Symposium EN14 : Thermoelectric Energy Conversion (TEC)—Complex Materials and Novel Theoretical Methods

Goniopolar materials are metals and degenerate semiconductors with a thermopower that is n-type in one crystallographic direction, but p-type in another. This effect is due to a particular geometry of their Fermi surface [1], which must be concave and cannot be 1-simply connected in the Brillouin zone. Very interestingly, in some cases, the sign of the Hall effect is exactly the reverse of the sign of the Seebeck effect, which can again be explained by the topology of the Fermi surface. The same properties can arise in (pxn) materials [2], composites made from alternate p and n-type layers. They also arise in solids in which the Fermi surface consists of electron pockets and hole pockets. When single crystals of such materials are cut at a particular angle with respect to the axes that have positve and negative thermopower, a transverse thermopower arises, in the geometry a Nernst thermopower would appear, but in zero magnetic field. This can be used to design transverse thermoelectric power generators and transverse Ettingshausen-like coolers. In this talk, several members of this new genus of materials will be described, beyond the original materials (NaSn2As2) on which the effect was discovered [1].
[1] B. He et al., Nat. Mater. published online https://doi.org/10.1038/s41563-019-0309, (2019)
[2] C. Zhou et al., Phys. Rev. Lett. 110, 227701 (2013).

Classical view of thermoelectricity (TE) requires a junction of two differing conductors – two dissimilar metals, p and n doped semiconductors, etc. in order to create the electric potential when the junction is heated (Seebeck effect), or to cool/heat the junction depending on the direction of current flow (Peltier effect). With the efficiency of TE devices defined by both the electronic and heat transport properties of materials, contradictory requirements have to be satisfied, with nanostructuring offering additional “degrees of freedom” allowing to find optimal solutions.

Here we studied atomically flat 2D materials that include zero-gap semiconductor graphene (GR) and semiconducting transition metal dichalcogenides known for their extraordinary electronic and thermal properties. With relativistic carrier dynamics and exceptionally large mean-free path (MFP) of carriers in GR, combined with its highly anisotropic and environment dependent thermal conductance, one can expect unusual thermoelectric properties in such nanostructures. In order to study TE phenomena in these, we applied scanning thermal microscopy (SThM) that uses a sharp tip in point contact (few nm across) with the probed surface. SThM can measure the local temperature of the heated sample or create a controlled local sample temperature rise, while measuring the resulting heat flow. We used e-beam lithography patterned GR devices of different geometry with attached electrodes that allow to either inject the current into the device, while mapping and measuring the local heat rise (Joule heating and Peltier effect) or to actively heat the SThM probe while providing nanoscale maps of the resulting “thermovoltage” (Seebeck effect). The latter method (we call it scanning thermal gate microscopy, or SThGM) developed for the first time in these experiments turned out to be very efficient in studying TE materials. We used high vacuum environment to eliminate spurious heat dissipation channels boosting accuracy and sensitivity down to mK range, and allowing measurements of samples down to cryogenic temperatures.

Using this methodology, we investigated TE phenomena in GR devices with varying geometry – starting with the simple bow-tie shape with the constriction of about 100 nm wide and 100 nm long connected to the triangular shaped areas expanding to the Au electrodes. While mapping and measuring Peltier cooling at the Au electrode – Gr interface (fully expected in the classical TE phenomena), we completely unexpectedly found that a much stronger Peltier cooling was observed around the constriction area. This phenomenon was confirmed by measuring the Seebeck thermovoltage map. This constriction had no dissimilar materials and we confirmed that no local doping was present by manipulating the back gate of the device to shifting the Dirac point, suggesting novel phenomenon of “geometrically” defined TE.

We then conducted a series of experiments on different geometries (multiple “necks, long constrictions, holes in the uniform ribbon) and varying current densities, comparing these with the MFP and heat flow models, for matching thermovoltage and cooling-heating measurements. These allowed to confirm that the new effect can be explained solely by the geometrical effect of the constriction on the MFP of the carriers in the GR layer. We also analyse experimental measurements of the effect for varying GR layer number, doping as well as nonlinear TE phenomena of “electron wind” effect observed at higher current densities when carrier drift velocity in 2D material becomes comparable with the carriers Fermi velocity.

In summary, we report the novel TE phenomena in 2D materials where geometry of the device alone can produce significant Peltier and Seebeck TE effects, offering new solutions for the efficient and compact TE devices. Additionally, a novel scanning thermal gate microscopy is presented allowing to very effectively study of TE phenomena in a broad range of nanostructures.

Recent research on thermoelectrics (TE) focuses not only on phonon engineering but also on electronic engineering. Generally, metallic materials have low TE performance as can be explained by Mott relation, where the thermoelectric power (S) of metallic materials is related to the Fermi energy E_F and carrier density n depending on the dimension of the system. Consequently, a large number of carrier density n in metallic materials gives a quite small value of S in contrast to its large electrical conductivity s, resulting in a very small value of power factor. Such a general conclusion, however, can be expected to be broken by introducing nonconventional electronic states such as the special surface states in topological materials. Dirac fermions existing on topological materials can be one of the candidates for non-trivial electronic systems, which attract much attention as the unique research platform in this decade. Three-dimensional topological insulators (3D-TIs) show intriguing gapless helical massless Dirac fermions on a two-dimensional (2D) surface of insulating bulk materials. The electronic band of Dirac fermions is characterized by a linear energy dispersion, which gives an extremely high carrier mobility that could enhance conductivity s without decreasing thermoelectric power to be compared with many conventional metals. This is because the thermopower in a 2D Dirac system is related to the carrier oncentration n with the special relation and can be different from the relation in the conventional metals, indicating a larger S with the same value of n. Such a unique n dependence of S has been observed for graphene first in the past. In the case of 3D-TIs, however, the contribution from bulk carriers prevents such observations and discussions on the TE properties of topological surface Dirac states (TSDSs) had been very limited and still ambiguous. In order to have clear understanding for thermoelectrics (TE) of 3D-TIs, it is essential to observe simultaneously the pure surface TE properties and the accurate band structure. Here, we demonstrate clear observations of both thermopower and quantum spin Hall effect (QSHE) using high quality single crystal flakes of 3D-TIs. According to the high bulk insulation of our 3D-TIs, we succeeded to observe clear QSHE of the TSDS at high temperatures and in the thickness of mm. Our high quality 3D-TIs enable one to determine accurate thermoelectric parameters. The details of band picture of surface states can be evaluated from both Shuvnikov-de-Hass (SdH) oscillations and the QSHE. By employing the fine and accurate experimental data, we will discuss how the high efficient TE properties of 3D-TIs can be available.

Development of efficient thermoelectric materials requires a designing approach that leads to excellent electronic and phononic transport properties. Tetragonal chalcopyrites materials have recently attracted significant attention in optoelectronic and photovoltaic applications. Using first principles density functional theory and semiclassical Boltzmann transport theory, we report unprecedented enhancement in electronic transport properties of chalcopyrites A^IIB^IVC₂^V (group II = Be, Mg, Zn, and Cd; group IV = Si, Ge, and Sn; and group V = P and As) via isoelectronic substitution. Multiple valleys in conduction bands, present in these compounds, are tuned to converge by substitution of group IV dopant. Additionally, this substitution improves the convergence of valence bands, which is found to have a direct correlation with the tetragonal distortion of these chalcopyrites. Furthermore, several chalcopyrite compounds with heavy elements such as Zn, Cd, and As possess low phonon group velocities and large Gruneisen parameters that lead to low lattice thermal conductivity. Combination of optimized electronic transport properties and low thermal conductivity results in maximum ZT of 1.67 in CdGeAs2 at moderate n-type doping. The approach developed here to enhance the thermoelectric efficiency can be generalized to other class of materials.

Complex Materials and Novel Theoretical Methods Researchers have recently developed processes for synthesizing monolithic Si with improved thermoelectric figure-of-merit, ZT. These materials obtain higher ZT through a finely controlled array of secondary phase inclusions. In this work we elucidate the role these particles play in two processes to increase ZT: (1) scattering of phonons to reduce thermal conductivity, and (2) energy selective scattering of electrons to increase the seebeck coefficient. For (1) Boltzmann transport simulations are used to predict the collective effect of the scattering mechanisms on thermal conductivity of different silicon samples and compared with equilibrium molecular dynamics counterpart. For (2) Fermi’s golden rule is used to compute electron scattering interactions with nanoparticles. There scattering rates are combined with electron-phonon and electron-impurity scattering rates using Mathessen’s method. The results are used to predict the electrical properties of the Sl with different volume fraction of nanoparticles. We further present a semiclassical model of thermoelectric transport properties in the presence of energy selective electron scattering — electron energy filtering. The model is validated against a set of Si based thermoelectrics containing silicon carbide dispersoids. The model is extrapolated to explore how energy filtering can be used to enhance the thermoelectric figure of merit, ZT, in a wide range of materials. Counterintuitively, this model predicts that the highest ZTs can be achieved in the tails of the Fermi-dirac distribution of thermoelectrics doped to have high carrier concentration. Boltzmann transport simulations are used to predict the collective effect of the scattering mechanisms on thermal conductivity of different silicon samples. Together, these models present a new strategy for optimization of thermoelectrics that breaks materials engineers free from the traditional paradigm of engineering the fermi energy at the band edge.

Half-Heusler compounds have recently been developed as the most promising high-temperature thermoelectric materials due to their excellent power factor. However, experimental investigations of their intrinsic electronic structures, underpinning the high power factor, are still rare. Herein, high-resolution angle-resolved photoemission spectroscopy, electrical and optical measurements, and first-principles calculations were performed to explore the intrinsic electronic structure of ZrNiSn, the archetypical n-type half-Heusler thermoelectric material. For the first time, the real bandgap of ZrNiSn was revealed as 0.5–0.6 eV using various methods, which are significantly larger than the values reported in previous literature. Additionally, a large anisotropic conduction band was directly identified to contribute to the high power factor. This successful observation of intrinsic electronic structure relies on the synthesized high-quality single crystals, which have much fewer Ni interstitial defects and negligible in-gap states. This work demonstrates the significance of processing-structure-property relationships in materials research and provides new insights to improve the thermoelectric performance of half-Heusler compounds.

The anomalous Nernst effect (ANE), which is magneto-thermo-electronic transport, manifests itself experimentally as an anomalous transverse voltage perpendicular to both the heat current and magnetization.

We report a robust ANE in Co₂MnGa thin films in the thickness series between 10 and 50 nm at 300 K. We further employ this thickness series to study the Mott relation and show that the anomalous Hall coefficient, which is thermoelectric counterpart of ANE, and the N_ANE measured in identical structures under equal conditions exhibit a similar dependency. Moreover, in selected samples, we measure all four transport coefficients – longitudinal and transversal resistivity, Seebeck and N_ANE – and we show that the values of measured and calculated Nernst coefficients and conductivity are comparable when assuming the validity of the Mott relation.

Bi₂Te₃, Sb₂Te₃, and Bi₂Se₃, are well established thermoelectric materials. To improve the thermoelectric performance of these materials, nanoparticles from a wet-chemical synthesis using metal-organic precursors and ionic liquids were synthesized as starting materials. By hot pressing these nanoparticles under inert conditions, record figure of merit (zT) values were demonstrated, for example of zT=1.5 at 470 K for Sb₂Te₃ nanoparticles from this synthesis. But these materials are also three-dimensional (3D) topological insulators (TI) exhibiting a bulk bandgap and highly conductive, robust, gapless surface states. Using the same nanoparticles, but slightly different compaction parameters, highly porous bulk materials were obtained. The high porosity and high interface density creates a situation in which surface transport from the topological states becomes visible in the transport properties at low temperatures. The high surface-to-volume ratio is hereby beneficial, because usually the transport properties of Bi₂Te₃, Sb₂Te₃, and Bi₂Se₃ are dominated by the bulk carriers. Using this nanoparticle-based materials’ design, the highly porous macroscopic sample features a carrier density of the surface states in a comparable order of magnitude as the bulk carrier density. Further, the sintered nanoparticles impose energetic barriers for the transport of bulk carriers at the interfaces (hopping transport), while the connected surfaces of the nanoparticles provide a 3D percolation path for surface carriers. This also helps to emphasis the topological transport properties. Within this article, both the synthesis and nanoparticle processing as well as the transport properties of these combined thermoelectric and 3D TI samples will be discussed.

Tin chalcogenides and specifically SnSe have recently gathered significant attention thanks to reports of record-high thermoelectric efficiency while devoid of toxic elements that might hinder commercial usage. The origin of the large thermoelectric figure of merit of SnSe, i.e. the ZT factor, is still subject of investigations, due to a number of interesting properties combined in this material. The large lattice anharmonicity is responsible for the low values of lattice thermal conductivity, whereas the presence of multiple valleys in the electronic band structure close to the chemical potential enhances the electrical transport characteristics. So far, theoretical modeling has mostly focused on describing the transport properties of SnSe using a semiclassical approach based on the Boltzmann transport equation. However, such model may not be sufficient to accurately describe the properties of this material. In this work, we study corrections to the Boltzmann transport equation, that can be derived more generally from many-body perturbation theory and parametrized using first-principles calculations. In particular, these corrections become more important for crystals with complex band structures (e.g. multi-valleys) and large temperatures, which routinely occur in thermoelectric materials, where electrons and holes are not a well-defined material excitations, and a description based on the density-matrix becomes necessary. We test these corrections in SnSe and show how they improve estimates of thermoelectric properties (by up to a factor 2), leading to an overall better agreement with experiments and providing a general path towards improved simulations of thermoelectric materials.

The ternary perovskite oxides with good stability and high power factor, epitomized by SrTiO₃, have been studied for high temperature thermoelectrics. Their relatively large lattice thermal conductivity remains an obstacle towards achieving high thermoelectric figure of merit, ZT. Binary chalcogenides such as PbTe benefit significantly from reduction of lattice thermal conductivity due to heavy elements and give rise to several materials with record ZT values about 2. Ternary chalcogenides with perovskite and relates structures could potentially marry the attractive features of both materials systems. We have recently explored these semiconductors for optoelectronic applications.^1,2 We chose the model system of BaTiS₃ with a low bandgap and a hexagonal perovskite structure to study the thermoelectric properties. Low thermal conductivity around 0.6 W/mK, Seebeck coefficient of around 170 mV/K and electrical conductivity of about 1 W/cm were obtained at room temperature. Differential scanning calorimetry and thermogravimetric analysis indicate stability up to 1000K in air. The thermoelectric figure of merit was extracted as a function of temperature and a ZT value up to 0.03 was obtained at 700K. We attempted doping BaTiS₃ with Nb and La, but no significant improvement on electrical conductivity was observed. More in-depth doping studies are needed to achieve higher ZT in these materials.
References:
1. Niu, S. et al. Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenides. Adv. Mater. 29, 1604733 (2017).
2. Niu, S. et al. Giant optical anisotropy in a quasi-one-dimensional crystal. Nat. Photon. 12, 392–396 (2018).

In this work, we report a simultaneous increase in Seebeck coefficient and electrical conductivity, that results in increasing power factor, of Ca₃Co₄O₉ (C-349) ceramic by forming a composite system together with another promising oxyselenide; BiCuSeO (BCSO). Composite engineering is a promising method to develop the existing properties of thermoelectric materials. The aim of this work is therefore to explore the unique thermoelectric properties of promising C-349 and BCSO oxides, then to investigate the new properties of their composites.
Pristine C-349 and BCSO were synthesized using sol-gel and solid-state reaction methods, respectively and mixed by several ball-milling steps followed by cold compaction. Bulk composites as pellets have been prepared and thermoelectric properties have been investigated. The electrical properties of samples were investigated by Hall effect measurements at room temperature. Lakeshore 7700A system was used with van der Pauw geometry between 0.15 and 1.5 T. Non-destructive measurements of the Seebeck coefficient and the electrical conductivity were carried out under vacuum in a temperature interval between 300 and 900 K using a lab-made system.
As the BCSO content increased, the electrical conductivity and Seebeck coefficient were simultaneously enhanced. The highest power factor was obtained as 0.17 mW/mK² in the composite system that contains 5 wt % BCSO. This value represents nearly 41% improvement in the power factor of pristine C-349 at 900 K. The addition of BCSO phase causes a slight decrease in carrier concentration and at the same time creates more porous structure. Both consequences have a positive impact on Seebeck coefficient. The reason for enhancement in electrical conductivity is related with a high increase in electron mobility without significant decrease in carrier concentration. These results prove that composite engineering technology will provide a promising advantage in developing thermoelectric materials with higher efficiencies. However, most works carried out about composite engineering intend to reduce the thermal conductivity. Therefore, this work shows the potential of power factor enhancement of composite materials in order to produce highly efficient thermoelectric materials

As the concern about depletion of non-renewable resources is growing, waste heat harvesting has become one of the effective approaches for solving the energy shortage issue. Thermoelectric materials are promising candidates for solid-state energy generators for converting the huge amount of industry and automobile generated waste heat into electricity. The material’s performance is characterized by the figure of merit zT = S²σT/κ, where S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ the thermal conductivity of the material.
Oxide thermoelectric materials have the advantages of low cost, environment-friendly manufacturing and chemical stability at high temperatures. However, oxides have been historically regarded as out of the question for good thermoelectric materials, because they are mostly poor conductors due to their high ionic nature and good heat conductors due to the light oxygen. Thus, alternatives such as presence of dopants, densification mechanisms, introduction of a second phase, and nanostructuring are considered to enhance the thermoelectric properties in oxides. The possibility of controlling the pores for optimization of materials properties motivates studies relating to the presence of dopants and densification mechanisms. Bi-layered pellets contain a layer of tin oxide doped with antimony, Sb-SnO₂, and a sintering additive, manganese oxide, MnO₂ allowed the production of materials with lower thermal conductivity and possible enhanced electrical conductivity. Thereby, these materials have the potential to improve the energy conversion and contribute to the expansion of new thermoelectric oxides materials.

Silicon germanium (SiGe) is a high-temperature thermoelectric material. Due to alloy scattering of short-wavelength phonons in SiGe, long-wavelength phonons make an important contribution to the lattice thermal conductivity of SiGe. The effects of nanostructures on suppressing the lattice thermal conductivity have been extensively studied for different materials including SiGe. However, the phonon mean free paths and lattice thermal conductivities of SiGe bulk crystals and nanowires have remained one of the outstanding questions in the study of phonon transport and nanostructured thermoelectric materials due to inconsistent results obtained in previous experiments. Here we report multi-probe measurements of the intrinsic thermal conductivities of different segments of the same SiGe nanowires. Our measurement results reveal a weak length dependence of the thermal conductivity of the SiGe nanowires, and are in agreement with a Monte Carlo simulation of phonon transport in SiGe nanowires with diffuse surface.

Thermoelectric properties of the materials originate from thermal conduction by free charge carriers instead of by collective lattice vibrations (phonons). Understanding the scattering mechanism, or dissipation, of phonons allows the engineering of materials with higher coefficient of Seebeck [1], i.e. higher difference of electric potential created for a given thermal gradient. In skutterudite type of thermoelectric materials, dissipation of phonons are attributed to localized, low frequency and anharmonic vibrations of heavy ions of the lanthanide family introduced into the large voids of the crystalline structure. Since most of these materials can be synthesized in the form of high quality single crystals, we have the possibility of revisiting the phonon scattering mechanism through x-ray phase measurements via dynamic diffraction effects [2]. Structure factor phases are susceptible to the differences between the vibration amplitudes of the atoms (root mean square atomic displacements), or in other words, phase values are invariants with temperature only when all occupied sites of the unit cell have the same Debye-Waller factor.

