Oxygen ion transport and dopant segregation in strained oxide thin films

摘要

Modified ionic transport at interfaces in oxide conductors has received significant interest recently, yet previous studies have provided inconsistent and often controversial results. Lattice strain occurring at heterogeneous interfaces or surrounding dislocations is often speculated as the cause for enhanced transport properties, yet direct evidence for this is lacking. The aim of this work was to investigate the possibility for modified oxygen conductivity at interfaces and develop a greater understanding of the underlying mechanisms. ududIn this work, it was shown that the crystallographic orientation of yttria-stabilised zirconia (YSZ) films fabricated by pulsed laser deposition (PLD), could be altered by the roughness and the temperature of the substrate during deposition. Furthermore, the films were seen to feature an expanded out-of-plane lattice parameter, which was found to be dependent upon the deposition temperature, substrate, the orientation of film, and the thermal history after growth. ududThe ionic transport of YSZ films fabricated on a range of substrates was assessed as a function of film thickness using electrochemical impedance spectroscopy (EIS) and a novel method of oxygen isotope exchange tracer diffusion (IETD). No enhancement over the conductivity of bulk YSZ was observed, despite expanded out-of-plane lattice parameters, substrates representing both positive and negative lattice mismatch, and a high density of interfacial dislocations. Instead, all interface regions were found to be more resistive due to a higher density of grain boundaries. ududFinally, gadolinia-doped ceria (CGO) films were analysed by low energy ion scattering (LEIS), to investigate the effects of dopant segregation to the surface and sub-surface regions. Gd segregation was found to occur for all films and observed to increase for higher annealing temperatures. Investigations into the affect of the lattice parameter on the rate of dopant segregation showed a possible link between segregation and lattice strain.