Dopant dependent microstructure of hot-pressed SiGe alloys and its implications on thermoelectric properties

SiGe is a proven high temperature thermoelectric material for space application. We have investigated the role of dopants (P and B) on the densification of SiGe alloys. It was observed that on P doping there is a formation of liquidous SiP phase in the SiGe matrix during hot pressing that improves the diffusion across grain boundaries and as a result ∼98 % of theoretical density was achieved. In case of identically prepared B doped samples, the segregation of B resulted in porous microstructure and low density ∼89 %. Such a difference in microstructure resulted a figure-of-merit of ∼1.29 (at 1100K) for n-type SiGe (P doped) and ∼0.86 (at 1100K) for p-type SiGe (B doped) at a hot press temperature of 1273 K. The study suggests that in case of SiGe apart from doping the densification of materials plays a very important role in improving its thermoelectric performance.