Nonmonotonic change in the structural grain size of the Bi0.4Sb1.6Te3 thermoelectric material synthesised by spark plasma sintering

A nonmonotonic variation of crystallite size in thermoelectric materials depending on spark plasma sintering (SPS) temperature was found in this work by using X-ray diffraction. The crystallite size grows with increasing temperature from 250 °C to 400 °C and decreases at temperatures from 400 °C to 500 °C. The transmission electron microscopy results suggest that the decrease in the average crystallite size is associated with an intense formation of fine grains at an SPS temperature of 450 °C as a result of repeated recrystallisation. New grains in the structure precipitate faster than the old grains grow. The total density of the defects including the twins decreases. The initiation of new repeated recrystallisation centres occurs on the grain boundaries, on the dislocation defects and in the grain bulk, most likely on the subgrains. The pattern that has been discovered solves the problem of nano state preservation at elevated SPS temperatures.