materials. First, I will discuss several studies related to heat conduction across interfaces. Secondly, I will demonstrate in several material systems how defects can soften a materials lattice which reduces the phonon group velocity and thus decreases thermal conductivity. Lattice softening is a fundamentally different avenue to engineer thermal conductivity than phonon-defect scattering. I will then explain why lattice softening is theoretically more effective in anharmonic materials and at high temperatures, and how this is a dominate mechanism at play in several high thermoelectric efficiency (zT>2) PbTe based materials. Finally, I will discuss heat conduction in the ‘fully defective’ or amorphous limit. In this case the lack of sufficient periodicity leads to the concept of a minimum thermal conductivity defined by diffusonquasiparticles instead of phonons.

If they existed, solid-state heat switches with a large switching ratio between their off and on states would have many applications [1]. They would be particularly useful in combination with magnetocaloric engines and adiabatic demagnetization coolers, which are capable of reaching temperatures of 1 to 100 mK. In this talk, a new physical mechanism will be described for magnetically controlled entropy and heat transport based on the topological properties of Weyl semimetals [2]. Weyl semimetals are electrically conducting solids in which the bulk band structure consists of Dirac bands that come in pairs, with the Dirac points of each pair separated by a wavevector k₀. When the Fermi energy is at the Dirac points, charge is carried in the bulk by both electrons and holes. The carriers also have a Berry curvature Ω_z. The surface states that correspond to this dispersion have themselves a dispersion that forms an arc in k-space, one on each of the top and bottom surfaces. An applied magnetic field normal to these arcs accelerates the electrons and holes, giving rise to a conveyor-belt flow of particles between the top surface, the bulk, the bottom surface, and the bulk again to the top surface. No net charge current flows since the carriers are of equal density and opposite polarity. Thermal transport along this conveyer belt stands in strong contrast to electrical transport. We show how in the presence of a temperature gradient, these electrons and holes carry a non-zero entropy along the conveyer belt. This leads to an additional field-driven electronic thermal conductance between top and bottom surfaces. Experimental data will be shown on semiconducting Bi-Sb alloys, in which band inversion and a Weyl semimetallic state is induced by Zeeman splitting of the Landau levels. The field-induced thermal conductance dominates the electronic thermal conductivity in longitudinal magnetic fields. The effect is macroscopic and visible on millimeter-sized samples up to 200 K.

Recent experiment has proved that superionic Ag₂Te can achieve a figure of merit as high as 1.39 due to its extremely low thermal conductivity. However, the traditional lattice vibration concept, i.e., phonons regarded as heat carriers, fails to explain the governing mechanism in such structures where anions vibrate around their equilibrium positions while cations flow like a liquid. As a result, the underlying physics for thermal transport properties in superionic Ag₂Te is still a mystery. In this study, two-channel heat transport in such ordered−disordered systems (i.e., lattice vibrations and liquid-like mobile ions coexist) is quantitatively characterized on the basis of the heat flux linear response theory. Our results show that the convective thermal conductivity is dominant in the system, which results from the free movement of Ag ions. As a consequence, the total thermal conductivity increases abnormally with temperature due to the strengthened cations’ mobilities at elevated temperatures. Meanwhile, the effect of experimentally observed nanotwin boundaries, which facilitate electrical transport in crystals, on heat carriers are also investigated. In contradiction to the classical heat transport theory, in which heat carriers are hindered by boundaries, the nanotwin boundary in superionic Ag₂Te unexpectedly improves thermal transport due to the enhanced movement of Ag ions around the grain boundary. This exhaustive explanation of thermal transport properties in superionic Ag₂Te will support the future design of superionic conductors based on thermoelectrics and more broad energy systems composed of ordered−disordered materials.

Solid-state cooling is a sustainable and energy efficient refrigerant technology that exploits field-induced reversible transformations in materials. Solid-state cooling is a promising alternative to traditional refrigeration technologies based on compression cycles of greenhouse gases, which in addition to their obvious environmental threats cannot be scaled down to small sizes (e.g., microchip dimensions). However, most caloric materials known to date (ferroelastic, ferroelectric, and magnetic compounds) display only modest refrigeration performances and/or operate at temperatures that are distant from ambient conditions.

Recently, we have demonstrated with state-of-the-art simulation methods that giant caloric effects occur in fast-ion conductors [1-3], a class of materials with high ionic conductivity below their fusion points that commonly are exploited in electro-chemical devices. The giant caloric effects disclosed in fast-ion conductors can be understood in terms of field-driven changes on their ionic conductivity, which entail large entropy changes and are highly reversible and responsive to external fields. A giant cooling effect of about 20 K has been just measured directly in the archetypal superionic compound AgI [4]. We argue then that solid-state cooling could benefit immensely from the intensive research already undertaken in the field of solid-sate batteries.

[1] C. Cazorla and D. Errandonea, Nano Letters 16, 3124 (2016)
[2] A. K. Sagotra, D. Errandonea, and C. Cazorla, Nature Communications 8, 963 (2017)
[3] A. K. Sagotra, D. Chu, and C. Cazorla, Nature Communications 9, 3337 (2018)
[4] A. Aznar, P. Lloveras, C. Cazorla et al., Nature Communications 8, 1851 (2017)

A critical design parameter of thermal switches is the ratio of between the thermal conductance in its on and off states; this parameter is denoted as the switching ratio. Most passive thermal switches rely on phase change material and operate only in small temperature regions. Active thermal switches usually require moving parts or vacuum seals. Here we propose an all-solid-state thermal switch with an adjustable switching ratio that can operate over a wide temperature range based on thermoelectric effects.
A switch was constructed from Bi₂Te₃-based materials and was characterized by its thermal conductance and response time. The switching ratio was calculated from the effective conductance measured in both open (no activation current) and closed switch (optimal activation current) configurations. This switching ratio correlates directly with the thermoelectric figure of merit ZT and diverges at small temperature difference. Switching ratios as large as 100 were found within the temperature range of -20 [^oC] to 140 [^oC]. The timed response can be predicted simply from the thermal resistance and capacitance.

While most materials expand when heated, materials that exhibit negative thermal expansion (NTE) shrink when heated. NTE materials are critical in applications involving extreme thermal environments, such as inside gas turbine engines, computing hardware and spacecraft. However, while first observed experimentally over a century ago, the fundamental driving mechanisms behind NTE remain largely unknown for all but a handful of systems. One mechanism often invoked to explain NTE in inorganic framework materials containing stiff polyhedral groups is the concept of rigid unit phonon modes (RUMs) – if these groups are modeled as perfectly rigid and corner-connected, relative rotations of these groups with respect to each other can decrease the volume of the unit cell. If there are many low-frequency RUM or RUM-like phonon modes in the system, they can drive bulk NTE behavior, as is the case in ZrW₂O₈, as well as in a number of zeolites and Prussian blue materials. RUMs are often associated with negative mode Grüneisen parameters, indicating that their frequency decreases with compressive strain or with a decrease in volume. Negative Grüneisen parameters are often seen as an indicator, and sometimes even an essential ingredient, to NTE. But do these rules of thumb hold for all inorganic materials that exhibit NTE? We use theory and first-principles calculations to demonstrate that neither RUMs nor phonons with negative Grüneisen parameters are necessary (or sufficient) in order for a material to undergo NTE. We elucidate the microscopic mechanism of NTE in the ferroelectric perovskite PbTiO₃ and show that few of the modes critical to driving NTE have negative Grüneisen parameters and that they are not RUMs, even though PbTiO₃ contains many RUM-like modes. Indeed, PbTiO₃ is unique among well-known NTE materials in that it appears to be the only materials that exhibits volumetric NTE well above room temperature and with positive Grüneisen parameters along all unique crystallographic axes. We find that the symmetry breaking distortion that drives the ferroelectric transition in PbTiO₃ also has a significant effect on both the elastic properties and lattice dynamics in general, and the NTE behavior of PbTiO₃ in particular. Our work has implications for the understanding of, discovery and design of NTE in perovskites and other families of inorganic materials.

Time-resolved measurements of the magneto-optic Kerr effect (TR-MOKE) are commonly used to study how the magnetization dynamics of ferromagnetic materials respond to fast temperature excursions and heat currents generated by ultrafast pulses of light. Here, we flip the situation around and describe how TR-MOKE provides an ultrafast thermometer for studies of the physics of heat conduction. TR-MOKE, i.e., measurements of transient changes in the polarization of a reflected probe beam, has some unique advantages over the more conventional time-domain thermoreflectance (TDTR) approach that is based on measurements of the transient changes in the intensity of a reflected probe beam. For example, the signal in a TR-MOKE measurement is localized to the magnetic layer while the signal from a TDTR measurement is dependent on entire temperature field that interacts with the probe beam. Thus, TR-MOKE can accurately report the temperature of semi-transparent magnetic layers or magnetic layers that are buried within multilayer structures. We are using TR-MOKE to test if the two-temperature model can be applied with consistent parameters to describe ultrafast heat transport in metallic multilayers. Our model system is Pt/Co/Pt trilayers deposited by magnetron sputtering on sapphire substrates. The thickness of the buried magnetic Co layer only 3 atomic layers and yet provides good signal-to-noise and fast time response to the local electronic temperature. We find that the two-temperature model provides a good description of heat transport in Pt over a wide range of length and time-scales.

Understanding heat transfer across organic-inorganic heterojunctions is important for emerging applications such as photothermal cancer therapy and organic-inorganic hybrid thermoelectric materials. In this study, we employ a new integrated approach to study heat dissipation from gold nanorods (GNRs) conjugated with organic ligands and suspended in water, with an atomic spatial resolution. In the integrated approach, we employ the picosecond transient absorption to monitor the time-resolved temperature evolution of GNRs after being heated by ultrafast laser pulses. In addition, we employ the time resolved Raman spectroscopy to monitor the temperature evolution at multiple locations along ligands (i.e., 4-Nitrothiolphenol and 4-Aminothiolphenol) conjugated to gold nanorods. We find that the bonds in the conjugated ligands are heated almost instantaneously and reach a peak temperature within ~1 ps after heating by the laser pulses, suggesting that the bonds are directly heated by hot electrons across the Au-thiol heterojunction the bonds. We also found that the bonds cool a lot faster than the gold nanorods, with a relaxation time of only ~3 ps (vs 300 ps for the gold nanorods). This indicates that coupling of Au-thiol heterojunctions is much stronger than coupling of the ligands to water. Our work facilitates better understanding of heat transfer at atomic scale.

Anisotropic thermal transport is of both fundamental and practical importance. Orientation-dependent thermal conductivity has been observed in many materials systems because of their highly asymmetric crystal structures. Importantly, the interaction between 2D lattices and external defects has been revealed to be highly anisotropic and phonon mode dependent through in situ thermal-electrochemical characterizations. But the characterization of anisotropic thermal properties using conventional methods remains challenging. We will describe our recent development of a new metrology based on asymmetric beam time-domain thermoreflectance (AB-TDTR) to measure three-dimensional anisotropic thermal transport by extending the conventional TDTR technique. Using an elliptical laser beam with controlled elliptical ratio and spot size, the experimental signals can be exploited to be dominantly sensitive to measure thermal conductivity along the cross-plane or any specific in-plane directions. An analytic solution for a multi-layer system is derived for the AB-TDTR signal in response to the periodical pulse, elliptical laser beam, and heating geometry, to extract the anisotropic thermal conductivity from experimental measurement. Examples with experimental data are given for various materials with thermal conductivity from 5 W/mK to 2000W/mK, including isotropic materials (silicon, boron phosphide, boron nitride), transversely isotropic materials (graphite, quartz, sapphire) and transversely anisotropic materials (black phosphorus). The developed AB-TDTR metrology provides a new approach to accurately measure anisotropic thermal phenomena for rational materials design and thermal applications^1,2.
References:
1. M. Li, J.S. Kang, and Y. Hu, Review of Scientific Instruments 89, 084901 (2018).
2. J. Kang, M. Ke, and Y. Hu, Nano Letters 17(3), 1431 (2017).
3. J. Kang, M. Li, H. Wu, H. Nguyen, and Y. Hu, Science 361, 575-578 (2018).

