This thesis examines melting in superheated silicon films in contact with SiO₂ under pulsed laser irradiation. An excimer-laser pulse was employed to induce heating of the film by irradiating the film through the transparent fused-quartz substrate such that most of the beam energy was deposited near the bottom Si-SiO₂ interface. Melting dynamics were probed via in situ transient reflectance measurements. The temperature profile was estimated computationally by incorporating temperature- and phase-dependent physical parameters and the time-dependent intensity profile of the incident excimer-laser beam obtained from the experiments. The results indicate that a significant degree of superheating occurred in the subsurface region of the film. Surface-initiated melting was observed in spite of the internal heating scheme, which resulted in the film being substantially hotter at and near the bottom Si-SiO₂ interface. By considering that the surface melts at the equilibrium melting point, the solid-phase-only heat-flow analysis estimates that the bottom Si-SiO₂ interface can be superheated by at least 220K during excimer-laser irradiation. It was found that at higher laser fluences (i.e., at higher temperatures), melting can be triggered internally. At heating rates of 10¹⁰ K/s, melting was observed to initiate at or near the (100)-oriented Si-SiO₂ interface at temperatures estimated to be over 300K above the equilibrium melting point. Based on theoretical considerations, it was deduced that melting in the superheated solid initiated via a nucleation and growth process. Nucleation rates were estimated from the experimental data using Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis. Interpretation of the results using classical nucleation theory suggests that nucleation of the liquid phase occurred via the heterogeneous mechanism along the Si-SiO₂ interface.

Electronic properties of solids are vitally important for modern technology, one may think of microelectronics, magnetic data storage, communication technologies and others. Sometimes, it seems possible to postpone the step of fundamental understanding when starting the application of a new material. However, lasting success and discovery of principally new approaches is based on it ...

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 191-206).

Este trabalho procurou identificar o defeito que ocorria no teste de estanqueidade de uma peça e apresentar a correlação entre o nível de porosidade encontrado no produto fundido com a liga de alumínio A356 e os processos de fusão e de manutenção sob temperaturas controladas, o teor de hidrogênio, o sistema de enchimento e de alimentação, obedecendo ao princípio de solidificação direcional e o posicionamento do modelo em relação à linha de divisão. Foram efetuadas algumas experiências com diferentes tempos de introdução de nitrogênio para a purificação do metal, com a inversão do modelo em relação ao plano de partição do molde, com a modificação das dimensões do sistema de enchimento, com a substituição dos massalotes laterais por outros com luvas exotérmicas e utilizando-se resfriadores. Tentou-se verificar os benefícios que os filtros cerâmicos proporcionam aos fundidos, principalmente quanto à diminuição do nível de porosidade, menor turbulência e maior capacidade de reter as inclusões e parte dos filmes de óxidos. Para comprová-los, propôs-se a fundição de peças, em moldes confeccionados pelo processo de areia à verde, com diferentes tipos de filtros cerâmicos nos canais de distribuição. As amostras foram analisadas no Laboratório Metalográfico do Departamento de Engenharia Metalúrgica e de Materiais da Escola Politécnica da Universidade de São Paulo e no Laboratório de Metalurgia e de Materiais Cerâmicos do Instituto de Pesquisas Tecnológicas de São Paulo. Após a análise dos resultados em microscópio óptico, microscópio eletrônico de varredura e em espectrômetro por emissão de energia observou-se que o problema está relacionado principalmente com a alimentação e a solidificação direcional e não somente com o teor de hidrogênio. / The objective of this work is to study the type of defect identified during the pressure tightness test of aluminum parts and to establish a relationship between porosity level of an A356 aluminum alloy and the production processes, hydrogen level, feeding system, and directional solidification. Different nitrogen blowing times, inversion of the mould pattern, relative to the partition line, changing of the feeding system dimensions, replacement of the lateral feeding by exothermic gloves and the use of chills where tested. Ceramic filters were also tested, in order to evaluate their influence on the formation of porosities and on the porosity level, through a decrease of turbulence and an increase in the capacity of retaining non-metallic inclusions and part of the oxide films. Casting was performed by the green sand process using different ceramic filters. The obtained parts were metallographicaly analyzed through optical and scanning electronic microscopy using also EDS analysis. The results showed that porosity is related with the feeding conditions and directional solidification besides the hydrogen content of the alloys.

