ES - Artigos em revistas internacionais
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- Assessment of dye distribution in sensitized solar cells by microprobe techniquesPublication . Barreiros, M. Alexandra; Corregidor, V.; Alves, L. C.; Guimarães, Fernanda; Mascarenhas, João; Torres, Erica; Brites, Maria JoãoDye sensitized solar cells (DSCs) have received considerable attention once this technology offers economic and environmental advantages over conventional photovoltaic (PV) devices. The PV performance of a DSC relies on the characteristics of its photoanode, which typically consists of a nanocrystalline porous TiO2 film, enabled with a large adsorptive surface area. Dye molecules that capture photons from light during device operation are attached to the film nanoparticles. The effective loading of the dye in the TiO2 electrode is of paramount relevance for controlling and optimizing solar cell parameters. Relatively few methods are known today for quantitative evaluation of the total dye adsorbed on the film. In this context, microprobe techniques come out as suitable tools to evaluate the dye surface distribution and depth profile in sensitized films. Electron Probe Microanalysis (EPMA) and Ion Beam Analytical (IBA) techniques using a micro-ion beam were used to quantify and to study the distribution of the Ru organometallic dye in TiO2 films, making use of the different penetration depth and beam sizes of each technique. Different 1D nanostructured TiO2 films were prepared, morphologically characterized by SEM, sensitized and analyzed by the referred techniques. Dye load evaluation in different TiO2 films by three different techniques (PIXE, RBS and EPMA/WDS) provided similar results of Ru/Ti mass fraction ratio. Moreover, it was possible to assess dye surface distribution and its depth profile, by means of Ru signal, and to visualize the dye distribution in sample cross-section through X-ray mapping by EPMA/EDS. PIXE maps of Ru and Ti indicated an homogeneous surface distribution. The assessment of Ru depth profile by RBS showed that some films have homogeneous Ru depth distribution while others present different Ru concentration in the top layer (2 lm thickness). These results are consistent with the EPMA/EDS maps obtained.
- Biomarkers of exposure to metal dust in exhaled breath condensate: methodology optimizationPublication . Félix, P. M.; Franco, C.; Barreiros, M. Alexandra; Batista, B.; Bernardes, S.; Garcia, S. M.; Almeida, A. B.; Almeida, S. M.; Wolterbeek, H. Th.; Pinheiro, TeresaIn occupational assessments where workers are exposed to metal dust, the liquid condensate of exhaled breath (EBC) may provide unique indication of pulmonary exposure. The main goal of this study was to demonstrate the quality of EBC to biological monitoring of human exposure. A pilot study was performed in a group of metal dust–exposed workers and a group of nonexposed individuals working in offices. Only metal dust–exposed workers were followed along the working week to determine the best time of collection. Metal analyses were performed with inductively coupled plasma mass spectrometry (ICP-MS). Analytical methodology was tested using an EBC sample pool for several occupationally exposed metals: potassium, chromium, manganese, copper, zinc, strontium, cadmium, antimony, and lead. Metal contents in EBC of exposed workers were higher than controls at the beginning of the shift and remained augmented throughout the working week. The results obtained support the establishment of EBC as an indicator of pulmonary exposure to metals.
- CdTe nano-structures for photovoltaic devicesPublication . Corregidor, V.; Alves, L. C.; Franco, N.; Barreiros, M. Alexandra; Sochinskii, N. V.; Alves, E.CdTe nano-structures with diameter of ∼100 nm and variable length (200–600 nm) were fabricated on glass substrates covered with conductive buffer layers such as NiCr, ZAO (ZnO:Al2O3 + Ta2O5) or TiPd alloys. The fabrication process consisted of the starting vapour deposition of metal catalyst dropped layer followed by the isothermal catalyst-prompted vapour growth of CdTe nano-structured layer of controllable shape and surface filling. The effect of buffer layers on the crystallographic orientation and thickness of CdTe nano-structured layers is investigated by means of IBA techniques, SEM and X-ray diffraction. It was shown that the formed CdTe nano-layers have a cubic structure, mainly oriented towards the [1 1 1] crystallographic direction, except for those grown on ZAO layer where the X-ray diffraction signal is very weak to be associated to any crystallographic form. The RBS spectra recorded on different areas of each sample type showed an almost constant thickness and SEM images revealed an homogeneous and dense distribution of the structures. It was also possible to study the first stage of the nano-structures grown on the Bi2Te3 seeds.
