Microstructure of thermoelectric SiGe alloys containing fullerite

A series of thermoelectric materials has been characterized microstructurally. The materials are Si80Ge20 (at.% alloys with additions of fullerite in the concentrations 0, 0.2 and 2.0 wt.%. The alloys were fabricated by a hot sintering technique, where the feed power was produced by mechanical alloying (MA). Fullerite was added either before or after MA. The experimental results have demonstrated that a high density of microtwins, some of them a few nanometers thick, exists in the alloys. The number density of the microtwins has been found to be over 108 cm−2 The addition of fullerite greatly reduced the grain size of the hot sintered alloys. The alloy without fullerite has a grain size of between 0.5 and 5 μm, whereas the grain size of the alloy with 2.0 wt% fullerite is decreased by one order of magnitude. Owing to a fine grain structure, the dislocation density in the alloy with fullerite is much less than 109 cm−2. It has also been found that the added fullerite reacted with the surrounding SiGe matrix to produce an f.c.c. silicon carbide of lattice parameter 0.4359 nm. It is concluded from analysis of Moiré fringes that the interface between the small SiC particles and the matrix is not coherent.