Comparison of thermoelectric properties of p-type nanostructured bulk Si_(0.8)Ge_(0.2) alloy with Si_(0.8)Ge_(0.2) composites embedded with CrSi_2 nano-inclusisons

摘要

P-type nanostructured bulk Si_(0.8)Ge_(0.2) and Si_(0.8)Ge_(0.2) composites with CrSi_2 nano-crystallite inclusions were synthesized via sintering approach. The composite structure showed power factor enhancement compared with nanostructured Si_(0.8)Ge_(0.2) alloy. The experimental data for both structures were modeled with solving the multiband Boltzmann transport equation in the relaxation time approximation for charge carriers and phonons. The Si_(0.8)Ge_(0.2) crystallite boundary scattering was modeled by a cylindrical potential barrier at the interfaces and the effects of CrSi_2 nano-inclusions were modeled by spherical potential barriers in the Si_(0.8)Ge_(0.2) lattice. The model calculations revealed that the enhancement in power factor is not an effect of hot carrier energy filtering, but it is due to the enhancement in charge carrier mobility in the composite structure. The analysis of charge carrier mobility components showed that while in nanostructured Si_(0.8)Ge_(0.2) the ionize impurities and acoustic phonons are dominant scatterers, in the composite structure the scattering by CrSi_2 nano-inclusions and acoustic phonons are dominant. The optimum size of the CrSi_2 nano-inclusions for enhancing ZT was predicted with the characteristic that ZT drops rapidly when the crystallite size decreases, but it changes slowly as it is increased above its optimum value.