Chemical stability of CaCo O /CaMnO p-n junction for oxide-based thermoelectric generators.

An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. CaCo O and CaMnO exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 °C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a CaCo Mn O interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in CaCo Mn O varies and is close to 1 ( = 0) at the CaCo Mn O-CaMnO boundary. The existence of a wide homogeneity range of 0 ≤ ≤ 1 for CaCo Mn O is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.