Band Engineering and Synergistic Modulation Doping for Excellent Thermoelectric Performance in Composites Ti_1–xNb_xCoSb–Nb_0.8+δCoSb

Abstract

We report on the dramatic improvement in the thermoelectric performance of TiCoSb by introducing threedimensional (3D) modulation doping and synergistic band engineering in the composites of the form (1 − f)A + f B, where A and B refer to the phases Ti 1−x Nb x CoSb and Nb 0.8+δ CoSb, respectively, and f is the volume fraction of phase B. We show that the electrical conductivity and Seebeck coefficient of these composites increase simultaneously due to modulation doping, giving rise to colossal power factor (PF) enhancement from 0.3 μW cm −1 K −2 (TiCoSb) to 18 μW cm −1 K −2 (x = f ≈ 0.05) at 300 K and exceeding 25 μW cm −1 K −2 over a broad temperature range (T > 600 K). Due to the Ti−Nb point mass fluctuation in phase A, high concentration of defects in phase B, and interfacial phonon scattering between A and B, these composites also exhibit very low lattice thermal conductivity (κ L ), resulting in a high zT of 0.81 near 970 K. The simulation of κ L using the Klemens model successfully describes the significant reduction of κ L for these composites, observed experimentally. Our ab initio DFT calculations show that Ti 1−x Nb x CoSb exhibits band convergence as x increases, which contributes to improving the charge transport. Thus, benefiting from the synergistic effect of band convergence and 3D modulation doping, a high zT is obtained.

Thermoelectric Performance Improvement in the ZrNiSn-Based Composite via Modulating Si Addition

“…Modulation doping has been effectively employed in SiGe alloy 36 and HH compound TiCoSb. 37 This work incorporates Si particles of varying types (either n-or p-type) and doping concentrations into ZrNiSn to adjust the thermoelectric properties. The secondary phases consist of in situ semimetallic ZrNiSi and the residual Si.…”