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Diffusion in mesoporous glass : simulations and experiments

Citation

Fernandes, Neil Edward (1997) Diffusion in mesoporous glass : simulations and experiments. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01092008-135803

Abstract

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The diffusivities of simple gases in mesoporous glass were studied by experiment and simulation.

Porous Vycor[...] glass was modified by deposition of silica on the internal pore surface using consecutive cycles of liquid phase silylation with silicon tetrachloride, and hydrolysis. Macroscopically uniform deposition was achieved by exploiting the self limiting nature of the reaction and the extent of deposition was monitored by the weight change of the samples. Weight increases as high as 24% were recorded and the average pore diameter was estimated to decrease from [...]. Permeation measurements were conducted in the Henry's law region at various levels of deposition for hydrogen, methane, isobutane and nitrogen, at temperatures between 60[degrees]C and 180[degrees]C. The measurements were compared to values calculated with a model using the effective medium approximation to treat network effects and Clausing's correction to account for conductances in pores of finite aspect ratio. The calculated values proved to be inaccurate for hydrogen, overestimating the permeance by a factor of two at high levels of deposition possibly because of non-ideal pore shapes accentuated by the deposition. For nitrogen and methane the agreement between calculations and measurements was better due to a fortuitous cancellation of deviations caused by the enhanced potential energy well within the pores and the non ideal pore shape. The intrapore potential energy effect was especially strong for isobutane and as a result the calculated flux was always less than the experimental.

In an effort to understand the importance of the intrapore potential and pore surface roughness on diffusion, molecular dynamics simulations of nitrogen and isobutane in a mesoporous glass pore, under free molecular flow conditions, were conducted for pores of diameter [...], and for temperatures between 200K and 800K. To study the effect of the intrapore potential, the gases were treated as simple Lennard-Jones atoms and the pore was simulated as a perfect cylinder exerting a 9-3 potential, but with its surface roughened by the superposition of spherical Lennard-Jones atoms representing silica tetrahedra. The molecular trajectories were calculated by the application of Nose-Hooverian mechanics and no momentum transfer was allowed between the pore walls and the gas molecules. Random walk behavior resulted from the resulting specular collisions. The effect of the intrapore potential was decoupled into two contributions. The effective diffusivity was respectively increased and decreased by a partitioning effect (or Henry's law adsorption) and a path curvature effect (the trapping of molecules near the surface). In pores of radius [...], both effects were present for temperatures as high as 500K, and were enhanced as the temperature decreased. For nitrogen, the combination of effects canceled over the temperature range of 500-200K and resulted in a temperature dependence similar to that of Knudsen diffusion. For isobutane, the partitioning effect overwhelmed the path curvature effect, resulting in significant surface flows at temperatures as high as 500K. At a temperature of 393K, as the pore radius was reduced from [...], the path curvature effect decreased and the partitioning effect increased. Although the intrapore potential becomes more negative as the pore size decreases, the magnitude of the potential energy barrier trapping molecules near the surface also decreases. The effect of surface roughness was studied through a hard sphere dynamics version of the above simulation. Diffusivities were obtained for various surface coverage of the silica tetrahedra. The specular reflection condition resulted in diffusivities at least twice that of the Knudsen value.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Gavalas, George R.
Thesis Committee:
  • Gavalas, George R. (chair)
  • Giapis, Konstantinos P.
  • Davis, Mark E.
  • Flagan, Richard C.
Defense Date:24 April 1997
Record Number:CaltechETD:etd-01092008-135803
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-01092008-135803
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:09 Jan 2008
Last Modified:26 Dec 2012 02:27

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