Alloys and Compounds for Thermoelectric and Solar Cell Applications X: On-Demand Oral Presentations
Sponsored by: TMS Structural Materials Division, TMS Functional Materials Division, TMS: Alloy Phases Committee
Program Organizers: Hsin-Jay Wu, National Chiao Tung University; Sinn-wen Chen, National Tsing Hua University; Franck Gascoin, CNRS Crismat Unicaen; Philippe Jund, Montpellier University; Yoshisato Kimura, Tokyo Institute of Technology; Takao Mori, National Institute For Materials Science; Alexandra Zevalkink, Michigan State University; Wan-Ting Chiu, Tokyo Institute of Technology; Pai-chun Wei, National Taiwan University
Monday 8:00 AM
March 14, 2022
Room: Electronic Materials
Location: On-Demand Room
Defect Engineering for Enhancement and Control over Thermoelectric Materials: Takao Mori1; 1National Institute for Materials Science
I will present several examples of effective defect engineering from our lab. Cr doping lowered formation energy of Ge defects, leading to homogenously distributed Ge precipitations and Ge vacancies, enabling a ZT~2 [1]. High-entropy alloying into GeTe, stabilized the cubic phase and defects, thereby enabling enhanced doping of Bi, leading to the first stable n-type conduction in GeTe [2]. The hidden role of rhombohedral distortion degree on Ge-vacancy formation energy was revealed and utilized leading to high power factor and ZTav [3]. Defects also enhanced power factor of Mg2Sn films. Minute Cu doping into grain boundaries and interstitial sites led to ZT enhancement, and an 8 pair module of Mg-Sb-based materials achieving efficiency of 7.3% at a hot side of 320°C rivalling long-time champion Bi2Te3-based module [4]. [1] Small, 16, 1906921 (2020). [2] NPG Asia Mater., 12:66 (2020). [3] Adv. Energy Mater., 10, 2002588 (2020).[4] Joule, 5, 1196 (2021).
Interfacial Reactions in Ni/PbSe and Ni/SnSe2 and Couples: Sinn-wen Chen1; Zhi-Kai Hu1; Hsu-Hui Chen1; Yohanes Hutabalian1; 1National Tsing Hua University
PbSe and SnSe2 are important thermoelectric materials. Ni is commonly used as barrier layer material. The interfacial reactions in the Ni/PbTe and Ni/SnSe2 couples are examined. PbSe and SnSe2 substrates are prepared with pure constituent elements and electroplated with Ni layer. The Ni/PbSe couples were reacted at 300, 350 and 400℃, and Ni/SnSe2 couples at 200, 300 and 500℃. For better understanding of the reaction paths, phase equilibria isothermal sections of Ni-Pb-Se and Ni-Se-Sn systems at respective temperatures are determined. A ternary phase, Ni3Pb2Se2, is formed in the Ni/PbSe couples reacted at all these three temperatures. The reaction layer grows thicker with higher reaction temperature and linearly with square root of reaction time. Much more complicated interfacial reaction results were observed in the Ni/SnSe2 couples. Three different reaction phase layers are observed in the couples reacted at 500℃. Four layers are observed for those reacted at 300 and 200 ℃.
Metallic Thermoelectric Materials as Active Heat Sinks: Mona Zebarjadi1; Shuai Li1; Kyle Snyder2; 1University of Virginia; 2CCAM
Traditional thermoelectric materials with a large thermoelectric figure of merit, ZT, are appropriate for power generation and refrigeration. On the other hand, high power factor and high thermal conductivity materials are a better alternative for active cooling applications where the heat is pumped from a hot source to a cold sink. Applications such as heat management in batteries and electronic cooling fall in this category. Metallic alloys have large thermal and electrical conductivity compared to traditional thermoelectric semiconductors. This combination is a good start point for designing high power factor, high thermal conductivity thermoelectric materials that are ideal for active cooling applications. Here, I will summarize our group’s work to design high power factor metallic alloys using melting, mechanical alloying, and additive manufacturing techniques. In particular, we studied copper-nickel alloys as a base for our optimization.
