Experiments with liquid bridges in simulated microgravity

A feature of stability diagrams of liquid bridges between unequal disks subjected to small axial gravity forces is that, for each separation of disks, there is a value of microgravity for which an absolute minimum volume limit is reached. The dependence of such microgravity values on the liquid bridge geometry has been experimentally checked by using the neutral buoyancy technique, experimental results being in complete agreement with theoretical ones. Analytical background assuring the experimental procedure used is presented, and a second order analytical expression for the equilirium interface is also calculated.

Axisymmetric long liquid bridges stability and resonances

In this paper mathematical expressions for minimum-volume stability limits and resonance frequencies of axisymmetric long liquid bridges are presented. These expressions are valid for a wide range of liquid bridge configurations, accounting for ef-fects like unequal disks and axial microgravity in the case of minimum-volume stability limits,and unequal disks, axial microgravity,non-zero viscosity and liquid bridge volume different from the cylindrical one in the case of resonance frequencies.

Advances on MBE selective area growth of III-nitride nanostructures: from nanoLEDs to pseudo substrates

The aim of this work is to provide an overview on the recent advances in the selective area growth (SAG) of (In)GaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs. The first three sections deal with the basic growth mechanisms of GaN SAG and the emission control in the entire ultraviolet to infrared range, including approaches for white light emission, using InGaN disks and thick segments on axial nanocolumns. SAG of axial nanostructures is eveloped on both GaN/sapphire templates and GaN-buffered Si(111). As an alternative to axial nanocolumns, section 4 reports on the growth and characterization of InGaN/GaN core-shell structures on an ordered array of top-down patterned GaN microrods. Finally, section 5 reports on the SAG of GaN, with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the templates is strongly reduced as indicated by a dramatic improvement of the optical properties. In the case of SAG on nonpolar (11-22) templates, the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of these nanostructures.

Measurement of the intrinsic damping constant in individual nanodisks of Y3Fe5O12 and Y3Fe5O12|Pt

We report on an experimental study on the spin-waves relaxation rate in two series of nanodisks of diameter ϕ=300 , 500, and 700 nm, patterned out of two systems: a 20 nm thick yttrium iron garnet (YIG) film grown by pulsed laser deposition either bare or covered by 13 nm of Pt. Using a magnetic resonance force microscope, we measure precisely the ferromagnetic resonance linewidth of each individual YIG and YIG|Pt nanodisks. We find that the linewidth in the nanostructure is sensibly smaller than the one measured in the extended film. Analysis of the frequency dependence of the spectral linewidth indicates that the improvement is principally due to the suppression of the inhomogeneous part of the broadening due to geometrical confinement, suggesting that only the homogeneous broadening contributes to the linewidth of the nanostructure. For the bare YIG nano-disks, the broadening is associated to a damping constant α=4 × 10−4 . A threefold increase of the linewidth is observed for the series with Pt cap layer, attributed to the spin pumping effect. The measured enhancement allows to extract the spin mixing conductance found to be G↑↓=1.55 × 1014 Ω−1 m−2 for our YIG(20nm)|Pt interface, thus opening large opportunities for the design of YIG based nanostructures with optimized magnetic losses.

Self assembled and ordered group III nitride nanocolumnar structures for light emitting applications

