Alloys and Compounds for Thermoelectric and Solar Cell Applications XI: Session I
Sponsored by: TMS Functional Materials Division, TMS Structural 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; Wan-Ting Chiu, Tokyo Institute of Technology; Chenguang Fu, Zhejiang University
Monday 8:30 AM
March 20, 2023
Room: Sapphire A
Location: Hilton
Session Chair: Hsin-Jay Wu, National Yang Ming Chiao Tung University; Takao Mori, National Institute for Materials Science
8:30 AM Introductory Comments
8:35 AM Invited
Interface and Grain Boundary Effects on Thermal and Electrical Transport: G. Jeffrey Snyder1; 1Northwestern University
Grain boundaries have a remarkable effect on the thermal and electrical transport properties of polycrystalline materials but are often ignored by prevailing physical theories. Grain boundaries and interfaces can adversely alter the properties of Solar Cells, Batteries and Thermoelectrics such as interfacial electrical and thermal resistance (Kapitza resistance) and even an interfacial Seebeck effect. Electrical grain boundary resistance can be so high in some thermoelectric materials it is the dominant property that limits zT. While small grains are usually considered beneficial for thermoelectric performance due to reduced thermal conductivity, Mg₃Sb₂ based thermoelectric materials, so far at least, contradict that trend. Indeed, atomic segregation has been recently observed at the nanometer scale in grain boundaries in many materials suggesting interfacial or complexion phases should be specifically considered when understanding nearly all thermoelectric materials.
8:55 AM Invited
Novel Mechanisms to Lower Thermal Conductivity and Enhance ZT: Takao Mori1; 1National Institute For Materials Science
We have developed novel mechanisms to lower thermal conductivity. Minute Cu doping in Mg3Sb2 resulted in interstitial doping which especially scattered phonons, leading to high conversion efficiency rivalling the best Bi2Te3-type modules [1]. The simple crystallographic parameter, partial occupancy was demonstrated to be an effective indicator to identify a new material catalogue with low thermal conductivity [2]. Electronic properties tuning resulted in very high ZT for a sulfide. Particular doping into SnTe was shown to lead to softening of the lattice, and a dramatic reduction of thermal conductivity largely exceeding the contribution from phonon scattering, resulting in a particularly high ZTav [3]. Heterogeneous bonding in mixed anion compounds was demonstrated to result in exceptionally low thermal conductivity [4]. [1] Joule, 5, 1196 (2021), Nature Commun. 13, 1120 (2022). [2] Energy Environ. Sci., 14, 3579 (2021). [3] Adv. Energy Mater., 11, 2101122 (2021).[4] J. Mater. Chem. A, 9, 22660 (2021).
9:15 AM Invited
Crystal Structure, Phase Stability, and Thermoelectric Properties of Medium-Temperature IV-VI Thermoelectric Materials: Hsin-Jay Wu1; Szu-Chien Wu1; 1National Yang Ming Chiao Tung University
Thermoelectric materials can directly convert thermal energy into electrical energy, becoming one of the promising green energies in recent years. Two medium-temperature IV-VI thermoelectric materials, SnTe and GeTe, are brought together to form the (SnTe-GeTe)-based alloys to tune the transport properties using phase diagram engineering. Herein, ternary Sn-Ge-Te and part of binary Ge-Te phase diagrams were constructed experimentally. Regarding thermoelectric properties, the Bridgman-grown Ge-doped Sn1-yGeyTe alloys show enhanced Seebeck coefficients and reduced thermal conductivity. The Pb-doped Sn0.7Ge0.3Te alloy performed an improved zT = 0.82, showing 600 % enhancement compared with the pristine SnTe. The TEM analysis of Pb-doped Sn0.7Ge0.3Te revealed the presence of dislocation networking, unpuzzling the mechanism for the enhanced phonon scattering. In short, this study successfully utilizes a synergistic approach of phase diagram engineering, carrier optimization, and defect controlling to optimize the TE performance of IV-VI compounds.
9:35 AM
Shapes of Phase Boundaries in Isothermal Phase Diagrams: Adetoye Adekoya1; G. Jeffrey Snyder1; 1Northwestern University
Doping/alloying is essential to controlling the electrical and thermal properties of thermoelectric materials. The solubility of these dopants in the bulk system is often in such dilute concentrations that they can best be described as 0-D defects in an otherwise essentially uniform crystal. Though dilute, the defects can nevertheless be visualized on a phase diagram as providing some width to nominally single-phase regions. The area/width of this phases are known to be set directly by the defect energy of the defects, however little attention is paid to the shape of these regions even though they can have tremendous impact on the maximum defect solubility achievable and offer a thermodynamic guide towards engineering of materials. The thermodynamic factors that impact the shape of the phase diagrams is used to derive an analytic expression for the expected shape and show how they can provide a guide to optimal doping of semiconductors.
9:55 AM
Redissolution of Ge precipitates Boosts Thermoelectric Performance and Self-tunes the Carrier Concentration in Homogenous GeTe materials: Yi-Fen Tsai1; Hsin-Jay Wu1; 1National Yang Ming Chiao Tung University
The germanium-telluride (GeTe) is one of the attractive thermoelectric (TE) materials due to its outstanding TE performance. However, the Ge vacancy with low formation energy results in high vacancies concentration in GeTe ( ~10-21 cm-3) which exceeds the optimal region for p-type TE alloys. Controlling the Ge vacancies and Ge precipitates become crucial in manipulating the carrier concentration and TE performance. This work utilizes pulsed hot-pressing to enable the redissolution of Ge precipitates and reduction in Ge vacancies sites for an In-doped GeTe. In the meantime, the nano-scale defects are introduced to lower the thermal conductivity. The average PF shows a 200% enhancement and the peak zT is 144% higher than the pristine GeTe.
