Developing contacting solutions for n-type Mg3Sb1.5Bi0.5 based thermoelectric materials

Abstract

n-type Mg3Sb1.5Bi0.5 intermetallic solid solution is a promising thermoelectric (TE) material for medium temperature range (room temperature to 450⁰ C) applications due to its high figure of Merit (peak zT value of 1.5-1.7) and cost-effective constituent elements. Extensive research on materials development has been carried out on this material system but there are limited reports related to its contacting. In this work, a detailed study of the contacting in Mg3.0+ySb1.49Bi0.49Te0.02(y=0.3) has been carried out using both single layer and multilayer approaches. For single layer contacting studies, the contact metal (Ni/Fe) is directly in contact with TE material while in case of the multilayer contacting process, the TE material powder is sandwiched between two layers (with different functionalities). The top layer is a metal foil (Cu/Ni) and is followed by a composite layer consisting of a mixture of TE material and Ni powder. Compaction in both cases is carried out by simultaneous application of temperature and pressure using the mono-block sintering process. It is observed that the electrical contact resistance (Rc) values for all cases lie in the range of 30-50μΩcm2. However, a marked difference in the microstructure and nature of bonding is observed between the various contacting approaches. For single layer contacting, Fe is observed to form an abrupt interface with absence of any reactive phases at the interface. However, Ni results in the formation of various intermetallic phases and a diffuse interface. For both Fe and Ni, cracks or delamination are observed at the interface. The multilayer contacting studies result in well bonded and crack-free interface for all the studied compositions. Various intermetallic phases have been identified from an elemental analysis of the interface region. Further, no diffusion of the metal layers is observed into the thermoelectric matrix.