Realizing high figure of merit plateau in Ge1-xBixTe via enhanced Bi solution and Ge precipitation

We reported in this work that both a broad plateau (623-773 K) of high figure of merit ZT (over 1.8) and a decent peak ZT similar to 2.0 at 773 K can be simultaneously achieved in Bi-doped Ge1-xBixTe sample as x = 0.07 when it experienced a water quench process. Distinct from Bi2Te3 alloying, Bi-doping in GeTe not only produces Ge "neutral vacancies" which eventually evolve into planar van der Waals gaps, but also generate residual Ge atoms which precipitate intrinsically as secondary phase. Additional water quench process is concluded to be of two main functions: (i) increasing the solution of Bi in GeTe matrix, and (ii) suppressing the Ge phase aggregation. The former function further modulates the matrix structure and electronic band, resulting in an improved Seebeck coefficient; while the latter one results in increased Ge/GeTe phase boundaries hence enhanced phonon scattering. Besides, the structure of intrinsic Ge precipitation and enhanced Bi solution is thermally stable; the water quenched Ge0.93Bi0.07Te sample can retain the high thermoelectric performance after 2-days annealing at 773 K. Our finding in this work might shed light on future researches in GeTe and related thermoelectric material systems. (C) 2019 Elsevier B.V. All rights reserved.