Chemical stability of Ca(3)Co(4−x)O(9+δ)/CaMnO(3−δ) 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. Ca(3)Co(4−x)O(9+δ) and CaMnO(3−δ) 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 Ca(3)Co(1+y)Mn(1−y)O(6) interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca(3)Co(1+y)Mn(1−y)O(6) varies and is close to 1 (y = 0) at the Ca(3)Co(1+y)Mn(1−y)O(6)–CaMnO(3−δ) boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca(3)Co(1+y)Mn(1−y)O(6) is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.