Nano/microstructures and thermoelectric properties of silicon germanium manufactured using laser powder bed fusion

Abstract

Silicon germanium alloys are high-temperature thermoelectric materials that convert heat to electricity at ∼1000 °C. Yet, the current application of silicon germanium-based thermoelectric devices is limited to niche areas due to unwieldy manufacturing processes that limit the shape of thermoelectric materials to simple cuboids and thus limit power generation potential. Laser powder bed fusion of thermoelectric materials offers the potential to fabricate freeform shapes and induce nano- and microstructures favorable for thermoelectric energy conversion. Here, we successfully fabricated undoped Si₅₀Ge₅₀ and Si₈₀Ge₂₀ parts using laser powder bed fusion and investigated the resulting structure and properties. The undoped Si₈₀Ge₂₀ alloy had a maximum thermoelectric figure of merit, ZT, of 0.06 at 400 °C. The laser manufactured parts exhibited p-type behavior with a measured Seebeck coefficient that changed based on the stoichiometry. Si₅₀Ge₅₀ reached a Seebeck coefficient of 588 µV/K at 50 °C while Si₈₀Ge₂₀ reached 513 µV/K at 400 °C. Oxidation during processing contributed to balling and lack-of fusion defects and was alleviated in single melt lines by washing the powder in hydrofluoric acid prior to laser processing. Processing related defects remained in bulk samples fabricated with acid treated powder, suggesting that the processing atmosphere is a primary cause of processing-induced defects. This work advances the processing of silicon germanium alloys for thermoelectric devices by uncovering the structures and thermoelectric properties of silicon germanium processed via laser-based additive manufacturing.