R. Gray, S. LeBlanc
The LeBlanc Lab,
Keywords: energy materials, powder characterization, thermoelectric generators, selective laser melting
Summary:This study examined magnesium silicide stannide (Mg2Si0.4Sn0.6) powder, a thermoelectric material optimal for high temperature applications. Mg2Si0.4Sn0.6 was characterized to establish the feasibility of using this powder in the selective laser melting process to produce thermoelectric generators. Thermoelectric generators are solid state devices capable of waste heat recovery in combustion and heat process systems and have the potential to radically change the energy industry. These devices are currently being manufactured using bulk material processing with multiple integration and assembly steps. Leading to decreased product efficiency, high manufacturing costs, and offering little flexibility in device geometry. Selective laser melting an additive manufacturing technique, on the other hand, could provide a unique solution to these manufacturing challenges. However, current additive manufacturing techniques exist only for limited materials- namely polymers, ceramics, and metals- which do not include semiconductors (like thermoelectric materials). As well as require specific starting powder characteristics: desired particle size distribution, and high levels of circularity and convexity. Powder parameters such as convexity, circularity, and particle size distribution not only effect the flowability through the selective laser melting process but also the density of the final thermoelectric device. With a higher density, thermoelectric generators are more efficient and resilient to internal fractures. If particles are elongated and nonuniform, there is an increase in particle friction and possible particle interlocking which in return decreases spreadability and flowability. These characteristics also decrease the final density as they can lead to uneven sintering, and interlayer voids, which cause internal fractures. Powder morphology and particle size distribution were analyzed through optical microscopy and the image analysis software FIJI. The ability to spread was assessed across 100 μm and 500 μm thick grooves using two rolling techniques, as well as a blade. Flowability was examined through the measurements of, angle of repose, angle of spatula, and compressibility. Each of these measurements follows the United States Pharmacopeia flowability standards. The powder used was crushed by hand and thus had a wide particle size distribution with an average convexity value of 0.89 and an average circularity value of 0.57. Both these values should be close to 1 to optimize flowability and spreadabilty. It was found that the spreadabilty trials were very repeatable and that the optimal particle size distribution was 46 μm < particle diameter < 53 μm. This qualitative data was based on packing density and surface uniformity. It was also concluded that as nominal sieve size decreased packing density and surface uniformity increased. However, as nominal sieve size decreased, agglomeration increased, causing cracks in the powder bed. Counter rolling resulted in the best surface uniformity and packing density across all tested mesh sizes. Overall the powder indicated fair flowability as indicated by the United States Pharmacopeia flowability standards which matches with initial observations. This study showed that Mg2Si0.4Sn0.6 has the potential to be integrated into selective laser melting and paves the way for future studies looking to use Mg2Si0.4Sn0.6 in thermoelectric devices utilizing advanced manufacturing techniques.