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Despite the many attractive features of carbon fiber reinforced polymers (FRPs) composites, they are prone to failure due to delamination. The ability to tailor the fiber/matrix interface FRPs is crucial to the development of composite materials with enhanced structural performance. In this dissertation, ZnO nanowires (NWs) were grown on the surface of carbon fibers utilizing low temperature hydrothermal synthesis technique prior to the hybrid composite fabrication. The scanning electron microscopy revealed that the ZnO nanowires were grown uniformly on the surface of the carbon fabric. The surface grown ZnO NWs functionally-graded the composite material properties and ensured effective load transfer across the interface. To assess the influence of the ZnO NWs growth, reference samples were also prepared by exposing the carbon fabric to the hydrothermal conditions. The damping properties of the hybrid ZnO NWs-CFRP composite were examined using the dynamic mechanical analysis (DMA) technique. The results showed enhanced energy dissipation within the hybrid composite. Quasi-static tensile testing revealed that the in-plane and out-of-plane strengths and moduli of the hybrid FRP composite were also boosted. The interlaminar shear strength (ILSS) measurements suggested the improvement in the mechanical properties of the composite to the enhanced adhesion between the ZnO nanowires and the other constituents (carbon fiber and epoxy). It was necessary thus, to utilize the molecular dynamics simulations (MD) to investigate the adhesion within the CFRP structure upon growing the ZnO nanowires on the surface of the carbon fibers. Molecular models of the carbon fibers, the epoxy matrix and the ZnO nanowires were built. The resulting molecular structures were minimized and placed within a simulation box with periodic boundary conditions. The MD simulations were performed using the force field COMPASS to account for the empirical energy interactions between the different toms in the simulation box. Proper statistical thermodynamics were employed to relate the dynamics of the molecular model to the macroscale thermodynamic states (pressure, temperature and volume). Per the computed potential energies of the different components of the composite, it was found that the polar surfaces in the ZnO structures facilitates good adhesion properties in the graphite-epoxy composite. Besides the attractive mechanical properties of the ZnO nanowires, their piezoelectric and semiconductor properties were sought to design an energy harvesting device. To ensure sufficient charges collection from the mechanically stressed individual ZnO nanowires, a copper layer was sputtered on top of the ZnO nanowires which introduced also a Schottky effect. The mechanical excitation was provided by exposing the device to different vibration environment. The output voltage and currents were measured at the conditions (in terms of frequency and resistive load). It was demonstrated that the electrical output could be enhanced by stacking up similar devices in series or in parallel. Finally, in an attempt to exploit the reversibility of the electromechanical coupling of the energy harvesting device, the constitutive properties of the hybrid ZnO nanowires-CFRP composite were estimated using the Mori-Tanaka approach. This approach was validated by a finite element model (FEM). The FEM simulations were performed on a representative volume element (RVE) to reduce the computational time. The results demonstrated that the mechanical properties of the hybrid ZnO NWs-CFRP composite were better than those for the baseline CFRP composite with identical carbon fiber volume fraction (but with no ZnO NWs) which confirmed the experimental findings. Furthermore, the electro-elastic properties of the hybrid composite were determined by applying proper boundary conditions to the FE RVE. The work outlined in this dissertation will enable significant advancement in the next generation of hybrid composites with improved structural and energy harvesting multifunctionalties. / Ph. D.

Various nano-fillers can introduce specific functions into polymer and expand their application areas. Myriad properties, such as mechanical, electrical, thermal, or magnetic properties can be combined with original polymer characteristics, including flexible, light weight, and ease of use. These composites can be used to produce multi-functional fibers as the next generation textile or fabrics. In this research, Polyacrylonitrile (PAN) is adopted as the main polymer with different nano-fillers, such as carbon nanotube (CNT), iron oxide nanoparticle, and graphene oxide nanoribbon (GONR). Using gel-spinning technology, PAN-based composite fibers are fabricated in single- or bi-component fibers. Fibers are also characterized for their structure, morphology, mechanical properties, as well as for their electrical, thermal, or magnetic properties. For example, bi-component fibers with polymer sheath and polymer-CNT core as well as polymer-CNT sheath and polymer core are processed. With electrical and thermal conductivity introduced by CNT, such bi-components fibers can be applied for wearable electronics or for thermal management. Joule-heating effect owing to applied electrical current on single component PAN/CNT fibers is also investigated. With controllable electrical conductivity and fiber temperature, this active functional fiber can be applied for temperature regulation fibers or new carbon fiber manufacturing process. Another example is magnetic fiber with superparamagnetic iron oxide nano-particles. These novel magnetic fibers with high strength can be used for actuator, inductors, EMI shielding, or microwave absorption. GONR is also discussed and used to reinforce PAN-based fibers. Several theoretical models are considered to analyze the observed results.

