30,384 research outputs found
Small Angle Neutron Scattering of Aerogels: Simulations and Experiments
A numerical simulation of silica aerogels is performed using
diffusion-limited cluster-cluster aggregation of spheres inside a cubic box
(with periodic boundary conditions). The volume fraction is taken to be
sufficiently large to get a gel structure at the end of the process. In the
case of monodisperse spheres, the wavevector dependent scattered intensity
is calculated from the product of the form factor of a sphere by
the structure factor , which is related to the Fourier transform of
, where is the pair correlation function between sphere centers.
The structure factor exhibits large- damped oscillations
characteristics of the short range (intra-aggregate) correlations between
spheres. These oscillations influence the curve in the -region
between the fractal regime and the Porod regime and quantitative comparisons
are made with experiments on colloidal aerogels. Moreover, at small- values,
goes through a maximum characteristic of large range (inter-aggregate)
correlations. Quantitative fits of the maximum in the experimental
curves of base-catalyzed aerogel are presented. In the case of polydisperse
spheres, is calculated directly from a single aggregate simulation. It
is shown that increasing polydispersity shifts the location of the cross-over
between the fractal and Porod regimes towards low -value.Comment: RevTex, 9 pages + 11 postscript figures, compressed using "uufiles".
Proceeding of the 4th International Simposium on Aerogels (To appear in J. of
Non-Cryst. Solids
Irradiation-induced Ag nanocluster nucleation in silicate glasses: analogy with photography
The synthesis of Ag nanoclusters in sodalime silicate glasses and silica was
studied by optical absorption (OA) and electron spin resonance (ESR)
experiments under both low (gamma-ray) and high (MeV ion) deposited energy
density irradiation conditions. Both types of irradiation create electrons and
holes whose density and thermal evolution - notably via their interaction with
defects - are shown to determine the clustering and growth rates of Ag
nanocrystals. We thus establish the influence of redox interactions of defects
and silver (poly)ions. The mechanisms are similar to the latent image formation
in photography: irradiation-induced photoelectrons are trapped within the glass
matrix, notably on dissolved noble metal ions and defects, which are thus
neutralized (reverse oxidation reactions are also shown to exist). Annealing
promotes metal atom diffusion, which in turn leads to cluster nuclei formation.
The cluster density depends not only on the irradiation fluence, but also - and
primarily - on the density of deposited energy and the redox properties of the
glass. Ion irradiation (i.e., large deposited energy density) is far more
effective in cluster formation, despite its lower neutralization efficiency
(from Ag+ to Ag0) as compared to gamma photon irradiation.Comment: 48 pages, 18 figures, revised version publ. in Phys. Rev. B, pdf fil
The Sol-Gel Process Simulated by Cluster-Cluster Aggregation
The pair-correlation function and its Fourier transform, the
structure factor , are computed during the gelation process of
identical spherical particles using the diffusion-limited cluster-cluster
aggregation model in a box. This numerical analysis shows that the time
evolution of the characteristic cluster size exhibits a crossover close
to the gel time which depends on the volumic fraction . In this model
tends to infinity when the box size tends to infinity. For systems of
finite size, it is shown numerically that, when , the wave vector ,
at which has a maximum, decreases as , where is
an apparent fractal dimension of clusters, as measured from the slo pe of
. The time evolution of the mean number of particles per cluster is also investigated. Our numerical results are in qualitative agreement
with small angle scattering experiments in several systems.Comment: RevTex, 13 pages + 9 postscript figures appended using "uufiles". To
appear in J. of Non-Cryst. Solid
Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime
Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean and a standard deviation . The wave vector dependent
scattered intensity is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given , the location of the
crossover decreases as increases. The dependence of on
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994
Exploiting limited valence patchy particles to understand autocatalytic kinetics
Autocatalysis, i.e., the speeding up of a reaction through the very same molecule which is produced, is common in chemistry, biophysics, and material science. Rate-equation-based approaches are often used to model the time dependence of products, but the key physical mechanisms behind the reaction cannot be properly recognized. Here, we develop a patchy particle model inspired by a bicomponent reactive mixture and endowed with adjustable autocatalytic ability. Such a coarse-grained model captures all general features of an autocatalytic aggregation process that takes place under controlled and realistic conditions, including crowded environments. Simulation reveals that a full understanding of the kinetics involves an unexpected effect that eludes the chemistry of the reaction, and which is crucially related to the presence of an activation barrier. The resulting analytical description can be exported to real systems, as confirmed by experimental data on epoxy-amine polymerizations, solving a long-standing issue in their mechanistic description
Evolution of Nanoporosity in Dealloying
Dealloying is a common corrosion process during which an alloy is "parted" by
the selective dissolution of the electrochemically more active elements. This
process results in the formation of a nanoporous sponge composed almost
entirely of the more noble alloy constituents . Even though this morphology
evolution problem has attracted considerable attention, the physics responsible
for porosity evolution have remained a mystery . Here we show by experiment,
lattice computer simulation, and a continuum model, that nanoporosity is due to
an intrinsic dynamical pattern formation process - pores form because the more
noble atoms are chemically driven to aggregate into two-dimensional clusters
via a spinodal decomposition process at the solid-electrolyte interface. At the
same time, the surface area continuously increases due to etching. Together,
these processes evolve a characteristic length scale predicted by our continuum
model. The applications potential of nanoporous metals is enormous. For
instance, the high surface area of nanoporous gold made by dealloying Ag-Au can
be chemically tailored, making it suitable for sensor applications,
particularly in biomaterials contexts.Comment: 13 pages, PDF, incl. 4 figures. avi movies of simulations available
at http://www.deas.harvard.edu/matsci/downdata/downdata.htm
Emergence of fractal behavior in condensation-driven aggregation
We investigate a model in which an ensemble of chemically identical Brownian
particles are continuously growing by condensation and at the same time undergo
irreversible aggregation whenever two particles come into contact upon
collision. We solved the model exactly by using scaling theory for the case
whereby a particle, say of size , grows by an amount over the
time it takes to collide with another particle of any size. It is shown that
the particle size spectra of such system exhibit transition to dynamic scaling
accompanied by the emergence of fractal of
dimension . One of the remarkable feature of this
model is that it is governed by a non-trivial conservation law, namely, the
moment of is time invariant regardless of the choice of the
initial conditions. The reason why it remains conserved is explained by using a
simple dimensional analysis. We show that the scaling exponents and
are locked with the fractal dimension via a generalized scaling relation
.Comment: 8 pages, 6 figures, to appear in Phys. Rev.
Experimental evidence on the development of scale invariance in the internal structure of self-affine aggregates
It is shown that an alternative approach for the characterization of growing
branched patterns consists of the statistical analysis of frozen structures,
which cannot be modified by further growth, that arise due to competitive
processes among neighbor growing structures. Scaling relationships applied to
these structures provide a method to evaluate relevant exponents and to
characterize growing systems into universality classes. The analysis is applied
to quasi-two-dimensional electrochemically formed silver branched patterns
showing that the size distribution of frozen structures exhibits scale
invariance. The measured exponents, within the error bars, remind us those
predicted by the Kardar-Parisi-Zhang equation.Comment: 11 pages, 4 figure
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