All power plants, automobiles and industrial manufacturing are very limited in efficiency because most (>50%) of the total energy that is produced is lost as heat. Directly recovering waste heat with thermoelectric materials, which produce a voltage in the presence of a thermal gradient, is thus a promising avenue to increase energy efficiencies, decrease costs, and reduce impact on the environment. To date, efficient thermoelectric materials have been challenging to produce since they require the elusive combination of low thermal conductivity and high electrical conductivity. However, recent advances in nanofabrication technology are providing new abilities to structure materials on the nanoscale with atomic precision, enabling new metamaterial systems with promising thermoelectric characteristics [1]. Since traditional techniques struggle to probe energy transport in functioning nanosystems, novel characterization techniques are required to make thermoelectric devices widespread and economically feasible.
We present thermal transport measurements on hybrid molecule-nanoparticle arrays and additional advanced thermal nanomaterials that are promising for next-generation thermoelectrics. The arrays consist of monolayers of 10 nm gold nanospheres, functionalized with oleylamine, and self-assembled into a hexagonal lattice [2]. Varying numbers of these layers are stamped onto various substrates, providing a versatile fabrication platform. In these systems, we observe up to two orders of magnitude reduction in thermal conductivity compared to bulk gold. To measure thermal transport without disrupting the arrays, we have extended our established extreme ultraviolet (EUV) nanometrology technique to be fully non-contact.
Our EUV nanometrology method is a pump-probe technique, where the pump pulse is an ultrafast laser and the probe pulse is an ultrafast, coherent EUV beam generated by tabletop high harmonic generation [3]. By using an EUV probe with 29 nm wavelength, we directly diffract from nanostructures that are far below the diffraction limit of visible light and track their thermal and acoustic dynamics with picometer-scale axial sensitivity [4]. Moreover, by choosing an EUV wavelength far from any electronic resonances in the sample, we are minimally sensitive to hot electrons, enabling measurements of lattice dynamics at ultrafast timescales [4]. In past work, we launched the nanoscale dynamics with laser heated nanolines [5]. To measure transport in the new, complex thermal materials, we have replaced the nanoline transducers with a micron scale transient thermal grating (TTG) excitation, generated by crossing two 400 nm laser beams [6]. We can thus nondestructively measure the composite thermal transport in these novel nanomaterials, which is relevant for device engineering. In the near future, we will extend TTG excitation to EUV wavelengths. This will directly launch ultrafast dynamics in the deep nanoscale regime in a fully non-contact modality, which is not possible with existing visible techniques that are diffraction limited to several hundreds of nanometers.

[1] M. Martin-Gonzalez, et al., Renew. Sust. Energ. Rev. 24, 288–305 (2013).
[2] C. E. McCold, et al., Mol. Syst. Des. Eng. 2, 4 (2017).
[3] A. Rundquist, et al., Science 280, 5368 (1998).
[4] R. Tobey, et al., Opt. Lett. 32, 3 (2007).
[5] K. Hoogeboom-Pot, et al., Proc. Natl. Acad. Sci. 112, 16 (2015).
[6] R. Tobey, et al., Appl. Phys. Lett. 89, 9 (2006).

Zirconium pentatelluride (ZrTe₅) has recently attracted renewed interest owing to many of its newly discovered extraordinary physical properties, such as 2D and 3D topological-insulator behavior, pressure-induced superconductivity, Weyl semimetal behavior, Zeeman splitting, and resistivity anomaly. The quasi-1D structure of single-crystal ZrTe₅ also promises large anisotropy in its thermal properties which have not yet been studied. In this work, via time-domain thermoreflectance measurements, ZrTe₅ single crystals are discovered to possess a record-low thermal conductivity along the b-axis (through-plane), as small as 0.33 ± 0.03 W m^-1 K^-1 at room temperature. This ultralow b-axis thermal conductivity is twelve times smaller than its a-axis thermal conductivity (4 ± 1 W m^-1 K^-1) owing to the material’s asymmetrical crystalline structure. First-principles calculations are further conducted to reveal the physical origins of the ultralow b-axis thermal conductivity, which can be attributed to: 1) the resonant bonding and strong lattice anharmonicity induced by electron lone pairs, 2) the weak interlayer van der Waals interactions, and 3) the heavy mass of Te atoms, which results in low phonon group velocity. This work sheds light on the design and engineering of high-efficiency thermal insulators for applications such as thermal barrier coatings, thermoelectrics, thermal energy storage, and thermal management.

Several experiments have been carried out recently to better understand lattice and electronic thermal transport behaviors in three material systems. Based on steady-state differential measurements, the electronic thermal conductivity of h-BN/graphene/h-BN heterostructures deviate considerably from the Wiedemann-Franz law for both the bi-polar and single carrier type regimes and at both low and high temperatures, and the lattice thermal conductivity of graphene appears to increase when it is encased in h-BN compared to when it is supported on amorphous oxide. In addition, steady state electro-thermal and lock-in Raman thermometry measurements have found that the average thermal conductivity of millimeter size boron arsenide bulk crystals approaches the theoretical limit set by four-phonon scattering. Meanwhile, a multiple probe thermal transport measurements have observed a much weaker length dependence of the thermal conductivity of silicon germanium nanowires compared to a previous report, and the measurement results are in agreement with a first principles calculation.

In the last 5 years the Kondo insulators have drawn a great interest in the field of strongly correlated systems. Special interest has been devoted to SmB6 that has been proposed to be a topological Kondo insulator at low temperature. Various electrical and thermal measurements have been performed on this material to explore its low-temperature insulating phase. These measurements often gave conflicting results due to the quality of the material used. To overcome these problems, we use micron-size single crystals prepared by plasma Focus Ion Beam (FIB) system and measure the thermal and electrical properties by specially designed experimental configuration. During the talk, I will present our experimental setup focusing on recent transport measurements of micron-size SmB₆ crystals at low temperatures and high magnetic fields. I will discuss our results in the context of the presence of metallic surface states in this material. In addition, the thermal conductivity of the micron-size SmB₆ crystals, measured by the 3w-method, will be presented and discussed.

The spin-Seebeck effect mediated thermoelectric energy conversion can provide an efficient alternative to traditional thermoelectrics for waste heat recovery. To achieve this goal, efficient spin to charge conversion using earth-abundant materials is essential. Proximity-induced Rashba effect arises from the charge potential mediated by structural inversion asymmetry, which has been reported in Si thin films and can be manipulated by controlling the thickness of the Rashba layer. We demonstrate a giant Rashba spin-Seebeck effect in NiFe/p-Si (polycrystalline) bilayer thin films. The bilayer thin film specimens have the p-Si layer thickness of 5, 25, and 100nm while keeping the NiFe layer thickness at 25 nm. The Rashba spin-Seebeck coefficient has been estimated to be 0.266 μV/K for 100nm p-Si, and increases by an order of magnitude to 2.11 μV/K for 5 nm p-Si. The measured spin-Seebeck coefficient in 5 nm p-Si specimen is one of the largest coefficients ever reported. The measured voltage of 100.3 μV is one of the largest reported spin-Seebeck voltages, with the smallest area of ≈160×10 μm² used in any spin-Seebeck measurement. This scientific and technological breakthrough using earth-abundant elements brings the spin mediated thermoelectric energy conversion for waste heat recovery closer to reality.

Heat conduction in confined geometries and around small sources is known to no longer obey standard diffusion models. In contemporary electronic devices, where nanoscale active regions and extremely thin layers are widely prevalent, both effects combine and can induce junction temperatures that substantially exceed conventional predictions. Even so, virtually all commercial TCAD software carries out device-level thermal simulations using classical Fourier diffusion models with either nominal or phenomenologically adjusted thermal conductivities.

Advances in both theoretical understanding and computational power over the past decade have brought refined microscopic treatments within conceptual reach. Computational DFT and phonon Boltzmann codes such as VASP and ShengBTE have enabled researchers to model and predict direction and temperature dependent bulk thermal conductivity for a wide range of semiconductor compounds from first principles. The almaBTE platform subsequently extended these capabilities to analysing the thickness-dependent effective conductivity reduction in thin films and 1D Monte Carlo simulations of multilayer substrates.

In typical device geometries, however, thermal transport has a pronounced multi-dimensional character and occurs through a variety of material systems in which phonons are not always the dominant heat carriers.

Here, we report on the development of a custom thermal simulation platform that addresses the limitations of currently available tools. Our IMPALA framework brings quasiballistic modeling capabilities to realistic TCAD settings. The solvers perform Monte Carlo simulation of 2D and 3D modular geometries using a hybrid materials modeling approach. The latter enables us to tackle, within one seamless particle-based simulation environment, the three types of compounds typically encountered in complex structures. Firstly, for semiconductors we employ wavevector-resolved phonon dispersions and scattering rates computed from first principles. These inherently capture the less efficient heat removal that arises in thin layers and around small active junctions. Secondly, metal contacts and electrodes are treated under the isotropic grey-medium BTE using estimated or measured values for the electron mean free path. Doing so naturally captures the size-dependent excess resistivity observed in narrow lines. Finally, heat flow in amorphous regions such as passivation/insulation layers is simulated by Brownian motion, thereby replicating diffusive transport at nominal bulk conductivity.

Practical applications for analysing front-end and back-end of line (FEOL/BEOL) self-heating and resulting thermal cross-talk in state-of-the-art finFET architectures will be discussed. 2D simulations already suggest the quasiballistic heat flow to have intricate repercussions on FEOL and BEOL reliability. The suppressed thermal performance of the transistor fins induces both a substantial increase in junction thermal resistance and stronger heat spreading along the drain electrodes. In turn, this raises the temperature in overhead metal lines, even near idle devices. Simulations of 3D structures, which are currently underway, can further elucidate the resulting impact on electromigration in the BEOL.

Reliable affordable thermal management technologies remain a major packaging challenge driven by the continuous miniaturization of integrated circuits, increase of functionality of mobile devices, and growing computing density in data centers. Development of the next generations of thermal interface materials (TIMs) is important for packaging and thermal management of all electronic and optoelectronic devices. The discovery of the unique heat conduction properties of graphene [1] motivated numerous studies on the use of graphene and few-layer graphene in various composites. The most recent studies of thermal composites with graphene have been limited to curing epoxy-based composites [2-3]. However, TIM applications in thermal management of computers and other electronic gadgets, require non-cured TIMs. The Semiconductor Research Corporation (SRC) target for the thermal conductivity of TIMs is ~10 W/mK near room temperature. Despite substantial efforts and investments in research of new TIMs in the last couple of decades, there is no commercial thermal greases available with the verifiable thermal conductivity above 10 W/mK and the thermal interface resistances below 10 Kmm²/W. Commercial TIMs filled with conventional conductive particles require large filler loading, f>60% to achieve the thermal conductivity of the composite in the range of ~0.5 – 6 W/mK. In this presentation we report on development of the exfoliation and mixing techniques for preparation of the non-cured mineral-oil based composite with the commercially produced graphene flakes. The viscosity of the composites was kept in the range required for easy dispersion on the surfaces of interest. The lateral sizes of the few-atomic-layer fillers of graphene ranged from ~2 μm to ~15 μm while the thickness varied from single atomic planes of 0.35 nm to ~12 nm. The thermal conductivity of TIMs was determined using two types of industry standard TIM Testers (LonGwin LW-9389 and Analysis Tech, Inc.), following the ASTM D5470-06 standard. It was found that the thermal conductivity of the graphene-based non-cured TIMs can exceed that of the best commercially available TIMs at substantially lower loading fraction. We will discuss the dependence of the thermal conductivity on the graphene loading characteristics, below and above the thermal percolation threshold. Brillouin – Mandelstam spectroscopy was used to determine the acoustic phonon velocities before and after graphene incorporation. Taking into account recent technological developments in the liquid phase exfoliation of graphene and various methods of reduction of graphene oxide, we argue that our results can open up new application domain for graphene in non-cured TIMs.
This work was supported, in part, by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE award 1433395, and by the UC – National Laboratory Collaborative Research and Training Program LFR-17-477237.
[1] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials. Nature Materials, 10, 569 (2011).
[2] F. Kargar, Z. Barani, M.G. Balinskiy, A.S. Magana, J.S. Lewis, A.A. Balandin, “Graphene composites with dual functionality: electromagnetic shielding and thermal management,” Advanced Electronic Materials (in print, 2018) arXiv:1808.03401.

