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Abstract
A new evaluation method for preparing silica membranes by counter diffusion chemical vapor deposition (CVD) was proposed. This is the first attempt to provide new insights, such as the decomposition products, membrane selectivity, and precursor reactivity. The permeation of the carrier gas used for supplying a silica precursor was quantified during the deposition reaction by using a mass spectrometer. Membrane formation processes were evaluated by the decrease of the permeation of the carrier gas derived from pore blocking of the silica deposition. The membrane formation processes were compared for each deposition condition and precursor, and the apparent silica deposition rates from the precursors such as tetramethoxysilane (TMOS), hexyltrimethoxysilane (HTMOS), or tetraethoxysilane (TEOS) were investigated by changing the deposition temperature at 400–600 °C. The apparent deposition rates increased with the deposition temperature. The apparent activation energies of the carrier gas through the TMOS, HTMOS, and TEOS derived membranes were 44.3, 49.4, and 71.0 kJ mol−1, respectively. The deposition reaction of the CVD silica membrane depends on the alkoxy group of the silica precursors.
Highlights
Membrane separation technology is an energy-saving technology with no phase transition
At 500 ◦C and 600 ◦C, there was no correlation with the TMOS-derived peak at m/z = 121, suggesting that those detections are not a fragment of the precursor due to its decomposition in the mass spectrometer, but possibly a decomposition product or fragment of a product resulting from the reaction
The membrane formation behavior of the counter diffusion chemical vapor deposition (CVD) method was analyzed by measuring the diffusion properties of carrier gas through the porous substrates during the CVD
Summary
Membrane separation technology is an energy-saving technology with no phase transition. Nomura et al reported TMOS-derived membrane prepared by counter diffusion CVD treatment at 600 ◦C with the He/N2 permeance ratio of 5700 [14]. The PhTEOS- or DPhDEOS-derived membranes showed an N2 (0.36 nm)/sulfur hexafluoride (SF6, 0.55 nm) permeance ratio of 11 or 18 at 200 ◦C, indicating that the pore size is larger than that of the TEOS-derived membrane. To optimize the performance of the silica membrane for gas separation, it is necessary to study the membrane formation conditions and optimize the deposition and decomposition reaction rates through several experimental investigations. Substrate pore closure by silica deposition reduced the diffusion amount of the carrier gas This phenomenon was applied to investigate the effect of the experimental conditions, including the precursor structure and deposition temperature.
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