Structure and aging characteristics of H₂-permselective SiO₂-Vycor membranes

Hydrogen-permselective membranes were prepared by atmospheric pressure chemical vapor deposition (APCVD) of SiO₂ layers in porous Vycor tubes. The deposition was carried out by passing the reactants SiCl₄ and H₂O through the bore of the support tubes at temperatures ranging from 600 to 800°C. The deposit layers were examined by TEM, SEM, and EPMA. When the deposit was confined inside the pores of the Vycor substrate, the membranes were mechanically stable but when it extended substantially outside of the porous matrix the stresses induced by thermal cycling led to crack formation and propagation. Electron microscopy revealed that the SiO₂ deposit density is maximum in a region ≈0.5 μm thick adjacent to the bore surface and gradually declines to zero within a depth of ≈10 μm from the surface. The thin region of maximum deposit density is responsible for permselectivity, for it essentially blocks the permeation of nitrogen and larger molecules while allowing substantial permeation of hydrogen. This region contains ≈10% by volume trapped voids and as a result has relatively high permeability as suggested by the percolation theory. Annealing at high temperatures causes densification of the deposited material as evidenced by increased activation energy for H₂ permeation and correspondingly reduced permeance. The presence of H₂O vapor accelerates the densification process. The densified membranes had a H₂ permeance as high as 0.1 cm³(STP)/min-atm-cm² at 500°C and a H₂/N₂ permeance ratio above 500.