ESTG - Artigos em revistas internacionais
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- Bidentate Urea Derivatives of p-tert-Butyldihomooxacalix[4]arene: Neutral Receptors for Anion ComplexationPublication . Marcos, Paula M.; Teixeira, Filipa A.; Segurado, Manuel A. P.; Ascenso, José R.; Bernardino, Raul J.; Michel, Sylvia; Hubscher-Bruder, VéroniqueThree new bidentate ureidodihomooxacalix[4]- arene derivatives (phenyl 5a, n-propyl 5b, and tert-butyl 5c) were synthesized in four steps from the parent compound ptert- butyldihomooxacalix[4]arene and obtained in the cone conformation, as shown by NMR studies. The binding ability of these neutral receptors toward spherical, linear, trigonal planar, and tetrahedrical anions was assessed by 1H NMR and UV-vis titrations. The structures and complexation energies of some complexes were also studied by DFT methods. The data showed that the association constants are strongly dependent on the nature of the substituent (aryl/alkyl) at the urea moiety. In general, for all the receptors, the association constants decrease with decrease of anion basicity. Ph-urea 5a is the best anion receptor, showing the strongest complexation for F- (log Kassoc = 3.10 in CDCl3) and also high binding affinity for the carboxylates AcO- and BzO-. Similar results were obtained by UV-vis studies and were also corroborated by DFT calculations.
- Error probability upper bound for perfect sequences implemented with super‐structured fibre Bragg gratingsPublication . Pereira, João S.; Silva, Henrique J. A. daThe success of coherent optical code‐division multiple‐access (OCDMA) systems is strongly dependent on the optical encoder/decoder technology and on the selection of the correct OCDMA codes/sequences. For this reason, in this study, the authors present a method to implement perfect sequences with Super‐Structured Fibre Bragg Gratings (SSFBGs). A new SSFBG power reflection model has been found. They have also derived a property that explains why the SSFBGs should use codes derived from m-sequences. Usually, OCDMA researchers try many different codes into SSFBGs in order to select the SSFBG encoders that result in lower error probability. In the authors work, they show that a SSFBG can be considered to be a perfect sequence encoder. For this reason, the codes written into the SSFBGs should be selected based on their new property. This property permits to design and select quickly the correct codes with low power contrast ratios. In addition, a new error probability upper bound, which is a function of the code family and of its power contrast ratio is also presented. With this new bound, it is not necessary to use an optical simulator to estimate the maximum bit error rate of an OCDMA system, if some power contrast ratios of the selected SSFBG code set are known.
- A new electric conductance conversion method suitable for very low power applicationsPublication . Miranda, Nuno; Serôdio, Carlos; Morais, RaulThis article describes a new electrical conductance converter method suitable for low power applications and an implementation in standard CMOS technology. Despite being designed to meet specific measurements requirements, this converter is intended for applications where device power requirements are determinative such self powered sensors networks and implantable devices. The topology is described and an implementation is presented. Results show the possibility of being powered by a single 1.2 V accumulator cell with a consumption of 8 nJ per conversion.
- Influence of Superposition Length on Transverse Impact Response of Single-Strap Adhesive JointsPublication . Reis, P. N. B.; Ferreira, J. A. M.; Pereira, A. M.; Antunes, F. J. V.Adhesive joints are usually designed to carry in-plane loads, but in many cases they are also prone to transverse loading. On the other hand, the impact response of adhesive joints has received limited attention compared to quasi-static loading. Therefore, the present paper aims to study the influence of superposition length on transverse impact response of single-strap adhesive joints. For this purpose, low-velocity impact tests were performed using a drop weight-testing machine with a hemispherical impactor falling at the center of a bi-clamped specimen. The specimens were manufactured using Docol 1000 high-elastic limit steel, with 1.5 mm of thickness, and an Araldite® 420 A/B adhesive (Huntsman Advanced Materials, Everberg, Belgium). The collapse thresholds obtained were 61.6 J, 75.1 J, and 77.5 J, respectively, for adhesive joints with gap length of ℓo = 0, 10, and 20 mm. An adhesive fracture occurred for the three geometries and the cracks initiated at the corner of the joint where the deflection is higher. Joints with higher ℓo have higher impact energies, despite the lower bonding area, as consequence of the lower local deformation. A numerical study was developed and the zero gap (ℓo = 0) gives maximum peel stress.
- Interlaminar Adhesive Strength of Nano-Reinforced Glass/Epoxy LaminatesPublication . Silva, H.; Ferreira, J. A. M.; Costa, J. D. M.; Capela, C.Interlaminar fracture is significantly influenced by the adhesive strength of fiber/matrix interfaces. Critical strain energy release rate (GC) is the most common parameter used to quantify the interfacial strength. However, subcritical debonding can occur at lower mechanical loads than those required for interlaminar fracture toughness (IFT). This study was performed using nanoclay-reinforced epoxy/glass fiber laminates in order to analyze the influence of the addition of nanoclay and hydro aging on IFT and subcritical crack growth. Hydro aging was done immersinga batch of specimens in distilled water at 25°C for 30 days. Mode I IFT was significantly improved by the incorporation of nanoclays into the resin, the improvement reaching 31% for 3% of nanoclays content. The results of subcritical debonding were plotted in terms of da/dt versus G curves, for dry materials and long term hydro aged composites. Hydro aged composites exhibit not only a reduction of GI c, of about 14% for 3% of nanoclays, but also a higher subcritical crack propagation rate. The addition of nanoparticles reduces subcritical crack propagation rate.
