Seminario Dr. Aldo Craievich

[charo en unsl.edu.ar]

Se invita a la comunidad universitaria al Seminario de interés general que
brindará el Dr. Aldo F. Craievich, del Instituto de Física, Universidad de
San Pablo, Brasil (e-mail: craievich en if.usp.br). El Seminario se realizará
el día 7 de noviembre de 2008, a las 10:30 h, en el Anfiteatro 2

Título(Presentación en castellano):
"Formación, fusión y cristalización de nanopartículas metálicas
inmersas en una matriz vítrea"

Abstract
The processes of nucleation and growth of liquid Bi nanodroplets embedded
in a soda-borate glass - submitted to isothermal annealing at different
temperatures - were studied by in-situ small-angle x-ray scattering (SAXS)
[1]. Our experimental results indicate that the formation of Bi droplets
occurs in two successive stages. The first stage is characterized by the
nucleation and growth of spherical droplets promoted by the diffusion of
isolated Bi atoms through the glass matrix and the second stage by droplet
coarsening. The experimental functions describing the time variation of
the droplet average radius and density number, at advanced stages of the
growth process, agree with those predicted by the classical
Lifshitz-Slyozov-Wagner theory for particle coarsening. A combined use of
the SAXS and WAXS techniques allowed us to establish that the melting
temperature of Bi nanocrystals strongly decreases for decreasing radius R
and is a linear function of 1/R [2]. The freezing temperature of Bi
nanodroplets was determined to be lower than the melting temperature of
nanocrystals with the same radius (overcooling effect). The freezing
temperature also decreases linearly for increasing values of (1/R), but,
in this case, the slope is lower than that determined for the melting
temperature. Thus the magnitude of the overcooling progressively decreases
for decreasing radius and it vanishes for a critical radius Rc=1.9A [3].
The above mentioned dependences of the melting and freezing temperatures
could be explained by a simple model that assumes the nanoparticles being
composed of a crystalline core surrounded by a disordered shell. On the
other hand, the reduction of the volume of Bi nanocrystals across the
melting transition was determined from the temperature dependence of the
integral of the SAXS intensity. This volume reduction - derived from SAXS
results - is smaller than that a priori expected for homogeneous Bi
nanocrystals. This finding corroborates the model that suggests the
heterogeneous nature of the (crystalline-amorphous) Bi nanoparticles with
R>Rc.  Our experimental results also indicate that Bi nanoparticles with
R<Rc are fully amorphous.  Thus, for Bi nanoparticles with sub-critical
radii, neither liquid-to-crystal nor crystal-to-liquid transitions are
expected to occur.

G. Kellermann and A. F. Craievich  [1] Phys. Rev. B 67, 085405 (2003), [2]
Phys. Rev. B 67, 085405 (2002),  [3] Phys. Rev. B 78, 054106 (2008).


Dra. María del Rosario Torres Deluigi
Lab. de Microscopía Electrónica y Microanálisis
Bloque 1
Universidad Nacional de San Luis
Ejército de los Andes 950
Tel: +54(2652)424027 Int: 130
- D5700HHW - San Luis
Argentina