Thermally-Induced Changes in the Defect Substructure of Pure and Doped Magnesium-Oxide

The defect substructure of nominally pure and doped magnesium oxide (MgO) includes voids and second phase particulates decorating subgrain boundaries and dislocation lines. Three independent nondestructive elastic light scattering and imaging techniques; ultramicroscopy, bulk scattering angular distributions, and large angle bulk scattering as a function of crystal orientation were used to self-consistently examine the precipitated defect substructure of MgO. Computational routines were developed to fit the bulk scattering angular distribution measurements and determine the particle size distributions, as well as the total particle population, using Mie theory or other approximation schemes. The aggregation, precipitation, growth, and development of structures beyond the point defect stages up to macroscopic dimensions were followed through a number of controlled heat treatments. Size distribution changes in the bulk and the actual motion of these particulates in boundary regions were observed during progressive sequences of thermal anneals applied to a single sample. Estimates of the volume and grain boundary diffusion constants of nominal impurities in undoped MgO were obtained from measurements on crystals annealed at 1000(DEGREES)C. The diffusion constants were D' (TURNEQ) 10('-9) cm('2)/sec in grain boundary regions and D (TURNEQ) 10('-11) cm('2)/sec in the bulk, in agreement with previous diffusion couple measurements on doped MgO. Scanning electron microscope imaging of freshly-cleaved surfaces showed structures of the type inferred from light scattering in the bulk. These results show that light scattering can be useful in investigations of the diffusion of residual impurities in transparent crystals at relatively modest temperatures. The progressive changes seen in the size of particulates within the bulk of the samples showed effects similar to coarsening or Ostwald ripening. The theory of Ostwald ripening predicts the growth of the larger particles in the size distribution at the expense of the smaller particles for the three rate-limiting cases of surface reaction controlled growth, bulk diffusion, and dislocation or grain boundary diffusion. The theoretical size distributions for each ripening case were used to predict the time evolution of the angular dependence of scattered light from precipitates in an ionic crystal at a particular annealing temperature. Magnesium oxide crystals doped with nickel and cobalt were used as samples to test the theoretical ripening model. A series of progressive thermal cycles applied to these doped crystals caused second phase oxide precipitates to undergo Ostwald ripening through a dislocation or grain boundary diffusion mechanism. The light scattering measurements also showed that the particles had a complicated structure composed of a layer of depleted solute surrounding each particle. This depletion layer is a product of the diffusion -controlled growth of the precipitates. Using a combination of theoretical models to explain the diffusion-controlled growth and ripening, light scattering data was fit to obtain the local diffusivities and activation energies in the vicinity of the precipitates. They were: D(,0) = (1.19 (+OR-) 0.8) x 10('-6) cm('2)/sec and E = 1.72 (+OR-) 0.3 eV for Ni, and D(,0) = (1.6 (+OR-) 0.9) x 10('-7) cm('2)/sec and E = 1.36 (+OR-) 0.45 eV for Co in MgO. After long annealing times, further growth of the precipitates was inhibited by a size effect, at least for the heavily-doped samples used in this study, due to the interaction of the depletion layers surrounding each of the particles.