Effects of locally-gradient Co-doping on the electron properties of Bi2Te2.1Se0.9+0.33wt.% Co composite

Abstract

Interconnected effects of the locally-gradient Co-doping and electron scattering by filler inclusions has been for the first time observed in thermoelectric Bi2Te2.1Se0.9 + 0.33 wt.% Co composite, prepared via spark plasma sintering (SPS) of matrix and filler powders at temperatures TS=573, 598, 623, 648, 673 and 698 K and at the same time t = 2.5 min. During SPS-process, initial Co inclusions, randomly distributed inside the Bi2Te2.1Se0.9 matrix, act as electron doping sources. High-temperature diffusion embedding of Co atoms in the Bi2Te2.9Se0.1 structure results in increasing electron concentration. This diffusion process corresponds to diffusion from limited doping sources, which are spatially separated inside the Bi2Te2.9Se0.1 matrix. Separate filler inclusion is each such source. Concentration profiles of Co distribution were applied to extract a diffusion coefficient, DCo, of Co atoms inside Bi2Te2.9Se0.1. The DCo increases with TS in accordance with Arrhenius law. Owing to locally-gradient distribution of doping Co atoms, the Co-doping is locally-gradient doping. Initial inclusions, consisting of only Co atoms, are gradually transforming into the “core Co@shell CoTe2” inclusions. This transformation is related to diffusion of Co atoms that is accompanied by subsequent solid-state Co+2Te→CoTe2 reaction. Room-temperature weighted mobility of electrons increases with growing TS. This behavior of mobility can be originated from electron scattering by magnetic moments of the Co cores.