Thermal dehydroxylation, gas diffusion, and viscous sintering during densification of sol-gel derived xerogels

Existing models for gas-solid reaction in porous solids and viscous sintering of porous glasses cannot be used to predict the full range of densification behavior in xerogels because of limitations in modelling either structural or compositional changes. Moreover, there is a need for a quantitative basis for predicting general trends in densification behavior as a function of thermal processing variables. A model of xerogel densification is developed which can be used to predict changes in composition and pore structure during thermal processing. The model incorporates surface reaction, skeletal phase diffusion, gas phase diffusion, and viscous sintering processes. A set of dimensionless groups are identified which characterize the relative rates of reaction, diffusion, and sintering processes. Values for these groups are readily calculated from data on xerogels and can be used to predict general densification behavior. Processes controlling xerogel densification are determined through the experimental study of silica xerogel densification as a function of sample size, processing temperature, heating rate, and atmosphere. Gas phase diffusion of water and skeletal phase diffusion of hydroxyl groups are shown to control the conversion rate in monoliths at short processing times and at temperatures below 873 K. Conversion between 800 K and 1000 K is affected by simultaneous gas and skeletal phase diffusion and viscous sintering of pores. Faster sintering in monoliths than powders is attributed to lower skeletal phase viscosity which results from higher water concentration in the pores and higher hydroxyl group concentration in the skeletal phase of monoliths. Values for the apparent surface dehydroxylation reaction rate constant, k$\sb{\rm f}$, and skeletal phase diffusivity, D$\sb{\rm sk}$, are obtained by fitting the densification model to mass conversion data