El objetivo de este trabajo es un estudio profundo del crecimiento selectivo de nanoestructuras de InGaN por epitaxia de haces moleculares asistido por plasma, concentrandose en el potencial de estas estructuras como bloques constituyentes en LEDs de nueva generación. Varias aproximaciones al problema son discutidas; desde estructuras axiales InGaN/GaN, a estructuras core-shell, o nanoestructuras crecidas en sustratos con orientaciones menos convencionales (semi polar y no polar). La primera sección revisa los aspectos básicos del crecimiento auto-ensamblado de nanocolumnas de GaN en sustratos de Si(111). Su morfología y propiedades ópticas son comparadas con las de capas compactas de GaN sobre Si(111). En el caso de las columnas auto-ensambladas de InGaN sobre Si(111), se presentan resultados sobre el efecto de la temperatura de crecimiento en la incorporación de In. Por último, se discute la inclusión de nanodiscos de InGaN en las nanocolumnas de GaN. La segunda sección revisa los mecanismos básicos del crecimiento ordenado de nanoestructuras basadas en GaN, sobre templates de GaN/zafiro. Aumentando la relación III/V localmente, se observan cambios morfológicos; desde islas piramidales, a nanocolumnas de GaN terminadas en planos semipolares, y finalmente, a nanocolumnas finalizadas en planos c polares. Al crecer nanodiscos de InGaN insertados en las nanocolumnas de GaN, las diferentes morfologias mencionadas dan lugar a diferentes propiedades ópticas de los nanodiscos, debido al diferente carácter (semi polar o polar) de los planos cristalinos involucrados. La tercera sección recoge experimentos acerca de los efectos que la temperatura de crecimiento y la razón In/Ga tienen en la morfología y emisión de nanocolumnas ordenadas de InGaN crecidas sobre templates GaN/zafiro. En el rango de temperaturas entre 650 y 750 C, la incorporacion de In puede modificarse bien por la temperatura de crecimiento, o por la razón In/Ga. Controlar estos factores permite la optimización de la longitud de onda de emisión de las nanocolumnas de InGaN. En el caso particular de la generación de luz blanca, se han seguidos dos aproximaciones. En la primera, se obtiene emisión amarilla-blanca a temperatura ambiente de nanoestructuras donde la región de InGaN consiste en un gradiente de composiciones de In, que se ha obtenido a partir de un gradiente de temperatura durante el crecimiento. En la segunda, el apilamiento de segmentos emitiendo en azul, verde y rojo, consiguiendo la integración monolítica de estas estructuras en cada una de las nanocolumnas individuales, da lugar a emisores ordenados con un amplio espectro de emisión. En esta última aproximación, la forma espectral puede controlarse con la longitud (duración del crecimiento) de cada uno de los segmentos de InGaN. Más adelante, se presenta el crecimiento ordenado, por epitaxia de haces moleculares, de arrays de nanocolumnas que son diodos InGaN/GaN cada una de ellas, emitiendo en azul (441 nm), verde (502 nm) y amarillo (568 nm). La zona activa del dispositivo consiste en una sección de InGaN, de composición constante nominalmente y longitud entre 250 y 500 nm, y libre de defectos extendidos en contraste con capas compactas de InGaN de similares composiciones y espesores. Los espectros de electroluminiscencia muestran un muy pequeño desplazamiento al azul al aumentar la corriente inyectada (desplazamiento casi inexistente en el caso del dispositivo amarillo), y emisiones ligeramente más anchas que en el caso del estado del arte en pozos cuánticos de InGaN. A continuación, se presenta y discute el crecimiento ordenado de nanocolumnas de In(Ga)N/GaN en sustratos de Si(111). Nanocolumnas ordenadas emitiendo desde el ultravioleta (3.2 eV) al infrarrojo (0.78 eV) se crecieron sobre sustratos de Si(111) utilizando una capa compacta (“buffer”) de GaN. La morfología y eficiencia de emisión de las nanocolumnas emitiendo en el rango espectral verde pueden ser mejoradas ajustando las relaciones In/Ga y III/N, y una eficiencia cuántica interna del 30% se deriva de las medidas de fotoluminiscencia en nanocolumnas optimizadas. En la siguiente sección de este trabajo se presenta en detalle el mecanismo tras el crecimiento ordenado de nanocolumnas de InGaN/GaN emitiendo en el verde, y sus propiedades ópticas. Nanocolumnas de InGaN/GaN con secciones largas de InGaN (330-830 nm) se crecieron tanto en sustratos GaN/zafiro como GaN/Si(111). Se encuentra que la morfología y la distribución espacial del In dentro de las nanocolumnas dependen de las relaciones III/N e In/Ga locales en el frente de crecimiento de las nanocolumnas. La dispersión en el contenido de In entre diferentes nanocolumnas dentro de la misma muestra es despreciable, como indica las casi identicas formas espectrales de la catodoluminiscencia de una sola nanocolumna y del conjunto de