Alternative Approaches to Group IV Thermoelectric Materials
In the pursuit of energy efficiency, there is a demand for systems capable of
recovering waste heat. A temperature gradient across a thermoelectric material
results in the thermal diffusion of charge carriers from the hot side to the cold
side, giving rise to a voltage that can be used to convert waste heat to electricity.
Silicon germanium (SiGe) alloys are the standard materials used for thermoelectric
generators at high temperatures.
We report an alternative method for preparing p-type Si1-xGex alloys from
a boron-doped silica-germania nanocomposite. This is the first demonstration of
the thermoelectric properties of SiGe-based thermoelectrics prepared at temperatures
below the alloy's melting point through a magnesiothermic reduction of the
(SiO2)1-x(GeO2)x. We observe a thermoelectric power factor that is competitive
with the literature record for the conventionally prepared SiGe. The large grain
size in our hot pressed SiGe limits the thermoelectric figure of merit to 0.5 at
800C for an optimally doped p-type Si80Ge20 alloy.
A phosphorus-doped oxide can yield n-type Si1-xGex; however, the current
processing method introduces a background boron content that compensates ~10%
of the donor impurities and limits the thermoelectric power factor.
Spark plasma sintering of the nano-Si1-xGex yields a heterogeneous alloy with
thermal conductivity lower than that of the hot pressed homogeneous alloy due
to a reduction in the average crystallite size. Magnesiothermic reduction in the
presence of molten salts allows some control over crystallite growth and the extent
of Si-Ge alloying.