Clathrate compounds are promising candidates for thermoelectric materials in terms of the “electron-crystal phonon-glass” concept. Because of strong anharmonic potential of guest atoms in cage structures of type-I clathrates, guest phonon modes lead to strong phonon scattering. Recently, first-principles simulations of phonon properties of a clathrate have been first demonstrated with using self-consistent phonon (SCP) theory that can include temperature-dependent fourth-order anharmonic interatomic force constants (IFCs) into harmonic IFCs [Tadano et al. Phys. Rev. Lett 120, 105901 (2018)]. Here, we reveal unusual anharmonic effects of phonons by comparing phonon properties of type-I Ba₈Ga₁₆X₃₀(X = Si, Ge, Sn; BGX) with first-principles simulations. We have first succeeded to calculate temperature-dependent thermal conductivities of BGSn with first-principles approach, which has the minimum thermal conductivity among the reported values of clathrates. Because the selection of atoms composing cage structures, namely Si, Ge, or Sn, manipulates the degree of the anharmonicity, we have clearly observed effects of anharmonicity of phonons in clathrates. For example, we have observed an unusual effect of grain boundaries on phonon transport in BGSn; polycrystallization decreases a thermal conductivity of BGSn more at higher temperature.

Energy and mass transport across grain boundary determines the physical property of materials. The emerging grain boundary engineering methods coupled with laser-based acoustic spectroscopy and thermal transport could offer a new route to study the microstructure of polycrystalline ceramics for novel applications.
Here, we employ the time-domain Brillouin spectroscopy (TDBS) to non-invasively monitor acoustic phonon propagation inside individual grains and detect the microstructure of polycrystalline ceria, an important electrolyte material in solid oxide fuel cells. TDBS employs a conventional pump-probe technique based on femtosecond laser. The pump beam launches bulk acoustic waves propagating away from the surface and a time-delayed probe beam detects propagation of these acoustic waves. Strong longitudinal waves are readily observed whose velocity agrees with theoretical calculations. Large amplitude shear waves are generated through mode conversion when a thin layer of gold is deposited with matched acoustic impedance. Amplitude of measured shear phonon exhibits strong dependence on optical polarization of the probe beam. A theoretical model that accounts for the generation, propagation, and detection of acoustic phonons is developed to explain these observations. Additionally, the capability of TDBS to image grain microstructure non-destructively through depth dependent measurement of Brillouin oscillation frequency is demonstrated. The grain orientations at the surface are obtained and compared with the results from electron backscatter diffraction technique(EBSD). The subsurface morphology of grain boundary is constructed in 3D nondestructively, unlike traditionally used EBSD that requires destructive sequential analysis. The current work paves the path for future study on phonon/grain boundary interaction and offers guidance for material design in advanced energy systems.

Recently, two-dimensional monolayer chromium triiodide (CrI₃) with intrinsic magnetism was experimentally discovered, which shows promising applications in many technologies ranging from sensing to data storage where thermal transport plays a critical role. However, so far, the effect of spin-lattice coupling on the thermal transport properties has not been explored yet. In this talk, I will present the giant effect of spin-lattice coupling on the lattice thermal conductivity (κ_L) of monolayer CrI₃. The lattice thermal conductivity is more than two orders of magnitude enhanced by considering the spin-lattice coupling. The effect is found to be especially stronger for the acoustic phonon modes, which dominates thermal transport with spin-lattice coupling. The mechanism lies in the weakened phonon anharmonicity by spin-lattice coupling. The bond angle and atomic position are changed due to the spin-lattice coupling, making the structure much stiffer and more symmetric, which lead to the weaker phonon anharmonicity, and thus the enhanced thermal conductivity. This study uncovers the effect of spin-lattice coupling on the thermal transport, which would deepen our understanding on thermal transport and shed light on future research of thermal transport in magnetic materials.

Efficient heat transfer is one of the major concerns in the design of micro- and nano-scale materials and devices. In metals, electrons and phonons both carry significant thermal energy, whereas in semiconductors and dielectrics phonons are the dominant carriers. Therefore, for metal-semiconductor composite structures, heat can transfer across an interface by coupling either between phonons in metal and dielectric or by direct coupling between electrons in metal and phonons in dielectric through electron-interface scattering. Phonon-phonon coupling at interfaces has been well studied, mainly by the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM).
For electron-phonon coupling at and near hetero-interfaces, a number of different viewpoints exist. Some studies have assumed that electron-phonon coupling across a metal-dielectric interface is negligible, and heat transfer occurs as electron-phonon coupling within metal and then phonon-phonon coupling across the interface. Electron-phonon coupling at metal-dielectric interfaces has been measured to exhibit negligible apparent thermal resistance in some cases, yet other times it has been suggested to be important [e.g., for Pd-diamond].
In recent work we have shown that, with accurate definition and understanding of atomic structure, the electron-phonon coupling across silicide-silicon interfaces must include rigorous first-principles understanding of electron-phonon coupling (e.g., via the Eliashberg function) bothwithin the metal andin the near-interface region of the semiconductor in order to obtain close quantitative agreement with experiments [1], [2]. In summarizing the current state of this topic, it would be reasonable to conclude that some qualitative understanding is emerging on cross-interface electron-phonon coupling, but that quantitative models validated by experiments remain scarce. Here, we discuss a new innovation – namely interfaces with tunable interfacial heat transfer characteristics to assess the relative contributions of different carriers and coupling mechanisms.
As another example, electron-phonon coupling in bilayer graphene controls its unique photoconduction response. We find that, in contrast to single-layer graphene, Γ point LA’ and TA’ modes in bilayer graphene couple strongly with electrons with energies lower than 200 meV, while optical modes dominate in the high electronic energy range. A Drude model of photoconductivity provides good agreement with experiments for both the real and imaginary components.
Finally, we discuss a rigorous temperature-dependent approach involving Green-Kubo equilibrium spiral and atomic dynamics to assess phonon-magnon interactions in magnetic solids. Resulting predictions of the non-electronic thermal conductivity of iron show very good agreement with experimental measurements. By computing the modal phonon-magnon scattering rates, the high-frequency phonon-magnon scattering rates are found to be one order magnitude larger than those at low frequencies regions due to energy conserving rules and the high density of states. This new framework fills existing gaps in simulating thermal transport across the ferro/para-magnetic transition.

References
[1] S. Sadasivam et al., “Thermal transport across metal silicide-silicon interfaces: First-principles calculations and Green’s function transport simulations,” Phys. Rev. B, vol. 95, no. 8, p. 085310, Feb. 2017.
[2] N. Ye et al., “Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces,” Phys. Rev. B, vol. 95, no. 8, p. 085430, Feb. 2017.

There has been great interest recently in predicting thermal properties at the nanoscale. Typical properties of interest include thermodynamic properties such as specific heat, the Gruneisen parameter, thermal expansion coefficient, macroscopic properties such as thermal conductivity, interface resistance, and phonon properties such as dominant mean free paths. Knowledge of these properties may be used to engineer novel materials or nanocomposites for applications such as thermal management and thermal energy storage. While these predicted thermal properties provide important information about material behavior, in order to optimize material design, one needs information about the sensitivity of these properties to design and operating variables. Sensitivities or derivatives are also used in thermal transport simulations to describe important crystal properties. For example, the Gruneisen parameter is the derivative of phonon frequency with respect to the volume of the crystal. The phonon group velocity is the derivative of the frequency with respect to the wave vector. A force constant is a derivative of the total potential energy of the system with respect to the displacement of a given atom. Accurate methods to compute these quantities are needed without the approximations inherent in finite difference methods.

In this paper, we present the automatic code differentiation technique to perform unintrusive sensitivity analysis and derivative computation in nanoscale phonon transport simulations. This method exploits the concepts of templating and operator overloading in C++ and other similar programming languages to unintrusively convert existing codes into those yielding derivatives of arbitrary order. The idea is demonstrated through the computation of phonon properties such as second and third order force constants, the Gruneisen parameter, group velocities, and the temperature sensitivity of specific heat for graphene nanoribbons. Derivative values so computed are compared with those obtained using finite difference approaches or with analytical values. The method is found to yield derivative values to machine accuracy, with none of the round-off issues associated with finite difference approaches.

Normal modes are non-interacting, non-decaying collective excitations of atomic vibrations. Many-body atomistic dynamics with an arbitrarily accurate anharmonic Hamiltonian provides a direct approach for probing the interactions among the crystal normal modes. The traditional normal mode analysis (NMA) framework, which employs complex normal mode coordinates, does not, however, distinguish the non-resistive normal (N) phonon interactions from the resistive Umklapp (U) interactions. By adopting a set of real asymmetric normal mode coordinates, we show that both N and U processes can be resolved directly from atomistic simulations.

Although the normal modes exchange energy continuously, they are localized in the reciprocal space and engage in discrete interactions. Interestingly, we observe that the harmonic and anharmonic energies do not remain constant. The time-dependent Hamiltonians, which is an intrinsic property of all anharmonic lattices even at low temperatures, can allow a multitude of normal mode interactions that are not possible with constant anharmonic energies. Using graphene as a model material, we show the emergence of normal modes that are created and annihilated simultaneously. We anticipate that such additional scattering channels can be discovered with more complex Hamiltonians using the current approach. Finally, we discuss the correspondence between the normal modes extracted from atomistic simulations and the quantum phonon modes that are employed in Boltzmann transport equation (BTE).

An accurate determination of thermal transport in thin film-on-substrate (FOS) architectures is crucial to optimum performance of nanostructured devices. A rigorous treatment of the nanoscale interfacial thermal coupling between materials accounting for dispersion mismatch, interfacial roughness and shadowing effects is imperative in studying the impact of substrates on thin-film conduction. We demonstrate novel thermal functionality in FOS systems, creating a new paradigm for thermal management in optoelectronic applications. In a breakthrough finding, we discover an increase in thermal conductivity with a reduction in thin-film thickness attributed to phonon injection from the substrate layer. We examine the in-plane and cross-plane configurations of thermal conduction in Ge and Al_0.1Ga_0.9As thin-films mounted over Si and GaAs substrates respectively. We provide an extensive analysis of phononic coupling and contrast the results with bulk and isolated thin film values. We present a detailed microscopic and spectral analysis by investigating the spatial thermal flux distribution, modal thermal conductivity, and mean free path and frequency spectra in the FOS architecture. We elucidate how interlayer phonon coupling opens new avenues for thermal conductivity manipulation in nanostructures and achieves desired thermal properties for rational thermal material design in microelectronics and optoelectronics.

Nanostructured thermoelectrics are currently emerging as promising materials for energy generation due to their improved thermoelectric efficiency via increasing contribution of complex nano-interfaces to the decreasing thermal transport through them. However, lack of 3D order and connectivity restricts the flow of electrons and phonons through the structure. But, theoretical predictions of electrical and thermal properties through periodically arranged nanoporous silicon suggests significant improvement in performance of such thermoelectric devices. In the present work, a 3D ordered and precisely doped Si nanostructure, namely an inverse opal metalattice, with periodicities on the order of 1-100 nm is synthesized using unique infiltration of Si into the voids of closely packed silica spheres nanotemplates through high pressure confined chemical vapor deposition.
Subsequently, Time domain thermoreflectance method is utilized to measure the thermal transport through these structures as a function of sphere size ranging from 14-100nm and observe the effect of interfacial thermal resistance and appropriately control it to strive towards higher thermoelectric figure of merit. Initial data on intrinsic Si inverse opals show a significant reduction in thermal conductivity with values lying close to or lower than fused silica indicating a dominant interface effect on phonon transport through these structures. Secondly, occurrence of a minima at 20nm sphere size further points towards the role that geometry has on phonon scattering mechanism while travelling through them. Theoretical understanding of the phonon transport mechanism and subsequent studies of thermal transport with respect to variation of geometry, composition and porosity will be performed to aim towards potential thermoelectric applications of these structures.
References
[1] Arun Majumdar, “Thermoelectricity in semiconductor nanostructures” Science(2004), Vol. 303, Issue 5659, pp.777-778
[2] Sanghamitra Neogi, J. Sebastian Reparaz, Luiz Felipe C. Pereira, Bartlomiej Graczykowski, Markus R. Wagner, Marianna Sledzinska, Andrey Shchepetov, Mika Prunnila, Jouni Ahopelto, Clivia M.Sotomayor-Torres,and Davide Donadio “Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering” ACS Nano, 2015, 9(4), pp 3820-3828
[3] <span style="font-size:10.8333px"> </span>J.H. Lee, G.A. Galli, & J.C. Grossman, "Nanoporous Si as an Efficient Thermoelectric Material".Nano Lett 8 (11), 3750-3754 (2008) http://dx.doi.org/10.1021/nl802045f
[4] Han, J.E. & Crespi, V.H., Tuning fermi-surface properties through quantum confinement in metallic metalattices: New metals from old atoms. Physical Review Letters 86 (4), 696-699 (2001)
[5] Han, J.E. & Crespi, V.H., Abrupt topological transitions in the hysteresis curves of ferromagnetic metalattices. Physical Review Letters 89 (19) (2002)

