Efficient graphene-based photodetector with two cavities

We present an efficient graphene-based photodetector with two Fabri-P\'erot cavities. It is shown that the absorption can reach almost 100% around a given frequency, which is determined by the two-cavity lengths. It is also shown that hysteresis in the absorbance is possible, with the transmittance amplitude of the mirrors working as an external driving field. The role of non-linear contributions to the optical susceptibility of graphene is discussed.Comment: 10 pages, 8 figures. published version: minor revisio

Betão autocompactável reforçado com fibras de aço: método de concepção e propriedades mecânicas

O betão autocompactável pode ser definido como um betão capaz de fluir quando colocado num molde, escoando por entre as armaduras, envolvendo-as e preenchendo o respectivo molde, apenas sob o efeito do seu peso próprio, dispensando o uso de equipamentos externos de vibração. Adicionando os benefícios associados à autocompactabilidade com os proporcionados pela incorporação de fibras de aço, é possível obter um material dotado de características de elevado desempenho: o betão autocompactável reforçado com fibras de aço (BACRFA). No entanto, a metodologia a implementar para quantificar a dosagem dos materiais constituintes deste tipo de betão é, forçosamente, mais complexa do que a necessária para um betão convencional, uma vez que as exigências no estado fresco são substancialmente mais restritivas, o número de componentes é mais elevado (adjuvantes e adições em maior número) e a sensibilidade de uns à presença dos outros é elevada. Neste sentido, é imperiosa a formulação de um método de composição racional que, de uma forma simples, seja sensível aos factores envolvidos e que seja eficaz na concepção de misturas com bom desempenho, tanto no estado fresco como endurecido, contemplando, se possível, aspectos relacionados com a necessária sustentabilidade económica e ambiental, reduzindo o mais possível o consumo de cimento. O presente trabalho insere-se neste contexto, descrevendo uma metodologia de composição racional, aplicável a BACRFA. A validade deste tipo de metodologia é aferida determinando experimentalmente uma composição de um BACRFA e procedendo à sua caracterização reológica e mecânica, por intermédio da avaliação do seu comportamento tanto no estado fresco como, quando endurecido, à compressão e à flexão

Self-compacting steel fibre reinforced concrete : a rational conception method

O conceito de betão auto-compactável é complexo, e fundamenta-se sobretudo num conjunto de propriedades que este material tem que demonstrar ser capaz de satisfazer, quando no estado fresco. É uma nova dimensão que se acrescenta ao conceito clássico de betão, e que se concentra sobretudo ao nível das suas propriedades reológicas. O incremento de complexidade que esta dimensão introduz na formulação e compreensão deste novo conceito de betão é evidente, e deve-se fundamentalmente à necessidade de compreender os fenómenos que orientam o comportamento de um material que, apesar de resultar da mistura dos mais diversos componentes e com granulometrias muito distintas, se comporta como um fluido estável. Desta forma, é difícil e nem sempre totalmente claro o percurso que conduz à afinação de uma mistura óptima, tanto mais que o número de componentes é mais elevado que o normal (adjuvantes e adições em grande número), e a sensibilidade de uns à presença dos outros é elevada. Torna-se, portanto, imperativa a formulação de um método racional que, de uma forma simples, seja sensível a toda esta problemática, e que seja eficaz na concepção de misturas com bom desempenho tanto mecânico como reológico, não esquecendo a sua sustentabilidade económica e ambiental. O método que aqui é proposto dá fortes sinais neste sentido, mostrando que é possível conceber misturas de betão auto-compactável reforçado com fibras de aço, capazes de satisfazer todas as exigências acima referidas.The concept of self-compacting concrete is complex, and stains above all in a number of properties that, while in the fresh state, it has to prove to be able to accomplish. It’s a new dimension that enriches classic concept of concrete, focused mainly on its rheological properties. The raise of complexity that this new dimension brings to the formulation and understanding of this new concept of concrete is clear, and is mainly the result of the need to justify the phenomena sustaining the behavior of a material that, despite resulting of the mixture of the most different components with different grain sizes, behaves like a stable fluid. In this way, the path that leads to the optimization of a mixture is hard and not always totally clear, being the number of components higher than in conventional cases ( higher number of admixtures and additives), and the sensibility of ones to the presence of the others is higher. This idea results in the strong need of a formulation of a new rational method that, in a simple manner, has to be sensible to all this aspects, and to be efficient in the conception of mixtures with good mechanical and rheological behavior, not forgetting his economical and environmental sustainability.. The method here proposed gives strong signs in this way, showing that is possible to conceive self-compacting concrete mixtures, reinforced with steel fibers, able to satisfy every demanding yet referred

