Physical Vapor Deposition of Yttria-Stabilized Zirconia and Gadolinia-Doped Ceria Thin Films for Fuel Cell Applications

Abstract: In this thesis, reactive sputter deposition of yttria-stabilized zirconia (YSZ) and cerium gadolinium oxide (CGO) thin films for solid oxide fuel cell (SOFC) applications have been studied. All films have been deposited under industrial conditions.YSZ films were deposited on silicon wafers as well as commercial NiO-YSZ fuel cell anodes. The texture, morphology, and composition of the deposited films were investigated with respect to deposition parameters such as bias voltage which was identified as a key parameter to tailor the texture of the film and promote less columnar coatings when depositing on Si. In contrast, films grown on NiO-YSZ fuel cell anodes were seen to be randomly orientated when deposited at low substrate bias voltages. When the bias voltage was increased the film took over the orientation of underlying substrate due to substrate template effects. The deposited coatings were found to be homogeneous large areas within the coating zone, which is highly important for industrial applications.The performance of sputtered CGO thin films as diffusion barriers for stopping Sr diffusion between SOFC cathodes and electrolytes was also studied. This was done by introducing the sputtered CGO films in a metal-based SOFC setup. The performance depended on the density of the barrier layer, signifying that Sr diffusion and SrZrO3 formation is an issue. Area specific resistances down to 0.27 ?cm2, corresponding to a maximum power density up to 1.14 W cm?2 at 650 °C could be obtained with sputtered CGO barrier layers in combination with Sr-doped lanthanum cobalt oxide cathodes which is a significant improvement compared conventional ceramic SOFCs.The diffusion mechanism of Sr through sputtered CGO films was investigated. For this purpose, a model system simulating a SOFC was prepared by depositing CGO and YSZ on cathode material. This setup allowed observation of Sr diffusion by observing SrZrO3 formation using X-ray diffraction while annealing. Electron microscopy was subsequently performed to confirm the results. It was found that Sr diffused along column/grain boundaries in the CGO films but by modifying the film thickness and microstructure the breaking temperature of the barrier could be increased.