The glass-forming area in the system SiO2-P2O5-Al2O3-MgO-Na2O is determined utilizing reagent grade chemicals. The obtained glasses are characterized using FT-IR, DTA, DSC, TMA and SEM. The multicomponent, mixed network glasses exhibiting low coefficients of thermal expansion, fairly high softening temperatures (at Al2O3/P2O5˃1) and good chemical durability, are made of SiO4, AlO4and PO4polymerized tetrahedral units. AlPO4groups incorporated in the network structure increase thermal stability and modify the crystallization behavior, rendering it a slow process requiring high temperatures and / or prolonged heating times. Ceramization is accomplished through a multistage process depending mainly on the AlPO4content which is determined by the Al2O3/P2O5ratio. At Al2O3/P2O5≈1, AlPO4-high cristobalitic form is the first to crystallize in bulk, followed by farringtonite Mg3(PO4)2. At Al2O3/P2O5˃ 2, platy corundum is the main crystallizing phase. Microporous glass-ceramics are obtained via acid leaching of either some or all of the crystallized phases, or through dissolving of the glassy matrix and preserving a network of one or more of the crystalline phases. Selected phases of less chemical durability have been removed, to leave a network of the most chemically resistant phases AlPO4and amorphous silica. On the other hand, the amorphous silica matrix containing Na2O, MgO and P2O5has been leached out in glasses having fibrous corundum as the main crystallizing phase to produce a microporous material with a unique morphology, being made of what looks like a house of cards structure made up of deformed, randomly oriented corundum blades or sheets. Such morphology, is believed to provide a microporous material with an extremely high surface area.
Interface structure of silicon nanocrystals embedded in an amorphous silica matrix