Orientador: Marco Antonio Robert Alves / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e Computação / Made available in DSpace on 2018-08-05T19:44:58Z (GMT). No. of bitstreams: 1 Porto_LesnirFerreira_M.pdf: 4190794 bytes, checksum: 1dec9fdc0b3dd1bffb34d78bccf2648b (MD5) Previous issue date: 2005 / Resumo: Este trabalho de mestrado teve como objetivos a fabricação e o estudo do comportamento elétrico das ponteiras de silício de emissão de campo a vácuo (PECV) recobertas com filme fino de carbono tipo diamante (DLC). Apresentamos o processo de fabricação das ponteiras de silício que é realizado através das etapas de fotolitografia, corrosão por íon reativo no plasma de SF6 (hexafluoreto de enxofre), oxidação térmica seca para afinamento, e deposição do filme DLC por PECVD (Plasma Enhanced Chemical Vapor Deposition). Mostramos os resultados obtidos da caracterização elétrica das ponteiras sem o filme e com o filme DLC, através do levantamento das curvas características I x V (corrente x tensão) e I x t (corrente x tempo). Verificamos que as curvas I x V obedeceram ao modelo de emissão de elétrons de Fowler-Nordheim. Comparamos estes resultados a fim de avaliarmos as mudanças na tensão de limiar, corrente emitida, e estabilidade de emissão. Neste estudo fabricamos PECV recobertas por filme DLC com espessura de aproximadamente 170 Å / Abstract: The objectives of this dissertation were the fabrication of silicon field emitter tips coated with diamond like carbon (DLC) thin films, and the study of its electrical behavior. We present the fabrication process of silicon tips that consists on four stages: photolithography, reactive ion etching SF6 plasma, thermal oxidation for sharpening, and the DLC deposition by PECVD (Plasma Enhanced Chemical Vapor Deposition). We show results obtained from the electrical characterization of tips without film and tips with DLC, by the characteristics curves I x V (current x voltage) and I x t (current x time). Current-voltage measurements followed a Fowler-Nordheim electron emission behavior. We compare these results to evaluate the change of the threshold voltage, emitted current, and emission stability, as a function of the coating with the film. In this study we fabricated silicon tips coated with DLC film with thickness of approximately 170 Å / Mestrado / Eletrônica, Microeletrônica e Optoeletrônica / Mestre em Engenharia Elétrica