Modular Flexible Thermoelectric Generators for Ambient Energy Recovery: Chien-Neng Liao1; Shih-Yao Chien1; 1National Tsing Hua University
Flexible thermoelectric generators (f-TEGs) prepared by printing technology are gaining increasing interests due to the advantages of material saving, structure flexibility and ease of fabrication. Despite their low power output (uW – mW), f-TEGs are highly suitable for massive wearable or environment-deployed IoT devices. To establish a thermoelectric energy harvester, the output electricity from the f-TEG needs to be regulated by a voltage-boosting electronic circuit. In this study, we demonstrate a modular thermoelectric TEG consisting of multiple dispenser-printed f-TEG modules. Each single TEG module can provide an electric power of 10s uW at a delta T of 10 – 20 K. A modular platform is designed to boost the output power for practical applications by integrating multiple f-TEGs. The technical issues regarding printing materials, sintering, thermoelement geometry optimization and contact electrode selection are discussed. The influences of various environment scenarios on the output performance of the modular f-TEG are evaluated.
Thermoelectric Properties of Nanostructured Al-doped ZnO Thin Films: Paolo Mele1; 1Shibaura Institute of Technology
Nanoengineering approach has been applied on thin films of Al-doped ZnO (AZO) prepared by several techniques to enhance their thermoelectric performance. Insertion of artificial nanodefects has been considered with the purpose of depressing thermal conductivity (k) and enhancing figure of merit (ZT). Several approaches have been tried: (i) insertion of hydroquinone nanolayers in AZO films prepared by atomic layer deposition (ALD): kALD (300 K) = 3.56 W/m×K ; (ii) addition of polymetylmetacrilate (PMMA) particles to AZO films prepared by multi-beam multi-target matrix-assisted PLD (MBMT/MAPLE-PLD): kMAPLE (300 K) = 5.9 W/m×K ; (iii) formation of nanopores in AZO films prepared by Mist-Chemical Vapor Deposition (Mist-CVD): ｋporous (300 K) = 0.60 W/m×K; (iv) dispersion of Al2O3 nanoparticulate in AZO films prepared by surface-modified target PLD: ｋnanoAl2O3 (300 K) = 3.98 W/m×K . All these successful examples highlight the promise of nanostructured AZO films for future energy harvesting applications.
Planar-type Nano Phononic Si Energy Harvesters: Masahiro Nomura1; 1University of Tokyo
Thermoelectric performance of planar-type uni-leg poly-Si energy harvesters will be presented. The device was fabricated using an SOI wafer with a 300-nm-thick n-doped Si with periodic circular holes to enhance the ZT value by reducing thermal conductivity. The output power density was enhanced by a factor of 10 by the phononic patterning, and showed 3 micro Watt per square centimeter with 1 K difference. We demonstrate that thermal design of the device is significantly important to maximize the device performance.
Fabrication of Nanostructured Half-Heusler Compounds via Crystallization of an Amorphous Precursor: Pyuck-Pa Choi1; Chanwon Jung1; Hail Park1; 1KAIST
Tailoring nanostructures is nowadays a common approach for enhancing the performance of thermoelectric Heusler compounds by decreasing the thermal conductivity without significantly affecting the electrical conductivity. However, the most widely reported method for obtaining nanostructured thermoelectrics, a top-down approach based on crushing as-cast alloy ingots followed by sintering of the debris, only gives limited control of the final nanostructure due to residual elemental segregation. Here, we present a bottom-up approach for fabricating nanostructured Heusler compounds, which is based on crystallizing an amorphous NbCoSn precursor. This method yields distinct nanostructures, comprising only half-Heusler grains or half-Heusler grains and full-Heusler nano-precipitates. Using high-resolution transmission electron microscopy, atom probe tomography, and characterization of transport properties, we give detailed insights into the structure-property relationship of the fabricated specimens.
Development of Low–defect Single Crystalline Ferromagnetic Shape Memory Alloy for Magnetic Cooling and Actuation: Wan-Ting Chiu1; Pimpet Sratong-On2; Masaki Tahara1; Volodymyr Chernenko3; Hideki Hosoda1; 1Tokyo Institute of Technology; 2Thai-Nichi Institute of Technology; 3Department of Electricity and Electronics, University of the Basque Country (UPV/EHU) & BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park
The internal degrees of freedom and shape change of the ferromagnetic shape memory alloys (FSMA) are controllable by the magnetic field. Therefore, magnetic cooling and high–speed actuators are considered to be practiced by using the FSMA–based materials. NiMnGa alloys are promising FSMA materials; however, the high brittleness issue of NiMnGa alloys restrains their applicability. In this study, the separators, such as S or Bi, were intentionally inserted into the grain boundaries of the polycrystalline NiMnGa alloys to further embrittle the NiMnGa alloys. The embrittled polycrystalline NiMnGa alloys were then mechanically crushed into single crystal powders at certain elevated temperature. The conventional difficulties, such as high concentration of defects and time–consuming processes for the preparations of single crystals, were solved by using this technology of grain powder preparation. Single crystalline NiMnGa powders were utilized to manufacture different composites for magnetic cooling and high–speed actuator applications.
