Synthesis and characterization of apatite structure sputter deposited coatings dedicated to intermediate temperature solid oxide fuel cells
Résumé
La-Si metallic coatings with La/Si ratios between 1.55 and 1.66 were synthesized by Direct Current Magnetron Sputtering of lanthanum and silicon targets in pure argon atmosphere. After the deposition stage, the ceramic apatite-structure coatings were obtained by thermal oxidation in air for 2 hours at 1173 K. The structural and chemical features of these films have been assessed by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS). The electrical properties were determined by Complex Impedance Spectroscopy in planar configuration. The films with near apatite structure La/Si atomic ratio in range of La10Si6027 and La9.33Si6026 deposited on different substrates were initially amorphous. After thermal oxidation at 1173 K in air, the coating crystallised under the expected apatite structure. SEM observation revealed that the film compactness and thickness increased after thermal oxidation. The film adhesion on SOFCs material is guaranteed if the anode support was not reduced before the deposition stage. A specific method was performed to measure the electrical conductivity, i.e. to determine the ionic conductivity of the film: it uses the four-point probe technique combined with ac impedance spectroscopy. The good agreement between measurements performed in this work (Ea = 0.75 eV, 973 K, = 3-4 10−3 S*cm−1) and literature data allows concluding that deposited layers are of good quality and should be used as electrolyte in an Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC).