Preparation of LaMnO3 perovskite thin films by suspension plasma spraying for SOFC cathodes

Perovskite type doped LaMnO3 is the most currently used cathode material for the solid oxide fuel cell, but when it is plasma-sprayed very often decomposition occurs. This study is devoted to the optimization of composition of starting particles and plasma conditions allowing, as much as possible, obtaining LaMnO3 perovskite without decomposition when spraying them by suspension plasma spraying. Suspension plasma spraying, used in this work, consists of mechanically injecting a stable suspension of particles in the micrometer size range in a direct current (dc) plasma jet. Particles are dispersed in a solvent (ethanol) by means of electrosteric repulsion. The large injected suspension drops (∼ 300 μm) are sheared into tiny ones (a few micrometers) by the dc plasma jet flow, the solvent is evaporated and the solid particles are melted resulting in perovskite droplets of about 1 μm impacting on the substrate. According to the perovskite particle size sprayed their decomposition is enhanced compared to conventional spraying. That is why in this work pure perovskite, the easiest to decompose, has been chosen (even if it is not recommended one for SOFCʹs cathode) to determine how its decomposition can be limited if not avoided. Both, the process conditions, including plasma-forming gases, mass flow rates, and current intensity and the powder characteristics including particle size and, percentage of MnO2 doping were found to strongly influence perovskite coating composition. Minimal perovskite decomposition, less than 5 wt.%, in coating is obtained with a perovskite powder doped with 20 mol% of MnO2, a mean particle size of 3 μm and by using an argon plasma-forming gas (60 slm) at 300 A current intensity for a 6-mm-internal-diameter (i.d.) anode-nozzle.