Alloys and Compounds for Thermoelectric and Solar Cell Applications VIII: Session IV
Sponsored by: TMS Functional Materials Division, TMS: Alloy Phases Committee
Program Organizers: Sinn-wen Chen, National Tsing Hua University; Franck Gascoin, Ensicaen University of Caen; Philippe Jund, Montpellier University; Yoshisato Kimura, Tokyo Institute of Technology; Lan Li, Boise State University; Takao Mori, National Institute For Materials Science; Hsin-jay Wu, National Chiao-tung University; Tiejun Zhu, Zhejiang University
Wednesday 2:00 PM
February 26, 2020
Room: Miramar
Location: Marriott Marquis Hotel
Session Chair: Sinn-wen Chen, National Tsing Hua University; Franck Gascoin, Ensicaen University of Caen
2:00 PM Invited
Thermoelectric Half Heusler Systems {Nb,Ta}FeSb - Ti1+xFe1.33-xSb: Peter Rogl1; Andrij Grytsiv2; Vitaliy Romaka2; Ali Tavassoli2; Gerda Rogl2; Ernst Bauer2; 1Institute of Materials Chemistry, Universitaet Wien; 2Universitaet Wien
Based on detailed phase diagram studies employing SEM, EPMA, XPD, the thermoelectric potential of Half-Heusler compounds in the title systems has been elucidated for 4.2K-900K. Investigations of physical properties (including mechanical behavior) were backed by DFT. Ti-Fe-Sb reveals an extensive homogeneity region: 800°C, Ti1+xFe1.33-xSb (-0.17-0.25). The TE performance increased with Fe/Co substitution and reached ZT = 0.42 for TiCo0.5Fe0.665Sb. We have synthesized the HHphases, TaFeSb, Ta1-xTixFeSb (0-0.1 and 4.2-823K) via melting. TaFeSb forms below 850°C via TaSb2+TaFe2 = 2TaFeSb. Physical properties of Ta1-xTixFeSb (0-0.1 and 4.2-823K) show a gradual change from semiconducting to metallic behaviour. Although Sv exhibits its largest values (above 230muV/K) for x=0.03, the peak of pf (above 6mW/mK2) was observed for x=0.06, due to rather low electrical resistivities. In combination with the suppressed phonon thermal conductivity, due to a unique role of Ti, ZT900K=1.0 for Ta0.94Ti0.06FeSb, close to the highest values reported for Hf-free p-type HH-systems.
2:20 PM Invited
Polar Tintermetallics as High Temperature Thermoelements: Franck Gascoin1; 1CRISMAT-UNICAEN-CNRS
The focus of this presentation is thermoelectric materials with potential industrial applications at high temperature (above 600°C) to harvest waste heat and convert it into usable energy. For example, facilities in the steel, non-ferrous and glass industries use a lot of energy, 50% of which gets lost during the production process. To meet these requirements, the research and development of high temperature stable thermoelectric materials is imperious.Besides the now well known Zintl phase 14-1-11, other chemical systems can withstand high temeprature, these will be discussed here.
2:40 PM Invited
Thermal Storage Effect on Thermoelectric Module: Albert T. Wu1; Chun-Hsien Wang1; Yu-Chien Wang1; 1National Central University
For Bi2Te3 and PbTe thermoelectric materials, the working temperature ranges are around 300 to 500 K and 400 to 800 K, respectively. Intermetallic compounds (IMC) would form at the interfaces between the thermoelectric materials and the electrodes when the modules are operating at elevated temperatures for long time. This study investigates the interfacial reaction and its corresponding thermoelectric properties as well as the reliability when the modules were aged at high temperature for different duration of times. For Bi2Te3, both Ni and Co are good diffusion barrier for enhancing the joint reliability. For PbTe, Co is required for inhibiting severe interaction with the electrodes. The growth of IMCs in different systems is examined by scanning electron microscope. The reliability of the joints is evaluated by shear test. The variation of thermoelectric properties with different aging conditions are also studied.
3:00 PM
Black Phosphorus and Semiconducting Transition Metal Dichalcogenides in Contact with Transparent Conductive Oxides for High Efficiency Solar Cells: Ravindra Mehta1; Avra Bandyopadhyay1; Anupama Kaul1; 1University of North Texas
Two-dimensional materials such as MoS2 are used as hole transport layer for increasing efficiency of solar cells. Transparent conductive oxides are commonly used as fore contacts in the solar cells. Monolayer MoS2 in contact with AZO has 85% transmittance in the visible region, a low threshold voltage (0.69V) and a large switching ratio (4 × 108). Studying contact of transparent conductive oxide with two-dimensional materials is essential for solar cells applications. Black phosphorus offers unique attributes of both transition metal dichalcogenides and graphene. We will report on chemical vapor deposition grown transition metal dichalcogenides such as MoS2 and WSe2 and mechanically exfoliated black phosphorus that have been transferred onto transparent conductive oxides. In this work, the opto-electronics performance of various transparent conductive oxides in contact with black phosphorus and various transition metal dichalcogenides will be evaluated and its role in improving the efficiency of solar cells will also be studied.