ellas. Para las nanocolumnas de InGaN/GaN crecidas sobre GaN/Si(111) y emitiendo en el rango espectral verde, la eficiencia cuántica interna aumenta hasta el 30% al disminuir la temperatura de crecimiento y aumentar el nitrógeno activo. Este comportamiento se debe probablemente a la formación de estados altamente localizados, como indica la particular evolución de la energía de fotoluminiscencia con la temperatura (ausencia de “s-shape”) en muestras con una alta eficiencia cuántica interna. Por otro lado, no se ha encontrado la misma dependencia entre condiciones de crecimiento y efiencia cuántica interna en las nanoestructuras InGaN/GaN crecidas en GaN/zafiro, donde la máxima eficiencia encontrada ha sido de 3.7%. Como alternativa a las nanoestructuras axiales de InGaN/GaN, la sección 4 presenta resultados sobre el crecimiento y caracterización de estructuras core-shell de InGaN/GaN, re-crecidas sobre arrays de micropilares de GaN fabricados por ataque de un template GaN/zafiro (aproximación top-down). El crecimiento de InGaN/GaN es conformal, con componentes axiales y radiales en el crecimiento, que dan lugar a la estructuras core-shell con claras facetas hexagonales. El crecimiento radial (shell) se ve confirmado por medidas de catodoluminiscencia con resolución espacial efectuadas en un microscopio electrónico de barrido, asi como por medidas de microscopía de transmisión de electrones. Más adelante, el crecimiento de micro-pilares core-shell de InGaN se realizó en pilares GaN (cores) crecidos selectivamente por epitaxia de metal-orgánicos en fase vapor. Con el crecimiento de InGaN se forman estructuras core-shell con emisión alrededor de 3 eV. Medidas de catodoluminiscencia resuelta espacialmente indican un aumento en el contenido de indio del shell en dirección a la parte superior del pilar, que se manifiesta en un desplazamiento de la emisión de 3.2 eV en la parte inferior, a 3.0 eV en la parte superior del shell. Este desplazamiento está relacionado con variaciones locales de la razón III/V en las facetas laterales. Finalmente, se demuestra la fabricación de una estructura pin basada en estos pilares core-shell. Medidas de electroluminiscencia resuelta espacialmente, realizadas en pilares individuales, confirman que la electroluminiscencia proveniente del shell de InGaN (diodo lateral) está alrededor de 3.0 eV, mientras que la emisión desde la parte superior del pilar (diodo axial) está alrededor de 2.3 eV. Para finalizar, se presentan resultados sobre el crecimiento ordenado de GaN, con y sin inserciones de InGaN, en templates semi polares (GaN(11-22)/zafiro) y no polares (GaN(11-20)/zafiro). Tras el crecimiento ordenado, gran parte de los defectos presentes en los templates originales se ven reducidos, manifestándose en una gran mejora de las propiedades ópticas. En el caso de crecimiento selectivo sobre templates con orientación GaN(11-22), no polar, la formación de nanoestructuras con una particular morfología (baja relación entre crecimiento perpedicular frente a paralelo al plano) permite, a partir de la coalescencia de estas nanoestructuras, la fabricación de pseudo-templates no polares de GaN de alta calidad. ABSTRACT The aim of this work is to gain insight into the selective area growth of InGaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs. Several nanocolumn-based approaches such as standard axial InGaN/GaN structures, InGaN/GaN core-shell structures, or InGaN/GaN nanostructures grown on semi- and non-polar substrates are discussed. The first section reviews the basics of the self-assembled growth of GaN nanocolumns on Si(111). Morphology differences and optical properties are compared to those of GaN layer grown directly on Si(111). The effects of the growth temperature on the In incorporation in self-assembled InGaN nanocolumns grown on Si(111) is described. The second section reviews the basic growth mechanisms of selectively grown GaNbased nanostructures on c-plane GaN/sapphire templates. By increasing the local III/V ratio morphological changes from pyramidal islands, to GaN nanocolumns with top semi-polar planes, and further to GaN nanocolumns with top polar c-planes are observed. When growing InGaN nano-disks embedded into the GaN nanocolumns, the different morphologies mentioned lead to different optical properties, due to the semipolar and polar nature of the crystal planes involved. The third section reports on the effect of the growth temperature and In/Ga ratio on the morphology and light emission characteristics of ordered InGaN nanocolumns grown on c-plane GaN/sapphire templates. Within the growth temperature range of 650 to 750oC the In incorporation can be modified either by the growth temperature, or the In/Ga ratio. Control of these factors allows the optimization of the InGaN nanocolumns light emission wavelength. In order