Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S47: Thermoelectrics -- Defects and NanostructuresFocus
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Sponsoring Units: DMP Chair: Bolin Liao, University of California, Santa Barbara Room: BCEC 213 |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S47.00001: Thermoelectric conversion enhanced by charged defects on a topological insulator surface Takahiro Chiba, Saburo Takahashi Thermoelectric (TE) conversion from thermal energy into electric energy has been attracting attention as a renewable energy harvesting. The conversion efficiency is evaluated by the dimensionless figure of merit ZT and for practical use ZT ≥ 1 (conversion efficiency is 10%) is required. Bismuth chalcogenide is not only known as a representative material with high ZT but also has received attention as a topological insulator (TI) phase in recent years. Here we theoretically study the thermoelectric effect on the surface of TIs that are ionically disordered as the consequence of hypothetical doping for systematic control of the Fermi levels. Coulomb type long range potential is introduced as the ionic disorders. Based on Boltzmann transport theory at finite temperature, we calculate the TE coefficient and ZT. As a result, we find that ZT can achieve substantially high values even for the thermal phonon. Our theory may help us understand recent TE transport measurements in TI thin films and could make an ionically disordered TI a promising material for TE conversion technology. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S47.00002: Density functional theory calculations of intrinsic defects in PbTe Sungjin Park, Min Ho Lee, Byungki Ryu PbTe is very noteworthy thermoelectric material working at intermediate temperatures. Most investigations of PbTe were focused on PbTe-based alloys to control band convergence, formation of resonant level, and nanostructuring. However, the intrinsic-defect nature of PbTe is still not well understood yet. The semiconductor type of PbTe-related alloys is known to be determined by Pb concentration with respect to Te concentration. In binary, Pb-rich or Te-rich PbTe typically exhibits n-type or p-type behavior, respectively. Our experiments showed that 10% excess-Pb doping merely changes the lattice parameter (<+0.3%). To understand such atomic and electrical behaviors in off-stoichiometric PbTe, we theoretically investigated the formation of intrinsic defects in PbTe by performing first-principle density functional theory calculations. We used PAW method and GGA-PBE, which are implemented in VASP code. The defect formation energies of various charged states were computed on the 128-atom FCC supercell and a 3x3x3 k-point mesh. We suggest that the formation of Pb interstitial or Te vacancy might be responsible for the off-stoichiometry in Pb-rich PbTe, while the formation of Pb vacancy or Te vacancy might be responsible in Te-rich PbTe. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S47.00003: Effect of intrinsic defects on the thermal conductivity of PbTe from classical molecular dynamics simulations Javier Fernandez Troncoso, Pablo Aguado-Puente Lead telluride (PbTe) is a reference high-performance thermoelectric which can exhibit low values of thermal conductivity in the presence of intrinsic defects. Here, we present an optimized Buckingham potential that provides an improved description of this material, especially for the lattice thermal conductivity. The latter is computed using two different methods: Green-Kubo method and direct thermostatting. Comparison with other classical force fields proposed in the literature shows substantial improvement. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S47.00004: Desirable dopants for thermoelectrics Jiawei Zhou, Qichen Song, Te-Huan Liu, Jun Mao, Hangtian Zhu, Ran He, Wuyang Ren, Zihang Liu, Zhifeng Ren, Gang Chen Doping is a common technique to achieve good electrical conduction in semiconductors, and is often used to optimize the performance of thermoelectric materials. While its effect on the electron mobility has been widely appreciated, the understandings still mostly rely on simplified models, which often neglect the chemical details of the dopants. On the other hand, experimental evidence has suggested that dopants with different chemical nature can behave quite differently. The lack of theory to explain such observations, however, has impeded our understanding for controlling dopants, and defects in general, for thermoelectric materials. Recently we have developed a first principles approach to quantitatively evaluate the the effects of dopants on the electron transport. We will discuss how the bonding environment of the atoms - a previously overlooked aspect - can significantly affect the electron transport. The study potentially provides guidelines for finding efficient dopants for thermoelectric materials. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S47.00005: Dopability on Complex Diamond-Like Semiconductors: new candidates for thermoelectric applications. Lidia Gomes, Jiaxing Qu, Brenden Ortiz, Eric Toberer, Elif Ertekin The diamond-like semiconductors (DLS) have recently garnered interest for the potential use as thermoelectric materials. DLS share the diamond structure, forming a chemically rich family of binary, ternary and quaternary compounds. To be a good thermoelectric, however, a material must be sufficiently dopable and the defect chemistry of this group is currently not well characterized. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S47.00006: Hierarchical design of nanomaterials for improving the power factor Neophytos Neophytou, Laura de Sousa Oliveira, Vassilios Vargiamidis Hierarchically nanostructured thermoelectric materials, where nanoinclusions are inserted at various length scales (grain boundaries, embedded quantum dots, atomistic defects), have shown the potential to provide much larger thermoelectric performance compared to pristine materials. This is to-date being attributed to drastic reductions in the thermal conductivity, but less on power factor improvements. In this work we combine Molecular Dynamics simulations for the thermal conductivity and Non-Equilibrium Green’s Function simulations for electronic transport to study thermoelectric transport through materials with hierarchically embedded nanoinclusions, namely superlattice-type barriers with quantum dots and voids in between them. We show that beyond the drastic reductions in thermal conductivity, the nanomaterials can be designed such that the power factor: i) is resilient to the presence of nanoinclusions, and ii) is even improved compared to pristine materials, independently of the density of the distorting nanoinclusions. |
Thursday, March 7, 2019 12:27PM - 1:03PM |
S47.00007: Utilizing Magnetism and Nanostructures to Enhance Performance of Thermoelectric Materials Invited Speaker: Takao Mori New principles and strategies are desired to overcome the traditional tradeoff between thermoelectric (TE) properties, i.e. electrical conductivity σ and thermal conductivity κ, and Seebeck coefficient α. We have been trying to develop TE enhancement principles which can be relatively easily implemented and applied to a wide range of materials [1]. Porosity had been considered detrimental for TE materials, with the penalty for σ usually being similar or larger than the κ reduction. Introducing a moderate volume of nano-micropores with size distribution by simple evaporation of a secondary phase, has led to effective phonon selective scattering, and 100% enhancement to figure of merit σα2/κ =ZT~1.6 in rare earth-free (“empty”) skutterudites [2]. This strategy has also led to ZT enhancement in other materials which will be presented. We have proposed to utilize magnetic interactions between carriers and magnetic moments to enhance the power factor σα2 [3]. Unlike magnon drag, TE enhancement via magnetic interaction is not solely dependent on ordering, and is effective at higher temperatures also. Magnetic ion doping for example, has led to TE enhancement for a variety of cases, CuGaTe2, BiCuSeO, Bi2Te3, SnTe, etc., if effective coupling is created. We have also demonstrated significant enhancement of the Seebeck coefficient via spin fluctuation. CREST project members are acknowledged. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S47.00008: Enhanced Seebeck effect in ion-gated FeSe Sunao Shimizu, Shiogai Junichi, Nayuta Takemori, Shiro Sakai, Hiroaki Ikeda, Ryotaro Arita, Tsutomu Nojima, Atsushi Tsukazaki, Yoshihiro Iwasa The reduction of the dimensionality leads to the manifestation of quantum phenomena and the development of electronic correlation. Such low dimensional effects often become even more pronounced in nano-scale materials including exfoliated atomic layers and their hetero structures, triggering the emergence of the novel electronic, optical, and magnetic properties. Here we report the thermoelectric effect of FeSe ultrathin films with an electric double layer configuration, which allows us to control not only the carrier density but also the film thickness down to monolayer [J. Shiogai et al., Nature Phys. 12, 42 (2016)]. By utilizing a chemical etching induced by applying a large gate bias, the thickness of thin films is reduced in a layer-by-layer manner. Accompanying the emergence of the high-Tc superconductivity, the ionic liquid gating on FeSe thin films induces the anomalous enhancement of the Seebeck effect. We will discuss the thickness and the temperature dependence of the thermoelectric properties in FeSe thin films in detail. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S47.00009: Performance Analysis of nanostructured Peltier coolers Aniket Singha, Bhaskaran Muralidharan Employing non-equilibrium quantum transport models [1], we investigate the details and operating conditions of nano-structured Peltier coolers embedded with an energy filtering barrier. Our investigations point out non-trivial aspects of Peltier cooling which include an inevitable trade-off between the cooling power and the coefficient of performance, the coefficient of performance being high at a low voltage bias and subsequently deteriorating with increasing voltage bias. We point out that there is an optimum energy barrier height for nanowire Peltier coolers at which the cooling performance is optimized. However, for bulk Peltier coolers, the cooling performance is enhanced with the height of the energy filtering barrier. Exploring further, we point out that a degradation in cooling performance with respect to bulk is inevitable as a single moded nanowire transitions to a multi-moded one. The results discussed here can provide theoretical insights into optimal design of nano Peltier coolers. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S47.00010: Doping effects and magnetic instabilities in full-Heusler Fe2YZ1-xAx thermoelectric compounds Sébastien Lemal, Fabio Ricci, Matthieu Verstraete, Philippe Ghosez Giant thermoelectric power factors have been theoretically predicted on n-doped Fe-based full-Heusler compounds, however, when dealing explicitly with dopants, they may undergo to a magnetic phase transition with a consequent power factor reduction. Using computer experiments, we demonstrate that such transitions is purely related to electronic effects. Exploiting the broader nature of 4d and 5d transition metal orbitals, the appearance of the magnetic instability can be shifted to higher doping regimes, recovering high power factors. The unveiled very large power factors and magnetic fluctuations are fascinatingly very promising respectively for standard thermoelectric and spin-caloritronic device applications. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S47.00011: NMR Study of Doped Tetrahedrite Thermoelectric Materials. Nader Ghassemi, Xu Lu, Xiaoyuan Zhou, Yanci Yan, Yefan Tian, Joseph Hansbro Ross Tetrahedrite thermoelectric materials with compositions between Cu12Sb4S13 and Cu10 Zn2Sb4S13 were analyzed from room temperature to 4.2 K by 63Cu and 65Cu NMR. Cu12Sb4S13 is known to have a metal-insulator phase transition at T = 88 K, however, structural details of the transformation are not very well known, particularly in the substituted materials of interest for thermoelectric devices. Cu NMR is very sensitive to changes in local structure, and we found the metal-insulator transition still present for Cu11ZnSb4S13, but absent in the Zn2 material. However, differences in high-temperature NMR spectra reveal a distinct local structure for Zn2 vs the undoped material. Also, we found NMR evidence for Cu-ion hopping, which disappears in the Zn2 material. Spin-lattice relaxation rates (1/T1) furthermore provide a measure of local anharmonic vibrational motions, through an electric quadrupolar process. Results indicate that the rattling behavior is little changed by Zn substitution up to Zn2, despite the structural differences. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S47.00012: Influence of the Mobility Ratio and Density of States Width on the Thermoelectric Properties of Polymer Blends Ashkan Abtahi, Yadong Zhang, Xuyi Luo, Jianguo Mei, Seth R. Marder, Kenneth Graham Conjugated polymers can be used in mechanically flexible and low cost thermoelectric (TE) devices, but their thermoelectric performance must be improved to make them commercially viable. The performance of thermoelectric materials depends on the electrical conductivity, Seebeck coefficient and thermal conductivity. The higher the doping concentration, the more electrically conductive the material becomes, but generally at the cost of a decrease in the Seebeck coefficient. Blending of π-conjugated polymers has been proposed as a method to minimize the tradeoff between electrical conductivity and the Seebeck coefficient. By blending two polymers, the total density of states (D.O.S.) will be manipulated, which may be used to alter the charge transport in the TE material. The major parameters that we expect to impact the power factor in polymer blends are the mobility ratios between the two polymers and the shape of D.O.S. Here, we use a model introduced by Bässler and Arkhipov to theoretically probe how these two parameters impact the thermoelectric performance. We find that a narrower D.O.S. and lower mobility of the added polymer with respect to host polymer can lead to an enhancement in the Power factor of the TE material. |
Thursday, March 7, 2019 2:03PM - 2:15PM |
S47.00013: Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure Seoung-Hun Kang, Kisung Chae, Seon-Myeong Choi, Duck Young Kim, Young-Woo Son Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high electric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si24 features highly anisotropic crystal structure with nanometer-sized regular pores. Here, based on first-principles study without any empirical parameter we show that the slightly doped Si24 can provide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores. |
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