(485b) Molecular Modeling of the Creation of Novel, Ultra-Thin, Nano-Porous Layers and Supported Membranes Using Chemical Vapor Deposition

This work is focused on modeling the chemical vapor deposition of ultra-thin, nano-porous inorganic membranes (NPIMs) on both non-porous and micro-porous substrates for use in the development of high-temperature gas separation membranes. Previous simulation [1,2] and experimental [3] work have demonstrated the remarkably high perm-selectivities of these membranes to key gas mixtures, such as N₂/O₂/CO₂.

Two model substrates were used, namely a non-porous surface which was created by slicing through an amorphous silica block followed by a relaxation, and a micro-porous surface which was generated through a simulated phase separation of a binary Lennard-Jones (LJ) fluid using the Quench Molecular Dynamics technique proposed by Gelb and Gubbins [4]. The pore size of the micro-porous substrate was tuned to give a structure representative of a borosilicate glass, such as a porous Vycor?, by adjusting the simulation time. In order to translate this structure to a fully connected silica structure, a configurational mapping was made between this model LJ structure and that of a melted crystal of b-cristabolite. During the mapping, regions corresponding to the ?boron?-rich phase were removed (The b-cristabolite melting procedure used a Monte Carlo routine with the implementation of bond-switching [5] to generate an amorphous structure.) Periodicity in the final system was then removed and a random deletion at the upper surface was conducted to produce a non-isotropic surface.

The subsequent deposition of a NPIM was simulated using a specially developed kinetic Monte Carlo (kMC) algorithm, generating silica layers of controlled thicknesses on the prepared substrate. The reactions scheme for deposition was as follows. Firstly, the deposition of Si(OH)₄ was modelled to occur as

         (1)

while the condensation (annihilation) reaction was given by

         (2)

With the above reactions, the associated KMC equations are given by the following

where P is the probability of observing a reaction event (r) in bin j (spatially resolved location), n* are scaled reaction frequencies and t_incr is the incremental time between reaction events (using uniform random number x₂).

Using the above equations, a series of depositions, using a silica-based precursor, were performed at a number of temperatures, pressures and H₂O partial pressures to observe the effects of these operating conditions on the deposited layer characteristics, including densities, pair distribution functions, occupied volume and connectivity maps for a series of gas molecules. In the analysis of the deposited layers we have also performed a cluster analysis in order to determine permeation pathways through the deposited layers. 

[1] NA Seaton, SP Friedman, JMD MacElroy, BJ Murphy, Langmuir, 13 (1997), 1199

[2] J.M.D. MacElroy, Molecular Physics 100 (2002), 2369

[3] L. Cuffe, J.M.D. MacElroy, M. Tacke, M. Kozachok, D.A. Mooney, J. Mem. Sci. 272 (2006), 6

[4] L. Gelb and K.E Gubbins, Langmuir 14 (1998), 2097

[5] C. Schumacher, J. Gonzalez, P.A. Wright and N.A. Seaton, J. Phys. Chem. B, 110 (2006), 319