Elevated Temperature Behavior of CuPb18SbTe20/Nano-Ag/Cu Joints for Thermoelectric Devices

Abstract

The thermal stability of the CuPb18SbTe20 thermoelectric legs and Cu electrodes bonded through sintering of nano-silver (n-Ag) paste was investigated after isothermal annealing of the joint at 400°C and 500°C for different times. The legs, prepared from spark plasma sintered (SPS) compacts of the pure CuPb18SbTe20 phase were electroplated with a 3-5 μm thick Ni barrier layer prior to bonding with the electrodes. The n-Ag layer, sintered to a relative density of around 95%, produced defect-free joints with interfaces excluding any reaction layers. The lowest specific contact resistance i.e., 136 μΩ cm2 was observed in joints with the 5 μm n-Ag layer, though it was increased about five times without a Ni barrier layer. Following thermal treatment, various interfaces of the n-CuPb18SbTe20/n-Ag/Cu joints underwent degradation reactions to produce compounds such as Cu2Te, Ag2Te in the TE legs. Under long duration of thermal treatment at 400°C, the Ni barrier layer also completely dissolved and formed a Ni (Sb1−xTex)1+y phase. The Seebeck coefficient and the electrical resistivity of the TE legs were degraded to the tune of 42 and 165%, respectively, due to the formation of secondary phases. The bonding of CuPb18SbTe20 legs and Cu electrodes through sintering of n-Ag paste necessitates a suitable barrier layer that is stable at 500°C, the intended maximum operating temperatures of PbTe-based modules.