Determinación espectroquímica de impurezas en plata

Con el fin de verificar el grado de pureza de la plata empleada principalmente para la fabricación de fusibles, se ha aplicado el análisis espectroquímico por su gran sensibilidad y rapidez. Debido a la gran variedad en el diámetro de los alambres y ante la imposibilidad de disponer de los patrones standard para cada medida, fue necesario disolver la muestra y realizar el análisis sobre las sales (nitratos). Las muestras standard se prepararon por el mismo procedimiento, empleando sustancias de pureza espectroscópica (Johnson Matthey). Esta técnica de análisis nos permite determinar once elementos (Cu, Cd, Pb, Ni, Sn, Al, Zn, Pt, Pd, Ir, Au). Se utilizo un espectrógrafo Z-3 (Jobin-Yvon), fuente de excitación G.M.60 (de Durr) y densitómetro 21-051 (Jarrell-Ash).Because of its great sensitivity and time-saving characteristics spectrochemical analysis has "been applied to the purity control of silver to he used mainly in fuse manufacture. As wires differ widely in their diameters and it is almost impossible to obtain standards por each size, it was necessary to disolve the sample and analize the resulting salts (nitrates). Standard samples were prepared making use of the procedure on spectroscopically pure materials (Johnson Matthey). This analytical technique allows the determination of eleven elements, i.e. Cu, Cd, Pb, Ni, Sn, Al, Zn, Pt, Pd, Ir, Au. Instrumentation used includes Jobin-Yvon Z-3 spectrograph, Durr G.M.60 excitation source and Jarrell-Ash,model 21-051 densitometer

Anisotropic intrinsic spin relaxation in graphene due to flexural distortions

We propose an intrinsic spin scattering mechanism in graphene originated by the interplay of atomic spin-orbit interaction and the local curvature induced by flexural distortions of the atomic lattice. Starting from a multiorbital tight-binding Hamiltonian with spin-orbit coupling considered non-perturbatively, we derive an effective Hamiltonian for the spin scattering of the Dirac electrons due to flexural distortions. We compute the spin lifetime due to both flexural phonons and ripples and we find values in the microsecond range at room temperature. Interestingly, this mechanism is anisotropic on two counts. First, the relaxation rate is different for off-plane and in-plane spin quantization axis. Second, the spin relaxation rate depends on the angle formed by the crystal momentum with the carbon-carbon bond. In addition, the spin lifetime is also valley dependent. The proposed mechanism sets an upper limit for spin lifetimes in graphene and will be relevant when samples of high quality can be fabricated free of extrinsic sources of spin relaxation.Comment: extended version with 7 pages, 4 figures and several new results; a numerical error has been corrected leading to longer spin lifetimes than in the previous versio

Aspectos de la no uniformidad de la irradiancia trasera en generadores fotovoltaicos bifaciales

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMEN: Desde hace algo más de un año se está monitorizando el funcionamiento de un generador fotovoltaico bifacial de 6.6 kWp conectado a la red. Durante este tiempo se ha analizado la irradiancia que el generador recibe en diferentes puntos de su parte trasera. Se ha observado que existen grandes diferencias en función de la posición de cada sensor, con valores de no uniformidad próximos al 50%. Con este estudio se pretende avanzar en el conocimiento del número mínimo de sensores que es recomendable instalar en la parte trasera de un generador y su ubicación más adecuada para que la medida de la irradiancia en esta parte posterior sea representativa y pueda ser correctamente utilizada en controles de calidad de sistemas fotovoltaicos bifaciales.ABSTRACT: Desde hace algo más de un año se está monitorizando el funcionamiento de un generador fotovoltaico bifacial de 6.6 kWp conectado a la red. Durante este tiempo se ha analizado la irradiancia que el generador recibe en diferentes puntos de su parte trasera. Se ha observado que existen grandes diferencias en función de la posición de cada sensor, con valores de no uniformidad próximos al 50%. Con este estudio se pretende avanzar en el conocimiento del número mínimo de sensores que es recomendable instalar en la parte trasera de un generador y su ubicación más adecuada para que la medida de la irradiancia en esta parte posterior sea representativa y pueda ser correctamente utilizada en controles de calidad de sistemas fotovoltaicos bifaciales.info:eu-repo/semantics/publishedVersio

On the constitution of polar fiber-reinforced materials

This article presents important constitutive refinements and simplifications in the theory of polar elasticity of materials reinforced by a single family of fibres resistant in bending. One of these simplifications is achieved by paying attention to forms of the strain energy which are symmetric with respect to the symmetric and the antisymmetric parts of the fibre gradient tensor. This leads to the identification of a restricted version of the theory that is predominantly influenced by the fibre-splay mode of deformation. The lack of ellipticity of the governing equations of polar elasticity and the anticipation of existence of weak discontinuity surfaces even in the small deformation regime are also investigated. The manner in which potential activation of such surfaces is related with the action of either the fibre-bending or the fibre-splay deformation mode, as well as with their conjoined combination and coupling with their fibre-twist counterpart, is examined. The proposed constitutive equations can be simplified via the use of a new set of fourteen independent spectral invariants of the deformation. This set serves as an irreducible functional basis of relevant invariants or as an irreducible integrity basis of polynomial invariants. For instance, its use here enables identification of fourteen classical invariants that emerge as mutually independent from the known set of thirty-three in total classical invariants. In the special case of polynomial invariants, this result paves the way for identification of a corresponding minimal integrity basis