Made available in DSpace on 2014-10-09T12:34:43Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:01:12Z (GMT). No. of bitstreams: 0 / Na elaboração do planejamento do tratamento de câncer com radiações ionizantes, o médico radioterapêuta, através dos protocolos clínicos, determina a dose de radiação diária para cada tipo específico de tumor e, junto com o físico, durante os procedimentos de simulação dos campos de tratamento, fazem a localização das áreas a serem tratadas. Em alguns casos, os campos de radiação apresentam dimensões extensas visando englobar todo o volume alvo, o que pode exigir a proteção de regiões anatômicas e órgãos vitais localizados no interior da área irradiada ou mesmo circunvizinhas ao volume alvo, a fim de se garantir o limite de dose absorvida tolerável por estes órgãos. Em geral, estes órgãos críticos localizam-se fora do eixo central do feixe de radiação, até mesmo próximo da periferia do campo, justificando a importância da determinação da dose de radiação em pontos situados fora do feixe central e do isocentro de tratamento, buscando dimensionar as colimações de proteção que dependem do seu posicionamento, da dose de tolerância do ponto anatômico e dos parâmetros radiométricos do equipamentos de radiação utilizados. Este trabalho apresenta uma análise da distribuição de dose absorvida em pontos situados fora do eixo central do feixe de radiação durante procedimentos de radioterapia com campos extensos e irregulares. O código computacional MCNP5 foi usado para construir duas modelagens do cabeçote de um acelerador linear clínico, utilizado como fonte de radiação, e simular o perfil radiométrico do feixe de tratamento para campos irregulares e alargados. Foram realizadas medidas experimentais da curva de Porcentagem de Dose Profunda (PDP) e perfil de dose utilizando câmara de ionização, detectores de diodos e filmes radiográficos. Os valores experimentais foram comparados com os perfis de dose simulados para realização do processo de validação dos cálculos. Após a validação, casos clínicos foram simulados como forma de aplicação da metodologia apresentada. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Orientador: Fernando Alvarez / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T22:51:20Z (GMT). No. of bitstreams: 1 SaezAcuna_JoseJavier_D.pdf: 8414775 bytes, checksum: 9dca5b5e710ad86a9f6375002fc43faa (MD5) Previous issue date: 2009 / Resumo: Este trabalho apresenta um estudo sobre a Emissão Eletrônica de amostras nanoestruturadas de Carbono sobre a ação de um campo elétrico controlado. Particularmente foram estudadas nanoes- truturas de Carbono tipo Nanotubos (CNTs). Com o intuito de aprimorar as propriedades elétricas destas nano~struturas, também são apresentados estudos sobre os efeitos da irradiação com feixes iônicos de 02+ sobre a superfície local dos emissores. Este processo, que por um lado é um pós- tratamento de purificação, ou remoção seletiva dos defeitos estruturais, e, ao mesmo tempo, um pós- tratamento de dopagem da superfície, ajudando assim na emissão eletrônica. Nos processos de pós-tratamento foram utilizados íons de 02+, N2+, H2+ e combinações destes, todos irradiando com um feixe iônico controlado e in situ após crescimento das nanoestruturas. Sem quebra de vácuo, estas irradiações foram analisadas por espectroscopia de elétrons fotoemitidos por raios-X incidentes, técnica conhecida como XPS. Uma vez as amostras fora do vácuo, elas foram anali- sadas por espectroscopia Raman, microscopia eletrônica de varredura (FEG-SEM) e microscopia eletrônica de transmissão de alta resolução (HR- TEM). Para a análise da Emissão Eletrônica várias complicações indesejáveis foram com sucesso supe- radas, como, por exemplo, efeitos de borda dos eletrodos que nos levam a cálculos errados da área efetiva de emissão; incertezas do número local de pontas emissoras produto do baixo controle do crescimento das nanoestruturas; como também muitas controvérsias sobre a forma correta de se analisar os dados experimentais. Em relação a este último ponto fizemos um experimento chave no discemimento do correto uso do modelo teórico para a Emissão Eletrônica a partir de sua dedução feita por Fowler e Nordheim. Demonstraremos que o campo elétrico limiar (the threshold electric field), o parâmetro usado para comparar os dados experimentais, não é um parâmetro confiável. Isto foi observado ao medirmos uma única amostra mudando apenas a configuração geométrica do eletrodo usado para efetuar as medições. Este estudo detalhado deixou em evidência fenômenos da emissão que antes não eram perceptíveis, e nos permitiu encontrar a forma correta de analisar os dados expe- rimentais. Propomos então uma norma comparativa para a Emissão Eletrôn1ca, a qual utilizamos nesta Tese. Esta norma correlaciona-se muito bem com todas as outras técnicas acima mencionadas / Abstract: This work presents a study of the Electron Field Emission of carbon nanostructured samples on the action of an Electric Field controlled. The Carbon Nanotubes (CNTs) was the nanostructures studied. In order to improve the electrical properties of these nanostructures are also presented studies on the effects of irradiation with O2+ ion beams on the surface emitter. This process, which is a post-purification treatment, or selective removal of structural defects, is also a doping-treatment of the surface helping in electronic properties. In cases of ion bean post-treatment were used O2+, N2+, H2+ ions and combinations of these, all radiating with an ion beam controlled in situ after growth of the nanostructures. Without breaking vacuum, the samples were analyzed by X-ray photoelectron spectroscopy, technique known as XPS. Once the samples were outside the vacuum, they were analyzed by Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (FEG-SEM), and high-resolution transmission electron microscopy (HR- TEM). For the analysis of the Electron Emission properties several undesirable complications have been successfully overcome, such as edge effects of the electrodes that lead to miscalculations of the effective area emission, uncertainties in the local number emitter tips due to poor control of the growth of nanostructures, as well as many controversies about the correct analysis of experimental data. On this last point, we have made a key experiment to discem the proper use of the theoretical model for Electron Emission from the deduction by Fowler-Nordheim. Here we demonstrate that the Threshold Electric Field, the parameter used to compare the experimental data, is not a reliable parameter. This was found when we made our measurements on one single sample, only changing the geometric configuration of the electrode used in the measurement. This performed with meticulous care and detail has left evidence of emission phenomena that were not obvious, and allowed us to find the correct way to analyze experimental data. From here we propose a comparative standard parameter for Electron Emission properties, and that we use in this thesis correlating very well with all the other techniques mentioned above / Doutorado / Física de Plasmas e Descargas Elétricas / Doutor em Ciências

