Uncovering the Structure and Stability of Thermoelectric La_3-xTe₄-Ni Composites Using High-Resolution andIn SituTEM

Lanthanum telluride (La_3-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 thezTcan be increased by 30% when nickel (Ni) nanoparticle inclusions are introduced to the La_3-xTe₄matrix. The coherent interfaces between La_3-xTe₄and Ni are likely a key factor determining the stability and performance of the La_3-xTe₄-Ni composites, but their role and nature are not well understood. It is important to determine the stability of the La_3-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 La_3-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 La_3-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, usingin situTEM.