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Advanced Half-Heusler Thermoelectric Materials (AHHTM)

Subject Area Synthesis and Properties of Functional Materials
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 392228380
 
Final Report Year 2023

Final Report Abstract

By focusing on single crystals studies, we reveal the intrinsic electronic structure and establish a scattering phase diagram of ZrNiSn‐based half‐Heusler thermoelectric materials. We found the intrinsic band gap of ZrNiSn can be much larger than previously reported in the polycrystalline samples with excess Ni‐induced in‐gap states. The elimination of Ni in‐gap states will help suppress the bipolar effect and thus, pave the way for further improvement of this famous n‐type half‐Heusler thermoelectric material. Furthermore, we also developed two types of n‐type half‐Heusler thermoelectric materials, i.e., (Zr,Hf)CoSb and NbCoSn. We found that isoelectronic alloying using period‐6 and period‐ 5 transition metals would result in strong phonon scattering while having a gentle effect on the carrier transport. Heavy‐element as the dopant can simultaneously optimize the electrical properties and suppress the lattice thermal conductivity, leading to improved thermoelectric performance. These results will be helpful for the development of high‐performance half‐Heusler thermoelectric materials for power generation applications. Using ab initio and classical simulation techniques, we have studied 18 electron and cation deficient 19 electron half‐Heusler thermoelectrics. Results give insights into electronic and atomic structures and material properties that help to improve thermoelectric materials. We studied how domain boundaries lead to a strong reduction of the heat transport, while the influence on the Young’s modulus is low. Our results predict that rough, broadened domain boundaries lead to a particularly strong reduction of heat transport. With numerical methods we studied cation deficient 19‐electron half‐Heusler materials V1‐xCoSb, Nb1‐xCoSb, and Ta1‐xCoSb, as well as the alloys, (V1‐zNb1‐z)1‐xCoSb and (Ta1‐zNb1‐z)1‐ xCoSb. In all cases we found most stable materials at 20% vacancies, i.e. x=0.2. Changing the composition and the vacancy fraction x, one can create p‐ and n‐type semiconductors with tuneable electronic properties. Cation deficient alloys show no large range domains but complex interlaced patterns. By combining experimental and theoretical approaches, we have comprehensively investigated the interstitial defects present in ZrNiSn and NbCoSn compounds. The correlations between the phase structure, microstructure, and thermoelectric properties of ZrNiCuxSn (x = 0–0.20) and NbCo1‐xNixSn (x = 0‐0.1) are investigated with X‐ray and neutron diffraction, transmission electron microscopy, atom probe tomography, and band structure and phonon spectra calculations. Our work analyses the possibility of interstitial defects from a thermodynamic point of view and highlights the defect engineering to positively tune the thermoelectric properties in half‐Heusler compounds. Besides, p‐type NbCoSn has been obtained by Sc substitution, making it possible to fabricate thermoelectric modules with both p‐ and n‐type NbCoSn compounds.

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