Modeling the Stress State of por-Si–H2O Structures in the Vicinity of the Phase-Transition Point of Water

Water-saturated mesoporous materials are characterized by a significant decrease in the melting/freezing temperature of liquids adsorbed in nanosized pores, which is due to the effect of spatial confinement. This effect at low temperatures significantly affects the performance of electrochemical devices developed on their basis. The paper considers the problem of predicting the stress state of water-saturated mesoporous materials based on silicon in the vicinity of the water crystallization point. A theoretical model has been constructed that takes into account the structure of the composite under study, in particular the orientation of the pores in the material space and the presence of a natural oxide layer on the surfaces of the pores in the silicon matrix, as well as the physical and mechanical characteristics and volume fractions of its components. It is shown that the developed model allows one to evaluate the resistance to mechanical destruction of water-saturated por-Si–H2O mesoporous structures during thermal cycling in the vicinity of the ice–water phase-transition point. Numerical model calculations of stresses arising in the silicon matrix during water crystallization were carried out. The dependences of the indicated stresses on changes in the volume fraction of water adsorbed in porous silicon membranes and the values of the dimensionless structural parameter—the ratio of the thickness of the silicon dioxide layer to the pore radius—were investigated. Modeling showed that an increase in both the volume fraction of water and the dimensionless structural parameter leads to an increase in the stress values in the silicon matrix. Using numerical calculations, it was established that freezing of water in pores during successive cooling–heating cycles should not have a destructive effect on the water-saturated mesoporous structures under consideration. The obtained conclusion does not contradict the available experimental data.