Plasma spraying of lanthanum silicate electrolytes for intermediate temperature solid oxide fuel cells (ITSOFCs)

A new challenge in the field of solid oxide fuel cells (SOFCs) concerns reducing their operating temperature to 973 K. Apatite ceramics are interesting candidates for SOFC electrolytes due to their high ionic conductivity at this temperature. The present work reports on the fabrication and characterization of La9SrSi6O26.5 coatings obtained by atmospheric plasma spraying with two different plasma spray powers. The microstructure and the composition of the as-sprayed and heat-treated coatings were investigated by several techniques including X-Ray Diffraction, Inductively Coupled Plasma-Atomic Emission Spectroscopy and Scanning Electron Microscopy. The open porosity of the coatings was evaluated by the Archimedean method. It was found that the as-sprayed apatite coatings were composed of an amorphous phase as well as of a crystalline apatite phase, and that they contained chemical heterogeneities resulting from Si volatilization in the high-temperature plasma. Furthermore, a heat treatment rendered it possible to obtain denser, fully crystallized apatite coatings. Ionic conductivity measurements carried out with impedance spectroscopy demonstrated that the conductivity of the apatite coatings - depending on the spraying conditions - increased with sintering.