Spinodal Decomposition by a Two-Step Procedure for Nano Porous Silica

The phase separation of the Na₂O–B₂O₃–SiO₂ system was explored both theoretically and experimentally in order to attain a spinodal structure having a narrowed periodic distance (<70 nm) with the porosity being kept at ∼60%. The phase separation was dealt with by two stages: an initial thermodynamic process of spinodal decomposition and a latter growth of the spinodal structure. The initial structural development was related to the interfacial energy and the change in free energy caused by phase separation. For the latter growth, a mathematical model was proposed to explain the kinetics by incorporating the effect of the inverse-square law in the diffusion of SiO₂, and a basic relation of $d^3-{d_0}^3\propto t$ (d: average periodic distance; t: time) was successfully derived. The phase separation was carried out accordingly by two steps: first for the phase separation forming durable silica skeletons at lower temperatures and second for the new equilibrium at the elevated temperature and the subsequent growth of the phase-separated structure. It was proven that the addition of Al₂O₃ in the glasses decreased the interfacial energy, leading to small periodic distances and the rapid establishment of the durable silica skeletons. In the two-step process, the fraction of borate-rich phase increased, and the structure grew depending on a modified period of time.