CHEMICAL-VAPOR-DEPOSITION OF HYDROGEN-PERMSELECTIVE SILICA FILMS ON POROUS-GLASS SUPPORTS FROM TETRAETHYLORTHOSILICATE

Hydrogen-permselective SiO2 films were formed within the pores of glas s support tubes by chemical vapor deposition from tetraethylorthosilic ate (TEOS). Deposition Of SiO2 involved the decomposition of TEOS both in the absence of and in the presence of oxygen at 200-700-degrees-C and 1 atm. The properties of the silica films produced were greatly af fected by the deposition conditions, the temperature, the contact sche me of reactants, and the amount of SiO2 deposited. In the absence of o xygen, the films produced appeared to be relatively labile and their s tabilities were dependent on the deposition conditions. In the presenc e of oxygen, however, stable and highly H-2-selective membranes were s ynthesized at temperatures above 300-degrees-C by one-sided deposition ; and unstable films were obtained at high concentration of TEOS by op posing-reactant deposition. The permeation rates of H-2 at 600-degrees -C through the SiO2 membranes were 0.20-0.42 cm3 (STP)/cm2 -min-atm an d the H-2:N2 permeation ratios varied between 500 and 3000. The SiO2 f ilms Produced using TEOS and oxygen were less dense than those deposit ed by SiCl4 hydrolysis. Thus the former membranes had higher H-2 perme ation rates but had lower H-2 selectivity as the pores of the substrat e were plugged with SiO2.