(742d) 2-Step Plasma-Enhanced CVD for Low-Temperature Fabrication of Silica Membranes with High Gas-Separation Performance

Amorphous silica network structure allows permeation of the smallest molecules such as helium (kinetic diameter: 0.26 nm) and hydrogen (0.289 nm), but not larger molecules such as nitrogen (0.364 nm).¹⁾ Since silica membranes for hydrogen separation were first reported in 1989 by thermal CVD²⁾ and in 1990 by sol-gel processing³⁾, porous silica membranes have drawn a great deal of attention. In general, thermal CVD membranes show high selectivity, while sol-gel silica membranes show high permeability. Although plasma-enhanced chemical vapor deposition (PECVD) has a great advantage of low temperature synthesis and has been extensively investigated in production of semi-conductors, only a limited number of papers were reported for gas separation.^4-6) To date, using silicon-precursors, including tetraethoxysilane (TEOS)⁴⁾, tetramethyldisilazane⁵⁾, and hexamethyldisiloxane (HMDSO)⁶⁾, silica membranes have been prepared on polycarbonate⁴⁾ and alumina^5,6) porous supports, and have been applied for pervaporartion⁴⁾ and gas separation^5,6). In this paper, sol-gel derived intermediate layers were prepared on cylindrical a-alumina supports, and subjected to PECVD using hexamethyldisiloxane (HMDSO). Gas permeability and thermal stability of plasma CVD membranes were evaluated.

PECVD was carried out in a flow-type reactor equipped with an RF coil induced at 13.56 MHz, and the gas permeation rate was measured in-situ without removal of the membranes from the PECVD apparatus. Hexamethyldisiloxane (HMDSO) in a mole fraction of 7% was fed under an argon flow rate of 10 ccm at a pressure of ~120Pa. Nanoporous TiO₂/a-Al₂O₃ membranes with average pore sizes of 4 nm were used as substrates for PECVD.

The single gas permeances of He (kinetic diameter: 0.26 nm), N₂ (0.36 nm) and SF₆ (0.55 nm) of Ar-CVD membranes, which were fabricated under Ar-flow in PECVD with TiO₂ nanoporous supports decreased on the order of 10^-2-10^-3 during the initial 10-20 minutes, and then approached steady values, showing a He permeance of 6 x 10^-9 mol/(m² s Pa) with a permeance ratio of 200 for He/SF₆ and 7 for He/N₂ at 25 °C.

After the Ar-CVD membrane was treated by O₂-CVD for 5 min in the second step of the 2-step membrane process, the permeance ratio for He/N₂ and He/SF₆ of the 2-step membrane increased up to 7,800 and 27,000, respectively. The 2-step CVD process was found to drastically increase the separation performance. Increased selectivity for small molecules is caused by the reduced pore sizes of the 2-step CVD membrane. Through the second step (O₂-CVD), the plasma-polymerized layer formed during Ar-CVD was converted to a silica-rich inorganic layer. The more rigid structure, which allowed less vibration of the network, led to high selectivity for hydrogen over nitrogen. In particular, the permeance ratio of He over H₂ was increased from 0.79 to 5.8 and 17 after second step reaction of 5 and 30 min, respectively. It should be noted that most polymers showed the separation factor for He/H₂ as approximate unity, and He/H₂ selectivities of inorganic membranes, including silica membranes fabricated by thermal CVD, were approximately several and less than 10 at maximum. Herein, it was confirmed that the present 2-step PECVD membrane showed an extremely high separation factor for He over H₂, which was also reproducible. Therefore, it is suggested that the Ar-CVD plasma polymerized layer, which is flexible and easy to form, was converted under O₂-CVD to a silica-rich phase with a high selective performance.

References:

1) Tsuru, J. Sol-Gel Sci. Technol. 46 (2008) 349. 2) Gavalas et al., Chem. Eng. Sci. 44 (1989) 1829. 3) Kitao et al., MAKU (Membrane) 15 (1990) 222. [4] C-H. Lo et al., J. Membr. Sci., 329 (2009) 138-145. [5] Kafrouni et al., J. Membr. Sci. 329 (2009) 130–137. [6] Wang et al., Microporous and Mesoporous Materials 77 (2005) 167–174.