Orientador: Iakov Veniaminovitch Kopelevitch / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-19T07:21:49Z (GMT). No. of bitstreams: 1 Merlo_RafaelBorges_M.pdf: 2866064 bytes, checksum: f84ae56edecef43c0c046f5d592e5baa (MD5) Previous issue date: 2011 / Resumo: Evidências experimentais e teóricas recentes de que a supercondutividade nas várias formas alotrópicas do carbono pode ocorrer em temperaturas próximas ou até acima da temperatura ambiente, desencadearam um grande interesse científico. Resultados do presente trabalho demonstraram a ocorrência de supercondutividade em compósitos de carbono vítreo-enxofre (CV-S) a T = 3 K. Nossas medidas revelaram que a supercondutividade ocorre em uma pequena fração da amostra, e que a grafitização do carbono amorfo parece ser uma condição necessária para disparar a supercondutividade. Apresentamos também evidências de supercondutividade à temperatura ambiente em sanduíches de grafite/Si, e demonstramos que a supercondutividade está associada à interface grafite/silício. O comportamento encontrado é semelhante ao conhecido para estruturas supercondutoras de baixadimensionalidade. Em particular, observamos oscilações do tipo Josephson em curvas características de corrente-tensão (I-V), bem como sua supressão pela aplicação de campo magnético. Além disso, o campo magnético perpendicular transforma as curvas características I-V do tipo supercondutor para tipo isolante, assemelhando-se à transição supercondutor-isolante induzida por campo magnético em redes de junções Josephson. Todos estes resultados indicam que a interface grafite/silício pode ser um material promissor para o desenvolvimento de dispositivos microeletrônicos sem dissipação à temperatura ambiente / Abstract: Recent both experimental and theoretical evidence that superconductivity in various allotropic forms of carbon can occur at temperatures near or even above room temperature, triggered a broad scientific interest. Results of the present work demonstrated the occurrence of superconductivity in carbon glassy-sulfur composites (CV-S) at T = 3 K. Our measurements revealed that the superconductivity occurs in a small fraction of the sample, and that the graphitization of the amorphous carbon seems to be a necessary condition to trigger the superconductivity. We also present evidence for the room temperature superconductivity in graphite/Si sandwiches and demonstrate that the superconductivity is associated with the graphite/silicon interface. The found behavior is similar to that known for low-dimensional superconducting structures. In particular, we have observed Josephson-type oscillations in current-voltage (I-V) characteristics as well as their suppression by applied magnetic field. Moreover, the perpendicular magnetic field transforms the superconducting-like to insulating-like I-V characteristics resembling the magnetic-field-driven superconductor-insulator transition in Josephsonjunction-arrays. All these results indicate that graphite-silicon interface can be a promising material for the development of microelectronic devices without dissipation at room temperature / Mestrado / Supercondutividade / Mestre em Física

This research involves the study of the short term loaded behaviour of mechanically degraded reinforced concrete (RC) flexural elements, which are strengthened with fibre reinforced polymer (FRP) composites. The two main objectives have been: (a) to conduct a series of realistic tests, the results of which would be used to establish the design criteria, and (b) to carry out analytical modelling and hence develop a set of suitable design equations. It is expected that this work will contribute towards the establishment of definitive design guidelines for the strengthening of reinforced concrete structures using advanced fibre composites. The experimental study concentrated on the laboratory testing of 30 simply supported, and 4 two-span continuous full size RC beams, which were strengthened by either FRP plates or fabric sheets. The failure modes of these beams, at ultimate limit state, were examined and the influencing factors were identified. A premature and extremely brittle collapse mechanism was found to be the predominant type of failure for beams strengthened with a large area of FRP composites. A modified semi-empirical approach was presented for predicting the failure load of such over strengthened beams. Despite the lack of ductility in fibre composites, it was found that the FRP strengthened members would exhibit acceptable ductile characteristics, if they were designed to be under strengthened. A new design-based methodology for quantifying the deformability of FRP strengthened elements was proposed, and its difference to the conventional concept of ductility was discussed. The available techniques for ductility evaluation of FRP strengthened concrete members were reviewed and a suitable method was recommended for determining ductility level of FRP strengthened members. A non-linear material based analytical model was developed to simulate the flexural behaviour of the strengthened and control beams, the results were seen to match very well. The parametric study provided an insight into the effects of various factors including the mechanical properties and cross sectional area of FRP composites, on the failure modes and ductility characteristics of the strengthened beams. Based on the findings of the experimental and analytical studies, design equations in the BS 8110 format were developed, and design case studies have been carried out. It was concluded that fibre composites could effectively and safely strengthen mechanically degraded reinforced concrete structures if appropriately designed. The modes of failure and the degree of performance enhancement of FRP strengthened beams depend largely on the composite material properties as well as the original strength and stiffness of the RC structure. If the FRP strengthened elements were designed to be under-strengthened, then the premature and brittle failure mode could be prevented and ductile failure mode could be achieved. It was also found that existing steel reinforcement would always yield before the FRP composite reached the ultimate strength. Furthermore, a critical reinforcement ratio, above which FRP strengthening should not be carried out, was defined. It was concluded that FRP strengthening is most suitable for reinforced concrete floor slabs, bridge decks, flanged beams and other relatively lightly reinforced elements. The study also revealed that to avoid a brittle concrete failure, existing doubly reinforced members should not be strengthened by FRP composites.