Strain Effects in Heterogeneous Oxide Electrolyte Films

A number of opportunities exist to use thin film deposition methods to nanoengineer materials so that we can understand and improve their ion transport behavior and other electrochemical properties. Here, we describe experiments on sputter-deposited ceramic films with controlled, heterogeneous chemical composition. Films are created using a technique developed in our lab where pure, single-element targets are reactively co-sputtered to create ceria-zirconia films where both the host and dopant atom concentrations can be controlled at the nanometer scale through the film thickness. The ionic conductivity of all samples was determined using impedance spectroscopy in the temperature range 573 K to 1073 K and over a range of oxygen partial pressures.

Two sets of results using this system will be presented. First, we will discuss films made to have heterogeneous dopant content. By placing the dopants in 2-D atomic planes, defects are trapped in engineered space charge zones surrounding the interface, and vacancy-dopant associations can be studied. Specifically, by comparing samples using heterogeneous La, Gd, or Y dopants, the role of strain in the mobility and concentration of defects in the space charge region is examined, with larger dopant ion correlated to increased space charge conductivity. A second system to be presented will involve modulation of the Ce/Zr ratio through the thickness. Since the lattice parameter is a linear function of Ce/Zr ratio, these samples can be used to smoothly and systematically control lattice strain. Strain was determined by x-ray diffraction and microscopy, including selected area electron diffraction. Clear differences in interfacial structure are found as the lattice parameter differential between the layers is adjusted. The strained multilayers indicate both a strain-based change in vacancy mobility and a general reduction in conductivity as the interfacial density increases.