Diferentes estratégias têm sido propostas a fim de melhorar o desempenho dos bioeletrodos utilizados nas biocélulas a combustíveis e nos biossensores. Por examplo, a funcionalização de eletrodos de carbono tem sido feita para esse fim. Neste estudo, propomos o desenvolvimento de fibras flexíveis de carbono (FFCs) funcionalizadas com grupos quinona e modificados com álcool desidrogenase (ADH) NAD-dependente para obter bioeletrodos para uma bio-eletrocatálise eficiente de etanol. Grupos quinona na superfície das FFCs foram obtidas utilizando o tratamento oxidativo com permanganato e também pelo ancoramento eletroquímico de antraquinona: ambas metodologias resultaram em bioeletrodos para a eletro-oxidação de NADH que pode aumentar a bio-eletrocatálise do etanol. De acordo dados espectroscópicos, microscópicos, e eletroquímicos, defeitos contendo grupos C=O nos eletrodos de FFCs são atribuídos à melhora na oxidação do NADH, aumentando a bio-eletrocatálise do etanol. Para se investigar o papel dos grupos quinona na eletro-oxidação do NADH, propomos uma configuração experimental baseado na espectroscopia de ressonância paramagnética de elétrons em modo operando (operando EPR). Com essa técnica, fomos capaz de mostrar a correlação entre o número de elétrons livres desemparelhados, a concentração superficial de quinonas e a oxidação do NADH com controle eletroquímico. Correlação para a concentração de spins revela um aumento no número de elétrons desemparelhados livres com o aumento do sobrepotencial aplicado e a oxidação do NADH, o que corrabora com a hipótese de que grupos quinona podem afetar a eletrocatálise rumo à oxidação do NADH a NAD+. É vislumbrado que operando EPR pode fornecer infromação útil para provar a dinâmica da transferência de elétrons em superfície de carbono e possa ser extendida a outros sistemas bioeletroquímicos. / There are several strategies to improve the performance of bioelectrodes applied in biosensors and biofuel cells. For instance, surface functionalization of the carbon-based electrodes has been used to this intend. Herein, we propose the development of flexible carbon fibers (FCFs) functionalized with quinone groups and modified with NAD-dependent alcohol dehydrogenase (ADH) to obtain bioelectrodes for efficient ethanol bio-electrocatalysis. Quinones groups on FCFs surfaces were obtained by using oxidative treatment with permanganate, and also by electrochemical grafting of anthraquinone: both these methodologies result in bioelectrodes for the electro-oxidation of NADH that can improve the ethanol bio-electrocatalysis. Based on spectroscopic, microscopic and electrochemical data, defects containing C=O groups on FCFs electrodes are attributed to improve the NADH oxidation, enhancing the ethanol bio-electrocatalysis. In order to investigate the role of quinone groups on the NADH electro-oxidation, we propose an experimental setup based on operando electron paramagnetic resonance spectroscopy (operando EPR). With this technique, we are able to show a correlation among the number of free unpaired electrons, surface concentration of quinones and NADH oxidation under electrochemical control. Correlation for the spin concentration reveals an increasing number of free unpaired electrons with increasing applied overpotential and NADH oxidation, which corroborates the hypothesis that quinone groups can act as electrocatalysts towards the oxidation of NADH to NAD+. It is glimpsed that operando EPR can provide useful information in probing the electron transfer dynamics on a carbon surface and may be extended to others bioelectrochemical systems.

