Transformation of Aluminosilicate Wet Gel to Solid State

Optically clear, homogeneous, monolithic aluminosilicate gels were prepared directly from solutions of inorganic aluminum salt, tetraethoxysilane, and alcohol. The dried aluminosilicate gels show new special properties, e.g. piezoelectricity and variable porosity (1-2000 m²/g) due to the fractal-like gel structure. Ceramic products, which keep the chemical bonds and structure of the gels, were produced by heat treatment at 300-350°C. The processes that occurred during heat treatment were investigated by thermal analysis and mass spectrometry. The temperature limits of the gel state were determined by differential scanning calorimetry and small angle X-ray scattering. The collapse of the gel structure began around 350°C; however, the amorphous character remained until the temperature (ca. 970°C) of the phase transmission.