687 research outputs found
Local structure of liquid carbon controls diamond nucleation
Diamonds melt at temperatures above 4000 K. There are no measurements of the
steady-state rate of the reverse process: diamond nucleation from the melt,
because experiments are difficult at these extreme temperatures and pressures.
Using numerical simulations, we estimate the diamond nucleation rate and find
that it increases by many orders of magnitude when the pressure is increased at
constant supersaturation. The reason is that an increase in pressure changes
the local coordination of carbon atoms from three-fold to four-fold. It turns
out to be much easier to nucleate diamond in a four-fold coordinated liquid
than in a liquid with three-fold coordination, because in the latter case the
free-energy cost to create a diamond-liquid interface is higher. We speculate
that this mechanism for nucleation control is relevant for crystallization in
many network-forming liquids. On the basis of our calculations, we conclude
that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely
in gaseous planets
Homogeneous nucleation: Comparison between two theories
The classical nucleation theory of Becker, D\"{o}ring and Zeldovich is
compared with the Langer coarse-grained field approach to the nucleation
phenomenon. Both formalisms have been applied to the condensation from a
supersaturated vapor. It is shown that the nucleation rate derived in the
classical theory can be expressed in a form equivalent to that of the field
nucleation theory. This equivalence serves as an explanation of the puzzling
fact that the numerical predictions of both theories for condensation of Xe and
CO are almost identical though the standard analytical expressions for the
nucleation rates are different. The results obtained can help to link the
theories of nucleation and their approximations.Comment: 12 pages, LaTeX, no figure
Classical Nucleation Theory of the One-Component Plasma
We investigate the crystallization rate of a one-component plasma (OCP) in
the context of classical nucleation theory. From our derivation of the free
energy of an arbitrary distribution of solid clusters embedded in a liquid
phase, we derive the steady-state nucleation rate of an OCP as a function of
the Coulomb coupling parameter. Our result for the rate is in accord with
recent molecular dynamics simulations, but it is greater than that of previous
analytical estimates by many orders of magnitude. Further molecular dynamics
simulations of the nucleation rate of a supercooled liquid OCP for several
values of the coupling parameter would clarify the physics of this process.Comment: 6 pages, 1 figure, accepted by PR
Crystallization Mechanism of Hard Sphere Glasses
In supercooled liquids, vitrification generally suppresses crystallization.
Yet some glasses can still crystallize despite the arrest of diffusive motion.
This ill-understood process may limit the stability of glasses, but its
microscopic mechanism is not yet known. Here we present extensive computer
simulations addressing the crystallization of monodisperse hard-sphere glasses
at constant volume (as in a colloid experiment). Multiple crystalline patches
appear without particles having to diffuse more than one diameter. As these
patches grow, the mobility in neighbouring areas is enhanced, creating dynamic
heterogeneity with positive feedback. The future crystallization pattern cannot
be predicted from the coordinates alone: crystallization proceeds by a sequence
of stochastic micro-nucleation events, correlated in space by emergent dynamic
heterogeneity.Comment: 4 pages 4 figures Accepted for publication in Phys. Rev. Lett., April
201
Gestão escolar ética na gestão do processo educativo da escola classe morro do sansão da rede pública do Distrito Federal
A ética na escola e a formação do caráter do aluno têm sido discutidas em âmbito mundial, este trabalho trata o tema com base na importância e necessidade de uma política educacional de qualidade. O trabalho relata o resultado de uma pesquisa feita com os seguimentos da área educacional: corpo docente, pais e servidores da educação da Escola Classe Morro do Sansão,por meio de um questionário, onde ficou evidente a problemática do ensino público nos dias de hoje. Aborda o papel cumprido pela escola na formação ética/moral do cidadão, assim como as propostas relativas à ética presentes nos Parâmetros Curriculares Nacionais são discutidas. O presente trabalho busca resgatar um pouco da cidadania na escola de hoje, trazendo valores morais, éticos e humanos para o cotidiano do ensino atual
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