Effect of composition on thermoelectric properties of Ca-Ir-O compounds prepared by SPS

Abstract

Ca-Ir-O compounds in various molar ratio of Ir to Ca (R_Ir/Ca) from 0.2 to 1.2 were prepared by spark plasma sintering (SPS) and the effect of compositions on phase structure, Seebeck coefficient, electrical conductivity and thermal conductivity were investigated. All specimens had density around 70% of theoretical density. Three stable phase compounds of Ca₄IrO₆, Ca₂IrO₄ and CaIrO₃ and a metastable phase of CaIrO₃ (m-CaIrO₃) were identified. Only a single phase of Ca₂IrO₄ were obtained at R_Ir/Ca = 0.5. The Seebeck coefficient of Ca-Ir-O compounds at room temperature was around 10 to 150 Sm^-1and increased with increasing temperature, exhibiting a semiconducting behavior for all samples. The Seebeck coefficient at R_Ir/Ca = 0.4 to 0.7 slightly increased with increasing temperature showing negative values. The Seebeck coeeficient at R_Ir/Ca = 0.9 to 1.2 showed positive values and decreased with increasing temperature at below 600 K and increased at higher than 600 K. The thermal conductivity was around 0.7 to 2.7 Wm^-1K^-1 at room temperature and slightly decreased with increasing temperature. The highest ZT was 0.006 at R_Ir/Ca = 1.2 (1023 K).