Este trabalho trata do estudo da introdução de fibras poliméricas como agente porogênico na manufatura de membranas cerâmicas de alta resistência mecânica. Membranas cerâmicas são utilizadas para a separação de substâncias onde a estabilidade química e a resistência à alta temperatura são requeridas; na engenharia mecânica é também aplicada como mancais aerostáticos. A escolha do processo de manufatura destas membranas e o projeto da porosidade da estrutura cerâmica é de grande importância. Quando se utiliza da adição de elementos porogênicos, que são substâncias que se decompõem durante a queima deixando poros (abertos e fechados), altera não somente a porosidade como também a seletividade e permeabilidade da membrana, bem como diminui suas propriedades mecânicas. Este trabalho objetiva membranas com poros micrométricos e submicrométricos para aplicações em microfiltração ou em mancais aerostáticos, porém que tenham a maximização da sua resistência mecânica. Para isso foi idealizada a obtenção de membranas permeáveis com a menor porosidade possível. Então, propôs-se a adição de fibras poliméricas (fibras de polipropileno, fibras de carbono e fibras de álcool polivinílico) como agentes porogênicos em uma massa cerâmica de alumina que após granulada foi prensada na forma de pastilhas e sinterizadas. As membranas foram caracterizadas por ensaios de permeabilidade ao ar, resistência mecânica por flexão a 3 pontos, volume de poros pelo Princípio de Arquimedes e morfologicamente por MEV. A caracterização morfológica das membranas e os resultados de porosidade indicaram que a percolação foi melhorada pela adição de fibras em relação a trabalhos equivalentes que se utilizaram de particulados. As melhores condições de permeação, integridade da matriz e resistência mecânica foram encontradas para as membranas com 1 vol. % de fibras de álcool polivinílico, seguida das membranas com 2 vol. % de fibras de carbono. Com base nos resultados obtidos, foi comprovado que a morfologia alongada das fibras aumenta a probabilidade de contato entre os poros, pois com apenas 2 vol. % de fibras de PVAl, obteve-se uma porosidade total de 33,3%, porosidade aparente de 17,4% e permeabilidade específica de 6,32x10-12 m², apresentando resistência à flexão de 134,3 MPa, valor este o dobro de quando utilizado 50 vol. % de sacarose como agente precursor com equivalente permeabilidade. As propriedades físicas do agente porogênico como: expansão térmica, dimensões, relação comprimento versus diâmetro afetam enormemente a porosidade, percolação dos poros e integridade da matriz. / This work deals with the study of the introduction of polymeric fibers as porogenic agent in the manufacture of ceramic membranes with high mechanical strength. Ceramic membranes are used for separation of substances where chemical stability and high temperature resistance are required, in mechanical engineering is also applied as an aerostatic bearings. The choice of these membranes manufacturing process and design the porosity of the ceramic structure is of great importance. When using addition of porogenic components, which are substances that decompose during the burning leaving pores (open and closed), not only alters the porosity and also the permeability and the selectivity of the membrane, as well as decreases their mechanical properties. This work aims membranes with micrometric pores and submicrometrics for microfiltration applications or aerostatic bearings, however they having the maximization their mechanic strength. To this was designed to obtain porous membranes with the lowest porosity possible. Then proposed the addition of polymeric fibers (polypropylene fiber, carbon fiber and polyvinyl alcohol fiber) as porogenic agents on a ceramic alumina mass that was granular and pressed the form of discs and sintered. The membranes were characterized by testing the air permeability, mechanical strength by three points flexural, pore volume by Archimedes principle and morphologically by SEM. Morphological characterization of membrane and the results of porosity indicated that the percolation has been improved by adding fibers in relation to the corresponding work where particulates were used. The best conditions of permeation, matrix integrity and mechanical strength were found for the membranes with 2 vol. % of polyvinyl alcohol fibers and membranes with 2 vol. % carbon fibers. Based on these results, it was proven that the elongated morphology of the fibers increases the probability of contact between the pores, because with only 2 vol. % of PVAl fibers, we obtained a total porosity of 33.3%, apparent porosity of 17.4% and the specific permeability of 6.32x10-12 m², flexural strength of 134.3 MPa, this value is twice than when it is used 50 vol. % sucrose as a precursor agent with equivalent permeability. The physical properties of porogenic agent such as thermal expansion, dimensions and relation length versus diameter affect greatly the porosity, pore percolation and integrity of the matrix.

