Influence of Composition and Thermal Treatments on Microhardness of the Filled Skutterudite Smy(FexNi1-x)4Sb12

Samples belonging to the series Sm-y(FexNi1-x)(4)Sb-12 were heated in Ar atmosphere up to 400 degrees C and cooled down to room temperature several times, with the aim to evaluate the effect of thermal cycles on microhardness. The treatment temperature was chosen in correspondence of the maximum ZT value, in order to simulate the operating conditions of the material in a thermoelectric device. Vickers measurements allowed to detect the effect of both composition and thermal treatments on the microhardness properties of the material. A decrease in the microhardness value was observed prior to thermal treatments with increasing Fe amount, due to the substitution of Ni by the larger Fe atom. Moreover, almost all compositions show an increase in the hardness of the skutterudite phase as a consequence of thermal cycles, accounting for the Sb enrichment of skutterudite. This evidence suggests also a possible improvement of the preparation procedure in order to obtain a stoichiometric Sb amount within the skutterudite. Samples were prepared by direct reaction of pure elements at 950 degrees C followed by thermal treatment at 620 degrees C. The composition and microstructure of the obtained samples were investigated by X-ray powder diffraction, and by optical and electronic microscopy.