Interfacial reactions at the joints of PbTe thermoelectric modules using Ag-Ge braze

Abstract

PbTe-based alloys are important mid-temperature thermoelectric materials. A suitable braze alloy and reliable joints are the basic requirements of a thermoelectric module. This study investigates interfacial reactions at the joints of the PbTe thermoelectric modules using Ag-Ge eutectic braze, Ag-24.5 at%Ge, with Ni or Co as barrier layer. These results indicate Co is a more suitable barrier layer candidate than Ni if Ag-Ge eutectic is used as braze. The Co/PbTe and Ni/PbTe interfacial reactions are negligible. Two reaction phases, CoGe and β-Co5Ge3, are formed in the Ag-Ge eutectic/Co couples reacted at both 400 °C and 750 °C, and Ge is the fastest diffusion species among the three elements. Only the ε-Ni5Ge3 phase is formed in the Ag-Ge eutectic/Ni couples reacted at both 400 °C and 750 °C. In the Ag-Ge eutectic/Ni couples, both Ni and Ge diffuse significantly, but Ni is faster. The reaction layer thickness is in proportion to the square root of reaction time, and the slopes of these lines of growth rates in the Ag-Ge eutectic/Co couples are 0.33 μm/hr and 40.1 μm/hr at 400 °C and 750 °C, and are 0.86 μm/hr and 124.9 μm/hr in the Ag-Ge eutectic/Ni couples, respectively.