Ultrafast high-temperature sintering and thermoelectric properties of n-doped Mg₂Si

Ultrafast high-temperature sintering (UHS) is a recently proposed technique able to synthesize and sinter dense materials within seconds. Although UHS has already proved its effectivity with a large set of materials, spanning from refractory ceramics to complex metal alloys, any application to thermoelectric materials is today still lacking. Mg₂Si is a well-established thermoelectric material. It is based on wide available non-critical raw materials, it is non-toxic, lightweight and it expresses its best thermoelectric performances in the intermediate temperature range (up to about 600 °C). Mg₂Si is typically produced with powder processing by Spark Plasma Sintering (SPS), partially limiting its widespread diffusion also due to the costly production technique. Here we present a simple route to sinter Mg₂Si pressed powders by UHS. The process allowed to obtain dense samples (with relative densities >95%) with 20 s heating up to about 1080 °C followed by a rapid free cooling, a total thermal history below 1 min, and with energy demand at the Wh scale. The high process rate proved its efficacy in preventing grain growth and in avoiding any significant Mg evaporation. A full thermoelectric functional characterization is presented for Mg₂Si and Bi-doped Mg₂Si, together with a comparison with SPS-produced properties.