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Self-confinement of nanoparticles in the grain boundary network of a
colloidal polycrystal
Auteur(s): Tamborini E. , Ghofraniha N., Oberdisse J., Cipelletti L., Ramos L.
(Affiches/Poster)
UKCOLLOIDS2011 (Londres, GB), 2011-07-04
Résumé: Composite materials, comprising nanoparticles dispersed in a matrix, are of great interest, since
the nanoparticles can enhance the matrix properties or impart new functionalities and because
the matrix can act as a template that structures the particles at the nanoscopic level. However,
controlling the spatial distribution of nanoparticles remains a challenging task, as it usually
depends crucially on the particles surface chemistry. We present here a general approach to
confine nanoparticles in colloidal materials in a controlled fashion, independent of the details of
the surface chemistry. We use a nanocomposite material obtained by dispersing small quantities
of nanoparticles (at most 2%) in a colloidal crystalline matrix composed of thermosensitive
micelles. The volume fraction of the micelles increases with temperature T, until crystallization
occurs due to entropic reasons, as in hard sphere colloidal systems. Hence our system allows
crystallization to be induced at the desired rate simply by varying T. By analogy with atomic
crystals, we expect the nanoparticles, which act as impurities, to be partially expulsed from the
growing lattice and to segregate in the grain boundaries. We use small angle neutron scattering
to probe the microscopic structure of the composite materials. We show that nanoparticles
do not perturb the crystalline structure of the micelles and that different crystallization rates
lead to cubic crystals with the same lattice parameter (30 nm) but different nanoparticles
rearrangements. In particular, we find that slow crystallization rates induce the formation
of regions, up to about tenfold more concentrated in nanoparticles, without macroscopic
phase-separation; these regions are the grain boundaries between crystallites. Consistent and
complementary results are obtained by confocal microscopy, where the network of grain-
boundaries enriched in nanoparticles can be visualized.
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Texture and Growth of Colloidal Crystallites
Auteur(s): Ghofraniha N., Ramos L., Tamborini E., Cipelletti L., Oberdisse J.
(Séminaires)
Université Rennes 1 (Rennes, FR), 2010-10-29
Résumé: Most metals and ceramics are aggregates of crystalline grains.
Grain-boundaries (GBs) are two-dimensional lattice defects that separate the different grains and that control the bulk mechanical properties of polycrystalline materials.
In particular, the sliding and the migration of GBs play important roles in the plastic deformation. Moreover the plastic behaviour depends crucially on the size of the crystallites.
Colloidal polycrystals behave as most metals and ceramics at larger time and length scales, allowing to investigate their morphology and mechanical properties by direct 3-D visualization.
In this talk I will show how
the texture of a colloidal model polycrystalline system is tailored by adding controlled amounts of impurity and by
changing the speed of crystallization. The morphology
is analyzed quantitatively by confocal microscopy
and for the first time the kinetics of growth of the crystallites, the distribution and the role of the impurities during solidification is observed proving theoretical predictions not detectable in molecular systems.
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