Dissertationen zum Thema „Ionised gases“

The electron energy distribution function (EEDF) in a microwave discharge in hydrogen was calculated and measured in the pressure range 1 to 100 Torr. Other discharge parameters such as electron density, H atom density and gas temperature were also measured. More generally7 diagnostics suitable for discharges in H2 at moderate pressure were developed and basic data to aid these diagnostics was obtained. The EEDF was calculated using the two term approximation to the Boltzmann kinetic equation subject to the constraint of ionization balance. The influence of the excited states of H2 and H was investigated and it was concluded that collisions with vibrationally excited H2 had significant efiects on the EEDF but only at gas temperatures above 1500 K. Criteria for estimating the importance of superelastic collisions with excited species were also proposed. The effects of variations in the electron density, H atom density and gas temperature on the EEDF were also studied. The gas temperature was measured in two ways: from rotational structure in the G —B emission band of H2 and from the Doppler profile of ground state atomic hydrogen, measured by two photon laser induced fluorescence (LIF). The G — B rotational temperature was at least 300 K lower than the H atom temperature over the pressure range 1 to 40 Torr. The hydrogen atom density was measured using two photon LIF. A new calibration method based on decay of the fluorescence was devised for this measurement. The decay of atomic hydrogen density in the afterglow of a pulsed discharge was measured to deduce the H atom wall recombination probability 7 which was found to be about 5 X 10—3. Another estimate of ”y was obtained from measurements of Ha emission during pulsing of the discharge and was found to be about two times larger. The hydrogen atom density measurements were used to deduce quenching rates for argon by H2, which should enable wider application of argon actinometry for studying relative changes in H atom density. Three spectroscopic methods were used to test the electron kinetic model. The first was to measure the populations of the n = 3, 4 and 5 excited states of atomic hydrogen. Generally, agreement between the measured populations and the predictions of a corona model was satisfactory. Another method based on pulsing of the discharge to separate the contributions to Ha emission by dissociative excitation of H2 and direct excitation of H was also used to test the electron kinetic model. Good agreement between theory and the electron mean energy inferred from these measurements was obtained at pressures above 10 Torr. The third method involved measurement of the H2 dissociation frequency from the time evolution of Ha emission during pulsing of the discharge and the equilibrium hydrogen atom density. Comparison with the predicted dissociation frequency showed agreement to within a factor of two. The excited state population measurements were also used to obtain quenching rate coefficients for the n = 4 and 5 excited states of H by H2. Langmuir probe methods of measuring the EEDF were also investigated (Chapter 6). Higher than expected mean electron energies were obtained with both double and single Langmuir probes. Comparison with experiments in an argon discharge led to the conclusion that the short electron energy relaxation length in H2 meant that the sampled EEDF corresponded to a region localised about the probe and that this region was probably disturbed by the probe7 leading to anomalous measurements. In general, the results indicated that a positive column model of the discharge with spatially averaged quantities is an adequate model for calculation of the EEDF.

Criteria for the confinement of plasmas consisting of a positive and negative component in Penning type traps with nested electric potential wells are presented. Computational techniques for the self-consistent calculation of potential and plasma density distributions are developed. Analyses are presented of the use of nested well Penning traps for several applications. The analyses include: calculations of timescales relevant to the applications, e.g. reaction, confinement and relaxation timescales, self-consistent computations, and consideration of other physical phenomenon important to the applications. Possible applications of a nested well penning trap include production of high charge state ions, studies of high charge state ions, and production of antihydrogen. In addition the properties of a modified Penning trap consisting of an electric potential well applied along a radial magnetic field are explored.

Thesis (Ph. D.)--University of Sydney, 2009.
Includes graphs and tables. Cotutelle thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Complex Plasma Laboratory, School of Physics, Faculty of Science, University of Sydney and the degree of Docteur de l'Université Orléans. Title from title screen (viewed May 5, 2009) Degree awarded 2009; thesis submitted 2008. Includes bibliographical references. Also available in print form.

Thesis (PhD (Electrical Engineering))--Cape Peninsula University of Technology, 2018.