Thermal interface materials (TIMs) are arguably the solution to faster and smaller computer chips. As integrated circuits get faster and smaller, it becomes more difficult and paramount to remove the excess heat. TIMs accomplish this by dramatically reducing the contact resistance between IC components so heat can be better conducted away from the chip. The most common way to make a TIM is to add high conductivity particles to a polymer matrix, which boosts the thermal properties of the base polymer material. However, the interfacial thermal boundary resistances between the particles and matrix (R_pm) and the particles themselves (particle-particle resistance, R_pp) prevent the high k particles from passing heat through the matrix effectively. In this presentation, we will discuss a two-step process for resolving this issue: inducing particle percolation using magnetic alignment and engineering of their contacts using shell structures that minimize R_pp.
Maximum particle-particle contact is achieved by magnetically aligning the fill material in the polymer matrix. The applied magnetic field segregates particles into aligned columns within the polymer matrix, enforcing percolation and ensuring good contact between particles. We show that magnetic alignment of the particles alone doubles the thermal conductivity of the composite (k_c) over the unaligned composites, even without any special consideration for the particle-particle interfaces. With the particles in good contact with one another, particle contact engineering can then have the greatest effect in boosting k_c.
We use silver and liquid metal particle coatings to decreaseR_pp of solid nickel particle fill by providing a layer that can plastically deform and increase the particle-particle contact area. Dry powder beds show a decrease of more than 4× in thermal resistance with a silver shell thickness of only a few hundred nanometer and 500 kPa force to “sinter” the powders together. Similarly, liquid metal is used as a coating for the nickel particles and shows a similar decrease in thermal resistance but requiring a larger amount of liquid metal for an equivalent reduction in thermal resistance.
Applying these engineered contacts in a magnetically aligned polymer composite maximizes the benefits of the silver and liquid metal coatings by enforcing good particle-particle contacts throughout the polymer matrix. We demonstrate that the magnetically aligned silver coated nickel particles produce almost an extra 2× increase in k_cover the aligned nickel particles at volumetric fill ratios of 0.4 and 0.5 with an 8vol% silver shell. The aligned liquid metal coated nickel show small increases in k_cover the aligned nickel particles until the volume of liquid metal to Ni reaches 50%, where a 2.5× increase is achieved over the aligned nickel composite at a particle volume fill fraction of only 0.3. The mechanisms are discussed and a full parameter space is explored to fully evaluate these engineered contacts and minimize R_pp.

Work has been conducted on the laser patterning of graphene and 2D thin transition metal dichalcogenide (TMDC) films. Laser thinning has been performed in conjunction with on-line diagnostics to precisely control the film thickness. Tuning the properties of atomic layer films by modulating the free carrier type and density and composition can offer an exciting new pathway to various practical nanoscale electronics. Our laser induced doping process is spatially local and selective, and the doping level is widely tunable by varying the duration and intensity of laser irradiation. Moreover, laser doping can be digitally controlled and is an air-stable process for achieving fabrication of high-quality, high fidelity nanodevices. Sharp and high quality P-N junctions have been demonstrated via this approach. A dual processing laser apparatus has been developed, aiming at decoupling the vacancy formation, precursor gas decomposition and dopant incorporation processes. Controlled doping of graphene and TMDC films has been demonstrated via this approach. Time-resolved probing with femtosecond resolution has been conducted to understand the transport properties of the ultrathin films and the effects of doping and defect formation.

The transient thermal grating (TTG) technique, in which two crossed laser pulses produce a spatially periodic temperature profile whose decay is monitored with a probe laser beam, is widely used for thermal transport measurements. On the one hand, this technique provides a non-contact method of measuring thermal diffusivity with intrinsically high absolute accuracy. On the other hand, it offers a tool for studying heat transport at small distances without the need to fabricate nanostructures, with the heat transfer distance controlled by the TTG period. However, the latter cannot be made smaller than half the optical wavelength, thus limiting the ability to study thermal transport on the nanometer scale. This difficulty can be circumvented by using an extreme ultra-violet (EUV) laser source. TTG measurements in the EUV range (13-50 nm) are now possible at the FERMI free electron laser facility (Elettra, Trieste, Italy). In this report, we will describe the first measurements of thermal transport with thermal gratings produced by EUV light performed at FERMI. In the initial series of measurements, we used EUV excitation (12.7 nm) and optical probe (400 nm) to study thermal transport in single crystal silicon and diamond sample at a TTG period of 280 nm, corresponding to a heat transfer distance of ~90 nm. We find that in silicon the heat transport is strongly non-diffusive but still does not reach the ballistic limit. In diamond the heat transport is found to be close to the purely ballistic regime. The results will be compared with calculations based on the Peierls-Boltzmann phonon transport equation using scattering rates calculated from first principles. We will also describe the first measurements with EUV light used for both excitation and probing, which made it possible to shrink the TTG period to 25 nm (heat transfer distance ~8 nm). The EUV probe has an additional advantage of being insensitive to electronic excitations, which simplifies the interpretation of the measurements. Further prospects for using EUV transient gratings to study thermal transport on the single-digit nanometer scale will be discussed.

Thermal energy dissipation and transport processes influence the physical properties of matter, control the kinetics of chemical reactions, and trigger bio-molecular mechanisms in living organism. Understanding how thermal phenomena arise and change as the characteristic length scale of structures becomes comparable to or even shorter than the mean free path of energy carriers such as electrons and phonons is therefore on of the key challenges in the physical sciences.
In this talk, we will first provide and overview about the numerous scanning probe based thermal nano-imaging and spectroscopy methods developed to quantify local energy dissipation and thermal transport phenomena in nanoscale systems and molecular structures. We will highlight the unique capabilities of thermal scanning probe techniques to unravel local dissipation processes down to few-nanometer length scale, and discuss prospects to extend existing thermal imaging approaches towards characterization of collective dissipation processes in quantum material systems. Secondly, we will present recent experiments extending traditional scanning thermal microscopy towards nanoscale spectroscopy. We will illustrate how local photo-thermal forces in tip-sample nanocavities enable to study energy dissipation in molecular systems with simultaneous chemical specificity. We will illustrate how to separate thermally induced expansion from optical forces and provide guidelines for the development of new thermal nanoscopy modalities combining local scanning probes with tunable laser excitations.

Individual luminescent nanoparticles enable temperature measurements with sub-diffraction limited spatial resolution. While single-particle measurements require excitation intensities orders of magnitude higher than nanoparticle ensembles, potential single-particle self-heating effects remain largely uninvestigated because thermal models predict negligible self-heating. We show that the common “ratiometric” thermometry single of individual 50 x 50 x 50 nm³ NaYF₄:Yb³⁺,Er³⁺ nanoparticles increases unexpectedly with excitation intensity, implying a temperature rise over 50 K if interpreted as thermal [1]. Luminescence lifetime thermometry results indicate a similar temperature rise. We resolve this apparent contradiction between model and experiment by systematically varying the nanoparticle’s thermal environment: the substrate thermal conductivity, nanoparticle-substrate contact resistance, and nanoparticle size. The apparent self-heating remains the same in all cases, demonstrating that this effect is an artifact rather than a true temperature rise. Using established rate equation models, we show that this artifact is instead due to increased radiative and non-radiative relaxation from higher energy Er³⁺ manifolds.

Following calibration of this apparent self-heating effect, individual NaYF₄:Yb³⁺,Er³⁺ nanoparticles can be employed as accurate nanothermometers. We demonstrate the ability of 50 x 50 x 50 nm³ NaYF₄:Yb³⁺,Er³⁺ nanoparticles to characterize nanoscale hotspots and sharp temperature gradients using two different modalities. First, by coating a low thermal conductivity substrate with an optically absorbing metal film, we can achieve substantial local heating and measure the peak temperature within a laser-heated spot using a single exciting laser beam. Our nanoparticle thermometer minimizes spatial averaging of the temperature profile, thereby increasing the accuracy of this temperature measurement relative to thermometry techniques with coarser resolution. We also measure the spatial temperature variation within the laser-heated spot. Additionally, we Joule heat microfabricated metal structures to create sharp temperature gradients via “current crowding” effects caused by geometric constrictions. We then quantify the temperature variation in these structures. Our measurements demonstrate that individual NaYF₄:Yb³⁺,Er³⁺ nanoparticles can be used as nanoscale thermometers in scenarios ranging from fundamental thermal measurements to device-level applications.

[1] A.D. Pickel, A. Teitelboim, E.M. Chan, N.J. Borys, P.J. Schuck, and C. Dames, “Apparent Self-Heating of Individual Upconverting Nanoparticle Thermometers,” accepted at Nature Communications.

Scanning thermal microscopy (SThM) is a key thermal characterization tool, where a micro to nanometer-scale probe tip measures the temperature close to a surface. The spatial and temperature resolutions of this technique are limited in particular by the size of the tip, the sample properties, and the various tip-sample heat transfer mechanisms [1] depending on the operation conditions. In this communication, we present a study of heat transfer between a sample surface and a SThM probe based on resistive thermometry: the probe electrical resistance depends on the probe temperature. Such probe operates either in thermometry (passive) mode with a minimal Joule self-heating to enable sample surface temperature measurement or in thermal-property measurement (active) active mode where a significant Joule heating is needed to allow thermal power flowing into the sample, depending on its thermal conductivity.

We first study the hot tip-cold sample (active mode) heat exchange as a function of the tip-sample distance [2]. When the tip is far from the surface (>100 micrometers), it is cooled by heat convection and there is no exchange with the sample. At lower distances, the diffusive heat exchange starts. The exact geometry is required to compute accurately the transfer, which can be done by means of Finite Element Modeling (FEM). In the sub-micrometer distance regime, air conduction cannot be modeled by usual FEM because the air mean free path (~60 nm) is on the order of the average distance between the hot object and the sample. In our simulations, we account for this deviation to the diffusive regime by adding thermal resistances on surfaces, which correspond to the ballistic transfer limit in air. This allows us to reproduce numerically the ballistic leveling off seen experimentally. Depending on the probe considered, a 1D approximation [3] or improved considerations based on the Boltzmann transport equation for air molecules are to be taken into account.
When the tip and the sample are in contact, heat transfer is enhanced, which is described by the contact thermal conductance. It includes contributions from the solid-solid mechanical contact, the water meniscus around the mechanical contact and the constrictions in the heat flux path. In the best cases, this thermal contact is responsible for 5% of the total tip temperature decrease due to the tip-sample exchange. We discuss the impact of various tips with curvature radii ranging from a micron down to 10 nm, especially the ballistic thermal dissipation in the sample, and the impact of vacuum-condition operation, where air transfer is removed. In addition, the modest impact of the applied force on the heat exchange is observed.

In a second step, we consider SThM thermometry (passive mode). There are two key phenomena to account for. First, the probe temperature is not equal to that of the sample, as the cantilever base acts as a heat sinks which imposes a temperature distribution in the tip. Second, since the tip acts partially as a heat sink which is placed close to the sample, it modifies the sample temperature distribution. Accounting for this effect can be crucial, as highlighted by experiments performed with samples heated by means of Joule-heated electrically-resistive serpentines.

We believe that this work, where both the understanding of the physical mechanisms responsible for thermal transport between the tip and the samples and precise knowledge of the energy balance of the tip-sample systems are targeted, provides a decisive step for determining quantitative data from the experiments.

References:
[1] S. Gomès et al., Phys. Status Solidi A 212, No. 3, 477–494 (2015)
[2] A.M. Massoud et al., submitted
[3] L. Shi et al., ASME J. Heat Transfer 124, 329-337 (2001)

Acknowledgements:
The support of projects TIPTOP (ANR-16-CE09-0023), DEMO-NFR-TPV (ANR-16-CE05-0013), QUANTIHEAT (FP7-2012 604668), EFINED (H2020-FETOPEN-1-2016-2017 766853), THERMOS (INSA-BQR-2017) is acknowledged.