Theory of Huge Thermoelectric Effect Based on Magnon Drag Mechanism: Application to Thin-film Heusler Alloy: Masao Ogata1; Hiroyasu Matsuura1; Takao Mori2; Ernst Bauer3; 1University of Tokyo; 2National Institute for Materials Science, and University of Tsukuba; 3Technische Universitat Wien
Recently, a thin-film Heusler alloy, Fe_2V_0.8W_0.2Al has attracted extensive attention because it shows a huge figure of merit (ZT~5) near the room temperature, derived from an unprecedentedly huge power factor, which we propose is actually related to the magnetism in this material. To understand the high thermoelectric performance in this thin-film Heusler alloy, we study the magnon drag effect, generated by the tungsten-based impurity band, as a possible source of this enhancement in analogy to the phonon drag observed in FeSb_2. Assuming that the thin-film Heusler alloy has a conduction band integrating with the impurity band, originated by the tungsten substitution, we derive the electrical conductivity based on the self-consistent t-matrix approximation and the thermoelectric conductivity due to magnon drag based on the linear response theory. We show that the origin of the huge thermoelectric properties is likely due to the magnon drag related with the tungsten-based impurity band.
Enhancing Room-temperature Thermoelectric Performance of SnSe with Excess Ag: Hsin-Jay Wu1; Meng-Yuan Ho1; 1National Chiao Tung University
Single-crystalline SnSe attracts attention owing to its high peak ZT > 2.5 at 923 K as measuring along with the b-axis and c-axis directions. In contrast, the poly-crystalline SnSe reveals a lower zT value but with a feasible synthesis procedure. The difference in the TE performance is attributed to the anisotropy and phase transition behavior in SnSe. This study aims to improve the TE performance of the low-temperature Pnma phase of SnSe by incorporating dilute amounts of In, Cu, and Ag into the single- and poly-crystalline SnSe substrates. Along with the Ag-excess condition, the room-temperature zT values of In-SnSe and Cu-SnSn crystals are greatly enhanced from 0.001 to 0.06 and from 0.001 to 0.03, respectively, showing nearly 30 to 60 times higher compared with their bare samples. This study uncovers the importance of phase diagram engineering and suggests that the electrical current sintering boosts the zT values for SnSe-based alloys.
Development of n-type Mg3Sb2-Mg3Bi2 Alloys for Thermoelectrics: Kazuki Imasato1; Michihiro Ohta1; G. Jeffrey Snyder2; 1National Institute of Advanced Industrial Science and Technology; 2Northwestern University
There has been significant interest in Mg3(Sb,Bi)2 since the demonstration of high zT in the n-type materials. In this talk, I will present the strategies to achieve a high thermoelectric performance of Mg3Sb2-Mg3Bi2. The thermoelectric performance of n-type Mg3Sb2-Mg3Bi2 alloys was systematically investigated and significantly improved with thorough experimental characterization of the effects of defects, composition, microstructure, and electronic band structure. The main mechanisms of the improvement are optimization of the band structure and microstructure. These results were compiled to establish foundational methodologies to optimize this material system. The understanding of these mechanisms is crucial to the cooling and wasted heat recovery applications. Considering the limited number of state-of-art n-type thermoelectric materials for the low-grade heat recovery and cooling technology, the further development of Mg3Sb2-Mg3Bi2 alloys is a significant step towards the commercial application of thermoelectric materials.
Giant Isotopic Shift in Phonons Keeps Charge Carriers Hot Longer in a Photovoltaic Perovskite: Michael Manley1; Kunlun Hong1; Panchao Yin1; Hsin Wang1; Songxue Chi1; Luke Daemen1; Andrew May1; Chengyun Hua1; Raphael Hermann1; Mark Asta2; Yao Cai2; Mahshid Ahmadi3; 1Oak Ridge National Laboratory; 2University of California, Berkeley; 3University of Tennessee
Hot-carrier solar cells provide a way to convert sunlight to electricity more efficiently than conventional solar cells by harnessing charge carriers before they lose energy to heat. It is therefore desirable to design materials with long hot charge carrier cooling times. Hot charge carriers thermalize through carrier–optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. A hot-phonon bottleneck, where hot charge carrier cooling through the phonons is blocked, can significantly extend this cooling time. We use neutron scattering and other techniques to show that thermal conduction can be reduced by swapping out a lighter isotope for a heavier one in the organic molecule of the high-performance photovoltaic material methylammonium lead iodide. The effect on phonons of the heavier isotope is to reduce the thermal conductivity by half and delay the cooling of hot carriers – extending their useful lifetime for energy conversion (https://doi.org/10.1126/sciadv.aaz1842).