Structure factor calculation in model structures revealed suitable Bragg reflections and x-ray energies to resolve the difference in atomic vibrations as a functions of temperature. It also revealed a giant resonante phase shift for the whole familiy of filled skutterudites RFe4P12 (R=Ce, La, Nd, Pr, Sm). By exploiting this resonant phase shift with synchrotron x-rays at different temperatures in CeFe4P12, we were able to demonstrated that rattling of Ce alone inside the icosahedral cage is not enough to explain phonon scattering and that the whole cage of 12 P is taking part of scattering mechanism.


[1] G. J. Snyder, E. S. Toberer. Complex thermoelectric materials. Nat. Mater. 7, 105-114 (2008).
doi = 10.1038/nmat2090

[2] S. L. Morelhão, C. M. R. Remédios, G. A. Calligaris, G. Nisbet. X-ray dynamical diffraction in amino acid crystals: a step towards improving structural resolution of biological molecules via physical phase measurements. J. Appl. Cryst. 50, (2017). doi: 10.1107/S1600576717004757

Understanding electron-phonon coupling and phonon-phonon interactions is critical for the design of new thermoelectric materials, in which the energy conversion efficiency is determined by the concurrent optimization of electrical and thermal conductivities. SnSe is a high-efficiency thermoelectric material, whose ultralow thermal conductivity originates predominantly from phonon anharmonicity [1,2], while SnS is of interest for both thermoelectric and photovoltaic applications. Previous inelastic neutron scattering (INS) measurements and first-principles simulations of SnSe identified the dramatic softening of TO (transverse optical) mode in the ambient Pnma phase, anisotropic dispersion of acoustic modes and velocities [2,3,4]. SnSe and SnS have similar crystal structures and electronic structures below and above a second-order structural phase transition to a Cmcm phase at T>800K. Our INS measurements of SnS and SnSe extended previous studies to higher temperatures and directly revealed the condensation of a zone boundary TA (transverse acoustic) mode in Cmcm phase, which yields the Brillouin zone folding and re-emerges as the zone-center TO mode in Pnma. By including anharmonic renormalization effects into the temperature-dependent force-constants, our first-principles calculations successfully reproduce the experimental phonon dispersions and the soft-mode condensation across the Cmcm-Pnma transition. The significant suppression of thermal conductivity near the phase transition temperature could be captured by including renormalized anisotropic group velocities into thermal conductivity calculations. Further, our ab initio molecular dynamics simulations could be used to understand electron-phonon coupling behaviors accounting for the response of SnSe and SnS under photo-excitation.

1. Zhao, L.-D., et al., Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature, 2014. 508(7496): p. 373-377.
2. C. Li, J. Hong, A. May, D. Bansal, J. Ma, T. Hong, S. Chi, G. Ehlers, and O. Delaire, “Orbitally-driven giant phonon anharmonicity in SnSe”, Nature Physics 11, 1063–1069 (2015).
3. D. Bansal, J. Hong, C. Li, A. May, W. Porter, M. Hu, D. Abernathy, and O. Delaire, "Phonon anharmonicity and negative thermal expansion in SnSe", Phys. Rev. B 94, 054307 (2016).
4. J. Hong and O. Delaire “Electronic Instability and Anharmonicity in SnSe”, Materials Today Physics 10, 100093 (2019).

The accurate characterization of thermal conductivity κ, particularly at high temperature, is of paramount importance to many materials, thermoelectrics in particular. The ease and access of thermal diffusivity D measurements allows for the calculation of κ when the volumetric heat capacity, ρc_p, of the material is known. However, in the relation κ = ρc_pD, there is some confusion as to what value of c_p should be used in materials undergoing phase transformations. Herein, it is demonstrated that the Dulong-Petit estimate of c_p at high temperature is not appropriate for materials having phase transformations with kinetic timescales relevant to thermal transport. In these materials, there is an additional capacity to store heat in the material through the enthalpy of transformation ΔH. It is shown in a model Zn₄Sb₃ system that the decrease in D through the phase transition at 250 K is fully accounted for by the increase in c_p. Importantly, κ changes smoothly through the phase transition. Consequently, reports of κ diverging through phase transitions have likely overlooked the effects of excess heat capacity on thermal properties measurements and overestimated the thermoelectric efficiency, zT.

Nano structuring thermoelectric materials has long been a technique to scatter phonons and lower a material’s lattice thermal conductivity¹. The lattice thermal conductivity of a material has historically been estimated by subtracting the electrical portion of thermal conductivity (calculated using the Weidman-Franz law) from the total thermal conductivity of a material (k_L=k-LσT). This method treats heat conducted by phonons and heat conducted by electrons as two separate and independent transport channels. While nano-structuring is known to scatter phonons, in some materials shrinking the grain size can have a detrimental effect on the electronic properties through additional grain boundary scattering². Herein we discuss how this grain-boundary scattering can lead to underestimating the electronic thermal conductivity, which necessitates a correction term to our estimation of lattice thermal conductivity. We demonstrate this correction using experimental data on Mg₃Sb₂, and show examples in the literature of CoSb₃, Bi₂Te₃, and other materials that might have benefitted from using this type of correction.

(1) Minnich A. et al., Energy Environ. Sci., 2009, 2, 466
(2) Kuo J. et al., Energy Environ. Sci., 2018, 11, 429

Dopants play an important role in engineering the electronic properties of semiconductor materials. At the same time they can strongly influence the phonon scattering processes and thereby the thermal conductivity. We have recently shown how Boron point defects in 3C-SiC act as “superscatterers” and exhibit resonant phonon scattering which is one to two orders of magnitude higher than other defects.[1] The increased scattering leads to a thermal conductivity that is suppressed by one to two orders of magnitude.

In order to understand the physics behind and the factors causing resonance scattering, we explain the results with the help of a simple 1D mono-atomic linear chain.[2] We show that small lattice distortions emanating from two or more close energy minima in potential energy surface lead to very large perturbations of the interatomic force constants. Such a behavior is characterized by a peak in the trace of imaginary part of the T matrix (which is closely related to the scattering rates) and a reflection coefficient approaching unity.

The strong influence of the potential energy surface surrounding the defect atom on the thermal conductivity opens a new path to tailor thermal conductivities where required values range from very low in thermoelectric materials to very high in power electronics applications. Using doping of GaAs as an example[3], we show how the provided insights can be used to identify potential superscatterers.

[1] Katre et al. Phys. Rev. Lett. 119 075902 (2017).
[2] Dongre et al. J. Mater. Chem. C 6, 4691 (2018).
[3] Kundu et al {\em arXiv:1905.11056}

Thermoreflectance microscopy is a noninvasive optical technique that is capable of mapping 2D temperature fields of surfaces with high spatial and time resolution. State of the art thermoreflectance microscopy set-ups use electrical current to thermally excite the device or sample under study and so generate temperature differences. This technique is particularly useful for investigating hot spots in integrated circuitry leading to failure or premature fatigue of microelectronic components [1]. It is also a powerful tool for investigating thermal properties of materials [1, 2] and thermal characterization of micro thermoelectric devices [3]. All-optical pump probe thermoreflectance microscopy, which thermally excites the sample by a pump laser spot focused on the sample’s surface, has recently been proposed as a promising tool for investigating anisotropic thermal properties of thin film hetero structures [4]. The later research efforts motivates the technique’s further development, since thermoelectric materials often present anisotropic thermal transport properties whose experimental characterization is challenging, especially in thin films. In this work an all-optical pump probe thermoreflectance microscopy is used to study a series of thin film multilayers of TiN/AlScN deposited on MgO substrates [5] which present tunable anisotropy. The temperature distribution on the surface of the sample was then analyzed by the probe laser and correlated to finite element modelling (FEM) simulations. FEM allowed us to systematically analyze, using realistic parameters, the impact of various experimental factors that affect the result of measurements. We gained understanding about the required experimental conditions in order to extract cross plane and in plane thermal conductivity of the thin films. Using FEM, a measure of the sensitivity required by the experimental device was obtained.


References:

[1] Maize, K. High Resolution Thermoreflectance Imaging of Power Transistors and Nanoscale Percolation Networks. PhD thesis, UC Santa Cruz. 2014.

[2] Ziabari, A., Torres, P., Vermeersch, B., Xuan, Yi, Cartoixà, X., Torelló, A., Bahk1, J.-H., Koh1, Y. R., Parsa, M., Ye1, P. D., Alvarez, F. X. & Shakouri, A. Full-field thermal imaging of quasiballistic crosstalk reduction in nanoscale devices. Nature communications. (2018) 9:255.

[3] Li, G., Garcia Fernandez, J., Lara Ramos, D. A., Barati, V., Pérez, N., Soldatov, I., Reith, H., Schierning, G. & Nielsch, K. Integrated microthermoelectric coolers with rapid response time and high device reliability. Nature Electonics. 2018, vol 1, 555–561.

[4] Yazawa, K., León Gil, J. A., Maze, K., Kendig, D., & Shakouri, A. Optical Pump-Probe Thermoreflectance Imaging for Anisotropic Heat Diffusion. 2018 17th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm). 2018, 2577-0799.

[5] Saha, B., Koh, Y. R., Feser, J. P., Sadasivam, S., Fisher, T. S., Shakouri, A., Sands, T. D. Phonon wave effects in the thermal transport of epitaxial TiN/(Al,Sc)N metal/semiconductor superlattices. Journal of Applied Physics. 2017, 015109.

Anomalous and nearly dispersionless phonons have recently been observed in many materials with a void (an inner space with large freedom of motion) inside a host cage containing a filler as the guest atom inside (usually called rattler), such as intermetallides, brownmillerite, skutterudite, pyrochlore and clathrate. The cage atoms are connected by covalent bonds, providing a robustly solid wall and a sufficient space for accommodating a guest atom. Since the guest atom in a cage is only weakly interacted with the surrounding atoms forming a cage, it can vibrate freely under a weakly-bound condition and shows an anomalous motion with low-energy excitation (ALE) and a large atomic displacement parameter (ADP). The rattling phonons of the ALE modes are scientifically important for: (1) large coupling with conduction electrons, giving rise to a significant modification of electron effective mass; (2) interactions with propagating phonons, leading to an enhancement of scattering probability of phonons and consequent low thermal conductivity which is useful for thermoelectric applications. ALE rattling phonons observed in cage materials have been both experimentally and theoretically investigated. The rattling phonon branches were shown to be optical-branch-like at around 5 meV, which was lower than the energies of the top of acoustic phonon branches, giving rise to ALE peaks in the phonon density of states (PDOS). The ALE vibration modes have also been detected and intensively discussed by Raman spectroscopy, optical conductivity, heat capacity (HC), and temperature dependent ADP obtained from crystallographic refinement of x-ray/neutron diffraction data. However, the origin of the low energy modes was still not perfectly clear. In the present paper, we address that van der Waals-type repulsive interactions between a guest atom and the atoms residing a host cage can be the origin of the ALE modes. The ALEs of three typical cage compounds of clathrates, skutterudites and pyrochlores have intensively been investigated. An exponential-form relationship between the force constant Fc and the free space derived based on the Morse-type van der Waals potential, gives very reasonable interpretations on ALEs. The Fc’s evaluated from the characteristic energies of the guest atoms are shown to vary exponentially as a function of the free space of guest atoms, being in good agreement with the unified relationship. By comparing the fitting parameters among clathrates, skutterudites, and pyrochlores, the features that specify each family of cage material are discussed. The anisotropic 3D van der Waals-type interactions significantly modify the physical properties of cage materials via phonon-phonon and electron-phonon interactions and play a very important role in thermoelectrics. The strategy of how to improve the figure of merits of these thermoelectric materials will be presented.

Structural defects have become a primary driver for materials design in thermoelectrics and (opto-)electrics, but the understanding of correlated disorder and their effects on material properties is still incomplete. In this work, we theoretically study the effects of correlated disorder on lattice conduction, taking graphene-hexagonal boron nitride composites as our model system. While the kinetic theory suggests that the effective conductivity is independent of structural correlation, our non-equilibrium Green’s function analysis and Green-Kubo formalism implemented within molecular dynamics simulations both show a strong dependence on the correlation functional, which we propose as a new designing degree of freedom viewing all defects collectively. Among all functionals considered, we show that a hyperuniform distribution of defects leads to the highest conductivity, while the Cauchy and Dagum correlation functions give conductivities as low as amorphous limit. Our spectral analysis shows the preservation of long-wavelength modes in the former, while the latter approach the amorphous limit where phonon concepts are no longer invalid. Aiming at thermoelectric applications, we apply Bayesian learning techniques to minimize the thermal conductivity at a given defect density, which could be extended to thermoelectric optimization with constraints in electrical properties.

The effect of compression on the thermal conductivity of CuGaS₂, CuInS₂, CuInTe₂, and AgInTe₂ chalcopyrites (space group I-42d) was studied at 300 K using phonon calculations. The lattice thermal conductivity (κ_ph) was evaluated by solving the Boltzmann transport equation with harmonic and third-order force constants.
Striking differences are obtained between the κ_ph behavior of CuGaS₂, CuInS₂, CuInTe₂, and AgInTe₂ under compression. The κ_ph value of CuGaS₂ always increases with pressure up to 9.5 GPa. A drastically different functional dependence is obtained as soon as heavier In is considered instead of Ga (B^III in A^IB^IIIC^VI₂). Under pressure up to 6 GPa, κ_ph of CuInTe₂ decreases from 7.6 to 4.1 W m^-1 K^-1, which is anomalous. This is consistent with the experimental data [1,2], implying that important physics is captured within the methodology employed herein and structural modulations are not indispensable to drive the anomaly. By exchanging Te with lighter S (C^VI in A^IB^IIIC^VI₂) and hence forming CuInS₂, κ_ph increases up to 2 GPa, which is again a common behavior and equivalent to that of CuGaS₂. Upon a further pressure increase, κ_ph begins to decrease and reaches a slightly lower value at 8 GPa than that at 0 GPa. To account for the effect of the transition metal constituent (A^I in A^IB^IIIC^VI₂), Cu in CuInTe₂ is exchanged with heavier Ag. AgInTe₂ exhibits a significantly lower κ_ph value and a steeper decrease in κ_ph under pressure, reaching 0.2 W m^-1 K^-1 at 2.6 GPa.
Using the Slack model, Gui et al. [3] have showed that the Thermoelectric Figure of Merit for CuInC^VI₂ (C^VI = S, Se, and Te) uniformly increases at elevated temperatures up to 850 K. However, the effect of pressure on κ has not thoroughly been studied. Using the quasi-harmonic Debye model, Sharma et al. have evaluated electronic, thermal, and mechanical properties of AgInC^VI₂ (C^VI = S, Se, and Te) under pressure and reported a noticeable reduction in the Grüneisen parameter and volumetric thermal expansion coefficient as well as bulk modulus [4]. Since the Grüneisen parameter and volumetric thermal expansion coefficient can be related to κ , it appears that pressure effects on κ are considerable. This is consistent with an experimental study reporting a decrease in κ by 30% for CuInTe₂ under pressure up to 2.3 GPa [1]. Generally, κ should increase under compression [5]. This implies that the behavior of CuInTe₂ is anomalous. Two possible mechanism have been proposed based on experiments: (i) anharmonic behavior of lattice vibrations [1] and (ii) structural modifications under high pressure (e.g. stacking faults) [2].The underlying physics of the κ reduction under compression of CuInTe₂ and possibly other A^IB^IIIC^VI₂ compounds is not fully understood.
This can be understood based on the phonon dispersion curves. Softening of the acoustic phonon modes occurs for these anomalous chalcopyrites. This leads to the negative Grüneisen parameter and negative volumetric thermal expansion coefficient. The decrease in phonon frequency upon compression is suggested to be due to the phonon oscillations in the form of a rotational motion rather than compressive waves. The physical origin of the anomalous thermal conductivity is thus identified in this work and AgInTe₂ with a very low thermal conductivity of 0.2 W m^-1 K^-1 at 2.6 GPa is proposed to be a promising thermoelectric compound.
Reference:
[1] Materials Today Physics 5, 1 (2018).
[2] Inorganic Chemistry 53, 6844 (2014).
[3] Applied Surface Science 458, 564 (2018).
[4] Physica B: Condensed Matter 438, 97 (2014).
[5] Proceedings of the National Academy of Sciences 104, 9192 (2007).

Electron-doped strontium titanate (SrTiO₃) is one of the candidate materials of thermoelectrics. In order to increase the carrier electron in SrTiO₃, usually, a part of Ti ions are substituted by Nb ions or a part of Sr ions are substituted by La ions. Although many researches on the electrical conductivity and thermopower of Nb or La-doped SrTiO₃ and Nb or La-doped SrTiO₃-based superlattices have been done thus far^[1,2], there are only a few researches on the thermal conductivity because of the low solubility limit of dopant ions in SrTiO₃ bulk. Recently, we successfully fabricated SrTi_1–xNb_xO₃ (0 ≤ x ≤ 1, STNO)^[1] and Sr_1–xLa_xTiO₃ (0 ≤ x ≤ 1, SLTO) full-range solid-solutions by PLD and found the anomalous behaviour of the electron transport properties. Since thermal conductivity is strongly correlated with the electron transport properties, we studied the thermal conductivity of the solid-solutions. Here we report high and low thermal conductivity phase boundary in SrTiO₃– SrNbO₃ solid-solution system. The x-dependent thermal conductivities (κ) of STNO and SLTO were measured by time-domain thermoreflectance (TDTR) method at room temperature. Steep increase in κ was observed around x ~0.5 for STNO, suggesting the existence of a phase transition. However, in SLTO case, all the samples show a κ at lower levels compared with SrTiO₃ single crystals, where no special behavior was observed. We attributed this special behavior to the polaron effect which only exists in STNO at x ≤ ~0.5 and SLTO and suppresses the κ. Furthermore, unlike the traditional alloy systems, SrNbO₃ with larger average atomic mass shows much higher κ than SrTiO₃. According to the phonon dispersion calculations, the phonon transport of SrTiO₃ shows much more intense scattering between optical and acoustic branches than SrNbO₃, which is likely to suppress phonon mean free path for SrTiO₃. The Grüneisen parameters of SrTiO₃ were much higher than those of SrNbO₃, which also will reduce the phonon transport properties. Our findings revealed important role of polaron in heat transport for system with strong electron-phonon couplings, which will be a fundamental contribution to the current knowledge in heat transportation and may be utilized to develop new thermoelectric materials.

References
Y. Zhang, H. Ohta et al. J. Appl. Phys. 121, 185102 (2017)
Y. Zhang, H. Ohtaet al. Nature Commun. 9, 2224 (2018).

Lead-tellurides are important mid-temperature thermoelectric (TE) materials. Nevertheless, the moderate zT of n-type PbTe limits the development of mid-temperature TE generator. Gallium (Ga) has been a successful n-type dopant for PbTe through contributing extrinsic carriers and optimizing the carrier concentration. In this study, the isothermal section of ternary Ga-Pb-Te is determined by various post-annealed alloys. In particular, the single-phase PbTe region could only tolerate a very small amount of Ga, and that single-phase region shows an asymmetrical homogeneity in the ternary Ga-Pb-Te. The combination between TE properties and phase diagram provided a better understanding in the dopant capability, which makes it serves as a crucial guideline for searching the high-efficient alloys where the compositional regions are seldom being explored before. With an as-determined isothermal section in hand as a map, the selective Ga-doped PbTe alloys are grown through the Bridgman method. In the temperature range of 550 K-673 K, an average zTave value ~1.21 (peak zT~1.5) could be achieved in the Ga-PbTe alloy with the existence of nano-scale strain field.