The generation of electricity through the conventional conversion system such as thermal and hydroelectric plants may no longer be sufficient to meet the increasing demands and usage. One of the major reasons for shortage supply of electric power is due to the lack of fossil fuel and other conventional resources that are currently being used in Africa. In addition, the conversion process of the conventional system often causes pollution which contributes to global warming. Therefore, there is a need for this research to develop novel and alternative methods of generating electric power. Among these methods is the Magnetohydrodynamics (MHD) conversion system, which is a direct energy conversion system. In this system, plasma or ionised gas is directly converted into electric power with generating efficiency of about 62 %. The conversion process of the MHD system is based on the principle of Faraday’s Law of electromagnetism and fluid dynamics. The focus of the present study is to investigate alternative methods through which an MHD power generator can be coupled to the existing thermal plants in South Africa. In doing so, the thermal cycle efficiency of these conventional plants can be improved. Another goal of this study is to investigate the behaviour of an MHD generator prototype under exposure to plasma through simulation and experimentation in a laboratory setting.

Over the past several decades, the inductively coupled plasma (ICP) has become one of the analytical chemist’s most popular tools. The principal use of the ICP has been as an excitation cell for atomic emission spectrometry (AES). More recently, it has been used as an atomization cell for atomic fluorescence spectrometry (AFS). Since the ICP is an energetic source, the vaporization process is efficient. This high temperature promotes transitions of the analyte and the argon support gas making spectral interference a problem. To alleviate this problem in AFS, it was necessary to look higher in the plasma tail, now the entrainment of oxygen and the formation of metal oxides was thought to be occurring. It was proposed that the addition of an organic gas may reduce the metal oxides, thus increasing the free atom concentration. The addition of propane produced enhancements of AFS signals in the ICP. In this study, the addition of propane and butane depressed many AES signals. In an attempt to elucidate a mechanism for the observed discrepancies, electron number density, excitation temperature, ion temperatures, atomic emission and atomic absorption measurements were considered. The system used enabled observations to be made on the effects of organic species in the plasma without altering the analyte transport efficiency. Using atomic absorption, scatter free data was obtained for the effects of propane on the ground state atom population, and it was observed to increase the ground state atom concentration for all elements attempted, with the exception of silver. With slurry introduction into the ICP, it was possible to control the composition of the plasma tail plume. The results from the slurries indicated that molecular formations can occur in the ICP. Finally, it was determined that a relationship between excitation energy and the effects of propane existed, and the increased ground state was due to propane hindering the excitation process of the plasma.
Ph. D.

Thesis (Ph. D.)--City University of Hong Kong, 2005.
"Submitted to Department of Physics and Materials Sciences in partial fulfillment of the requirements for the degree of Philosophy of Doctor." Includes bibliographical references.

Frequency domain analysis tools have been developed to analyse simultaneous multi-point measurements of developed space plasma turbulence. The Coherence Length technique enables the scale length for plasma wave structures to be measured from magnetic field measurements. The coherence length defines a length scale for the measurement of wave phenomena. Single satellite measurements can be used, the technique becoming more reliable with higher numbers of satellites. The technique is used to identify coherence lengths for waves observed in the magnetic field near the bow shock by the dual AMPTE-UKSIAMPTE-IRM satellites, and for mirror wave structures observed in the magnetic field in the magnetosheath by the dual ISEE-lIISEE-2 satellites. The Transfer Function Estimation technique enables the transfer of energy between plasma waves to be measured, from simultaneous dual-point measurements, resulting in linear growth / damping rates and second-order wave coupling. The technique is improved by replacing the Least Squares method for inversion with Regularisation. The technique is applied to simultaneous magnetic field measurements near the bow shock by the AMPTE-UKSIAMPTE-IRM satellites, where a linear instability in the wave field is identified, which is attributed to an ion anisotropy instability, and accompanying sequence of second-order three-wave coupling processes is also identified, which dissipates the energy from the linear instability. The Wave vector Determination technique enables the identification of wave vectors from simultaneous four-point measurements. The availability of four-point measurements means that the reliance on Minimum Variance Analysis, and that of only being able to use magnetic field measurements, is removed, the wave vector can be determined unambiguously directly from the magnetic field measurements. The technique can identify between waves of different frequency, and waves at the same frequency but propagating in different directions. The technique is applied to simultaneous observations of the electric field by the four-point ii Cluster II satellites, enabling the determination of the wave vector and the identification of a mirror mode structure, solely from the electric field measurements. Chapter 1 introduces the solar-terrestrial environment, briefly describing exploration of this environment by man-made satellites and listing some aims of the analysis of data collected by the satellites. Chapter 2 elaborates on what is meant by data analysis; Spectral Transforms are introduced and described, with a comparison made between Fourier Transforms and Wavelet Transforms, before a review is made of current data analysis techniques for satellite data. Chapter 3 defines and focuses attention on the objectives of this thesis, which are addressed in the following three chapters. Chapter 4 investigates the coherence length of plasma waves through use of the Wavelet Transform and the Fourier Shift Theorem. Chapter 5 makes estimates of wave Transfer Functions, replacing an established Least Squares inversion technique with a Regularisation inversion. Chapter 6 uses a method to determine wave propagation directions, from multi-satellite data, that has not been applied before due to the lack of availability of suitable data sets. Chapter 7 summarises the preceding chapters. The Appendices contain reprints of papers resulting from, and relating to, this research.

In this work, we investigate numerically the evolution of perturbed Vlasov equilibria. according to the full nonlinear system with particular emphasis on analyzing the asymptotic states towards which the system evolves. The simulations are carried out with the numerical code that we have implemented on the Cray X-MP of the Pittsburgh Supercomputing Center and which is based on the splitting scheme algorithm. Maxwellian symmetric and one-sided bump-on-tail and two-stream type of equilibrium distributions are considered: the only distribution which seems to evolve towards a BGK equilibrium is the two-stream while the asymptotic states for the other distributions are better described by superpositions of possible BGK modes. Perturbations with wave-like dependence in space and both symmetric and non-symmetric dependence on velocity are considered. For weakly unstable modes, the problem of the discrepancy between different theoretical models about the scaling of the saturation amplitude with the growth rate is addressed for the first time with the splitting scheme algorithm. The results are in agreement with the ones obtained in the past with less accurate algorithms and do not exhibit spurious numerical effects present in those. Finally, collisions are included in the splitting scheme in the form of the Krook model and some simulations are performed whose results are in agreement with existing theoretical models.
Ph. D.

This thesis describes several aspects of the development of an ignition system for modern pulverised. coal fired furnaces. This ignition system is novel in that its primary fuel is pulverised coal ignited directly by an electric arc plasma torch. The project has many facets including: plasma generation; pulverised coal collection, storage and delivery; and pulverised coal combustion using plasma as an ignition source. The first aspect of experimenta l work described herein is directed toward the production of a stable, highenthalpy, nitrogen plasma jet. Stability of the plasma jet was gained through modifying the geometry of the plasma torch anode. Evidence is provided which illustrates the nature of arc movement within the tubular anode of a plasma torch. It is shown that in regard to stability, the most important movement of the arc is that of restriking or shunting. The causes of shunting are investigated and a case is presented to suggest that the principal factor in the cause of arc restrike is the production of N2 ions in the gas surrounding the arc. Details are also given on the design of a plasma torch which can operate at the required. power level without water cooling of the cathode. Further to the design of the plasma torch itself, details are provided on the development of a plasma torch power supply which can contend with the rapid changes in arc voltage whilst maintaining an accurately regulated arc current. Information is also provided on several aspects of the pulverised coal feeding system. Novel techniques and components have been developed to provide the plasma igniter with a smooth, continuous flow of pulverised coal. When combined to form an ignition system, the plasma torch and a suitable pulverised coal supply may supplant the use of volatile fuels such as fuel oil and gas in many industrial applications and could have a significant impact on the development and use of plasmas as an industrial tool.

Spectroscopic investigations have been carried out on an argon inductively coupled plasma operating at non-atmospheric pressure. The relationship between torch pressure and a number of plasma operating characteristics was explored for torch pressures between 100 and 3000 torr. The plasma operating characteristics examined include observed analyte emission intensities, electron densities, ion to atom ratios, and the deviation of plasma conditions from local thermodynamic equilibrium. The effect of pressure on the observed analyte emission intensities was found to include factors in addition to the change in density of species within the torch. Emission lines originating from ions and atoms with high ionization potentials (greater than 7 eV) increased in intensity with increasing torch pressure, in excess of that predicted by the increase in density of species present. Conversely, emission lines originating from atoms of low ionization potential decreased in intensity with increasing torch pressure despite the increase in density. The results of the spatial determination of electron densities and ion to atom ratios indicate that excitation conditions within the central channel of the plasma are shifted towards conditions of local thermodynamic equilibrium as the pressure within the torch is increased. In addition, it is possible to obtain improved limits of detection by optimizing the torch pressure for the analyte element of interest.

Existe un extenso bagaje respecto al comportamiento de la materia tanto a nivel atómico y molecular como a nivel microscópico. Sin embargo, aún queda mucho por estudiar de las propiedades a escala nanométrica, donde las dimensiones de los sistemas son similares a las longitudes características de muchos fenómenos y procesos. Para el caso de las nanopartículas, han despertado gran interés propiedades tales como: la gran relación superficie/volumen que poseen, que provoca una reactividad química muy elevada; su elevada energía libre superficial, que causa una reducción en el punto de fusión; que los efectos del estrés en la superficie originan diferencias en la red cristalina, respecto del material en volumen; o que la presencia de nanoestructura puede dar lugar a una naturaleza electrónica única donde se modifican las propiedades electro-ópticas, ya que se rigen por las leyes de la mecánica cuántica en lugar de las de la física clásica que rige los materiales a granel. Esta tesis se centra en la obtención de nanopartículas de silicio a través de técnicas de PECVD a baja presión y temperatura ambiente, complementado por la caracterización morfológica (SEM-TEM), estructural (HRTEM-SAED-Raman) y el establecimiento del estado superficial de las mismas (TG-DTA-MS). Su principal objetivo es la generación de nanopartículas esféricas, de diámetros definidos y bajas dispersiones, a fin de controlar específicamente las propiedades superficiales y ópticasde las mismas. La optimización del proceso de generación y recolección de las muestras se consigue a través de la implementación de una recolección por plasma remoto y el uso de flujo laminar secuencial, con modulación del plasma sincronizado, con lo que se logra un mayor control de tamaño y producción máxima de nanopartículas. A través de un diseño experimental de Plackett-Burmanse evalúan los parámetros tecnológicos más importantes de nuestro sistema, que inciden sobre el tamaño de las nanopartículas y su respectiva dispersión. Su evaluación exhibe la importancia de los periodos de modulación de plasma para controlar el tamaño de las nanopartículas y determina a la presión como el único factor significativo para controlar la dispersión de las mismas. Se estudia un modelo cinético para la formación de las nanopartículas, a partir de analogías con la teoría clásica de la condensación de gases supersaturados y considerando la formación de núcleos como resultado de la disociación del silano y moléculas relacionadas. Al comparar estos cálculos con resultados experimentales se establecen las influencias de los distintos parámetros tecnológicos sobre el tamaño de las nanopartículas y se consigue el control del mismo, a través del tiempo de encendido del plasma, para rangos entre 2-15 nm, considerando dispersiones de hasta 10 %. Se establece que la formación de las nanopartículas está dominado por un proceso de coagulación, lo que provoca un alto ritmo de nucleación y crecimiento, hasta alcanzar el tiempo de residencia del gas en la cámara de descarga, donde la concentración de partículas decae, y se pasa a una fase de crecimiento más lenta, dominada por el aporte de monómeros sobre la superficie de las mismas. Además de que, la estructura de las nanopartículas está controlada por la temperatura durante su formación y ésta a su vez depende del bombardeo iónico presente en el plasma, así como de la cantidad de núcleos que se formen. Observaciones realizadas sobre el envejecimiento de nanopartículas en solución, indican que las nanopartículas recolectadas fuera del plasma y almacenadas en etanol, son las más estables (al menos 4 años). Esto queda determinado por menor estado de aglomeración y su oxidación natural al primer contacto con el ambiente, lo que permite una funcionalización, sobre la superficie de las nanopartículas, que confieren derivados del etanol. Se estudian los estados superficiales de las nanopartículas bajo tratamientos térmicos y se define un alto contenido de hidrógeno presente en la estructura polimérica y porosa de las nanopartículas, así como se revela la cristalización de las nanopartículas amorfas y que su cáscara oxidada evita la sinterización de los cristales (al menos hasta los 1000 ºC). Se establecen los elementos adsorbidos en las nanopartículas, así como su efusión (hasta 200 y 400 ºC, respectivamente), lo que una pérdida de masa total de alrededor del 38 %. Las nanopartículas poseen también hidrocarburos en su superficie, generados al primer contacto con el aire, los cuales se eliminan entorno a los 350 ºC y la efusión de su contenido de hidrógeno se produce en dos etapas distintas. La primera corresponde a hidrógenos superficiales, desorbidos en un tramo entre 400 y 600 ºC, y la segunda (la mayor cantidad de hidrógeno), correspondiente a hidrógeno inmerso al interior de las nanopartículas, presenta su desorción entorno a 1000 ºC. La segunda etapa de desorción de hidrógeno provoca la re-estructuración de las nanopartículas y el material resultante corresponde a centros cristalinos de silicio cúbico (5 % del material), cubiertos de una cascara oxidada que genera un estrés extensivo. Finalmente se estudian propiedades evaluadas para algunas aplicaciones. En el caso de superficiales, con un depósito de nanopartículas amorfas se consiguen tanto superficies superhidrofóbicas, con ángulos de contacto de 170º y con características de autolimpieza, como superficies superhidrofílicas, de ángulos de contacto de menos de 2º. En el caso de asociadas a su luminiscencia, se establece el mecanismo de emisión de la luminiscencia de las partículas cristalinas, el cual se rige por un efecto de confinamiento cuántico sumado a un corrimiento originado por las tensiones a las que están sometidas las nanopartículas, así como un corrimiento hacia el rojo, provocado por el etanol de la solución.
