Structure and aging characteristics of H_2-permselective SiO_2-Vycor membranes

Abstract

Hydrogen-permselective membranes were prepared by atmospheric pressure chemical vapor deposition (APCVD) of SiO_2 layers in porous Vycor tubes. The deposition was carried out by passing the reactants SiCl_4 and H_2O 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 SiO_2 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 H_2 permeation and correspondingly reduced permeance. The presence of H_2O vapor accelerates the densification process. The densified membranes had a H_2 permeance as high as 0.1 cm^3(STP)/min-atm-cm^2 at 500°C and a H_2/N_2 permeance ratio above 500.

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