Thermoelectric (TE) materials convert energy from waste heat into electricity and have been used, e.g., to charge batteries and for refrigeration. Thermoelectric properties are characterized by a figure of merit (ZT); ZT=S²σT/k. S is the Seebeck coefficient, k the thermal conductivity, σ the electrical conductivity, and T temperature. This investigation reports respective properties of lead ruthenate Pb₂Ru₂O_6.5 and some derivatives. Lead ruthenate has a defect pyrochlore crystal structure. Molar fractions of Ru have been substituted by Pb, using the formula of Pb_2+yRu_2-yO_6.5 with y increasing from 0 to 0.9. All samples were prepared by solid-state synthesis and characterized by XRF, X-ray diffraction, SEM/EDX, for composition, structure and phase content. Thermal conductivity, electrical conductivity, the Seebeck coefficient and the ZT values will be reported as a function of temperature (room temperature to 300°C) and stoichiometry.

The concept of thermoelectric (TE) power generation based on the principle of Seebeck effect has been demonstrated to be the most promising and efficient green energy approach to harvest electricity from waste heat sources. Achievement of a high figure of merit, defined as ZT = S²/ρκ, where, S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and S²/ρ is the power factor (PF), requires potential TE materials with large S for generation of large voltage from heat energy, low ρ for minimizing Joule heating, and a low κ for retaining the heat at the junctions of TE device. Various research groups have adopted oxide materials as active TE materials for their non-toxicity, natural abundance, chemical and thermal stabilities, and have demonstarted utilization in high temperature TE applications. Calcium cobalt oxide (Ca₃Co₄O₉) show large values for S and low values for κ, due to it’s layered misfit structure making it a suitable material for fabrication of durable TE devices.
Here, the calcium carbonate (CaCO₃) and cobalt oxide (Co₃O₄) in stoichiometric ratio of 3:4:9 for Ca, Co, and O are mixed using high energy ball milling (HEBM), processed for optimized time duration of 48 h followed by drying on hot plate, and calcined at 1123 K for 6 h to achieve Ca₃Co₄O₉ (CCO) powders. The resulting as-synthesized CCO powder was further ball-milled for 2, 6, 12, and 24 h and the FESEM images indicates of nanostructure formation after HEBM for 24 h. The hot-pressed pellets (HPPs) of bulk and nano-powders of CCO are prepared by placing the respective powders in a graphite die, heating the die to 1173 K inside a vacuum chamber, and applying a continuous pressure of 200 MPa for 15-20 mins. These processing parameters (pressure and temperature) are optimized to obtain pure, fully dense (99.2 ± 0.5%), and thermally stable pellets. The microscopic, crystallographic, thermal, and spectroscopic analysis suggest samples processed with longer HEBM time require less time for calcination during synthesis of highly pure crystalline phases of CCO.
The X-ray diffraction (XRD) patterns of all the CCO powders shows the signature (002) plane at ~16.5° along with other peaks and is in well-correlation with JCPDS #00-023-0110 assigned for CCO, consisting of alternate stacking sub-systems of rock-salt-type [Ca₂CoO₃] layers sandwiched between two hexagonal [CoO₂] layers along c-axis. However, XRD signals from both the pellets, B-HPP and N-HPP show peaks for (00l) crystallographic planes of CCO. The disappearance of other diffraction (hkl) planes corresponding to CCO crystal structure could be attributed to recrystallization and formation of textured growth due to simultaneous application of high temperature and pressure. The degree of orientation of crystallographic planes in the HPPs is evaluated by calculating the Lotgering factor (L_F) for the perfectly oriented HPP samples (B-HPP and N-HPP) and their respective randomly oriented CCO powders. The estimated value of L_F=1 supports the appearance of ideally aligned (00l) crystallographic planes of CCO and indicates achievement of texturing in B-HPP and N-HPP and further confirmed through FESEM.
The significant improvement of ~11 times in PF at 770 K for bulk nanostructured CCO (64.2 10^-3 Wm^-1K^-2) ceramics compared to the HPPs of as-synthesized CCO microstructures (5.8 10^-3 Wm^-1K^-2) is attributed to the enhancement in S triggered due to oxygen vacancies and structural deformations developed during HP process along with introduction of enlarged interface area at grain boundaries. This concept of structural modulation could be adopted for understanding the TE properties in other nanostructured materials as well. The currently prepared bulk nanostructured CCO could be further utilised commercially for mass scale production.

Phonon transports in thermoelectric materials are affected by various different factors. One of these factors is the electron-phonon coupling phenomenon, which serves as a scattering source not only to charge carriers but also to heat-conducting phonons, especially in highly-doped thermoelectric materials.
We propose a relatively simple numerical method for estimating lattice thermal conductivity, including the effect of the electron-phonon coupling phenomenon, and present statistical approaches for quantifying the uncertainty in the predicted values. The method employs a formulation similar to that of the EPA (electron-phonon averaged) method. The electron-phonon coupling matrix elements, which are explicitly dependent on the momentum of charge carriers and that of scattering phonons, are approximated as a function of two energies only. At the same time, the energy levels of the involved electron bands are averaged over the Brillouin zone. These simplifications enable us to estimate the effect of the electron-phonon coupling phenomenon in phonon transports. We also demonstrate that the quantification of undertainty in the predicted values can also be carried out by using statistical techniques like Gaussian process regression (GPR) and empirical bootstrapping, within our proposed method.

Thermoelectric oxide semiconductor thin films contribute to development of conversion devices due to their thermal and chemical stability in a wide temperature range, and property modification derived by to quantum effect^[1]. Nickel oxide (NiO) thin films with rocksalt-type crystal structure has also been researched as an p-type semiconductor for thermoelectric sensors, though layered cobalt oxides have acquired interests for the thermoelectric conversion^[2,3]. The p-type conductivity of NiO thin films has been modified by doping heterovalent cations such as Li⁺ (~10 atm%), and by epitaxial synthesis which improves the carrier mobility owing to structural consistency^[4]. Reduction of the epitaxy temperature contributes to development of multilayered thermoelectric devices by surface and interfacial flatness, and homogeneous crystallites boundary as phonon scattering sites. Furthermore, low-temperature epitaxial growth allows extraordinarily high doping ratio because of the suppressed re-evaporation and phase segregation, which results in generation of charge carriers associated with local strain. In this study, epitaxial NiO thin films heavily doped with monovalent or trivalent ions, e.g. M=Li and Fe, were grown as Ni_1-xM_xO (0≤x≤0.7) solid-solution oxide semiconductor at room-temperature, and the effect of dopant ratio on local structure and thermoelectric properties were investigated.
The thin films were grown on atomically stepped α-Al₂O₃ (0001) substrates by laser molecular beam epitaxy (Laser-MBE) technique equipped with KrF excimer laser (λ=248 nm, d=20 ns, E~1.5 J/cm²) and NiO–Li₂O or NiO–Fe₂O₃ targets. The growth took place in ultra-high vacuum to 10^–3 Pa of O₂ at room-temperature. As a result, the epitaxial growth of both Ni_1-xLi_xO (111) and Ni_1-xFe_xO (111) thin films with ultra-flat surface well reflecting morphology of the substrates was obtained. The remained rocksalt-type crystal structure of NiO and expansion of d₍₁₁₁₎-spacing along the increased doping ratio up to x=0.5 suggested substitution of Ni²⁺ with the dopants. The epitaxial thin films demonstrated semiconducting behavior at low-temperature (15—293 K), that electric resistivity decreased along the raised doping ratio (0≤x≤0.6) for either dopants of Li and Fe. The electric conductivity of Ni_1-xLi_xO and Ni_1-xFe_xO epitaxial thin films at room-temperature were ~3.3×10² Sm^–1and ~5.3×10¹ Sm^–1, which properties improved to ~5.0×10³ Sm^–1and ~2.9×10² Sm^–1 at 493 K, respectively. The Ni_1-xLi_xO film demonstrated thermoelectric power factor of ~16 μWm^–1K^–2 at relatively low-temperature of 593 K, and the Ni_1-xFe_xO film revealed negative thermopower which indicated n-type conduction above 493 K. Further detailed structural analyses, and effect of dopant species and ratio on the thermoelectric properties would also be presented.
[1] M.S. Dresselhaus et al., Adv. Mater., 19 (2007) 1043–1053.
[2] M. Matsumiya et al., Sens. Actuators B, 93 (2003) 309–315.
[3] W. Shin et al., Jpn. J. Appl. Phys., 38 (1999) L1336–L1338.
[4] A. Matsuda et al., Appl. Phys Lett., 90 (2007) 182107.

Argyrodites with a general chemical formula of A₈BX₆ (A = Cu, Ag; B = Si, Ge, Sn; and X = S, Se, and Te) are known for the intimate interplay among mobile ions, electrons, and phonons, which yields rich material physics and materials chemistry phenomena. In particular, the coexistence of fast ionic conduction and promising thermoelectric performance in Ag₈GeTe₆, Ag₈SnSe₆, Ag₈SiTe₆, Ag₈SiSe₆, Cu₈GeSe₆ at high temperatures ushered us to their chemical neighbor Ag₈GeSe₆, whose high temperature crystal structure and thermoelectric properties are not yet reported. In this work, we have employed a growth-from-the-melt technique followed by hot pressing to prepare polycrystalline Ag₈GeSe₆ samples, on which the crystal structure, micro-morphology, compositional analysis, UV-vis absorption, specific heat, speed of sound, and thermoelectric properties were characterized as a function of the Se-deficiency ratio and temperature. We found (i) the crystal structure of Ag₈GeSe₆ evolved from orthorhombic at room temperature to face center cubic above 410 K, with a region of phase separations in between; (ii) like other Argyrodite 816 phases, Ag₈GeSe₆ exhibited ultralow thermal conductivities over a wide temperature range as the phonon mean free path was down to the order of inter-atomic spacing; and (iii) Varying Se deficiency effectively optimized the carrier concentration and power factor, a figure of merit zT value ~ 0.55 was achieved at 923 K in Ag₈GeSe_5.88. These results not only fill a knowledge gap of Ag₈GeSe₆ but also contribute to a comprehensive understanding of 816 phase Argyrodites at large.

Reducing the lattice thermal conductivity without simultaneously decreasing the charge transport is a challenge to design high-performance thermoelectric materials. The rattling motion of fillers in cage structures has been verified to effectively reduce the lattice thermal conductivity recently. Here, we propose a rattling motion in layered structures to reduce the lattice thermal conductivity without suppressing the charge transport in a leading thermoelectric material----BiCuSeO (BCSO). Using a combination of inelastic x-ray scattering (IXS), inelastic neutron scattering (INS) measurements and first-principles calculations, we identify a Cu-dominated rattling mode, reflected as a low lying flat optic band in BCSO. The present results provide new evidence to clarify the origin of the intrinsic ultralow thermal conductivity of BCSO and pave a path for designing high-performance thermoelectric materials.

Recently, thermoelectric lead-free selenides have attracted great attention due to their earth-abundant, low-cost and environment-friendly characteristics. Here we report a new strategy to simultaneously enhance the electronic transport properties and reduce the thermal conductivity of polycrystalline SnSe₂. By combining weak van der Waals bonding with the mobile behavior of Ag⁺ ions, the carrier concentration is optimized over a wide temperature range, which can be attributed to the dynamic Ag⁺- intercalation into the van der Waals gap from the Ag⁺ ion reservoir AgSnSe₂. On account of additional electrical bridges between interlayers contributed by the intercalated Ag⁺ ions and weak anisotropy, an exciting high power factor of up to 7.46 mW cm^-1K^-2 at 789 K is achieved along the pressing direction. In addition, the thermal conductivity is simultaneously reduced to 0.57 W m^-1K^-1 at 789 K, owing to numerous line defects, phase interfaces, twin boundaries, dislocation and intercalated atomic layers generated after Ag introduction, as well as the anharmonic vibration of Ag+ ions. As a result, a record peak ZT of 1.03 at 789 K is realized along the pressing direction, which is 1.6 times larger than the highest reported value (0.63) of polycrystalline SnSe₂ and even comparable to that of p-type polycrystalline SnSe. This study opens a new way to achieve ultra-high thermoelectric performance, especially in layered materials.

Thermoelectric generators (TEG) are solid state heat engines which convert thermal energy to electrical energy. The efficiency of thermoelectric materials is related to Carnot efficiency and a material’s ability to convert heat into electricity. Thermoelectric efficiency is defined by the dimensionless thermoelectric figure-of-merit, zT, where zT = (S²/ρκ)T where low electrical resistivity (ρ), high Seebeck coefficient (S) and low total thermal conductivity (κ) are essential for a good thermoelectric materials. Rare earth (RE) tellurides have been studied extensively for use as high-temperature thermoelectric materials, with lanthanum and praseodymium tellurides (La_3-xTe₄ and Pr_3-xTe₄) having zT’s = 1.1 and 1.7 at 1273 K, respectively, with optimized carrier concentrations. The high-performance of these compounds warrants further investigation of additional RE tellurides.

While not part of the lanthanide series, scandium is classified as a RE element due to its chemical similarity to other elements of the lanthanide series. However, previous studies of scandium sesquitelluride (Sc₂Te₃) were limited to structural analysis, with the electronic and thermal properties remaining a mystery. In this study, we used a mechanochemical approach to synthesize Sc₂Te₃ and formed compacted pellets using spark plasma sintering (SPS). The thermoelectric properties were then measured from room-temperature to 1100 K. It was found to have a high power factor (S²/ρ) at low temperature and a low κ, leading to a zT = 0.3 from 500 – 750 K. Additionally, temperature-dependent resonant ultrasound spectroscopy was utilized to measure the temperature dependent elastic constants and thermal expansion of Sc₂Te₃.

Layered structure Bi₂Te₃ and MoS₂ thin films were successfully deposited on different substrates using radio-frequency magnetron sputtering technique. Structural, morphological and thermoelectric transport properties of Bi₂Te₃ and MoS₂ thin films have been investigated systematically to fabricate high-efficient thermal energy harvesting TE device. Magnitude of Seebeck coefficient of Bi₂Te₃ decreases with increase in film thickness. Bi₂Te₃ grown at 350 °C for 10 minutes which is approximately 120 nm display a maximum value of -126 µVK^-1 at 435 K. The performance shows strong temperature dependence when the films were deposited at 300 °C, 350 and 400 °C. Power factor increase from 0.91X10^-3W/mK² at 300 K to about 1.4x10^-3W/mK² at 350 K. MoS₂ films shows positive Seebeck coefficient values. The open-circuit voltage of the pn-junction TEG device increases with increase in ΔT to about 0.13 V at ΔT= 120 °C. We demonstrated high efficient pn-junction thermoelectric generator device for waste heat recovery applications.

As the featured material of the superionic thermoelectric (TE) material family, copper-chalcogenide Cu_2-xSe is attracting growing research interest for its excellent TE performance derived from the satisfactory power factor and the ultra-low thermal conductivity induced by the superionic effect. Various efforts have been made and proved to be effective to further enhance the TE performance for Cu_2-xSe. However, this material is still far from the application stage, which is mainly due to concerns regarding control of the properties and the costly complex fabrication technology. Here we report a scalable pathway to achieve high-performance and tunable Cu_2-xSe, utilizing conventional sintering technology and copper (Cu)-vacancy engineering with an effective mass model. The figure of merit zT is a competitive value of 1.0 at 800 K for the optimized binary Cu_2-xSe, based on the precise modeling prediction and Cu-vacancy engineering. The changes in TE properties of Cu_2-xSe under heating-cooling cycle tests are also revealed. Our work offers the referable method along with the decent parent material for further enhancement of TE performance, paving a possible route for the application and industrialization of Cu_2-xSe TE materials.

The recent interest in portable and wearable electronics lead flexible thin film thermoelectrics to be regarded as promising candidates for the power supply of self-powered systems. Herein, we report a cost-effective solution process to fabricate flexible Sb₂Te₃ thermoelectric thin films using molecular Sb₂Te₃ precursors, synthesized by the reduction of Sb₂Te₃ powder in ethylenediamine and ethanedithiol with superhydride. This synthetic route decreases the size of the Sb₂Te₃ precursor to the molecular level, thereby dramatically improving the uniformity and continuity of the thin film. Furthermore, thermally stable FePt nanoparticles are homogeneously embedded in the Sb₂Te₃ thin film by using the mixed ink solution. The fabricated Sb₂Te₃ thin films on flexible polyimide substrates exhibit a power factor of of ~8.5 μW cm^-1 K² at 423 K. Moreover, such thermoelectric performances of thin films are well preserved during 1000 bending cycles. The current study offers considerable potential for a cost-effective manufacturing for high-performance flexible thin film devices.

Thermoelectric (TE) materials can directly convert heat into electricity. The leading commercialized thermoelectric materials are mainly tellurium (Te) based, despite Te being extremely scarce in the Earth crust as well as highly toxic. Among them, bismuth telluride (Bi₂Te₃) based materials show the best efficiencies for near room temperature applications.
First, we measure electrochemically deposited Bi₂Te₃ films and show that a parallel circuit measurement technique is accurate to measure their temperature dependent Seebeck and electrical conductivity. This is in-line with expected properties for grain sizes and doping levels typical in electrochemical deposition. Next, in order to explore cheaper and less toxic alternative chalcogenide materials, we then perform direct nanoscale patterning of Bi₂Se₃ using a spin-coatable and electron beam sensitive bismuth selenide resist, tris[N,N-diisobutyl-N'-(benzoylselenoureato)]bismuth(III), without the lift-off or etching step. Subsequently, we perform temperature dependent TE characterization. The room temperature electrical resistivity of the film is measured by four probe I-V and is equal to 3400 Ω cm. This large value of resistivity is attributed to the pristine nature of the Bi₂Se₃ as well as to its low thickness (10 nm, as determined by Atomic Force Microscopy). Optimization of the film’s geometry as well as chlorine (Cl) and hafnium (Hf) doping of the Bi₂Se₃ are ongoing in order to obtain films with enhanced TE performance.

Cu₂Se thin films provide a promising route to relatively safe, sustainable and solution processable flexible thermoelectric (TE) modules in contrast to more expensive and toxic materials currently on the market such as Bi₂Te₃. Cu₂Se is known in the thermoelectric community for its high performance at high temperature and has recently attracted attention from its large theoretically predicted figure of merit at room temperature. If the performance can be optimized at room temperature, flexible Cu₂Se thin films will be the material of choice to utilize in TE modules for powering miniature electronics and sensors, which has been an increasingly popular and rapidly expanding market. Unfortunately, one of the main limitations encountered so far in Cu₂Se thin films is that the carrier concentrations are not optimized for TE operation after solution processing. In this work, we conduct a comprehensive study of the structural, optical and TE properties of Cu₂Se thin films and demonstrate that non-optimized carrier concentrations in these films lead to observations of poor performance at room temperature. Through a simple soaking procedure in a Cu⁺ ion solution for only a few minutes, we demonstrate a 200-300% increase in power factor. This soaking process pushes the carrier concentration of the Cu₂Se thin film towards its optimal value for TE operation and marks the highest TE performance for any solution processable Cu₂Se thin film at room temperature thus far.

