The Hubbard Dimer: A density functional case study of a many-body problem

This review explains the relationship between density functional theory and strongly correlated models using the simplest possible example, the two-site Hubbard model. The relationship to traditional quantum chemistry is included. Even in this elementary example, where the exact ground-state energy and site occupations can be found analytically, there is much to be explained in terms of the underlying logic and aims of Density Functional Theory. Although the usual solution is analytic, the density functional is given only implicitly. We overcome this difficulty using the Levy-Lieb construction to create a parametrization of the exact function with negligible errors. The symmetric case is most commonly studied, but we find a rich variation in behavior by including asymmetry, as strong correlation physics vies with charge-transfer effects. We explore the behavior of the gap and the many-body Green's function, demonstrating the `failure' of the Kohn-Sham method to reproduce the fundamental gap. We perform benchmark calculations of the occupation and components of the KS potentials, the correlation kinetic energies, and the adiabatic connection. We test several approximate functionals (restricted and unrestricted Hartree-Fock and Bethe Ansatz Local Density Approximation) to show their successes and limitations. We also discuss and illustrate the concept of the derivative discontinuity. Useful appendices include analytic expressions for Density Functional energy components, several limits of the exact functional (weak- and strong-coupling, symmetric and asymmetric), the Kohn-Sham hopping energy functional for 3 sites, various adiabatic connection results, proofs of exact conditions for this model, and the origin of the Hubbard model from a minimal basis model for stretched H$_2$

Caracterización del comportamiento retrógrado del sistema PMMA-CO2 mediante visualización directa

El objetivo principal de este trabajo es la caracterización del comportamiento de la temperatura de transición vítrea del sistema PMMA-CO2 a través de una ruta de caracterización alternativa, no utilizada hasta la fecha, que consiste en un estudio de carácter óptico denominado método óptico de visualización directa. Así mismo, se ha comprobado el carácter retrogrado de la temperatura de transición vítrea en este sistema. Por un lado, se ha realizado una investigación sobre la influencia de la variación de los parámetros de temperatura y presión sobre la Tg en tres sistemas PMMA-CO2. Para ello, se han empleado presiones inferiores a los 5.5 MPa, y se ha trabajado en un rango de temperaturas entre 25 y 110 oC. Por otro lado, se ha estudiado, a modo de comprobación de resultados, la transición entre ambas fases mediante la cantidad de gas, CO2, absorbida por muestras de PMMA a ciertas condiciones de presión y temperatura. Además, cada uno de los grados del material de partida, PMMA, tiene diferentes características, como el peso molecular, la temperatura de transición vítrea, la viscosidad, el índice de fluidez, etc. Se ha probado que dichas propiedades influyen y denotan la disposición y variación de la depleción de la temperatura de transición vítrea, proporcionando información para generar un modelo predictivo para este sistema.Grado en Físic

Estudio, evaluación y propuesta de mejora en los talleres de mecánica y carpintería en la Escuela de Educación Técnica Profesional y Secundaria N° 8013 San José de la Ciudad de Rosario, Provincia de Santa Fe, de acuerdo a las normativas vigentes

El presente trabajo propone evaluar el cumplimiento de la Ley de Higiene y Seguridad en el trabajo en el Taller de Mecánica y Carpintería en la Escuela de Educación Técnico Profesional y Secundaria N° 8013 San José de la ciudad de Rosario, sita en Presidente Roca 150, y sugerir medidas correctivas de las condiciones de trabajo mediante el análisis de riesgos y la medición de factores de contaminación ambientales existentes. Cabe destacar que si bien este trabajo fue basado en la Ley de Higiene y Seguridad en el trabajo N° 19.587 y sus decretos reglamentarios, se considera que los beneficiarios y destinatarios del mismo son todos los integrantes de la comunidad educativa, que tienen derecho a un ambiente seguro y saludable.Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensur

