Thermoelectric properties of polycrystalline Ca_0.9Yb _0.1MnO₃ prepared from nanopowder obtained by gas-phase reaction and its application to thermoelectric power devices

Ca_0.9Yb_0.1MnO₃ nanopowder prepared by a gas-phase reaction (GPR) consisted of well-dispersed particles with an average diameter of 47 nm. Sintering of this GPR powder proceeded rapidly and at a lower temperature than that required for a comparable powder prepared by conventional solid-state reaction (SSR). The sintered bulk material from the GPR powder (GPR-bulk) consisted of small grains with an average diameter of 620 nm; this morphology is completely different to that of the SSR-bulk in which larger grains bind together to form a network-like structure. A maximum power factor of 0.19mW.m^-1.K^-2 was obtained for GPR-bulk at 973 K; this value is higher than that of SSR-bulk, mainly as a result of the lower electrical resistivity of GPR-bulk. The thermal conductivity of GPR-bulk is also lower than that of SSR-bulk, possibly because of increased phonon scattering at the grain boundary. The maximum value of the dimensionless figure of merit of 0.13 was obtained for GPR-bulk at 1073 K; this value is about 1.5-fold higher than that for SSR-bulk at 773 K. A unicouple device consisting of a p-type Ca_2.7Bi_0.3Co₄O₉ leg and an ntype Ca_0.9Yb_0.1MnO₃ (GPR-bulk) leg was fabricated. Both oxide legs used for the measurement are 3.1- 3.5mm in both width and thickness and ̃5mm in height. The device generated up to 0.14W of power when the hot- and cold-side temperatures at the ends of the oxide legs were 1095 and 390 K, respectively.