Fenología, producción y conservación post cosecha de diferentes cultivares de cebolla en el sudeste de Buenos Aires

p.73-80Los cultivos de cebolla en el S.E. de la Provincia de Buenos Aires se realizan casi exclusivamente con el cultivar Valcatorce INTA. Aunque se ha propuesto el cultivo de cebollas de ciclo más corto, prácticamente no existe información acerca del comportamiento de estos cultivares en la región. En este trabajo se evalúa la fenología, producción y comportamiento postcosecha de seis cultivares de diferente longitud de ciclo, en comparación con Valcatorce. De acuerdo a nuestros ensayos, el empleo de los genotipos de ciclo corto evaluados sólo sería factible para consumo inmediato, debido a su mala conservación. Por otra parte, Valcatorce presentó la mejor conservación y una producción aceptable, mientras que Armada se destacó como una interesante alternativa. Se evalúan, asimismo, diferentes parámetros como indicadores de la longevidad post-cosecha

Subcategorias reflexivas y correflexivas de la categoria de los espacios topologicos

Often working in certain areas of mathematics and its applications in other branches of science, fortified building objects from universal properties of given objects. These ideas motivate the definitions of coreflective and reflective subcategories, expressing notions improvement, optimization and density. In this article we review the theory of reflective and coreflective subcategories of the category of topological spaces and proposes new methods of construction of these categories in the category of topological spaces and in the category of topological spaces with base pointEs frecuente en el trabajo de ciertas áreas de las matemáticas, y en sus aplicaciones en otras ramas de la ciencia y la ingeniería, construir objetos enriquecidos con propiedades universales, a partir de objetos dados. Estas ideas motivan las definiciones de subcategorías reflexivas y correflexivas, nociones que expresan mejoramiento, optimización y densidad. En este artículo se hace una revisión de la teoría relacionada con las subcategorías reflexivas y correflexivas de la categoría de los espacios topológicos, y a este respecto se proponen nuevos métodos de construcción de dichas categorías en las categorías de los espacios topológicos y en la de los espacios topológicos punteados

Changes in the growth mode of electrodeposited silver layers : Surface rearrangements induced by the presence of lead adatoms

The surface characteristics of Ag electrodeposits prepared on polyfaceted Pt(sc) electrodes have been followed through complete Pb upd/stripping voltammetry. The growth mode of Ag overlayers at constant temperature depends on both the cathodic electrodeposition overvoltage and the silver ion concentration in solution. Ag surface rearrangements can be induced by either holding the potential or Pb upd/stripping cycling in the potential range where the degree of surface coverage by Pb adatoms is between 0 and 1. The voltammograms of Ag overlayers at the monolayer level show considerable changes compared with those obtained for thicker Ag overlayers. The present results allow us to establish a correlation between the development of rough electrodeposits and the surface diffusion properties of metal atoms.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Electrochemical kinetics and growth modes of silver deposits on polyfaceted platinum spherical electrodes

The Ag electrodeposition on Pt spherical substrates was investigated over a wide range of experimental conditions to establish a correlation between the kinetics of the process and the different growth modes of the Ag overlayer. At Ed ≊ Erev, the electrodeposition of the first Ag layer obeys a combined adsorption-desorption and nucleation and 2-D growth process under diffusion control. At Ed ≊ Erev, the formation of the 2-D Ag layer proceeds through an adsorption mechanism, whereas the formation of the 3-D Ag layer fits a progressive nucleation and 3-D growth mechanism under diffusion control. Finally, when Ed rev, a potential threshold related to Ag dendritic growth, the kinetics of the reaction apparently obeys a progressive nucleation and 1-D growth. This process is triggered at edges and corners of large Ag crystals. Dendritic growth takes place outside the diffusion layer defined around the Pt substrate sphere. In this case migration plays a substantial role in the process.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Kinetics and growth modes of quasi-2d silver branched electrodeposits produced in the presence of a supporting electrolyte

