STRUCTURAL TRANSFORMATIONS OF ALUMINA BY HIGH-ENERGY BALL-MILLING

Room temperature, high energy ball milling was applied to various transition aluminas (gamma, kappa, and chi), producing thermodynamically stable alpha-alumina-a phenomenon that could otherwise be achieved only by high temperature (1100-1200-degrees-C) heat treatment. The transformation proceeds in two steps. The first one consists of rapid microstructural rearrangements with continuously increasing alpha-transformation rate. In the second step (1-2 h from the start), only relatively small changes in morphology are observed with a constant alpha-transformation rate. The rate is influenced only by the milling intensity. The presence or the absence of oxygen in the milling atmosphere has a large influence on the final surface area of alpha-alumina.