High-temperature thermoelectric properties of polycrystalline CaMn1-xNbxO3-δ

The structural and thermoelectric (TE) properties of polycrystalline CaMn1-xNbxO3-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 Nb5+ content. The CaMn1-xNbxO3-δ crystallized in an orthorhombic perovskite structure of the Pnma space group. The density and grain size of the CaMn1-xNbxO3-δ samples gradually decreased when Nb5+ ions substituted Mn4+ ions. The CaMn0.95Nb0.05O3-δ sample contained charge-ordered domains, stacking faults, and micro-twins. The substitution of Nb5+ for Mn4+ up to x = 0.15 led to an increase in electrical conductivity, mainly due to an increased electron concentration. The CaMn1-xNbxO3-δ samples with low Nb5+ contents (0.025 ≤ x ≤ 0.15) showed metallic behavior, whereas those with high Nb5+ contents (0.2 ≤ x ≤ 0.25) showed semiconducting behavior. The Nb5+ substitution lowered the absolute value of the Seebeck coefficient for the CaMn1-xNbxO3-δ samples due to an increased electron concentration. The largest power factor (1.19 × 10−4 W m−1 K−2) was obtained for CaMn0.95Nb0.05O3-δ at 800 °C. The partial substitution of Nb5+ for Mn4+ in CaMnO3-δ proved to be highly effective for improving high-temperature TE properties.