In our project, we are developing a planar-shape mW-class thermoelectric device with thick film thermoelectric materials for the advanced utilization of thermal energy by the recovery of abundant low-temperature and small-scale waste heat. We chose oxides as the thermoelectric material in favor of the high stability and low toxicity, and in particular Na_0.5CoO₂ as the p-type thermoelectric material. The Na_0.5CoO₂ thick film was formed by screen printing technique on an alumina substrate, but the simple printing and sintering process caused serious cracking and peeling from the substrate. We have developed the process to obtain a crack-free Na_0.5CoO₂ thick film with good adhesion to the substrate and its thermoelectric properties have been investigated.
The low chemical reactivity of Na_0.5CoO₂ and alumina made direct adhesion of the thick film to the substrate difficult. Therefore, we adopted the CuO interlayer to achieve good adhesion. Furthermore, we have found that the slight Co-Cu substitution in Na_0.5CoO₂ further improves the adhesion of the thick film to the interlayer. On the other hand, NaCl, which have been used as a flux for synthesizing of Na_0.5CoO₂ crystal, was mixed in Na_0.5CoO₂ in order to avoid the crack formation during sintering. Since the liquid phase NaCl promoted the sintering of the Na_0.5CoO₂ thick film, the film was densified and the cracks was removed. Through these developments, the thermoelectric power factor of the Na_0.5CoO₂ thick film reached 0.30 mW/K²m equivalent to the sintered body (0.31 mW/K²m). We will discuss the mixing effect of NaCl on morphology and properties of Na_0.5CoO₂ thick film at the presentation.
This work was financially supported by the Future Pioneering Program, "Thermal Management Materials and Technology," commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

IV-VI group Tin (Sn) chalcogenide-based nanomaterials (SnSe, SnS, and SnTe) have intensive interest in various research fields because of the unique characteristics distinct from the bulk counterparts. The introduction of a number of pores into the nanomaterials can manipulate their electrical and thermal transport characteristics of a material, which is considered as a promising strategy to improve the transport properties. Herein, we report on that the successful fabrication and optimization of porous SnSeS-based nanosheets and their thermoelectric enhancement. Two-dimensional SnSe_0.8S_0.2 (SnSeS) nanocrystals are chemically exfoliated from the layered bulk structure through a Li-intercalation and an exfoliation process. A large number of pores are successfully introduced into the SnSeS nanosheets through a solution-phase chemical transformation procedure, and the porosity of the materials can be optimized with different reaction time, resulting in the effective reduction of the thermal conductivity and the enhanced thermoelectric performance. The incorporation of nano and porous structure can boost the research advances regarding nano and porous materials and high-performance thermoelectrics.

Thermoelectric devices fit power generation applications by converting excess heat, either directly from solar energy or as a byproduct of fuel burn, into electricity. A thermoelectric device is made of both n-type and p-type semiconductors. Bismuth–telluride-based alloys are of great importance not only as the best thermoelectric materials with the maximal ZT values close to unity near room temperature, but also due to the potential for further performance improvement.
The performance of thermoelectric devices is assessed by the dimensionless figure of merit ZT of the material, defined as ZT =α²σT/k, where α, σ, k and T are the Seebeck coefficient, the electrical and thermal conductivities, and the absolute temperature, respectively. The thermal conductivity is a combination of thermal conductivity via electrons, κ_e, and via phonons, κ_l. The main difficulty in improvement of the efficiency of a thermoelectric device is due to the complex relation between σ, α and k.

In this study Bi₂Te_2.4Se_0.6 alloys were prepared and examined. The synthesis process included melting ingots via rocking furnace, melt spinning with two different wheel RPMs and hot pressing under optimal conditions. The samples were characterized in the direction parallel and perpendicular to the pressing direction. The anisotropic properties of the material were maintained throughout the process and a high ZT of 1.06 was achieved at ~65C.

Conversion of dissipated heat into electricity is the basic principle of thermoelectricity. It has a wide variety of applications in the areas such as automobile engineering, refrigerating coolants, satellite etc. In search of such materials, thermoelectrics has given wide scope to complex materials like Tellurides, Clathrates, Zintl compounds, Half Heusler alloys, Si-Ge, Skutterudite etc. The defining factor for thermoelectric materials is ZT, thermoelectric figure of merit. This attributes to the power factor (P=σS², σ-electrical conductivity S-Seebeck coefficient) enhancement and thermal conductivity reduction. Bandgap tuning, carrier concentration are a few ways to improve power factor whereas, in parallel, grain size reduction, point defects, dislocations are key ways to thermal conductivity reduction. These can be achieved experimentally through microstructural engineering and processing. The issues existing with available thermoelectric materials are associated with the stability of performance for a long range of temperatures. Fine tuning of the microstructure is the key factor in overcoming thermal stability issues.

Currently, we are exploring a program of microstructure- transport properties correlation of newly processed thermoelectric alloys by fine-tuning the eutectic microstructure. In this current work, we have synthesized a set of thermoelectric alloys which are eutectic and near-eutectic compositions in Sn-Te system. The microstructure of these alloys shows microstructure consisting of SnTe and Te phases. We varied the morphology, orientation, fraction, spacing of the eutectic phases (SnTe and Te) using different processing conditions. Variations of transport properties with respect to different processing conditions have also been studied. Third component additions with various at% to Eutectic microstructure has been studied. To these alloys, the effect of Directional solidification has been studied and has improved TE properties significantly.
The elemental distribution mapping and the compositional analysis using electron probe microanalysis (WDS), demonstrates that the matrix is rich in tellurium and the continuous phase has the composition of Sn₅₀Te₅₀ (at%). Transport properties of the current alloy are attractive in terms of standard thermoelectric material. Further elemental additions have enhanced properties. They were directionally solidified at various speeds and TE properties were studied with microstructural modification. The detailed microstructure-transport properties correlation will be presented.
Acknowledgement: The authors would like to acknowledge the facility of the AFMM.



References:
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Exceptionally high thermoelectric figure of merit ZT of ~2.6 at 923 K for p-type and ~2.8 at 773 K for n-type were reported in the single crystalline form of SnSe along the crystallographic b-axis and a-axis respectively. Due to its undesirable mechanical properties and strict demands of crystal growth conditions, its polycrystalline counterparts have been extensively investigated as an alternative. However, polycrystalline SnSe has poor thermoelectric performances with higher total thermal conductivity than single crystals.
Defects such as point defects, nanoprecipitates and dislocations can significantly affect charge and phonon transport behavior. Accordingly, defect engineering has been a proven tool to control charge-carrier and thermal transport properties of thermoelectric materials. However, its formation mechanism at the atomic level has not been fully understood. In this presentation, we synthesized new SnSe materials involving a high degree of vacancy to induce dense dislocations in the SnSe matrix. We traced the formation of atomic defects and edge dislocations with respect to annealing temperature at near the phase transition temperature employing spherical aberration corrected transmission electron microscope.
As a result of induced defect structure in SnSe, the resulting polycrystalline samples exhibit remarkably high power factor with a maximum of 6.3 μW cm^-1K^-2 at 800 K and substantially suppressed thermal conductivity. They synergistically contribute to remarkably high thermoelectric figure of merit over 2.0 at 800 K.

The introduction of heterogeneous dopants could not only selectively improve intrinsically-poor specific properties of the base materials but potentially break the physically coupled thermoelectric properties due to the created various functional interfaces for further enhancing the overall thermoelectric figure of merit (ZT).^[1] In this work, a series of novel heterogeneous nanocomposite films comprising amorphous carbon materials (a-C) and Bi_xSb_2-xTe₃ (BST) were successfully deposited on SiO₂/Si substrates using a dual-beam pulsed-laser deposition system. BST and pyrolytic carbon targets were individually ablated for fabricating the a-C/BST films with different a-C contents (0~20 wt%). The highest Seebeck coefficient of the a-C/BST film (4 wt% a-C) was found to approach 650 µVK^-1 which is over three times higher than that for intrinsic BST. According to Pisarenko plot, the significantly enhanced Seebeck coefficient originated from the a-C/BST hetero-interfaces induced energy filtering effect. Besides, the significantly suppressed thermal transport as evidenced by micro-Raman and thermal diffusivity characterizations reasonably came from the grain fining of BST and the intrinsically low thermal conductivity of a-C. To further adjust the preferential orientation and interfacial structure, the laser energy ratio for providing the highest Seebeck coefficient was maintained for film preparation with a higher deposition temperature of 450 ^oC and 500 ^oC. It was found that preferential orientation gradually shifted from (015) to (00l) with temperature increasing. Besides, the TEM images attested high-density and strongly oriented twin bands almost across the whole BST flake-like structure, clearly indicate the potential for scattering multi-wavelength phonons and further decreasing the thermal conductivity, without substantially interfering the carrier transportation. An enhanced Seebeck coefficient of 620 μVK^-1 and the corresponding power factor of ~40 µWcm^-1K^-2 obtained from the a-C/BST film prepared at 450 ^oC (4 wt% a-C) are comparable to or higher than previously reported values of BST or Bi-Sb-Te-based nanocomposites.


1. T. H. Chen, P. H. Chen, C. H. Chen, J. Mater. Chem. A, 6, 982-990 (2018).

Thermoelectric properties are governed by classic Boltzmann transport equations. This often sets limits for how much each material can be optimized for thermoelectric application. The resonant effect from dopants in certain systems presents one of a few successful strategies to work outside this classic limit. Using PbSe co-doped with Tl and Na as a platform, we have presented a method to describe the influence of resonant levels on thermoelectric properties at room temperature and above. We showed (and modeled) the impact of resonant levels on the Seebeck coefficient and mobility at different temperatures, which, especially for mobility, has not been shown before. In general, resonant dopants are not always beneficial for thermoelectric properties overall. This is because the enhanced Seebeck coefficient comes at the price of mobility reduction. Three parameters, namely the resonant level position E_res, the width of resonant levels G, and the percentage of participating resonant levels H, determine the observed trans- port properties. Among them E_res is crucial in deciding whether the resonant levels are beneficial (in terms of peak zT). Resonant levels could increase zT if E_res is inside the band but very close to the band edge. Our model also confirmed that it is possible to achieve moderate improvement of performance with resonant levels in systems similar to PbSe. This work could help the rational use of resonant levels to their full potential for thermoelectric materials.

In this work, we have investigated the thermoelectric properties of formamidinium tin iodide (FASnI₃), lead-free hybrid perovskite, in terms of electrical conductivity, Seebeck coefficient and thermal conductivity. By doping with small amount of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), both electrical conductivity and Seebeck coefficient of solution-processed FASnI₃ thin film are substantially improved, resulting in enhancement on overall thermoelectric performance. The optimal FASnI₃ thin film with 0.05 mg/mL of F4-TCNQ doping exhibits higher electron/hole mobilities and lower trap densities with finely modified thin film morphology by F4-TCNQ. An ultralow thermal conductivity (~0.45 W m^-1 K^-1) is found in FASnI₃ thin film by estimation based on quantitative scanning thermal microscopy (SThM). The thermoelectric figure of merit, value, of optimal FASnI₃ thin film is enhanced to be 0.085 at room temperature, which is almost 10 times of pristine FASnI₃. Our findings open a door for realizing room-temperature-operated thermoelectric applications by low-cost processing of less-toxic hybrid perovskite materials.

Thermoelectric materials have consistently been the focus of research efforts for energy conversion and harvesting. In recent years, organic-inorganic hybrid perovskite materials started to draw attention for thermoelectric applications, thanks to their low thermal conductivity and high Seebeck coefficient. Compared with the organic-inorganic hybrid perovskite materials, especially lead-based perovskites, the all-inorganic perovskite CsSnBr₃ is advantageous for its low toxicity, relative thermal stability, and higher electrical conductivity. In this work, we grew cubic CsSnBr₃ single crystals about 8 mm in size from ethylene glycol solutions, and measured its electrical conductivity to be ~64 S/m and p-type carrier concentration to be 10¹⁷–10¹⁸ cm⁻³ at room temperature. Its Seebeck coefficient increases steadily from ~400 μV/K at 325 K to ~700 μV/K at 575 K, while its thermal conductivity drops from 0.50 to 0.25 W/mK in the same temperature range. In summary, the highest figure-of-merit ZT that we recorded for single-crystal CsSnBr₃ is ~0.067 at 572 K, with a power factor (PF) of ~34 μW/(mK²). The electrical conductivity as well as ZT value can be further enhanced by heavy doping or replacing Br by I in CsSnBr₃.

The family of ternary Zintl phases with the 21-4-18 composition has several different structure types and has been explored for their electronic and magnetic properties. Yb₂₁Mn₄Sb₁₈ adds a new composition to this class of Zintl phases, crystallizing in the alpha-Ca₂₁Mn₄Sb₁₈ structure type (monoclinic, space group C2/c). The expected low thermal conductivity, due to the large primitive unit cell, and potentially high Seebeck, because of the significant number of states at the Fermi level, make this phase of interest for its thermoelectric properties. The complex crystal structure has been studied through synchrotron powder x-ray diffraction, single crystal x-ray diffraction and pair distribution function analysis using time-of-flight neutron diffraction that reveal positional disorder on several sites. The electrical and thermal transport properties show that the high efficiency of Yb₂₁Mn₄Sb₁₈ results from its large Seebeck coefficient (~290 µV/K at 650K) and extremely low thermal conductivity (~0.5 W/(mK) at room temperature). The optimum hole carrier concentration was tuned according to a single parabolic band model through Na doping which has improved zT over all temperature ranges compared to the parent compound yielding a maximum zT ≈ 0.8 at 800K, fairly high performance in the mid-to-high temperature regime. Electronic structure calculations of the band structure and partial density of states have revealed that states near the Fermi level are mostly contributed by the Mn and Sb atoms that participate in the [Mn₄Sb₁₀]^22- motif of the structure. Doping and results of structural variants will be presented and discussed.

Intermetallic compound of Mg₂Si and its solid solutions are promising thermoelectric materials since they exhibit high thermoelectric performance and consist of eco-friendly elements. In these materials, two types of Mg-related point defects (Mg vacancies and interstitial Mg atoms), as well as dopant elements, have significant influence on the carrier concentration (n) and resulting thermoelectric properties [1]. For example, Mg vacancies compensate the carrier from the dopant in heavily Sb-doped Mg₂Si (Mg_2–δSi_1–xSb_x) [2], where a decrease in the Mg content of 0.01 corresponds to a decrease in n of ca. 3×10²⁰ cm^–3. While the carrier concentration is very sensitive to the Mg content, its precise control has not been achieved because of the high vapor pressure of Mg.
Here, we have developed a new approach, Mg-pressure-controlled annealing [3,4], to achieve the precise control of the Mg content for ternary and quaternary Mg₂Si-based materials (Mg_2–δSi_1–xSb_x and Mg_2–δ(Si_0.5Sn_0.5)_1–xSb_x). Annealing under low and high Mg partial pressures (0.1 and 10 Pa) leads to low and high carrier concentrations (e.g. 1.4×10²⁰ and 6.3×10²⁰ cm^–3 for Mg_2–δSi_0.90Sb_0.10), respectively. The change in n is reversible and attributed to an equilibrium reaction, where the Mg vacancies in the sample are partly filled by Mg atoms in the gas phase. In addition, annealing under an intermediate Mg partial pressure (2 Pa) results in an intermediate value of n (e.g. 4.1×10²⁰ cm^–3 for Mg_2–δSi_0.90Sb_0.10), suggesting that the Mg content in Mg₂Si-based materials can be tailored via the Mg-pressure-controlled annealing.
The variable range of n (Δn) due to the Mg non-stoichiometry in Mg_2–δSi_1–xSb_x depends on the Sb content and the maximum Δn of 4.9×10²⁰ cm^–3 is obtained at x = 0.10. The same x-dependence of Δn is also observed for the quaternary compound Mg_2–δ(Si_0.5Sn_0.5)_1–xSb_x, where Δn at x = 0.10 is 3.5×10²⁰ cm^–3 (0.2×10²⁰ cm^–3 ≤ n ≤ 3.7×10²⁰ cm^–3). In the case of Mg_2–δ(Si_0.5Sn_0.5)_0.90Sb_0.10, the high value of n due to Mg-rich composition suppresses the bipolar diffusion, which decreases the Seebeck coefficient and increases the thermal conductivity, at high temperatures. Owing to the suppression of the bipolar diffusion, Mg_2–δ(Si_0.5Sn_0.5)_0.90Sb_0.10 shows zT value of 0.9 at 773 K at Mg-rich composition whereas zT value of 0.6 is obtained at Mg-poor composition.

1) X. Liu, L. Xi, W. Qiu, J. Yang, T. Zhu, X. Zhao, and W. Zhang, Adv. Electron. Mater. 2 (2016) 1500284.
2) G.S. Nolas, D. Wang, and M. Beekman, Phys. Rev. B 76 (2007) 235204.
3) D. Kato, K. Iwasaki, M. Yoshino, T. Yamada, and T. Nagasaki, J. Solid State Chem. 258 (2018) 93–98.
4) D. Kato, K. Iwasaki, M. Yoshino, T. Yamada, and T. Nagasaki, PCCP 20 (2018) 25939-25950.

The detailed understanding of the mechanism(s) responsible for the very low and sometimes even glass-like lattice thermal conductivity in thermoelectric clathrates continues to develop. Of particular interest is the role of the guest atoms and the other unique features of the structure of these materials. Despite a wide variety of novel mechanisms proposed over the past two decades to explain the unusual low temperature thermal transport, more recent theoretical and experimental analyses have suggested that phonon-phonon scattering, e.g. 3-phonon Umklapp-like processes, plays an important role in producing the low lattice thermal conductivity in select clathrate compositions, especially at elevated temperatures. We have shown this is supported by the application of a simple model due to Slack, which shows the lattice thermal conductivity values above the Debye temperature can be predicted relatively well assuming only 3-phonon scattering processes involving acoustic phonons, for a large number of clathrate compositions. Furthermore, our studies of thermal expansion using temperature dependent powder and single crystal X-ray diffraction from 20 K to 500 K on Na_xSi₁₃₆ clathrates with variable guest content (0 < x < 24) reveal the mode averaged Grüneisen parameter increases dramatically by more than a factor of 2 as the guest content increases from 0 to 100% filling, indicating the guest atoms have a pronounced effect on the anharmonicity of the interatomic interactions. Taken together, all of these results confirm the important role the guest atoms play in significantly influencing the lattice dynamics in these materials, and that anharmonic phonon-phonon scattering may play a leading role in the thermal transport and thermoelectric performance in the clathrates in the temperature range of interest for thermoelectric applications.

Since the 1960s, the state-of-the-art power systems for space applications has typically been based up on either SiGe alloys or PbTe. Although reliable and robust, the thermal to electric conversion efficiency of these systems remains fairly low at only 6.5% with an average thermoelectric figure of merit of about 0.55. A factor of 2 improvements in the thermoelectric conversion efficiency is needed to support future space missions. In recent years, complex Zintl phases such as n-type La_3-xTe₄ and p-type Yb₁₄MnSb₁₁ have emerged as new high efficiency, high temperature thermoelectric materials with peak ZTs of 1.2 at 1275K. The high performance of these materials is attributed to their combination of favorable characteristics such as: semi-metallic behavior due to small band gaps, low glass-like lattice thermal conductivity values due to structural complexity and reasonably large Seebeck values near their peak operating temperatures. However, significant enhancements in material systems such as the Yb₁₄MnSb₁₁ have remained stagnant since its initial discovery in 2010. We will present an overview of recent research efforts at JPL and UC Davis collaborators on thermoelectric properties of new 14-1-11 formulations phases as well their suitability for advanced TE devices.

Composite materials are conceptualized with the intent of obtaining a final material with improved characteristics. In thermoelectrics this has been achieved for organic-inorganic composites where conducting materials embedded in polymeric matrixes improved the thermoelectric properties. A completely different situation is the combination of two inorganic compounds. Although according to several studies a thermoelectric composite cannot have better properties than its constituents, a study from Bergman and Fel (1999) proved that the enhancement of the figure of merit of a composite material is possible under specific conditions. The purpose of this work is to show the effect of Co inclusions on the electronic and thermal property of Yb₁₄MnSb₁₁. Yb₁₄MnSb₁₁, with a zT of about 1.2 at 1200 K, is already the most performing p-type material for high temperature thermoelectric applications. Our intent is to further increase its thermoelectric performance by decoupling the electronic transport properties (σ, S). Even though cobalt is characterized by a metallic electrical conductivity, it shows a relatively high Seebeck coefficient (S ≈ -25 μV/K at 1100 K). Therefore, cobalt inclusions are expected to provide a boost to the electrical conductivity, while affecting only marginally S. At the same time, if the inclusion size can be kept in the nm-μm range, inclusions can act as active phonon scattering centers and hence reduce the lattice thermal conductivity. To verify the validity of our assumptions, we synthesized four samples with different inclusion density (0, 2, 5, 10 vol%). The samples have been chemically, electrically, and thermally characterized and the obtained results will be shown and compared with the baseline of the Jet Propulsion Laboratory for the Yb₁₄MnSb₁₁ ATEC (Advanced ThermoElectric Converter). In the end, the sample with 5vol% cobalt inclusions is the one that showed the best thermoelectric properties achieving a peak zT of 1.6 at 1250K.

