Thermoelectric Properties of Cold-Pressed Higher Manganese Silicides for Waste Heat Recovery

Polycrystalline higher manganese silicide (HMS) samples with different grain sizes have been obtained by cold-pressing HMS powder under high pressure of about 3 GPa and postprocessing annealing. It was found that the cold-pressing process can reduce the grain size of HMS to 120 nm. The cold-pressed pellets were then annealed at different temperatures to obtain a series of samples with different grain sizes. For comparison, an additional sample was prepared in a regular die under low pressure of 300 MPa, which resulted in lower density and higher porosity than the high-pressure process. For these samples, the effect of grain size and porosity on Seebeck coefficient was not as apparent as that on electrical conductivity and thermal conductivity. The electrical conductivity of the cold-pressed samples increases as the grains grow, and the grain boundary connection is improved during the postprocessing annealing. A significant reduction in the thermal conductivity of the cold-pressed samples was observed. The sample prepared with the low-pressure pressing shows the lowest thermal conductivity of 1.2 W m(-1) K-1 at 300 K, which can be attributed to its high porosity of 34% and low phonon transmission coefficient through the grain boundaries. The low-temperature thermal conductivity data of all samples were analyzed to obtain the phonon transmission coefficient and the Kapitza resistance at the grain boundaries.