Chemical stability of Ca₃Co_4- O₉₊ /CaMnO_3- 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. Ca₃Co_4- O₉₊ and CaMnO_3- 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 Ca₃Co₁₊ Mn_1- O₆ interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca₃Co₁₊ Mn_1- O₆ varies and is close to 1 (y = 0) at the Ca₃Co₁₊ Mn_1- O₆-CaMnO_3- boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca₃Co₁₊ Mn_1- O₆ is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.