Influence of in situ formed MoSi2 inclusions on the thermoelectrical properties of an N-type silicon-germanium alloy

An N-type Si92Ge08 (7000 at. ppm P doping) powder (matrix) prepared by mechanical alloying was homogeneously mixed with 1.3 vol.% nanosized molybdenum. The “composite” and matrix powders were sintered by spark plasma sintering, using optimized parameters. The as-sintered “composite” material exhibits a dimensionless thermoelectrical figure of merit (ZT) of ∼1.0 at 700 °C. For the same temperature, the ZT value is only 0.7 for the as-sintered matrix alone. In the composite material, transmission electron microscopy observations enable the detection of nanosized (average diameter of ∼30 nm) MoSi2 inclusions that formed in situ during sintering via the reaction between the native silicon-germanium matrix and the added molybdenum particles. The presence of such nanosized inclusions reduces significantly the lattice contribution to the thermal conductivity of the sintered “composite” material, explaining the strong increase of the ZT parameter in comparison to the sintered matrix alone.