Probing Radiation Damage in Fe-doped SrTiO3 using Micro-contact Impedance Spectroscopy

Radiation damage in ceramics has drawn significant attention due to their application as wasteforms for the immobilisation of nuclear waste and a requirement to understand the fundamentals that govern radiation damage tolerance. This project is based on determining whether a novel technique, micro-contact Impedance Spectroscopy, can be employed to characterise radiation damage. Firstly, a greater understanding of the measurements of local electrical properties using micro-contact measurements is achieved using an in-house finite element modelling package. The electrical response of a single crystal sample based on SrTiO3 was simulated using different electrode geometries and conditions of confinement. For two micro-contacts on the same surface, the spreading resistance equation overestimates the conductivity at small separations. The accuracy of this equation increases as the contact separation increases and is within 10% error when the contacts are separated by 8 times the micro-contact radius. Further convergence to errors lower than 10% becomes increasingly difficult and requires excessively large separations. The influence of confinement within the sample/electrode arrangements can be an important factor and increases the measured resistance and the spreading resistance equation becomes an underestimate of the conductivity. At extreme levels of confinement, the geometric correction becomes a better approximation for the conductivity. In some cases, the enhancement of conductivity from the close separation of the micro-contacts can counter-balance the reduction in conductivity from confinement producing fortuitously accurate calculated values of conductivity. The modelling was then extended to include a resistive surface layer of thicknesses ranging from 1 – 50 μm. Equations are proposed to calculate the conductivity of both the surface layer and the underlying bulk phase. If the surface layer is sufficiently thin with respect to the micro-contact size, a geometric factor and the spreading resistance equation produces accurate conductivity values for the surface layer and bulk, respectively. For large surface layer thicknesses, the spreading resistance equation produces an accurate conductivity for the surface layer, however, a bulk conductivity is not obtainable. Fe-doped SrTiO3 single crystals were characterised using X-ray diffraction and Impedance Spectroscopy prior to radiation damage studies. The conductivity increased with increasing Fe content, in agreement with previous studies. For micro-contact Impedance Spectroscopy, temperature corrections were made using relationships obtained from conventional Impedance Spectroscopy measurements. These included relationships obtained from the Arrhenius conductivity behaviour of the sample and also from comparison of the frequency maximum of the M” Debye peak associated with the bulk response with temperature. The Impedance Spectroscopy signature of a pristine sample included a bulk response and multiple responses at lower frequencies attributed to the sample/electrode interface. Bulk 5 MeV Au ion implantation of Fe-doped SrTiO3 single crystals was undertaken to a fluence of 5 x 10^15 ions cm-2. Glancing angle X-ray diffraction and cross-sectional transmission electron microscopy (XTEM) confirmed the samples had amorphised. Annealing studies revealed the amorphous surface had partially recrystallised to a polycrystalline perovskite after heating to 400 °C for 30 mins. XTEM confirmed that vacancy-type defects, cracks and regions of high strain still existed in the recrystallised layer after annealing. Cracks were a significant feature on the surface of annealed samples. Conventional Impedance Spectroscopy measurements revealed significant differences between the damaged and pristine samples that could not be easily resolved; however, as micro-contact Impedance Spectroscopy measurements are surface sensitive, these responses were more easily resolved. Two stages of the annealing process were observed. First, an intermediate frequency response initially shifted to higher frequency with a response that was more similar to the bulk response followed by a second stage where the intermediate frequency response shifted to lower frequency and away from the bulk response. The second stage was very sensitive to the number of cracks between the two micro-contacts. The response associated with the first stage of annealing indicates that micro-contact Impedance Spectroscopy may well be a useful tool characterise radiation damage in oxides; however, the changes in physical microstructure associated with the second stage of annealing limited the use of micro-contact Impedance Spectroscopy for characterising radiation damage induced in this study.