The effect of synthesis parameters on transport properties of nanostructured bulk thermoelectric p-type silicon germanium alloy

Nanostructured silicon germanium thermoelectric materials prepared by mechanical alloying and sintering method have recently shown large enhancement in figure-of-merit, ZT. The fabrication of these structures often involves many parameters whose understanding and precise control is required to attain large ZT. In order to find the optimum parameters for further enhancing the ZT of this material, we have grown and studied both experimentally and theoretically different nanostructured p-type SiGe alloys. The effect of various parameters of milling process and sintering conditions on the thermoelectric properties of the grown samples were studied. The electrical and thermal properties were calculated using Boltzmann transport equation and were compared with the data of nanostructured and crystalline SiGe. It was found that the thermal conductivity not only depends on the average crystallite size in the bulk material, but also it is a strong function of alloying, porosity, and doping concentration. The Seebeck coefficient showed weak dependency on average crystallite size. The electrical conductivity changed strongly with synthesis parameters. Therefore, depending on the synthesis parameters the figure-of-merit reduced or increased by 60% compared with that of the crystalline SiGe. The model calculation showed that the lattice part of thermal conductivity in the nanostructured sample makes 80% of the total thermal conductivity. In addition, the model calculation showed that while the room temperature hole mean free path (MFP) in the nanostructured sample is dominated by the crystallite boundary scattering, at high temperature the MFP is dominated by acoustic phonon scattering. Therefore, the thermal conductivity can be further reduced by smaller crystallite size without significantly affecting the electrical conductivity in order to further enhance ZT. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim