Sol–gel preparation and characterisation of dense proton conducting Sr3CaZr0.5Ta1.5O8.75 thin films

Dense proton conducting Sr₃CaZr_0.5Ta_1.5O_8.75 films (1.25 μm, with grain size in the 200–400 nm range) were deposited, using the sol–gel method, on Al₂O₃–8%Y₂O₃-stabilised ZrO₂ plates. The obtained gels were characterised by differential and thermogravimetric thermal analysis (DTA–TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results of a study of the structural and electrical properties of Sr₃CaZr_0.5Ta_1.5O_8.75 films deposited on the mentioned substrates are presented herein. The structural data for the gels and films were compared with those obtained for the same material prepared by solid state synthesis. Electrical properties of the sandwich-type structure were investigated by AC impedance conductivity measurements at different temperatures, in both dry and wet 5% H₂/Ar atmospheres. A careful analysis of the impedance spectra for this complex structure was performed, using a model with a series of five electrical circuits, having resistance and capacitance coupled in parallel. The specific responses observed in the impedance spectra were assigned to the corresponding substrate and layer contributions. A significant improvement, by an order of magnitude, in the electrode response was observed in the presence of the interleaving Sr₃CaZr_0.5Ta_1.5O_8.75 proton conducting layer, between the electrode and electrolyte. This enhancement is lost at temperatures above that at which the Sr₃CaZr_0.5Ta_1.5O_8.75 dehydrates and its protonic conductivity diminishes. Considering the structural and electrical characterisation results, these Sr₃CaZr_0.5Ta_1.5O_8.75 sol–gel derived films have a potential use for proton conducting electrolyte or intermediate layer in fuel cells.