Thermoelectric properties of plasma sprayed of calcium cobaltite (Ca₂Co₂O₅)

The thermoelectric properties of calcium cobaltite deposits produced by the plasma spray process are investigated from room-temperature to 873 K. Synthesis of Ca₃Co₂O₆ and Ca₂Co₂O₅ powders were prepared by the solid-state reaction from CaO and CoO_x starting powders. During their subsequent plasma spray Ca particles experience preferential evaporation within the plasma, resulting in a complex interplay among process conditions, stoichiometry, and resultant phases. The as-sprayed material predominantly contains amorphous and secondary phases. Upon annealing, the deposits show sensitivity to phase evolution and therefore thermoelectric properties. Through screening studies, optimal annealing conditions were identified which show a p-type Seebeck coefficient value of 180 μV K⁻¹, electrical conductivity of 1.09 × 10⁴ S m⁻¹, thermal conductivity of 1.16 W m^-1 K^-1 at 873 K. The resultant figure of merit value reached 0.266 for this combination of processing and thermal treatment and is in line with data reported from other techniques for this system.