Rapid Solidification via Drop-Tube Processing of Si-Ge Thermoelectric Alloy

Si-30 wt.% Ge material was rapidly solidified using a 6.5 m high vacuum drop-tube. Powders produced and sieved into eight size fractions (>850-μm to 75–53-μm) with corresponding estimated cooling rate of 1.4 × 102 K s−1 to 8.5 × 104 K s−1. Microstructural analysis revealed that rapid solidification increased, rather than decreased, the heterogeneity of the solid product. Contrary to typical behaviour observed in other alloy systems, the heterogeneity increased at grain boundaries with increasing cooling rates. with severe Ge segregation manifesting as series of regions with uniform stoichiometry. Step-mediated gradients were observed predominantly follow multiples of 5 at.% Ge (i.e., 5, 20, 25, 55, 65, 70, 85, and 90 at.% Ge). The TEM/SAED analysis of the most Ge-rich regions revealed superlattice spots indicative of chemical ordering. This suggested formation of stoichiometric compounds (e.g., Ge1Si4, Ge1Si7, Ge11Si9), contradicting the accepted thermodynamic description of the SiGe system as a continuous random solid solution.
Mass-balance calculations and SEM-EDX further corroborated these findings, showing consistency with the observed segregation. Grain size decreased with cooling rate, and Ge-rich regions occupied smaller volume fractions, from 20.80% (>850-µm) to 11.81% (75–53-µm). XRD was failed to resolve Ge-rich regions (>40 wt.% Ge) due to small grain-boundary regions and peak broadening. Similarly, Robert’s extrapolation and Cohen’s analytical methods confirmed this behaviour.