Our laboratory is interested in taking advantage of plasmonic properties for thermionic power converters. The benefit to using these metasurfaces comes from the additional population of photo-excited non-thermal ‘hot’ electrons on top of traditional photothermalization. Thermionic emission current increases as a function of temperature, however practically devices are limited by the melting point of the metal. This significantly hinders possible power conversion applications. By decoupling the electronic and phononic temperature in nanostructured films we can increase the electronic temperature to a regime where there is efficient emission without destroying the device.

We show how anti-Stokes Raman spectroscopy can independently measure the phononic and electronic temperature. This allows for us to demonstrate the changes in the electronic temperature in response to excitation wavelength but also changes in nanostructure geometry. Phononic temperatures from the nanostructures under laser excitation vary from 300-500 K, well below the melting point of the structures, while electronic temperatures range above 5000 K. These electronic temperatures are high enough to promote efficient electron emission in a temperature range where the device is still thermally stable.

We verify that device power conversion matches our temperature measurement by constructing a simple thermionic device by pairing the metasurface with an ITO anode and measuring current across the vacuum gap between the two. We demonstrate that a gold film, which has fewer hot electrons at electronic temperatures <2000 K, shows no measurable current. However, nanostructured surfaces lead to a short circuit current of 0.015 A*m² under continuous wave illumination of 10⁹ W/m². Correlating the temperature and the measurement gives information about the lifetime of a hot electron as well as the electron-phonon coupling that we correlate to other ultra-fast time resolved studies.

Thermoelectric Generators (TEGs) can continuously convert human body heat into electricity. Therefore, they can eliminate the usage of chargeable batteries in wearable devices. The next generation of self-powered devices come up with controlling the performance of thermoelectric materials and device together. For body wearable TEGs the performance is limited by the hot side (skin) and cold side (ambient) heat exchangers as they experience high parasitic resistances to the body [1]. This requires the TEG to be designed with not only careful consideration of material properties, but also specific consideration of module design, particularly the fill factor and contact resistance. Most of current research is focused on improving the thermoelectric material properties to enhance the efficiency of the TEGs while poor design of the TEG module can produce parasitic resistances that can reduce the generating power on body condition.
To improve the performance of the TEG device, the thermal resistance of the module should be much higher than parasitic resistances. The module thermal resistance can be increased by reducing the thermal conductivity of the legs, increasing the aspect ratio of the legs, and reducing the fill factor (number of the legs in unit area) of the module. Designing low fill factor TEGs will reduce the production cost, while it improves the generated output power in comparison to a high fill factor TEGs. The maximum output power for a wearable TEG made of high thermal conductivity materials (~1.5 W/mK) achieves at lower fill factors (<10%), however, making low fill factor modules is quite challenging. In this research, we designed a TEG with very small fill factor (5%) and compared it with other modules with different fill factors. The power generation characteristics of the modules were studied in practical environment. The results confirm the concept of generating high power with low fill factor module [1]. Therefore, the output power per unit mass can be maximized in comparison to the commercial off-the-shelf TEG.

Reference:
[1] F. Suarez, A. Nozariasbmarz, D. Vashaee, M.C. Öztürk, Designing thermoelectric generators for self-powered wearable electronics, Energy Environ. Sci., 9, 2099-2113, 2016.

Nanoporous metal films, typically synthesized by dealloying metal alloys, are used in a wide variety of applications from catalysis to electronics interconnects and thermal management. However, dealloying kinetics are complex and poorly understood, and the technique is limited to leaving the more noble constituent of the alloy as the final film [1]. Rather than rely on complex yet limited electrochemistry, hard templating with polymers provides an avenue to decouple composition from nanostructure. Self-assembled block copolymer films are an emerging category of organic templates that have enabled fabrication of intricate, highly ordered nanostructures [2]. However, the bicontinuous block copolymer state (gyroid) occupies a small portion of the equilibrium phase diagram and thus requires precise process control.
Annealing kinetics, i.e. the process through which a block copolymer film transitions from disordered to ordered, have also been studied extensively in literature, including non-equilibrium ordered states [3]. However, there is little analysis of disordered states, lacking ordering beyond a single natural period, even though order is unimportant in some applications.
Here we present an alternative to dealloying that uses evenly weighted PS-b-PMMA block copolymers in a disordered transient state. By stopping the anneal process before the lamellar formation is complete, we generate a film with separate PS and PMMA domains but no discernable ordering on length scales larger than the natural period. We find these films to be bicontinuous; thus, by selectively dissolving the PMMA we create nanoporous templates for electrodeposition of continuous nanoporous metal foams.
We demonstrate control of both pore size (through annealing time) and porosity (through the addition of homopolymers). We use the films as templates for electrodeposition of nickel. We measure electrical resistivity as a function of temperature and demonstrate good agreement with an analytical model based on Matthiessen’s rule, considering electron-surface and electron-phonon scattering. These results demonstrate that block copolymers can be used as templates to make continuous nanoporous films for high specific surface area applications without the need for extremely precise phase control. Finally, we demonstrate the impact of modulating the pore size, by adjusting annealing time, on the hydrophobicity/hydrophilicity of the porous films by measuring contact angle. This tunability, along with enhanced surface area, implies significant value for two-phase heat transfer. The synthesis methodology presented in this work represents a scalable, simple, and low-cost platform for the preparation of high surface area metal and polymer films, informing the development of future electrochemical and thermal systems.

[1] Erlebacher, et al., 2001, DOI: 10.1038/35068529
[2] Bates and Frederickson, 1990, DOI: 10.1146/annurev.pc.41.100190.002521
[3] Majewski and Yager, 2016, DOI: 10.1088/0953-8984/28/40/403002

The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC). However, recent computational works have suggested that interfacial defects can enhance this thermal boundary conductance through the emergence of unique vibrational modes intrinsic to the material interface and defect atoms, a finding that contradicts traditional theory and conventional understanding. By manipulating the local heat flux of atomic vibrations that comprise these interfacial modes, in principle, the TBC can be increased. In this work, we provide experimental evidence that interfacial defects can enhance the TBC across interfaces through the emergence of unique high frequency vibrational modes that arise from atomic mass defects at the interface with relatively small masses. We demonstrate ultrahigh TBC at amorphous SiOC:H/SiC:H interfaces, approaching 1 GW m^-2 K^-1 and are further increased through the introduction of nitrogen defects. The fact that disordered interfaces can exhibit such high conductances, which can be further increased with additional defects, offers a unique direction to manipulate heat transfer across materials with high densities of interfaces by controlling and enhancing interfacial thermal transport.

This talk presents new synthesis methods and characterization of thermally insulating and optically transparent mesoporous silica monoliths. The thermal conductivity of transparent materials (e.g., silica) can be reduced by introducing porosity. However, it is typically accompanied by the presence of large pores (> 30 nm) which tend to scatter visible light and render the material translucent instead of transparent. In addition, thick mesoporous slabs provide superior thermal insulation. However, they are prone to cracking during synthesis. Here, thick mesoporous silica monoliths were synthesized at room temperature and atmospheric pressure using silica nanoparticles, 5-10 nm in diameter, as building blocks. Their porosity ranged from 50 to 80% while the pore size distribution was narrow with all pores less than 20 nm in diameter resulting in excellent optical clarity. Finally, not only porosity but also pore size and mass fractal dimension affected the thermal conductivity of the mesoporous silica slabs.

Nanoarchitectured metamaterials that enable spatial control of heat flow has innumerable benefits in as daytime radiative cooling, thermal textile, and thermophotovoltaic systems. Their abilities to tune in plane heat transport and out of plane thermal emission has been of tremendous interest in adaptive tuning of optical channels in response to environmental changes.

In this invited talk, I will present our ongoing efforts on nanoarchitectured thermal metamaterials that are enabled by scalable 3D nanofabrication technology. For example, we recently demonstrated an on-site nano-kirigami method by programming the dosages of the ion beam irradiation in one step. A remarkable 3D optical chirality is achieved through the out-of-plane twisting by nano-kirigami, leading to selective angular emission of circularly polarized IR photons several orders of magnitudes higher than that in natural chiral materials. I will also discuss the potential of thermal metamaterials for detection of chemically induced phase transitions and 3D nanophotonic control of thermal radiation.

The specularity of phonons at rough crystal surfaces is a fundamental aspect of phonon transport in nanostructures. It directly impacts engineering problems such as heat conduction in nanostructures and dissipation in nanomechanical resonators. At room temperature, the available guidance from theory is limited to fully diffuse transport under the condition of small surface roughness. Recent experiments [1,2] in thermal transport suggest that there may exist large gaps in understanding phonon interactions with rough surfaces, especially when the roughness dimensions are comparable to phonon wavelengths. We report spectroscopic measurements of specularity in silicon nanostructures [3] with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and detect confined acoustic phonons (~ 18 - 200 GHz) in freely-suspended silicon membranes and nanowires. Surface scattering dominates intrinsic Akhiezer damping at frequencies > 60 GHz, thereby enabling us to probe phonon-boundary interactions over wavelengths ~ 42 - 140 nm. To quantitatively understand the dependence of boundary scattering on RMS roughness and correlation length, we obtained detailed statistics of the surfaces using HRTEM and AFM imaging. For silicon membranes, we find that the perturbation approximation [4] for roughness scattering successfully explains the nearly specular reflection of ~ 0.1 THz phonons from surface with ~ 1-nm scale roughness. In silicon nanowires, however, we find that measured phonon specularities are significantly lower in comparison to the predictions from perturbation-based approach. The reduction in specularity is caused by additional scattering from multiple surfaces introduced in a nanowire. Using a remarkably simple non-dimensionalization scheme, we show that the scattering from multiple surfaces can be effectively decoupled. The frequency dependence of the phonon specularities (after normalization) still cannot be completely explained within the existing framework of perturbation approach. This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.

[1] Ghossoub, M. G., Valavala, K. V., Seong, M., Azeredo, B., Hsu, K., Sadhu, J. S., ... & Sinha, S. (2013). Spectral phonon scattering from sub-10 nm surface roughness wavelengths in metal-assisted chemically etched Si nanowires. Nano letters, 13(4), 1564-1571.
[2] Ravichandran, N. K., Zhang, H., & Minnich, A. J. (2018). Spectrally Resolved Specular Reflections of Thermal Phonons from Atomically Rough Surfaces. Physical Review X, 8(4), 041004.
[3] Gelda, D., Ghossoub, M. G., Valavala, K., Ma, J., Rajagopal, M. C., & Sinha, S. (2018). Specularity of longitudinal acoustic phonons at rough surfaces. Physical Review B, 97(4), 045429.
[4] Maznev, A. A. (2015). Boundary scattering of phonons: Specularity of a randomly rough surface in the small-perturbation limit. Physical Review B, 91(13), 134306.

We computationally designed ultra-narrowband wavelength-selective thermal emitter and absorber via a materials-informatics method alternating between Bayesian optimization and thermal electromagnetic field calculation. For a given target infrared wavelength, the optimal structure was efficiently identified from over 8 billion candidates of multilayers consisting of multiple components (Si, Ge, and SiO₂). The resulting optimized structure is an aperiodic multi-layered metamaterial exhibiting high and sharp emissivity with the Q-factor of 273. The designed metamaterials were then fabricated, and reasonable experimental realization of the optimal performance was achieved with the Q-factor of 188, which is significantly higher than those of structures empirically designed and fabricated in the past. This is the first demonstration where metamaterials designed by Bayesian optimization is realized in experiments. The results facilitate the machine-learning-based design of metamaterials and advance our understanding in the narrow-band thermal emission mechanism of aperiodic multi-layered metamaterials.