3:20 PM Invited
Laser Additive Manufacturing of Bismuth Telluride and Silicide Thermoelectric Materials: Saniya Leblanc1; Haidong Zhang1; Ryan Welch1; George Nolas2; Yohann Thimont3; 1George Washington University; 2University of South Florida; 3Universite Paul SABATIER CIRIMAT
Traditional thermoelectric device manufacturing uses bulk material processing with machining, assembly, and integration steps which lead to material waste and performance limitations. This approach offers virtually no flexibility in designing the geometry of thermoelectric modules. Additive manufacturing can overcome these challenges. This talk will describe our progress in laser-based additive manufacturing of tellurides and silicides. Laser powder bed fusion (selective laser melting) is an additive process which locally melts successive layers of material powder to construct 3D objects. Applied to thermoelectric materials, this technique could enable new geometries and architectures, nano- to meso-scale structuring, and material-to-device integration. We will discuss the impact of laser processing on microstructure and phase, and we characterize the key thermoelectric properties (Seebeck coefficient, electrical resistivity, thermal conductivity) as a function of temperature for parts made via laser additive manufacturing. Finally, we demonstrate the impact novel geometries can have on thermoelectric device performance using multiphysics modeling.
3:40 PM Break
4:00 PM Invited
Lattice Hardening Due to Vacancy Diffusion in (GeTe)mSb2Te3 Alloys: Alexandra Zevalkink1; Wanyue Peng1; David Smiadak1; Michael Boehlert1; Spencer Mathers1; Jared Williams1; Donald Morelli1; 1Michigan State University
GeTe-Sb2Te3 alloys in recent years have emerged as excellent thermoelectric materials, with reports of zT > 2 for Ge-rich compositions. These alloys exhibit a solid-state phase transition from a layered rhombohedral structure to a cubic rocksalt structure at intermediate temperatures. It is typical for materials to soften with increasing temperature due to thermal expansion, which impacts the speed of sound and thermal conductivity. While we observe softening in GeTe, Sb2Te3, and Bi2Te3 with increasing temperature, our study reveals anomalous hardening of the elastic moduli in the alloy (GeTe)17Sb2Te3 leading up to the phase transition. We attribute this to the gradual diffusion of the vacancies preceding the phase transition from a layered compound to a cubic structure and we expect such stiffening is a general feature of all (GeTe)mSb2Te3 and (GeTe)mBi2Te3 alloys, playing a fundamental role in the thermal conductivity in this important class of thermoelectric materials.
4:20 PM
Realizing the Compositional Homogeneity in GeTe-based Thermoelectric Materials and Phase Transition Behavior: Yi-Fen Tsai1; Pai-chun Wei2; Hsin-Jay Wu1; 1National Chiao-Tung University; 2Computer, Electrical, and Mathematical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST)
Lately, the thermoelectric community has been fascinated by the Germanium-tellurides (GeTe-based alloys) which feature high electrical conductivity and high thermal conductivity. Additionally, the GeTe undergoes a phase transition from the low-temperature rhombohedral α-GeTe to high-temperature cubic β-GeTe at 673 K, which induces the discontinuities in temperature-dependent TE property curves. In this work, the GeTe is doped with multiple dopants (e.g., Sb, Cu, Se, Bi, Ga, Sn.) via long-term heat treatment, and we further realized that the α-to-β phase transition could be greatly affected by the dopants. For example, the addition of Sb, Se, and Mn effectively lower the α-to-β phase transition temperature and can be viewed as stabilizers for cubic β-GeTe. The compositional homogeneity and microstructural texture for α-GeTe and β-GeTe vary dramatically in those Sb-, Se- and Mn-doped GeTe, which eventually lead to a wide distribution in their zT peak values, ranging from 0.8 to above 2 (723 K).
4:40 PM
Optimizing Power Factor in Rare Earth-free CoSb3-Skutterudite Thin Films: Cedric Bourges1; Isao Ohkubo1; Naohito Tsujii1; Takao Mori1; 1NIMS
In the present study, we have focused on the elaboration of control CoSb3 thin films by magnetron sputtering which is an attractive technique for industrial development of thermoelectric (TE) thin films. We have successfully synthesized sputtering targets with a reliable approach in order to obtain high-quality films with control stoichiometry. TE properties were then probed and revealed a reliable n-type behavior characterized by poor electrical transport properties. Telluride substitution was realized by co-sputtering deposition and allowed to obtain a significant enhancement of the power factor with promising values of PF ≈ 0.3-0.5 mW/m.K2 near room temperature. It is related to the Te doping effect which leads to an increase of the Seebeck coefficient and the electrical conductivity simultaneously. However, despite this large improvement, the properties remained far from the bulk material and further development are engaged to improve the carrier mobility reduced by the thin film formatting.
5:00 PM
Severe Plastic Deformation (SPD) via High Pressure Torsion (HPT) a Perfect Tool not Only to Enhance ZT of Thermoelectric Materials but Also to Produce Them: Gerda Rogl1; Ernst Bauer2; Michael Zehetbauer3; Peter Rogl3; 1CDL University of Vienna; 2TU Wien; 3University of Vienna
SPD via HPT can enhance the values of the figure of merit, ZT, of thermoelectric materials due to the ultra fine-grained microstructure in combination with a high level of defects, which significantly increase the scattering of the phonons, thus leading to a minimum of lattice thermal conductivity, even if the electrical resistivity is enhanced. Especially for skutterudites, it is one of the major techniques to boost ZT of hot pressed samples. In addition it is possible to achieve high-ZT p- and n-type bulk skutterudites by processing cold pressed industrially produced raw powder, applying a modified HPT equipment at elevated temperature under argon atmosphere. This new method, especially using large HPT facilities is time and energy saving and therefore perfect for industrial use.Concerning mechanical properties, the application of SPD methods significantly raises the strength although the density becomes slightly lower, while leaving the elastic moduli more or less unchanged.
5:20 PM Concluding Comments