Seebeck Coefficient Enhancement in CoSb3-Skutterudite Induced by Constructive Magneto-composite Effects: Cedric Bourges1; Wenhao Zhang1; Bhuvanesh Srinivasan1; Naoyuki Kawamoto1; Masanori Mitome1; Kazuaki Kobayashi1; Jean-Francois Halet2; David Berthebaud2; Takao Mori1; 1National Institute for Materials Science; 2Laboratory for Innovative Key Materials and Structures, IRL 3629-CNRS
Filled skutterudite compounds have been extensively studied as one of the most promising TE materials based on the PGEC concept. However, non-desirable behavior has been reported upon the subtle oxidation of the rare-earth filled skutterudite make it attractive to develop high-performance skutterudite free of those elements. We were focused on the n-type Skutterudite to realize a high-performance TE material targeting room temperature. We adopted an original approach center on the co-doping of MxCo1-xSb2.85Te0.15 – skutterudite with magnetic elements. Through calculation, we identified elements susceptible to substitute the Co and induce a magnetic moment in the CoSb3 phase. We revealed that a significant enhancement of 45% of the overall PF can be achieved thanks to a spectacular increasing of the Seebeck coefficient in the co-doped CoSb3. Based on the combined experimental and theoretical findings, we highlighted that a plethora of factors constructively contributed to the large enhancement of the Seebeck coefficient.
Thermoelectric Transport and Crystal Morphology of the Quasi-1D Zintl Ca5M2Sb6: Alexandra Zevalkink1; 1Michigan State University
Zintls, with their vast range of structural patterns and excellent thermoelectric performance, stand out as an intriguing subject area for the study of transport anisotropy. In the present study, we use a combination of theory and experiment to investigate the anisotropic transport properties of Ca5In2Sb6, which is characterized by anionic sub-structures resembling infinite 1D chains of corner-linked MSb4 tetrahedra. Ca5In2Sb6 single crystals grown from an Sb-rich molten flux were found to form long, needle-like crystals oriented parallel to the polyanionic chains. Due to the small cross-section of the crystals, characterization of transport properties in the direction perpendicular to the chains is challenging. For this purpose, micro-ribbons were extracted from single crystals using focused ion beam milling, and laser photolithography was used to deposit contacts to measure electrical resistivity. The resistivity parallel to the growth direction was found to be nearly 20 times higher than the perpendicular direction, confirming predictions.
Integration of Si-based Micro Thermoelectric Generator Devices: Takanobu Watanabe1; 1Waseda University
We pursued the optimal design of large-scale integration of micro thermoelectric power generators and developed fabrication processes for the integrated module. We are proposing a new thermoelectric device architecture using Si nanowire thermoelement, which was patterned by top-down lithography of SOI substrates without forming a cavity space underneath it. The simple device structure allows us to enhance the power generation capacity by miniaturizing each element together with the high-density integration. We investigated the optimal width ratio of n-type and p-type legs in the bileg module, and demonstrated multi-stage thermoelectric modules of unileg and bileg architectures.
Impact of Cation Substitution in (AgxCu1-x)2ZnSnSe4 Absorber-based Solar Cells towards 10% Efficiency: Experimental and Theoretical Analyses: Li-Chyong Chen1; Shaham Quadir1; Mohammad Qorbani1; Ying-Ren Lai1; Ho-Thi Thong2; Amr Sabbah2; Michitoshi Hayashi1; Cheng-Ying Chen3; Kuei-Hsien Chen2; 1National Taiwan University; 2Academia Sinica; 3Ming Chi University of Technology
Solar cells based on kesterite Cu2ZnSnSe4 (CZTSe) compounds with earth-abundant elements are highly desirable for the low-cost and high-efficiency production of renewable energy. However, the occurrence of intrinsic defects substantially impairs the photovoltaic properties of CZTSe. In this work, we applied a cation substitution method to control and passivate the defect states of kesterite CZTSe by incorporating Ag ions. Intensity-dependent low-temperature photoluminescence measurements show that Ag incorporation could reduce the density and depth of intrinsic defects in CZTSe. The results reveal that (AgxCu1-x)2ZnSnSe4, with x = 0.10, provides the shallowest defect states and less non-radiative recombination. Based on first-principles calculations, Ag incorporation enables the formation and suppresses the beneficial and detrimental defects, respectively, and the observed sub-band photoluminescence peaks can be assigned to the intrinsic point and cluster defects. The best power conversion efficiency of 10.2% is achieved for the 10%-Ag-alloyed CZTSe cell, along with an enhanced open-circuit voltage.