Porous polymers, which vary in their intrinsic electromagnetic behavior and morphology, are a diverse platform for optical and thermal management. For instance, porous poly(vinylidene fluoride-co-hexafluoropropene) (P(VdF-HFP)) coatings exhibit near-perfect solar reflectance R_solar (> 0.96) and long-wave infrared (LWIR) emittance ε_LWIR (~ 0.97) for radiative cooling [1], while porous poly(ethene) (PE) films exhibit high R_solar (~ 0.8) and LWIR emittance ε_LWIR (~0.6) useful in thermal management designs with tuneable ε_LWIR [2]. However, making such coatings optically dynamic remains to be explored in detail.
In this presentation, we demonstrate that the optical behaviors of porous polymers can be dramatically altered by reversibly wetting and drying them with index-matched fluids. For porous P(VdF-HFP), the drastic reduction of the scattering efficiency of the pores upon wetting causes transition from a solar reflective to a transparent state. The coatings, which show remarkable ΔR_solar of 0.74, can be deployed in roofs, walls and windows for large scale adaptive radiative cooling, solar heating and daylighting depending on the time of day or season. Extending the concept to ethyl cellulose, we show that hydrophilic porous polymers can act as water-responsive coatings that can be used for warming houses after rain or light snow, sensing applications and underwater camouflaging. Lastly, we show that porous PE-based designs wetted with IR emissive fluids can achieve large and opposite ΔR_solar ~ 0.3 and Δε_LWIR ~ - 0.6, representing an “ice-house” to “green-house” transition useful for tuneable radiative cooling at night and potentially, thermal camouflaging. Promisingly, the optical performances are achieved over a fast switching time ~ 1 min, and are stable over large number of cycles.
The performances of the porous polymer coatings are either new, or better than or comparable to notable electrochromic and thermochromic designs [2-4]. However, unlike the latter, they combine a passive functionality with active switching. Importantly, the porous polymers and fluids used are readily available and inexpensive. Given these aspects, porous polymers with fluid-activated optical switching are a promising and sustainable paradigm for optical and thermal regulation.

[1] J. Mandal et. al., Science 362, 315 (2018)
[2] J. Mandal et. al. Adv. Funct. Mater. 28, 1802180 (2018)
[3] D. R. Rosseinsky et. al. Adv. Mater. 13, 11 (2001)
[4] J. Zhou et. al. Sci. Rep. 3, 3029 (2013)

A fundamental series of thermal conductivity measurements using time domain thermoreflectance (TDTR) have been performed on photonic (phonon-glass) HKUST-1 Metal-Organic Frameworks (HKUST-1 SURMOFs) thin films. We have determined the thermal conductivity of loaded and unloaded (pristine) HKUST-1 thin films in dependence of their film thickness. These monolithic, polycrystalline HKUST-1 SURMOF thin films are grown using liquid phase epitaxy (LPE) grown at different film thicknesses on specially modified Au and Al SiO2 substrates. In addition, infiltrated MOFs have been measured to determine the effect of loading with different organic solvents, water, and TCNQ. The experimental results demonstrate a reduction in the thermal conductivity upon the polar guest molecule infiltration compared to the pristine activated MOFs. The bulk modulus of the thin, pristine HKUST-1 SURMOF was also measured to understand how the negative thermal expansion of this material affects the temperature dependent mechanical and thermal properties. These materials have been presented here as tunable Designer Materials for the generation of novel porous phononic Nanomaterials with unexpected and tailorable physical properties for high-efficient thermoelectric generation and future applications.

Colloidal nanocrystal solids are widely used in a variety of applications due to their unique electronic, optical and magnetic properties¹. However, nanocrystal solids have very low thermal conductivities (0.1-0.5 W m^-1 K^-1), which presents a challenge for heat dissipation in nanocrystal solid-based electronic and photonic devices^{2, 3}. The low thermal conductivity of nanocrystal solids originates from numerous interfaces in their internal structure. The ligand-ligand interfaces between adjacent nanocrystals are especially problematic because of the weak van der Waals (vdW) interactions at these locations.
We show that forming covalent bonds at these ligand-ligand interfaces can significantly increase thermal conductivity in colloidal nanocrystal solids. More specifically, we demonstrate a thermal conductivity increase of 200-300% in iron oxide nanocrystal solids via crosslinking of adjacent oleic acid ligand molecules.
Crosslinking of the oleic acid molecules on nanocrystal surface is carried out through a moderate thermal anneal (350 °C)⁴ and is confirmed through a combination of x-ray photoelectron spectroscopy, mass spectrometry, and nanoindentation experiments. The nanoindentation experiments demonstrates that this cross-linking process increases the Young’s modulus of the nanocrystal solid from 1.4 GPa to 9.5 GPa. This correlates to a sound velocity increase from 1243 m s^-1 to 4290 m s^-1 that in turn helps increase thermal conductivity.
We use the differential 3 omega method to measure the cross-plane thermal conductivity of our nanocrystal solid samples as a function of nanocrystal diameter. These measurements indicate that ligand-cross-linking increases the thermal conductivity of these samples by a factor of 2-3. This is in approximate agreement with effective medium approximation (EMA) modeling that predicts a factor of 3. We also use molecular dynamic simulations to confirm that ligand cross-linking alleviates a key thermal transport bottleneck in these materials. These results collectively suggest that the key to improve the thermal transport in nanocrystal solids lies to improving thermal transport across the ligand-ligand interface.

Reference:
1. J.-H. Choi, H. Wang, S. J. Oh, T. Paik, P. Sung, J. Sung, X. Ye, T. Zhao, B. T. Diroll and C. B. Murray, Science, 2016, 352, 205-208.
2. W.-L. Ong, S. M. Rupich, D. V. Talapin, A. J. McGaughey and J. A. Malen, Nat Mater, 2013, 12, 410.
3. M. Liu, Y. Ma and R. Y. Wang, ACS nano, 2015, 9, 12079-12087.
4. A. Dreyer, A. Feld, A. Kornowski, E. D. Yilmaz, H. Noei, A. Meyer, T. Krekeler, C. Jiao, A. Stierle and V. Abetz, Nat Mater, 2016, 15, 522.

Wide bandgap electronics made from nitrides (e.g., Gallium Nitride (GaN)) and oxides (e.g., Hafnium Oxide (HfO₂) and Gallium Oxide (Ga₂O₃)) are currently under development due to their potential to create some of the most advanced RF, power electronic, and neuromorphic computing devices in the world. However, the thermal response of these devices under applied electric fields can create large power densities (RF and power electronics) or highly non-linear behavior (neuromorphic devices) that must be understood. For many of these devices, the electrothermal response plays a strong role in both the acceptable operation or long-term failure and reliability of the devices. Thus, tools that can help elucidate these responses and provide a method to help design better devices is of critical need for this field.
In this talk we will discuss advancements in modeling and thermal characterization techniques that have allowed new insights into electrothermal behavior in HfO₂, GaN and Ga₂O₃ materials and devices. Specifically, a focus on the role of device architecture and material processing will be addressed. For GaN and Ga₂O₃ RF and power electronics, the role of thermal interfaces in the heat dissipation of both lateral and vertical device architectures will be discussed. Unique thermoreflectance imaging of cross sectioned devices will show for the first time the experimental picture of the electrothermal response of devices such as pn diodes as current and heat flow through the electrical contacts. For neuromorphic devices, the use of electrically driven filament formation will be elucidated through multiphysics modeling and high spatial resolution thermal imaging. These models will be used to determine methods in which low power formation and operation of neuromorphic devices can be obtained.
Wide bandgap electronics made from nitrides (e.g., Gallium Nitride (GaN)) and oxides (e.g., Hafnium Oxide (HfO₂) and Gallium Oxide (Ga₂O₃)) are currently under development due to their potential to create some of the most advanced RF, power electronic, and neuromorphic computing devices in the world. However, the thermal response of these devices under applied electric fields can create large power densities (RF and power electronics) or highly non-linear behavior (neuromorphic devices) that must be understood. For many of these devices, the electrothermal response plays a strong role in both the acceptable operation or long-term failure and reliability of the devices. Thus, tools that can help elucidate these responses and provide a method to help design better devices is of critical need for this field.
In this talk we will discuss advancements in modeling and thermal characterization techniques that have allowed new insights into electrothermal behavior in HfO₂, GaN and Ga₂O₃ materials and devices. Specifically, a focus on the role of device architecture and material processing will be addressed. For GaN and Ga₂O₃ RF and power electronics, the role of thermal interfaces in the heat dissipation of both lateral and vertical device architectures will be discussed. Unique thermoreflectance imaging of cross sectioned devices will show for the first time the experimental picture of the electrothermal response of devices such as pn diodes as current and heat flow through the electrical contacts. For neuromorphic devices, the use of electrically driven filament formation will be elucidated through multiphysics modeling and high spatial resolution thermal imaging. These models will be used to determine methods in which low power formation and operation of neuromorphic devices can be obtained.

We present experimental measurements of the thermal boundary conductance (TBC) from 78 − 500 K across isolated heteroepitaxially grown ZnO films on GaN substrates. This data provides an assessment of the underlying assumptions driving phonon gas based models, such as the diffuse mismatch model (DMM), and atomistic Green’s function (AGF) formalisms used to predict TBC. Our measurements, when compared to previous experimental data, suggest that TBC can be influenced by long wavelength, zone center modes in a material on one side of the interface as opposed to the “vibrational mismatch” concept assumed in the DMM; this disagreement is pronounced at high temperatures. At room temperature, we measure the ZnO/GaN TBC as 490[+150, −110] MW m−2 K−1 . The disagreement among the DMM and AGF, and the experimental data at elevated temperatures, suggests a non-negligible contribution from other types of modes that are not accounted for in the fundamental assumptions of these harmonic based formalisms, which may rely on anharmonicity. Given the high quality of these ZnO/GaN interfaces, these results provide an invaluable, critical, and quantitative assessment of the accuracy of assumptions in the current state of the art computational approaches used to predict phonon TBC across interfaces. We then extend our understanding to measurements across other interfaces of GaN.

Solid-state interfaces are pivotal to the high performance in semiconducting electronic devices ranging from GaN-based high-electron mobility transistors to light-emitting diodes. While unprecedented electrical loads are on such electronic devices to maximize their capability, the interfaces usually impede efficient heat removal with large thermal boundary conductance (TBC). For example, in power electronics, generated heat fluxes from semiconducting channels have reached (or exceeded) 1 GWm^-2. It leads to temperature rises in the channels up to 100 ^oC, where half of the temperature rise can be from various interfaces. Recently, it has been reported that nanostructured interfaces with periodic nanopillar arrays significantly increase thermal transport across semiconductor/metal interfaces.
In this study, we demonstrate that random-distributed nanopillar interfaces fabricated by low-cost Au islands metallic etching masks can enhance TBC of semiconductor/metal interfaces, where the Au islands mask is prepared by thermal solid-state dewetting (TSSD) properties of Au on solid substrates. We first confirm that Si/Al nanostructured interfaces with randomly and periodically distributed Si nanopillars have similar enhanced thermal transport across the interfaces when their interfacial area enhancement -- which are accurately controlled by electron-beam lithography -- are almost same. Such finding suggests distribution of nanopillars at interfaces are not critical to thermal transport enhancement, motivating us to find low cost methods to fabricate randomly distributed nanopillars on solid substrates. As a result, we apply the TSSD method to prepare random nanostructured Si surfaces where the Si nanopillars have characteristic lengths ranging from 10 nm to 70 nm. By capping these nanostructured Si surfaces with Al layer, it is found that the TBC of these randomly nanostructured Si/Al interfaces is ~ 90 % higher than that of planar interfaces. In addition, the nanostructured Si/Al interfaces maintain the enhancement at a temperature range from 30 ^oC to 110 ^oC, in which semiconducting electronics usually work. Moreover, we successfully extend the TSSD technique to GaN surfaces and demonstrate thermal transport enhancement across GaN/Al interfaces similar to the cases for Si/Al interfaces. The TSSD technique can potentially reduce the cost to achieve random nanostructured interfaces over large areas. Commercial applications can take advantage of our results in enhanced interfacial thermal transport to improve thermal management.

Moore’s law of packaging calls for several innovations in advanced packaging. One approach is the direct placement of silicon dies, heterogeneously, on a single interconnected platform, termed ‘Silicon Interconnect Fabric’ (Si-IF) [1-2]. In this approach, interconnect pitch can be reduced by almost 100 times (<10 µm) [3], which is impossible to achieve by conventional packaging technologies. However, such high placement density of dies requires enormous power density (~ 1-10 Wmm^-2) which in turn generates huge heat load on the system. Therefore, such a dense but large system will need efficient thermal dissipation to avoid thermal degradation of performance and to ensure reliability. This calls for an efficient power interconnect architecture to the Si-IF that delivers not only power but also manages thermal loads generated in Si-IF. Here, we propose an approach, called PowerTherm that combines these two functions uniquely and efficiently.
The PowerTherm concept involves a network of current-delivering blocks with built-in cooling that is dynamic and addressable. The architecture of PowerTherm consists of metallic terminal blocks such as Cu bonded to Si-IF by thermo-compression bonding and optimized copper-filled electrically insulated through-wafer-vias (TWVs) that pass through the entire thickness of the Si-IF (500 µm) to contact the terminal block. For thermal management, macroscopic channels are constructed into the metallic terminal blocks through which cooled liquid is passed. We use a dynamic and ultra-high-flux capable flash cooling method with two-phase coolants that produces peak heat transfer coefficients of 50,000 W/m²K [4]. For testing of thermal management of Si-IF by flash cooling, we developed a dynamically controlled flow loop. The loop has been designed to be flexible and robust at various test conditions to quantify and benchmark the flash cooling of Si-IF. A detailed characterization of heat dissipation density and transient thermal response of the Si-IF were carried out in this study.