Filled skutterudites such as Yb-Co₄Sb₁₂ have a high potential for medium temperature thermoelectric energy conversion due to their high power factor. Their high thermal conductivity, however, limits large scale practical application. In order to enhance both power factor and figure-of-merit of Yb filled Co₄Sb₁₂, type-I clathrates and modified type-I clathrates Ba₈Al₁₆Si₃₀ and (DyBa)₈Al₁₆Si₃₀ which also exhibits open cage structure have been added. The skutterudite and clathrates were synthesized separately by vacuum alloying, and the mixed materials were obtained by hot pressing the requisite amount of powders at 873 K for 10 min and 60 MPa. All the materials, Yb_0.3Co₄Sb₁₂, (Ba₈Al₁₆Si₃₀)_x(Yb_0.3Co₄Sb₁₂)_100-x and (Ba_7.5Dy_0.5Al₁₆Si₃₀)_y(Yb_0.4Co₄Sb₁₂)_100-y with x = 2,4 and y = 1,2 wt.% respectively, exhibit a single phase X-ray diffraction pattern corresponding to the skutterudite phase. The X-ray diffraction peaks corresponding to the clathrates are extremely weak due to the small amount present in these materials. All the materials exhibit a negative Seebeck coefficient indicating electrons to be the major charge carriers. The materials with 1 wt.% Dy-modified clathrates addition has the highest Seebeck coefficient at all temperatures and has a peak value of 210 μVK^-1 at ~725 K, 15 % higher than Yb_0.3Co₄Sb₁₂.The electrical conductivity of this material is lowest compared to other materials, 1150 Scm^-1 at room temperature, which decreases to ~ 700 Scm^-1 at 750 K. The high value of Seebeck coefficient coupled with good electrical conductivity results in good power factor of 3.2 mW^-1K^-2 , 25 % increase compared to single filled skutterudite at 650 K. The power factor for T > 650 K decreases due to the onset of bipolar conduction. The thermal conductivity of this composite, however, does not decrease significantly compared to Yb_0.3Co₄Sb₁₂ indicating that the interfaces between Yb_0.3Co₄Sb₁₂ and Ba_7.5Dy_0.5Al₁₆Si₃₀ are band matched for charge transport as well as phonon transport. The figure-of-merit of y = 1 wt.% material reaches a value of ~ 0.85 compared to 0.7 for Yb_0.3Co₄Sb₁₂ at ~ 750 K, an increase of 20 %.

Mg₃Sb₂-Mg₃Bi₂ alloy is recently found to have high performance n-type thermoelectric (TE) properties for low-grade waste heat recovery near room temperature. It rivals current best thermoelectric materials (e.g. Bi₂Te₃-Bi₂Se₃ alloy) for thermoelectric performance in the temperature range of 0°C to 250°C and features a higher fracture toughness and lower cost. While previous first-principles studies on Mg₃Sb₂ and Mg₃Bi₂ have revealed their complex electronic band structures, the effect of alloying, particularly on the electron transport properties has not been thoroughly understood. In this study, we perform first-principles calculations on Mg₃Bi₂ to investigate the major scattering mechanisms for both electrons and phonons in this system. Our results can provide guidelines for the design of better materials for low-temperature thermoelectrics.

To be practical, semiconductors need to be doped; in thermoelectric applications to nearly degenerate levels. However, many materials are not dopable at all, especially those with wide band gaps, while many other exhibit strong preference toward either p- or n-type conductivity, but not both. These doping bottlenecks and doping tendencies are well known but our present understanding of these phenomena is fairly qualitative and phenomenological. In this talk I will describe our recent work dedicated to advancing our understanding of the intrinsic materials properties that govern dopability of semiconductors. This we do by developing an analytic (model) description of the defect formation in semiconductors which allows formulation of rigorous design principles. Our approach, which builds upon the semiconductor defect theory applied to a suitably devised (tight-binding) model system, reveals analytic relationships between intrinsic materials properties and semiconductor dopability, and elucidates the role of previously suggested heuristic descriptors such as the absolute band edge positions and/or the branch point energy. We validate our model against the present state-of-the-art defect calculations as well as experimental data on a number of classic binary semiconductors. Finally, I will discuss the model extension to more complex chemistries and its utility in large-scale material searches.

The thermoelectric effect that is drawing attention from the viewpoint of energy saving is typically the Seebeck effect in which an electromotive force is generated in the same direction as the thermal gradient. In magnetic materials, there is also an anomalous Nernst effect that generates an electromotive force in the direction perpendicular to the thermal gradient due to spontaneous magnetization, and has recently been attracting attention for the improvement of energy conversion efficiency[1]. In each effect, it is important to examine the contribution of thermal conductivity in evaluating the thermoelectric figure of merit ZT that represents the performance of the material. Since the thermal conductivity can be described for each electron and lattice, in this research, we theoretically computed both electronic and lattice thermal conductivity, and then evaluated ZT.
Theoretical analysis of lattice thermal conductivity is conducted by Boltzmann transport theory with relaxation time approximation. In order to do this, it is necessary to determine the inharmonic atomic force constant exactly, and an accurate analysis is carried out by first-principles calculations based on density functional theory. As computational codes, OpenMX[2] is used as a density functional calculation, and ALAMODE[3] is used as a lattice thermal conductivity calculation.
The half-Heusler compounds are ternary compounds, have composition formula of XYZ, and are expected as thermoelectric materials that exhibit high power factor in a medium temperature range of about 600 to 1000 K. In this study, we analyzed thermoelectricity and thermal conductivity of ferromagnetic half-metallic half-Heusler MnCoSb[4]. We evaluated the ZT for the Seebeck and anomalous Nernst effect. We will discuss carrier and temperature dependence of ZT.

[1] Y. Sakuraba, Scripta Mater., 111, 29 (2016).
[2] T. Ozaki et al., Open source package for Material eXplorer [www.openmx-square.org]
[3] T. Tadano, and S. Tsuneyuki, Phys. Rev. B 92, 054301 (2015).
[4] S. Minami, F. Ishii, Y.P. Mizuta, and M. Saito, Appl. Phys. Lett. 113, 032403 (2018),

We develop and investigate fast automated first-principles methods for computing electronic and thermal transport properties of complex semiconductors and low-dimensional quantum materials, without empirically fitted parameters. Using these electron-phonon averaged (EPA) methods we performed screening and discovered new thermoelectric alloy compositions with leading performance and stability. The computational approaches achieve good predictive accuracy and transferability while greatly reducing complexity and computation cost compared to the existing methods. The first-principles calculations of the electron-phonon coupling demonstrate that the energy dependence of the electron relaxation time varies significantly with chemical composition and carrier concentration, suggesting that it is necessary to go beyond the commonly used approximations to screen and optimize materials' composition, carrier concentration, and microstructure. The new methods are verified using high accuracy computations and validated with experimental data before applying it to screen and discover promising compositions in the space of half-Heusler alloys. We discuss the universality of the Wiedemann-Franz law and deviations from it in semiconductors, computing the Lorenz number from first principles.

Anisotropic electronic and thermal transport properties can be harnessed to enhance the thermoelectric (TE) performance of materials. There is also a growing interest in exploring the functionality of single crystals, especially of materials with layered motifs, for thermoelectrics. Layered materials often exhibit anisotropic transport properties. Therefore, it is crucial to account for the anisotropy in transport properties in computational searches for TE materials. Traditional computational approaches are expensive and not amenable to high-throughput searches. In this work, we build upon our intutition from prior semi-empirical models to create a new anisotropic model of carrier mobility by utilizing the elastic stiffness and the conducitivity effective mass tensors. Similarly, we extend our prior semi-empirical lattice thermal conductivity model to account for anisotropy by calculating the speed of sound tensor. By combining the models for anisotropic mobility and lattice thermal conductivity, we predict the anisotropic TE performance quantified by the thermoelectric quality factor. We apply these models to predict the TE performance of a large number of layered materials (>2000) and identify candidates with predicted high performance.

Topology, a mathematical concept, recently became a hot and truly transdisciplinary topic in condensed matter physics, solid state chemistry and materials science. In magnetic materials the Berry curvature and the classical anomalous Hall and the anomalous Nernst effect helps to identify potentially interesting candidates. As a consequence, the magnetic Heusler compounds have already been identified as Weyl semimetals: for example, Co₂YZ, Mn₃Sn and Co₃Sn₂S₂. The Anomalous Hall angle also helps to identify materials in which a QAHE should be possible in thin films.

The reduction of lattice thermal conductivity (κ_L) can be realized by introducing large mass contrast through atomic substitution within pristine compounds. Such process are usually termed as “point-defect scattering” and has been proved effective in the improvement of thermoelectric figure-of-merit (zT). Recently we synthesized two sets of half-Heusler compounds: ZrCoSb_1-xSn_x and Zr_1-yTi_yCoSb, and compared their lattice thermal conductivity with respect to the substitution level. Contrary to the general conception that larger mass difference are more effective in phonon scattering, we find a much lower κ_L in compounds with Sn substitution at the Sb site than the ones with Ti substitution at the Zr site. The origin of phonon scattering in the Sn-containing compounds will be discussed in great detail based on transport property measurement and microstructure characterization. Our work propose a novel strategy for increasing phonon scattering without large mass contrast so that a high carrier mobility can be preserved.

This study shows that ultrahigh bulk thermoelectric performance across all temperatures is physically possible and within reach, and also that pocket multiplicity can hurt at high temperatures when the pockets have disparate scattering rates. Using state-of-the-art ab initio approaches to explicitly treat electron-phonon and phonon-phonon scattering for accurate electronic and phonon transport, we predict that full-Heusler Sr₂BiAu and Sr₂SbAu are theoretically capable of delivering ultrahigh n-type thermoelectric performance at cryogenic-to-high temperatures: zT=0.3-5.1 at 100-800 K. The feature critical to such high performance is a set of dispersive conduction band pockets, at the L-point in addition to those along Γ-X, for a total of ten. Relative to Ba₂BiAu, the additional L-pockets significantly increase the power factor at low temperatures, generating as high as 10 mW m^-1 K^-2 at 200 K. However at high temperatures, because the less dispersive L-states experience much heavier scattering, their existence damages the overall electron lifetime, mobility, and hence the power factor. The dominant intrinsic defect at play in these compounds is Bi/Sb_Au antisites, which limit their n-dopabilitie. Nevertheless, Sr₂SbAu potentially has both a large enough stability region and high enough Sb_Au formation energies to retain some chance at experimental realization as a high-performance thermoelectric.

Thermoelectrics are unique materials which can transform heat into electricity and vice versa. They have applications ranging from renewable energy generation to heating and cooling units. Thus far, however, thermoelectric efficiency remains low, in part because of the contradictory requirements of high electrical conductivity and high Seebeck coefficient coupled with low thermal conductivity. Most TE materials possess small band gaps. A transparent TE that can operate near room temperature was discovered in 2014 [1], and the first inorganic one in 2017. [2,3] The discovery of a high performance transparent, room temperature TE would open up new fields of research in a range of novel applications such as smart windows (or screens) with energy harvesting, cooling and thermal sensing functionalities. Using density functional theory (DFT) with hybrid functionals we have screened the novel p-type transparent half-Heusler TaIrGe [4] for its thermoelectric ability. We will demonstrate that TaIrGe possesses an ultra-low lattice thermal conductivity and has the potential to yield the highest ZT ever for a transparent TE.

[1] Lee, S. H. et al., J. Mater. Chem. A 2014, 2, 7288-7294.
[2] Yang et al., Nat. Commun. 2017, 8, 16076.
[3] Couderc, Nat. Energy 2017, 2, 17137.
[4] Yan et al., Nat. Commun. 2015, 6, 7308.

Half-Heusler alloys are among the most promising candidates for applications in thermoelectric power generators, since they are thermally and mechanically robust, do not contain toxic elements and exhibit some of the largest ZT values among bulk systems. In particular, exceptionally high ZT values (1.52 at 973 K) have been recently reported from the experimental measurements of transport properties of TaFeSb-based compounds, following up on theoretical predictions. However, the microscopic origin of such high ZT values in these systems is still not fully understood. In this work we use ab initio calculations to investigate the transport properties of TaFeSb in terms of semiclassical Boltzmann transport equation, beyond the constant relaxation time approximation. We take into account the effects due to electron-phonon interaction by calculating corresponding scattering rates using Wannier-Fourier interpolation of electron-phonon matrix elements as well as the recently developed electron-phonon averaged approximation. We discuss the likely mechanisms of high ZT in TaFeSb-based systems and possible ways to further improvement of these materials.

Over the last decade, significant effort has been devoted to creating computational predictive frameworks for predicting the thermoelectric performance of materials. Such efforts shine light into reciprocal space, where we often have less chemical intuition, and thus enable the accelerated down-selection of candidate materials. Computational predictions, interrogated by experiment, has revealed new classes of thermoelectric materials, thereby validating this approach as a key tool in the search for thermoelectric materials. However, we now face a new problem: We have more predicted candidates than experimental bandwidth to test these candidates. Further, the expansion of thermoelectric training sets for machine learning remains quite slow. As such, our current efforts are focused on developing the infrastructure to accelerate synthesis and characterization of thermoelectric materials by x50 without significant sacrifices in material or data quality. These efforts are inspired by develops over the last three decades in combinatorial thin film growth, but are focused instead on bulk materials. Efforts to develop high throughput weighing, milling, consolidation, and transport measurements will be discussed in the context of pnictide and chalcogenide thermoelectric materials.

In the past, we have developed thin-film P-type Bi₂Te₃/Sb₂Te₃ single period superlattice structures [1] with ZT ~2.4 at 300K and N-type Bi₂Te₃/Bi₂Te_3-xSe_x structures, with more modest ZT ~0.7-0.8. These thin-film superlattices have been utilized to demonstrate extraordinarily large (>1000 W/cm²) heat-pumping in microelectronic devices [2]. There is a need to advance the 300K ZT of N-type materials, in particular, as well as optimize both the P- and N-type materials for the temperature range of 300K to 100K to enable solid state cryogenic cooling devices. In this presentation, we will describe the concept of hierarchically engineered superlattices in thin-film thermoelectrics to improve the ZT of both P- type and N-type materials, to values approaching ~3 and ~2, respectively. We will present the electrical conductivity, Seebeck coefficient and thermal conductivity data in these materials in the 300K-100K range. Additionally, we will delineate the advantages of hierarchically engineered superlattices, as compared to conventional single-period superlattices, for designing efficient cooling down to cryogenic temperatures. The electrical transport of these hierarchically engineered superlattices has been studied by Hall-effect from 300K-77K. Parabolic band is assumed with energy dispersion relation of E =0.5 (h/2p)²k²/m* and carrier velocity ~ (h/2p)k/m*, with a relaxation time (t) approximation model where the scattering dependence on energy and temperature leading are given by t (E,T) = t_o (E/k_BT)^r-1/2. This model of transport data suggests that the scattering is dominated by optical phonons (T^-1/2) and less of acoustic phonons (T^-3/2); the hierarchical structuring seems to result in more optical phonon and less acoustic phonon carrier scattering than superlattices, consistent with lower K_lattice seen with hierarchically engineered nanostructures. The thermal conductivity measurements carried out by time domain thermo reflectance, between 300K and 100K, also indicates the benefit for scattering of a range of heat-conducting phonon wavelengths in such hierarchically engineered superlattices. The ZT advancements obtained with these hierarchically engineered superlattices will be presented along with cooling device advancements.
[1] R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature 413, 597-602, (2001).
[2] I. Chowdhury, R. Prasher, K. Lofgreen,.R. Venkatasubramanian, Nature Nanotech 4, 235-238 (2009).

Thermoelectric (TE) energy harvesting or Peltier cooling devices will be prospectively used in a broad range of applications from industry to consumer products. Successful optimization of the TE figure of merit, i.e., zT, is a key enabler for the introduction of these devices to application. While zT of bulk materials is accessible by a variety of well-established measurement set-ups that are commercially available, exact determination of zT for thin and thick films remains a great challenge. This is especially true for thick films grown by the electroplating technique, which is favored for the fabrication of many micro TE devices. In this method the TE material is deposited onto an electrically conductive seed layer. Such layer causes an in-plane short-circuit that makes the determination of transport coefficients almost impossible. Here, we develop a platform for full in-plane zT characterization of TE materials produced by electroplating technique, eliminating the impact of the electrically conducting seed layer. Such characterization is enabled by using a suspended TE material within a transport device which is prepared by photolithography and etching processes. This full in-plane zT characterization provides an inevitable mile stone for a materials optimization under realistic conditions in a micro TE device. In addition, a comparative study is performed between an electrochemical deposited Co₇₅Ni₂₅ film and a bulk ingot with identical composition. The validation of the thermal conductivity measurement is done by Comsol simulation and analytic approach using heat transfer equation with specific boundary conditions dedicated to experimental setup.

Seebeck coefficient (S) is the ratio of open circuit voltage generated per unit degree of temperature difference across a semiconductor. It is of essential importance to thermoelectric materials. In a broader context, S is a fundamental transport parameter. It can be used to extract various information including carrier type, position of Fermi level, effective mass, scattering mechanism, even the complex shape of electronic bands. Seebeck coefficient measurements on typical semiconductors are routinely done in labs with fairly simple requirement on methodology and hardware. However, at extreme cases, reliable measurement of a small voltage of 100 nV to 1 mV can be very challenging. We present here an AC measurement technique which has several advantages over DC methods. The first is the ability to measure high impedance samples: We demonstrate the successful measurement of S on a halide perovskite CH₃NH₃PbI₃ film, with resistance over 100Gohms, can be determined whereas DC signal is totally obscured by random voltage fluctuations. The second, when a sample is inhomogeneous like often seen in lightly doped semiconductors, the AC method with a little modification can measure both the average S between the two points and the difference. In contrast, DC measurement under the same condition will only yield non-linear voltage behavior. Lastly, the great accuracy of phase sensitive detection allows the use of very small temperature gradients, for instance, DT = 0.1°C, revealing true S at a given temperature, which is useful when studying material behavior close to a phase transition.

Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest because of the potential energy filtering effect, which can enhance Seebeck coefficient and thermelectric performance due to the alteration of energy-dependent scattering. The thermoelectric properties of mixed-phase LixMoS2 are studied as a function of its phase composition tuned by in-situ thermally driven deintercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T′ phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of deintercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase LixMoS2. Our work shows that careful phase engineering is a promising route toward achieving thermoelectric performance in TMDCs.

Thermoelectric research has been significantly activated during the past two decades by the quantum confinement effect, a characteristic of low-dimensional (1D & 2D) materials, thanks to the pioneering work by Hicks and Dresselhaus [1]. The confinement effect leads to a enhancement of the density of states at the Fermi energy, which enhances thermoelectric power factor (PF) of low-dimensional materials. In particular, for 2D material, the PF is enhanced only when the thickness of the material is smaller than the thermal de Broglie wavelength [2]. 2D semiconductors naturally satisfy the condition so that they are a good candidate for thermoelectricity. However, the thermoelectric performance of 2D semiconductors still needs to be improved for industrial applications because of large thermal conductivity. In this work, we propose two possible strategies to improve the thermoelectric performance of the 2D materials beyond the confinement effect. Firstly, we show that the thermoelectric performance of the 2D semiconductors can be improved by the band convergence, in which some distinct valleys become degenerate in energy [3]. This technique is possible to realize, for example, by applying mechanical strain in monolayer InSe [3,4]. Further, we show that the nonparabolic Kane bands and large anharmonicity of phonons could lead to better thermoelectric performance of the 2D tetradymites [5].

