Experimental and numerical study of cw green laser crystallization of a-Si:H thin films

Crystallization and grain growth technique of thin film silicon are among the most promising methods for improving efficiency and lowering cost of solar cells. A major advantage of laser crystallization and annealing over conventional heating methods is its ability to limit rapid heating and cooling to thin surface layers. Laser energy is used to heat the amorphous silicon thin film, melting it and changing the microstructure to polycrystalline silicon (poly-Si) as it cools. Depending on the laser density, the vaporization temperature can be reached at the center of the irradiated area. In these cases ablation effects are expected and the annealing process becomes ineffective. The heating process in the a-Si thin film is governed by the general heat transfer equation. The two dimensional non-linear heat transfer equation with a moving heat source is solve numerically using the finite element method (FEM), particularly COMSOL Multiphysics. The numerical model help to establish the density and the process speed range needed to assure the melting and crystallization without damage or ablation of the silicon surface. The samples of a-Si obtained by physical vapour deposition were irradiated with a cw-green laser source (Millennia Prime from Newport-Spectra) that delivers up to 15 W of average power. The morphology of the irradiated area was characterized by confocal laser scanning microscopy (Leica DCM3D) and Scanning Electron Microscopy (SEM Hitachi 3000N). The structural properties were studied by micro-Raman spectroscopy (Renishaw, inVia Raman microscope).

Mejora y optimización de un código de simulación hidrodinámico con transporte de radiación para el estudio de sistemas en condiciones de alta densidad de energía

Se presentan las mejoras introducidas en un código de transporte de radiación acoplada a la hidrodinámica llamado ARWEN para el estudio de sistemas en el rango de física de alta densidad de energía (High Energy Density Physics). Los desarrollos introducidos se basan en las siguientes áreas: ít>,~ Ecuaciones de estado: se desarrolla una nueva metodología mediante la cual es posible ajustar los resultados de un modelo simple de ecuaciones de estado como QEOS a datos experimentales y resultados de AIMD. Esta metodología tiene carácter general para poder ser aplicada en gran cantidad de materuales de interés y amplia la flexibilidad de ajuste de los métodos de los que ha partido como base este trabajo. En segundo lugar, se ha desarrollado una librería para la gestión de tablas de datos de ecuaciones de estado que también incluye la gestión de tablas con datos de opacidades y de ionización. Esta nueva librería extiende las capacidades de la anterior al tener llamadas más específicas que aceleran los cálculos, y posibilidad de uso de varias tablas a la vez. Solver de difusión: se ha desarrollado un nuevo paquete para resolver la ecuación de difusión que se aplicará a la conducción de calor dentro del plasma. El método anterior no podía ser ejecutado en paralelo y producía resultados dependientes de la resolución de la malla, mientras que este método es paralelizable y además obtiene una solución con mejor convergencia, lo que supone una solución que no depende del refinamiento del mallado. Revisión del paquete de radiación: en primer lugar se ha realizado una revisión de la implementación del modelo de radiación descubriendo varios errores que han sido depurados. También se ha incluido la nueva librería de gestión de tablas de opacidades que permiten la obtención de las propiedades ópticas del plasma en multigrupos de energía. Por otra parte se ha extendido el cálculo de los coeficientes de transporte al esquema multimaterial que ha introducido David Portillo García en el paquete hidrodinámico del código de simulación. Por último se ha revisado el esquema de resolución del transporte y se ha modificado para hacerlo paralelizable. • Se ha implementado un paquete de trazado de rayos para deposición láser que extiende la utilidad del anterior al ser en 3D y poder utilizar entonces diferentes configuraciones. • Una vez realizadas todas estas tareas se ha aplicado el código ARWEN al estudio de la astrofísica de laboratorio simulando los experimentos llevados a cabo en la instalación PALS por Chantal Stehlé acerca de ondas de choque radiativas. Se han comparado los resultados experimentales frente a las predicciones del código ARWEN obteniéndose una gran concordancia en la velocidad de la onda de choque generada y en las dimensiones del precursor. El código de simulación sobre el que se ha trabajado, junto con los desarrollos aportados por otros investigadores durante la realización de esta tesis, ha permitido participar en colaboraciones con laboratorios de Francia o Japón y se han producido resultados científicos publicados basados en el trabajo descrito en esta tesis. ABSTRACT Improvements in radiation hydrodynamic code ARWEN for the study of systems in the range of physics high energy density (High Energy Density Physics) are presented. The developments introduced are based on the following áreas: • Equations of state: a new methodology was developed to adjust the results of a simple Equation of State model like QEOS to experimental data and results of AIMD. This methodology can be applied to a large amount of materials and it increases the flexibility and range of the previous methods used as basis for this work. Also a new computer library has been developed to manage data tables of thermodynamic properties as well as includes the management of opacity and ionization data tables. This new library extends the capabilities of the previous one with more specific routines, and the possibility of using múltiple tables for several materials. • Diffusion solver: a new package has been developed to solve the diffusion equation applied to the heat conduction of the plasma. The previous method is not parallelizable and it produced mesh dependent results, while this new package can be executed in parallel and achieves a more converged solution that does not depend on the refinement of the mesh. • Radiation package: the check of the radiation model rose several bugs in the implementation that had been removed. The new computer library for EOS managing includes capabilities to store opacity tables for multigroups of energy. Moreover the transport coefficients calculations have been extended for the new multimaterial hydrodynamic package developed by David Portillo García. Also the solving methodology for the transport equation has been modified to make the code run in parallel. • A new ray tracing package has been introduced to extend the previous one to 3D. Once all these tasks has been implemented, the ARWEN code has been applied to study laboratory astrophysics systems. Simulations have been done in order to reproduce the results of the experiments carried out in PALS facility by Chantal Stehlé in radiative shock production. Experimental results are in cióse agreement to the ARWEN estimations of the speed of the shock wave and the length of the precursor. The simulation code used in this thesis, including the work done in ARWEN by other colleagues at the time of this research, allowed the collaboration with other research institution in France and Japan and some of the results presented in this thesis have been published in scientific journals.