References:
1. Bajwa, A. A.; Jangam, S.; Pal, S.; Vaisband, B.; Irwin, R.; Goorsky, M.; Iyer, S. S. In Demonstration of a Heterogeneously Integrated System-on-Wafer (Sow) Assembly, 2018 IEEE 68th Electronic Components and Technology Conference (ECTC), 29 May-1 June 2018; 2018; pp 1926-1930.
2. 2. Iyer, S. S., Heterogeneous Integration for Performance and Scaling. IEEE Transactions on Components, Packaging and Manufacturing Technology 2016, 6, 973-982.
3. Bajwa, A. A.; Jangam, S.; Pal, S.; Marathe, N.; Bai, T.; Fukushima, T.; Goorsky, M.; Iyer, S. S. In Heterogeneous Integration at Fine Pitch (≤ 10 µM) Using Thermal Compression Bonding, 2017 IEEE 67th Electronic Components and Technology Conference (ECTC), 30 May-2 June 2017; 2017; pp 1276-1284.
4. Engerer, J. D.; Fisher, T. S., Flash Boiling from Carbon Foams for High-Heat-Flux Transient Cooling. Applied Physics Letters 2016, 109, 024102.

Because of its ultra-wide bandgap, high electrical breakdown strength, and potential for large area substrates, b-Ga₂O₃has attracted great attention recently for applications of high power electronics. However, its thermal conductivity is at least an order of magnitude lower than those of other wide bandgap semiconductors, such as SiC, GaN, AlN, and diamond. Proper thermal management is essential to avoid device degradation for high-voltage applications. A possible solution is to integrate thin Ga₂O₃with diamond to fabricate Ga₂O₃-on-diamond devices by taking advantage of the ultra-high thermal conductivity of diamond as a heat spreader and wide bandgap dielectric substrate. When dissipating heat generated in a device through a diamond substrate, the thermal boundary resistance (TBR) of the Ga₂O₃-diamond interface can play an important role in the thermal response. The contribution of the TBR in this heterostructure has not been studied yet. In this work, we study thermal transport across several Ga₂O₃-diamond interfaces, including exfoliated Ga₂O₃-single crystal (SC) diamond van der Waals interface, chemical-vapor-deposited diamond grown on a SC (-201) Ga₂O₃commercial substrate (Tamura Corp.), as well as atomic-layer-deposited Ga₂O₃on SC diamond. The thermal boundary conductance (TBC) of these interfaces was measured by time-domain thermoreflectance (TDTR) and the structures of these interfaces are studied with transmission electron microscopy (TEM). For instance, the TBC of exfoliated Ga₂O₃-SC diamond van der Waals interface is measured to be 17 MW/m²-K. To understand the measured TBC values of different Ga₂O₃-diamond interfaces, a Landauer approach is used to calculate the theoretical TBC of Ga₂O₃-diamond interfaces. An electron energy loss spectroscopy (EELS) study is performed across the Ga₂O₃-diamond interfaces with nanometer spatial resolution to probe the vibrational mode. Our work is the first effort to study thermal conductance across Ga₂O₃-diamond interfaces. These experimental and theoretical studies would not only contribute to developing new theories of interfacial thermal transport, but also impact applications such as high power electronics.

A novel fabrication method for an imperceptible transparent electrode based on copper random micro patterns is introduced in this work. This fabrication method enables minimum pattern width of 2.4μm and elimination of Moiré and starburst effect by employing the randomness of the pattern, in turn an imperceptible transparent electrode especially for display devices or thermoelectric devices can be made. The cost of fabrication of the transparent electrode introduced in this study is much lower than other well-known fabrication methods which use noble materials such as silver nanowires since the process is all solution based process and only Cu nanoparticles are used as a conducting medium. Thermal conducting layer is introduced to successfully sinter Cu nanoparticles. The performance of imperceptible transparent electrode based on copper random nanopatterns made by fabrication method introduced in this work, a resistive type touch panel application and defroster application is also demonstrated.

Understanding the scattering mechanisms of charge carrier is critical to the electrical properties of two-dimensional materials such as graphene, transition metal dichalcogenide monolayers, black phosphorene, and tellurene. Beyond pristine two-dimensional materials, further tailored properties can be achieved by nanoporous patterns such as nano- or atomic-scale pores (antidots) across the material. As one example, structure-dependent electrical/optical properties for graphene antidot lattices (GALs) have been studied in recent years.^1,2,3However, detailed charge carrier scattering mechanism is still not fully understood, which hinders the future improvement and potential applications of such metamaterials.
In this work, the energy sensitivity of charge-carrier scattering and thus the dominant scattering mechanisms are revealed for GALs by analyzing the maximum Seebeck coefficient with a tuned gate voltage and thus shifted Fermi levels.⁴It is found that charge carriers are mainly scattered by the potential field due to pore-edge-trapped charges, particularly at elevated temperatures. For thermoelectric interests, the gate-voltage-dependent power factor of different GAL samples are measured and can reach 554 at 400 K for a GAL with the square pattern. For on-chip cooling applications, such a high power factor is improved by more than one order of magnitude from the values for the state-of-the-art bulk thermoelectric materials.⁵With their high thermal conductivities and power factors, these GALs can be well suitable for “active coolers” within electronic devices, where heat generated at the hot spot can be removed with both passive heat conduction and active Peltier cooling.⁶
Reference:
T. G. Pedersen, C. Flindt, J. Pedersen, N. A. Mortensen, A.-P. Jauho and K. Pedersen, Physical Review Letters, 2008, 100, 136804.
2. A. Sandner, T. Preis, C. Schell, P. Giudici, K. Watanabe, T. Taniguchi, D. Weiss and J. Eroms,Nano letters, 2015, 15, 8402-8406.
3. R. Yagi, R. Sakakibara, R. Ebisuoka, J. Onishi, K. Watanabe, T. Taniguchi and Y. Iye, Physical Review B, 2015, 92, 195406.
4. S. Tang, Scientific reports, 2018, 8, 10597.
5. G. S. Nolas, J. Sharp and J. Goldsmid, Thermoelectrics: basic principles and new materials developments, Springer Science & Business Media, 2013.
6. M. Zebarjadi, Applied Physics Letters, 2015, 106, 203506.

Two-dimensional (2D) semiconducting MoS₂ is a promising material for end-of-roadmap technologies [1], flexible and transparent electronics [2], and thermoelectric applications [3]. However, it is known that electrical and thermal conductivities in other 2D materials, like graphene, degrade when in contact with a substrate, e.g. due to scattering with substrate impurities or phonons [4, 5]. Such insulating substrates cannot be avoided in modern nanoelectronics; therefore, understanding the role of the substrate is crucial to know the realistic thermal limitations of 2D semiconductors like MoS₂.

Here, we present a detailed molecular dynamics (MD) study on the effects of an amorphous SiO₂ (a-SiO₂) substrate on the in-plane thermal conductivity of single-layer MoS₂. Unlike continuum models, the atomistic nature of MD accurately captures phonon scattering physics in 2D materials and their interfaces. The GPUMD package [6, 7] is used for all MD computations and thermal conductivity calculations use the recently-developed homogenous non-equilibrium MD (HNEMD) method [8]. HNEMD results are consistent with the equilibrium Green-Kubo and non-equilibrium MD methods but require fewer computational resources. The in-plane thermal conductivity is decomposed into its in-plane and out-of-plane contributions to elucidate which phonons are suppressed. We also present a spectral decomposition of the thermal conductivity for insight into the phonon wavelengths affected by the substrate.

To facilitate comparison, we compute the in-plane thermal conductivity of both isolated, single-layer MoS₂, and single-layer MoS₂ on an a-SiO₂ substrate. The simulation cell area is 10 × 10 nm², the single-layer MoS₂ is 6.15 Å thick, and the a-SiO₂ substrate is 5.4 nm thick. The in-plane dimensions are periodic, with vacuum in the out-of-plane direction. All simulations are at 300 K with a time step of 0.5 fs.

Running 20 independent simulations for each structure, we first calculate the suspended single-layer MoS₂ in-plane thermal conductivity to be ~115 Wm^-1K^-1 (consistent with experimental measurements on bulk MoS₂ [9]) with the in- and out-of-plane contributions as ~84 Wm^-1K^-1 and ~31 Wm^-1K^-1 respectively. The in-plane thermal conductivity of MoS₂ supported by an a-SiO₂ substrate is calculated to be ~29 Wm^-1K^-1 (~74% decrease) with the in- and out-of-plane contributions as ~25 Wm^-1K^-1 (~70% decrease) and ~4.7 Wm^-1K^-1 (~85% decrease) respectively. While a greater proportion of the out-of-plane component is suppressed on a-SiO₂, the in-plane component’s reduction drives the overall reduction in thermal conductivity. This contrasts the suppression of the in-plane thermal conductivity of graphene on a-SiO₂, which suffers ~91% degradation mostly from the suppression of the out-of-plane component [10]. Our results underscore the strong effect of the substrate on thermal conductivity of single-layer 2D semiconductors like MoS₂, for the first time.

1. S. Salahuddin et al., Nature Electronics 1, 442-450 (2018).
2. G.-H. Lee et al., ACS Nano 7, 7931-7936 (2013).
3. K. Hippalgaonkar et al., Physical Review B 95, 115407 (2017).
4. J. H. Seol et al., Science 328, 213-216 (2010).
5. V. E. Dorgan et al., Applied Physics Letters 97, 082112 (2010).
6. Z. Fan et al., Computer Physics Communications 218, 10-16 (2017).
7. Z. Fan et al., Computer Physics Communications 184, 1414-1425 (2013).
8. Z. Fan et al., arXiv:1805.00277 [cond-mat.mtrl-sci].
9. J. Liu et al., Journal of Applied Physics 116, 233107 (2014).
10. Z.-Y. Ong, E. Pop, Physical Review B 84, 075471 (2011).

Nowadays, there is an ever-growing wave of reports providing cumulative evidence about a behavior of heat transport in solids at nanoscale that may substantially divert from the well-known physical laws governing thermal conductivity at macroscopic (bulk) size range¹. Thus, the opportunity is wide open to unveil a much more comprehensive insight on how the adage “more is different” can work also in the opposite direction. If so, a scientific turning point lies ahead, once ascertained if certain laws of nature may work through a different path when considered at ultrashort range domain.

In the other hand, luminescent nanothermometry has become a powerful investigative tool able to report temperature real-time, from inside complex systems in a smart non-contact way. Based in a thorough understanding of how emission lines from the luminescent nanoparticles change depending on temperature variations^2,3, these so-called nanothermometers report local temperature readouts grasped from the spectroscopic measurement of the light emitted from them, upon optical excitation. This research field has produced substantial progress and applications⁴, especially at the biomedical sensing arena⁵

In this work, we present a simple nanothermometry set up -already used for succesful studies on brownian movement⁶ - to measure the average time profile of temperature changes inside a given kind of luminescent nanoparticles. Appropriately, the nanoparticles has been engineered to constitute high-resolution nanothermometers, able to distinctively report in real-time the differentiated temperature from two physically separated points, at the inside of the nanoparticles. Thus, that described framework allows us to record the (averaged) time evolution of the temperature inside the nanoparticles.

A quartz cuvette containing core-shell nanoparticles suspended in water was heated from one side in thermal contact with a thermofoil heater. The temperature of the suspension was increased by adjusting the temperature of the heater and the evolution of corresponding upconversuion emission spectra of the suspended nanoparticles was recorded upon excitation with a 980nm-laser. The core and shell temperature profiles were determined, respectively, by using the integrated intensity ratios of the Er^{3+ 2}H_11/2-->⁴I_15/2/⁴S_3/2-->⁴I_15/2 and Tm^{3+ 1}G₄ -->³H₆/¹D₂-->³F₄ transitions as thermometric parameters, with temperature resolution <0.5K and relative thermal sensitivity up to 1.25%K^-1. Hence, this procedural method allows to measure precisely two distinct onset times with delay time up to 25s for the core and shell temperature increments when a core-shell nanoparticle is heated by an external heating source and to throroughly study the corresponding thermal transport dynamics in nanosystems. The experimental process is also gifted with high repeatability of the measure values. The data mining, once carefully treated with error correction numerical protocols, translates into a great wealth of heat transport insight (20-50 nm distance through cristalline lattice).