Uncovering Design Principles for Amorphous-like Heat Conduction Using Two-channel Lattice Dynamics: Raphael Hermann1; Riley Hanus1; Michael Manley1; Janine George2; 1Oak Ridge National Laboratory; 2Universite Catholique de Louvain
Neutron scattering and atomic dynamics simulations reveal that complex crystals can conduct heat like a glass through diffusive quantum hopping. A crossover in conduction from the conventional phonon-gas channel to the unusual diffuson channel occurs near room temperature in Yb14MnSb11, a complex Zintl phase with outstanding thermoelectric properties at high temperature. Following a recently developed framework which quantifies how mode overlap forces heat conduction through the diffuson channel[1], i.e., diffusive quantum energy hopping we show that the diffuson channel can overwhelm the phonon-gas channel thermal transport. Inelastic neutron scattering reveals the extent of mode overlap and validates lattice dynamics calculations. Our findings[2] suggest mechanisms to down tune the diffuson channel by controlling mode overlap through crystal chemistry and varying the atomic masses. [1]Simoncelli et al., DOI: 10.1038/s41567-019-0520-x (2019).[2]Hanus et al., DOI: 10.1016/j.mtphys.2021.100344 (2021).
Liquid-like Ion Diffusion Leading to the Ultralow Thermal Conductivity of Ag8SiTe6 Argyrodite: Pai-Chun Wei1; 1Center for Condensed Matter Sciences, National Taiwan University
Combining ultrahigh-resolution synchrotron X-ray diffraction and first-principle molecule dynamics simulations, here we report the liquid-like behavior of Ag ions leading to the ultralow thermal conductivity of Ag8SiTe6 argyrodite and a falloff in isobaric specific heat to ~ 50% of the classic Dulong-Petit limit. We show that the augmented ion diffusion at T > 260 K triggers the observed liquid-like thermal transport behavior, while the gradual localization of Ag ions at T < 260 K gives rise to consecutive symmetry breaking, lattice distortion, more solid-like transport properties. We also demonstrate the presence of soft and damped vibration modes and anomalous Stokes-anti-Stokes ratios that infers the persistently excited Ag cages with unusual statistics. These findings provide valuable insights into the argyrodite family and disclose the new state of matter that shall beckon further characterization.
Relationship between Lattice-site Preference and Phase Stability in Thermoelectric Compounds Mg2M (M: Si, Ge, Sn) and RNiSn (R: Ti, Zr, Hf): Yoshisato Kimura1; Yaw Wang Chai1; Manabu Watanabe1; Yonghoon Lee2; 1Tokyo Institute of Technology; 2KELK Ltd.
We focus on the Anti-Fluorite (AF) Mg2(Si,Ge,Sn) and Half-Heusler (HH) (Ti,Zr, Hf)NiSn systems, in which solid solution formation and related two-phase separation can be considered as important factors to control and enhance thermoelectric performance. The objective is to understand the relationship between site-preference of solute atoms at specific lattice-site and phase stability based on phase equilibria for both systems. Two-phase separation from random solid solution occurs between Mg2(Si,Ge) and Mg2Sn, and between TiNiSn and (Zr,Hf)NiSn, according to large lattice misfits as the dominant cause in common. In the case of AF, the partitioning of doping elements was traced through the phase separation, which affects thermoelectric performance. In the case of HH, the specific site occupation is the key to control conduction type between N and P, which is related to the substitution for the Ni-site as well as the vacancy-site occupation with proper solute atoms.
Combustor/Heat Exchanger-integrated Thermoelectric Power Generation System for Autonomous Robots: Yuji Suzuki1; Minhyeok Lee1; Shintaro Uchida1; Hajime Asama1; Keiji Nagatani1; Shunsuke Hamasaki1; Shota Chikushi1; Chul-Ho Lee2; Yong Fan2; Kazuo Nagase2; Hirokuni Hachiuma3; Shinichi Fujimoto4; Naoki Yokoo5; Hiroyuki Sato5; Kenichi Odagawa5; 1The University of Tokyo; 2National Institute of Advanced Industrial Science and Technology; 3Komatsu Ltd.; 4KELK Ltd.; 5Dainichi Co. Ltd.
