Effect of Mixed Grain Sizes on the Thermoelectric Properties of Ca0.9Yb0.1MnO3

Ca0.9Yb0.1MnO3 has been identified as a material that might be suitable for thermoelectric applications. We fabricated micro/nanograined Ca0.9Yb0.1MnO3 composites, with the aim of controlling the passage of electrons and phonons simultaneously. Micro/nanograined Ca0.9Yb0.1MnO3 composites containing various fractions of nanosized powder were prepared by sintering mixtures of microparticulate and nanoparticulate Ca0.9Yb0.1MnO3, obtained by solid-state reaction and by gas-phase reaction, respectively. The electrical resistivity increased markedly when the weight fraction of nanosized powder exceeded 50%, probably as a result of a percolation phenomenon. However, the thermal conductivity was considerably reduced when the weight fraction of nanosized powder exceeded 25%, but then remained almost constant. The absolute values of the Seebeck coefficient of micro/nanograined Ca0.9Yb0.1MnO3 composites were larger than those of monolithic micro- or nanograin Ca0.9Yb0.1MnO3, probably as a result of the effects of potential-barrier scattering. The highest dimensionless figure of merit ZT value of 0.09 at 973 K was achieved with a sample containing 50% nanosized powder, and this value is 10% larger than that of monolithic micrograined Ca0.9Yb0.1MnO3.