A técnica caracterizada pela colagem de polímeros reforçados com fibras de carbono (PRFC) em elementos estruturais de concreto vem sendo aplicada com sucesso no reforço de estruturas em todo o mundo. Resistência à corrosão, elevada resistência à tração, baixo peso, facilidade e rapidez de aplicação, são algumas das características interessantes que têm contribuído para a sua disseminação. Nesta pesquisa propõe-se uma inovação construtiva fundamentada no desenvolvimento de um compósito de alto desempenho à base de cimento Portland e fibras de aço (macro + microfibras), destinado a constituir o que está sendo preliminarmente chamado de substrato de transição. A finalidade desse substrato é a de controlar melhor a fissuração do concreto da viga e retardar ou até evitar o desprendimento prematuro do reforço polimérico. Devido à carência de pesquisas semelhantes a aqui proposta, foi realizado um estudo preliminar em vigotas moldadas com fibras de aço e reforçadas externamente com manta de PRFC, onde se verificou que a concepção do substrato de transição é válida. Partiu-se então para a realização de ensaios visando à obtenção de um compósito cimentício com características apropriadas para constituir o substrato de transição. Os resultados e as análises efetuadas mostram que foi possível desenvolver um material de elevado desempenho, traduzido por um comportamento de pseudo-encruamento, com elevados ganhos de resistência e tenacidade ao fraturamento. A aplicação do reforço com manta sobre a superfície do substrato de transição, formado a partir da reconstituição do banzo tracionado da viga com o compósito cimentício, mostrou melhorar significativamente os níveis de desempenho da peça reforçada. Do estudo realizado foi possível comprovar a eficiência da técnica de reforço proposta, além de reunir uma série de informações que podem ser exploradas para se tornarem úteis como critérios de projeto de estruturas recuperadas e reforçadas. / The technique characterized by bond of the carbon fibers reinforced polymer (CFRP) in structural elements of concrete comes being applied successfully in the strengthening of structures in the whole world. Resistance to the corrosion, high tensile strength, low weight, easiness and rapidity of application, is some of interesting characteristics that have contributed for its dissemination. The objective of this research is to develop an innovate strengthening method for RC beams, based on a high performance cement-based composite of steel fibers (macro + microfibers) to be applied in a transition layer. The purpose of this transition layer is to better control the cracking of concrete and to be late or until avoid the premature detachment of strengthening. Due to lack of similar research here the proposal, was carried through a preliminary study in short beams molded with steel fibers and strengthened with CFRP sheet, where if it verified that the conception of the transition layer is valid. Tests were developed to get a cement-based composite with characteristics to constitute the layer transition. The results shown that were possible to develop a material of high performance with a pseudo strain-hardening behavior, high strength and fracture toughness. The application of the strengthened about the layer transition surface showed significantly to improve the levels of performance of the strengthening beam. Of the carried through study it was possible to prove the efficiency of the new strengthened technique and describe various information that can be explored to become useful as criteria of project of repaired and strengthened structures.

