Sol → gel → glass: III. Viscous sintering

Analysis of isothermal sintering data reveals that the viscosity of a gel increases with time. If the experiment is continued long enough, a limiting value of viscosity, η (final), is reached at each temperature. The increase in viscosity is attributed to two concurrent processes: condensation of hydroxyl groups and structural relaxation. Analysis of heat capacity (C_p) data permits an estimate of the viscosity (η) of the sintered glass. The results are in good agreement with η (final) from the sintering data. The approximate dependence of viscosity on hydroxyl content ([OH]) was established using η obtained from (C_p) data and [OH] from IR transmission spectra. The results indicate that structural relaxation contributes significantly to the rise in η during sintering.