Thermal expansion and high temperature structural features of the filled skutterudite Sm-beta(Fe alpha Ni1-alpha) Sb-4(12)

A structural study was performed by high temperature x-ray diffraction on samples of the filled skutterudite Sm-beta(Fe alpha Ni1-alpha)(4)Sb-12 between 303 K and 773 K, i.e. in the range including the highest values of the dimensionless figure of merit ZT for this system. Samples were prepared by direct reaction of the pure elements at 1223 K, followed by an annealing treatment at 873 K. Morphology and composition of the obtained alloys were checked by SEM-EDS; the coefficient of thermal expansion (CTE) was calculated from the refined values of the cell parameter, and its trend as a function of the Fe amount revealed a discontinuity in proximity of the composition separating p- and n-skutterudites. Since the reduction of thermal stresses is a fundamental requirement for the lengthening of the thermoelectric devices lifetime, this evidence suggests useful hints for the choice of the most proper skutterudite compositions to be used in p- and n-legs. The effect of increasing temperature on other structural parameters, such as the shape of the Sb-4 rings, is considered too and discussed in terms of filling degree of the cell.