- Human Cranial SimulationPublication . Perestrelo, Pedro; Bártolo, Paulo; Torres, Maurício; Noritomi, Pedro; Silva, JorgeBeing the traumatic brain injury (TBI) a devastating cause of death, it grows day after day on a worldwide basis. So, the effort to prevent, detect and understand it must be pushed forward. In order to improve the technics applied today and achieve a much stronger effort, biomechanical and clinical theories need to be joined, leading to better technical approaches of the problem and to an improved understanding of it. In the light of this acknowledgement, it is proposed the development of a virtual platform using the BioCAD protocol, computer aided design software (CAD) and finite element method (FEM) analysis software. The computational model to achieve has to be adapted to the needs of the user and/or patient and will result in an innovative tool most needed to an improved development of the research and prevention of TBI. As a consequence this area will be pushed to a new level of understanding.
- Structure and ionic conductivity of reactively sputtered apatite-type lanthanum silicate thin filmsPublication . M.M. Vieira; Oliveira, J. C.; Shaula, A. L.; Trindade, B.; Cavaleiro, A.La–Si–O thin films with Si/(La + Si) atomic ratios ranging from 0.36 to 0.43 were produced by magnetron sputtering in a reactive Ar/O discharge gas. The as-deposited films have large X-ray diffraction peak characteristic of quasi-amorphousmaterials and oxygen contents from29 to 35 at.%. The Apatite-type lanthanumsilicate phase was formed in all the as-deposited films upon annealing at 900 °C for 1 h. The lanthanumsilicate films obtained by annealing the as-deposited filmswith lower Si/(La+Si) atomic ratios have a preferential orientationwith the c-axis perpendicular to the substrate while low intensity diffraction peaks ascribed to La2Si2O7 phase were detected in the films deposited with higher Si content. The preferentially oriented films have higher activation energy and lower ionic conductivity as the ionic conductivity measurements were performed in the direction perpendicular to the c-axis. The highest ionic conductivity was obtained for the film deposited with a Si/(La + Si) atomic ratio of 0.42, with a value of 1.2 × 10−2 S·cm−1 at 750 °C.
- A Novel Time-Domain CAD Technique Based on Automatic Time-Slot Division for the Numerical Simulation of Highly Nonlinear RF CircuitsPublication . Ferreira, David; Oliveira, Jorge F.; Pedro, José C.This paper presents a new computer-aided design tool specifically developed for the efficient time-domain simulation of strongly nonlinear heterogeneous RF circuits whose stimuli are turned off for unknown periods of time, hence leaving the systems in a natural response state. The proposed technique is implemented in a completely different approach in comparison to the techniques previously published in the literature. It is based on a time-slot division strategy with automatic switching between different algorithms along the simulation process. Significant gains in computational speed over commercial simulation tools are reported without compromising the accuracy of the results, as this technique takes advantage of the unknown on-off state of the stimulus, while commercial tools would have to rely on classical SPICE-like simulation methods.
- Rate control for unbalanced multiple description video streamingPublication . Correia, Pedro; Assuncao, Pedro; Silva, VitorUnbalanced multiple description coding (U-MDC) of video signals aims to obtain different coding rates for transmission over channels with asymmetric available bandwidth. This paper proposes a new rate control method, based on Multiple Description Scalar Quantisation (MDSQ), to generate unbalanced descriptions for asymmetric multipath video streaming. A new set of index assignment tables combined with an extension of the ρ model is devised to generate two unbalanced descriptions with controlled rate, using U-MDC under the general coding framework of H.264/AVC. This work demonstrates that a linear relationship between the rate and percentage of zeros in the transform coefficients of each description is maintained across different description domains, i.e., when moving from the single description domain (SDC) into the MDC domain. The simulation results show that rate-distortion performance of U-MDC is better than its equivalent balanced MDC. The proposed rate control algorithm exhibits high accuracy in achieving different target bitrates and unbalanced ratios between two descriptions. Improved performance is also achieved in video streaming over asymmetric transmission paths, where each description rate can be adapted according to different channel conditions.
- A multidisciplinary framework to support the design of injection mold toolsPublication . Ferreira, Irene; Cabral, J. A.; Saraiva, Pedro; Oliveira, M. C.The design of injection mold tools is a complex process for which market pressures demand ever-shorter development time and higher quality level. Thus, it is considered imperative to adopt new methods and tools to support the design process, in order to achieve a more effective mold design solution. Based on this assumption, a multidisciplinary framework was developed, centered on Design for Six Sigma methodology and reinforced with a set of highly valued techniques, namely: the European Customer Satisfaction, the Axiomatic Design and the Multidisciplinary Design Optimization. As a result, a platform was built to support the design of any mold tool without undercuts, which tackles the design of an injection mold as a fully integrated multidisciplinary optimization problem, oriented by customer preferences and their impositions. A set of specific analysis sub-modules is integrated through an overseeing optimization code system, responsible for the numerical simulation of the injection process. This platform was validated through the comparison with an existing mold, whose results attained highlight the great potential of the proposed framework to achieve mold design improvement. In particular, the value of mold solutions generated led to a global improvement on mold performance by 5 %.