Electronic properties of solids are vitally important for modern technology, one may think of microelectronics, magnetic data storage, communication technologies and others. Sometimes, it seems possible to postpone the step of fundamental understanding when starting the application of a new material. However, lasting success and discovery of principally new approaches is based on it ...

Bifilms is the oxide layer created between two surfaces in the melt of light metals that form an oxide layer. These become planar inclusions in the final casting and are problematic for the mechanical properties such as cracks and crack initiations. Bifilms are too thin to be viewed properly in two dimension cross-section method as they will only appear as thin lines. Because of this, it is relevant to test if it is possible to use electrolytic extraction (EE) as a alternative method to investigate bifilms. Both the deeply etched surface and the inclusions on a filter from the extraction are looked at in the scanning electron microscope (SEM) to get an understanding of the size and shape of the inclusions. With this, a greater understanding of these types of defects can be achieved. After both the filtered inclusions and the surface are examined in SEM with images and Energy-dispersive X-ray spectroscopy (EDS), the images are measured in the software ImageJ. The measurements and analysis show that it is probably bifilms and that they can be relatively large, and not so circular. However, the measurements with the filter have shown high levels of oxygen and carbon. Some levels of chlorine, nitrogen and iron have also been found. But if the surface is compared to the metal surface, it can be concluded that it is likely that bifilms have been found. There are sufficient levels of aluminum and oxygen present. Images in SEM also show the appearance of film-like inclusions. If the method of electrolytic extraction is to be improved to investigate bifilms,optimizations such as filters of other compositions are recommended. / Bifilms är det oxidskikt som skapas mellan två ytor i smältan hos lätta metaller som bildar ett oxidskikt. Dessa blir sedan plana inneslutningar i den slutliga gjutningen och är problematiskt för de mekaniska egenskaperna i form av sprickor och sprickinitieringar. Bifilms är för tunna för att de ska kunna ses korrekt i en tvärsnittsmetod, eftersom de bara kommer att visas som tunna linjer. På grund av detta är det relevant att testa om det är möjligt att använda elektrolytisk extraktion (EE) som en alternativ metod. Både den djupt etsade ytan och inneslutningarna på ett filter från extraktionen tittas på i svepelektronmikroskop för att få en förståelse för inneslutningarnas storlek och form. I och med det kan en högre förståelse uppnås för dessa typer av defekter. Efter att både de filtrerade inneslutningarna och ytan granskats i SEM med bilder och Energy-dispersive X-ray spectroscopy (EDS), mäts bilderna i en mjukvara, ImageJ. De mätningarna och analyserna visar att det antagligen hittats bifilms och att de kan vara förhållandevis stora, samt inte så cirkulära. Däremot har mätningarna med filtret visat höga halter av syre och kol. Även vissa halter av klor, kväve och järn har hittats. Men om ytan jämförs med metallytan så kan en slutsats dras om att det är troligt att bifilms har hittats. Där finns tillräckliga halter av aluminium och syre. Även bilder i SEM påvisar filmliknande inneslutningar till sitt utseende. Om metoden med elektrolytisk extraktion ska förbättras för att undersöka bifilms behövs optimeringar, som till exempel filterav en annan sammansättning.

