Influence of growth conditions on structure, composition homogeneity and mechanical properties of thermoelectric crystals for coolers

The object of this work is to produce by economical method (vertical zone melting) the polycrystalline textured ingots of p- and n-type thermoelectric materials (TEM) based on antimony and bismuth tellurides and selenides solid solutions with high performance parameters (thermoelectric figure-of-merit Z) and diameter not less than 20 mm. Following problems were to be solved: a) the formation of favorable crystallographic orientation with minimum scatter which allows to make use of properties anisotropy to achieve maximum value of Z; b) preparation of solid solutions with reasonably homogeneous composition (along and across the ingot) and reproducible alloying level; c) elucidation of causes of mechanical stresses, that are responsible for cracking, to produce material with adequate mechanical properties. The texture, phase composition, macro- and microscale composition inhomogeneity of solid solution have been studied with X-ray diffraction methods by evaluation of lattice parameter and smearing (broadening) of diffraction maxima. In addition, microheterogeneity was assessed by microhardness measurements coupled with local elemental microanalysis (local X-ray microprobe). The cracking pattern was correlated with microstructure features (using optical metallography and X-ray topography). The slip lines formed by indenter prints during microhardness measurements were used for evaluation of cross-sectional disorder of adjacent grains, whose cleavage planes are parallel to ingot axis.