259 research outputs found
From wave function to crystal morphology: application to urea and alpha-glycine
In this paper the relation between the molecular electron density distribution and the crystal growth morphology is investigated. Accurate charge densities derived from ab initio quantum chemical calculations were partitioned into multipole moments, to calculate the electrostatic contribution to the intermolecular interaction energy. For urea and alpha-glycine the F-faces or connected nets were determined according to the Hartman-Perdok PBC theory. From attachment energy and critical Ising temperature calculations, theoretical growth forms were constructed using different atom-atom potential models. These were compared to the Donnay-Harker model, equilibrium form and experimental growth forms. In the case of alpha-glycine, the theoretical growth forms are in good agreement with crystals grown from aqueous solution. Crystals obtained by sublimation seem to show some faces which are not F-faces sensu stricto
Evaporation and growth of crystals - propagation of step density compression waves at vicinal surfaces
We studied the step dynamics during crystal sublimation and growth in the
limit of fast surface diffusion and slow kinetics of atom attachment-detachment
at the steps. For this limit we formulate a model free of the quasi-static
approximation in the calculation of the adatom concentration on the terraces at
the crystal surface. Such a model provides a relatively simple way to study the
linear stability of a step train in a presence of step-step repulsion and an
absence of destabilizing factors (as Schwoebel effect, surface electromigration
etc.). The central result is that a critical velocity of the steps in the train
exists which separates the stability and instability regimes. When the step
velocity exceeds its critical value the plot of these trajectories manifests
clear space and time periodicity (step density compression waves propagate on
the vicinal surface). This ordered motion of the steps is preceded by a
relatively short transition period of disordered step dynamics.Comment: 18 pages, 6 figure
Delineamento esquemático dos solos do Brasil.
Os dados sobre a natureza e distribuição geográfica dos solos do Brasil são ainda insuficientes para permitir um delineamento razoavelmente preciso, mesmo de um tipo esquemático ou generalizado como é o presente caso. A disponibilidade e detalhe de informações sobre os solos são heterogêneas e dispersas, sendo mesmo inexistentes para certas zonas do país. Com a finalidade de suprir essa deficiência, foram colecionadas informações e dados que permitiram, tentativamente, fazer um esboço dos solos do Brasil. Tal esboço e do tipo esquemático, foi desenvolvido por processos de compilação, generalização, extrapolação e dedução, conforme as correlações conhecidas para as condições do Brasil. Tal esboço esta constituído por associações amplas, formadas por número variável de diversas classes de solos. Da apreciação integrada combinando o ponto de vista pedológico e o geográfico resultou o estabelecimento de vinte associações gerais de solos; basicamente individualizadas pelos solos dominantes e tanto quanto possível, por solos exclusivos da associação. Para tornar mais objetiva a definição tentativa de cada uma das associações, foram descritas preliminarmente de modo sumário as classes de solos fundamentais componentes das mesmas. As denominações utilizadas para as diversas classes de solos foram basicamente referidas a sistema americano de classificação de solos.bitstream/item/212985/1/DPFS-BT-1-1966.pdf; bitstream/item/213810/1/Mapa-Delineamento-esquematico-dos-solos-do-Brasil-1966.pdfSeparata de: Pesquisa Agropecuária Brasileira, v. 1, p. 47-54, 1966. Acompanha 1 mapa, color. Escala 1:10.000.000
Equilibrium morphology of incommensurately modulated crystals: a superspace description
Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array
When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an efficient trigger
implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
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Impact of meteorological and environmental factors on the spatial distribution of Fasciola hepatica in beef cattle herds in Sweden
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