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Modeling growth and stability of nanoporous materials
We have modeled the thermodynamics and polymerization of silica, in order to understand the formation of complex materials such as zeolites. We have developed a simple molecular model of silica that is both physically realistic and computation ally efficient. We performed isobaric-isothermal Monte Carlo simulations to study the mechanical and phase behavior of this model, finding that it can be used as a qualitative representation of silica. Ratios of zero pressure bulk moduli and thermal expansion coefficients are shown to be in good agreement with experimental values for alpha-quartz, alpha-cristobalite, and coesite. A pressure-temperature phase diagram was constructed that shows three phases corresponding to cristobalite, quartz and coesite, and a fluid or glass phase, in good agreement with experiment. This phase diagram was extended to include the zeolitic polymorph silicalite-1. We found that silicalite-1 is a thermodynamically stable polymorph at high temperatures (>1000 K) and low pressures. This is surprising considering that zeolites were previously thought to be at most metastable. We then examined whether templated zeolites are thermodynamically stable under synthesis conditions (≈425 K). We found that an unphysically high template-zeolite attraction is needed to make this so, suggesting that zeolite formation is kinetically controlled. ^ We have extended this model to simulate the early stages of silica polymerization in order to determine atomistic structures of silica nanoparticles. The chemistry was simulated with the reaction ensemble Monte Carlo method. We introduce a new sampling method based on altering topological graphs to move between different networked states. We find that this method overcomes the sampling difficulties present in networked systems. Using an experimentally derived thermochemistry, we studied the effect of pH on the growing networks. We found that at high pH, the networks are dissolved in the basic solution leaving highly ionized monomers. We also found that the nanoparticles exhibit radial structure: a core of bridging oxygens (Si-O-Si) surrounded by a shell of silanol groups (Si-OH), surrounded further by an outer shell of charged Si-O- groups. ^
Chemistry, Physical|Engineering, Chemical
Matthew H Ford,
"Modeling growth and stability of nanoporous materials"
(January 1, 2006).
Electronic Doctoral Dissertations for UMass Amherst.