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Full-Text Articles in Physics

Pore Diameter Dependence Of Catalytic Activity: P-Nitrobenzaldehyde Conversion To An Aldol Product In Amine-Functionalized Mesoporous Silica, Andres Garcia, Igor I. Slowing, James W. Evans Jul 2018

Pore Diameter Dependence Of Catalytic Activity: P-Nitrobenzaldehyde Conversion To An Aldol Product In Amine-Functionalized Mesoporous Silica, Andres Garcia, Igor I. Slowing, James W. Evans

Chemistry Publications

The reaction yield for conversion of p-nitrobenzaldehyde (PNB) to an aldol product in amine-functionalized mesoporous silica nanoparticles (MSN) exhibits a 20-fold enhancement for a modest increase in pore diameter, d. This enhanced catalytic activity is shown to reflect a strong increase in the “passing propensity,” 𝒫, of reactant and product species inside the pores. We find that 𝒫 ≈ 0, corresponding to single-file diffusion, applies for the smallest d which still significantly exceeds the linear dimensions of PNB and the aldol product. However, in this regime of narrow pores, these elongated species must align with each other and with the pore ...


Application Of The Fokker-Planck Molecular Mixing Model To Turbulent Scalar Mixing Using Moment Methods, Ehsan Madadi-Kandjani, Rodney O. Fox, Alberto Passalacqua Jun 2017

Application Of The Fokker-Planck Molecular Mixing Model To Turbulent Scalar Mixing Using Moment Methods, Ehsan Madadi-Kandjani, Rodney O. Fox, Alberto Passalacqua

Chemical and Biological Engineering Publications

An extended quadrature method of moments using the beta kernel density function (beta-EQMOM) is used to approximate solutions to the evolution equation for univariate and bivariate composition probability distribution functions (PDFs) of a passive scalar for binary and ternary mixing. The key element of interest is the molecular mixing term, which is described using the Fokker-Planck (FP) molecular mixing model. The direct numerical simulations (DNSs) of Eswaran and Pope ["Direct numerical simulations of the turbulent mixing of a passive scalar,"Phys. Fluids 31, 506 (1988)] and the amplitude mapping closure (AMC) of Pope ["Mapping closures for turbulent mixing and reaction ...


Stochastic Modeling Of Catalytic Processes In Nanoporous Materials: Beyond Mean-Field Approach, Andres Garcia Jan 2017

Stochastic Modeling Of Catalytic Processes In Nanoporous Materials: Beyond Mean-Field Approach, Andres Garcia

Graduate Theses and Dissertations

Transport and reaction in zeolites and other porous materials, such as mesoporous silica particles, has been a focus of interest in recent years. This is in part due to the possibility of anomalous transport effect (e.g. single-file diffusion) and its impact in the reaction yield in catalytic processes. Computational simulations are often used to study these complex non-equilibrium systems. Computer simulations using Molecular Dynamics (MD) techniques are prohibitive, so instead coarse grained one-dimensional models with the aid of Kinetic Monte Carlo (KMC) simulations are used. Both techniques can be computationally expensive, both time and resource wise. These coarse-grained systems ...


Nanostructure Control: Nucleation And Diffusion Studies For Predictable Ultra Thin Film Morphologies, Matthew Hershberger Jan 2017

Nanostructure Control: Nucleation And Diffusion Studies For Predictable Ultra Thin Film Morphologies, Matthew Hershberger

Graduate Theses and Dissertations

This thesis covers PhD research on two systems with unique and interesting physics. The first system is lead (Pb) deposited on the silicon (111) surface with the 7x7 reconstruction. Pb and Si are mutually bulk insolubility resulting in this system being an ideal case for studying metal and semiconductor interactions. Initial Pb deposition causes an amorphous wetting layer to form across to surface. Continued deposition results in Pb(111) island growth. Classic literature has classified this system as the Stranski-Krastanov growth mode although the system is not near equilibrium conditions. Our research shows a growth mode distinctly different than classical ...


Point Island Models For Nucleation And Growth Of Supported Nanoclusters During Surface Deposition, Yong Han, Émilie Gaudry, Tiago J. Oliveira, James W. Evans Dec 2016

Point Island Models For Nucleation And Growth Of Supported Nanoclusters During Surface Deposition, Yong Han, Émilie Gaudry, Tiago J. Oliveira, James W. Evans

Physics and Astronomy Publications

Point island models (PIMs) are presented for the formation of supported nanoclusters (or islands) during deposition on flat crystalline substrates at lower submonolayer coverages. These models treat islands as occupying a single adsorption site, although carrying a label to track their size (i.e., they suppress island structure). However, they are particularly effective in describing the island size and spatial distributions. In fact, these PIMs provide fundamental insight into the key features for homogeneous nucleation and growth processes on surfaces. PIMs are also versatile being readily adapted to treat both diffusion-limited and attachment-limited growth and also a variety of other ...


Catalytic Conversion In Nanoporous Materials: Concentration Oscillations And Spatial Correlations Due To Inhibited Transport And Intermolecular Interactions, Andrés García, James W. Evans Nov 2016

Catalytic Conversion In Nanoporous Materials: Concentration Oscillations And Spatial Correlations Due To Inhibited Transport And Intermolecular Interactions, Andrés García, James W. Evans

Physics and Astronomy Publications

We show that steady-state catalytic conversion in nanoporous materials can occur in a quasi-counter-diffusion mode with the reactant (product) concentration strongly decaying (growing) into the pore, but also with oscillations in the total concentration. These oscillations reflect the response of the fluid to the transition from an extended to a confined environment near the pore opening. We focus on the regime of strongly inhibited transport in narrow pores corresponding to single-file diffusion. Here, limited penetration of the reactant into the pores and the associated low reaction yield is impacted by strong spatial correlations induced by both reaction (non-equilibrium correlations) and ...


Transition Metals On The (0001) Surface Of Graphite: Fundamental Aspects Of Adsorption, Diffusion, And Morphology, David Victor Appy, Huaping Lei, Cai-Zhuang Wang, Michael C. Tringides, Da-Jiang Liu, James W. Evans, Patricia A. Thiel Jan 2014

Transition Metals On The (0001) Surface Of Graphite: Fundamental Aspects Of Adsorption, Diffusion, And Morphology, David Victor Appy, Huaping Lei, Cai-Zhuang Wang, Michael C. Tringides, Da-Jiang Liu, James W. Evans, Patricia A. Thiel

Chemistry Publications

In this article, we review basic information about the interaction of transition metal atoms with the (0001) surface of graphite, especially fundamental phenomena related to growth. Those phenomena involve adatom-surface bonding, diffusion, morphology of metal clusters, interactions with steps and sputter-induced defects, condensation, and desorption. General traits emerge which have not been summarized previously. Some of these features are rather surprising when compared with metal-on-metal adsorption and growth. Opportunities for future work are pointed out.


Flux Requirements For The Growth Of Rfeaso (R = Rare Earth) Superconductors, J.-Q. Yan, Brandt A. Jensen, Kevin W. Dennis, R. William Mccallum, Thomas A. Lograsso Feb 2011

Flux Requirements For The Growth Of Rfeaso (R = Rare Earth) Superconductors, J.-Q. Yan, Brandt A. Jensen, Kevin W. Dennis, R. William Mccallum, Thomas A. Lograsso

Ames Laboratory Publications

Millimeter-sized LaFeAsO single crystals have been successfully grown out of NaAs flux starting with (LaAs+1/2Fe2O3):20NaAs. The factors which allow the growth of these crystals out of NaAs but not out of many other fluxes, such as FeAs, have been investigated. X-ray powder diffraction found that NaAs synthesized for the growth ofRFeAsO superconductors has monoclinic LiAs structure. Thermal analysis confirmed that NaAs melts congruently at about 600 °C. The ability to extract RFeAsO crystals from this NaAs flux suggests that NaAs has a significant oxygen solubility, possibly due to the formation of ...


From Initial To Late Stages Of Epitaxial Thin Film Growth: Stm Analysis And Atomistic Or Coarsegrained Modeling, James W. Evans, Yong Han, Barış Ünal, Maozhi Li, K. J. Caspersen, Dapeng Jing, Anthony R. Layson, C. R. Stoldt, T. Duguet, Patricia A. Thiel Aug 2010

From Initial To Late Stages Of Epitaxial Thin Film Growth: Stm Analysis And Atomistic Or Coarsegrained Modeling, James W. Evans, Yong Han, Barış Ünal, Maozhi Li, K. J. Caspersen, Dapeng Jing, Anthony R. Layson, C. R. Stoldt, T. Duguet, Patricia A. Thiel

Chemistry Conference Papers, Posters and Presentations

Epitaxial thin film growth by vapor deposition or molecular beam epitaxy under ultra‐high vacuum conditions generally occurs in two stages: (i) nucleation and growth of well‐separated islands on the substrate; (ii) subsequent formation of a thicker continuous film with possible kinetic roughening. For homoepitaxial growth, two‐dimensional (2D) monolayer islands are formed during submonolayer deposition. Typically, the presence of a step‐edge barrier inhibits downward transport and leads to the formation of mounds (multilayer stacks of 2D islands) during multilayer growth. For heteroepitaxial growth, islands formed in the initial stages of deposition sometimes have a 2D monolayer structure ...


Low-Temperature Adsorption Of H2s On Ag(111), Selena M. Russell, Da-Jiang Liu, Maki Kawai, Yousoo Kim, Patricia A. Thiel Jan 2010

Low-Temperature Adsorption Of H2s On Ag(111), Selena M. Russell, Da-Jiang Liu, Maki Kawai, Yousoo Kim, Patricia A. Thiel

Chemistry Publications

H2S forms a rich variety of structures on Ag(111) at low temperature and submonolayer coverage. The molecules decorate step edges, exist as isolated entities on terraces, and aggregate into clusters and islands, under various conditions. One type of island exhibits a (×)R25.3° unit cell. Typically, molecules in the clusters and islands are separated by about 0.4 nm, the same as the S–S separation in crystalline H2S. Density functional theory indicates that hydrogen-bonded clusters contain two types of molecules. One is very similar to an isolated adsorbed H2S molecule, with both ...


Modeling Of Island Formation During Submonolayer Deposition: A Stochastic Geometry-Based Simulation Approach, Maozhi Li, James W. Evans Jan 2005

Modeling Of Island Formation During Submonolayer Deposition: A Stochastic Geometry-Based Simulation Approach, Maozhi Li, James W. Evans

Physics and Astronomy Publications

Homogeneous nucleation and growth of two-dimensional islands during submonolayer deposition has been analyzed extensively by kinetic Monte Carlo (KMC) simulation of atomistic models and more recently by Burton--Cabrera--Frank-type continuum formulations for diffusion and aggregation of the deposited adatoms. Here, we develop an alternative geometry-based simulation (GBS) approach. This approach replaces an explicit treatment of adatom diffusion (either within an atomistic or continuum framework) with a formulation based on the stochastic geometry of "depletion zones" or "capture zones" surrounding islands. We consider models with a prescribed critical size, i, above which islands are stable. For canonical models with small i, we ...


From Atomic Scale Reactant Ordering To Mesoscale Reaction Front Propagation: Co Oxidation On Pd(100), Da-Jiang Liu, James W. Evans Nov 2004

From Atomic Scale Reactant Ordering To Mesoscale Reaction Front Propagation: Co Oxidation On Pd(100), Da-Jiang Liu, James W. Evans

Mathematics Publications

We utilize a heterogeneous coupled lattice-gas (HCLG) approach to connect the length scales from a realistic atomistic description of surface reactions to the associated mesoscale spatiotemporal behavior. This method is applied to describe reaction front structure in a model for CO oxidation on Pd(100) which incorporates complex ordering of CO and O adlayers, and a precise treatment of the chemical diffusion for interacting CO adlayers in an environment of coadsorbed O.


Fluctuations And Bistability In A "Hybrid" Atomistic Model For Co Oxidation On Nanofacets: An Effective Potential Analysis, Da-Jiang Liu, James W. Evans Oct 2002

Fluctuations And Bistability In A "Hybrid" Atomistic Model For Co Oxidation On Nanofacets: An Effective Potential Analysis, Da-Jiang Liu, James W. Evans

Mathematics Publications

An analysis on the existence of bistability in the hybrid reaction model for CO oxidation in finite systems is demonstrated. As such, numerical evidence to indicate the existence of an effective potential characterizing this behavior is provided. This potential is determined by exact kinetic Monte Carlo (KMC) simulation analysis, but reasonable estimate available from approximate master equations, and from Fokker-Planck equations which are obtained from further approximation of the master equations.


Sintering Of Two-Dimensional Nanoclusters In Metal(100) Homoepitaxial Systems: Deviations From Predictions Of Mullins Continuum Theory, Da-Jiang Liu, James W. Evans Oct 2002

Sintering Of Two-Dimensional Nanoclusters In Metal(100) Homoepitaxial Systems: Deviations From Predictions Of Mullins Continuum Theory, Da-Jiang Liu, James W. Evans

Mathematics Publications

We present a comparison of the predictions of atomistic and continuum models for the sintering of pairs of near-square two-dimensional nanoclusters adsorbed on the (100) surface in fcc metal homoepitaxial systems. Mass transport underlying these processes is dominated by periphery diffusion (PD) of adatoms along the edge of the clusters. A Mullins-type continuum model for cluster evolution incorporates anisotropy in the step edge stiffness (reflecting the energetics and adsorption site lattice structure in the atomistic model), and can also account for anisotropy in the step edge mobility (reflecting details of the kinetics). In such continuum treatments, the characteristic time τeqfor ...


Nuclear Magnetic Resonance Determination Of The Activation Volume For Self-Diffusion In Aluminum , Ralph Duane Engardt Jan 1970

Nuclear Magnetic Resonance Determination Of The Activation Volume For Self-Diffusion In Aluminum , Ralph Duane Engardt

Retrospective Theses and Dissertations

No abstract provided.


Nuclear Spin Relaxation Of Hydrogen In Thorium Hydrides , Jerald Duane Will Jan 1968

Nuclear Spin Relaxation Of Hydrogen In Thorium Hydrides , Jerald Duane Will

Retrospective Theses and Dissertations

No abstract provided.


A Monte Carlo Study Of Turbulent Diffusion , Dale William Kirmse Jan 1964

A Monte Carlo Study Of Turbulent Diffusion , Dale William Kirmse

Retrospective Theses and Dissertations

No abstract provided.


The Pressure Dependence Of Self-Diffusion In Lithium And Sodium , Roland Arthur Hultsch Jan 1961

The Pressure Dependence Of Self-Diffusion In Lithium And Sodium , Roland Arthur Hultsch

Retrospective Theses and Dissertations

No abstract provided.


Diffusion In Alpha Brass , Felix S. Palubinskas Jan 1952

Diffusion In Alpha Brass , Felix S. Palubinskas

Retrospective Theses and Dissertations

The present investigation was undertaken for the purpose of determining the diffusion coefficients of radioactive zinc in copper and alpha brass single crystals at selected high temperatures. To carry out this program the author designed and constructed various pieces of equipment for the operations required in the investigation;A Bridgman furnace was built for growing single crystals. Approximately half of the runs with this furnace were successful in producing large single crystals of alpha brass. In the remainder of the runs either bicrystals or very coarse-grained polycrystals were produced. (Abstract shortened by UMI.)