Nucleation model for mesopore-confined water freezing kinetics

Abstract

Cyclic freezing of water is the main reason of microstructure damages in porous materials exploited in cold climate. It is usually assumed that water freezes much faster than temperature changes, therefore the kinetic effects are neglected. Nevertheless, in the reality such assumption might be often violated. In this paper the kinetics of water freezing in mesopores of two silica gels with dominant pore diameter equal 11 and 36 nm has been investigated. To obtain the freezing rate functions the differential scanning calorimetry test with multiple cooling rate program has been performed. The activation energy is calculated using both, differential and local integral methods. It is found that the activation energy is proportional to the reciprocal temperature. The most suitable kinetic model has been selected by means of two methods: the classical procedure recommended by ICTAC KC and the linear model-fitting method. The former method indicates that two models are appropriate: the empirical Šesták–Berggren model and physical Johnson-Mehl-Avrami model. The SB model demonstrates high accordance with the experimental data, whereas the JMA model allows to approximate the experimental data with high accuracy only if the activation energy is calculated using differential approach. The application range of linear model-fitting method has been extended with the fractional numbers for JMA model parameter. The best correlation between experimental data and theoretical model is obtained for the non-integer values of JMA parameter. It indicates that nucleation and nuclei growth for both silica gels are of fractional dimensions.