Ageing of two 5kW PV arrays at the IES-UPM after 8 years of operation

This work analyses the degradation of two 5 kW PV arrays which are located on the roof of the Campus Sur of the Universidad Politécnica de Madrid. These systems were installed in March 2013 and they have been continuously monitored while they have been injecting power into the grid, storing at the same time its DC power and the operation conditions (effective irradiance and cell temperature). These variables allow to calculate their power at Standard Test Conditions over time and, consequently, the degradation rate for each PV array in these 8 years of operation. The linear degradations obtained are similar to the ones presented by other authors, but we have found that there is not apparent degradation in the first years of operation. Besides, it seems that shading could increase the degradation rates. These figures and phenomena can be useful for a better understanding of the actual behavior of PV systems in order to obtain more accurate energy predictions.This work has received funds from the European Union’s Horizon2020 research and innovation programme under Grant Agreement 787289. This work is part of the Project MADRID-PV2 (P2018/EMT-4308) funded by the Comunidad de Madrid with the support from FEDER Funds

On the characterisation of polar fibrous composites when fibres resist bending

This study aims to initiate research for the invention of methods appropriate for characterisation of fibre-reinforced materials that exhibit polar material behaviour due to fibre bending resistance. It thus focuses interest in the small strain regime, where there are examples of particular deformations for which non-polar linear elasticity fails to distinguish clearly the nature of a fibrous composite or even to account for the presence of fibres. Particular attention is accordingly given to the solution of the polar material version of the pure bending problem of transverse isotropic or special orthotropic plates with embedded fibres resistant in bending. It is seen that pure bending deformation enables polar fibre-reinforced materials to generate constant couple stress-field which, in turn, endorses uniqueness of the solution of the corresponding boundary value problem. In this context, by appropriately extending the validity of Clapeyron’s theorem within the regime of polar linear elasticity for fibre-reinforced materials, it is shown that the solution of well-posed linear elasticity boundary value problems that generate a constant couple-stress field is unique. The well-known uniqueness of solution of conventional, non-polar linear elasticity boundary value problems is, in fact, a particular case in which the generated constant value of the couple-stress field is zero

A unified approach for a posteriori high-order curved mesh generation using solid mechanics

The paper presents a unified approach for the a posteriori generation of arbitrary high-order curvilinear meshes via a solid mechanics analogy. The approach encompasses a variety of methodologies, ranging from the popular incremental linear elastic approach to very sophisticated non-linear elasticity. In addition, an intermediate consistent incrementally linearised approach is also presented and applied for the first time in this context. Utilising a consistent derivation from energy principles, a theoretical comparison of the various approaches is presented which enables a detailed discussion regarding the material characterisation (calibration) employed for the different solid mechanics formulations. Five independent quality measures are proposed and their relations with existing quality indicators, used in the context of a posteriori mesh generation, are discussed. Finally, a comprehensive range of numerical examples, both in two and three dimensions, including challenging geometries of interest to the solids, fluids and electromagnetics communities, are shown in order to illustrate and thoroughly compare the performance of the different methodologies. This comparison considers the influence of material parameters and number of load increments on the quality of the generated high-order mesh, overall computational cost and, crucially, the approximation properties of the resulting mesh when considering an isoparametric finite element formulation

A new restriction for initially stressed elastic solids

We introduce a fundamental restriction on the strain energy function and stress tensor for initially stressed elastic solids. The restriction applies to strain energy functions W that are explicit functions of the elastic deformation gradient F and initial stress τ⁠, that is W:=W(F,τ)⁠. The restriction is a consequence of energy conservation and ensures that the predicted stress and strain energy do not depend upon an arbitrary choice of reference configuration. We call this restriction initial stress reference independence (ISRI). It transpires that most strain energy functions found in the literature do not satisfy ISRI, and may therefore lead to unphysical behaviour, which we illustrate through a simple example. To remedy this shortcoming, we derive three strain energy functions that do satisfy the restriction. We also show that using initial strain (often from a virtual configuration) to model initial stress leads to strain energy functions that automatically satisfy ISRI. Finally, we reach the following important result: ISRI reduces the number of unknowns in the linear stress tensor for initially stressed solids. This new way of reducing the linear stress may open new pathways for the non-destructive determination of initial stresses through ultrasonic experiments, among others

Spin transport and spin torque in antiferromagnetic devices

Ferromagnets are key materials for sensing and memory applications. In contrast, antiferromagnets which represent the more common form of magnetically ordered materials, have found less practical application beyond their use for establishing reference magnetic orientations via exchange bias. This might change in the future due to the recent progress in materials research and discoveries of antiferromagnetic spintronic phenomena suitable for device applications. Experimental demonstration of the electrical switching and detection of the Néel order open a route towards memory devices based on antiferromagnets. Apart from the radiation and magnetic-field hardness, memory cells fabricated from antiferromagnets can be inherently multilevel, which could be used for neuromorphic computing. Switching speeds attainable in antiferromagnets far exceed those of ferromagnetic and semiconductor memory technologies. Here we review the recent progress in electronic spin-transport and spin-torque phenomena in antiferromagnets that are dominantly of the relativistic quantum mechanical origin. We discuss their utility in pure antiferromagnetic or hybrid ferromagnetic/antiferromagnetic memory devices