Enhanced densification and thermoelectric properties of Ca3Co4O9 ceramics fabricated by solid-state combustion and hot-pressing

The Ca3Co4O9 powders were synthesized by the solid-state combustion method and then calcined at 775-875 °C for 6 h. The combination of combustion-derived fine powders and hot pressing was adopted to improve the densification and thermoelectric (TE) performance of Ca3Co4O9 ceramics. Dense Ca3Co4O9 ceramics were subsequently fabricated by hot-pressing to investigate the influence of hot-pressing temperature (800-950 °C, 2 h) on phase formation, microstructure, electrical, and TE properties. The Ca3Co4O9 powder was obtained after calcination at 800 °C for 6 h and exhibited an average particle size of 0.55 μm. XRD analysis confirmed that Ca3Co4O9 was the predominant phase in all hot-pressed samples. XPS analysis further confirmed the presence of oxygen vacancy (VO⁺⁺)-related defects and mixed-valence cobalt species. FESEM observations revealed a dense microstructure composed of plate-like grains with an average grain size ranging from 0.61 to 0.96 μm. The bulk density ranged from 4.31 to 4.46 g/cm³, indicating dense ceramics. The electrical resistivity (ρ) decreased with increasing measured temperature for all samples. Among all samples, the ceramic hot-pressed at 900 °C exhibited the lowest ρ at 600 °C. The Seebeck coefficient (S) significantly increased from 150 μV/K to 223 μV/K, while the thermal conductivity (κ) decreased with increasing temperature. Due to the favorable combination of low ρ and high S, the sample hot-pressed at 900 °C achieved the highest power factor (PF) and a maximum dimensionless figure of merit (ZT) of 0.17 at 600 °C.