Optimization of Components of Superstructure of High-Speed Rail: The Spanish Experience

The performance of rail transport has increased significantly in recent decades, in particular due to the gradual introduction of high-speed rails worldwide. In 1981, the first high-speed line of the world was inaugurated; nowadays, high-speed is operating in more than 20 countries, the high-speed network covering more than 35,000 kms (with more than 25,000 additional kms under construction). Spain is the second country by total distance of railways installed (only behind China) and the first in terms relative to the population and surface. Since the installation of the first high-speed line in Spain in 1992, the elements of the superstructure have undergone a continuous evolution, in order to improve the performance, the durability of the components and the comfort of the passengers. This evolution rests on an adequate selection of materials based on the characterization of their physical and mechanical properties to ensure the optimum in-service conditions. This chapter includes an overview of the different elements present in the railway superstructure of the high-speed lines in Spain. Throughout the text, the innovations incorporated over time are analyzed, as well as the methods used to validate them. In particular, a description of the mechanical characterization procedures is presented

Unconventional magnetism in small gold organic molecules

We present a theoretical study of the magnetic properties of dicyclopentadienyl metallocene and phthalocyanine molecules, that contain the transition metal atoms M = Fe, Co, Ni, Cu, Zn, Ir, Pt and Au. Our most important prediction is that gold and copper molecules are magnetic. We find that the magnetism of these molecules is fairly unconventional: the gold atom itself is weakly magnetic or even non-magnetic. Its role is rather to induce magnetism in the surrounding carbon and nitrogen atoms, producing a sort of spin density wave.Comment: 4 pages, 5 figure

La falta de conocimiento de los emprendedores ocañeros para exportar sus productos

En la provincia de Ocaña existen emprendimientos con ideas de alto impacto en la región, contribuyendo a la creación de nuevos empleos y a la economía local, sus productos han permitido que la provincia se dé a conocer en Colombia, dando a que el empresario quiere llevar su mercancía aun mercado fuera del país, pero nace la duda de si cuentan con la información necesaria para cumplir con este objetivo, que en cuestiones de ingresos ayudaría a que la misma se sostenga no solo con las ventas del entorno rural y urbano. Por lo cual esta investigación se enfoca en la falta de conocimiento de los emprendedores Ocañeros para exportar sus productos, es por ello que el objetivo está basado en analizar la ausencia de un aprendizaje previo de los emprendedores Ocañeros para exportar sus productos por primera vez, así también esto por medio del cumplimiento de los objetivos específicos; Inspeccionar en la cámara de comercio acerca de cómo esta entidad repercute su asesoría en los emprendedores ocañeros de cómo deben exportar sus productos, determinar que emprendedores ocañeros están interesados en comercializar sus productos a otros países, indagar el apoyo o información que los emprendedores reciben de parte de la alcaldía de Ocaña y por ultimo diseñar un proceso de exportación básico para llegar a un mercado internacional. El enfoque fue de tipo cualitativo el cual es un método de investigación que alude a las cualidades y realizando un muestreo no probabilístico por conveniencia. Se concretó una entrevista con emprendedores ocañeros, cámara de comercio y alcaldía para conocer los resultados en cuanto al grado de conocimiento de los emprendedores de la región en procesos de exportación en la actualidad, concluyendo de esta actividad una ausencia de apoyo de las entidades públicas de Ocaña para con las empresas de la región

Mechanical behaviour of the cellulosic dielectric materials of windings in power transformers in operation

Power transformers are crucial elements in electrical systems, and the end of their useful life is commonly conditioned by the degradation of the cellulosic insulation materials inside them. These materials are subjected to elevated temperatures and mechanical stresses, generated by electrical solicitations which deform the copper conductors and subsequently the paper, and also to the chemical reactions which take place in the dielectric oil in which the paper is impregnated. In order to better understand the behaviour of cellulosic insulation, we have studied the previous bibliography analysing the mechanical behaviour of cellulosic materials. At present, there are no experimental results analysing how the paper responds to the deformation suffered by the copper conductor in a realistic situation. We have developed a simulation model describing the mechanical behaviour of a standardised copper conductor wrapped with four layers of dielectric paper, using ANSYS Workbench Static Structural, which will be compared with experimental results afterwards