to achieve white light emission two approaches are used. First yellow-white light emission can be obtained at room temperature from nanostructures where the InGaN region is composition-graded by using temperature gradients during growth. In a second approach the stacking of red, green and blue emitting segments was used to achieve the monolithic integration of these structures in one single InGaN nanocolumn leading to ordered broad spectrum emitters. With this approach, the spectral shape can be controlled by changing the thickness of the respective InGaN segments. Furthermore the growth of ordered arrays of InGaN/GaN nanocolumnar light emitting diodes by molecular beam epitaxy, emitting in the blue (441 nm), green (502 nm), and yellow (568 nm) spectral range is reported. The device active region, consisting of a nanocolumnar InGaN section of nominally constant composition and 250 to 500 nm length, is free of extended defects, which is in strong contrast to InGaN layers (planar) of similar composition and thickness. Electroluminescence spectra show a very small blue shift with increasing current, (almost negligible in the yellow device) and line widths slightly broader than those of state-of-the-art InGaN quantum wells. Next the selective area growth of In(Ga)N/GaN nanocolumns on Si(111) substrates is discussed. Ordered In(Ga)N/GaN nanocolumns emitting from ultraviolet (3.2 eV) to infrared (0.78 eV) were then grown on top of GaN-buffered Si substrates. The morphology and the emission efficiency of the In(Ga)N/GaN nanocolumns emitting in the green could be substantially improved by tuning the In/Ga and total III/N ratios, where an estimated internal quantum efficiency of 30 % was derived from photoluminescence data. In the next section, this work presents a study on the selective area growth mechanisms of green-emitting InGaN/GaN nanocolumns and their optical properties. InGaN/GaN nanocolumns with long InGaN sections (330-830nm) were grown on GaN/sapphire and GaN-buffered Si(111). The nanocolumn’s morphology and spatial indium distribution is found to depend on the local group (III)/N and In/Ga ratios at the nanocolumn’s top. A negligible spread of the average indium incorporation among different nanostructures is found as indicated by similar shapes of the cathodoluminescence spectra taken from single nanocolumns and ensembles of nanocolumns. For InGaN/GaN nanocolumns grown on GaN-buffered Si(111), all emitting in the green spectral range, the internal quantum efficiency increases up to 30% when decreasing growth temperature and increasing active nitrogen. This behavior is likely due to the formation of highly localized states, as indicated by the absence of a complete s-shape behavior of the PL peak position with temperature (up to room temperature) in samples with high internal quantum efficiency. On the other hand, no dependence of the internal quantum efficiency on the growth conditions is found for InGaN/GaN nanostructures grown on GaN/sapphire, where the maximum achieved efficiency is 3.7%. As alternative to axial InGaN/GaN nanostructures, section 4 reports on the growth and characterization of InGaN/GaN core-shell structures on an ordered array of top-down patterned GaN microrods etched from a GaN/sapphire template. Growth of InGaN/GaN is conformal, with axial and radial growth components leading to core-shell structures with clear hexagonal facets. The radial InGaN growth (shell) is confirmed by spatially resolved cathodoluminescence performed in a scanning electron microscopy as well as in scanning transmission electron microscopy. Furthermore the growth of InGaN core-shell micro pillars using an ordered array of GaN cores grown by metal organic vapor phase epitaxy as a template is demonstrated. Upon InGaN overgrowth core-shell structures with emission at around 3.0 eV are formed. With spatially resolved cathodoluminescence, an increasing In content towards the pillar top is found to be present in the InGaN shell, as indicated by a shift of CL peak position from 3.2 eV at the shell bottom to 3.0 eV at the shell top. This shift is related to variations of the local III/V ratio at the side facets. Further, the successful fabrication of a core-shell pin diode structure is demonstrated. Spatially resolved electroluminescence measurements performed on individual micro LEDs, confirm emission from the InGaN shell (lateral diode) at around 3.0 eV, as well as from the pillar top facet (axial diode) at around 2.3 eV. Finally, this work reports on the selective area growth of GaN, with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the GaN templates is strongly reduced as indicated by TEM and a dramatic improvement of the optical properties. In case of SAG on non-polar (11-22) templates the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of the nanostructures.