In this study, we studied a small thermoelectric power generation system using a Butane cassette cylinder as fuel, which can continuously drive an autonomous robot for a long time. In the SiGe topper thermoelectric module, which is directly heated by catalytic combustion on the Pd/porous alumina layer, the maximum temperature at the high temperature end can reach 900 ºC. The heat conduction loss to the low temperature end is surpressed by 1/5 by using fuel mixture flow from the low temperature end to the high temperature end. We have also developed an early prototype of a thermoelectric power generation system that includes a pair of SiGe topper thermoelectric module, a BiTe exhaust heat recovery thermoelectric module, and all necessary auxiliary equipment. By using this system, we demonstrated successful long-term operation of a wheel-type mobile robot.
Intermetallic and Chalcogenide Materials for Mid-range Temperature Thermoelectric Applications: David Berthebaud1; 1CNRS - Saint-Gobain - NIMS, IRL3629, Laboratory for Innovative Key Materials and Structures (LINK),
We will present our recent results on intermetallic and chalcogenide materials which were selected for their low cost and/or advantageous properties. For example, the incommensurate compounds MnSiγ (γ ~ 1.73) and FeGeγ (γ ~ 1.52) were studied through high temperature XRD to assess their crystal structure and thermoelectric properties stability. MnSiγ is particularly interesting as it exhibits sufficiently high ZT values (≥ 0.5 at 500 °C) in a wide temperature range, which is an advantage for stable electricity production. These advantages render them ideal candidates for the development of industrial prototypes. We also focused on the case of Ti/Zr doped SnTe which were found to be efficient dopants for SnTe to realize a stable and improved thermoelectric performance for mid temperature regime (323 – 723 K) and by codoping the optimized Ti/Zr-doped SnTe with Mn, remarkably enhancing the device/average zT (zTave).
Ternary and Quaternary Thermoelectric Sulphides: Mechanical-alloying, Order/Disorder Phenomena, Conductive Network, Transport Properties, and Theory: Emmanuel Guilmeau1; 1CRISMAT/CNRS
Among the various possibilities offered by the periodic table, copper-rich ternary and quaternary sulphides represent a formidable source for the discovery of low cost and environmentally benign thermoelectric materials. Copper-rich sulphides form an important class where univalent copper is the dominant element with the potential to generate hole carriers within the conductive “Cu–S” network for the generation of p-type thermoelectrics. Those materials represent a fantastic playground for solid state chemists. Some peculiar structural features in connection with chemical bonding, such as the existence of metal complexes within the structure and/or structural order/disorder phenomena, were carefully examined in different materials to establish rules and correlations between the crystal structures, electronic structures, vibrational and thermoelectric properties.
Enhanced Thermoelectric Performance with High Thermal Stability in Cu-based n-type PbTe: Ping-Yuan Deng1; Hsin-Jay Wu1; Kung-Kuo Wang2; 1National Yang Ming Chiao Tung University; 2National Sun Yat-sen University
Lead-tellurides have been the well-established mid-temperature thermoelectric (TE) materials since the 1960s. Nevertheless, the conversion efficiency of the n-type PbTe has a lower TE still has room to improve. Incorporation of Cu in PbTe not only brings the n-type conduction but elevates the power factor as a result of enhancing carrier mobility. The largest drawback for the Cu-doped PbTe arises from the mobile Cu, especially under activation of the electrical current. The electromigration effect caused by Cu in PbTe could deteriorate the TE performance, falling to meet satisfactory thermal stability after cycling. Benefited from lightly Cu doping and VIA-element alloying, the Cu0.012M0.006(PbTe)0.988 (M = VIA elements) exhibits a peak zT ~ 1.5 at 598 K with high thermal stability between the temperature range of 300 – 598 K after multiple thermal cycling. The co-existence of interstitial-Cu and VIA nano-precipitate maintain the large carrier mobility with a reduced kL.
Enhancing Thermoelectric Performance via Microstructure Engineering in GeTe Alloys: Yi-Fen Tsai1; Hsin-Jay Wu1; 1National Chiao Tung University
GeTe-based alloys, which exhibit a vast amount of Ge vacancies, are promising mid-temperature thermoelectric (TE) materials with excellent electrical conductivity. Contradictorily, the high thermal conductivity limits the thermal-to-electrical conversion efficiency in rhombohedral GeTe. Microstructure engineering coupled with doping suppress the thermal conductivity while retain the intrinsically high electrical conduction. Examples can be given in In-GeTe alloys which show 200% reduction in thermal conductivity compared with undoped GeTe. Meanwhile, the Seebeck coefficient of In-GeTe gradually increases, leading to an outstanding power factor in rhombohedral phase region. In short, the In-GeTe shows superior TE performance due to the synergistic approaches of microstructural and doping engineering.
