Plasma-sprayed lanthanum-doped strontium titanate as an interconnect for solid oxide fuel cells: Effects of powder size and process conditions

As a key component of the novel bi-layer interconnect for solid oxide fuel cells (SOFC), La-doped SrTiO₃ (LST) has been deposited by atmospheric plasma spraying in this study. The effects of powder particle size, hydrogen flow rate and deposition temperature on stoichiometry, microstructure, and properties of LST deposits have been systematically investigated. The preferential vaporization loss of Sr is quantified by the relationship between the droplet size and splat composition, while the droplet size prior to impact is measured by a confocal laser microscopy and the composition of splat is analyzed by the energy-dispersive spectrum of scanning electron microscopy. Thermogravimetric analysis is used to determine the oxygen non-stoichiometry of the deposit. It is found that when the droplet diameter is smaller than 30 µm, the preferential vaporization induced Sr loss is significantly increased up to 45% with decreasing particle size, while it is greatly reduced if the droplet size is larger than 30 µm. Two mechanisms including diffusion-dominated mass transfer and convection-dominated mass transfer are proposed to explain the particle size effect on Sr loss. With increasing H₂ flow in plasma gas, the oxygen deficiency in LST deposit is increased. Moreover, the effective bonding at lamellar interfaces is realized by increasing the deposition temperature to higher than the critical bonding temperature. The electrical conductivity of LST deposits is remarkably improved by decreasing Sr loss, increasing oxygen vacancy concentration, and enhancing the lamellar bonding. The LST coatings prepared at a deposition temperature of 450 °C with coarse powder particles exhibits a dense microstructure and a conductivity of ~100 S/cm at 850 °C in the reducing atmosphere. The results show that by properly optimizing the process conditions, the plasma-sprayed LST coating is a promising candidate to be a part of the bi-layer interconnect for SOFCs.