Radioisotope thermoelectric generators produce electricity for space exploration. These generators use thermoelectric material, usually a silicon-germanium alloy, to turn heat from radioactive decay into electricity. Recently, the thermoelectric performance (ZT) of Si80Ge20 was improved by reducing the grain size to the nanometer range, significantly lowering the thermal conductivity.
To optimize the properties of silicon-germanium we studied the effect of doping concentration and processing parameters on the microstructure and properties of boron-doped Si80Ge20 produced by arc melting, ball milling and spark plasma sintering. The thermal conductivity was estimated with a model. The electrical conductivity and Seebeck coefficient were measured.
Some conclusions from this work are that the current production process can be used to produce nanostructured Si80Ge20Bx with a crystallite size of 50-100 nm. This material reaches a maximum, but not necessarily optimal, doping concentration when x=1 due to limited solubility. The material suffers from grain growth when exposed to high temperatures for several days. The use of iron in the ball milling process significantly affected the microstructure and properties.