- Characterization and electrochemical behaviour of nanostructured calcium samarium manganite electrodes fabricated by RF-Magnetron SputteringPublication . Barrocas, B.; Sério, S.; Rovisco, A.; Nunes, Y.; de Sá, A.I.; Pereira, M. I. da Silva; Jorge, M. E. MeloCa1-xSmxMnO3(0 . x . 0.4) films were successfully fabricated on Indium Tin Oxide (ITO) coated quartzglass substrates by radio frequency magnetron sputtering technique (RF- magnetron sputtering) fromcompacted nanosized powder targets, and subsequent annealing at 800.C in air, for 6 h. X-ray diffrac-tion shows a pure typical perovskite phase for x . 0.1. Scanning electron microscopy and atomic forcemicroscopy revealed that the films surface is dense, with low roughness, depending on the Sm content, even though a few cracks were observed. Crystallite size was found to decrease with the Sm content. Theelectrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The oxide electrode fs capacitance was estimated using both techniques and the corresponding roughness factors evaluated. The values obtained from the two methods show a good agreement. A com-parison between the voltammetric data and those referred in the literature allowed finding out that theredox reaction occurring at the electrode surface involves the pair Mn4+/Mn3+. EIS measurements con-firm the voltammetric data and they also give additional information about the film porosity and the charge transfer resistance. This last parameter is associated with the oxidation and reduction of the pairMn3+/Mn4+and after normalized by the roughness factor shows an increase with samarium content.
- Chemical Activities, a(H) and a(X), of Constituents in H2X Type Gas Molecules (X = O or S) at Arbitrary Degree of DissociationPublication . Shohoji, NobumitsuChemical activities, a(X) and a(H), of constituents, X and H, in H2X type gas molecules (X = S or O) were evaluated as functions of temperature T and extent α of dissociation adapting a thermodynamic analysis procedure developed by Katsura for interpreting enhanced a(N) and a(H) in NH3 gas molecules with suppressed α by flowing. Present analysis results showed that both H2S and H2O gas molecules are chemically rather inert even at comparatively low α unlike nitrogen-family tri-hydrides XH3 that were proved to yield high chemical activity of each constituent in a state being away from thermodynamic equilibrium. The parameter α referring to the extent of dissociation of HnX type gas molecules appears to be a significant parameter in evaluating the chemical activities, a(X) and a(H), in the HnX gas molecules that are remained non-dissociated.
- Computational model of a Calcium-looping fluidized bed calcination reactor with imposed concentrated solar irradiancePublication . Rodrigues, Diogo; Rivero, Mayra Alvarez; Pinheiro, Carla I.C.; Cardoso, João P.; Mendes, Luís FilipeABSTRACT: The Calcium-looping process is a promising option for thermochemical energy storage in concentrating solar power plants. A crucial element of this process is the solar calcination reactor, where the endothermic reaction of CaCO3 calcination occurs with formation of CaO and CO2. The solar energy that is chemically stored in the reaction products can be retrieved by the exothermic reaction of CaO carbonation when needed. In this article, a new computational model is developed for the solar calcination reactor in this Calcium-looping process. The calcination reaction takes place in the riser of a continuous circulating fluidized bed that corresponds to an absorber tube exposed to concentrated solar radiation, which allows the reaction chamber to be indirectly heated. A core-annulus heat transfer model and a modified version of the Kunii-Levenspiel fluid dynamics model are used. In contrast to previous models found in the literature, the change in the mass flow rate of the species and in the density of the phases due to the reaction is considered. Simulation studies are performed with a fixed and imposed concentrated solar irradiance on the reactor wall, which varies in both the axial and angular directions. Wall conduction in the angular direction is also considered. The results show that nearly complete calcination can be achieved with a reactor of 4 m of height. A sensitivity analysis with respect to the model parameters and inlet conditions shows that the calcination conversion is mostly affected by the solids mass flow rate and the bed temperature at the inlet.
- Cork: a strategic materialPublication . Gil, LuísCork is a material whose applications have been known since Antiquity, especially in floating devices and as stopper for beverages, mainly wine, whose market, from the early twentieth century, had a massive expansion, particularly due to the development of several cork based agglomerates. Cork is closely related to the maintenance of biodiversity, the heart of sustainable development, and the reduction of emissions and sequestration of CO2, aspects that, additionally to the environmental importance, are also economically very important (Gil, 2011). Other services such as the formation of the landscape, soil protection, regulation of hydrological cycle, are also very sound (Pereira, 2007). Cork regenerates after each stripping, and the cork tree survives the lost of an important quantity, often more than 50% of the total trunk and branches' surface. The fact that corks are made of the bark harvested from living trees has lead environmentalists to encourage the use of cork over other, less natural, alternatives.
- Cork: sustainability and new applicationsPublication . Gil, LuísCork is a strategic material used in multiple applications and its use has accompanied mankind since the days of Ancient Egypt. The cork oak forests are extremely well adapted to the semi-arid regions of southern Europe and northern Africa (western Mediterranean). These forests help to prevent the advance of desertification, improve water penetration into the soil and hydrological regulation, promote soil conservation, and being the perfect habitat for many animal and vegetables species. Consequently, these forests promote biodiversity. Save the cork forests, increasing the area and the quantity and quality of cork produced and develop new products with high added value are essential actions. The loss of the economic importance of the cork activity would lead to an uncertain future of cork oak forests, promoting biodiversity loss, land abandonment (social desertification), and also social imbalance. This could lead to the disappearance of one of the most sustainable industries based on forest products, beyond the environmental and nature problems created. One of the ways to decrease the amount of carbon dioxide in the atmosphere is the production of long-life products based on plant biomass, which include without doubt, cork products. Furthermore, these materials are “carbon neutral” at the time of decomposition or energy recovery.
- Coumarin dye with ethynyl group as p-spacer unit for dye sensitized solar cellsPublication . Torres, Erica; Sequeira, S.; Parreira, P.; Mendes, Paulo; Silva, Tiago J. L.; Lobato, Killian; Brites, Maria JoãoC1-LEN coumarin dye, containing an ethynyl conjugated bridge was tested as a new chromophore for dye-sensitized solar cells (DSSCs). Investigation on the relationship of dye structure, optical properties, electrochemical properties and performance of DSSCs is described. The C1-LEN-sensitized solar cells showed an overall conversion efficiency of 2.2% (JSC = 6.11 mA/cm2, VOC = 547 mV, FF = 0.66), which corresponds to 68% of N719-based device efficiency, fabricated under similar conditions. Electrochemical impedance spectroscopy (EIS) analysis reveals that charge recombination at the nanocrystalline TiO2/dye/redox electrolyte interface is similar for C1-LEN and N719 sensitized solar cells, i.e., charge recombination is not the factor limiting the performance of C1-LEN device.
- Development and performance analysis of a metallic passive micro-direct methanol fuel cell for portable applicationsPublication . Falcão, D. S.; Pereira, J. P.; Rangel, C. M.; Pinto, A. M. F. R.Due to the growing interest on miniaturization for application on portable devices, the Micro Direct Methanol Fuel Cells (Micro-DMFC) proved to have great benefits. Passive fuel cells have extra advantages leading to less complex and cheaper systems. In the present work, an experimental study on the performance of a passive Micro-DMFC with an active area of 2.25 cm2 working at ambient conditions is described. Several commercially available materials for Membrane Electrode Assembly (MEA) are tested including materials with low platinum content to achieve lower prices. The effect of methanol concentration on the cell performance is evaluated. The performance is compared with the one obtained using an active Micro-DMFC with the same active area. A optimized design is proposed corresponding to a maximum power density, 19.2 mW/cm2, obtained using a Nafion 117 membrane, 3 mg/cm2 Pt–Ru and 0.5 mg/cm2 Pt as, respectively, anode and cathode catalyst loading, carbon paper as anode gas diffusion layer (GDL) and Sigracet carbon paper with micro porous layer (MPL) as cathode GDL at methanol feed concentration of 3 M. This result higher than the optimal power obtained with the active Micro-DMFC clearly demonstrates that membranes with low catalyst content could be used in passive MicroDMFC with success. This is an important result bearing in mind the use of micro-DMFCs in portable applications.