C. Korte

Influence of interface structure on mass transport in phase boundaries between different ionic materials: Experimental studies and formal considerations

AbstractInternal and external interfaces in solids exhibit completely different transport properties compared to the bulk. Transport parallel to grain or phase boundaries is usually strongly enhanced. Transport perpendicular to an interface is usually blocked, i.e., transport across an interface is often much slower. Due to the high density of interfaces in modern micro- and nanoscaled devices, a severe influence on the total transport properties can be expected. In contrast to diffusion in metal grain boundaries, transport phenomena in boundaries of ionic materials are still less understood. The specific transport properties along metal grain boundaries are explained by structural factors like packing densities or dislocation densities in the interface region. In most studies dealing with ionic materials, the interfacial transport properties are merely explained by the influence of space charge regions. In this study the influence of the interface structure on the interfacial transport properties of ionic materials is discussed in analogy to metallic materials. A qualitative model based on the density of misfit dislocations and on interfacial strain is introduced for (untilted and untwisted) phase boundaries. For experimental verification, the interfacial ionic conductivity of different multilayer systems consisting of stabilised ZrO2 and an insulating oxide is investigated as a funtion of structural mismatch. As predicted by the model, the interfacial conductivity increases when the lattice mismatch is increased.Graphical abstract

Magnetoresistance in Ag2+δSe with high silver excess

In the present study, we investigated the galvanomagnetic transport properties of polycrystalline AgxSe thin films with silver excess in the range from x=1.5 to 18. The results prove that the silver excess controls the transition from linear magnetoresistance (MR) behavior to the quadratic ordinary MR and the temperature for the metal–semiconductor transition. Analyzing the MR effect by Kohler’s rule and comparing the results with the field-free resistivity we observe for 2<x<2.3 a steep rise of the product of mean free path and electron concentration (λ·n2∕3). We interpret this result as a consequence of the percolation of nanoscale silver networks within the semiconducting matrix, i.e., as a consequence of the two-phase character of the system.

Preparation and magnetoresistance of Ag2+xSe thin films deposited via pulsed laser deposition

The preparation of Ag 2+x Se thin films with thicknesses between 4 nm and 3000 nm by pulsed laser deposition on single crystalline NaCl and MgO substrates is reported. The films are perfectly dense and show a good lateral uniformity with a small number of defects. The microstructure of the films corresponds to a nanoparquet, being composed of two different phases of silver selenide. One phase is identified as the Naumannite low temperature phase of silver selenide, the structure of the other phase has not been reported in detail before and probably represents a metastable phase. Silver-rich films contain silver precipitates with typical sizes on the nanoscale. Their presence and their size appears to be responsible for the large and linear magnetoresistance effect of silver-rich silver selenide.

Thermodiffusion in Ionic Solids —Model Experiments and Theory

The current state of theory and experimental work on thermotransport (diffusion in a temperature gradient) in solids and the heat of transport is reviewed and the results of the authors’ own studies are summarized. Results from molecular dynamical simulations of vacancy migration in solid noble gases suggest that the heat of transport is temperature dependent and that it differs substantially from the migration enthalpy of the mobile point defect, in disagreement with some pre-existing models. Experimental results on mixed conducting solids support the theoretical results. The difficulties of thermal diffusion experiments are analysed, and possible experimental strategies are discussed.

Electric field driven solid state reactions—microscopic investigation of moving phase boundaries in the system MgO/MgIn2O4/In2O3

An external electric field acts on the mass transport in ionic solids as a second driving force in addition to the chemical potential gradient. The acceleration of a spinel-forming reaction between two binary oxides under the influence of an external electric field is studied in the model system MgO/MgIn2O4/In2O3. A simple kinetic model based on defect thermodynamics and linear transport theory is presented. The results indicate that the morphology during the early stages of growth of MgIn2O4 in the electric field is determined mainly by the initial grain structure of the In2O3 layer. The reaction front advances faster in the vicinity of grain boundaries. Morphological instabilities predicted by a continuum model of the different transport properties of MgO, MgIn2O4 and In2O3 are less important. The grain boundaries act as fast diffusion paths, resulting in large MgIn2O4 thickness variations along the interface. Accordingly the MgIn2O4/MgO interface adopts a highly curved morphology. Depending on the curvature of the MgIn2O4/MgO interface, the advancement of the interface proceeds either via a dislocation mechanism or via a ledge mechanism. Most probably, these two mechanisms result again in locally different growth rates, so that a feedback mechanism is established via the local micromorphology of the interface.

Electric field driven solid state reactions—reaction kinetics and the influence of grain boundaries on the interface morphology in the system MgO/MgIn2O4/In2O3

Under working conditions ceramic materials are often exposed to high electrical fields in addition to high temperatures. For this reason we investigate the reaction kinetics and the morphological evolution of the moving interfaces of a spinel forming solid state reaction under the influence of an external electric field. A model, based on linear transport theory and defect thermodynamics is used to analyse the results. Compared to a reference reaction without external electric field, the reaction rate is strongly enhanced and the morphology of the product layer has significantly changed. Systematic kinetic studies confirm a linear law with a constant growth rate for the electric field-driven spinel formation. The role of grain boundaries as fast diffusion paths is highly emphasised. A relationship between large angle grain boundaries in the product phase and the local reaction kinetics is pointed out. An analysis of the reaction rate indicates an unusually high transference for the trivalent ions compared to the divalent ions in the grain boundaries of the spinel phase.

Synthesis and calorimetric studies of oxide multilayer systems: Solid oxide fuel cell cathode and electrolyte materials

The authors used differential scanning calorimetry and high temperature oxide melt calorimetry to investigate the interface energies in various multilayer systems. For yttria stabilized zirconia (YSZ)/Al2O3 multilayers, the presence of interfaces is shown to affect the temperature and the enthalpy of crystallization; and therefore these interfaces play an important role in phase stability. From the thermal analysis results, it can be concluded that in YSZ/Al2O3 multilayers, the Al2O3 crystallization temperature increases and the enthalpy becomes less exothermic compared to the values for single alumina films. It is also shown that crystalline perovskite films of La(Sr)MnO3 can be deposited on NaCl substrates and can be collected from the substrate after the deposition which makes them suitable for high temperature calorimetry.