High temperature thermoelectric properties of nano-bulk silicon and silicon germanium

Point defect scattering via the formation of solid solutions to reduce the lattice thermal conductivity has been an effective method for increasing ZT in state-of-the-art thermoelectric materials such as Si -Ge, BiTe -SbTe and PbTe-SnTe. However, increases in ZT are limited by a concurrent decrease in charge carrier mobility values. The search for effective methods for decoupling electronic and thermal transport led to the study of low dimensional thin film and wire structures, in particular because scattering rates for phonons and electrons can be better independently controlled. While promising results have been achieved on several material systems, integration of low dimensional structures into practical power generation devices that need to operate across large temperature differential is extremely challenging. We present achieving similar effects on the bulk scale via high pressure sintering of doped Si and Si-Ge nanoparticles. The nanoparticles are prepared via high energy ball milling of the pure elements. The nanostructure of the materials is confirmed by powder X-ray diffraction and transmission electron microscopy. Thermal conductivity measurements on the densified pellets show a drastic reduction in the lattice contribution at room temperature when compared to doped single crystal Si. The combination of low thermal conductivity and high power factor leads to an unprecedented increase in ZT at 1275 K by a factor of 3.5 in n-type nanobulk Si over that of single crystalline samples. Experimental results on both n-type and p -type Si are discussed in terms of the impact of the size distribution of the nanoparticles, doping impurities and nanoparticle synthesis processes. © 2009 Materials Research Society.