Production of Monodisperse Si Nanoparticles Obtained by Modulated Plasma There is much remains to study of the properties of materials at the nanoscale, where the dimensions of the systems are similar to the characteristic lengths of many phenomena and processes. In the case of nanoparticles, great interest have attracted properties such as: the large surface/volume ratio possessing, which causes a very high chemical reactivity; their high surface free energy, which causes a reduction in melting point that the effects of stress on the surface differences originate in the crystal lattice, with respect to the bulk material, or that the presence of nanostructure can lead to unique electronic nature where they modify the electro-optical properties, as they are governed by the laws of quantum mechanics rather than classical physics that governs the bulk materials. This thesis focuses on the production of silicon nanoparticles via PECVD techniques at low pressure and room temperature, supplemented by morphological (SEM-TEM) and structural (HRTEM, SAED, Raman) characterization, and the establishment of the surface state of thereof (TG-DTA-MS). The main purpose is the generation of spherical nanoparticles of defined diameters and low dispersions, specifically to control their surface and optics properties. The optimization of the generation process and the collection of samples is accomplished through the implementation of a remote plasma collection and use of laminar flow sequentially synchronized modulation of plasma, which achieve greater control of size and maximum production of nanoparticles. The evaluation of an experimental design of Plackett-Burman shows the importance of plasma modulation periods to control the size of the nanoparticles and determines the pressure as the only significant factor for controlling their dispersion. We studied a kinetic model for the formation of nanoparticles based on analogies with the classical theory of supersaturated-gas condensation and considering the nuclei formation as the result of the dissociation of silane and related molecules. Comparing these calculations with experimental results were established the influence of technical parameters on different nanoparticle size and the control of it is achieved through the time of plasma ignition, for ranges between 2-15 nm, considering dispersions up to 10 %. It is established that the formation of nanoparticles is dominated by a coagulation process, which causes a high rate of nucleation and growth, until reach the residence time of the gas in the discharge chamber, where the particle concentration decays, and passes to a slower growth phase dominated by the contribution of monomers on their surface. Besides that, the structure of the nanoparticles is controlled by the temperature during formation and this in turn depends on the ionic bombardment in plasma as well as the number of nuclei that are formed. Observations on aging of nanoparticles in solution indicate that the nanoparticles collected out of the plasma and stored in ethanol are the most stable, for at least 4 years. This is determined by a lower state of agglomeration and the natural oxidation that nanoparticles suffer at the first contact with the atmosphere, allowing the surface functionalisation of nanoparticles conferred by ethanol derivatives. We studied the surface states of the nanoparticles under heat treatments and establishes a high content of hydrogen present in the porous polymeric structure of the nanoparticles. It also discloses the crystallization of amorphous nanoparticles and that their oxidized shell prevents the crystals sinterization (at least up 1000 °C). Physisorbed and chemisorbed elements on the nanoparticles surface are established as well as their effusion (up to 200 to 400 °C, respectively), which involves a total mass loss of about 38 %. Nanoparticles also have hydrocarbons on its surface, that were generated at the first contact with the air, and which are eliminated around 350 °C. The effusion of the hydrogen content in the nanoparticles are detected in two distinct stages. The first stage corresponds to surface hydrogens, which are desorbed in a section between 400 and 600 °C. The second stage, the greater amount of hydrogen corresponds to hydrogen immersed into nanoparticles and their desorption environment presents to 1000 °C. This last desorption cause the restructuring of the nanoparticles and the resulting material corresponds to crystalline cubic silicon centers (5 % of the material) covered by an oxide shell which generates an extensive stress. Finally properties evaluated are studied for some applications. In the case of surface with a deposit of amorphous nanoparticles are achieved both superhydrophobic surfaces with contact angles of 170° and with self-cleaning characteristics as superhidrofílicas surfaces, contact angle less than 2°. For its luminescence associated sets the emission mechanism of the luminescence crystal particle, which is governed by a quantum confinement effect in addition to a shift caused by the stresses to which are subject the nanoparticles.

Plasma Polymerization is a novel technique for the preparation of polymer-like thin film coatings at low temperatures onto almost any type of substrates: plastic, metal, semiconductors, wood, textile fibers or membranes, to cite just a few. The films can be grown directly from liquid monomers that are introduced in the vapor phase into a vacuum chamber equipped with one or more electrodes that generate the plasma after a high voltage in continuous current mode (DC), low-frequency (AC) or high frequency (RF) is applied. The plasma state is a high-energy gas state in which the density of electrons, ions, excited species and radical fragments is abundant. The introduction of an organic monomer vapor into the plasma triggers the formation of molecular fragments capable of initiating multiple reactions: in the gas phase, recombination of radicals, oligomerization of high-weight molecules and aggregation into nanoscopic dust can occur, whereas adsorption and reaction onto any solid surface will result in the growth of highly adherent thin films. The structural, chemical and functional properties of these coatings are determined by the composition of the precursor gas mixture and the type of monomer, and also by several technological parameters that can be fine-tuned, such as the pressure, plasma power, frequency of change of electrode polarization, substrate location, flux of gas, etc... By controlling these technological parameters it is possible to modulate the value of the magnitudes that govern the physico-chemical mechanisms which are responsible for film growth: residence time of molecules, available energy per molecule, degree of monomer fragmentation, density and energy of ion bombardment on the substrates, and gas transport in the reactor, among others. Plasma polymerization allows to grow films from virtually any kind of organic molecule which can be evaporated at low temperatures (<80 °C) and introduced in the reactor at sufficient flow rates (> lsccm), even when that molecule would not be the characteristic repeating unit of any conventional polymer synthesized by other physical or chemical means. The technique is also applicable to other types of monomers (non-carbon based), such as organosilicon or organometallic molecules. The use of organosilicon monomers allows to obtain films with a wide spectrum of properties, from those frequently attributed to an elastomeric polymer such as silicone (polydimethylsiloxane, PDMS) to those associated to a hard inorganic material, such as glass (amorphous silicon dioxide, silica). Regardless of their apparently opposed nature, these two materials share an extremely similar chemical backbone based on silicon-oxygen chemical bonds. During the investigations conducted in our study, different organosilicon monomers have been employed for plasma polymerization: hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDSN) and tetraethoxysilane (TEOS), but the only results presented within the scope of this Thesis are those obtained for ppHMDSO films. This is due to the fact that HMDSO is the only monomer allowing the growth of polymer-like films, inorganic-like films, intermediate stoichiometry films and even graded films with properties varying with depth in a single plasma process. With respect to power sources for plasma generation, most published works choose high frequency electrical power sources, such as radiofrequency (RF) or microwaves (MW), although plasma polymerization can also be carried on with direct-current high-voltage sources (HV-DC), from 500 V to 3000 V. In our investigations, these three types of sources have been employed, as can be found in our related publications, but again only results with the DC plasma source will be presented due to their simple design and use in industrial applications. A main objective of this Thesis is to establish its limitations, such as the limited film thickness attainable or the excessive heating of substrates, depending on the reactor configuration and the operating parameters. As a consequence, the scope of this Thesis covers two main objectives: first, the study of DC plasma polymerization of hexamethyldisiloxane (DC ppHMDSO) with and without addition of carrier gases in the precursor mixture, in order to obtain polymer-like or inorganic silica-like coatings with specific mechanical, optical and corrosion protective functional properties, for further application to solving some practical problems of industrial interest; secondly, the study of modifications produced by a different non-additive post-treatments in a polymer-like ppHMDSO film, in order to obtain a film with graded properties varying with depth.