Minimum volume stability limits for axisymmetric liquid bridges subject to steady axial acceleration

In this paper the influence of an axial microgravity on the minimum volume stability limit of axisymmetric liquid bridges between unequal disks is analyzed both theoretically and experimentally. The results here presented extend the knowledge of the static behaviour of liquid bridges to fluid configurations different from those studied up to now (almost equal disks). Experimental results, obtained by simulating microgravity conditions by the neutral buoyancy technique, are also presented and are shown to be in complete agreement with theoretical ones.

Dynamic stability of long axisymmetric liquid bridges

This paper deals with the non-linear forced oscillations of axisymmetric long liquid bridges between equal disks. The dynamics of the liquid bridge has been analyzed by using a self-similar, one-dimensional model already used in similar problems. The influence of the dynamics on the static stability limits, as well as the main characteristics of the non-linear behaviour of long liquid bridges, have been studied with in the range of validity of the mathematical model used here.

On the use of liquid bridges as accelerometers

The shape of the interface of a drop of liquid held by surface tension forces between two solid disks,a liquid bridge, depends on the geometry of the supporting disks, the volume of liquid and the external forces acting on the drop. Therefore, once the geometry of the supporting disks and the volume of liquid are fixed, and assuming that the value of the surface tension is known, a way to measure such external forces could be by measuring the deformation of the liquid bridge interface.

On the stability limits of long nonaxisymmetric cylindrical liquid bridges

There is a self-similar solution for the stability limits of long, almost cylindrical liquid bridges between equal disks subjected to both axial and lateral accelerations. The stability limits depend on only two variables; the so-called reduced axial, and lateral Bond numbers. A novel experimental setup that involved rotating a horizontal cylindrical liquid bridge about a vertical axis of rotation was designed to test the stability limits predicted by the self-similar solution. Analytical predictions compared well with both numerical and experimental results.

An experimental study of the breakage of liquid bridges at stability limit of minimum volume

An experimental apparatus to study the breaking process of axisymmetric liquid bridges has been developed, and the breaking sequences of a large number of liquid bridge configurations at minimum-volume stability limit have been analyzed. Experimental results show that very close to the breaking moment the neck radius of the liquid bridge varies as t1/3, where t is the time to breakage, irrespective of the value of the distance between the solid disks that support the liquid column.

Stability of liquid bridges between an elliptical and a circular supporting disk

A numerical method has been developed to determine the stability limits for liquid bridges held between noncircular supporting disks and the application to a configuration with a circular and an elliptical disk subjected to axial acceleration has been made. The numerical method led to results very different from the available analytical solution which has been revisited and a better approximation has been obtained. It has been found that just retaining one more term in the asymptotic analysis the solution reproduces the real behavior of the configuration and the numerical results.

Aerodinámica experimental de elementos de parques energéticos

La creciente demanda de energía eléctrica y la necesidad de implementar energías no contaminantes hace que las llamadas tecnologías verdes sean cada día más solicitadas. Entre estas tecnologías encontramos la energía solar y la energía eólica; ambas tienen una trayectoria de uso e investigación bastante amplia, sin embargo aún presentan problemas de fondo que impiden dar mayor impulso a su uso. El objetivo de la presente tesis es presentar soluciones a problemas de optimización en campos conversores de energía. Para ello se analizan y resuelven dos problemas por medio de técnicas de aerodinámica experimental: el primero sobre campos de colectores solares y el segundo sobre campos eólicos. Las técnicas de medición utilizadas en aerodinámica, y en el presente trabajo, son: medición de cargas, anemometría de hilo caliente, velocimetría por imagen de partículas y escaneo de presiones; además de un análisis estadístico de los datos. En el primer caso se ensayan experimentalmente colectores solares parabólicos en donde, por cuestiones de seguridad o por protección contra el viento, se utilizan cercas. Éstas modifican el comportamiento del flujo corriente abajo y se ha encontrado que la distancia a la cual se colocan, así como el tipo de cercas (sólida o permeable), modifican las cargas estructurales a las que los colectores están expuestos. Los resultados demuestran que existe una distancia crítica en la cual la presencia de la cerca aumenta la carga en lugar de disminuirla, por lo cual la selección adecuada del parapeto y la distancia a la cual se coloca son de suma importancia para la optimización de la estructura. En el segundo caso se ensaya experimentalmente y simula numéricamente la estela de turbinas eólicas por medio de discos porosos. En donde un disco permeable simula el rotor de una turbina. El disco es capaz de semejar la estela y los efectos que ésta puede causar corriente abajo. Los resultados muestran que seleccionando adecuadamente la porosidad, es posible simular coeficientes de empuje similares a los desarrollados por los aerogeneradores, además la estela y sus efectos son semejantes a los medidos en campo. ABSTRACT The called green energy technologies are increasingly required because of the growing demand for electricity and the need to implement nonpolluting energy. Among the green energy technologies it is found the solar and the wind energy, both have a history of use and fairly extensive research; however they still have problems which limit to give them further impetus to its use. The aim of this thesis is to present solutions to optimization problems in energy harvesting. To this end it is analysed, and solved, two problems by means of techniques in experimental aerodynamics: the first issue with regard to parabolic troughs and the second about wind farms. The measurement techniques commonly used in aerodynamics, and used in this research work, are: measurement of loads, hot wire anemometry, particle image velocimetry and scanning of pressures; where data are collected and then an statistical analysis is done. In the first case it is tested parabolic troughs where, either for security reasons or protection against the wind actions, fences are used. These fences modify the behaviour of flow downstream and it was found that the distance at which they are placed, and the type of fences (solid or permeable) modify the structural loads to which the parabolic troughs are exposed. The results show that there is a critical distance at which the presence of the fence increases the load instead of protecting the parabolic trough, hence making the proper selection of the parapet and the distance at which it stands are paramount for the optimization of the structure. In the second case it is tested, experimentally and numerically, the wake of wind turbines by means of porous disks; where the permeable disc simulates a turbine rotor. The disc is able to mimic the wake and the effects that it can cause downstream. The results show that by properly selecting the porosity, it is possible to simulate drag coefficients similar to those developed by wind turbines; moreover the porous disk wake and its effects are similar to those measured on field.

