Uncovering the Structure and Stability of Thermoelectric La3–xTe4–Ni Composites Using High-Resolution and In Situ TEM

Lanthanum telluride (La3–xTe4) is a high-performance, next-generation thermoelectric material with a thermoelectric dimensionless figure of merit (zT) of 1.1 at 1273 K (x = 0.23) and has potential applications in radioisotope thermoelectric generators (RTGs) to power the space missions conducted by National Aeronautics and Space Administration (NASA). It has been shown that the zT can be increased by 30% when nickel (Ni) nanoparticle inclusions are introduced to the La3–xTe4 matrix. The coherent interfaces between La3–xTe4 and Ni are likely a key factor determining the stability and performance of the La3–xTe4–Ni composites, but their role and nature are not well understood. It is important to determine the stability of the La3–xTe4/Ni interface in deep-space conditions, in addition to the effect of heat and oxygen on the mechanisms and kinetics of interface degradation. Here, we show the high-resolution structural characterization and the epitaxial crystallographic relationship of La3–xTe4–Ni thermoelectric composites and their interfaces at high-vacuum and ambient-temperature conditions using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). We further demonstrate the La3–xTe4/Ni interface degradation and Ni diffusion over its operating temperature range (25–1000 °C), in the presence and absence of oxygen, in real time, using in situ TEM.