J. H. Harding

Hartree-Fock cluster computations of defect and perfect ionic crystal properties

Abstract Recent progress is reported on a collaboration to develop a convenient program package for point defect calculations in ionic crystals, incorporating unrestricted Hartree-Fock self-consistent field cluster calculations with correlation correction, interfaced with a surrounding shell-model lattice. Electric multipole consistency between cluster and lattice and appropriate lattice boundary conditions for the cluster are features of the method. Application to specific defects and questions of consistency between cluster treatment and perfect lattice properties will be discussed.

Modelling the interfaces between calcite crystals and Langmuir monolayers

Langmuir monolayers are good model systems for investigating the use of organic templates to control the growth and morphology of calcium carbonate. We investigate the structure of the organic–mineral interface using molecular dynamics methods. The monolayer consists of octadecanoic (stearic) acid molecules; the calcium carbonate is the calcite phase. Seven interfaces were chosen to demonstrate the various types of behaviour possible. We show that simple epitaxial arguments based on the ideal, unrelaxed monolayer and mineral surfaces can be very misleading, particularly when considering polar directions. Furthermore, such arguments cannot predict the relative stability of the various interfaces. We show that the polar (001) direction (with a Ca termination) produces the most stable interface and discuss the implications for mineral growth on organic monolayers.

The simulation of general polar boundaries

We show that there is a general method for quenching the macroscopic dipole that is found when a stack of planes is set up along a polar direction. We therefore show how simulations of these polar interfaces can be performed. We also show that there is a category of polar interface that cannot be stabilised in any simple way that both quenches the dipole and is chemically reasonable. We illustrate the method by calculating the energies of a number of boundaries in spinel.

Comparative theoretical study of the Ag–MgO (100) and (110) interfaces

We have calculated the atomic and electronic structures of Ag–MgO(100) and (110) interfaces using a periodic (slab) model and an ab initio Hartree–Fock approach with a posteriori electron correlation corrections. The electronic structure information includes interatomic bond populations, effective charges, and multipole moments of ions. This information is analyzed in conjunction with the interface binding energy and the equilibrium distances for both interfaces for various coverages. There are significant differences between partly covered surfaces and surfaces with several layers of metal, and these can be understood in terms of electrostatics and the electron density changes. For complete monolayer (1:1) coverage of the perfect MgO(100) surface, the most favorable adsorption site energetically for the Ag atom is above the surface oxygen. However, for partial (1:4) coverage of the same surface, the binding energies are very close for all the three likely adsorption positions (Ag over O, Ag over Mg, Ag over a gap position). For a complete (1:1) Ag monolayer coverage of the perfect MgO(110) interface, the preferable Ag adsorption site is over the interatomic gap position, whereas for an Ag bilayer coverage the preferred Ag site is above the subsurface Mg2+ ion (the bridge site between two nearest surface O2− ions). In the case of 1:2 layer coverage, both sites are energetically equivalent. These two adhesion energies for the (110) substrate are by a factor of two to three larger than over other possible adsorption sites on perfect (110) or (100) surfaces. We compare our atomistic calculations for one to three Ag planes with those obtained by the shell model for 10 Ag planes and the Image Interaction Model addressing the case of thick metal layers. Qualitatively, our ab initio results agree well with many features of these models. The main charge redistributions are well in line with those expected from the Image Model. There is also broad agreement in regard to orders of magnitude of energies.

Modelling of silver adhesion on MgO(100) surface with defects

We show how surface defects (especially Fs 0 and Vs 0 centres) can play a major role in the adhesion of Ag (at 1:4 and 1:1 coverages) on the MgO(100) surface. Our calculations use a periodic (slab) model and an ab initio Hartree-Fock approach with a posteriori electron correlation corrections. We are able to analyse the interatomic bond populations, effective charges and multipole moments of ions, in combination with the interface binding energy and the equilibrium distances. Both surface defects cause strong redistributions of the electron density which increase the binding energy of metal atoms by more than an order of magnitude. This implies radiation-induced strengthening of metal adhesion on oxide substrates and clarifies defect mechanisms in nucleating film growth. We compare our atomistic predictions with those from simpler methods which might be used for complex technologically interesting systems. There is good general agreement with the image interaction model; differences arise partly from different treatments of dispersion and partly from subtle but significant charge redistribution in the Ag. Further, a simple Born-Haber analysis of charge transfer is consistent with the several cases predicted in the atomistic calculations.

Calculations of interionic potentials in oxides

Abstract Calculations of the electronic structure of a pair of O−2 ions in a point-ion lattice are analysed in the manner of a shell model. Induced dipole moments on the highly polarizable ions are explicitly accounted for in fitting the repulsive potential to a Born form. The resulting potential, together with an empirical Mg2+−O2− potential, is used to calculate bulk and defect properties of MgO. The values obtained for bulk dielectric properties using this method are worse than those found from empirical potentials, but defect properties are very similar.

Calculated cell discharge curve for lithium batteries with a V2O5 cathode

Electronic structure techniques now enable accurate estimates of cell voltages of solid state batteries employing intercalation cathodes: we calculate the variation of cell potential with the degree of discharge—a crucial factor determining the applicability and utility of cathode materials—for the LixV2O5 cathode system.

Computer simulation of the reactive element effect in NiO grain boundaries

Diffusion within the grain boundaries of ceramics is an important mechanism for the growth of oxide films at moderate temperatures. Experiments show that the addition of impurities can drastically reduce the rate at which the film grows. We have investigated this by atomistic computer simulation. Since the migration energies of grain boundary processes in ionic systems are too high for a conventional molecular dynamics simulation to be used, we have used a modified simulation. The structure of the boundary is equilibrated and a vacancy introduced. A migration trajectory is chosen and a small force pulling the ion along this trajectory added to the ion that is to hop. A counter-force is added to the remaining ions to prevent the whole cell moving and the velocities are scaled to remove the energy introduced by doing work on the moving ion. The effect of this is to enable the hopping trajectory to be plotted out and the migration energy found. We have investigated the effect of Mg2+, Ca2+, Sr2+ and Ba2+ impurities on the migration energies and diffusion pathways of cation vacancies in the {310}/[001] and {410}/[001] tilt grain boundaries of NiO at moderate temperatures. We show that there is a correlation between the size of the impurity and its favoured position within the boundary. The presence of impurities increases the migration energies and alters the diffusion pathways. We conclude that impurities will bind vacancies to themselves, reducing the rate of diffusion.

Novel exchange mechanisms in the surface diffusion of oxides

We use temperature-accelerated dynamics to show the importance of exchange mechanisms in surface diffusion and growth of simple oxides. Such mechanisms can dominate transport processes both on terraces and steps for both homoepitaxial and heteroepitaxial growth. We suggest that the mixing inevitable when an exchange mechanism is present must be considered when attempts are made to grow sharp interfaces in oxide nanostructures.

Modelling the production and performance analysis of plasma-sprayed ceramic thermal barrier coatings

SummaryThe present review describes current research on the numerical modelling of ceramic thermal sprayed coatings, both their manufacture and their performance during thermal cycling. Micromechanical approaches are described and discussed for the coating deposition process, and for the constitutive description of the material produced. A decision support system based on the numerical models is shown to be an effective way of collecting the knowledge produced and feeding back the results to industrial users. Finite element models are used to simulate the thermal and mechanical response to thermal cycling of coated components. Experimental test data is used to develop and validate the models. We consider two industrial applications; coated pistons (from automotive engineering), and coated turbine blades (from aerospace engineering).