EFFECT OF PHYSICAL STRUCTURE ON THE PHASE DEVELOPMENT OF ALUMINOSILICATE GELS

The surface area and density of aluminosilicate xerogels containing a one-to-one Al/Si molar ratio (47 wt% alumina) can be varied dramatically by changing the pore fluid prior to drying. The surface area of ethanol-washed 47 wt% alumina gels was more than 500 m2/g, while gels dried from the mother liquor (approximately 75 vol% ethanol, 25 vol% water) had less than 1 m2/g surface area. Changes in the physical structure of the dried gels had dramatic effects on subsequent phase evolution and densification behavior during heat treatment. NMR, X-ray diffraction, and DTA were used to follow the phase evolution of different gels. Differences in the amorphous gel structure were identified using Al-27 and Si-29 MAS NMR. Gels of identical composition prepared from the same precursor solutions crystallized to different phases, depending upon the surface area of the gel prior to heating. The high surface material (ethanol washed) formed mullite and amorphous silica, while the low surface area gel (unwashed) crystallized to mullite and cristobalite. These gels were prepared from alkoxide precursors. A low surface area gel with a different degree of chemical homogeneity was prepared by the nitrate method for comparison. Results indicate that the physical structure of aluminosilicate gels, i.e., pore structure and chemical homogeneity, has a dramatic influence on phase evolution.