Oxygen-diffusion characteristics of loosely-sintered polycrystalline MgO

Self-diffusion coefficients of oxygen in well-sintered and in loosely-sintered polycrystalline MgO have been measured at an ambient oxygen pressure of about 35 mm Hg over the temperature range 1020–1450°C. The oxygen diffusion coefficient as a function of temperature of the former specimen is expressed by a single Arrhenius equation with an activation energy of 55.8 kcal/mole, while that of the latter is composed of two intersecting lines (E = 60.2 kcal/mole from 1020–1250°C; E = 102.8 kcal/mole from 1250–1450°C). The diffusivity for single-crystal grain of the well-sintered polycrystals is two orders of magnitude larger than that in the low-temperature portion of the loosely-sintered one. This fact is interpreted in terms of an insufficient dissolution of monovalent cation impurities into the loosely-sintered crystal lattice. Two ways of interpretation are given for the presence of the high-temperature portion characteristic of the loosely-sintered polycrystal.