Stability of H_2-Permselective SiO_2 Films Formed by Chemical Vapor Deposition

Thin SiO_2 films were heat treated in different gas mixtures to determine their stability in functioning as high-temperature hydrogen permselective membranes. The films were formed within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing reactants geometry. Film deposition was carried out at 450 °C in the presence and absence of water vapor. Immediately after formation, the films were highly selective to hydrogen permeation having a H_2:N_2 permeability ratio of about 3000 : 1. The films were subsequently heat treated at 450-700°C in dry N_2, dry O_2, N_2- H_2O, and O_2-H_2O mixtures. The permeation rates of H_2 and N_2 changed depending on the original conditions of film formation as well as on the heat treatment. Heating in dry N_2 reduced slowly the permeation rates of both H_2 and N_2. Heating in N_2-H_2O atmosphere led to a steeper decline of H_2 permeability. But the permeation rate of N_2 increased or decreased according to whether the film deposition had been carried out in the absence or presence of H_2O vapor, respectively. Thermal treatment in O_2 caused rapid decline of the permeation rates of H_2 and N_2 in films that were deposited under dry conditions. The decline was moderate in films deposited under wet conditions