47 research outputs found
Computational study of uniaxial deformations in silica aerogel using a coarse-grained model
Simulations of a flexible coarse-grained model are used to study silica aerogels. This model, introduced in a previous study (J. Phys. Chem. C 2007, 111, 15792), consists of spherical particles which interact through weak nonbonded forces and strong interparticle bonds that may form and break during the simulations. Small-deformation simulations are used to determine the elastic moduli of a wide range of material models, and large-deformation simulations are used to probe structural evolution and plastic deformation. Uniaxial deformation at constant transverse pressure is simulated using two methods: a hybrid Monte Carlo approach combining molecular dynamics for the motion of individual particles and stochastic moves for transverse stress equilibration, and isothermal molecular dynamics simulations at fixed Poisson ratio. Reasonable agreement on elastic moduli is obtained except at very low densities. The model aerogels exhibit Poisson ratios between 0.17 and 0.24, with higher-density gels clustered around 0.20, and Young's moduli that vary with aerogel density according to a power-law dependence with an exponent near 3.0. These results are in agreement with reported experimental values. The models are shown to satisfy the expected homogeneous isotropic linear-elastic relationship between bulk and Young's moduli at higher densities, but there are systematic deviations at the lowest densities. Simulations of large compressive and tensile strains indicate that these materials display a ductile-to-brittle transition as the density is increased, and that the tensile strength varies with density according to a power law, with an exponent in reasonable agreement with experiment. Auxetic behavior is observed at large tensile strains in some models. Finally, at maximum tensile stress very few broken bonds are found in the materials, in accord with the theory that only a small fraction of the material structure is actually load-bearing
Lattice-gas Monte Carlo study of adsorption in pores
A lattice gas model of adsorption inside cylindrical pores is evaluated with
Monte Carlo simulations. The model incorporates two kinds of site: (a line of)
``axial'' sites and surrounding ``cylindrical shell'' sites, in ratio 1:7. The
adsorption isotherms are calculated in either the grand canonical or canonical
ensembles. At low temperature, there occur quasi-transitions that would be
genuine thermodynamic transitions in mean-field theory. Comparison between the
exact and mean-field theory results for the heat capacity and adsorption
isotherms are provided
Simple Model of Capillary Condensation in porous media
We employ a simple model to describe the phase behavior of 4He and Ar in a
hypothetical porous material consisting of a regular array of infinitely long,
solid, parallel cylinders. We find that high porosity geometries exhibit two
transitions: from vapor to film and from film to capillary condensed liquid. At
low porosity, the film is replaced by a ``necking'' configuration, and for a
range of intermediate porosity there are three transitions: from vapor to film,
from film to necking and from necking to a capillary condensed phase.Comment: 14 pages, 7 figure
Lattice model of gas condensation within nanopores
We explore the thermodynamic behavior of gases adsorbed within a nanopore.
The theoretical description employs a simple lattice gas model, with two
species of site, expected to describe various regimes of adsorption and
condensation behavior. The model includes four hypothetical phases: a
cylindrical shell phase (S), in which the sites close to the cylindrical wall
are occupied, an axial phase (A), in which sites along the cylinder's axis are
occupied, a full phase (F), in which all sites are occupied, and an empty phase
(E). We obtain exact results at T=0 for the phase behavior, which is a function
of the interactions present in any specific problem. We obtain the
corresponding results at finite T from mean field theory. Finally, we examine
the model's predicted phase behavior of some real gases adsorbed in nanopores
Voluntary disclosure of corporate strategy: determinants and outcomes. An empirical study into the risks and payoffs of communicating corporate strategy.
