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$_2$ 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