The conclusions produced from the collected data will undoubtedly help clarifying trends on the different behavior of the thermal diffusivity at nanoscale in solids, compared to the one foreseeably expected from the (macroscopic) heat diffusion laws in solids.This presented method paves the way for adapting new theoretical formulations of heat transport, also featuring different implications spread into physics, microelectronics, photonics and biology.

REFERENCES

¹ Kathleen M. Hoogeboom-Pot, et al PNAS , 2015 112 4846
² D. Jaque, F. Vetrone, Nanoscale 2012, 4, 4301;
³ C. D. S. Brites et al, Nanoscale 2012 , 4, 4799
⁴ C. D. S. Brites et al, in Handbook on the Physics and Chemistry of Rare Earths, Vol. 49 (Eds: J.-C. G. Bünzli, V. K. Pecharsky), Elsevier Science, B. V., Amsterdam 2016, 339
⁵ H. D. A. Santoset al Adv. Functional Mater. 2018, 10.1002/adfm.201803924
⁶ C. D. Brites, et al Nat. Nanotechn. 2016, 11, 851

Materials with tunable phonon transport are a key ingredient to achieve logic operations using heat currents. Moreover, the corresponding variation of the thermal conductivity can be used to greatly enhance the thermoelectric properties and, therefore, promoting the development of new thermoelectric materials. In this regard, ferroelectrics are excellent candidates to be explored as they possess intrinsic interfaces –domain walls– which effectively act as phonon scattering centers and their distribution and type can be engineered, for instance, by epitaxial strain. In pursuit of this goal, we report the effect of the strain-engineered domain and superdomain structures in ferroelectric PbTiO₃ on the thermal conductivity.
PbTiO₃ thin films were grown by reactive molecular-beam epitaxy on SrTiO₃, DyScO₃, TbScO₃, GdScO₃, and SmScO₃ substrates, covering epitaxial strain values from compressive -1.36% to tensile +0.71%. For large compressive strains PbTiO₃ thin films with only c-domains are obtained. At these compressive strain values, the absence or the presence of an electrostatic boundary condition (an epitaxially grown bottom electrode) switches from a c-up/c-down configuration with 180° domain walls to a single c-domain sample that contains no domain walls. On reducing the compressive strain, a gradual increase in the population of a-domains embedded in a matrix of c-domains takes place, giving rise to a/c domain architectures. The density of these a/c 90° domain walls increases on reducing compressive strain. For tensile strains a competing scenario between a/c and a₁/a₂ superdomains is found in which the ratio between both populations can be tuned by varying strain. This enabled the superdomain architecture to be engineered at will. The density of 90° domain walls between the a domains within the a₁/a₂ superdomains was found to increase with increasing tensile strain.
The thermal conductivity of PbTiO₃ thin films is strongly influenced by these designed ferroelectric nanostructures. When no domain walls are present, relatively high thermal conductivity is obtained: ≈3.4 Wm^-1K^-1, similar to bulk values. The thermal conductivity, however, dramatically drops (≈70% reduction) when a high a₁/a₂ domain wall density in an a₁/a₂ superdomain configuration is achieved. A clear correlation between thermal conductivity and domain wall density/type is found, giving rise to a huge control of further tuning the thermal conductivity of PbTiO₃ through epitaxial-strain-engineered domain configurations. These results agree with the theoretical prediction from phase-field modeling, which were performed without previewing the experimental data. The combination of theory and experiment allows gaining a deeper understanding of this phenomenon that relates strain-designed domain structures and thermal conductivity.

In this contribution, we shall illustrate through two examples how the addition of a tiny amount of nanoparticles may significantly change the thermal transport properties of soft materials. Of particular importance here are the coupling between phonons in the solid and high frequency vibrations in the liquid [1].

First, we analyze heat transport between gold and a model ethanol nanofluid using molecular dynamics simulations [2]. We show that the presence of the nanoparticles increases the Kapitza resistance typically by a factor 2. Surprisingly, the interfacial resistance is found to depend only weakly on the interactions between the nanoparticles and the gold surface. This increase has important consequences as regards ultrafast explosive boiling, and we report on a significant shift of the nanofluid/gold boiling temperature as compared to pure ethanol. The corresponding superheating shift is found to be strongly correlated with the increase of Kapitza resistance.

Second, we demonstrate how the presence of a few nanoparticles may enhance the thermal conductivity of amorphous polymers [3]. Indeed, polymer are renowned for their low cost, low weight and resistance to corrosion. Yet, a barrier which limits the spread and use of polymers in a wide range of applications, and especially in microelectronics, is their relative poor thermal conduction properties. Dispersing highly conductive nanoobjects in a polymer matrix is a common strategy employed to level off polymer conductivity. However, the results are often mitigated by the high Kapitza resistance between the polymer matrix and the dispersed fillers. Here, we report a significant increase of the bulk conductivity of polystyrene matrix filled with silica nanoparticles. We show that the relative enhancement is essentially explained by the relative good matching between the vibrational modes of the polymer and the silica nanoparticles. We discuss also the role of polymer chains grafted on the surface of the nanoparticle.

References:
[1] H. Han, S. Merabia, F. Müller-Plathe, J. Chem. Phys. Lett., 8 (2017), 1946-1951
[2] H. Han, S. Merabia*, F. Müller-Plathe*, Nanoscale 9 (2017), 8314-8320
* equal contribution
[3] C. Ma, S. Merabia et al. under redaction

The availability of fabricating new mesoporous materials opens the possibility of having materials with tunable properties such as thermal conductivity. In this work, we present two applications of a generalized effective medium approximation that allows modeling of the thermal properties of porous systems.
A simple composite system is made of a homogeneous matrix with a fixed number of parallel cylindrical holes of circular cross-section. By defining the two-dimensional depolarization factor for the cross-section of the cylinders, we calculate the effective thermal conductivity. For a system with a fixed number of pores, we show how the heat transport depends on the eccentricity of the pores. This is a full calculation that includes the heat resistance at the pore-matrix interface (Kapitza resistance). The method is applied to deformable porous media. By applying a mechanical stress to the system, the distance between the nanopores changes as well as their shape, initially with a circular cross-section. We find an expression for the effective heat coefficient as a function of the applied mechanical stress.
For a set of layered systems, each layer made of a mesoporous system. In particular, we consider as a second example the calculation of the heat transport coefficient of a stack of a mesoporous TiO2 layer and mesoporous SiO2 layer, validating our model with recent measurements (1).
(1) C. D. S. Brites, M. C. Fuertes et al. NanoLetters, vol. 17, 4746 (2017).

Encompassing diffusive, quasi-ballistic and ballistic regimes, heat conduction is inherently a multiscale problem. At the atomistic level, the phonon mean-free-path (MFP) distribution is typically obtained by density functional theory. On the other side, phonon-boundary interaction is conveniently captured by continuum models, such as the Boltzmann transport equation (BTE). Finally, short-MFP phonons will mostly travel diffusively, so they can be accounted for by a standard diffusive solver. OpenBTE [1] attempts bridging these three regimes. Specifically, the MFP distribution, computed by first-principles, is used as input to the MFP-BTE [2], a recently introduced version of the BTE that uses the phonon MFP as a control variable. Then, using a parameter-free multiscale approach, the BTE solver integrates a modified diffusive model that calculates heat carried by short-MFP phonons. After a brief introduction to the software architecture, we will show example applications, including thin films, nanoporous materials, Graphene nanoribbons, Si serpentines, phonon focusing and large-scale screening of nanostructured materials for thermoelectric applications. Conclusions and final remarks will conclude the talk.
[1] G. Romano and J. C. Grossman. "Heat conduction in nanostructured materials predicted by phonon bulk mean free path distribution." Journal of Heat Transfer, 137.7 (2015): 071302.
[2] www.openbte.org

Defects engineering has been a widely used approach for developing low thermal conductivity materials. Also, for some materials (e.g., IV-VI compound), considerably large amount of vacancies was often observed in single crystalline samples. The atomic vacancy in dilute case was assumed as a point defect that can cause the Rayleigh scattering with its rate proportional to the fourth power of phonon frequency. Due to the large dependence of scattering rate on phonon frequency, a vacancy was considered to only weakly suppress the mode thermal conductivity of low frequency acoustic phonons. We show that this does not hold for IV-VI materials. We compare the phonon scattering by a single vacancy in group IV (diamond and Si) and IV-VI (PbTe and GeTe) crystals using an exact ab initio Green’s function approach. The phonon-vacancy scattering rates in diamond and Si follow the well-known fourth-power dependence on phonon frequency. However, PbTe and GeTe exhibit phonon scattering rates that have much weaker frequency dependence. As a result, a vacancy scatters low frequency acoustic phonons more strongly in PbTe and GeTe than in Si and diamond. This unusual behavior in PbTe and GeTe originates from the significant changes of interatomic force constants that extend up to the distance of 10 angstrom from the vacancy site. Therefore, a vacancy in IV-VI materials cannot be assumed as a point defect but its finite size should be considered for phonon scattering. In contrast, a vacancy in Si and diamond changes interatomic force constants only up to 3 angstrom range. Our study provides new insights on the phonon spectral contribution to thermal transport when vacancies exist in IV-VI materials and will help to develop low thermal conductivity materials by engineering vacancies.

In semiconductors, heat is carried by phonons, which are quantized vibrations of the crystal lattice. Standard first principles methods to obtain the thermal properties of solids rely on the phonon quasiparticles being well-defined, and assume that the lowest order interactions among three phonons are sufficient to describe thermal transport. Here we show that this is not the case for strongly anharmonic semiconductors, where phonon scattering is so strong that the standard phonon quasiparticle picture can break down, and three-phonon scattering is insufficient to explain the experiments. To address this issue, we present a novel first principles method that features an anharmonic many-body renormalization scheme to create well-defined phonon quasiparticles with weakened interactions, and rigorously accounts for both three-phonon and four-phonon scattering to obtain thermal transport properties [1]. Using a showcase strongly anharmonic material – sodium chloride (NaCl), we demonstrate that our first-principles method simultaneously captures the apparently contradictory experimental measurements of low thermal conductivity and large lattice thermal expansion of NaCl on the one hand, and the relatively temperature-independent phonon frequencies on the other, while the standard first principles theory fails in all three cases. Furthermore, we show that the higher-order four-phonon scattering significantly lowers the thermal conductivity of NaCl and should be rigorously included for a proper comparison to measurements. Finally, we show that our anharmonic renormalization theory, along with four-phonon scattering, also successfully captures the measured phonon frequencies and thermal transport properties of a weakly anharmonic material – diamond, although the four-phonon scattering and renormalization effects are weak in this material. Our work presents a unified first principles framework to accurately predict the thermal properties of solids with varying bond anharmonicities.

[1] Navaneetha K. Ravichandran and David Broido, Phys. Rev. B. 98, 085205 (2018).

This work was partially supported by Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001299/DE-FG02-09ER46577 (phonons and thermal expansion) and by the Office of Naval Research MURI, Grant No. N00014-16-1-2436 (thermal conductivity).

Thermally conductive molecular crystals are of fundamental and practical interest in part because they are unlike typical complex crystals, which conduct heat poorly owing to their large phonon scattering phase space. While molecular crystals with high thermal conductivity in the range of tens of Wm^-1K^-1 have been known experimentally for decades, the microscopic origin of this property has remained unclear. Ab-initio methods that have been successfully applied to simple crystals have proved difficult to adapt to molecular crystals due to quantum nuclear motion and their complex primitive cells. Here, we report the thermal transport properties of crystalline polythiophene with 28 atoms per primitive cell using an ab-initio approach that rigorously includes finite temperature anharmonicity and quantum nuclear effects. The calculated room temperature thermal conductivity is 176 Wm^-1K^-1, a high value that arises from exceptional phonon focusing along the chain for certain branches and despite short lifetimes in the picosecond range. Our finding suggests that many complex molecular crystals with stiff intra-chain bonds are intrinsically good thermal conductors as phonon focusing occurs in any crystal with anisotropic bonds.