Phase decomposition of Ni5Pb2Te3 in diffusion bonded PbTe-Ni interfaces – A TEM and thermodynamic study

The study of metallurgical phase transformations is essential for developing reliable interconnects in thermoelectric devices. PbTe-based devices are used in mid-temperature range applications, with Ni being the most preferred contact metal. Several studies have reported the direct joining of PbTe and Ni through rapid hot-pressing routes. However, the study of equilibrium phase transformations in these processes is complex owing to many external variables (applied pressure, electrical current, and simultaneous sintering of PbTe). Furthermore, the long-term phase transformation behavior of Ni ₅Pb ₂Te ₃ and (Ni _3±xTe ₂) β ₂ phases generated from PbTe/Ni interaction has not been examined. This paper reports the phase decomposition of the high-temperature Ni ₅Pb ₂Te ₃ phase in diffusion bonded PbTe/Ni heterointerfaces and presents a general thermodynamic framework to predict such transformations. The phase decomposition is studied through advanced analytical TEM/STEM techniques. The Ni ₅Pb ₂Te ₃ phase transforms into monoclinic β ₂ and PbTe phases with a lamellae-like arrangement accompanied by the formation of extrinsic stacking faults in the β ₂ lamellae. Our work highlights an effective experimental technique (solid-state diffusion bonding) that can be applied to study a metal-contact problem in thermoelectrics and outlines a framework to derive essential thermodynamic data in the absence of any experimental measurements and crystallographic information.