Abstract
Sol-gel silica monoliths have been prepared with 16 mol water per mol tetraethylorthosilicate (TEOS) under acid conditions. These materials have been converted to a physical duplicate of fused silica by employing viscous sintering. The densification of these gels has been accomplished by various heat treatments. An understanding of the densification behavior is drawn from constant rate heating (CRH), isothermal treatments (single 4 h step) and isothermal treatments in steps (multistep). The number of steps for the multistep treatments was as many as five in 50 or 100° increments. During isothermal treatments at 600°C and above, the apparent viscosity was calculated from the cylinder model for viscous sintering. The effects of dehydroxylation and structural relaxation were studied in the context of densification. All in all, the kinetics of the gel-to-glass conversion depend strongly on thermal history.
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