Microinformática en entornos profesionales : instalación, configuración, mantenimiento y reparación

Los sistemas microinformáticos se componen principalmente de hardware y software, con el paso del tiempo el hardware se degrada, se deteriora y en ocasiones se avería. El software evoluciona, requiere un mantenimiento, de actualización y en ocasiones falla teniendo que ser reparado o reinstalado. A nivel hardware se analizan los principales componentes que integran y que son comunes en gran parte estos sistemas, tanto en equipos de sobre mesa como portátiles, independientes del sistema operativo, además de los principales periféricos, también se analizan y recomiendan algunas herramientas necesarias para realizar el montaje, mantenimiento y reparación de estos equipos. Los principales componentes hardware internos son la placa base, memoria RAM, procesador, disco duro, carcasa, fuente de alimentación y tarjeta gráfica. Los periféricos más destacados son el monitor, teclado, ratón, impresora y escáner. Se ha incluido un apartado donde se detallan los distintos tipos de BIOS y los principales parámetros de configuración. Para todos estos componentes, tanto internos como periféricos, se ha realizado un análisis de las características que ofrecen y los detalles en los que se debe prestar especial atención en el momento de seleccionar uno frente a otro. En los casos que existen diferentes tecnologías se ha hecho una comparativa entre ambas, destacando las ventajas y los inconvenientes de unas frente a otras para que sea el usuario final quien decida cual se ajusta mejor a sus necesidades en función de las prestaciones y el coste. Un ejemplo son las impresoras de inyección de tinta frente a las laser o los discos duros mecánicos en comparación con y los discos de estado sólido (SSD). Todos estos componentes están relacionados, interconectados y dependen unos de otros, se ha dedicado un capítulo exclusivamente para estudiar cómo se ensamblan estos componentes, resaltando los principales fallos que se suelen cometer o producir y se han indicado unas serie tareas de mantenimiento preventivo que se pueden realizar para prolongar la vida útil del equipo y evitar averías por mal uso. Los mantenimientos se pueden clasificar como predictivo, perfectivo, adaptativo, preventivo y correctivo. Se ha puesto el foco principalmente en dos tipos de mantenimiento, el preventivo descrito anteriormente y en el correctivo, tanto software como hardware. El mantenimiento correctivo está enfocado al análisis, localización, diagnóstico y reparación de fallos y averías hardware y software. Se describen los principales fallos que se producen en cada componente, cómo se manifiestan o qué síntomas presentan para poder realizar pruebas específicas que diagnostiquen y acoten el fallo. En los casos que es posible la reparación se detallan las instrucciones a seguir, en otro caso se recomienda la sustitución de la pieza o componente. Se ha incluido un apartado dedicado a la virtualización, una tecnología en auge que resulta muy útil para realizar pruebas de software, reduciendo tiempos y costes en las pruebas. Otro aspecto interesante de la virtualización es que se utiliza para montar diferentes servidores virtuales sobre un único servidor físico, lo cual representa un importante ahorro en hardware y costes de mantenimiento, como por ejemplo el consumo eléctrico. A nivel software se realiza un estudio detallado de los principales problemas de seguridad y vulnerabilidades a los que está expuesto un sistema microinformático enumerando y describiendo el comportamiento de los distintos tipos de elementos maliciosos que pueden infectar un equipo, las precauciones que se deben tomar para minimizar los riesgos y las utilidades que se pueden ejecutar para prevenir o limpiar un equipo en caso de infección. Los mantenimientos y asistencias técnicas, en especial las de tipo software, no siempre precisan de la atención presencial de un técnico cualificado, por ello se ha dedicado un capítulo a las herramientas de asistencia remota que se pueden utilizar en este ámbito. Se describen algunas de las más populares y utilizadas en el mercado, su funcionamiento, características y requerimientos. De esta forma el usuario