This is the first study comparing morphologies of CNT films on Kanthal wire, with their field emission properties, and as such offers ways to design better cylindrical emitter devices. A low turn-on field was achieved (0.35 V/µm), the field emission results have been explained using a simple model, and a fluorescent lamp was fabricated. Whilst previous work has been done on the link between “as grown” CNT films and their respective field emission properties on flat substrates, very little work has been done on linking morphology to emission performance on wire substrates, where the morphology can be very different. Microscopic structures such as towers, ridges and clumps consisting of many aligned or entangled CNTs were grown using an aerosol chemical vapour deposition (a-CVD) technique. Hydrogen added to the carrier gas resulted in a decrease in defect density in the growth of undoped CNTs, and an increase in defect density in the growth of nitrogen doped CNTs (N-CNTs) and boron doped CNTs (BCNTs). In-situ transmission electron microscopy (TEM) studies show that damage to CNT tips results in a significantly higher turn-on field compared to undamaged tips. This can be recovered by making the CNT emit current for several minutes which makes the tip recrystallize due to heat caused by the Nottingham effect. The field emission properties of the “as grown” CNT films are dominated by protruding CNTs found at the edges of ridge and tower microscopic structures. The field emission properties are also related to the dimensions of these structures with the longest ridges (hence those with the longest protruding CNTs) resulting in the lowest turn-on electric field. The ridge and tower structures act to accommodate protruding CNTs at their edges and their physical dimensions (mainly width) act to separate these emitters so that screening is minimised. This work shows that efficient emitters can be fabricated effectively from simple a-CVD techniques and microscopic structures act to improve, not degrade, field emission properties.

Ce travail s’inscrit dans le cadre de la caractérisation physique de nanofils (NF) semiconducteurs (SC) qui est un domaine en plein essor ces dernières années. Plus précisément, nous explorons l’émission de champ (EC) de NFs individuels de Carbure de Silicium (SiC) pour leur potentialité comme source d'électrons, mais surtout pour étudier leurs propriétés de transport électrique, optiques et mécaniques.Le rôle important joué par la surface dans ces NFs a été prouvé par des traitements in situ qui ont eu des conséquences radicales sur l’EC dévoilant ainsi des propriétés d’émission propres aux SCs. En particulier, un régime de saturation, en accord avec la théorie d’EC des SCs, associé à une forte dépendance de l'émission à la température et à l’illumination laser a été révélé pour la première fois pour un NF. Ces mesures ouvrent des perspectives importantes tant pour la recherche fondamentale que pour les applications telles que la réalisation de photocathodes et de sources d’électrons pilotées optiquement ou par la température. Les caractéristiques courant-tension-température associées à l’analyse en énergie des électrons émis nous ont permis de déterminer le mécanisme de transport dans ces NFs, qui est limité par le nombre de porteurs dans le volume et contrôlé par les pièges présents dans la bande interdite par l’effet Poole-Frenkel. Finalement, la caractérisation mécanique a révélé des valeurs du facteur de qualité élevé (160000) et du module de Young allant jusqu’à 700GPa. Ces valeurs sont très prometteuses pour l’utilisation de ces NFs dans les systèmes nano-électro-mécaniques et dans les composites. / We use field emission (FE) from individual silicon carbide nanowires (NWs) to explore their potential as electron sources, and especially as a versatile tool for studying transport, optical and mechanical properties of NWs. These studies fall within the larger framework of the physics of semiconducting (SC) nanowires, which is presently a large and rapidly expanding domain. The important role played by the surface in the transport and optical properties of NWs was clearly demonstrated by the radical consequences induced by in situ treatments on the FE properties. This permitted the observation of the specific behavior expected for SCs, particularly, a current saturation regime in agreement with the theory of FE for SCs. We found that the saturation was concomitant with a strong dependence of the emission on temperature and laser illumination, revealed for the first time for a NF. These measurements open important perspectives for both fundamental research and applications such as the realization of optically or thermally controlled FE electron sources. The current-voltage-temperature characteristics were carried out in parallel with measurement of the energy distributions of the emitted electrons, thus permitting the determination of the transport mechanism in the NWs. We found that the transport was limited by the carrier density in the volume and by the traps in the gap that generate current through the Poole-Frenkel effect. Finally, the mechanical characterization revealed high quality factors, as high as 160,000, and a Young’s modulus up to 700 GPa. These values are very promising for the use of these NWs in nano-electro-mechanical systems (NEMS) and composites.

Beyond Atlas is a Swedish private company with the goal of exploring the solar system with cheap and reliable spacecraft. Part of their maiden mission, Hugin, aims to demonstrate navigation, propulsion, and communication technology on a 3U CubeSat. This thesis aims to investigate the feasibility of using the Enpulsion NANO electric propulsion (EP) system for deep-space applications and how to best demonstrate its capabilities in low-Earth orbit. Literature reviews of scientific papers and software simulations were conducted to gain an understanding of the underlying processes involved in EP in-orbit operations. Analyses were made on orbital maneuvers, momentum unloading, power and thermal restrictions. The results suggest that the EP system's capabilities is mainly limited by the saturation time of the reaction wheels restricting longer duration orbital maneuvers. Orbital maneuvers for demonstrating the capabilities are proposed based on the limitations imposed on the EP system by the rest of the spacecraft. On the basis of the results of this research, it can be concluded that the Enpulsion NANO thruster's operational range can be utilized both as a low thrust efficient main drive and as a high thrust maneuvering thruster for deep-space applications but is limited by the high power consumption and low thrust-to-power ratio.

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Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.