Abstract
The aim of this work was to develop innovative a-SiCxNy:H membranes prepared by PECVD, with good selectivity for gas with small kinetic diameters (H2/He) and with good hydrothermal stability. For this purpose, membrane depositions were carried out in two types of reactors (low frequency LF and microwave MW) using both HMDSN and NH3 as precursors and argon as a carrier gas. First, different synthesis conditions were investigated in order to obtain a large range of materials deposited as thin layers on monocrystalline silicon (for physicochemical characterizations) and mesoporous planar substrates (for membrane characterization). In the LF reactor, parameters such as deposition temperature and NH3 pressure during the synthesis were studied. In MW reactor, these studies were supplemented by the change of electric power applied to the plasma. The influence of these parameters on the microporous structure of the materials has been proven, and correlations with the membranes ability to selectively permeate hydrogen have been established. For both types of reactors, a molecular sieving behavior was obtained with the following performance: single gas He permeance > 10-7 mol.m-2.s-1.Pa-1 and He/CO2 ideal selectivity = 50 at 400°C. Secondly, the first steps for a transfer of this lab scale technology towards industry are investigated. At first, single gas (He, N2 and CO2) were replaced by a gas mixture containing H2, CO, CO2 and CH4 for studying separation factors both in dry and wet conditions, while keeping a planar mesoporous substrate. The results showed that, despite a slight decrease in H2 permselectivity, the membrane was still a good candidate for this purpose. In addition, the presence of moisture at 150°C seemed to slightly increase performance and proved the hydrothermal stability of the membrane at this temperature. The second step corresponds to the transfer from planar to tubular geometry, with the PECVD membranes deposited on the outer surface of the tubes. The results showed that the geometric stresses and the possible mesoscopic defects of these substrates greatly reduced the H2 selectivity of the deposited a-SiCxNy:H membrane materials, although their molecular sieving behavior and thermal stability at 400°C were kept.