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, RRu/A = 1.0 and 0.5, respectively. A non-stoichiometric solid solution range was identified in the Ca–Ru–O system at RRu/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.