Quasi-2d silver electrodeposits were grown electrochemically at constant potential from aqueous Ag+ ion-containing solutions in the presence of a supporting electrolyte, at room temperature, using a three-electrode quasi-2d circular electrochemical cell. Open branching and dense radial branching patterns were distinguished on the centimetre scale, and growth mode transitions could be observed during the process. Branching patterns exhibited a mass fractal behaviour with a mass fractal dimension increasing from that expected for a DLA-like pattern to that of a dense branching pattern as either the cathodic overpotential (ηc) or the Ag+ ion concentration in the solution (c) was increased. The electrodeposition current increased with time exhibiting different regimes depending on whether an open branching or a dense radial branching growth mode prevailed. When the electrodeposition time exceeded a certain critical value, the radial growth rate of electrodeposits (vr) approached a vr α ηcc relationship. The experimental morphologies and growth kinetics were reproduced by Monte Carlo simulations of a growth model in which depositing particles follow a biased random walk.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Growth mode transition involving a potential-dependent isotropic to anisotropic surface atom diffusion change. Gold electrodeposition on HOPG followed by STM

The electrodeposition of gold on highly oriented pyrolytic graphite (HOPG) from acid aqueous solutions was studied by using electrochemical techniques complemented with ex-situ scanning tunneling microscopy (STM). The kinetics of gold electrodeposition is consistent with a nucleation and three-dimensional growth process under diffusion control from the solution side. As the applied potential moves in the negative direction, the gold crystal density increases, and the crystal shape changes from a Euclidean to a dendritic fractal morphology. This transition can be assigned to the anisotropic surface diffusion of gold adatoms induced by the applied electric potential. A model including a potential-dependent energy barrier at step edges accounts for the morphology transition for gold electrodeposition on HOPG.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Electrochemical formation of palladium islands on HOPG: Kinetics, morphology, and growth mechanisms

Scanning tunneling microscopy (STM) and conventional electrochemical techniques were utilized to investigate the growth kinetics and mechanism of palladium island electroformation on highly oriented pyrolitic graphite (HOPG) from aqueous acid palladium chloride solutions at 298 K. Initially, the electrodeposition reaction at low cathodic overpotentials involves an activation process in which a PdCl2 surface intermediate is formed. At intermediate overpotentials, the growth of palladium islands involves a progressive nucleation and growth model under diffusion control, whereas at high overpotentials, the bulk discharge of soluble palladium species undergoes a free convective-diffusion process. As the cathodic overpotential is shifted negatively, the aspect ratio of the islands, defined as the ratio of the maximum height of the island to the island radius, and the island size decrease, whereas the island density increases. As the cathodic overpotential becomes a few millivolts more positive than the threshold potential of the hydrogen evolution reaction, the island shape changes from a compact disk to a quasi-2D dense radial Pd(111) island. The formation of a dense radial morphology and its small departure from a perfect 2D pattern indicates the presence of weak step-edge energy barriers, as expected from theoretical calculations for Pd(111). These results stress the key role of step-edge energy barriers in determining growth patterns during metal electrodeposition.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

Negative pressures in CaWO4 nanocrystals

Tetragonal scheelite-type CaWO4 nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO4 nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO4 nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predictions from our ab initio calculations. CaWO4 nanocrystals exhibit a reconstructive-type mechanism for the scheelite-to-zircon phase transition that seems to follow the tetragonal path that links both structures. This result is in contrast with the mechanism recently proposed for this transition in bulk ZrSiO4 where the transition goes through an intermediate monoclinic [email protected]

Growth of three-dimensional silver fractal electrodeposits under damped free convection

Quasispherical three-dimensional Ag electrodeposits grown at constant potential and damped free convection exhibit a quasi-isotropic radial growth up to a critical radius rc≃0.3 cm and a mass fractal dimension DM=2.5. For r≫rc, DM→3. The product vrη’ is nearly constant, vr being the radial growth velocity, and η’ the apparent medium viscosity. It appears that vr is determined by the Laplacian field operating between cathode and anode.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada