Nanoporous SiO2/vycor membranes for air separation
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- Vycor tubes; low pressure; chemical vapor deposition (LPCVD); Materials Science and Engineering
thesis / dissertation description
Porous Vycor tubes with 40Å initial pore diameter were modified using low pressure chemical vapor deposition (LPCVD) of silicon dioxide N02). Diethylsilane (DES) in conjunction with nitrous oxide (N2O) was used as a precursor to synthesize these SiO2 films. The aim of this study was to obtain a considerable selectivity between species of comparable size and hence N2O was used. The use of N2O was believed to make the process self-limiting. DES was allowed to flow through the tube and N2O on the outside in the chamber at 550°C in a counter-flow mechanism. This deposition geometry provided an optimum pore narrowing rate and eliminated the possibility of film cracking. The pore size of the Vycor tube was reduced with successive depositions and the stage at which maximum selectivity between oxygen and nitrogen was obtained was recorded. The value of selectivity was confirmed using mass spectroscopy and reproducing the results using another Vycor tube. The temperature dependence on selectivity was also studied. Characterization of the Vycor membranes was carried out to observe the SiO2 coating. Calculation of permeability was done using ASTM standards.