Tailoring electronic and thermal transport properties of CaO(CaMnO₃)_m-based (m=1 and m=∞) composites for thermoelectric power generation

Oxide thermoelectric (TE) materials are promising for waste heat recovery at high temperatures thanks to their good chemical stability at elevated temperatures and low cost. We study Nb-doped n-type TE oxides of the CaO(CaMnO₃)_m-series. The CaMnO₃ (m = ∞) and Ca₂MnO₄ (m = 1) derivatives feature extremely opposite transport coefficients, where the m = ∞ structure exhibits high electrical and thermal conductivity, and the m = 1 derivative exhibits the opposite combination. We synthesize composite materials based on these two phases of different ratios to draw correlations between the TE properties, microstructure evolution, and composition of the material. We determine the optimum sintering temperature to be 1373 K, and measure both thermal and electronic transport coefficients, then perform a thorough general effective medium (GEM) analysis. Interestingly, we find that most ratios obey to the GEM behavior, where deviations are elucidated in terms of interfacial effects. This study provides us with tools for identifying the significance of bulk vs. interfacial effects in design of composite materials with controllable transport properties.