Gas permeation of PECVD membranes inside alumina substrate tubes

Abstract

The purpose of this work is the gas permeation study of silicon carbide membranes inside asymmetric porous alumina tube. Membranes have been synthesized by the Plasma Enhanced Chemical Vapor Deposition (PECVD). The reactor we have set up allows the deposition of amorphous hydrogenated inorganic thin films on the inner surface of tubular substrates and involves the reaction between an organosilicon vapor (Diethylsilane: SiH2(C2H5)2) and a transported glow discharge of argon. In a previous study, the composition and morphology of a:SixCyHz layers have been investigated as a function of different plasma conditions (electric power applied to the plasma, gases flow rates, substrate temperature). It appears that the well controlled homogeneity and composition of the layers result in a good control of the substrate temperature induced by the glow discharge heating during the deposition. The good mechanical properties and thermal stability of the amorphous hydrogenated silicon carbide layers, and the alumina substrate allow an important heating without any damage. In this work the permeation and separation experiments are performed using a laboratory made gas permeation cell by testing H2, CO2, and N2 gases through thin films synthesized with different conditions on two kind of alumina tubes whose average pore size is 5 or 200 nm. The nitrogen permeance at room temperature is in the range of 10−6–10−7 mol m−2 s−1 Pa−1 for membranes with a thickness above 1 μm. This permeability remains quite constant regardless pressure, which means that our membranes permeate gases principally according the Knudsen diffusion mechanism.