INFLUENCE OF THE SILICA MATRIX ON THE FORMATION OF ALPHA-ALUMINA IN AMULLITE ALUMINA COMPOSITE FROM A DIPHASIC PRECURSOR

In mullite-alumina composite precursors, interaction between the silic a matrix and the fine gamma-alumina texture strongly influences the pr ecursor phase transformation, the nucleation and the crystal geometry both of the mullite and of the alpha-alumina. The mullite-alumina comp osite precursor calcined at 1000 degrees C has a layered structure pro bably derived from the layered texture of the gamma-alumina. The phase transition of this layered texture is retarded by the presence of the silica matrix and a metastable mullite phase is formed before nucleat ion of alpha-alumina. By leaching away the silica matrix, the remainin g layered texture is readily transformed into very fine, thin alpha-al umina platelets by calcination at 1000 degrees C. This seems to be one reason for the appearance of elongated mullite grains in a pure mulli te matrix and the platelet shaped alpha-alumina grains in the mullite- alumina composite prepared from diphasic precursors. (C) 1998 Chapman & Hall.