Examination of NCSL (near-coincidence site lattice) structures formed during β2 (Ni3-xTe2) precipitation in diffusion-bonded PbTe-Ni interfaces – A TEM study

Abstract

Establishing dependable interconnects within thermoelectric (TE) modules is of utmost significance in ensuring the longevity and functionality of these solid-state devices. PbTe-based devices are used in mid-temperature range applications, with Ni being the most preferred interconnect. The available literature has focused primarily on optimizing the joining parameters. However, fine microstructural features have not been reported that can affect the local semiconducting and mechanical properties. This research paper presents novel findings regarding the formation of previously undiscovered fine (Ni3-xTe2) β2 precipitates resembling Widmanstätten morphology within the PbTe matrix during joining processes. PbTe and Ni discs are diffusion bonded at 600 °C for various holding times. The precipitation is studied through advanced analytical TEM/STEM techniques. The current study leads to two noteworthy observations: 1) specific orientation relationships were discovered between the two phases: [001] PbTe ‖ [001] β2, (020) PbTe ‖ (210) β2 and growth directions were along 〈100〉 in PbTe matrix phase. 2) Formation of near-coincidence site lattice (NCSL) structures occurring periodically at the growing fronts of these precipitates. Atomic scale lattice strain mapping indicates a cyclic state of compression and tension at these interfaces. These findings shed light on local composition changes and strain fields within the PbTe matrix during fine β2 precipitation, which has the potential to impact interface reliability.