2020同步年報

080 ACTIVITY REPORT 2020 A neutron-scattering experiment has been performed on SIKA for Ge 0.92 Sb 0.08 Te single crystals that produced a record high zT , 2.2 at 740 K, with an optimal hole carrier concentration ≈ 4 × 1020 cm −3 that simultaneously maximizes the power factor (PF) ≈ 56 µW cm −1 K −2 and minimizes the thermal conductivity ≈ 1.9 Wm −1 K −1 . The Ge 0.92 Sb 0.08 Te material exhibits a sig- nificant modification for phonon dispersion with an extra phonon excitation about ≈ 5–6 meV at the Γ point of the Brillouin zone as confirmed. The theoretical support with density-functional theory (DFT) confirmed this phonon excitation, and pre- dicted another higher-energy phonon excitation ≈ 12–13 meV at the W point. These phonon excitations collectively increase the number of phonon decay channels leading to a softening of the phonon frequencies such that a three‐phonon process is dominant in Ge 0.92 Sb 0.08 Te, in contrast to a dominant four‐phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improve thermoelectric (TE) performance. Fig. 1 : Inelastic neutron scattering tests of GeTe and Sb-doped GeTe. Phonon dispersion relation from S ( Q , E ) with function of energy transfer E and q along [0K0] for (a) pristine GeTe, and (c) Ge 0.92 Sb 0.08 Te crystals with TA and LA branches. The solid circles in (a) were determined with a multi-peak Gaussian function from panel (b), the red dashed lines are for visual guidance. (b) and (d) show phonon energy spectra for energy scans along [0K0] with a constant Q of k = 1–1.5 for GeTe and Ge 0.92 Sb 0.08 Te crystals respectively. The open symbols represent the data collected from the triple-axis spectrometer of SIKA ; the solid lines in (b) are numerical ts with a multi-peak Gaussian function, and in (d) are for visual guidance. [Reproduced from Ref. 1] High zT and Its Origin in Sb‐Doped GeTe Single Crystals A record high zT, 2.2 at 740 K, is reported for Ge 0.92 Sb 0.08 Te single crystals, with an optimal hole carrier concentration ≈ 4 × 1020 cm −3 that simultaneously maximizes the power factor (PF) ≈ 56 µW cm −1 K −2 and minimizes the thermal conductivity ≈ 1.9 Wm −1 K −1 . Neutron inelastic scattering and DFT calculations revealed the origin.