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Thermodynamique des agrégats bimétalliques : surface, volume et effet de taille finie

Abstract : The finite size of the nanoparticles confers specific characteristics, both in the surface and in the core. In this context, we studied the thermodynamics of bimetallic clusters using an Ising model by both Monte Carlo simulations and also the mean-field approximation. We considered the Cu-Ag system, a prototype of systems with a strong tendency to phase separate. For a cuboctahedron of thousands of atoms, Monte Carlo simulations show that the different facets of the surface undergo a dynamical equilibrium at a sufficiently low temperature. We can then predict, in the same cluster, the coexistence of copper-rich and silver-rich (001) facets, similar results are obtained for (111) facets, and even edges. Also a dynamical equilibrium is observed for the bulk layers, analogous of the phase separation for the infinite system. By studying the linear chain we determined the parameters that control the competition between the wetting (Ag enrichment from the surface that progressively reachs the core) and the installation of a dynamical equilibrium initiated in the core. The influence of the chain size on this competition was investigated, yielding the draft of the phase diagram of a finite structure that takes into account superficial segregation. This rigid-lattice study was later completed taking in account atomic relaxations, via a N-interatomic potential, for the icosahedron case. We illustrate the stress effects inherent in this structure, that induce strong heterogeneities of the intra - and inter-layer composition.
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Submitted on : Friday, March 6, 2009 - 7:01:41 PM
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Florence Lequien. Thermodynamique des agrégats bimétalliques : surface, volume et effet de taille finie. Physique [physics]. Université Paris Sud - Paris XI, 2008. Français. ⟨tel-00366428⟩

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