Effect of dehydration on the viscosity of sol-gel processed silica

For short times and low temperatures, the dehydration of silica gels is limited by the diffusion of water species. For long times and temperatures 600°C and higher, dehydration shows first order kinetics. That is dehydration rates decrease with increasing temperature where silica gels have less surface area and consequently fewer hydroxyls. For comparison with samples treated isothermally, samples were heated to 600, 700, 800 and 850°C at 100°C intervals and held 4 h at each temperature starting with 500°C. Comparison of isothermal and multistep treatments shows the contributions of structural relaxation and dehydration to the viscosity. At a given temperature, two gels of different water content may show the same viscosity, because the difference in water is matched by the difference in degree of structural relaxation. More complete dehydration is accomplished by constant rate heating to a single isothermal treatment rather than heating in steps.