Sheath vaporization of a monodisperse fuel-spray jet

The group vaporization of a monodisperse fuel-spray jet discharging into a hot coflowing gaseous stream is investigated for steady flow by numerical and asymptotic methods with a two-continua formulation used for the description of the gas and liquid phases. The jet is assumed to be slender and laminar, as occurs when the Reynolds number is moderately large, so that the boundary-layer form of the conservation equations can be employed in the analysis. Two dimensionless parameters are found to control the flow structure, namely the spray dilution parameter 1, defined as the mass of liquid fuel per unit mass of gas in the spray stream, and the group vaporization parameter e, defined as the ratio of the characteristic time of spray evolution due to droplet vaporization to the characteristic diffusion time across the jet. It is observed that, for the small values of e often encountered in applications, vaporization occurs only in a thin layer separating the spray from the outer droplet-free stream. This regime of sheath vaporization, which is controlled by heat conduction, is amenable to a simplified asymptotic description, independent of ε,in which the location of the vaporization layer is determined numerically as a free boundary in a parabolic problem involving matching of the separate solutions in the external streams, with appropriate jump conditions obtained from analysis of the quasi-steady vaporization front. Separate consideration of dilute and dense sprays, corresponding, respectively, to the asymptotic limits λ<<1 and λ>>1, enables simplified descriptions to be obtained for the different flow variables, including explicit analytic expressions for the spray penetration distance.

Desarrollo de un método numérico para flujos multimaterial en 2D en ARWEN

En esta tesis se presenta un método numérico para resolver las ecuaciones de Euler para flujos multimaterial en malla euleriana. Este solver se ha acoplado en el código hidrodinámico en dos dimensiones con transporte de radiación desarrollado en el Instituto de Fusión Nuclear de la UPM bajo la dirección del profesor Pedro Velarde, ARWEN. Los objetivos de este trabajo son: Desarrollo e implementación de un método de Godunov unsplit de alto orden multimaterial en 2D para malla euleriana en geometría cartesiana y geometría cilíndrica. Se presenta una extensión del trabajo realizado por Miller y Puckett (36) a una formulación unsplit. Además, se ha prestado especial atención al acoplamiento con el transporte de radiación y la conducción de calor. El método presentado se ha probado en una gran cantidad de problemas. Aplicación del código multimaterial al estudio de experimentos reales: • Simulación de una propuesta de experimento de laboratorio para reproducir la etapa de arrancamiento de material de la interacción entre el gas proveniente de la explosión de una supernova y la estrella secundaria en un escenario degenarado (SD). • Formación de jets en el laboratorio producidos por la colisión de dos plasmas. ABSTRACT We present a solver for the Euler equations for multimaterial flows in eulerian mesh. This solver has been coupled in the 2D AMR radiation transport code developed at Instituto de Fusión Nuclear (UPM) under the direction of professor Pedro Velarde, ARWEN. The main goals of this thesis are: Development and implementation of an 2D unsplit high-order Godunov method for multimaterial flows in eulerian mesh for cartesian and axialsimetry geometry. We present an extension of the work of Miller and Puckett (36) to an unsplit formulation. Also, we have paid special attention to the coupling with radiation transport and heat conduction. The method has been tested in a wide variety of problems. Application of the multimaterial solver to the study of real experiments: • Simulation of a proposal of a laboratory experiment aimed to reproducing the stripping stage of the interaction between the gas ejected during a supernova explosion and the secondary star in the Single Degenerate scenario. • Experiments of plasma jets in the laboratory obtained by the collission of two hot plasmas.

Joule heating in ferromagnetic nanostripes with a notch

The temperature in a ferromagnetic nanostripe with a notch subject to Joule heating has been studied in detail. We first performed an experimental real-time calibration of the temperature versus time as a 100 ns current pulse was injected into a Permalloy nanostripe. This calibration was repeated for different pulse amplitudes and stripe dimensions and the set of experimental curves were fitted with a computer simulation using the Fourier thermal conduction equation. The best fit of these experimental curves was obtained by including the temperature-dependent behavior of the electrical resistivity of the Permalloy and of the thermal conductivity of thesubstrate(SiO2). Notably, a nonzero interface thermal resistance between the metallic nanostripe and thesubstrate was also necessary to fit the experimental curves. We found this parameter pivotal to understand ourresults and the results from previous works. The higher current density in the notch, together with the interface thermal resistance, allows a considerable increase of the temperature in the notch, creating a large horizontal thermal gradient. This gradient, together with the high temperature in the notch and the larger current density close to the edges of the notch, can be very influential in experiments studying the current assisted domain wall motion.

(Table 1) Thermal conductivities of lithified core samples from ODP Leg 166 sites

The geothermal regime of the western margin of the Great Bahama Bank was examined using the bottom hole temperature and thermal conductivity measurements obtained during and after Ocean Drilling Program (ODP) Leg 166. This study focuses on the data from the drilling transect of Sites 1003 through 1007. These data reveal two important observational characteristics. First, temperature vs. cumulative thermal resistance profiles from all the drill sites show significant curvature in the depth range of 40 to 100 mbsf. They tend to be of concave-upward shape. Second, the conductive background heat-flow values for these five drill sites, determined from deep, linear parts of the geothermal profiles, show a systematic variation along the drilling transect. Heat flow is 43-45 mW/m**2 on the seafloor away from the bank and decreases upslope to ~35 mW/m**2. We examine three mechanisms as potential causes for the curved geothermal profiles. They are: (1) a recent increase in sedimentation rate, (2) influx of seawater into shallow sediments, and (3) temporal fluctuation of the bottom water temperature (BWT). Our analysis shows that the first mechanism is negligible. The second mechanism may explain the data from Sites 1004 and 1005. The temperature profile of Site 1006 is most easily explained by the third mechanism. We reconstruct the history of BWT at this site by solving the inverse heat conduction problem. The inversion result indicates gradual warming throughout this century by ~1°C and is agreeable to other hydrographic and climatic data from the western subtropic Atlantic. However, data from Sites 1003 and 1007 do not seem to show such trends. Therefore, none of the three mechanisms tested here explain the observations from all the drill sites. As for the lateral variation of the background heat flow along the drill transect, we believe that much of it is caused by the thermal effect of the topographic variation. We model this effect by obtaining a two-dimensional analytical solution. The model suggests that the background heat flow of this area is ~43 mW/m**2, a value similar to the background heat flow determined for the Gulf of Mexico in the opposite side of the Florida carbonate platform.

