Deposition of H_2-permselective SiO_2 films

Abstract

Films of amorphous SiO_2 were deposited within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing-reactants geometry: SiH_4 was passed inside the tube while O_2 was passed outside the tube. The two reactants diffused opposite to each other and reacted within a narrow front inside the tube wall to form a thin SiO_2 film. Once the pores were plugged the reactants could not reach each other and the reaction stopped. At 450°C and 0.1 and 0.33 atm of SiH_4 and O_2, the reaction was complete within 15 min. The thickness of the SiO_2 film was estimated to be about 0.1 μ. Measurements of H_2 and N_2 permeation rates showed that the SiO_2 film was highly selective to H_2 permeation. The H_2: N_2 flux at 450°C varied between 2000 and 3000. Thermal annealing at 600°C reduced somewhat that selectivity. Thermal annealing in the presence of H_2O vapor decreased further the flux of H_2 and increased the flux of H_2. Permeation of H_2 is believed to occur through an activated diffusion mechanism. Applications of such H_2-permeable films to membrane reactors for equilibrium-limited reactions are discussed.

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