D. J. Harris

Atomistic simulation of dislocations, surfaces and interfaces in MgO

A new simulation code for modelling extended defects e.g. linear (dislocations) and planar (surfaces and grain boundaries) at the atomistic level is introduced. One of the key components is the ability to calculate the Coulombic potential of a solid with one-dimensional periodicity. This approach has been applied to screw dislocations in MgO and we have evaluated the structure (including core size) and stability of the 〈100〉 and 1/2〈110〉 screw dislocations. The 1/2〈110〉 dislocation, which has the shortest Burgers vector, was found to be more stable, as predicted by elasticity theory, although the simulations show that elasticity theory underestimates the energy difference.In addition, it has been shown that by using this new computer simulation code METADISE, following the approach of Tasker, the structure and energetics of surfaces and interfaces can be calculated. This method has been applied to modelling micro-faceting and it was found that micro-facetted {110} and {111} surfaces of MgO are the most stable forms of these surfaces. The formation energy of tilt grain boundaries in MgO ({h10} and {h20}) as a function of misorientation angle was also investigated and it was found that for the {h10} series the formation energy is proportional to the interfacial bond density while no such pattern can be found for the {h20} series.

Atomistic simulation of the effect of temperature and pressure on the [001] symmetric tilt grain boundaries of MgO

Abstract Computer simulation has been used to model a series of grain boundaries of MgO. The aim of the work is to establish a procedure for generating grain boundaries and to calculate the effect of high pressures and temperatures upon the structure, energies and ultimately diffusivities associated with these boundaries. The simulations predict that temperatures up to 1200K have little effect upon the structure and energies, whereas pressures of up to 40 GPa have a large effect, causing the free energy of formation to increase with pressure for all the grain boundaries studied. In addition the (310), (410) and (510) grain boundaries undergo structural changes to minimize the dilation of the grain boundary. In the case of the (410) and (510) grain boundaries this structure change is found to be irreversible when the pressure is released.

Computer simulation of pressure-induced structural transitions in MgO [001] tilt grain boundaries

Abstract Atomistic simulations using lattice and molecular dynamics were carried out on the {210}, {310}, and {410} tilt grain boundaries of MgO as a function of pressure up to 100 GPa at a single temperature of 600 K. The calculations show a significant change in the structure of the tilt grain boundaries as pressure increases. The results show that, beyond the previously identified reversible pressure induced collapse of the channel structure, an irreversible shear was identified that forms a mirror grain boundary, which does not possess well-defined dislocation cores and is consequently denser. These mirror boundaries are energetically more favorable than the symmetric boundaries at elevated pressures. As applied pressure approaches 100 GPa a reversible structural transition occurs causing the boundaries to shear perpendicular to the boundary plane forming highly dense structures that contained seven coordinate ions at the boundary and the presence of an edge dislocation-like structure. We suggest that differences between the boundary structures seen in the HREM studies with those predicted by simulations may result from stresses upon the crystal during preparation causing irreversible shearing.

UNDERGRADUATE PROJECT WORK

ABSTRACT A study of the stages and processes involved in final‐year Undergraduate projects at the University of Bath was conducted over a two‐year period. The phases and the factors which influence them were identified in the first year of the investigation and this scheme for a project formed the framework for the following year. Both student and staff opinions, based on questionnaires and interviews, form the data on which discussion of the phases of a project is based.

Atomistic simulation of steps on the MgO(100) surface

Static lattice calculations have been used to calculate the surface energy of a large number of surfaces along two arcs; for surface normals from [100] to [110] in the (001) plane and from [001] to [110] in the (110) plane. Results for vicinal surfaces have been analysed in terms of steps and step–step interactions. Both repulsive and attractive interactions are observed, showing that this effect is strongly dependent on the configuration of step and terrace.

Atomistic simulation of oxide dislocations and interfaces

Abstract Atomistic simulation techniques have been used to study screw dislocations, grain boundaries, thin films and surfaces. The results show that the a/2〈110〉 screw dislocations in bulk MgO and NiO are more stable than the a〈100〉 although the latter are stabilised by vacancies. Adsorption of MgO units at the a〈100〉 spiral dislocation shows a complicated two-layer growth mechanism. Self-diffusion through MgO grain boundaries is shown to be faster than in the bulk crystal, with pipe diffusion the energetically most favourable route. Study of thin iron oxide films on MgO found that the most stable MgO/Fe3O4 {001}/(001) interface is an open structure with closely matching spacing between substrate Mg ions and oxygens of the film. The interaction of water with oxides MgO and SiO2 has been investigated. The dominance of the MgO {100} surface is shown through facetting of the less stable {110} and {111} surfaces. The low-coordinated surface sites hence formed are the most reactive towards adsorption of water...

AN ILLUMINATIVE EVALUATION OF AFIRST YEAR LABORATORY COURSE – A CRITICAL APPRAISAL

ABSTRACT The paper outlines techniques used in an illuminative evaluation of a laboratory course. A brief outline of the organisation of the course is given, followed by details of the methods of collecting information and the method of sampling the groups of students. Methods were constrained by the time available to carry out the evaluation. Comparisons are made between methods of collecting data. The dynamic aspect of the methods and the possibility of rapid feedback and reporting are discussed. The paper attempts to show the advantages of this style of evaluation for an idiosyncratic evaluation exercise carried out within an establishment using an evaluator external to the department involved.