TRANSPORT PROPERTIES OF THERMOELECTRIC CaO(CaMnO₃)_m-BASED COMPOSITES

Oxide thermoelectric (TE) materials are promising for waste-heat recovery at high temperatures. We study Nb-doped n-type CaO(CaMnO₃)_m-based oxides. The CaMnO₃ (m=∞) and Ca₂MnO₄ (m=1) derivatives are of our particular interest, since they feature extreme transport coefficients, i.e. both high values of electrical and thermal conductivities for m=∞, and the opposite for m=1. We investigate the thermal stability and temporal evolution of composite samples comprised of the m=1 and m=∞ phases of different weight ratios, and determine the optimized sintering conditions to be 24 h at 1373 K. Electronic and thermal transport coefficients are analyzed, and results are compared to a simple rule-of-mixture model in order to discern interfacial effects from phase mixture effects. We find decent agreement for most ratios, and certain deviations for others; the latter are indicative of predominating interfacial effects. The TE figure of merit is found to increase with increasing temperature and amount of the m=∞ phase constituent. These research outcomes may serve as guidelines for the design of further improved composite materials with optimized TE figure of merit.