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.

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[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