Wheat grain cooking process as investigated by modulated temperature differential scanning calorimetry

The thermal properties of soft and hard wheat grains, cooked in a steam pressure cooker, as a function of cooking temperature and time were investigated by modulated temperature differential scanning calorimetry (MTDSC). Four cooking temperatures (110, 120, 130 and 140 degrees C) and six cooking times (20, 40, 60, 80, 100 and 120 min) for each temperature were studied. It was found that typical non-reversible heat flow thermograms of cooked and uncooked wheat grains consisted of two endothermic baseline shifts localised around 40-50 degrees C and then 60-70 degrees C. The second peaks of non-reversible heat flow thermograms (60-70 degrees C) were associated with starch gelatinisation. The degree of gelatinisation was quantified based on these peaks. In this study, starch was completely gelatinised within 60-80 min for cooking temperatures at 110-120 degrees C and within 20 min for cooking temperatures at 130-140 degrees C. MTDSC detected reversible endothermic baseline shifts in most samples, localised broadly around 48-67 degrees C with changes in heat capacity ranging from 0.02 to 0.06 J/g per degrees C. These reversible endothermic baseline shifts are related to the glass transition, which occurs during starch gelatinisation. Data on the specific heat capacity of the cooked wheat samples are provided. (C) 2005 Elsevier Ltd. All rights reserved.

The drying of small drops of particulate slurries

The literature on the evaporation of drops of pure liquids, drops containing solids and droplet sprays has been critically reviewed. An experimental study was undertaken on the drying of suspended drops of pure water and aqueous sodium sulphate decahydrate with concentrations varying from 5 to 54. 1 wt. %. Individual drops were suspended from a glass filament balance in a 26 mm I.D. vertical wind tunnel, designed and constructed to supply hot de-humidified air, to simulate conditions encountered in commercial spray driers. A novel thin film thermocouple was developed to facilitate the simultaneous measurement of drop weight and core temperature. The heat conduction through the thermocouple was reduced because of its unique design; using essentially a single 50μ diameter nickel wire. For pure water drops, the Nusselt number was found to be a function of the Reynolds, Prandtl and Transfer numbers for a temperature range between 19 to 79°C.                  Nu = 2 + 0.19 (1/B)0.24 Re0.5 Pr0.33 Two distinct periods were observed during the drying of aqueous sodium sulphate decahydrate. The first period was characterised by the evaporation from a free liquid surface, whilst drying in the second period was controlled by the crust resistance. Fracturing of the crust occurred randomly but was more frequent at higher concentrations and temperatures. A model was proposed for the drying of slurry drops, based on a receding evaporation interface. The model was solved numerically for the variation of core temperature, drop weight and crust thickness as a function of time. Experimental results were in excellent agreement with the model predictions although at higher temperatures modifications to the model had to be made to accommodate the unusual behaviour of sodium sulphate slurries, i.e. the formation of hydrates.

Mechanisms of drying of skin forming materials

The literature relating to evaporation from single droplets of pure liquids, and to the drying of droplets containing solids and of droplet sprays has been reviewed. The heat and mass transfer rates for a single droplet suspended from a nozzle were studied within a 42mm I.D. horizontal wind tunnel designed to supply hot dry air, to simulate conditions encountered in a practical spray dryer. A novel rotating glass nozzle was developed to facilitate direct measurements of droplet weight and core temperature. This design minimised heat conduction through the nozzle. Revised correlations were obtained for heat and mass transfer coefficients, for evaporation from pure water droplets suspended from a rotating nozzle. Nu = 2.0 + 0.27 (l/B)°-18Re°-5Pr°-83 Sh = 2.0 + 0.575 ((T0-T.)/Tomfc) -o.o4Reo.5 ^0.33 Experimental drying studies were carried out on single droplets of different types of skin-forming materials, namely, custard, gelatin, skim milk and fructose at air temperatures ranging from 19°C to 198°C. Dried crusts were recovered and examined by Scanning Electron Microscopy. Skin-forming materials were classified into three types according to the mechanisms of skin formation. In the first type (typified by droplets of custard and starch) skin formed due to gelatinisation at high temperatures. Increasing the drying temperature resulted in increased crust resistance to mass transfer due to increased granule swelling and the crust resistance was completely transferred to a skin resistance at drying temperatures > 150°C. In the second type e.g. gelatin droplets the skin formed immediately drying had taken place at any drying temperature. At drying temperature > 60° C a more resistant skin was formed. In the third type (typified by droplets of skim milk and fructose) the skin appeared on the droplet surface at a certain stage of the drying process under any drying conditions. As the drying temperature was increased the resistance of the skin to mass transfer increased. The drying rate history of any material depended upon the nature of the skin formed which, in turn, depended upon the drying conditions. A mathematical model was proposed for the drying of the first type of skin-forming material. This was based on the assumption that, once all the granules gelatinised at the gelatinisation temperature, a skin appeared instantaneously on the droplet surface. The experimentally-observed times at which the skin appeared on the droplets surfaces were in excellent agreement with those predicted from the model. The work should assist in understanding the fundamentals of paniculate drying processes, particularly when skin-formation occurs and may be a crucial factor in volatiles retention.

