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 tube s. The deposition was carried out by passing the reactants SiCl4 and H 2O through the bore of the support tubes at temperatures ranging from 600 to 800-degrees-C. The deposit layers were examined by TEM, SEM, an d EPMA. When the deposit was confined inside the pores of the Vycor su bstrate, the membranes were mechanically stable but when it extended s ubstantially outside of the porous matrix the stresses induced by ther mal cycling led to crack formation and propagation. Electron microscop y revealed that the SiO2 deposit density is maximum in a region approx imately 0.5 mum thick adjacent to the bore surface and gradually decli nes to zero within a depth of approximately 10 mum from the surface. T he thin region of maximum deposit density is responsible for permselec tivity, for it essentially blocks the permeation of nitrogen and large r molecules while allowing substantial permeation of hydrogen. This re gion contains approximately 10% by volume trapped voids and as a resul t has relatively high permeability as suggested by the percolation the ory. Annealing at high temperatures causes densification of the deposi ted material as evidenced by increased activation energy for H-2 perme ation and correspondingly reduced permeance. The presence of H2O vapor accelerates the densification process. The densified membranes had a H-2 permeance as high as 0.1 cm3 (STP)/min-atm-cm2 at 500-degrees-C an d a H-2/N2 permeance ratio above 500.