Thermoelectric properties of alkaline earth ruthenates prepared by SPS

Alkaline earth ruthenates ARuO₃ (A = Ca, Sr and Ba) and Sr₂RuO₄ in a single phase were obtained at molar ratio of Ru to A, R_Ru/A = 1.0 and 0.5, respectively. A non-stoichiometric solid solution range was identified in the Ca-Ru-O system at R_Ru/Ca = 0.7-1.0. The relative densities of CaRuO₃, SrRuO₃, BaRuO₃ and Sr₂RuO₄ compacted by spark plasma sintering (SPS) were 82%, 95%, 97% and 96%, respectively. The electrical conductivity (σ) of ARuO₃ showed a metallic behavior whereas Sr₂RuO₄ exhibited a semiconducting behavior. The σ increased in the order of Sr₂RuO₄, BaRuO₃, CaRuO₃ and SrRuO₃. The Seebeck coefficient (S) of ARuO₃ was around 25-40 μV K^-1 and almost independent of temperature whereas that of Sr₂RuO₄ decreased with increasing temperature. The thermal conductivity (κ) of ARuO₃ slightly increased with increasing temperature, whereas that of Sr₂RuO₄ decreased with increasing temperature. SrRuO₃ showed much higher κ than the other compounds. The highest dimensionless figure of merit (ZT) values of ARuO₃ were around 0.02-0.025 at 1023 K whereas Sr₂RuO₄ was 0.001 at 600 K.