Thermoelectric properties of Ca-Ir-O compounds prepared by spark plasma sintering

Ca-Ir-O Compounds in various molar ratios of Ir to Ca (R_Ir/Ca) from 0.2 to 1.2 were synthesized by solid-state reaction and compacted by spark plasma sintering (SPS). Ca₄IrO₆ and CaIrO₃ with a small amount of second phase were obtained at R_Ir/Ca = 0.25 and 1.0, respectively, whereas Ca₂IrO₄ in a single phase was obtained at R_Ir/Ca = 0.5. The electrical conductivity was 4 × 10^-4 to 150 Sm^-1 at room temperature and exhibited a semiconducting behavior for all the samples. The Seebeck coefficient at R _Ir/Ca = 0.4 to 0.7 showed negative values whereas samples with R _Ir/Ca = 0.25 and 0.9 to 1.2 showed positive values. The thermal conductivity was 1.1 to 2.7 Wm^-1K^-1 at room temperature and slightly decreased with increasing temperature. The highest ZT value of p-type was 0.006 at 1023 K and R_Ir/Ca = 11 and 1.2, while that of n-type was 0.0022 at 800 K and R_Ir/Ca = 0.5.