Effect of processing routes on the synthesis and properties of Zn₄Sb₃ thermoelectric alloy

In this work, sintering of Zn₄Sb₃ was performed by open die pressing at 558 K, investigating the role of different starting materials (powders obtained from slowly cooled master alloy ingots or rapidly solidified ribbons) on microstructure, phase stability, thermoelectric properties and mechanical properties. With respect to the master alloy, sintered samples show lower porosity, a slight decrease in Zn content and a partial decomposition of Zn₄Sb₃ into ZnSb and Zn. Sintering of ground rapidly solidified ribbons further reduces porosity and partially inhibits the decomposition of Zn₄Sb₃. As sintered samples show a temperature behaviour of the Seebeck coefficient that is intermediate between those of Zn₄Sb₃ and ZnSb. Thermal cycling between 300 K and 700 K favours the homogenization of the microstructure, due to the recombination of ZnSb and Zn into Zn₄Sb₃, leading to a Seebeck coefficient typical of Zn₄Sb₃ single phase. The phase evolution observed upon sintering and thermal cycling is described in terms of competing reactions ruled by thermodynamics and kinetics, and the different behaviour of the samples is explained by the different free energy level of the starting materials. All samples show a brittle behaviour upon stress-strain compressive test and microindentation measurements. A remarkable increase of the fracture strength is observed as the porosity decreases, whereas microhardness values are similar for all samples (220-240 HVN). For the sintered samples, values of the indentation fracture toughness (0.8-0.9 MPa m^1/2) are estimated from the length of Palmqvist-type radial cracks.