Ag-Ni-Sb phase equilibria and Ag-Sb/Ni interfacial reactions

Abstract

Ag-Sb eutectic alloy is a potential braze for mid-temperature thermoelectric device, and Ni is frequently used as a barrier layer. This study attempts to generate information about Ag-Ni-Sb phase equilibria and Ag-Sb/Ni interfacial reactions which is fundamentally important for thermoelectric devices but is yet not available. The Ag-Ni-Sb phase equilibria isothermal section at 550 °C is experimentally determined. No ternary compound is found, and all the binary compounds have very limited ternary solubilities. Using the Calphad approach, this study develops thermodynamic descriptions of the Ag-Ni-Sb system, and calculates the isothermal sections at 350 °C and 550 °C. The interfacial reactions in the Ag-41.0 at.%Sb/Ni couples at 350 °C and 550 °C are examined. Their reaction paths are Ni/NiSb/NiSb2/Ag-41.0 at.%Sb and Ni/Ni3Sb/Ni5Sb2/NiSb/Ag-41.0 at.%Sb. These paths are illustrated with the Ag-Ni-Sb isothermal sections. It is concluded that Sb is the fastest diffusion species in the Ag-Sb/Ni couples. The growth rates of the reaction phases are 0.2 μm/h and 2 μm/h in the first 24 h at 350 °C and 550 °C, respectively. These rates are comparable to those of Sn-based solder.