Light scattering measurements of diffusional growth of precipitates in nickel- and cobalt-doped MgO

Changes in the sizes of second-phase precipitates in nickel- and cobalt-doped MgO single crystals have been examined in samples which were subjected to progressive thermal anneals at temperatures ranging from 600 to 1500 °C. Precipitate sizes and spatial locations were measured by means of light scattering before and after anneals in flowing argon. Absolute light scattering angular distribution measurements were used to obtain size distributions from a Mie theory deconvolution procedure. Comparison of the Ostwald ripening theory with the annealing time evolution of these precipitates indicates that the predominant growth can be described by a dislocation or grain boundary diffusion ripening mechanism. This mechanism is also consistent with ultramicroscope measurements taken before and after each anneal. The depletion region surrounding these ripening particles has been observed by means of backscattering in the measured angular distributions. Our results for diffusion within boundary regions are D′=(1.19±0.8×10−6 cm2/s)exp(−1.72 eV/kT) for nickel ions and D′=(1.60±0.9×10−7 cm2/s)exp(−1.36 eV/kT) for cobalt ions in MgO. These equations are in agreement with previous diffusion-couple measurements of nonenhanced diffusion in grain-boundary regions in these materials.