Enhanced Thermoelectric Properties in Sb2Te3 Thin Film Modules: Zhen-Wei Sun1; Albert T. Wu1; 1National Central University
Sb2Te3 is a well-known thermoelectric (TE) material due to its excellent zT values at low temperature region. The transition efficiency drastically reduces when the TE materials are assembled to modules. Cu and Ni were selected as the electrode material. Sb2Te3 has a hexagonal crystal structure that allows fast diffusion along the direction perpendicular to c-axis. In this study, TE module is aged for different duration of time to simulate the operation conditions. For the Cu/Sb2Te3 module, a great amount of Cu diffuses into Sb2Te3 layers and reacts with Sb2Te3. The formation of CuTe compounds deteriorates TE properties of the modules. Selecting Ni as the electrode enhances TE properties with less formation of Ni-Te compounds in the film. The stoichiometry of the Sb2Te3 greatly affects the TE property.
Enhanced Thermoelectric Performance of n-type Silver Chalcogenide with Excess Ag
: You-Cheng Du1; Wan-Ting Yen1; Hsin-Jay Wu1; 1National Yang Ming Chiao Tung University
The n-type silver chalcogenides (Ag2X, X= Se, Te) are ideal alternative to the bismuth-tellurides that are widely used for thermoelectric (TE) cooler. Comparing to the state-of-the-art bismuth tellurides, the silver chalcogenides comprise earth-abundant and low-toxic elements. The highly mobile silver ions in a silver-chalcogenide lattice could introduce various scale of defects that enhance the phonon scattering and reduce the lattice thermal conductivity. Single crystalline Ag2Se and Ag2Te were obtained from the Bridgman method while the excess Ag was introduced by sputtering. We found that the slightly off-stoichiometric silver chalcogenides yield dramatic change in the resultant transport properties. The excess Ag in our Ag2+xX crystals bring high power factors and enhanced peak figure-of-merit near the room-temperature region, making the n-type silver chalcogenides as ideal candidates for n-type TE cooler.
Phase Equilibria of Cu-Se-Te System and Interfacial Reactions in Cu/Se, Cu/Se-20at.%Te and Cu2Te/Se Couples: Yohanes Hutabalian1; Sinn-wen Chen1; 1National Tsing Hua University
The Cu2Se-based and Cu2Te-based alloys are promising thermoelectric materials. To provide fundamental information which is lacking in literature, the isothermal sections at 500oC, 300oC and liquidus projection of Cu-Se-Te ternary system are experimentally determined. Se-Te is an isomorphous system, but the phase relationships in the Cu-Se and Cu-Te systems are complicated. In the Cu-Se-Te system, there are two ternary compounds, and a continuous solid solution between the congruently melting Cu2Se and Cu2Te phases. The Cu-Se-Te liquidus projection has eleven primary solidification phases. Reaction couples of Cu/Se, Cu/Se-20at%Te, and Cu2Te/Se couples at 300oC are prepared and examined, and complicated reaction phases are observed. For example, after reaction for only 10 min, a very thick layer of 147 mm thickness with various phases is found in the Cu2Te/Se couple. The diffusion path proceeds in the following phase sequence: Se/CuSe2/CuSe/Cu2Se/Te/CuTe/Cu3Te2/Cu2Te. It was found that the diffusion of Cu is faster than other elements.
Light Impurity Doping and Entropy Engineering Eliciting High-performance n-type Bi2Te3 Thermoelectrics: Wan-Ting Yen1; Hsin-jay Wu1; 1National Yang Ming Chiao Tung University
Bismuth telluride (Bi2Te3) alloys have been the well-established thermoelectric (TE) cooler since the 1960s. To date, the Bi2Te3-based alloys still attract numerous attention as they are the best TE materials near the room temperature region. This study aims to boost the TE performance of n-type Bi2Te3 crystals by light impurity doping and entropy engineering. Dilute dopants of Ag, Cu, S, Se are incorporated into Bi2Te3 crystals, aiming to boost the carrier mobility while leaving the band structure of Bi2Te3 unaffected. When it comes to the thermal conductivity reduction, entropy engineering is exploited by doping with Ag, Cu, Sand Se. The synergistic approaches of light impurity doping and entropy engineering enable the tradeoff between high power factor and low thermal conductivity, advancing the TE performance for n-type Bi2Te3.