Steel fiber reinforced self-compacting concrete: experimental research and numerical simulation

Over the last few decades, the astonishing developments of super plasticizers technology allowed great achievements on the conception of concrete mixes exhibiting self-compacting ability. Since the eighties, some methodologies have been proposed to achieve self-compacting requirements in fresh concrete mixes, based on the evaluation of the flowing properties of these mixes. There still persist, however, some doubts about the most appropriate strategy to define the optimum composition of a self-compacting concrete (SCC) mix, based on a required performance. The behavior of SCC as a structural material can be improved if adequate steel fiber reinforcement is added to SCC mix composition. In fact, the fiber reinforcement mechanisms can convert the brittle behavior of this cement based material into a pseudo-ductile behavior up to a crack width that is acceptable under the structural design point-of-view. Fiber addition, however, increases the complexity of the mix design process, due to the strong perturbation effect that steel fibers cause on fresh concrete flow. In the present work, a mix design method is proposed to develop cost effective and high performance Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC). The material properties of the developed SFRSCC are assessed as well as its potentiality as a structural material, carrying out punching and flexural tests on panel prototypes. A material nonlinear analysis is carried out, aiming to address the possibility of calibrating the constitutive model parameters by obtaining, with an inverse analysis, the fracture parameters using forcedeflection relationships recorded in simpler laboratory tests, like the three point notched beam bending test. The contribution of steel fibers for punching resistance is also, by this means, discussed

Light scattering by a medium with a spatially modulated optical conductivity: the case of graphene

We describe light scattering from a graphene sheet having a modulated optical conductivity. We show that such modulation enables the excitation of surface plasmon-polaritons by an electromagnetic wave impinging at normal incidence. The resulting surface plasmon-polaritons are responsible for a substantial increase of electromagnetic radiation absorption by the graphene sheet. The origin of the modulation can be due either to a periodic strain field or to adatoms (or absorbed molecules) with a modulated adsorption profile.Comment: http://iopscience.iop.org/0953-8984/24/24/24530

Painéis aligeirados em betão autocompactável reforçado com fibras de aço

O Betão auto-compactável reforçado com fibras de aço é um material que alia as vantagens inerentes à adição de fibras na sua composição (incremento da sua ductilidade), com as que resultam do facto de se tratar de um material que possui elevados níveis de trabalhabilidade. Trata-se, deste modo, de um material extremamente versátil, para o qual se apresenta o método de concepção e a caracterização mecânica de uma composição formulada.Fundação para a Ciência e a Tecnologia (FCT) - PABERFIA (Painéis sandwich prefabricados de betão reforçado com fibras). FEDER - PABERFIA (Painéis sandwich prefabricados de betão reforçado com fibras). Universidade do Minho. Prégaia. Civitest. Secil. Bettor MBT. Bekaert. Comital

Assessing the life cycle of existing maritime structures – application to a harbour bridge in Porto port

