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Non-Tech : US Global Nanospace (USGA) -- Ignore unavailable to you. Want to Upgrade?

To: scion who wrote (14)	2/18/2004 9:14:23 PM
From: scion	Read Replies (1) \| Respond to of 132

".....silica membrane, referred to as Nanosil, was prepared by chemical vapor deposition of a thin SiO2 layer on a porous Vycor glass support." Mechanism of permeation in hydrogen-selective silica membranes S. Ted Oyama and Doohwan Lee. Department of Chemical Engineering, Virginia Tech, 140 Randolph Hall, Blacksburg, VA 24061 Hydrogen separation silica membranes have attracted much interest in recent years. These membranes are usually in the form of silica layers placed on ceramic supports such as porous Vycor glass or alumina, and are deposited by methods such as sol-gel techniques or chemical vapor deposition of silica precursors. The hydrogen permeance of these membranes varies over a wide range (10-7 - 10-9 mol m-2 s-1 Pa-1) showing a trade off with the selectivity over N2 which ranges from 100 to 3000. However, details of the gas transport mechanism through the supported silica membranes were not discussed in earlier studies. In this work the gas permeance on a deposited silica layer was obtained by applying a series analysis of gas permeation on the combined silica layer and Vycor support composite system. The order of permeance through the silica layer was He > H2 > Ne, which did not follow the molecular weight or size. The order of permeation is explained for the first time using a statistical approach, which takes into consideration the density of solubility sites for the various species and the vibrational frequency of the species within the sites. For this work a highly hydrogen permeable silica membrane, referred to as Nanosil, was prepared by chemical vapor deposition of a thin SiO2 layer on a porous Vycor glass support. The composite membrane showed good permeance (~10-8 mol m-2 s-1 Pa-1) for the small gas molecules (He, Ne and H2) at 873 K with excellent selectivity (~104) over other larger gas molecules (CO2, CO, and CH4). The characteristics of gas transport on the Vycor and Nanosil membrane were investigated with several gas diffusion models. The experimental gas permeation data on Vycor glass could be explained by the occurrence of Knudsen diffusion in parallel with surface diffusion. The permeance of the small gas molecules (He, Ne and H2) on the Nanosil membrane was activated, and increased as temperature increased. However, this permeance was limited at high temperature because of the limited permeance on the Vycor support. tinyurl.com ragingbull.lycos.com By: lineolator 18 Feb 2004, 07:23 PM EST Msg. 24554 of 24606 (This msg. is a reply to 24553 by BusyBodys.) Jump to msg. #