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

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 degrees C and 500 degrees C for different times. The legs, prepared from spark plasma sintered (SPS) compacts of the pure CuPb18SbTe20 phase were electroplated with a 3-5m 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., 136cm(2) was observed in joints with the 5m 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 degrees 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 degrees C, the intended maximum operating temperatures of PbTe-based modules.