Characterization and gas permeation properties of amorphous silica membranes prepared via plasma enhanced chemical vapor deposition

Abstract

Plasma enhanced chemical vapor deposition (PECVD) was applied to the fabrication of hexamethyldisiloxane (HMDSO) derived amorphous silica membranes. Three different PECVD procedures, namely, Ar-PECVD, O2-PECVD, and a 2-step PECVD sequence involving Ar-PECVD followed by O2-PECVD, were employed to determine the effect of carrier gas on membrane structure and gas permeation property. The membrane prepared by Ar-PECVD showed moderate molecular sieving properties at room temperature (permeance ratio of 15 for He/N2 and 220 for He/SF6), but was easily decomposed at high temperatures. For the case of the O2-PECVD membrane, the permeance of small molecules such as He and H2 increased with increasing temperature, while the permeance of large molecules such as N2 and SF6 was consistent with Knudsen diffusion, and thus the selectivity of He/N2 increased up to 81 at 500°C. Compared with Ar- and O2-PECVD, 2-step PECVD membrane showed a excellent molecular sieving properties. Since the 2-step membrane was treated by O2-PECVD in the second step of the sequence, a good thermal stability was confirmed at temperatures as high as 400°C. The 2-step PECVD membrane showed high He and H2 permeances of 5.2×10−7 and 2.4×10−7mol/(m2sPa), respectively, with He/N2 and H2/N2 permeance ratios of 4200 and 1900 at 400°C, respectively. The results suggested that the 2-step PECVD is applicable to the low-temperature fabrication of thermally stable molecular sieving amorphous silica membranes.