Experimental dataset on the tensile and compressive mechanical properties of plain Kraft and crepe papers used as insulation in power transformers after ageing in mineral oil

The solid insulation in the windings of power transformers, which generally consists of oil-impregnated thin paper, is one of the key elements for the performance and durability of these electrical machines. Insulation paper is subjected to static and dynamic forces of electromagnetic origin, in combination with high temperatures and chemical reactions, during the operating life of a power transformer. The mechanical properties of the cellulosic insulation are relevant parameters because its breakage could result in the electric failure of the transformer. Indeed, paper manufacturers usually provide values of the tensile strength and elongation at breakage of the insulating paper in its two principal material directions, the MD (machine direction) and CD (cross-direction). However, paper is a highly anisotropic material and its material properties evolve as the paper insulation ages. The paper insulation in an operating transformer is subjected to a multiaxial stress state field including compressive and shear stresses. This article reports experimental data on the tensile and compressive mechanical properties of two types of paper, plain Kraft and crepe paper, typically used as insulation in power transformers, under different ageing states (which were induced through accelerated thermal ageing and quantified by means of the degree of polymerisation). These data could be reused for several purposes. They can improve the current understanding of the mechanical response and degradation processes of the cellulosic insulation in power transformers, and give some reference values that can be compared with others obtained in the factory by manufacturers. In the field of engineering failure analysis, those values could be reused for the assessment of mechanical failure of paper materials used in power transformers, see [1]

A modification of the Norris failure criterion for the prediction of the mechanical failure of the aged paper insulation in the windings of a power transformer

The deterioration of the insulation in the windings of power transformers affects their lifespan. A commercial insulated Continuously Transposed Conductor (CTC) was studied experimentally, numerically and analytically. The purpose was to understand the mechanisms governing the mechanical failure of the insulating paper, and to achieve a criterion for predicting failure under different conditions. Samples of that insulated CTC were extracted from a coil and aged at 150∘C for different durations inside vessels filled with naphthenic oil. Then the degree of polymerisation and tensile, compressive and shear mechanical properties of the insulation were measured/estimated. Aged insulated CTC samples were subjected to three-point bending tests, producing deformations compatible with a short circuit, and the fractures in the insulation were analysed. The bending test over a CTC sample was simulated by means of a FEM Program. The Norris failure model, applicable to a lamina, was adapted to the studied insulation materials. The predictions of that failure criterion agreed with experimental observations

Kinetic assessment of mechanical properties of a cellulose board aged in mineral oil and synthetic ester

In oil-immersed power transformers, the insulation system is constituted of a dielectric oil-solid combination. The insulation oil generally used is mineral oil; however, this fluid has started to be substituted by natural and synthetic esters due to their higher biodegradability and flash point. The introduction of a new fluid in the insulation system of power transformers requires kinetic models that can estimate the degradation rate of insulation solids. The aim of this work was to go further in quantifying through different kinetic models the deterioration suffered by a commercial cellulose board (PSP 3055), which is one of the solid materials used in the insulation system of oil-filled transformers. The aging study was extended to cellulose board specimens immersed in two different oils (mineral and synthetic ester). It was obtained that there is a lower degradation when synthetic ester is used in the insulation system. Additionally, it can be concluded that the use of mechanical properties to quantify the degradation of the cellulose board through kinetic models provides information about the different behavior shown by PSP 3055 when different fiber direction angles are considered.This research was funded by State Scientific and Technical Research and Innovation Plan under the PID2019-107126RB-C22/AEI/10.13039/501100011033 grant agreement, financed by the Government of Spai