Since the construction industry is the one that bears most expenses, both in financial and environmental terms, it is of the upmost importance that these expenses originate a product with a long term exploitation, so as to mitigate them. Having this problem in consideration, in this paper the bridge located in the north of Portugal, near Porto, in the oil tanker terminal at the Leixões port was studied. This structure is located in one of the most aggressive environments for concrete structures, a maritime zone. The most accepted durability models in the country, related to deterioration induced by sea chlorides penetration, were then implemented. Thereby, it was possible to identify which model better reflects reality, since the structure in analysis is now at the end of its lifetime, after 50 years of service and shows advanced degradation due to chloride attack. In the context of this work, the structure and the test methods relevant to the theme being studied were described by inspection reports and in situ test results, made available by the Douro and Viana do Castelo Port Authority. From this data it was possible to study the structure deterioration by introducing them into durability models. These models range from prescriptive to performance based approaches, being possible to identify, from the later ones, a deterministic model, based on the Model Code 2010, a semi-probabilistic based on the E465 specification from the Portuguese National Laboratory for Civil Engineering (LNEC) and two probabilistic models, based on the same standards, for which a computer code was developed during this work. Through these deterioration prediction tools, different project scenarios were established, originating a list of minimal concrete covers to ensure 100 years of lifetime to a structure built in the studied exposure zone.(undefined)info:eu-repo/semantics/publishedVersio

Post-cracking behaviour of selfcompacting steel fibre reinforced concrete

Self-compacting concrete (SCC) can be defined as concrete that is able to flow in the interior of the formwork, passing through the reinforcement and filling it in a natural manner, being consolidated under the action of its own weight. Adding the benefits of SCC to those resulting from the addition of discrete fibres to cement based materials, a high performance material, designated by steel fibre reinforced self-compacting concrete (SFRSCC), is obtained. In the present work the strategy followed to design SFRSCC is described, as well as, the experimental research carried out to characterize its flexural and compression behaviour. A special effort is done to assess the post-cracking behaviour, carrying out three point notched beam tests. The equivalent and the residual flexural tensile strength parameters, proposed by RILEM TC 162-TDF to characterize the toughness of steel fibre reinforced concrete (SFRC), are determined. To evaluate the stress-crack opening diagram of the designed SFRSCC, an inverse analysis is performed fitting the obtained experimental force-deflection curves.FEDER. MCT. Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BSAB/291/2002-POCTI. Fundo Social Europeu (FSE) - SFRH/BSAB/291/2002-POCTI. MBT. SECIL. Bekaert. Comital. “Central do Pego”. Civitest Lda

Transport properties of graphene with one-dimensional charge defects

We study the effect of extended charge defects in electronic transport properties of graphene. Extended defects are ubiquitous in chemically and epitaxially grown graphene samples due to internal strains associated with the lattice mismatch. We show that at low energies these defects interact quite strongly with the 2D Dirac fermions and have an important effect in the DC-conductivity of these materials.Comment: 6 pages, 5 figures. published version: one figure, appendix and references adde

Electrically tunable resonant scattering in fluorinated bilayer graphene

Adatom-decorated graphene offers a promising new path towards spintronics in the ultrathin limit. We combine experiment and theory to investigate the electronic properties of dilutely fluorinated bilayer graphene, where the fluorine adatoms covalently bond to the top graphene layer. We show that fluorine adatoms give rise to resonant impurity states near the charge neutrality point of the bilayer, leading to strong scattering of charge carriers and hopping conduction inside a field-induced band gap. Remarkably, the application of an electric field across the layers is shown to tune the resonant scattering amplitude from fluorine adatoms by nearly twofold. The experimental observations are well explained by a theoretical analysis combining Boltzmann transport equations and fully quantum-mechanical methods. This paradigm can be generalized to many bilayer graphene-adatom materials, and we envision that the realization of electrically tunable resonance may be a key advantage in graphene-based spintronic devices.We thank X. Hong for helpful discussions. A.S., J.L., and J.Z. are supported by ONR under Grant No. N00014-11-1-0730 and by NSF CAREER Grant No. DMR-0748604. A.F. and N.M.R.P. acknowledge EC under Graphene Flagship (Contract No. CNECT-ICT-604391). A.F. gratefully acknowledges the financial support of the Royal Society (U.K.) through a Royal Society University Research Fellowship. We acknowledge use of facilities at the PSU site of NSF NNIN