Search for: All records

Abstract Thermoelectric materials offer a unique solution for active cooling or conversion of heat to electricity within a thermal protection system due to their solid-state nature. Yet, the integration of thermoelectrics into thermal protection systems is hindered by conventional manufacturing processes, which limit the material’s shape. Laser additive manufacturing can enable freeform shapes that allow integration of thermoelectrics into systems that are favorable for thermoelectric energy conversion. Through modeling and experimentation, this work presents single melt line processing and structures of silicon germanium, a high-temperature thermoelectric material, for laser powder bed fusion. Experiments consisted of single melt lines with an Nd-YAG laser and 50-µm spot size on Si₅₀Ge₅₀and Si₈₀Ge₂₀powder compacts. We found that laser processing of silicon germanium alloys causes oxidation and processing defects that are resolved through rescanning strategies. Rapid cooling results in a microstructure with silicon-rich grains and germanium entrapped near grain boundaries for Si₈₀Ge₂₀and dendritic structures in Si₅₀Ge₅₀which are linked to the degree of undercooling during solidification. Laser-processed silicon germanium contains crystalline defects, nanoscale precipitates, and an average grain size of 24 µm. This work informs laser additive manufacturing of silicon germanium parts and uncovers process-structure relationships of laser-processed silicon germanium alloys. 

Additive manufacturing allows fabrication of custom-shaped thermoelectric materials while minimizing waste, reducing processing steps, and maximizing integration compared to conventional methods. Establishing the process-structure-property relationship of laser additive manufactured thermoelectric materials facilitates enhanced process control and thermoelectric performance. This research focuses on laser processing of bismuth telluride (Bi 2 Te 3 ), a well-established thermoelectric material for low temperature applications. Single melt tracks under various parameters (laser power, scan speed and number of scans) were processed on Bi 2 Te 3 powder compacts. A detailed analysis of the transition in the melting mode, grain growth, balling formation, and elemental composition is provided. Rapid melting and solidification of Bi 2 Te 3 resulted in fine-grained microstructure with preferential grain growth along the direction of the temperature gradient. Experimental results were corroborated with simulations for melt pool dimensions as well as grain morphology transitions resulting from the relationship between temperature gradient and solidification rate. Samples processed at 25 W, 350 mm/s with 5 scans resulted in minimized balling and porosity, along with columnar grains having a high density of dislocations.