22 research outputs found
Scaling Theory and Numerical Simulations of Aerogel Sintering
A simple scaling theory for the sintering of fractal aerogels is presented.
The densification at small scales is described by an increase of the lower
cut-off length accompanied by a decrease of the upper cut-off length ,
in order to conserve the total mass of the system. Scaling laws are derived
which predict how , and the specific pore surface area should
depend on the density . Following the general ideas of the theory,
numerical simulations of sintering are proposed starting from computer
simulations of aerogel structure based on a diffusion-limited cluster-cluster
aggregation gelling process. The numerical results for , and
as a function of are discussed according to the initial aerogel density.
The scaling theory is only fully recovered in the limit of very low density
where the original values of and are well separated. These numerical
results are compared with experiments on partially densified aerogels.Comment: RevTex, 17 pages + 6 postscript figures appended using "uufiles". To
appear in J. of Non-Cryst. Solid
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
Fluctuating Bond Aggregation: a Model for Chemical Gel Formation
The Diffusion-Limited Cluster-Cluster Aggregation (DLCA) model is modified by
including cluster deformations using the {\it bond fluctuation} algorithm. From
3 computer simulations, it is shown that, below a given threshold value
of the volumic fraction , the realization of all intra-aggregate
bonding possibilities prevents the formation of a gelling network. For ,
the sol-gel transition occurs at a time which, in contrast to DLCA,
doesnot diverge with the box size. Several results are reported including small
angle scattering curves and possible applications are discussed.Comment: RevTex, 9 pages + 3 postscript figures appended using "uufiles". To
appear in Phys. Rev. Let
Knowledge socialization: strategies in the field of nanoscience and nanotechnology in venezuela
Since 2009, the Venezuelan Network of Nanotechnology has developed some activities for the dissemination of this knowledge area, and also promoted joint working initiatives between members interested in advancing in such task. This paper presents an overview of the activities of the network and other groups related to the subject, and also present the network strategies for the future which arises in the context of a broad notion of the meaning of socialization of knowledge as the axis of transformation of society.Desde el año 2009, la Red Venezolana de Nanotecnología ha desarrollado diversas actividades para la formación, divulgación y popularización de esta área del conocimiento, fomentado además el trabajo conjunto entre los miembros interesados en adelantar iniciativas en este ámbito. Este trabajo presenta un balance de las actividades realizadas por la red y otros grupos relacionados con el tema, así como también se propone analizar las estrategias que la red se plantea a futuro en el marco de una noción amplia de lo que significa la socialización del conocimiento como eje de transformación de la sociedad
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
A Numerical Study of Phase Transitions Inside the Pores of Aerogels
Phase transitions inside the pores of an aerogel are investigated by
modelizing the aerogel structure by diffusion-limited cluster-cluster
aggregation on a cubic lattice in a finite box and considering -states Potts
variables on the empty sites interacting via nearest-neighbours. Using a finite
size scaling analysing of Monte-Carlo numerical results, it is concluded that
for the transition changes from first order to second order as the
aerogel concentration (density) increases. Comparison is made with the case
(where the first order transition is weaker in three dimensions) and with
the case but for randomly (non correlated) occupied sites. Possible
applications to experiments are discussed.Comment: RevTex, 12 pages + 10 postscript figures compressed using "uufiles",
To appear in J. of Non-Cryst. Solid