Laser Powder Bed Fusion of Silicon Germanium Alloys: An Investigation of Processing Dynamics, Material Structure, and Thermoelectric Properties
Open Access DepositedThermoelectric materials provide solid-state energy conversion of heat to electricity or use electricity to pump heat. Silicon-germanium is a high-temperature thermoelectric material that provides energy conversion at 900 °C. The ability to convert energy at 900 °C makes silicon germanium an attractive choice to convert heat to electricity or thermal management for high-temperature applications, such as in hypersonic vehicles. Conventional methods of consolidating silicon-germanium alloys severely hinder the shape of devices, making it challenging to integrate thermoelectrics into practical applications for waste heat recovery or localized thermal management. Moreover, thermal and electrical transport in silicon germanium is dependent on the micro- and nano-structures within the material. Laser powder bed fusion, an additive manufacturing technique offers a solution by enabling freeform shapes and structuring of the material during consolidation, potentially providing a new capability for high-temperature energy conversion within structures. In this dissertation, I investigate laser powder bed fusion of silicon germanium alloys, Si50Ge50 and Si80Ge20, by creating single melt lines on powder compacts and fabricating bulk samples for thermoelectric property measurements. Two different rescanning strategies alleviated balling and void defects, which enabled sufficient melting to fabricate bulk samples that had a 59% relative density, on average. Due to rapid cooling, the microstructure had substantial germanium segregation. The amount of segregation and morphology depended upon the starting stoichiometry of the powder and the amount of undercooling during solidification. An analysis of the grain structure showed a polycrystalline material with a grain diameter distribution skewed towards grain diameters < 40 µm. Nanoscale features observed within Si50Ge50 and Si80Ge20 alloys were crystalline defects from lattice strain due to a change in germanium content. Hierarchical structuring from laser processing typically causes a reduction of thermal conductivity, a desirable trait for high-performance thermoelectric materials. The micro- and nanoscale features resulted in a high electrical resistivity when compared to a silicon germanium standard reference material. The bulk samples exhibited p-type Seebeck behavior, indicating an unintentional dopant resulted in p-type conduction. I posit that trace amounts of milling media caused p-type behavior in Si80Ge20. Additionally, the formation of vacancy-oxygen defects during processing contributed to an increase in hole charge carriers. This research demonstrates that laser powder bed fusion is a viable means to produce silicon germanium ingots that display the thermoelectric effect. The solidification structure indicates that hierarchical structuring from laser additive manufacturing of silicon germanium alloys is promising for providing a means to produce high-performance, high-temperature thermoelectric material with non-cubic shapes. The work presented here provides understanding of the process-structure-property relationship of silicon germanium alloys processed by laser powder bed fusion.
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