Thermoelectric performance of Si80Ge20−xSbx based multiphase alloys with inhomogeneous dopant distribution

Si80Ge20−xSbx based alloys were synthesized employing high energy ball milling and induction hot pressing with change in milling time and Sb content. With increasing milling time, the grain size decreases and the elements become more evenly distributed for both the as-milled powder and the hot pressed samples. Microstructure analysis on the hot pressed samples revealed presence of nano-features of 10-200 nm and existence of multiphases, including Si-rich matrix, Ge-rich phase and Si-Ge-Sb ternary precipitates. Moreover, for the dry-milled samples the intended doping element, Sb, prefers to locate in the Ge-rich phase or in the Si-Ge-Sb ternary area rather than in the Si-rich matrix. Sb doping leads to n-type conduction, but Sb content has little influence on the carrier concentration. The milling time has double effects on the thermoelectric properties. On the one hand, the inhomogeneous dopant distribution in the samples prepared by shorter milling time leads to higher carrier mobility and higher power factor due to modulation doping effect. But on the other hand, the thermal conductivity decreases with increasing milling time due to enhanced boundary scattering of middle-to-long term phonons at the nano-to-micro sized boundaries and impurity interfaces. As a result, the 2 h dry-milled and hot pressed sample obtains the maximum power factor of 2.5 × 10−3 Wm−1 K−2 at 780 K, the more homogeneous samples prepared by 16 h of dry-milling or wet-milling exhibit the lowest power factor and lowest ZT, while the 6 h dry-milled and hot pressed sample with nominal composition Si80Ge18Sb2 possesses the maximum ZT value of 0.67 at 1000 K.