[1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 and 16631(R) (1993).
[2] N. T. Hung et al., Phys. Rev. Lett. 117, 036602 (2016).
[3] N. T. Hung et al., J. Appl. Phys. 125, 082502 (2019).
[4] N. T. Hung et al., Appl. Phys. Lett. 111, 092107 (2017).
[5] N. T. Hung et al., Nano Energy 58, 743 (2019).

It is well known that the thermoelectric properties of materials are critically affected by the scattering due to impurities. Empirical or semi-empirical formulations like the effective-mass theory are typically being used for studying impurity scattering, and a fully ab initio study of the phenomenon remains a rather difficult task even today. Only a few detailed ab initio studies have been reported for relatively simple materials. Application of these ab initio methodologies to realistic, comlex thermoelectric materials is still far from realistic. In this presentation, a method for quick and rough estimation of electron-defect interactions in thermoelectric materials is proposed. The method is based on Gaussian process regression (GPR), which is a statistical procedure used widely in machine learning communities. By using the GPR procedure, relatively coarse observations of electron-defect interaction coefficients in the Brillouin zone can be effectively interpolated in an appropriate energy window, whose results can be further utilized to estimate the scattering rate due to impurities. A numerical example to demonstrate the feasibility of our proposed method is also provided.

The half-Heusler NbFeSb-based semiconductors have been of considerable interest due to their excellent thermoelectric performance, especially an enhanced power factor [1]. For enhanced performance, various substitutional elements can be incorporated, with Ti substitution, in particular, providing outstanding performance as a p-type material. However, intrinsic and extrinsic defects play an important role in adjusting the carrier concentrations and mobilities. To better understand the inherent defects in these materials and then improve the thermoelectricity, we used experimental and computational methods to study NbFeSb-based semiconductors.

To investigate the defects in NbFeSb-based semiconductors, we have performed XRD, ⁹³Nb NMR, and magnetic measurements on pure NbFeSb and a series of Ti-substituted (Nb, Ti)FeSb samples with different substitution levels. Magnetic measurements combined with an observed Schottky anomaly and changes in the NMR line width indicate the presence of a 0.2% concentrated native magnetic defect in stoichiometric NbFeSb sample [2]. For Ti-substituted samples, the breaking of the 18-electron rule leads to heavily p-type materials. A small but consistently increasing paramagnetic defect density is observed with the increase of titanium substitution concentrations revealing the existence of additional Ti-induced paramagnetic defects. Also, with Ti substitution, the ⁹³Nb NMR spectra show progressive increases in Knight shift providing a measure of band-edge carrier densities, and we have also mapped changes in the NMR line shapes to local carrier density distributions connected to different local configurations.

To further identify the origin of the defects in NbFeSb, DFT calculations have been utilized. In this work, we applied the pyCDT package [3] on p-type half-Heusler thermoelectric NbFeSb, combined with DFT calculations. The Fe(Nb) antisite is identified as an important n-type defect, and this defect was also identified to match the magnetic signature seen in undoped samples in NMR and magnetic measurements. In addition, we identified the Fe interstitial and Nb vacancy as other important native defects, and we discuss their relation to the observed NMR shift behavior. The iron interstitial is identified as an n-type native defect with relatively low formation energy, even though Ti-substituted NbFeSb forms as nearly a line compound, with very little interstitial formation. This stands in contrast to the end member of the series, TiFe_1+xSb, in which a significant number of interstitials form spontaneously to balance the 18-electron rule. In addition, we discuss different local configurations in Ti-substituted alloys and their effect on the transport behavior.

[1] He et al., Proc. Natl. Acad. Sci. U. S. A. 113, 13576 (2016).
[2] Tian et al., Phys. Chem. Chem. Phys. 20, 21960 (2018).
[3] Broberg et al., Comput. Phys. Commun. 226, 165 (2018).

There have been several attempts in literature to provide a variational formalism for solving quantum transport problems. However, the asymptotic boundary conditions used in such methods do not take the evanescent modes into account, which is crucial for applications in meso- and nano-scale devices. We develop a novel method based on sources and absorbers to study quantum transport in confined geometries in the presence of defects [1]. This method overcomes the limitations of the currently prevalent approaches to provide a complete non-asymptotic variational description for quantum transport in meso- and nano-scale systems. We apply this method to study electron transport in disordered waveguides, and examine the impact of evanescent modes in enhancing the Seebeck coefficient and power factor beyond the earlier proposed limits. We also discuss a new way of fabricating current rectifiers using tapered electron waveguides [2].

References:
[1] Sathwik Bharadwaj and, L. R. Ram-Mohan, Electron scattering in quantum waveguides with sources and absorbers. I. Theoretical formalism, J. Appl. Phys. 125, 164306 (2019).
[2] Sathwik Bharadwaj and, L. R. Ram-Mohan, Electron scattering in quantum waveguides with sources and absorbers. II. Applications, J. Appl. Phys. 125, 164307 (2019).

To deal with an unprecedented energy crisis nowadays, humongous attention is concentrated on the developing high-performance thermoelectric material to effectively recover a lot of waste heat. However, intrinsic trade-off relationship between thermoelectric properties make it harder to improve the performance. Especially, improving Seebeck coefficient while preserving electrical conductivity is challenging as both of them are electron-related phenomena. Therefore, simultaneous enhancement of the both properties is required for efficient thermoelectric material.
Here, we present the peculiar behavior of both electrical conductivity and Seebeck coefficient, which show a constant value along wide range of temperature, in Na- and Ag-doped PbTe. This temperature-robust electronic transport properties yield a broad plateau in power factor, while thermal conductivity keeps decrease as temperature increases. As a result, sharp increase of thermoelectric figure of merit (zT) is achieved.
This unconventional thermoelectric property is mainly attributed to the re-dissolution of the Na segregated at the grain boundary. Segregated Na can be incorporated into the lattice at the high temperature, increasing hole concentration significantly. As the optimum hole concentration in p-type PbTe shows monotonic increase as temperature increases, inducing segregation at low temperature and re-dissolution at high temperature can be an effective strategy to keep up with it.
It is further founded that Ag promotes the solubility drop of the incorporated Na, leading to the temperature-robust transport property even at the lower Na doping concentration. Formation of Ag_i defect induces a huge lattice strain on the already shrunk Na-doped PbTe, thus phase separation between Na and PbTe lattice occurs. Microstructure analysis using SEM and TEM reveals that the segregated Na indeed exists at the grain boundary, and the defective structures such as dislocation and precipitation are uniformly distributed through the lattice.

Graphyne is one of the two-dimensional carbon allotrope and consisting of sp and sp² hydridized carbon atoms. Herewith we investigate the lattice thermal conductivities of α-, β- and γ-graphyne by perform large-scale non-equilibrium molecular dynamics simulations and solve linearized phonon Boltzmann transport equation. Up to the length of 1.5 μm along the heat transport direction is investigated, at which the converging behavior is found. The previous molecular dynamics simulations have been only limited up to few hundred nanometer scale in length, which may result in a large discrepancy with the experiments. Based on the results, the intrinsic macroscopic lattice thermal conductivities of α-, β- and γ-graphyne is evaluated to be 24.7, 23.8 and 42.0 W/m-K at room temperature. The previous theoretical studies with about few hundred length of graphyne are suggested here to have still ballistic thermal transport components and can give a smaller value than the macroscopic thermal conductivity of graphyne. We also calculate lattice thermal conductivity, mode-dependent phonon relaxation times, group velocities and dispersions based on density functional perturbation theory and linearized phonon Boltzmann transport equation. The calculated results are consistent with molecular dynamics calculation results, and we found that number of acetylene linkages in graphyne play significant roles on lattice thermal conductivities.

Silicide-based materials, such as Mg₂Si and MnSi_1.73, attract attention as promising candidate for thermoelectric materials because they have advantages such as high compatibility with Si process and good thermoelectric properties. SrSi₂ (α phase) is also a promising candidate as a thermoelectric material because it consists of abundant nontoxic elements and a good thermoelectric power factor (S²ρ^-1) over 1 mWm^-1K^-2 near the room temperature. It is reported that the thermoelectric performance is improved by substituting Ca or Ba for Sr. In this study; we prepared (Ca_xSr_1-x)Si₂ and (Ba_xSr_1-x)Si₂ thin films on insulating substrates and measured their thermoelectric properties. Films are deposited at nonequilibrium condition that possible to increase the solubility limit of Ba and Ca into α phase. This larger solubility of Ba and Ca expects to improve the thermoelectric properties of α phase.
(Ca_xSr_1-x)Si₂ and (Ba_xSr_1-x)Si₂ thin films were deposited on (001) Al₂O₃ substrates at various deposition temperatures by using co-sputtering method.
Constituent phases strongly depended on the deposition temperature and film composition. At 700 °C, the single phase of α phase was observed up to 26% of Ba/(Ba+Sr) ratio, while main phase of BaSi₂ structure phase (stable phase of BaSi₂) was observed over 30%. When the deposition temperature decreased to 650 °C, the Ba/(Ba+Sr) ratio of main phase of α phase expanded to 30% and BaSi₂ structure phase was observed over 35%. Note that this solubility limit was larger than that of reported value of the powder. Linear relationship between the lattice parameter and the Ba/(Ba+Sr) ratio in the film was observed for the films deposited at both temperatures. This result shows that the Ba soluble for Sr site of α-phase and its solubility limit increased as the deposited temperature decreased. When the deposition temperature father decreased to 600 °C, the metastable phase with CaSi₂ structure became a main constituent phase up to 30% of Ba/(Ba+Sr) ratio and α phase was hardly observed. In case of (Ca_xSr_1-x)Si₂ films, the CaSi₂ structure phase was the main phase except for the film with 6% of Ca/(Ca+Sr) ratio for the film deposited at 700 °C that consist of α phase. This shows that the solubility limit of Ba in α phase is larger than Ca. However, α phase was hardly observed at 600 °C.
Power factor of α-phase was beyond 700 mWm^-1K^-2 at room temperature and was not strongly depend on the film composition. This value is larger than (111) one-axis-oriented Mg₂Si films prepared by the same deposition process, maximum 130 mWm^-1K^-2 at 300 °C. The present result shows that α-phase of M_xSr_1-xSi₂ (M = Ca, Ba) is one of the promising candidates as thin film thermoelectric materials.

Fused deposition modeling (FDM) type of 3D printing is widely used for manufacturing complex shaped polymer products. Recently, the metal/polymer composite products can be made by 3D printer using metal/polymer composite filament. Now, we are planning to develop a new manufacturing process of the thermoelectric (TE) elements or modules by combining the FDM type of 3D printing and the degreasing-sintering process. In this work, we focused on the degreasing-sintering process of the mixture of Mg₂Si and polylactic acid (PLA) powders. Mg₂Si compound powder was synthesized by a liquid-solid phase reaction (LSPR) method. Mixture powder of Mg₂Si, Al and PLA was pressed and heated in a pulse discharge sintering (PDS) chamber under a vacuum in various degreasing conditions. Following the degreasing, the sintering of Mg₂Si was carried out in the same PDS chamber at various starting sintering temperatures. Sintered density, Seebeck coefficient and electrical resistivity of the consolidated Mg₂Si were measured and power factor as TE performance was estimated from the TE properties. The optimum conditions of degreasing-sintering process maximizing the sintered density and the TE performance of Al-doped Mg₂Si were investigated. Furthermore, influences of additive amount of Al on the sintered density and the TE performance of Mg₂Si fabricated by the optimized degreasing-sintering process were investigated.

Huge energy demand and pollution constitute two major problems of modern society. A large amount of normally wasted heat is present in many places around us, from human and animal bodies, natural thermal springs, combustion engines, up to numerous friction sources. The solid-state thermoelectric devices allow a direct conversion of heat into electricity without harmful contamination and those based on nanowires have demonstrated to be a promising route to reach a high thermoelectric efficiency [1,2]. Furthermore, branches attached to a nanowire may significantly modify the transport of excitations along it due to the wave interference [3]. In this work, we study the thermoelectric properties of thin nanowires with quasiperiodically placed branches by means of a real-space renormalization plus convolution method developed for the Kubo-Greenwood formula, in which tight-binding and Born models are respectively used for the calculation of electrical and lattice thermal conductivities [4]. The results show a substantial growth of the thermoelectric figure of merit (ZT) induced by the long-range quasiperiodic disorder, because it diminishes the thermal conduction of long wavelength acoustic phonons being such phonons usually not altered by local defects neither impurities [5]. A remarkable reduction of the thermal conduction can be further achieved by using low sound velocity materials [6].

This work has been partially supported by UNAM-IN115519, UNAM-IN106317 and CONACyT-252943. Computations were performed at Miztli of DGTIC, UNAM.

[1] G. J. Snyder and E. S. Toberer, Nat. Mater. 7, 105 (2008).
[2] R. Chen, J. Lee, W. Lee and D. Li, Chem. Rev. (2019) doi: 10.1021/acs.chemrev.8b00627
[3] C. Wang, C. Ramirez, F. Sanchez and V. Sanchez, Phys. Status Solidi B 252, 1370 (2015).
[4] J. E. Gonzalez, V. Sanchez and C. Wang, J. Electron. Mater. 46, 2724 (2017).
[5] F. Sanchez, C. Amador-Bedolla, V. Sanchez and C. Wang, J. Electron. Mater. (2019) doi: 10.1007/s11664-019-07298-0.
[6] J. E. Gonzalez, V. Sanchez and C. Wang, MRS Commun. 8, 248 (2018).

The aim of this study was to establish the feasibility of using magnesium silicide stannide (Mg₂Si_0.4Sn_0.6) powder in the selective laser melting process to produce thermoelectric generators (TEGs). Mg₂Si_0.4Sn_0.6 powder is a thermoelectric material optimal for high temperature applications and shows potential for use in TEGs. Currently TEGs are being manufactured using bulk material processing with multiple integration and assembly steps. Leading to decreased product efficiency, high manufacturing costs, and offering little flexibility in device geometry. Selective laser melting (SLM) an additive manufacturing technique, on the other hand, could provide a unique solution to these manufacturing challenges. However, current additive manufacturing techniques exist only for limited materials- namely polymers, ceramics, and metals- which do not include semiconductors (like thermoelectric materials). As well as require specific starting powder characteristics: desired particle size distribution, and high levels of circularity and convexity. Powder parameters such as convexity, circularity, and particle size distribution not only effect the flowability through the selective laser melting process but also the density of the final thermoelectric device. With a higher density, thermoelectric generators are more efficient and resilient to internal fractures.

Powder morphology and particle size distribution were analyzed through optical microscopy and the image analysis software FIJI. The ability to spread was assessed across 100 μm and 500 μm thick grooves using two rolling techniques, as well as a blade. Flowability was examined through the measurements of, angle of repose, angle of spatula, and compressibility. Each of these measurements follows the United States Pharmacopeia flowability standards. The results of these measurements show the potential for Mg₂Si_0.4Sn_0.6 to be integrated into SLM and paves the way for future studies looking to use Mg₂Si_0.4Sn_0.6 in thermoelectric devices.

Thermoelectric energy conversion is of great interest since most processes produce waste heat. Thus much effort has been devoted to enhancing the comparatively low conversion efficiency.
Sufficient evidence has been provided that nanostructuring can enhance the thermoelectric conversion efficiency through size effects on the thermoelectric transport. These include phonon boundary scattering at grain boundaries, surfaces or interfaces within heterostructures, quantum confinement effects and modification of the phonon dispersion. While some of these effects have been investigated in simple low-dimensional systems, three-dimensional nanostructured materials are not as well understood, even though they are more relevant with regard to applications. Moreover most of the demonstrated enhancements of the figure of merit are due to a decrease of the lattice thermal conductivity, exploiting quantum confinement has been difficult, mostly due to strict requirements with regard to feature sizes.
In this work we apply electrochemical methods together with colloidal crystal lithography to tackle these problems, since this allows the fabrication of nanostructured and yet well-defined periodic materials. This facilitates relating the thermoelectric to structural properties and thus more accurate quantification of size effects.
Additionally electroplating allows the growth of heterostructures by making use of the potential dependence of the composition in binary systems, in our case Bi-Te, which is expected to lead to enhanced phonon scattering due to the large acoustic mismatch and improved electrical properties (thermopower and mobility).
The structural and thermoelectric characterization of the obtained materials is carried out both in- and cross-plane and numeric and analytic modelling is used to determine the material properties and quantify size effects.

Thermoelectric materials, capable of directly and reversibly converting thermal energy into electricity, present a promising direction for renewable energy conversion.^[1] The effectiveness of a thermoelectric material is measured using the dimensionless figure of merit ZT, with the world record set at 2.6 for single-crystal SnSe along the through-plane direction.^[2] Realising reasonable conversion efficiencies generally requires high electrical conductivity and low thermal conductivity, with the maximum ZT of a material often limited by the strong correlation between these properties.

Despite efforts to find promising thermoelectrics containing earth abundant and non-toxic elements, bismuth telluride (Bi₂Te₃) and lead chalcogenides remain the champion materials, with the performance of oxide thermoelectrics generally lagging behind the efficiencies achieved by chalcogenide-based materials.^[3] Oxides generally present properties valuable for thermoelectric applications, including low cost, thermal and chemical stability and environmental benignity, but broadly thermoelectric performance has been hindered by the inherent high lattice thermal conductivities.^[4] However significant progress has been made through novel materials with complex crystal structures which possess lower lattice thermal conductivities than usually seen in oxides.

In this study, we calculate the electronic band structures and corresponding ZTs of a range of novel mixed-anion quaternary oxypnictides using hybrid density functional theory, including spin-orbit effects. We have identified that LaZnOP and YZnOP possess promising properties for thermoelectric applications, including low intrinsic lattice thermal conductivities predicted from third order lattice dynamics calculations, and low charge carrier effective masses. In addition, we have examined the intrinsic defect chemistry of LaZnOP, to predict optimal carrier concentrations and an insight into ideal growth conditions for maximized performance. We conclude that these materials possess ZT’s greater than many existing oxide thermoelectric materials, and have the potential to perform as high-performance thermoelectric components.

1. L. E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 321, 1457–1461 (2008)
2. L-D, Zhao, S-H. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V. P. Dravid and M. G. Kanatzidis, Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 508, 373-377 (2014)
3. G. Tan, L-D. Zhao, and M. G. Kanatzidis, Rationally designing high-performance bulk thermoelectric materials, Chemical Reviews 116, 12123–12149 (2016)
4. J. He, Y. Liu and R. Funahashi, Oxide thermoelectrics: The challenges, progress and outlook, J. Mater. Res. 26, 1762-1772 (2011)

A thermophotovoltaic (TPV) mounted on a thermoelectric generating (TEG) conceptual device is modeled analyitically and numercially with the final goal developing a Hybrid Thermophotovoltaic (HTPV-TEG) system that offers improvements on efficiencies obtained with standard Thermophotovoltaic devices. The HTPV-TEG shall use inexpensive and simple components such as commercial photovoltaic cells and absorber/emitters, thermoelectric modules, and a microchannel-based cooling device. Special attention is given to the selection of specific absorber/emitter components, placed between a radiative heat source and a receiving Photovoltaic Cell (PVC).
The goal is to predict the temperatures that will be reached by the absorbing/emitting element, the lower PVC and the TEG stacked under a controlled heat concentrating device. The objective is to determine heat input and resulting surface temperatures that will be reached under steady state conditions. Conduction, convection and radiative heat transfer will be present in the device. The resulting power generated by the PVC and TEG will be used to determine the overall device efficiency. Heat transfer simulations of conduction, natural convection and radiation of the device are performed using Computational Fluid Dynamics software in order to predict temperatures that will be reached by the test device as a function of input power, before its actual construction. Simulations allow for the selection of appropriate components and results are compared to studies by other authors.
Preliminary results show that at 410 K a maximum theoretical improvement in efficiency of 7% can be obtained. These results are compared to studies conducted by others. HTPV-TEG improved efficiency therefore exceeds the performance of common solar photovoltaic cells, and TPV devices, and may be used for applications in the housing, agriculture and mining sectors, that do not necessarily require solar radiation.

