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.
Competing Interests: There are no conflicts to declare.
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