puede ser atendido de una forma rápida, minimizando los tiempos de respuesta y reduciendo los costes. ABSTRACT Microcomputer systems are basically made up of pieces of hardware and software, as time pass, there’s a degradation of the hardware pieces and sometimes failures of them. The software evolves, new versions appears and requires maintenance, upgrades and sometimes also fails having to be repaired or reinstalled. The most important hardware components in a microcomputer system are analyzed in this document for a laptop or a desktop, with independency of the operating system they run. In addition to this, the main peripherals and devices are also analyzed and a recommendation about the most proper tools necessary for maintenance and repair this kind of equipment is given as well. The main internal hardware components are: motherboard, RAM memory, microprocessor, hard drive, housing box, power supply and graphics card. The most important peripherals are: monitor, keyboard, mouse, printer and scanner. A section has been also included where different types of BIOS and main settings are listed with the basic setup parameters in each case. For all these internal components and peripherals, an analysis of their features has been done. Also an indication of the details in which special attention must be payed when choosing more than one at the same time is given. In those cases where different technologies are available, a comparison among them has been done, highlighting the advantages and disadvantages of selecting one or another to guide the end user to decide which one best fits his needs in terms of performance and costs. As an example, the inkjet vs the laser printers technologies has been faced, or also the mechanical hard disks vs the new solid state drives (SSD). All these components are interconnected and are dependent one to each other, a special chapter has been included in order to study how they must be assembled, emphasizing the most often mistakes and faults that can appear during that process, indicating different tasks that can be done as preventive maintenance to enlarge the life of the equipment and to prevent damage because of a wrong use. The different maintenances can be classified as: predictive, perfective, adaptive, preventive and corrective. The main focus is on the preventive maintains, described above, and in the corrective one, in software and hardware. Corrective maintenance is focused on the analysis, localization, diagnosis and repair of hardware and software failures and breakdowns. The most typical failures that can occur are described, also how they can be detected or the specific symptoms of each one in order to apply different technics or specific tests to diagnose and delimit the failure. In those cases where the reparation is possible, instructions to do so are given, otherwise, the replacement of the component is recommended. A complete section about virtualization has also been included. Virtualization is a state of the art technology that is very useful especially for testing software purposes, reducing time and costs during the tests. Another interesting aspect of virtualization is the possibility to have different virtual servers on a single physical server, which represents a significant savings in hardware inversion and maintenance costs, such as electricity consumption. In the software area, a detailed study has been done about security problems and vulnerabilities a microcomputer system is exposed, listing and describing the behavior of different types of malicious elements that can infect a computer, the precautions to be taken to minimize the risks and the tools that can be used to prevent or clean a computer system in case of infection. The software maintenance and technical assistance not always requires the physical presence of a qualified technician to solve the possible problems, that’s why a complete chapter about the remote support tools that can be used to do so has been also included. Some of the most popular ones used in the market are described with their characteristics and requirements. Using this kind of technology, final users can be served quickly, minimizing response times and reducing costs.