There is a pressing need for durable energy harvesting techniques that are not limited by intermittency, and capable of persistent and continuous operation over extended periods of time in a variety of environments. Our laboratory has identified ambient thermal fluctuations of various frequencies as potentially abundant, ubiquitous sources of such energy. We have developed a type of novel devices called thermal resonator for transient thermal energy harvesting, tested them in various environments and are working on integrating them with energy storage systems.^{1, 2} In this work, we present a mathematical theory for the operation and design of a thermal resonator interfaced with optimized thermal diodes that we show is capable of much higher productivity than previous possible. This work is a significant advance over resonators that we have previously introduced using high thermal effusivity materials for a dual polarity electrical output from input temperature fluctuations. Herein, we outline the optimal design of a thermal resonance device incorporating optimal thermal diodes on its external boundaries with the environment, and we show that such a configuration is able to produce single polarity electrical generation drastically exceeding the performance of previous thermal resonators by a factor of 5. We also characterize additional thermal fluctuations in the environment, expanding the scope of resonator application. Lastly, we analyze the capability of thermal diodes available in the literature to realize such an enhancement in thermal resonance performance, and we conclude that current thermal diodes in existence could produce approximately a factor of 2 improvement in thermal resonator performance. This work points out the significant role of thermal diodes in ambient energy harvesting, and outlines important directions for next generation thermal diodes, independent of advances in thermoelectric or pyroelectric materials.

Reference:
1. A. L. Cottrill, A. T. Liu, Y. Kunai, V. B. Koman, A. Kaplan, S. G. Mahajan, P. W. Liu, A. R. Toland, M. S. Strano, Nat. Commun. 2018, 9, 664.
2. A. L. Cottrill, G. Zhang, A. T. Liu, A. Bakytbekov, K. S. Silmore, V. B. Koman, A. Shamim, M. S. Strano, Appl. Energy 2019, 235, 1514.

Understanding the thermal conductance of solid-solid interfaces is important for applications like the thermal management of nanoelectronic devices and the engineering of novel thermoelectric materials. An ideal method to investigate interfacial thermal conductance with minimal assumptions is to explicitly model the atomistic interactions and their subsequent dynamics in a molecular dynamics simulation. However, prior attempts to perform such simulations often describe atomistic interactions using simple but unrealistic classical potentials that are not explicitly fit to capture thermal properties at an interface. An ideal interatomic potential used for this application would be able to replicate the interatomic force constants (IFCs) obtained from first-principles density functional theory calculations, remain stable at high temperatures, and would be quick to evaluate so as to enable large simulations over long time scales. Systems with heterointerfaces are especially challenging, as there are three distinct sets of IFCs: two for the different bulk regions surrounding the interface and one for the interface itself. We explore this challenge by developing an interatomic potential for a GaAs-Ge interface, a model III-V/IV heterointerface with minimal lattice mismatch. Our potential directly incorporates a Taylor expansion of the energy landscape of the different regions of our system. Comparisons are made with recent machine-learning inspired potentials, especially with respect to training data size and evaluation speed. Using this potential, the interface conductance modal analysis (ICMA) method is then applied to study the modal contributions to thermal conductance at the Ge-GaAs interface.

The thermoelectric community strives to find new materials with better performance by looking for higher peak zTs. However, in real devices it is often more important to have a high average zT over a broad range of temperatures in order to maximize the devices Carnot efficiency. Mg₃Sb₂-Mg₃Bi₂ alloys have recently garnered renewed attention to due their high performance at mid-range temperatures (zT »1.5 at 700K). While having a relatively high performance at 700K, these materials suffer from grain boundary resistance at lower temperatures leading to an overall lousy performance at room temperature¹. Synthesis and optimization process to mitigate these grain boundary effects have been limited due to the loss of Mg, which hinders a samples n-type dopability². Herein we demonstrate that a Mg vapor anneal can preserve a sample's n-type dopability, while allowing us to grow the grains within our material. We show that with different processing steps we can change the room temp performance of Mg₃Sb_1.49Bi_0.5Te_0.01 from zT = 0.2 to zT = 0.8.

(1) Kuo J. et al., Energy Environ. Sci., 2018, 11, 429
(2) Ohno S. et al., Joule, 2018, 2, 141–154

Recently, wearable and sustainable self-powering technologies have become increasingly important due to the development of wearable electronics such as sensors, health monitoring, and smart gears. Renewable energy sources include solar light, mechanical motion, thermal energy, and can be developed in the form of solar cells, piezoelectric and thermoelectric generators (TEGs), respectively. Among these, the wearable TEGs (WTEGs) operating based on the temperature gradient between the body and ambient temperature have generated significant amount of interest because the body heat can be used at any time and stable with time unlike other renewable energy sources.
WTEGs are fabricated either by printing TE inks directly onto flexible substrates, or by embedding rigid TE legs, made small enough not to compromise the flexibility, in flexible matrixes. Chen et al. synthesized printable thick-film TE ink by mixing Bi₂Te₃ and Sb₂Te₃ powders with epoxy resin and fabricated TE legs using a dispenser printing on a polyimide (PI) substrate. On the other hand, Wang et al. reported a vertical-type WTEG using BiTe-based rigid TE pellets embedded in polydimethylsiloxane (PDMS) between the top and bottom PI thin films. The WTEG, composed of 52 pairs of p-n legs, generated an open-circuit voltage (V_OC) of 37.2 mV and output power (V_OUT) density of 16.7 μW cm^-2 at a ΔT of 50 °C. Suarez et al. enhanced the flexibility of a WTEG by replacing stiff electrodes with eutectic gallium-indium (EGaIn) liquid metal interconnects and filling the empty space between TE legs with PDMS in a commercial BiTe-based TE modules. These WTEGs exhibited relatively high TE material performances, because they were fabricated based on proven inorganic materials such as Bi₂Te₃ and Sb₂Te₃. However, a limit in temperature difference is observed when the body heat is used as a heat source. When the ambient temperature, at which the TEGs are driven, is in the range of 15 to 25 °C, the ΔT values of the body-heat-based WTEGs are only 1.5 – 4.1 °C.
Previously, we first demonstrated a wearable solar TEG (WSTEG) with a solar absorber in WTEG and reported a high ΔT of 33.4 °C. The TE legs were fabricated by dispenser-printing a BiTe-based ink, prepared by dissolving mechanically alloyed BiTe powders and Sb2Te3 sintering additive in glycerol. However, despite the high temperature difference, high power density was not obatined due to low thermoelectric properties of Printed TE leg compared to the rigid TE legs. Herein, we proposed a dual-mode wearable thermoelectric that can obtain both body heat and solar energy by placing a solar absorber on the bottom surface of a bulk TEG. The WTEG can improve the temperature difference compared to conventional TEGs using only the body heat. In addition, a PDMS was filled between the rigid TE legs obtaining both flexibility and high value of power density.

In this research, we report the growth of nanoscale multilayered thermoelectric thin films and fabrication of integrated thermoelectric devices for high-efficiency energy conversion and solid-state cooling. Nanoscale multilayered thin films such as Sb/Sb₂Te₃ and Te/Bi₂Te₃ thin films were grown using the e-beam evaporation with DC bias on the substrate. The multilayered films were prepared to have 150 to 300 layers, where each layer was about 5 nm thick. The effects of DC substrate bias on the growth of films such as the crystalline structures in the films were studied. Integrated thermoelectric devices with a high density of thermoelectric elements were fabricated with the nanoscale multilayered thin films using the clean room-based microfabrication techniques such as UV lithography. X-ray diffraction and reflection and high-resolution tunneling electron micrograph (HR-TEM) were used to analyzed the e-beam-grown nanoscale multilayered thin films. SEM was used to image and analyze the fabricated devices. The thermoelectric characteristics of the fabricated devices such as the open-circuit voltage and output power were measured and analyzed. The effects of cooling and annealing on the nanoscale multilayered thin films and the integrated thermoelectric devices were studied. Highly-efficient thermoelectric thin-film materials and integrated devices will be demonstrated and reported.

Interlayer interaction can introduce profound effects on thermal conductivity, which is strongly dependent on the properties of the coupling itself. Here we develop a model showing the ratio between the high-order and harmonic terms plays the dominate role in the phonon scattering behavior and demonstrated this relationship by carrying out comparison between the layered materials MoS₂ and PtS₂, using first principles calculation and solving phonon Boltzmann transportation equations. Moreover, series of materials with BiCuSeO structure are also analyzed, where geometry factor was constructed to indicate the delicate variance of the interlayer distance of these structure. These understanding can be used as guidelines for designing materials of particular phonon properties, for example low thermal conductivity.

Photovoltaic electrical energy has experimented a significant increase in the last decade motivated mostly for the reduction of the PVC costs, the emergence of new PVC technology and increase amount of new large projects, particularly in areas with large levels of irradiance. However, despite the above factors, the worldwide electricity generated by photovoltaic technologies continue being low compared to other renewable technologies such as the ones based on water, biomass, and wind and with traditional fossil-based technologies. One of the main reasons that can explain this low participation is the low conversion efficiency of traditional PV technologies, which ranges from 10% to 25%. This low participation has pushed to developer and researchers to create novel and more efficient photovoltaic technologies that combine multi-junction solar cells with high solar concentration to achieve a conversion efficiency equivalent to 40%. By using a concentrated photovoltaic (CPV) system, it is possible to reduce the use of expensive and not easy disposable semiconductor materials. However, since only a fraction of the concentrated direct normal irradiance (DNI) is converted into electricity, a cooling system is completely necessary to avoid solar cell overheating, which reduces the solar cell efficiency and accelerates the solar cell degradation. Additionally, we consider integrating a thermoelectric generator (TEG) to produce extra electrical energy by mean of Seebeck and Peltier effect that can convert part of the solar cell heat into electricity. Several researchers have proposed and evaluated the use of microchannel heat sinks as a cooling system solution for CPVs, due to its high thermal performance at low Reynolds numbers and small heat transfer area, low weight, and its possible incorporation to the solar cell system structure ((Leonardo Micheli et al. 2013, M Di Capua H et al. 2018, Radwan et al. 2019).
The present work seeks to evaluate a CPV-TEG hybrid system cooled by a microchannel heat sink (MCHS) under variable DNI, ambient temperature, and wind velocity by developing an analytical model that is numerically simulated and solved. A time-dependent thermodynamic model has been developed to analyze the behavior of the CPV-TEG-MCHS system considering both, clear and cloudy day. The principal objective of our current work is to evaluate the thermal inertia of the hybrid system under variable surrounding conditions, as well as the coolant flow rate that allows achieving the maximum conversion efficiency and safe solar cell temperature. The system evaluated is composed of a concentration photovoltaic system that uses a Fresnel lens to concentrate the solar radiation over a multi-junction (GaInP/GeAs/Ge) solar cell with a concentration factor equivalent to 1000x. In the bottom solar cell face, a TEG is integrated ann an MCHS with several microchannels of Wc and Hc, the width and height, respectively, in a parallel flow configuration is incorporated in the cold side of the TEG. The considered PV cell areas in the analyzes are 3x3 mm2 and 10x10 mm2. The MCHS material is copper and the coolant is deionized demineralized single phase water.

Keywords: CPV, cooling, microchannel

References:

Leonardo Micheli, Tapas Mallick. Opportunities and challenges in micro- and nanotechnologies for concentrating photovoltaic cooling: A Review. Renewable and Sustainable Energy Reviews. 2013
Mario Di Capua H, Rodrigo Escobar,A.J. Diaz, Amador M. Guzmán. Enhancement of the cooling capability of a high concentration photovoltaic system using microchannels with forward triangular ribs on sidewalls. Applied Energy. 2018
Ali Radwan, ShinichiOokawara, Mahmoud Ahmed.Thermal management of concentrator photovoltaic systems using two-phase flow boiling in double-layer microchannel heat sinks. Applied Energy.2019

Efficient silicon-based thermoelectric materials compatible with the existing electronic and photovoltaic technologies would provide excellent energy harvesting opportunities. Although the high bulk thermal conductivity of silicon has been successfully reduced during the past decade through nanostructures such as nanowires, nanograins, and nanomeshes, the fundamental mechanisms that reduce the thermal conductivity through nanometer-sized inclusions of other materials still require further investigation. Here, we report the thermal conductivity of silicon nanowires with epitaxial nickel silicide nanoinclusions (Si-NiSi₂ NWs) and develop a model to explain the observed thermal conductivity trend. We synthesize silicon nanowires using metal-assisted chemical etching and deposit nickel nanoparticles on their surface with electroless deposition. Annealing the nanowires allows the nickel to diffuse into the silicon, resulting in silicon nanowires embedded with metallic nickel silicide nanoinclusions grown in the diffusion plane [1 1 1], which results in diamond or rectangular-shaped inclusions. We measure the thermal conductivity of single nanowires using suspended micro-membranes in a cryo-chamber from 40 to 300 K in high vacuum. We report the thermal conductivity of two Si-NiSi₂ NWs with 30% inclusion density and diameters of 120 and 170 nm, respectively, which increases with temperature and saturates at 150 K with ~15 Wm^-1K^-1 with little difference between both wires. The consistent thermal conductivity among Si-NiSi₂ NWs wires with the same inclusion density differs from the reports on the thermal conductivity of silicon nanowires synthesized using metal-assisted chemical etching depending on their surface roughness, which can have large variability even within the same batch. This indicates that the nanoinclusions consistently decrease the phonon mean free path and dominate thermal transport. To better understand the underlying mechanisms that maximize the thermal conductivity reduction through diamond and rectangular nanoinclusions, we integrate Monte Carlo ray-tracing simulations with the kinetic theory and Landauer formalism, including the Diffuse Mismatch Model to calculate the thermal boundary resistance and the two-temperature model to estimate the electron-phonon coupling. We demonstrate that by incorporating nanoinclusions into a film with high interfacial density, the matrix phonon mean free path and the film’s thermal conductivity can be minimized without compromising the electrical properties. Additionally, metal/silicon junctions produce chemically stable Schottky barriers at the nanoinclusion interface that can filter low energy electrons and increase the Seebeck coefficient without damaging the electrical conductivity. The results of this work expand the understanding of transport phenomena in complex nanoengineered materials and open promising optimization paths for silicon-based thermoelectric materials.

This talk will discuss our recent work to simulate electron and phonon thermoelectric transport in complex materials and across interfaces based on the density-functional theory and atomic Green’s function. One example of the complex material is Mg₂Sb₃, which has been recently reported to have high figure of merit. We compare the charge defect scattering due to different dopants using first principle calculation and find weak impact of defect on mobility. Instead, the intrinsic carrier-phonon scattering is the major scattering mechanism. Only specific polar optical phonon modes make dominant contribution to the scattering rates, which we attribute to the atomic structure and electrostatic interactions in Mg₂Sb₃. For transport across interfaces, an important question to ask is if electron and phonon scatterings conserve lateral momentum, i.e., if they experience specular or diffuse scattering, in the presence of interfacial atomic mixing. We employ atomic Green’s function to simulate electron and phonon transmission across interfaces for SiGe interface with mode-by-mode resolution. We find the angular dependent interfacial transport for electron and phonon are behaving differently, which provides new opportunities for optimizing thermoelectric performance. This work is supported by DARPA MATRIX program (Grant No. HR0011-16-2-0041).

Thermoelectric materials (TEs) could play an important role in future energy management
through environmentally sound cooling and power generation, e.g., converting waste heat into
electricity. Efficient TEs inhibit the propagation of heat (low thermal conductivity, κ) but
conduct electricity well (high power factor, PF). Although κ in a given material can be reduced
via alloying and nanostructuring, identifying materials with intrinsically low κ is still needed.
It has already been known that soft phonon modes due to weakly bonded atoms and s² lone-
pair electron are common to materials with low-κ. Here, we predict a series of new materials
which are weakly bonded systems with same constituent elements but different stoichiometry
either with s² lone-pair electron or high mass density. Due to giant phonon anharmonicity and
low phonon group velocities, they offer extremely low κ (0.3-0.6 W/mK) at 300K approaching
to those found in the amorphous/disordered regime. In addition to low-κ, high Seebeck
coefficients and electrical conductivities in these materials may provide a new opportunity for
designing high-efficiency thermoelectrics at room temperature.

For the next revolution in thermoelectrics, development of new physics as well as new materials needs to go hand-in-hand. In this talk, I will introduce new perspectives such as correlated electron physics, wave effects in phonons as well as opportunities for new inorganic-organic (hybrid) materials¹. Then, I will introduce specifically the case of CuTe:PEDOT thin films and using this as a test case, describe the design principles for creating the next generation of hybrid materials². In addition, I will describe how data-driven approaches can augment our knowledge: used in conjunction with the right material and transport descriptors, these can prove prescient in predicting new TE materials. I will end with a specific example of how we’ve used a Crystal Graph Convolutional Neural Network (CG-CNN) with training data from Materials Project to learn about TE properties. In addition, we use high-throughput, high-fidelity DFT calculations with the Electron Phonon Averaging (EPA) approximation to calculate carrier relaxation times, we propose that the transport effective mass remains an effective single descriptor that can guide inorganic TE material screening.

¹ JJ Urban, AK Menon, Z Tian, A Jain, K Hippalgaonkar, Journal of Applied Physics 125, 180902 (2019)
² P Kumar, EW Zaia, E Yildirim, DVM Repaka, SW Yang, JJ Urban, K Hippalgaonkar, Nature Communications 9, 5347 (2019)

With the maturity of sophisticated, high throughput computational tools and the advent of machine learning applications in materials science, identifying the appropriate fundamental level descriptors has never been more relevant and pressing in data-driven materials discovery domain. In this work, we first present the inertial effective mass as an important descriptor in thermoelectric transport. The conclusion was obtained by analyzing data from 1617 compounds of all crystal stuctures mined from the materialsproject.org. Data was analyzed using polycrystalline averaging Seebeck, effective mass, as well as electrical conductivity. In addition, constant relaxation time approximation was used in all data analysis, which is known not to necessarily reflect experimentally measured compounds.
Subsequently, we present a state-of-the-art approach of using electron-phonon averaging (EPA) to obtain contribution of all possible scattering mechanisms to the relaxation time and challenge the initial assumption of constant relaxation time. More interestingly, we sought to evaluate the veracity of insights and conclusions drawn under constant relaxation time approximation from the EPA point of view and indeed confirm that effective mass is an important descriptor. Further insights gained from analysis of scattering times provide us additional insight towards prediction of new thermoelectric compounds.