Biocélulas a combustível (BFCs) são definidas como dispositivos bioeletroquímicos que utilizam componentes biológicos, como enzimas ou microrganismos, para converter energia química em energia elétrica. Neste estudo, reporta-se o desenvolvimento de um bioeletrodo composto por fibras flexíveis de carbono (FFC) modificadas com a enzima álcool desidrogenase (ADH), o qual foi utilizado juntamente com a levedura Saccharomyces cerevisiae , sendo enzima e microrganismos usados como biocatalisadores cooperativos para bioprodução e oxidação de etanol. A glicose é oxidada pelas células de levedura sob condições anaeróbias, e o etanol formado pela fermentação alcóolica é, em seguida, oxidado a acetaldeído pela enzima ADH. A oxidação de etanol pela ADH resulta ainda, na redução da molécula de nicotinamida adenina dinucleotídeo, NAD+ a NADH. Posteriormente, o NADH formado nessa reação é eletroquimicamente oxidado a NAD+ na superfície do bioeletrodo de FFC baseado em ADH (FFC-ADH). Avaliou-se a influência da temperatura e do pH na bioeletrocatálise de etanol pela ADH e a melhor resposta obtida foi em 40 ºC e pH 8,5. Além disso, obteve-se uma excelente correlação linear entre os valores de concentração de etanol e densidade de corrente, indicando que a resposta bioeletrocatalítica da ADH é diretamente proporcional à concentração de etanol produzido a partir da fermentação. O conceito de que microrganismos e enzimas podem trabalhar cooperativamente para produzir uma nova classe de bioeletrodos, foi introduzido nesse trabalho. Por fim, demonstrou-se, que o bioeletrodo cooperativo pode ser aplicado com sucesso em uma BFC, utilizando o biocátodo de difusão de gás contendo a enzima bilirrubina oxidase (BOx) imobilizada em sua superfície. / Biofuel cells (BFCs) are defined as bioelectrochemical devices that use biological components, such as enzymes or microorganisms, to convert chemical energy into electric energy. In this study, we report the development of a bioelectrode composed of flexible carbon fibers (FCF) modified with the enzyme alcohol dehydrogenase (ADH) together with Saccharomyces cerevisiae yeast, being enzyme and microorganisms used as cooperative biocatalysts for bioproduction and oxidation of ethanol. Glucose is oxidized by the yeast cells in anaerobic conditions, and ethanol is produced through alcoholic fermentation and then it is oxidized to acetaldehyde by the ADH enzyme. The ethanol oxidation by ADH also results in the reduction of the nicotinamide adenine dinucleotide molecule, NAD+ to NADH. Subsequently, the NADH produced in this reaction is electrochemically oxidized to NAD+ on the surface of the FCF bioelectrode based on ADH (FCF-ADH). The influence of temperature and pH on the bioelectrocatalysis of ethanol was evaluated and the best performance was found at 40 ºC and pH 8.5. Additionally, the results demonstrated an excellent linear correlation between the ethanol concentration and the current generated, which indicates that the bioelectrocatalytic response of ADH is directly proportional to concentration of ethanol produced from the fermentation. The present study has introduced the concept that microorganisms and enzymes can work cooperatively to produce a new class of bioelectrodes. Finally, it has been demonstrated that the cooperative bioelectrode can be applied successfully to BFC using a gas-diffusion biocathode containing the bilirubin oxidase enzyme (BOx) immobilized on its surface.

Este trabalho trata do estudo da introdução de fibras poliméricas como agente porogênico na manufatura de membranas cerâmicas de alta resistência mecânica. Membranas cerâmicas são utilizadas para a separação de substâncias onde a estabilidade química e a resistência à alta temperatura são requeridas; na engenharia mecânica é também aplicada como mancais aerostáticos. A escolha do processo de manufatura destas membranas e o projeto da porosidade da estrutura cerâmica é de grande importância. Quando se utiliza da adição de elementos porogênicos, que são substâncias que se decompõem durante a queima deixando poros (abertos e fechados), altera não somente a porosidade como também a seletividade e permeabilidade da membrana, bem como diminui suas propriedades mecânicas. Este trabalho objetiva membranas com poros micrométricos e submicrométricos para aplicações em microfiltração ou em mancais aerostáticos, porém que tenham a maximização da sua resistência mecânica. Para isso foi idealizada a obtenção de membranas permeáveis com a menor porosidade possível. Então, propôs-se a adição de fibras poliméricas (fibras de polipropileno, fibras de carbono e fibras de álcool polivinílico) como agentes porogênicos em uma massa cerâmica de alumina que após granulada foi prensada na forma de pastilhas e sinterizadas. As membranas foram caracterizadas por ensaios de permeabilidade ao