La polimerització assistida per plasma és una tècnica novedosa per a la obtenció de recobriments polimèrics a baixa temperatura sobre qualsevol tipus de substrat: plàstic, metall, semiconductors, fusta, fibra tèxtil o membranes, per citar-ne només alguns. Els recobriments són obtinguts directament a partir de monòmers líquids que s'introdueixen controladament en fase vapor dins una cambra de buit equipada amb un o més elèctrodes que, en aplicació d'una tensió elèctrica constant (DC), alterna (AC) o d'alta freqüència (RF, MW), generen el plasma. Les propietats estructurals, químiques i funcionals d'aquests recobriments venen determinades per la composició de la mescla gasosa precursora i la naturalesa del monòmer, i per diferents paràmetres tecnològics controlables, com ara la pressió, la potència acoplada al plasma, la freqüència d'oscil•lació de la polarització, la posició dels substrats, el flux circulant de gas, etc. L'ús de monòmers orgànics de base silici permet obtenir propietats amb característiques molt amples, des de les més pròpies d'un polímer elastomèric com la silicona (polidimetilsiloxà, PDMS) a les d'un material inorgànic dur, com el vidre (òxid de silici amorf, Si02. Aquests dos materials comparteixen una base química extraordinàriament similar fonamentada en un esquelet d'enllaços Silici-Oxigen. Durant el treball de desenvolupament d'aquest estudi s'han emprat diferents monòmers de base silici: l'hexametildisiloxà (HMDSO), l'hexametildisilazana (HMDSN) i el tetraetoxisilà (TEOS), però només es presentaran els resultats obtinguts en els dipòsits de polimerització plasma del primer, HMDSO, degut a ser l'únic amb capacitat de generar recobriments de caire polimèric, inorgànic, intermedi o fins i tot amb propietats variables amb la profunditat en un únic recobriment. L’objectiu principal d’aquesta tesi és l’estudi de la polimerització assistida per plasma en corrent contínua de l’hexametildisiloxà (DC ppHMDSO) amb i sense addició de gasos portadors, per a l’obtenció de recobriments polimèrics o inorgànics de base silici amb especials propietats funcionals mecàniques, òptiques i protectores contra la corrosió, i l’aplicació pràctica d’aquests recobriments a la solució d’alguns problemes d’interès industrial. El segon objectiu és l’estudi de les modificacions de les capes de naturalesa polimèrica ja dipositades mitjançant un segon plasma o post-tractament sense contingut de monòmer per tal de modificar la superfície del recobriment i aconseguir un gradient en profunditat de les propietats del material.

The goal of the present investigation is to examine the processing-structure-property relationships of multilayer graphene nanowall materials. Various plasma enhanced chemical vapor deposition (PECVD) processing parameters were altered to control the structure and morphology of the material. Growth parameters and substrate material were the major structural features studied, and these were characterized by spectroscopic techniques. The direct synthesis of graphene without catalysis on dielectric substrates, compatible with the complementary metal oxide semiconductor technology, is a stimulating but complex task. The goal of the thesis has different tasks consisting of: a) The design and construction of a new inductively coupled plasma remote chemical vapor deposition reactor in the PECVD-FEMAN laboratory of the Universitat de Barcelona. b) Fabrication of vertical graphene nanostructures at low temperature on different conductive and nonconductive substrates. c) Characterization of the vertical graphene obtained through different synthesis parameters in order to optimize their physical and surface properties; such as structural and morphological studies by Raman spectroscopy, SEM and TEM. d) Functionalization of MLGNWs by MnO nanoparticles for hybrid supercapacitor systems. The thesis consists of the following main parts: In the first part of the thesis provides a brief introduction of carbon materials, graphene, graphene nanowalls and their history, discovery, outstanding properties and all the technologies that prompted their development during these years until the first application. Moreover, in this section the methods for the synthesis of carbon nanostructures and brief explanation of the fundamentals of each technique explains. In the second part the concepts and technologies of plasma, plasma enhanced chemical vapor deposition (PECVD) and PECVD related techniques are exposed. In addition, in this section a deposition reactor designed by us are described, where all experiments carried out during this thesis took place. Also, the basics and work principles of different characterization