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Atomistic Simulation of the Formation of Nanoporous Silica Films Via Chemical Vapor Deposition

Thomas C. McDermott, M. Kozachok, and J. M. D. Macelroy. School of Chemical and Bioprocess Engineering, University College Dublin, UCD Engineering and Materials Science Centre, Belfield, D4, Dublin, Ireland

To date, membranes which could be used to separate gas mixtures at high temperatures (notably combustion or other process gas streams at temperatures T > 750K) have only been developed with limited success and it is apparent that this modest pace of growth is, in part, due to a lack of a detailed understanding of the membrane fabrication process at an atomistic level. One particular family of materials which has been shown to exhibit high permselectivities at elevated temperatures are thin films of amorphous silica glasses formed via chemical vapor deposition (CVD) techniques. In earlier work reported by this group [1-3] the possibility of generating membranes with both high kinetic selectivities for CO2 capture and acceptable permeability has been demonstrated. However, the influence of the experimental conditions employed during the CVD fabrication process on the fundamental microscopic details of the membranes formed still needs to be clarified.

In this work, we investigate the creation of moderate density silica films via direct simulation of the CVD process at intermediate to high temperatures. To model the creation of nanoporous silica layers via CVD we apply a hybrid kinetic Monte-Carlo (KMC) method [4]. Lattice KMC [5] is used for the elementary reactions and an off-lattice method [6,7] is employed for silica network relaxation and bond switching moves. The sensitivity of the resulting layer structure (nanoporosity, cavity size distribution, bond angle distributions, ring size distributions) to the substrate temperature, composition of the vapour, and the initial distribution of substrate seed sites is examined. The outcome of this work will assist in providing guidelines for the protocols needed to fabricate high temperature permselective membranes for the separation of CO2 from combustion gas mixtures.

References [1] MacElroy, J.M.D., 2002, Molecular simulation of kinetic selectivity of a model silica system. Mol.Phys., 100, 2369-2376. [2] Mooney, D.A., MacElroy, J.M.D., Kozachok, M., Cuffe, L., and Tacke, M., Atomistic Modeling of Permselective Membranes for CO2 Recovery. 16th International Congress of Chemical and Process Engineering, CHISA 2004, 22-26 August, Prague, the Czech Republic. [3] Cuffe, L., MacElroy, J.M.D., Tacke, M., Mykola Kozachok, M., Mooney D.A., 2006, The development of nanoporous membranes for separation of carbon dioxide at high temperatures, J.Membr.Sci., 272, 6-10. [4] Knizhnika, A.A., Bagaturyantsa, A.A., Belova, I.V., Potapkina, B.V., Korkin, A.A., 2002, An integrated kinetic Monte Carlo molecular dynamics approach for film growth modelling and simulation: ZrO2 deposition on Si(1,0,0) surface, Comp.Mat.Sci., 24, 128-132. [5] Battaile, C.C., Srolovitz, D.J., 2002, Kinetic Monte Carlo Simulation of Chemical Vapor Deposition, Ann.Rev.Mat.Res., 32, 97-319. [6] Schumacher C., J. Gonzalez, J., Wright P.A., N.A. Seaton, N.A., 2006, Generation of atomistic models of periodic mesoporous silica by kinetic Monte Carlo simulation of the synthesis of the material, J.Phys.Chem.B , 110, 319-333.

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