Sintering Crystalline Solids. II. Experimental Test of Diffusion Models in Powder Compacts

During sintering in alumina powder compacts, the density has been found to increase linearly with the logarithm of time, and the grain size increases with the one-third power of time. Incorporation of the time dependence of grain size increase into latestage bulk diffusion sintering models (from Part I) [R. L. Coble, J. Appl. Phys. 32, 787 (1961)] leads to corrected models by which a semilogarithmic behavior is predicted. The presence of density gradients in normally fabricated pellets makes impossible the deduction of whether theoretical density will be achieved from the early stages of the course of densification. Diffusion coefficients calculated from the intermediate and later stages of sintering bear order-of-magnitude agreement with those calculated from the initial-stage sintering measurements in alumina. All diffusion coefficients from sintering data are higher than Kingery's measured diffusion coefficients for oxygen. It is hypothesized that the sintering process must then be controlled by bulk diffusion of aluminum ions while the oxygen transport takes place along the grain boundaries. In controlling the sinterability of alumina to theoretical density, it appears that magnesia does not ``inhibit'' discontinuous grain growth, but instead increases the sintering rate such that discontinuous growth nuclei do not have time to form.