Phase Diagram Engineering Enables Lightly Impurity Doping for High-performance GeTe Thermoelectrics: Szu-Chien Wu1; Yi-Fen Tsai1; Hsin-Jay Wu1; 1National Yang Ming Chiao Tung University
Phase diagram engineering has opened a new thermodynamic route for optimizing thermoelectric performance. The SnTe-GeTe alloys form a cubic solid-solution in a wide compositional region when the annealing temperature is below the rhombohedral-GeTe to cubic-GeTe phase transition point (T < 700 K). On the basis of Sn-Ge-Te phase diagram, a structural transition zone exists between the rhombohedral- and cubic-(SnTe, GeTe) solid solution, together with another six multi-phase regions. This structural transition zone enables the lightly impurity doping for SnTe-GeTe alloys, which show the decent electrical conductivity σ and power factor PF = S2σ. The slightly Sb and Bi co-doping could further decrease the thermal conductivity, making the lightly co-doped rhombohedral-GeTe alloys promising as thermoelectric generators.
Thermal Evolution of Internal Strain in Doped PbTe: James Male1; Riley Hanus2; G. Jeffrey Snyder1; Raphael Hermann2; 1Northwestern University; 2Oak Ridge National Laboratory
Recent improvements in heat-to-electricity energy conversion efficiency in lead chalcogenide thermoelectrics involve reducing thermal conductivity by incorporating large amounts of internal strain. The extent to which typical thermoelectric processing techniques (like doping, ball milling, and densification) increase internal strain and dislocation density must be quantified to improve materials design. In this study, temperature dependent neutron powder diffraction is leveraged to evaluate the internal strain introduced by ball milling in doped and undoped PbTe from 50 K to 700 K. Na and/or Eu doping increases internal strain beyond ball milling alone with the greatest increase from combining the two dopants. Strain recovery occurs in each powder above 400 K but can be suppressed by co-doping – indicating strong dopant-dislocation interactions. This work provides key guidance for defect engineering to maximize internal strain and thermoelectric performance in PbTe thermoelectrics.
The Effects of Hot-compression at Working Temperatures on the Physical and Mechanical Behavior of the Thermoelectric GeTe Alloy: Gilad Guttmann1; Shmuel Samuha1; Reuven Gertner1; Shlomo Harush1; Yaniv Gelbstein2; 1NRCN; 2Ben-Gurion University of the Negev
Thermoelectric generators (TEGs) are devices capable of transmitting heat energy into electricity and vice-versa. One major setback for TEGs to be achieve higher technological readiness level is the lack of understanding of the thermo-mechanical properties of the constructing thermos-electric (TE) materials, which will help prevent in-service failures and support develop advanced manufacturing procedures. GeTe is a base alloy for forming a series of efficient TE materials in the 200 ÷ 500ºC temperature range, which have been intensively researched and successfully applied in several TEGs. This study is innovative in this respect, for focusing on the mechanical behavior of the GeTe alloy under compression tests conducted at ambient to high temperatures. This study also incorporated extensive electron microscopy and crystallographic characterization methods – and reveals a unique microstructure modification, resulting in a brittle-ductile transition at temperature within the expected service temperature range.
Achieving High Thermoelectric Performance in Hierarchical Structured Bi0.5Sb1.5Te3 Alloys via Controlling the Hot Pressing Temperature: Peyala Dharmaiah1; Sungjae Jo1; Yeeun Lee1; Soon-Jik Hong1; 1Kongju National University
Over the last decades, intensive research has been devoted to optimize and enhance the thermoelectric (TE) properties of BiSbTe-based materials for energy generation and solid-state cooling applications. However, commercial applications are constrained by the low efficiency of TE materials. In this work, an economically viable and single-step water atomization process is employed to fabricate BiSbTe-Cu based alloy powder followed by hot pressing (HP) at different temperatures. The electrical conductivity was increased significantly with increasing HP temperature due to the simultaneous increase of carrier concentration and mobility, while the Seebeck coefficient was nearly similar. Consequently, the maximum power factor of 4.28 mW/mK2 at 300 K was obtained for the perpendicular direction of HP490 sample. A peak ZT of 1.39 was attained at 400 K for the HP490 sample. This remarkable enhancement in ZT for the HP490 sample is due to enhanced power factor and reasonable reduction of thermal conductivity.