Microstructure and electrical conductivity of nanocrystalline nickel- and nickel oxide/gadolinia-doped ceria thin films

NiO/Ce0.8Gd0.2O1.9−x (NiO/CGO) films with thicknesses between 150 and 800 nm were prepared by spray pyrolysis. Average grain sizes were 5–260 nm after annealing in air at 600–1200 °C. After reduction, the Ni/CGO cermet microstructures were stable up to temperatures of 600 °C for grain sizes of 53 nm. Nickel coarsening was observed for films with smaller grains. The development of a rigid CGO grain network helped to prevent nickel growth. The electrical conductivities of the films were comparable to state-of-the-art Ni–YSZ cermets and reached 3000 S cm−1 at 600 °C. Films with stable microstructures showed no degradation in electrical conductivity over 1400 h at 570 °C and upon thermal cycling. A transition from three-dimensional metallic percolation of cermets with small grains and large thickness to two-dimensional percolation for films with grain sizes in the range of the layer thickness was observed.