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All-scale hierarchical nanostructures and superior valence band convergence lead to ultra-high thermoelectric performance in cubic GeTe
Energy & Environmental Science ( IF 32.5 ) Pub Date : 2022-09-13 , DOI: 10.1039/d2ee02752f
Debattam Sarkar, Manisha Samanta, Tanmoy Ghosh, Kapildeb Dolui, Subarna Das, Kumar Saurabh, Dirtha Sanyal, Kanishka Biswas

GeTe is among the most fascinating inorganic compounds for thermoelectric (TE) conversion of waste heat into electricity. However, the TE performance in its ambient rhombohedral phase is strongly impeded by natural excessive Ge vacancies resulting in high hole concentration, and the rhombohedral to cubic phase transition at high temperature (T ∼ 700 K) deteriorates its mechanical robustness. Thus, stabilization of the high T cubic phase near ambient conditions would resolve many of these unwarranted challenges. Importantly, the higher symmetric cubic phase is beneficial for large Seebeck coefficient (S) due to its higher valence band (VB) degeneracy. Here, we show a simple innovative strategy of using high energy ball-milling (BM) and spark plasma sintering (SPS) to promote the crystal symmetry in Sb doped GeTe, which stabilizes in a near-cubic phase under ambient conditions. Consequently, the energy gap between the primary and secondary VBs drastically decreases to ∼0.06 eV and the band degeneracy enhances, leading to high S. BM followed by SPS simultaneously lead to the formation of hierarchical nano/meso architectures comprising solid solution point defects, Ge and GeSb4Te7 nanoprecipitates and nano/mesoscale grains, which efficiently scatter broad length scales (few Å–200 nm) of phonons responsible for thermal transport. As a result, the lattice thermal conductivity (κlat) is suppressed to ∼0.59 W m−1 K−1. This combined effect of VB convergence due to enhanced crystal symmetry and ultra-low κlat via hierarchical nanostructuring results in an ultra-high TE figure of merit (zT) ∼2.5 at 662 K in Ge0.9Sb0.1Te-BM + SPS. Furthermore, the fabricated double leg thermoelectric device shows promising output power density of ∼570 mW cm−2 for a ΔT of 442 K.

中文翻译:

全尺度分级纳米结构和优异的价带收敛性导致立方 GeTe 的超高热电性能

GeTe 是用于将废热热电 (TE) 转化为电能的最令人着迷的无机化合物之一。然而,在其环境菱面体相中的 TE 性能受到天然过量 Ge 空位的强烈阻碍,导致空穴浓度高,并且在高温(T ∼ 700 K)下菱面体向立方相变降低了其机械强度。因此,在环境条件附近稳定高T立方相将解决许多这些无根据的挑战。重要的是,较高的对称立方相有利于较大的塞贝克系数(S) 由于其较高的价带 (VB) 简并性。在这里,我们展示了一种简单的创新策略,即使用高能球磨 (BM) 和放电等离子烧结 (SPS) 来促进 Sb 掺杂的 GeTe 中的晶体对称性,其在环境条件下稳定在近立方相。因此,初级和次级 VB 之间的能隙急剧减小至 0.06 eV,能带简并性增强,导致高S。BM 和 SPS 同时导致形成包含固溶点缺陷、Ge 和 GeSb 4 Te 7的分级纳米/介观结构纳米沉淀物和纳米/中尺度晶粒,它们有效地散射负责热传输的广泛长度尺度(几 Å–200 nm)的声子。结果,晶格热导率(κ lat)被抑制到~0.59 W m -1 K -1。由于增强的晶体对称性和通过分级纳米结构实现的超低κ lat导致的 VB 收敛的组合效应导致在 Ge 0.9 Sb 0.1 Te-BM + SPS中在 662 K 时的超高 TE 品质因数 ( zT ) ~2.5。此外,制造的双腿热电器件显示出有希望的输出功率密度~570 mW cm -2 ΔT442 K。
更新日期:2022-09-13
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