Engineering : Chemical / M. Tech. (Chemical Engineering)

Submitted by Wagner Henrique Rabelo (wagner144@bol.com.br) on 2018-07-26T20:18:44Z No. of bitstreams: 1 MESTRADO - WAGNER RABELO - VERSÃO FINAL.pdf: 3489307 bytes, checksum: 98e6a7b1b48eed69e255358e3a62fdd9 (MD5) / Approved for entry into archive by Lucilene Cordeiro da Silva Messias null (lubiblio@bauru.unesp.br) on 2018-07-30T12:45:51Z (GMT) No. of bitstreams: 1 rabelo_mh_me_bauru.pdf: 3537069 bytes, checksum: 6434660c8d96cc051a92066c85136ebc (MD5) / Made available in DSpace on 2018-07-30T12:45:51Z (GMT). No. of bitstreams: 1 rabelo_mh_me_bauru.pdf: 3537069 bytes, checksum: 6434660c8d96cc051a92066c85136ebc (MD5) Previous issue date: 2018-05-28 / O avanço das técnicas de deposição de filmes finos sobre as superfícies dos materiais tem permitido agregar valor e dar novas funcionalidades aos produtos. Atualmente, os filmes finos de óxido de estanho dopado com índio (ITO) têm encontrado grande aplicação no mercado. Entretanto, devido à pouca disponibilidade do índio na natureza e aos altos custos envolvidos na sua aquisição, elementos alternativos estão sendo estudados para sua substituição. Nesse contexto, destaca-se o óxido de zinco dopado com alumínio (AZO) como um promissor substituto, devido às características de elevada transmissividade, baixa resistividade e band gap da ordem de 3,37 eV, que permitem sua aplicação na síntese de filmes finos semicondutores. Com base no exposto, neste trabalho, foi projetado e desenvolvido o protótipo de uma fonte amplificadora de potência (FAP) de corrente alternada (AC) em baixa frequência, operando entre 15 a 40 kHz, responsável por iniciar e sustentar o campo elétrico utilizado para a geração do plasma. Esta FAP foi utilizada para a deposição de filmes finos de (AZO) por meio da técnica de magnetron sputtering. A análise das características morfológicas, ópticas e elétricas dos filmes de AZO produzidos neste estudo resultaram em uma transmitância superior a 80%, energia de band gap de 3,82 eV, e resistividade de 1,46.10-3 .cm, permitindo concluir que o filme produzido se comporta como um TCO (óxido transparente condutivo). A comparação desses resultados com trabalhos disponíveis na literatura, permite concluir que a fonte amplificadora de potência desenvolvida nesta dissertação possibilita a obtenção de filmes finos de AZO com condutividade e transparência superiores àqueles produzidos com fontes operando em radiofrequência, técnica atualmente disponível e amplamente utilizada no mercado. / The development of thin films deposition techniques allows to increase value and give new features to the materials. Currently, indium doped zinc oxide (ITO) is widely used in the market. However, due to the low availability of the indium in the nature and the high costs involved on its acquisition, alternative elements are being studied for its replacement. Aluminum doped zinc oxide (AZO) stands out as a promising substitute, mainly because of its characteristics, such as high transmissivity, low resistivity and band gap value of 3.37 eV. That allow the application of AZO in the synthesis of thin films semiconductors. In this work, it was developed a prototype of a plasma power source amplifier (FAP) to operate in alternating current (AC) and low frequency (15 - 40 kHz), responsible for initiating and sustaining the electric field used for plasma generation. This FAP was used to deposit AZO thin films by the technique of magnetron sputtering. The analysis of the morphological, optical and electrical characteristics of the AZO films produced in this study resulted in more than 80% transmittance, band gap energy value of 3,82eV, and resistivity of 1,46.10-3 .cm. The thin films synthetized was classified as transparent conductive oxide (TCO). The comparison of these results with the characteristics of similar films avaiable in the bibliography, allows to conclude that the power amplifier source developed in this dissertation makes it possible to obtain thin films of AZO with conductivity and transparency superior to those produced with RF magnetron sputtering, technique currently available and widely used in the market.

The electrochemical corrosion behavior of mild steel in apple, grape, mango, orange and the mixture of these juices was investigated in this study. Open circuit potential and weight loss analytical techniques were employed to establish the comparative corrosion rates of this material in the agro media over the interval of five days for a sixty-day immersion period at an ambient temperature. The chemical compositions of both mild steel and the agro media were determined to ascertain corrosion mechanism for the reaction. Polarization behavior of mild steel in the agro media were determined by Tafel extrapolation curves. The analysis showed that cathodic polarization curves were almost identical irrespective of the concentration of the various media while the anodic polarization curves exhibited varying active and passive corrosion behaviour due to passivating oxide films. The analysis of the results further showed that the corrosion rate of the metallic sample decreased with longer immersion periods which could be attributed to a gradual decline in acidity of these media as revealed by the pH results. Hence, the evolution of hydrogen gas and reduction of dissolved oxygen molecules from the reacting system were presumed to be major factors retarding corrosion of the solution involved. Similarly, the presence of suspended particles on the surface of the test steel sample could have also led to the impediment to corrosion rates from the surrounding atmosphere. Microscopic analysis of the corroded mild steel specimens revealed uniform and localized corrosion with Fe2O3 spotted as the main corrosion product. The result obtained from the electrochemical study showed that the corrosion rate of the mild steel sample was highest in the orange medium (1.53mm/yr), followed by grape medium (1.40mm/yr), mixture of these media (0.67mm/yr), mango medium (0.40mm/yr) while the metal corroded least in the apple medium (0.30mm/yr) over the duration of immersion. / Engineering : Chemical / M. Tech. (Chemical Engineering)

