Porous glass is a widely-used inorganic porous membrane material, characterized by its small pore size and sharp distribution. Therefore, it has been the subject of much research since the 1950s. In recent years, inorganic membranes have drawn growing attention, and their application studies have been carried out using porous glass as a typical inorganic membrane. The advantages of inorganic membranes in thermal and chemical stabilities over organic membranes have been verified through research, while some disadvantages have also been pointed out, such as their poor selectivity and difficulty in thinning. At this stage, however, only limited research is available regarding the improved selectivity of inorganic membranes. Schnabel and Vaulont (1 978) improved the desalination property of porous glass membrane by surface modification. The authors (1 987, 1988) modified the surface condition of a porous glass membrane to control the surface transport property and reported the possibility of improving the gas selectivity.
It is agreed generally that a membrane with uniform pores on the order of angstroms and the level of molecular size would have excellent selectivity. For a long time, the order of the uniform pore size of inorganic membranes had been limited to the order of a few tens of angstroms for porous glass, where the selectivity by gas-phase flow only had an upper limit predicted by Knudsen's equation.
Recently, some experimental research into fine pore control was reported. Niwa et al. (1984) presented a method in which