ar, resistência mecânica por flexão a 3 pontos, volume de poros pelo Princípio de Arquimedes e morfologicamente por MEV. A caracterização morfológica das membranas e os resultados de porosidade indicaram que a percolação foi melhorada pela adição de fibras em relação a trabalhos equivalentes que se utilizaram de particulados. As melhores condições de permeação, integridade da matriz e resistência mecânica foram encontradas para as membranas com 1 vol. % de fibras de álcool polivinílico, seguida das membranas com 2 vol. % de fibras de carbono. Com base nos resultados obtidos, foi comprovado que a morfologia alongada das fibras aumenta a probabilidade de contato entre os poros, pois com apenas 2 vol. % de fibras de PVAl, obteve-se uma porosidade total de 33,3%, porosidade aparente de 17,4% e permeabilidade específica de 6,32x10-12 m², apresentando resistência à flexão de 134,3 MPa, valor este o dobro de quando utilizado 50 vol. % de sacarose como agente precursor com equivalente permeabilidade. As propriedades físicas do agente porogênico como: expansão térmica, dimensões, relação comprimento versus diâmetro afetam enormemente a porosidade, percolação dos poros e integridade da matriz. / This work deals with the study of the introduction of polymeric fibers as porogenic agent in the manufacture of ceramic membranes with high mechanical strength. Ceramic membranes are used for separation of substances where chemical stability and high temperature resistance are required, in mechanical engineering is also applied as an aerostatic bearings. The choice of these membranes manufacturing process and design the porosity of the ceramic structure is of great importance. When using addition of porogenic components, which are substances that decompose during the burning leaving pores (open and closed), not only alters the porosity and also the permeability and the selectivity of the membrane, as well as decreases their mechanical properties. This work aims membranes with micrometric pores and submicrometrics for microfiltration applications or aerostatic bearings, however they having the maximization their mechanic strength. To this was designed to obtain porous membranes with the lowest porosity possible. Then proposed the addition of polymeric fibers (polypropylene fiber, carbon fiber and polyvinyl alcohol fiber) as porogenic agents on a ceramic alumina mass that was granular and pressed the form of discs and sintered. The membranes were characterized by testing the air permeability, mechanical strength by three points flexural, pore volume by Archimedes principle and morphologically by SEM. Morphological characterization of membrane and the results of porosity indicated that the percolation has been improved by adding fibers in relation to the corresponding work where particulates were used. The best conditions of permeation, matrix integrity and mechanical strength were found for the membranes with 2 vol. % of polyvinyl alcohol fibers and membranes with 2 vol. % carbon fibers. Based on these results, it was proven that the elongated morphology of the fibers increases the probability of contact between the pores, because with only 2 vol. % of PVAl fibers, we obtained a total porosity of 33.3%, apparent porosity of 17.4% and the specific permeability of 6.32x10-12 m², flexural strength of 134.3 MPa, this value is twice than when it is used 50 vol. % sucrose as a precursor agent with equivalent permeability. The physical properties of porogenic agent such as thermal expansion, dimensions and relation length versus diameter affect greatly the porosity, pore percolation and integrity of the matrix.

The research conducted in this study was concerned with the development of low-cost continuous production of carbon fiber/aluminum composites. Two coatings, alumina and zirconia, were applied to the fibers to protect against interfacial degradation. They were applied using a sol-gel method and common metal salts. The fibers were infiltrated with molten aluminum using an ultrasound sonicator. The resultant composites were well-infiltrated and were tested in tension to determine their mechanical properties. Strengths were only 15-35% of the theoretical values predicted by the rule of mixtures. The composite microstructure revealed a sizable void fraction and that the fibers within the composites did not contain any coating on their surface. It was hypothesized that this was a result of few exposed graphite plane edges on the fiber surface, causing poor adhesion of the oxide coating to the fiber surface. To improve adhesion, an amorphous carbon coating was applied to the fiber surface, but still the oxide coatings were removed from the fibers upon infiltration. It was found, however, that the carbon coating on its own did strengthen the interface between the fiber and the aluminum.