It was recalled that diffusion within the grain boundaries of ceramics was an important mechanism for the growth of oxide films at moderate temperatures, and that the addition of impurities could sharply reduce the rate at which the film grew. This process was investigated here by means of atomistic computer simulation. Because the migration energies of grain boundary processes in ionic systems were too high for conventional molecular dynamics, a modified simulation was used. The structure of the boundary was equilibrated, and a vacancy was introduced. A migration trajectory was chosen, and a small force which pulled the ion along this trajectory was added to the hopping ion. A counter-force was applied to the remaining ions in order to prevent the entire cell from moving. The velocities were scaled in order to remove the energy which was introduced by doing work on the moving ion. The effect of this was to permit the hopping trajectory to be plotted. An investigation was made of the effect of Mg, Ca, Sr and Ba impurities upon the migration energies and diffusion pathways of cation vacancies in the {310}/(001) and {410}/(001) tilt grain boundaries of NiO at moderate temperatures. It was shown that there was a correlation between the size of the impurity and its favoured position within the boundary. The presence of impurities increased the migration energies and altered the diffusion pathways. It was concluded that impurities would bind vacancies to themselves and thus reduce the diffusion rate.

Computer Simulation of the Reactive Element Effect in NiO Grain Boundaries. D.J.Harris, J.H.Harding, G.W.Watson: Acta Materialia, 2000, 48[12], 3039-48