Elaboración y caracterización de concreto polimérico: efecto del contenido de fibra de luffa y de la radiación gamma

A pesar de que el concreto convencional (elaborado con cemento Portland) es un material con gran versatilidad en cuanto a su funcionalidad y economía, presenta algunas limitaciones tales como: alta porosidad y baja resistencia a esfuerzos de flexión, tensión y a reactivos químicos altamente corrosivos. Algunas de estas limitaciones se han subsanado mediante la incorporación de fibras. Se han podido mejorar la resistencia a la tracción, compresión, durabilidad, resistencia a la fatiga, resistencia al impacto y a la abrasión, la contracción, la expansión, la estabilidad térmica, y resistencia al fuego. Sin embargo, muy pocas investigaciones ha abordado el efecto de fibras en concreto poliméricoEn el presente trabajo se estudian los efectos de la incorporación de fibras y de la radiación gamma en las propiedades mecánicas del concreto polimérico, en particular las resistencias a la compresión y a la flexión, la deformación mecánica y los módulos de elasticidad. Los concretos son elaborados con resina de poliéster insaturada, arena sílice y fibras de Luffa como agregados.CONACY

Fate of the spin-\frac{1}{2} Kondo effect in the presence of temperature gradients

We consider a strongly interacting quantum dot connected to two leads held at quite different temperatures. Our aim is to study the behavior of the Kondo effect in the presence of large thermal biases. We use three different approaches, namely, a perturbation formalism based on the Kondo Hamiltonian, a slave-boson mean-field theory for the Anderson model at large charging energies and a truncated equation-of-motion approach beyond the Hartree-Fock approximation. The two former formalisms yield a suppression of the Kondo peak for thermal gradients above the Kondo temperature, showing a remarkably good agreement despite their different ranges of validity. The third technique allows us to analyze the full density of states within a wide range of energies. Additionally, we have investigated the quantum transport properties (electric current and thermocurrent) beyond linear response. In the voltage-driven case, we reproduce the split differential conductance due to the presence of different electrochemical potentials. In the temperature-driven case, we observe a strongly nonlinear thermocurrent as a function of the applied thermal gradient. Depending on the parameters, we can find nontrivial zeros in the electric current for finite values of the temperature bias. Importantly, these thermocurrent zeros yield direct access to the system's characteristic energy scales (Kondo temperature and charging energy).Comment: 14 pages, 11 figures, revised versio

Maize Production and Agricultural Policies in Central America and Mexico

This paper reviews trends in maize production and consumption in Central America and Mexico in the context of the political and economic changes taking place in the region since the 1970s. The authors focus on the effects of the structural adjustment programs in the 1980s and 1990s. The analysis begins by reviewing the economic context in which maize production occurs in the region and the main economic policy instruments affecting the maize economy. Next, trends in maize consumption and production are analyzed, along with the main factors influencing maize production, including trends in the public financing of maize research and extension. The authors find that several factors related to structural adjustment have defined--and are still defining--the course of agriculture, including maize production, in the countries of the region. The impact of these factors on maize production, consumption, and import trends has been different in Central America and in Mexico. In particular, the reduction or complete elimination of production incentives, the reduction of trade barriers, the liberalization of input and product prices, the deregulation of the currency exchange rate, the control of inflation, and the restructuring of agricultural research systems between the public and the private sectors have determined how basic grains are produced in the region and how they will be produced in the future. Furthermore, the visible and increasing deterioration of the natural resource base has raised great concern about the need to promote more sustainable, environmentally friendly uses of production systems and natural resources.Agricultural and Food Policy, Crop Production/Industries,

Quasi-Exactly Solvable Potentials on the Line and Orthogonal Polynomials

In this paper we show that a quasi-exactly solvable (normalizable or periodic) one-dimensional Hamiltonian satisfying very mild conditions defines a family of weakly orthogonal polynomials which obey a three-term recursion relation. In particular, we prove that (normalizable) exactly-solvable one-dimensional systems are characterized by the fact that their associated polynomials satisfy a two-term recursion relation. We study the properties of the family of weakly orthogonal polynomials defined by an arbitrary one-dimensional quasi-exactly solvable Hamiltonian, showing in particular that its associated Stieltjes measure is supported on a finite set. From this we deduce that the corresponding moment problem is determined, and that the $k$-th moment grows like the $k$-th power of a constant as $k$ tends to infinity. We also show that the moments satisfy a constant coefficient linear difference equation, and that this property actually characterizes weakly orthogonal polynomial systems.Comment: 22 pages, plain TeX. Please typeset only the file orth.te