Business leaders increasingly face pressure from stakeholders to be transparent. There
appears however little consensus on the risks and payoffs of disclosing vital information
such as corporate strategy. To fill this gap, this study analyzes firm-specific determinants
and organisational outcomes of voluntary disclosure of corporate strategy. Stakeholder
theory and agency theory help to understand whether companies serve their interest to
engage with stakeholders and overcome information asymmetries. I connect these
theories and propose a comprehensive approach to measure voluntary disclosure of
corporate strategy. Hypotheses from the theoretical framework are empirically tested
through panel regression of data on identified determinants and outcomes and of
disclosed strategy through annual reports, corporate social responsibility reports,
corporate websites and corporate press releases by the 70 largest publicly listed
companies in the Netherlands from 2003 through 2008. I found that industry,
profitability, dual-listing status, national ranking status and listing age have significant
effects on voluntary disclosure of corporate strategy. No significant effects are found for
size, leverage and ownership concentration. On outcomes, I found that liquidity of stock
and corporate reputation are significantly influenced by voluntary disclosure of corporate
strategy. No significant effect is found for volatility of stock. My contributions to theory,
methodology and empirics offers a stepping-stone for further research into understanding
how companies can use transparency to manage stakeholder relations
Molecular Dynamics Simulations of the Polymerization of Aqueous Silicic Acid and Analysis of the Effects of Concentration on Silica Polymorph Distributions, Growth Mechanisms, and Reaction Kinetics
A Monte Carlo Simulation Study of Methane Clathrate Hydrates Confined in Slit-Shaped Pores
Monte Carlo simulations are used to study the structure,
stability,
and dissociation mechanisms of methane hydrate crystals inside carbon-like
slit-shaped pores. The simulation conditions used mimic experimental
studies of the dissociation of methane and propane hydrates in mesoporous
silica gels (Handa, Y. P.; Stupin, D. <i>J. Phys. Chem. </i><b>1992</b>, <i>96</i>, 8599). Simulations are performed
under conditions of fixed methane pressure and fixed water loading,
with the temperature increased in steps, with long equilibrations
at each temperature. The initial structures of the confined hydrates
are taken to be bulk-like, and pore widths chosen to accommodate integer
or half-integer numbers of hydrate unit cells. Density profiles and
orientational order parameter profiles are obtained and used to understand
the structural changes associated with hydrate dissociation. Three
different common water models, SPC/E, TIP4P, and TIP4P/2005, are used
and the results compared. For water modeled using either the TIP4P
or TIP4P/2005 potentials, dissociation temperatures are depressed
proportionally to the inverse pore width, as predicted by the macroscopic
Gibbs–Thomson equation. This behavior is observed for pores
small enough that only half-cages of the clathrate structure are present.
Experimental work has verified Gibbs–Thomson behavior for pores
as small as 2 nm (Seshadri, K.; Wilder, J. W.; Smith, D. H. <i>J. Phys. Chem. B</i> <b>2001</b>, <i>105</i>, 2627); micropores of the size studied here have not yet been studied
by experiment. Interestingly, the dissociation of hydrates modeled
using the SPC/E water potential does not display the predicted pore-size
dependence, and the dissociation mechanisms in this model seem to
be quite different than those in the TIP4P-type models. In the SPC/E
hydrates, with increasing temperature, cage dissocation occurs before
methane desorption. In TIP4P-type hydrates, these processes occur
either at the same temperature (to within the resolution of this study)
or with dissociation occurring at higher temperatures than desorption.
These simulations show that a variety of interesting clathrate structures
and phase behaviors may be accessed in suitably designed microporous
materials, with potentially useful applications in gas storage or
separations
Structural and Transport Properties of Tertiary Ammonium Triflate Ionic Liquids: A Molecular Dynamics Study
Ammonium-based protic
ionic liquids (PILs) have shown promising
physicochemical properties as proton conductors in polymer membrane
fuel cells. In this work, molecular dynamics simulations are used
to study the structural, dynamic, and transport properties of a series
of tertiary ammonium triflate PILs. Nonpolarizable all-atom force
fields were used, including two different models for the triflate
anion. Previous simulation studies of these PILs have yielded poor
results for transport properties due to unrealistically slow dynamics.
To improve performance, polarization and charge-transfer effects were
approximately accounted for by scaling all atomic partial charges
by a uniform factor, Îł. The optimum scaling factor Îł =
0.69 was determined by comparing simulation results with available
experimental data and found to be transferable between different ammonium
cations and insensitive to both the temperature and choice of experimental
data used for comparison. Simulations performed using optimized charge
scaling showed that the transport properties significantly improved
over previous studies. Both the self-diffusion coefficients and viscosity
were in good agreement with experiment over the whole range of systems
and temperatures studied. Simulated PIL densities were also in good
agreement with experiment, although the thermal expansivity was underestimated.
Structural analysis revealed a strong directionality in interionic
interactions. Triflate anions preferentially approach the ammonium
cation so as to form strong hydrogen bonds between sulfonate oxygen
atoms and the acidic ammonium hydrogen. The ionic conductivity was
somewhat underestimated, especially at lower temperatures. Analysis
of conductivity data suggests that there is significant correlated
motion of oppositely charged ions in these PILs, especially at short
times. These results overall indicate that the transport properties
of this class of PILs are accurately modeled by these force fields
if charge scaling is used and properly calibrated against selected
experimental data