HIGH-SURFACE-AREA ALUMINA AEROGEL AT ELEVATED-TEMPERATURES

In order to obtain an alumina with a high surface area at elevated temperatures, a suppression of the theta-alpha phase transformation of alumina by decreasing the bulk density has been attempted. The abundance of the lattice defects and dislocations in the neck region would be a favourable site for nucleation of the alpha-phase. Therefore the decrease of the neck region, that is, the decrease in bulk density would lead to a suppression of the theta-alpha phase transformation. In order to decrease the bulk density, the alumina aerogel was prepared from boehmite sol. The bulk density was varied from 0.06 to 1.1 g cm-3 by decreasing the zeta-potential of the boehmite particles during ageing. The phase transformation and the surface area of alumina from aerogels with various bulk densities were examined at elevated temperatures. The phase-transformation temperature increased with decreasing bulk density. While the alumina with its bulk density of 1.1 g cm-3 transformed to the alpha-phase at 1443 K, alumina aerogel with a bulk density of 0.06 g cm-3 transformed to the alpha-phase at 1573 K. The shift of the transformation to a higher temperature can be understood in terms of the decrease in the neck region. As a result of the suppression of phase transformation, a large surface area was maintained at elevated temperatures. After heating at 1473 K for 5 h, the alumina aerogel, with a bulk density of 0.06 g cm-3, was composed of the theta-phase and its surface area was 90 m2 g-1. After completion of the phase transformation, i.e. at 1673 K for 5 h, alumina aerogel with a bulk density of 0.06 g cm-3 still exhibited a surface area of 10 m2 g-1, larger than that of the conventional alpha-phase.