Les procédés de photodégradation, de conversion photocatalytique du CO2 et la technologie des supercondensateurs représentent des options intéressantes pour palier aux problèmes environnementaux et pour apporter des réponses à la crise énergétique. Dans ces trois domaines, les matériaux à base d'oxydes métalliques sont très prometteurs. Cependant, les voies classiques (c'est-à-dire par voies hydrothermales ou de sol-gels) présentent un certain nombre d’inconvénients tels que leur longues durées de préparation et leurs rendements limités. En outre, la mise en oeuvre de nanopoudres suppose une opération de post-filtration dans les procédés de photodégradation et l’utilisation supplémentaire d’un liant dans la confection d’électrodes de supercondensateurs, ce qui non seulement réduit les performances respectives de ces deux procédés mais entrave également leur développement au niveau industriel. Dans cette thèse, les technologies SPPS (Solution Precursor Plasma Spray) et SPFS (Solution Precursor Flame Spray) ont été introduites pour préparer des films d'oxydes métalliques à base de ZnO, en bénéficiant des avantages de rapidité et simplicité de ces techniques de formation de dépôts. Les films ainsi obtenues ont ensuite été testés dans des expériences de photodégradation, de conversion photocatalytique du CO2 et sous forme de supercondensateurs. Tout d'abord, à notre connaissance, c'est la première fois que l'on synthétise directement des nanostructures de ZnO (par exemple des nanotubes ou des nanofils) par des procédés SPPS. Ces films nanostructurés et hiérarchisés présentent non seulement une croissance préférentielle le long du plan cristallin (002), mais contiennent aussi des lacunes d'oxygène dans leurs réseaux. La mise en œuvre de simulations DFT a permis de proposer un mécanisme possible de croissance des nanostructures de ZnO lors de leur synthèse par la voie SPPS. Deuxièmement, divers films “composites”, constitués de ZnO et d’un second oxide “MO” ont également été préparés par SPPS afin de réduire les bandes d'énergie interdites. Dans cette partie de la thèse, nous avons étudié l’effet du rapport molaire MO/ZnO sur la structure des films MO/ZnO obtenus (M = Mn et Cu); nous avons aussi préparé, par cette nouvelle méthode, des films fins CuO/ZnO et CeO2/ZnO ainsi que des films de ZnO “décorés”. Troisièmement, des films mettant en œuvre des structures du type spinelle (tels que les oxydes mixtes ZnFe2O4, NiCo2O4, ZnCo2O4 et Co3O4) ont également été synthétisés et déposés à l’aide des techniques SPPS et SPFS, ceci en raison de leur grand intérêt pour les applications susmentionnées. Il a été constaté que les structures obtenues sont très sensibles aux rapports MO/ZnO et que les morphologies de surface dépendent davantage des paramètres d'injection des solutions. En outre, la puissance de la torche joue un rôle plus critique dans la synthèse in situ de la phase spinelle binaire que la température de préchauffage du substrat. Enfin, la voie de préparation par SPPS favorise la formation de dépôts de textures floconneuses, notamment dans le cas des films de NiCo2O4 et Co3O4, tandis que des particules de formes sphériques ont été plutôt observées dans les échantillons préparés par la voie SPFS... / The fields of research dealing with photodegradation, photocatalytic conversion of CO2 and supercapacitors are important to address environmental problems and respond to the energy crisis. Metal oxides are promising materials in these three domains. However, the conventional routes (i.e. hydrothermal, sol-gel) suffer from major deficiencies, namely their multi-step natures, their long preparation duration and small-scaled yields. Moreover, the usage of nanopowders implies a post-filtration operation at the end of the photodegradation processes and requires an additional binder in supercapacitor electrodes. In this thesis, “Solution Precursor Plasma Spray” (SPPS) and “Solution Precursor Flame Spray” (SPFS) technologies have been introduced to develop metal oxide films in view of the three aforementioned applications, benefiting from the facility and rapidity advantages of this one-step process.Firstly, to our best knowledge, it is the first time that films composed by ZnO nanostructures (e.g. nanorods, nanowires) are directly synthesized via a SPPS process. These hierarchical ZnO nanostructured films not only exhibit preferential orientation growth along the (002) crystal plane, but also feature in-situ oxygen vacancies. As a result, a possible growth mechanism of ZnO nanostructures via SPPS route was proposed.Secondly, various metal oxides composite films containing ZnO and a second metal oxide were also prepared by SPPS in an effort to narrow the energy bandgaps. In this work, not only the effect of the molar CuO/ZnO and MnO/ZnO ratio was investigated, but also laminated CuO/ZnO and CeO2/ZnO films and CuO, Co3O4 and Fe2O3 decorated ZnO nanorods films were pioneeringly deposited via this novel route.Thirdly, films involving spinel-type materials (including ZnFe2O4, NiCo2O4, ZnCo2O4 and Co3O4) were also synthesized and deposited by the SPPS and SPFS technologies, owing to their high-interest in the aforementioned applications. We found that the phase compositions are more sensitive to the Fe/Zn and Ni/Co ratios and that the surface morphologies are more dependent on the patterns of the solution injection. In addition, the power of the torch plays a more critical role on the in-situ synthesis of binary spinel phase. Besides, the SPPS route promotes the formation of flake-like particles both in the NiCo2O4 and Co3O4 films, while sphere-like particles were observed in the SPFS-prepared samples.Finally, some as-prepared films were selected to evaluate their performances within the three applications. On the one hand, Orange II was successfully (100%) degraded within 2h under UV irradiation and about 85% was removed within 6h under visible light irradiation. On the other hand, Co3O4 samples exhibited specific capacitances up to 1190 F g−1 with a retention capacity of 136% after 2500 cycles at a 20 mV/s scanning rate in 2 M KOH electrolyte. Finally, when using ZnCo2O4 as photocatalyst, CO2 was converted into CO by visible light irradiation with a maximum turnover number as high as 61.38 and a selectivity as high as 90.5 %.Overall, this work not only improves the performances of the three studied processes thanks to the use of novel, fast preparation methods, but also suggests that “Solution Precursor Thermal Spray” should be a highly promising technology for further, alternative functional applications that involve finely structured metal oxides film.