10:15 AM Break
10:35 AM Invited
Unexpected Reactions Observed in Ni/SnSe2 Couples: Sinn-wen Chen1; Chao-hong Wang2; Jia–Ruei Chang1; He-Cheng Yang2; 1National Tsing Hua University; 2National Chung Cheng University
Unexpected reactions are observed on the bare surface of an Ni layer which has no direct contact with the SnSe2 substrate in the Ni/SnSe2 reaction couple reacted at 500℃ and 300℃. The reaction phase is determined to be an NiSe phase. It has been found, although both Se and SnSe2 are not gas phases at 500℃ and 300℃, Se vapor from the non-plated SnSe2 substrate reacts with the bare surface of the Ni layer. With the continuous consumption of Se vapor, parts of the SnSe2 substrate decompose and transform into SnSe. A separate Ni/Se reaction experiment is carried out to verify the assumption that Ni reacts with Se vapor.
10:55 AM
Phase Diagrams of the Ag-Cu-Se-Te Quaternary System: Yohanes Hutabalian1; Sinn-wen Chen1; 1National Tsing Hua University
The Ag-Cu-Te-Se quaternary system has many well-known thermoelectric compounds and is an important material system. Phase diagrams of the Ag-Cu-Se-Te quaternary system are determined in this study. Phase diagrams of its six binary constituent systems are adopted from assessed literatures. Determinations of the isothermal sections and liquidus projections of its four constituent ternary systems, Ag-Cu-Se, Ag-Se-Te, and Ag-Cu-Te, Cu-Se-Te, are carried out. Ternary compounds, AgCuSe and AgCuTe, are found at 400 and 600℃ in the Ag-Cu-Se and Ag-Cu-Te systems, respectively. The compositional regimes of the miscibility gaps in the Ag-Cu-Se and Ag-Cu-Te ternary systems are quite significant at 400 and 600℃. There is no ternary compound in the Ag-Se-Te system, and there are two ternary compounds in the Cu-Se-Te system. A Calphad-type thermodynamic modeling is carried out. A good agreement has been obtained between the experimental and the calculated phase diagrams.
11:15 AM
Concluding Maximum Solubility Using Impurity Phase Stoichiometry: Shashwat Anand1; Chris Wolverton2; Jeff Snyder2; 1Lawrence Berkeley National Laboratory; 2Northwestern University
Solubility of dopant species is an important materials property which determines its potential for optimizing its thermoelectric figure of merit. Experimental reports often conclude reaching the solubility limit as soon as any impurity phases are observed. However, this approach does not necessarily ensure reaching the actual maximum solubility in compounds. As a result, solubility limit in many thermoelectric materials were mistakenly underestimated in the past, thereby unknowingly discouraging further attempts and delaying the realization of better and reproducible thermoelectric performance. In this talk, we summarize in simple graphical guidelines --- rooted in thermodynamics --- how the maximum dopant solubility can be concluded using the stoichiometry of the impurity phase observed.
11:35 AM
Enhanced Thermoelectric Performance by Compositional Modulation in AgSbTe2: Bo-Chia Chen1; Hsin-Jay Wu1; 1National Yang-Ming Chiao Tung University
AgSbTe2 is a promising thermoelectric compound in the mid-temperature range. Even slightly changing the ratio of each element can optimize their thermal and electrical properties effectively. However, the high electrical resistivity (ρ) affects its power factor (S2ρ), resulting in the figure-of-merit zT only ~1 at 623 K. In this research, the stoichiometric ratios for the pristine AgSbTe2 is tuned, resulting in the enhanced Seebeck coefficient (S) as well as the higher electrical conductivity. Combined with the low thermal conductivity (κ), the figure-of-merit zT increased to ~1.4. Secondly, the κ and κL are further reduced after proper doping, while the power factor remains unchanged. Our AgSbTe2-based alloys attain the peak zT ~1.7 at 623 K and an average zTave ~1.15 within 300 - 623 K, outperforming most AgSbTe2-based materials.
11:55 AM
Phase Diagram of Ternary Zn-Sb-Cu System and Thermoelectric Properties of Copper Doped Zn4Sb3: I-Lun Jen1; You-Kai Su2; Hsin-Jay Wu1; 1National Yang Ming Chiao Tung University; 2National Sun Yat-Sen University
Thermoelectric (TE) materials have attracted growing attention in recent years. β-Zn4Sb3 is a promising mid-temperature TE material because it comprises cost-effective and earth-abundant elements. Nevertheless, the disordering Zn-interstitial in the Sb-framework induces the structural instability, limiting the utilization of Zn4Sb3-based alloys in TE generators. In this study, the thermodynamic approach coupled with Bridgman growth were aided in boosting the TE performance while settling the instability issue. The 623 K isothermal section Zn-Sb-Cu system is constructed in this work. In addition, Phase Diagram Engineering is an effective strategy to determine the solubility range of Cu content. At 623 K, the maximum solubility in Zn4Sb3 is less than 4 at.%Cu. Consequently, the Cu-doped Zn4-xSb3 alloys are prepared, and the Cu-substituted alloy achieves a figure of merit zT of 0.9 at 623 K, resulting from the enhanced power factor S2ρ-1 ~1.2 (mW/mk2).