High-temperature thermoelectric properties of polycrystalline CaMn 1-x Nb x O 3-δ

Abstract The structural and thermoelectric (TE) properties of polycrystalline CaMn 1- x Nb x O 3-δ (0.025 ≤  x  ≤ 0.25) were studied with X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and electrical transport measurements, with an emphasis placed on the Nb 5+ content. The CaMn 1- x Nb x O 3-δ crystallized in an orthorhombic perovskite structure of the Pnma space group. The density and grain size of the CaMn 1- x Nb x O 3-δ samples gradually decreased when Nb 5+ ions substituted Mn 4+ ions. The CaMn 0.95 Nb 0.05 O 3-δ sample contained charge-ordered domains, stacking faults, and micro-twins. The substitution of Nb 5+ for Mn 4+ up to x  = 0.15 led to an increase in electrical conductivity, mainly due to an increased electron concentration. The CaMn 1- x Nb x O 3-δ samples with low Nb 5+ contents (0.025 ≤  x  ≤ 0.15) showed metallic behavior, whereas those with high Nb 5+ contents (0.2 ≤  x  ≤ 0.25) showed semiconducting behavior. The Nb 5+ substitution lowered the absolute value of the Seebeck coefficient for the CaMn 1- x Nb x O 3-δ samples due to an increased electron concentration. The largest power factor (1.19 × 10 −4 W m −1 K −2 ) was obtained for CaMn 0.95 Nb 0.05 O 3-δ at 800 °C. The partial substitution of Nb 5+ for Mn 4+ in CaMnO 3-δ proved to be highly effective for improving high-temperature TE properties.