Genetics of superionic conductivity in lithium lanthanum titanates

Using a genetic algorithm in conjunction with molecular dynamics we describe diffusion mechanisms and provide evidence that there is a 3D percolated network of Li diffusion pathways in La_2/3−xLi_3xTiO₃.

Chemistry and structure of homoepitaxial SrTiO3 films and their influence on oxide-heterostructure interfaces

The properties of single-crystal SrTiO3 substrates and homoepitaxial SrTiO3 films grown by pulsed laser deposition have been compared, in order to understand the loss of interfacial conductivity when more than a critical thickness of nominally homoepitaxial SrTiO3 is inserted between a LaAlO3 film and a SrTiO3 substrate. In particular, the chemical composition and the structure of homoepitaxial SrTiO3 investigated by low-energy ion-scattering and surface X-ray diffraction show that for insulating heterointerfaces, a Sr-excess is present between the LaAlO3 and homoepitaxial SrTiO3. Furthermore, an increase in the out-of-plane lattice constant is observed in LaAlO3, indicating that the conductivity both with and without insertion of the SrTiO3 thin film originates from a Zener breakdown associated with the polar catastrophe. When more than a critical thickness of homoepitaxial SrTiO3 is inserted between LaAlO3 and SrTiO3, the electrons transferred by the electronic reconstruction are trapped by the formation of a Sr-rich secondary phase and Sr-vacancies. The migration of Sr towards the surface of homoepitaxial SrTiO3 and accompanying loss of interfacial conductivity can be delayed by reducing the Sr-content in the PLD target.

An Investigation of Buried Layer Formation by 40keV Oxygen Implantation Into Silicon

Device grade single crystal <100> silicon wafers have been implanted with 40keV oxygen ions over a dose range of 2×1017 to 8×1017/cm2 at a temperature of 5507±10°C. The specimens have been analyzed, both before and after high-temperature annealing at 1320°C for six hours, using SIMS, RBS, and TEM. During implantation a continuous buried SiO2 layer forms when the dose after which the buried SiO2 layer grows primarily toward the surface. After annealing samples implanted with the doses of both 2×1017 and 3×1017/cm2 it was observed that a defective SIMOX like structure was formed. Implanting with a dose of 4×1017/cm2 results in pit formation in the silicon overlayer. For the sample implanted with a higher dose of 5×1017/cm2, a non-continuous poly-crystalline silicon “overlayer” was formed. It would thus appear that when a single step implantation route is chosen, there is a dose (or layer thickness) limit for the production of a thin film SIMOX structure.

Improving Gd-Doped Ceria Electrolytes for Low Temperature Solid Oxide Fuel Cells

Gadolinia-doped ceria electrolyte is being investigated as an alternative electrolyte for solid oxide fuel cells operating at temperatures below 700°C. Measurements were made to determine the effects that small additions of Ca, Pr and Fe to gadolinia-doped ceria have on the bulk and grain boundary conductivities. These small additions (1–2%) of dopant did not alter the bulk conductivity significantly but resulted in a large increase in the grain boundary conductivity. The grain boundary conductivity was similar for all three electrolyte compositions. These results are explained by the possible formation of second phases at the grain boundary, which can incorporate impurity elements. The electronic conductivity in these electrolyte materials was also evaluated, but it was found that the Ca, Pr and Fe additions do not reduce the electronic conductivity compared to a standard Gd-doped ceria sample.

Optimisation of oxygen ion transport in materials for ceramic membrane devices

Oxygen transport in ceramic oxide materials has received much attention over the past few decades. Much of this interest has stemmed from the desire to construct high temperature electrochemical devices for energy conversion, an example being the solid oxide fuel cell. In order to achieve high performance for these devices, insights are needed in how to achieve optimum performance from the functional components such as the electrolytes and electrodes. This includes the optimisation of oxygen transport through the crystal lattice of electrode and electrolyte materials and across the homogeneous (grain boundary) and heterogeneous interfaces that exist in real devices. Strategies are discussed for the optimisation of these quantities and current problems in the characterisation of interfacial transport are explored.