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. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.