Estabilidad de puentes líquidos bajo el efecto de una rotación no axilsimétrica

El estudio de la influencia de perturbaciones de distinta naturaleza en configuraciones de puentes líquidos apoyados en dos discos coaxiales en rotación encuentra una importante motivación en el uso de dicha configuración en la fabricación de cristales semiconductores ultra-puros por la denominada técnica de zona flotante, en la que la rotación de los discos se utiliza para alcanzar temperaturas uniformes. El presente estudio muestra los resultados obtenidos mediante la aplicación de un método numérico en el análisis de la estabilidad de puentes líquidos en isorrotación sometidos al efecto de una fuerza axial uniforme (gravedad axial) y una excentricidad entre el eje de giro y el eje de los discos. Se analiza el efecto de la aplicación de estos factores tanto de forma conjunta como por separado. Aunque existen numerosos estudios previos sobre puentes líquidos sometidos a diversos efectos, el análisis del efecto combinado de la rotación con excentricidad y gravedad axial no ha sido realizado con anterioridad. Este estudio permite además entender los resultados del experimento a bordo de la misión TEXUS-23, en el que un puente líquido sujeto entre dos discos circulares y coaxiales es sometido al efecto de una rotación creciente en torno a un eje desplazado respecto al eje de los discos. Aunque en el experimento no se impone una fuerza axial controlada, la desestabilización y rotura del puente se produce de forma notablemente asimétrica, lo que no puede ser explicado con los estudios precedentes y sugiere una posible presencia de una aceleración axial residual. Se ha desarrollado por tanto un método de análisis de imágenes que permite comparar las formas obtenidas en el experimento con las calculadas numéricamente. En este estudio se muestran los detalles del procesado realizado en las imágenes de la misión TEXUS-23, y los resultados de su comparación con el análisis numérico, que permiten determinar el valor de la gravedad axial que mejor reproduce los resultados del experimento. Estos resultados ponen de manifiesto la importancia del conocimiento y la modelización de efectos cuya presencia (intencionada o no) afectan de forma visible a la estabilidad y la morfología de los puentes líquidos. ABSTRACT The study of the influence of various disturbances in configurations consisting of a liquid bridge supported by two co-axial disks in rotation has an important motivation in the use of this configuration in the fabrication of ultrapure semiconductor crystals via the so-called floating zone technique, in which the rotation of the disks is used to achieve a uniform temperature field. The present study shows the results obtained through the application of a numerical method in the analysis of the stability of liquid bridges in isorotation under the effect of a uniform axial force field (axial gravity) and an offset between the rotation axis and the axis of the supporting disks (eccentricity). The analysis studies the effect of both the combined and separate application of these factors. Although there are numerous studies on liquid bridges subject to various effects, the analysis of the combined effect of rotation with eccentricity and axial gravity has not been done before. Furthermore, this study allows us to understand the results from the experiment aboard the TEXUS-23 mission, in which a liquid bridge supported between two circular-shaped, co-axial disks is subject to the effect of an increasing rotation around an axis with an offset with respect to the axis of the disks. Although the experiment conditions do not include a controlled axial force field, the instability and breakage of the bridge occurs with a marked asymmetry, which cannot be explained by previous studies and suggests the possible presence of a residual axial gravity. Therefore, an image analysis method has been developed which allows to compare the shapes obtained in the experiment with those calculated with the numerical method. This study shows the details of the processing performed on the images from the TEXUS-23 mission and the results from their comparison with the numerical analysis, which allow to determine the axial gravity value which best recovers the experimental results. These results highlight the importance of the understanding and modelling of effects which, when present (intentionally or not), noticeably affect the stability and shape of the liquid bridges.

Hydrostatic instabilities in floating zone cristal growth process

If only Fluid Mechanics aspects are considered, the configuration appearing in the floating zone technique for crystal growth can be modelled as a mass of liquid spanning between two solid rods. Besides, if now the influence of temperature gradients and heat flow are not considered, the simplest fluid model consists of an isothermal liquid mass of constant properties (density and surface tension) held by capillary forces between two solid disks placed a distance L apart: the so called liquid bridge. As it is well known, if both supporting disks were parallel, coaxial and of the same diameter, 2R, the volume of liquid, V, were equal to that of a cylinder of the same L and R (V=KR~L) and no body forces were acting on the liquid column, the fluid configuration (under these conditions of cylindrical shape) will become unstable when the distance between the disks equals the length of the circumference of the supporting disks (L=2KR, the so-called Rayleigh stability limit). One should be aware that the Rayleigh stability limit can be dramatically modified when the geometry differs from the above described cylinder (due to having non-coaxial disks, different diameter disks, liquid volume different from the cylindrical one, etc) or when other external effects like accelerations either axial or lateral are considered. In this paper the stability limits of liquid bridges considering different types of perturbations are reviewed.