Population lensing vs thermal lensing in Yb:YAG rods and disks

The induced lenses in the Yb:YAG rods and disks end-pumped by a Gaussian beam were analyzed both analytically and numerically. The thermally assisted mechanisms of the lens formation were considered to include: the conventional volume thermal index changes ("dn/dT"), the bulging of end faces, the photoelastic effect, and the bending (for a disk). The heat conduction equations (with an axial heat flux for a disk and a radial heat flux for a rod), and quasi-static thermoelastic equations (in the plane-stress approximation with free boundary conditions) were solved to find the thermal lens power. The population rate equation with saturation (by amplified spontaneous emission or an external wave) was examined to find the electronic lens power in the active elements.

Mathematical Modeling and Simulation of Radial Temperature Profile of Strontium Bromide Lasers

For metal and metal halide vapor lasers excited by high frequency pulsed discharge, the thermal effect mainly caused by the radial temperature distribution is of considerable importance for stable laser operation and improvement of laser output characteristics. A short survey of the obtained analytical and numerical-analytical mathematical models of the temperature profile in a high-powered He-SrBr2 laser is presented. The models are described by the steady-state heat conduction equation with mixed type nonlinear boundary conditions for the arbitrary form of the volume power density. A complete model of radial heat flow between the two tubes is established for precise calculating the inner wall temperature. The models are applied for simulating temperature profiles for newly designed laser. The author’s software prototype LasSim is used for carrying out the mathematical models and simulations.

Application of low temperature plasmas for restoration/conservation of archaeological objects

The low-temperature low-pressure hydrogen based plasmas were used to study the influence of processes and discharge conditions on corrosion removal. The capacitive coupled RF discharge in the continuous or pulsed regime was used at operating pressure of 100-200 Pa. Plasma treatment was monitored by optical emission spectroscopy. To be able to study influence of various process parameters, the model corroded samples with and without sandy incrustation were prepared. The SEM-EDX analyzes were carried out to verify corrosion removal efficiency. Experimental conditions were optimized for the selected most frequent materials of original metallic archaeological objects (iron, bronze, copper, and brass). Chlorides removal is based on hydrogen ion reactions while oxides are removed mainly by neutral species interactions. A special focus was kept for the samples temperature because it was necessary to avoid any metallographic changes in the material structure. The application of higher power pulsed regime with low duty cycle seems be the best treatment regime. The low pressure hydrogen plasma is not applicable for objects with a very broken structure or for nonmetallic objects due to the non-uniform heat stress. Due to this fact, the new developed plasmas generated in liquids were applied on selected original archaeological glass materials.

PHYSICA: A multiphysics computational framework and its application to casting simulations

Metal casting is a process governed by the interaction of a range of physical phenomena. Most computational models of this process address only what are conventionally regarded as the primary phenomena – heat conduction and solidification. However, to predict other phenomena, such as porosity formation, requires modelling the interaction of the fluid flow, heat transfer, solidification and the development of stressdeformation in the solidified part of the casting. This paper will describe a modelling framework called PHYSICA[1] which has the capability to stimulate such multiphysical phenomena.

Modeling and analysis of thermoelectric energy conversion efficiency in nanostructures.

116 p.

Introduction to the theory of Fourier's series and integrals and the mathematical theory of the conduction of heat,

Bibliography: Fournier's series: p. [411]-418; The conduction of heat: p.[419]-429.