772 research outputs found
R. Brousse, C. Lefèvre, Le volcanisme en France
No full textMottet Gérard. R. Brousse, C. Lefèvre, Le volcanisme en France . In: Revue de géographie de Lyon, vol. 66, n°3-4, 1991. A propos de la séquence neigeuse de décembre 1990 : Une approche des évènements externes. p. 222
R. Brousse, C. Lefèvre, Le volcanisme en France
No full textMottet Gérard. R. Brousse, C. Lefèvre, Le volcanisme en France . In: Revue de géographie de Lyon, vol. 66, n°3-4, 1991. A propos de la séquence neigeuse de décembre 1990 : Une approche des évènements externes. p. 222
Freezing of silver nanodroplets
No full textThe freezing of silver nanodroplets is studied by molecular dynamics simulations on time scales which mimic typical experimental situations. At small sizes (2-3 nm) both crystalline and non-crystalline structures (icosahedra and decahedra) are observed: while at large sizes, also around magic icosahedral numbers, we show that silver droplets solidify preferentially as non-icosahedral clusters. We compare two different ways of growing clusters: the solid-state growth from a very small seed and the freezing of a droplet. We find that the final shape of clusters depends on the growth process, and that the solid-state growth gives results in better agreement with the experiments. (C) 2002 Elsevier Science B.V. All rights reserve
Adsorption and diffusion on nanoclusters of C-60 molecules
No full textThe adsorption and the energy barriers for diffusion on clusters of C60 molecules are studied by means of quenched molecular dynamics simulations. The interaction among the C60 molecules is modeled by means of the Girifalco potential. Three different clusters structures are considered: a truncated octahedron of 38 molecules, an icosahedron of 55 molecules and a truncated decahedron of 75 molecules. © 2003 Elsevier Science B.V. All rights reserved
A Dichotomy for First-Order Reducts of Unary Structures
No full textMany natural decision problems can be formulated as constraint satisfaction
problems for reducts of finitely bounded homogeneous structures.
This class of problems is a large generalisation of the class of CSPs over
finite domains. Our first result is a general polynomial-time reduction from
such infinite-domain CSPs to finite-domain CSPs. We use this reduction to
obtain new powerful polynomial-time tractability conditions that can be
expressed in terms of the topological polymorphism clone of .
Moreover, we study the subclass of CSPs for structures
that are reducts of a structure with a unary language. Also this
class properly extends the class of all finite-domain CSPs. We
apply our new tractability conditions to prove the general tractability
conjecture of Bodirsky and Pinsker for reducts of finitely bounded homogeneous
structures for the class
Growth simulations of silver shells on copper and palladium nanoclusters
No full textThe growth of Ag-Pd and Ag-Cu core-shell nanoclusters is studied by molecular-dynamics simulations on realistic time scales. The metals are modeled by many-body potentials derived in the framework of the second-moment approximation to the tight-binding model. First, the energy barriers of the most relevant diffusion processes of Ag adatoms on Pd and Cu cores are calculated, and then growth simulations are performed for several values of the deposition flux and of the temperature. We find that well-defined Ag shells on both Pd and Cu cores are grown. In particular, single-layer shells with a few defects can be obtained in a wide range of temperatures. The main difference between the Ag-Pd and the Ag-Cu systems is that in the former better structures are obtained at low temperatures, while in the latter, the best shells are grown at high temperatures. These behaviors are explained in terms of the ability (inability) to incorporate Ag adatoms into the Pd or Cu core
Time evolution of Ag-Cu and Ag-Pd core-shell nanoclusters
No full textThe possibility of obtaining core-shell nanoparticles by depositing
adatoms of a different element B above a cluster of an element A (system B/A)
is studied
by Molecular Dynamics simulations. We consider the four cases Ag/Cu, Cu/Ag,
Ag/Pd and Pd/Ag, which present very different behaviours, and investigate
whether it is possible to build up of well-defined core-shell structures
Growth of three-shell onionlike bimetallic nanoparticles
No full textWe show by molecular dynamics simulations on three systems (B/A = Pd/Ag, Cu/Ag, and Ni/Ag) that three-shell metallic nanoparticles made by a core of a metal A, an intermediate shell of metal B and an external shell of metal A (A-B-A nanoparticles) can be grown by deposition of B atoms onto an A core. The growth of the intermediate B shell is triggered by the fact that the most favorable positions for isolated B impurities inside A clusters are located just one layer below the cluster surfac
Molecular dynamics simulations of surface diffusion and growth on silver and gold clusters
No full textWe report a systematic study of the diffusion of single adatoms and of the growth of fee silver and gold clusters (Wulff polyhedra) by molecular dynamics simulations. Both metals have been modelled by many-body right-binding potentials. The energy barriers for adatom diffusion on the cluster facets are calculated by the nudged elastic band method. Concerning single-adatom diffusion, we have studied the mechanisms connecting different facets [(111) and (100)] that play the main role during growth. We have found that the diffusion among different facets takes place by exchange processes in both metals; however, we have found that the mobility from a (111) to a (100) facet is much easier in gold than in silver. This has important consequences for the growth modes of the clusters. In fact, our growth simulations have shown that the transition from the Wulff shape to the octahedron is possible with gold even at low temperatures and quite fast deposition rates, whereas for silver much higher temperatures are required
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