Porous glasses thermal treatment

The attack of most glasses in water and acid is diffusion controlled and the thickness of the porous layer formed on the glass surface consequently depends on the square root of the time. There is ample evidence that the diffusion of alkali ions and basic oxides is thermally activated, suggesting that diffusion occurs either through small pores or through a compact body. The reacted zone is porous and can be further modified by attack and dissolution, if alkali is still present, or by further polymerisation. Consolidation of the structure generally requires thermal treatment. [Pg.880]

Zhdanov (Institute of Silicate Chemistry, the U.S.S.R. Academy of Sciences, Leningrad) (154) showed (1949) that the adsorption of water vapor by SiC>2 (porous glasses, silica gels) strongly depends on the temperature of the preliminary thermal treatment of the adsorbent. Calcination of 300-500 °C resulted in a sharp decrease in the adsorption of H2O at low values of pressure over initial pressure p/p0 (<0.3), and the adsorption isotherms were found to be irreversible. On the other hand, the adsorption isotherms of water on silica subjected to calcination in vacuo at <500 °C (but after the sample was kept in contact with water vapor or liquid water at room temperature) again became reversible that is, the adsorption activity of Si02 was restored. [Pg.614]

Fig. 18.1. Schematic representation of rigid skeletons in hydrous oxides (a) a three-dimensional lattice in which small cation sites are interconnected (e.g. Nasicon, H2Sb40,. nHjO, hydrogen P-alumina) (b) crystalline heteropolyacids with Keggin anions and interconnected liquid containing protonic species particle hydrate structure is assumed to be of the same type but disordered (c) layer structure (as in e.g. clays, VjOj) (d) skeleton of a gel obtained by alkoxide hydrolysis of a complex composition (solvent in the pores is a water-ethanol mixture) (e) porous glass obtained by thermal treatment of the above gel.

Volkova AV, Ermakova LE, Sidorova MP, Antropova TV, Drozdova IA (2005) The Effect of thermal treatment on the structural and electrokinetic properties of porous glass membranes. Colloid J. 67 263-270. [Pg.227]

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. [Pg.273]

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