Microstructure characterisation of drop tube processed SiGe semiconductor alloy

Si-Ge based thermoelectric materials are of interest due to the challenges associated with energy recovery from waste heat in industrial processing. A directionally solidified, chill cast ingot of Si70Ge30 was broken into pieces before being subject to rapid solidification in reduced gravity. Drop-tube processing was employed to produce a powder sample with particle diameters in the range 850-150 μm. Solidification occurred under reduced gravity conditions during free-fall down the tube. Cooling rates of between 1800 and 20000 K s-1 were achieved among the various particle sizes. EDX analysis was used to confirmed that the starting material, drop-tube particles and a small amount of residual material left in the crucible were all the same composition. Scanning Electron Microscopy (SEM) was used to analyse the resulting microstructure as a function of cooling rate. The as-solidified microstructure consists of relatively large Si-rich grains with Ge localised at the boundaries, in line with the expected solidification pathway. The Ge is found to form numerous small Ge rich grains which decorate the boundaries of the much larger Si-rich grains, resulting in highly bimodal grain size distribution. This has not previously been reported in the scientific literature. The effect of cooling rate on grain size was studied using SEM and quantitative image analysis, with grain size being found, as expected, to decrease with increasing cooling rate. Point Energy Dispersive X-Ray (EDX) was conducted on all samples at areas of interest; with the larger Si rich grains showing more Si than the original alloy composition (approx. 5-15% more). The small Ge rich grains showed relatively small amounts of Si (between 3-15%). The segregation and heterogeneity found in the microstructure of rapidly solidified particles, alongside the correlation in grain sizes determined by cooling rate, would be important in understanding and improving the conversion efficiency of SiGe whilst also maximising the cost-efficiency.