The two-dimensional layered materials with interlayer van der Waals (vdWs) bonding such as graphene have attracted tremendous interests in scientific community. Recently, layered materials composed of transition metal trichalcogenides with strong in-plane anisotropy, which show unusual properties based on theory, have been successfully synthesized so that their properties can be explored through experiment. For instance, due to theoretically calculated ZT value (3.1) of monolayer titanium trisulfide along y direction at moderate carrier concentration, layered titanium trisulfide nanoribbons with intralayer covalent bonding and interlayer vdWs bonding burst extensive research, which inspires us to investigate the lattice thermal conductivity of the nanoribbon and the size effects on the thermal conductivity because lowering the thermal conductivity can increase the ZT value significantly. In order to better understand the thermal transport in the titanium trisulfide nanoribbon, the theoretical calculation combined with the experiment are adopted. Through solving the phonon Boltzmann transport equation (implemented in ShengBTE package) with the first-principles second- and third-order force constant (implemented in VASP package), the thermal properties of titanium trisulfide are obtained. The calculation results reveal that the thermal conductivity monotonously decreases with the number of titanium trisulfide layers increasing from one to three and displays a strong in-plane anisotropy as the temperature range from 40 to 500 K. For example, the thermal conductivity along y direction is 2.7 times larger than x direction at room temperature. Compared to layered graphite, due to the weaker interlayer coupling strength, the out-of-plane thermal conductivity (around 1.1 W/m-K) at the room temperature is 5 times lower.
Based on the results of theoretical calculation, we synthesized the titanium trisulfide nanoribbons with different thicknesses and measured the thermal conductivity using a suspended micro-thermometry from 20 to 300 K. The experimental results show that the in-plane thermal conductivity of titanium trisulfide nanoribbons decreases monotonously as the thickness increases, which is same as the theoretical calculation. Interestingly, the thermal conductivity keeps decreasing as the thickness is smaller than 272 nm, which is quite different from multi-layer graphene. In detail, the thermal conductivity decreases rapidly as the thickness is below 60 nm and becomes slowly beyond 150 nm. Due to the computational cost, 4 layers or thicker titanium trisulfide cannot be calculated. The giant large layer thickness dependent in-plane thermal conductivity is surprising and confusing. We speculated that this phenomenon may stem from weak interlayer coupling strength and unique atomic structure, which needs further investigation.

Thermoelectric semiconductors directly convert heat into electricity. These solid-state devices have been use reliably in space for over 40 years without maintenance. Temperature gradients and heat flow are omnipresent in natural and human-made settings and offer the opportunity to scavenge energy from the environment recovering waste heat from industry or replacing the need for batteries in remote sensor networks or mobile devices. Particularly attractive is the ability to generate electricity from body heat that could power medical devices or implants, personal wireless networks or other consumer devices. This talk will discuss the design principles for thermoelectric generators using a generalized electrical transport model combined with an effective thermal conductivity approach [1, 2]. Such design principles provide good estimates of the power that could be produced and the size and complexity of the thermoelectric generator that would be required.
In addition these design principles can guide the search and optimization new thermoelectric materials. In organic and polymer semiconductors the optimum doping is identified and the peak zT for a given class of materials can be determined.
Also, a complete system design shows the misconconception that power factor rather than zT should be optimized for a TE generator. The power factor misconception also leads to misleading strategies for optimization and discovery of new TE materials.
The materials requirements for flexibility, leading to a flexibility figure-of-merit will be discussed.
[1] L. Baranowski, GJS, Eric S. Toberer “Effective Thermal Conductivity in Thermoelectric Materials” J. Applied Physics113, 204904 (2013)
[2] S. Kang, GJS “Charge Transport Model for Conducting Polymers” Nature Materials 16, 252 (2017)

Nanopatterned holey silicon materials follow the concept of “phonon glass and electron crystal” and possess potential for thermoelectric device applications as it is also compatible with the well-developed Si-based industry. We fabricate a boron-implanted holey Si thin-film device, and study the thermoelectric transport properties. A novel method to measure in-plane thermal conductivity of thin film materials using thermal reflectance imaging is successfully conducted. Subsequently, we fabricate a hybrid F4TCNQ-silicon, which utilizes a non-destructive doping mechanism by charge transfer at the heterointerface. We conduct transport measurements and validate that organic
F4TCNQ molecules effectively p-type dope the Si surface, resulting in power factor enhancement. Outcomes of this study serve as proof of concept in designing 3D hybrid structures with closely packed interfaces towards an efficient thermoelectric device, using organic-inorganic transfer doping mechanism.

Hybrid materials consisting of inorganic nanostructures embedded in conducting polymer matrices have emerged as promising systems for room temperature thermoelectric applications. They are attractive due to their intrinsic low thermal conductivities, the ability to engineer interfaces for energy filtering effects and phonon scattering, and their ability to take advantage of high-throughput and solution processable manufacturing. Most polymer and hybrid materials reported in the literature have been p-type, owing to difficulties in n-type doping of conducting polymers in conjunction with the nature of the applied nanocrystals. This has resulted in a strong drive to develop new n-type materials, since both are necessary for module development. Here we explore our recent developments in the synthesis chalcogenide nanowires encapsulated in poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) that are used as templates for the synthesis of Ag_2-xE (where E=Te, Se) via topotactic chemical transformation processes. This synthetic method allows us to engineer the composition of our hybrids, whereby we are able to directly influence the thermoelectric properties, including the production of both p-type and n-type materials from the same parent material. We detail the structural and morphological development of our materials with changing stoichiometry during aqueous based synthesis via X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) and relate this to their thermoelectric performance. We take the example of one of our n-type Ag₂Te/PEDOT:PSS hybrid materials and further manipulate its performance via post-deposition (de)doping schemes, whereby the PEDOT:PSS is reduced by treatment with Tetrakis(dimethylamino)ethylene (TDAE). This process is followed by UV-Vis spectroscopy and electron paramagnetic resonance spectroscopy (EPR) to detail how this (de)doping process effects the charge carrying species. Control of the competing charge carrier concentration allows further improvement in the n-type hybrid device performance.

Porous modification is general approach to endowing the rigid inorganic thermoelectric (TE) materials with considerable flexibility, however, by which the TE performances are severely sacrificed. Thus, there remains a struggling against the trade-off between the TE properties and flexibility. Herein, we develop a novel strategy to combine the Bi₂Te₃ thick film with the ubiquitous cellulose-fiber (CF) print-paper via unbalanced magnetron sputtering technique (ACS Appl. Mater. Interfaces 10, 1743, 2018). The Bi₂Te₃/CF TE composites with tailorable shapes and dimensions were successfully obtained by our approach, which have reasonable internal resistance as components of TE devices with in-plane configurations. Owing to the hierarchical nano-micro porous microstructures and the excellent fraction resistance of the Bi₂Te₃/CFs constructions, the prepared TE composites with Bi₂Te₃ nominal deposition thickness of tens of micrometers exhibit mechanically reliable flexibility, of which the bending deformation radius could be as small as a few millimeters. Meanwhile, the thermal conductivity was remarkably reduced, due to the phonon-nanopore scattering effects. Enhanced Seebeck coefficients were observed comparing with the dense films and the power factors of ~250 to 400 μW/mK² were obtained for the composites from room temperature (RT) to 473 K, which can be further improved by optimizing the carrier concentrations. As a result, the TE figure of merit, ZT, is as high as ~0.38 at 473K.
Moreover, Fig. 1 depicts the photograph, structure, and working principle diagram of the flexible TE device consisting of 12 pairs of p- and n-type legs integrated on the double sides of CF paper sheet employing a laser beam micro-cutting system, which displays great potential as flexible TE device for thermal energy harvesting. When the light bulb works for about a few minutes, the temperature of the central side rises quickly, the temperature difference between the center and periphery is about 50 K from the infrared image, and the TE device generates an output voltage of 0.144 V. This kind of flexible TE device can realize the collection and recovery of heat energy in daily life to power flexible electronics, such as wearable devices and environmental monitors, which also promotes the development of paper-based and thin-film electronics.

Since the 1960’s, NASA has implemented Radioisotope Thermoelectric Generators (RTGs) to supply energy for many of its satellites and space probes. Similar generators for industrial and automotive waste heat recovery have been proposed and many new thermoelectric generator materials have been investigated. Nonetheless, mechanical integrity for the full operational life of the thermoelectric modules, which can extend to decades, has not been given much consideration in such applications. Among many contributors, clamping forces, vibrational stresses, and thermally-induced mismatch stresses may combine to give stress levels high enough to deform the thermoelectric module by creep, thus diminishing its useful lifetime. To date, few thermoelectric materials have been tested for creep, including Bi₂Te₃, PbTe, Mg_1.96Al_0.04Si_0.97Bi_0.03, and TAGS-85.

In the present talk, we show the case of the compressive creep deformation behavior of two thermoelectric materials; half-Heusler n-type Hf_0.3Zr_0.7NiSn_0.98Sb_0.02 and n-type skutterudite (Yb-CoSb₃) alloys, at 500-705 ^oC. When subjected to uniaxial compressive stresses at 600^oC, the n-type half-Heusler alloy Hf_0.3Zr_0.7NiSn_0.98Sb_0.02 exhibits Newtonian flow, consistent with diffusional creep of its fine-grain (1-7 μm) microstructure achieved via spark-plasma sintering of powders. In addition to its promising thermoelectric performance at high temperatures, this alloy sustains very high compressive stresses at 600^oC (from 21 to 359 MPa, for ~ 23 days) without macroscopic failure. However, the brittle nature of the alloy leads to the formation of numerous cracks at such high stresses, which in turn deteriorate the thermoelectric performance. A more realistic creep stress range (15-46 MPa, for ~ 4 days) preserves the high thermoelectric figure of merit zT. Among thermoelectric materials mechanically creep-tested to date, the ZrNiSn-based Half Heusler alloy has the highest creep resistance. On the other side, the n-type skutterudite (Yb-CoSb₃) alloy showed acceptable creep resistance under protected environment.

Given their high melting temperature, stiffness, and creep resistance, half-Heusler alloys appear uniquely suited for long-term thermoelectric applications where high stresses and temperatures are present.

Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions as well as terrestrial applications where unattended operation in remote locations is required. The discovery of new, more efficient materials, and the development of practical, robust elements and device technologies are the key to improving existing space power system performance and versatility and expanding the use of thermoelectrics into efficient, cost-effective terrestrial applications using medium to high grade heat sources.
We present an overview of NASA-funded collaborative research efforts to identify advanced bulk thermoelectric materials, capable of quadrupling current state-of-practice average ZT values over the available operating temperature range of 1275 K to 475 K, through the exploration of structurally complex compounds allowing for a wide range of chemical tuning and the possibility of forming stable nano- and micro-scale composites. Materials- and device-level experimental performance validation accomplished to date, technical challenges, progress and plans for technology infusion into future thermoelectric power systems are discussed.

Energy harvesting and thermal management are required for applications in the internet-of-things, autarkic sensors, or highly integrated electronic devices. Thermoelectric generators and coolers are promising technologies for localized energy harvesting and thermal management. These devices are curretnly well optimized for near-room temperature operation at the macroscopic scale. However, the high integration density of today’s most significant applications requires an increasing degree of miniaturization. Using finite element calculations, we gained insight on the design guidelines for micro thermoelectric devices with realistic material properties, and with concurring size and geometry constraints [1]. Understanding the interplay between thermal and electrical heat fluxes at the micro scale allowed our group to fabricate micro thermoelectric devices for thermal management, efficient enough that can be integrated in electronic packages [2]. Our relevant findings about the design of micro thermoelectric devices will be presented.

[1] D. A. Lara Ramos et al. Adv. Sustainable Syst. 1800093 (2019)
[2] G. Li et al. Nature Electronics1, 555 (2018).

The detection of temperature and dynamics of working fluids, including water, is important for experimental and industrial applications, such as detection of precise changes in fluid environment, monitoring of drainage or cooling water at factory facilities, micro / nano flow analysis. However, due to the spatial limitations or complex geometry of fluid channel interfaces, it is important to accurately detect multiple parameters that are optimally integrated into the operating platform. Also, many fluid sensors necessarily disrupt the natural flow within the platform, and even though the installation of the sensing device should not affect the intrinsic properties of the target element in fluid applications. In this work, we develop a flexible, attachable dual-output sensor for fluid temperature and transfer dynamics based on structural design of thermoelectric materials (SDTM). The SDTM flexible substrates using PET was developed to detect real-time changes in temperature and peak voltage reflecting fluid dynamics. Simple sputtering deposition of Bi2Te3 through a patterning mask has allowed the fabrication of dual-power sensors which do not affect the fluid flow shape. When the working fluid contacts the surface, thermoelectric structure pattern induces continuous double thermal wave and time interval without disturbing the natural flow of working fluid. The raw voltage signal induced by thermal gradient wave provides the magnitude of the first peak voltage and the duration between the two peaks, which show the real time temperature and the moving velocity of the working fluid. Furthermore, as a demonstration of an expandable platform using SDTM, a scalable sensor array comprising multiple SDTMs was fabricated as a large-area device for sensing a fluid temperature flow dynamic sensing device. Its performance with respect to sensing the fluctuation of working fluid temperature and kinematics was verified using a 4 × 4 SDTM array. As a result, the new methodology using SDTM can contribute to the development of entirely new technologies for next-generation sensors that require advanced features such as multi-element detection and a variety of integrated flexible and removable features.

Bi₂Te₃-based p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se₃ have been the only materials used for thermoelectric cooling for decades. Even though the progress on advancing the thermoelectric figure-of-merit (ZT) has been significant especially the materials with peak ZT at high temperatures, materials with high enough ZT around room temperature are very rare. Up to now, in addition to Bi₂Te₃-based ones, the only reported is p-type MgAgSb with ZT of ~0.8 at room temperature. There is no report on any n-type material exhibiting ZT similar to that of the n-type Bi₂Te_2.7Se₃. In this talk, I will present a new n-type material that has a ZT of ~0.7 at room temperature, which is comparable to that of n-type Bi₂Te_2.7Se₃. The cooling performance of a unicouple consisting of the new n-type material and the p-type Bi_0.5Sb_1.5Te₃ is also in par with the commercial legs consisting of the p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se₃.

Thermoelectric effects play a significant role in phase change memory (PCM) and Ovonic threshold switch (OTS) devices used as access devices in PCM cells [1]. Typical PCM cells are two-terminal nanometer-scale resistive memory devices which can be reversibly switched between a low-resistance crystalline and a high-resistance amorphous state via nanosecond electrical pulses. Amorphization in PCM devices is achieved by self-heating the phase change material, typically a chalcogenide, close to its melting temperature, followed by a sudden quench. Crystallization is achieved by self-heating the phase change material above its glass-transition temperature. OTS devices typically use amorphous chalcogenides that do not crystallize during normal device operation.
The local current densities in PCM and OTS devices can reach 10⁸ A / cm², giving rise to local temperatures in excess of 900 K and thermal gradients as high as 50 K / nm; hence, Peltier effects at material interfaces and Thomson heating within the active area are substantial. Accurate modeling of thermoelectric effects requires knowledge of temperature dependent electrical resistivity and Seebeck coefficients of these materials. These parameters can be measured at low temperatures on as-deposited amorphous films [2]. However, melt-quenched amorphous materials’ parameters tend to differ from as-deposited films, and PCM materials rapidly crystallize at higher temperatures. High-speed metastable electrical resistivity measurements can be performed on nanoscale devices using electrical pulses to uniformly amorphize devices up to approximately 200 nm in diameter [3], but larger devices tend to form current filaments and hence do not amorphize uniformly. On the other hand, measurement of the Seebeck coefficient (S), which is vital to understanding thermoelectric effects, is very difficult at small scales.
In this work, we model the Seebeck coefficient for metastable amorphous Ge₂Sb₂Te₅ (aGST) based on high-speed experimental results [3] and an energy band diagram proposed by Muneer et. al. [4] from 300-850 K [3], [5], and we analyze thermoelectric effects in PCM cells using finite element phase change device simulations [6]–[9]. We calculate the electron and hole Seebeck contributions S_e and S_h in metastable aGST with the band diagram in [4] and find that S_h is similar in both magnitude and slope to S measurements on as-deposited aGST thin films in 300-400 K range [5], [10], consistent with the unipolar conduction assumed when deriving the band gap in [4]. We use S_h as the Seebeck coefficient in metastable aGST and simulate reset and set operations in a PCM double mushroom cell and find that the Seebeck differential between crystalline and amorphous GST results in significant heating/cooling at amorphous-crystalline junctions during both crystallization (set) and melting (reset).
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[2] L. Adnane et al., J. Appl. Phys., 122, 12, p. 125104, Sep. (2017) DOI: 10.1063/1.4996218.
[3] F. Dirisaglik et al., Nanoscale, 7, 40, pp. 16625–16630, Oct. (2015) DOI: 10.1039/C5NR05512A.
[4] S. Muneer et al., AIP Adv., 8, 6, (2018) DOI: 10.1063/1.5035085.
[5] L. Adnane et al., APS Meet., (2012).
[6] Z. Woods and A. Gokirmak, IEEE Trans. Electron Devices, 64, 11, pp. 4466–4471, Nov. (2017) DOI: 10.1109/TED.2017.2745506.
[7] Z. Woods et al., IEEE Trans. Electron Devices, 64, 11, pp. 4472–4478, Nov. (2017) DOI: 10.1109/TED.2017.2745500.
[8] J. Scoggin et al., Appl. Phys. Lett., 112, 19, p. 193502, (2018) DOI: 10.1063/1.5025331.
[9] J. Scoggin et al., Appl. Phys. Lett., 114, 4, pp. 1–6, Oct. (2019) DOI: 10.1063/1.5067397.
[10] S. A. Baily et al., Solid State Commun., 139, 4, pp. 161–164, (2006) DOI: 10.1016/j.ssc.2006.05.031.

Flexible materials with high thermoelectric performance have attracted growing interest recently. By intercalating organic molecules into the van der Waals gap of varieties of inorganic two dimensional layered compounds, we developed a large family of thermoelectric materials with excellent mechanical flexibility. The inorganic and organic monolayers are alternatively stacked to form an inorganic/organic superlattice, in which the high electronic transport properties of the inorganic component has been maintained and the thermal conductivity was dramatically suppressed by the organic components, finally resulting in boosted ZT value. We have demonstrated this idea in several two dimensional host materials, including TiS₂, Bi₂Se₃ and TaS₂, etc. The abundant choice of the organic molecules also brings new opportunities to optimize the thermoelectric performance, such as the dielectric screening effect and the quantum confinement effect. We finally developed a solution-processed strategy to fabricate large area flexible thermoelectric foil based on this inorganic/organic superlattice, which enables easy integration into energy-harvesting electronic devices.

Recently, our group developed a Bi–Te-based TEG device by using conventional semiconductor packaging technology. In that device, Ni/Au was used as the diffusion barrier, and Sn–Ag-based solder was used as the bonding material. Although Bi–Te-based TE materials can be used up to approximately 300°C, the maximum operating temperature of the device was limited to approximately 150°C owing to the low melting point of the Sn–Ag-based solder (~150°C) that bonds Bi–Te-based TE materials and Cu electrodes. Therefore, in order to apply higher temperature differences to the device, in this study, we attempted to develop a bonding interface resistant to heat up to 250°C. Ni/Au was retained as the diffusion barrier layer, but Ag paste was chosen as the bonding material, instead of the Sn–Ag-based solder, because it has many advantages such as high-temperature stability (melting point ~ 960°C), printability, low electrical resistivity, high thermal conductivity (150 Wm^-1K^-1), and a temperature requirement of less than 300°C for sintering-bonding. The Ni/Au diffusion barrier layer was prepared using two different deposition processes, sputtering and electroplating, and the element diffusion behaviors of these two bonding interfaces at 250°C were investigated.
In the bonded sample with sputtered Ni/Au, the Cu electrode diffused violently into the chip and formed a Cu–Te-rich phase. On the other hand, in the bonded sample with electroplated Ni/Au, the diffusion of Cu was blocked, but a NiTe phase formed on the chip side. The above results demonstrate that, with sputtered Ni/Au, grain-boundary diffusion is predominant and Cu electrodes diffused along the grain boundaries, while with electroplated Ni/Au, self-diffusion is predominant because of the instability of the monocrystalline–amorphous phase. These results suggest that the highly crystalline Ni layer, which has dense grain boundaries and no pores, is a highly effective diffusion barrier.