techniques are briefly described. Furthermore, this part discusses the growth mechanism of MLGNWs synthesized by PECVD. In the third part, which is the main results part, discusses the study of growing material along the entire length of the tube and the importance of sample location inside a tubular quartz reactor. The influence of the substrate material, growth time and growth temperature on the MLGNWs growth process have been examined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques. The chemical characteristics of as grown structures were studied by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectrometer. The hydrogen and carbon contents in grown samples were determined by elemental analysis (EA). To study the photoluminescent properties of the carbon structure grown in the whole length of the tubular reactor, the room temperature PL spectra were conducted. In addition, the chemical reactions inside the tube under plasma deposition were controlled by optical emission spectroscopy (OES). Also, in the part of results synthesis and characterization of MLGNWs/CNTs hybrid structure are discussed. The goal of MLGNWs/CNTs hybrid structure is increased chemical activity of CNTs. The morphological and structural characterization was carried out using SEM, High resolution TEM and Raman scattering analysis. Electrochemical properties of transferred MLGNWs/CNTs were studied by CV and charge/discharge measurements. The last section of the part of the results are exposed information about application of MLGNWs in supercapacitors, in particular, supercapacitive performance of manganese dioxide/ graphene nanowalls electrodes deposited on stainless steel current collectors and annealing temperature effect are discussed. Composite electrodes MLGNWs/MnO2 were characterized by FESEM and Raman shift spectroscopy. The electrochemical properties of the MLGNWs/MnO2 for supercapacitor applications were investigated by cyclic voltammetry (CV), charge/discharge and electrochemical impedance spectroscopy (EIS). The influence of annealing temperature on the electrochemical performance has been studied as well.
El grafeno, como material basado en el carbono, es un logro del desarrollo y los avances de la Nanotecnología. La síntesis directa de grafeno sin catálisis sobre sustratos dieléctricos, compatible con la tecnología de los semiconductores complementarios de óxido metálico, es una tarea estimulante pero compleja. La técnica PECVD, permite la síntesis directa de nanoestructuras de carbono a temperaturas más bajas y es el método principal utilizado en esta tesis. El objetivo de esta tesis es la síntesis y optimización de nanoparedes verticales de grafeno y su posible extensión a aplicaciones en sistemas que requieran superficies macroscópicas. Para ello, se han realizado diferentes tareas: a) Se ha diseñado y construido un reactor prototipo con plasma remoto en el laboratorio PECVD-FEMAN de la Facultad de Física (Universidad de Barcelona) con el fin último de crecer grafeno en forma de paredes/tabiques verticales nanométricos mediante la técnica PECVD. b) Se ha desarrollado un proceso PECVD modificado con el fin de mejorar los resultados actuales en términos de: 1) el tiempo de crecimiento, 2) la temperatura, 3) la naturaleza del substrato, 4) la presión, y 5) la cantidad de gas precursor para crecer grafeno vertical. Las muestras obtenidas fueron caracterizadas mediante microscopía TEM, SEM, XPS, XRD y mayormente mediante espectroscopia Raman, con el objetivo de optimizar el proceso y las propiedades físico-químicas y del grafeno vertical. c) Se ha desarrollado una estructura híbrida con nanoparedes y nanotubos de carbono. Para ello, se utilizaron tres equipos: el reactor “PEDRO” para la preparación del substrato, el reactor “CNTs” para el crecimiento de nanotubos de carbono y el reactor ICP-CVD para el crecimiento de nanoparedes de grafeno. En esta tesis se investigaron las caracterizaciones morfológicas y electroquímicas, pero aún se necesitan más estudios para confirmar posibles futuras aplicaciones. d) Para mejorar las propiedades de los supercapacitores basados en los electrodos desarrolladas con nanoparedes de grafeno y acero inoxidable, se ha realizado el crecimiento de capas delgadas de MnO2 mediante el método de electrodeposición. El efecto de la temperatura de recocido (annealing) en las propiedades electroquímicas de las muestras se ha estudiado en el rango de 70° C a 650° C.

Thesis (Ph. D.)--Indiana University, Dept. of Chemistry, 2006.
"Title from dissertation home page (viewed June 28, 2007)." Source: Dissertation Abstracts International, Volume: 67-06, Section: B, page: 3105. Adviser: Gary M. Hieftje.