MODEL FOR THE CRYSTALLIZATION AND SINTERING OF UNSEEDED AND SEEDED BOEHMITE GELS

Boehmite gels crystallize into alpha-alumina with a dendritic morphology and, in thin films, whiskers can be obtained. In bulk gels, vermicular morphologies are observed, their scale depending on the temperature of conversion to alpha-alumina. In seeded gels, where the phase change to a occurs at a lower temperature, the scale is much finer than in unseeded gels where a higher alpha-crystallization temperature is required. A model which explains the dendritic morphology is presented; it rests on the hypothesis that the porosity in the gel must be redistributed into the dense alpha-dendrite and the empty interdendritic space at the crystallization front. The dendrite spacing is then determined by the kinetic balance between the rate of crystallization and the rate at which the local porosity is redistributed. Measurements of the crystallization velocity, combined with the model, successfully explain the scale of the dendritic morphologies in the seeded and unseeded bulk gels, and in the seeded gel films. The model predicts that the scale of the dendritic structure should increase rapidly with crystallization temperature, also in agreement with experimental results. The model suggests processing paths for controlling the whisker diameter in seeded films, and for low-temperature sintering of seeded bulk gels.