Espectroscopia de impedância eletroquímica da biodissolução da calcopirita na presença de íons ferrosos

Pós-graduação em Química - IQ

Estudo do efeito do transporte de massa nas curvas de polarização potenciodinâmica em ligas de alumínio da série 7XXX de aplicação aeronáutica

Aluminum Alloys are widely used as structural materials in the aerospace industry due to low weight, high mechanical strength and enduring corrosion resistance. Their resistance to corrosion is attributed to the rapidly formed stable oxide film (Al2O3) which spontaneously forms itself on the surface of the material. However, in the presence of aggressive ions, such as halide, Aluminum Alloys are subject to a localized process of corrosion. The electrochemical behavior of 7081-T73511 and 7050-T7451 Aluminum Alloys employed in the aerospace industry was investigated using a 0.6 M NaCl solution under the conditions of a controlled mass transport employing a rotating disk electrode. The theoretical limiting current density was determined by the Kouteki-Levich equation. The results confirmed that the inter-metallic Al7Cu2Fe acts as preferential cathode generating the galvanic coupling and the dissolution of the Aluminummatrix around it.

Polymer-liquid crystal interface: a molecular dynamics study

Liquid crystals (LCs) are an interesting class of soft condensed matter systems characterized by an unusual combination of fluidity and long-range order, mainly known for their applications in displays (LCDs). However, the interest in LC continues to grow pushed by their application in new technologies in medicine, optical imaging, micro and nano technologies etc. In LCDs uniaxial alignment of LCs is mainly achieved by a rubbing process. During this treatment, the surfaces of polymer coated display substrates are rubbed in one direction by a rotating cylinder covered with a rubbing cloth. Basically, LC alignment involves two possible aligning directions: uniaxial planar (homogeneous) and vertical (homeotropic) to the display substrate. An interesting unresolved question concerning LCs regards the origin of their alignment on rubbed surfaces, and in particular on the polymeric ones used in the display industry. Most studies have shown that LCs on the surface of the rubbed polymer film layer are lying parallel to the rubbing direction. In these systems, micrometric grooves are generated on the film surface along the rubbing direction and also the polymer chains are stretched in this direction. Both the parallel aligned microgrooves and the polymer chains at the film surface may play a role in the LC alignment and it is not easy to quantify the effect of each contribution. The work described in this thesis is an attempt to find new microscopic evidences on the origin of LC alignment on polymeric surfaces through molecular dynamics (MD) simulations, which allow the investigation of the phenomenon with atomic detail. The importance of the arrangement of the polymeric chains in LCs alignment was studied by performing MD simulations of a thin film of a typical nematic LC, 4-cyano-4’-pentylbiphenyl (5CB), in contact with two different polymers: poly(methyl methacrylate)(PMMA) and polystyrene (PS). At least four factors are believed to influence the LC alignment: 1. the interactions of LCs with the backbone vinyl chains; 2. the interactions of LCs with the oriented side groups; 3. the anisotropic interactions of LCs with nanometric grooves; 4. the presence of static surface charges. Here we exclude the effect of microgrooves and of static surface charges from our virtual experiment, by using flat and neutral polymer surfaces, with the aim of isolating the chemical driving factors influencing the alignment of LC phases on polymeric surfaces.

Shelling mechanism

A corn shelling mechanism comprising an elongated concave means having one end thereof positioned below a rotatable cylinder. A continuous belt sheller is positioned above the concave in a spaced relationship with respect thereto. The corn ears are fed to the rotating cylinder by a feeder housing means with the ear receiving one or more impacts by the rasp bar of the cylinder before it travels out of range of the cylinder. The partially shelled ear passes between the belt sheller and the concave with the belt sheller urging the ears into engagement with the concave to complete the shelling operation.

The colloidal tool-box approach for fuel cell catalysts: Systematic study of perfluorosulfonate-ionomer impregnation and Pt loading

In this study, the correlation between the impregnation of proton exchange membrane fuel cell catalysts with perfluorosulfonate-ionomer (PFSI) and its electrochemical and electrocatalytic properties is investigated for different Pt loadings and carbon supports using a rotating-disk electrode (RDE) setup. We concentrate on its influence on the electrochemical surface area (ECSA) and the oxygen reduction reaction (ORR) activity. For this purpose, platinum (Pt) nanoparticles are prepared via a colloidal based preparation route and supported on three different carbon supports. Based on RDE experiments, we show that the ionomer has an influence both on the Pt utilization and the apparent kinetic current density of ORR. The experimental data reveal a strong interaction in the microstructure between the electrochemical properties and the surface properties of the carbon supports, metal loading and ionomer content. This study demonstrates that the colloidal synthesis approach offers interesting potential for systematic studies for the optimization of fuel cell catalysts.

Electrochemical corrosion of unalloyed copper in chloride media–a critical review

The literature dealing with the electrochemical corrosion characteristics of unalloyed copper in aqueous chloride media is examined. The enormous quantity of polarisation and mixed/corrosion potential data that has been made available in the literature over the last 50 years has been compiled and discussed in a comprehensive review. For a wide range of electrode geometries, the importance of the chloride ion and the mass transport of anodic corrosion products on the corrosion behaviour of copper are made clear for both freshly polished and 'filmed' surfaces. (C) 2003 Elsevier Ltd. All rights reserved.

The performance of stabilized and unstabilized minestone in an aqueous environment with emphasis on its erosion resistance

Britain's sea and flood defences are becoming increasingly aged and as a consequence, more fragile and vulnerable. As the government's philosophy on resources shifts against the use of prime quarried and dredged geo-materials, the need to find alternative bulk materials to bolster Britain's prone defences becomes more pressing. One conceivable source for such a material is colliery waste or minestone. Although a plethora of erosion-abrasion studies have been carried out on soils and soil-cements, very little research has been undertaken to determine the resistance of minestone and its cement stabilized form to the effects of water erosion. The thesis reviews the current extent to which soil-cements, minestone and cement stabilized minestone (CSM) have been employed for hydraulic construction projects. A synopsis is also given on the effects of immersion on shales, mudstones and minestone, especially with regard to the phenomena of slaking. A laboratory study was undertaken featuring a selection of minestones from several British coalfields. The stability of minestone and CSM in sea water and distilled water was assessed using slaking tests and immersion monitoring and the bearing on the use of these materials for hydraulic construction is discussed. Following a review of current erosion apparatus, the erosion/abrasion test and rotating cylinder device were chosen and employed to assess the erosion resistance of minestone and CSM. Comparison was made with a sand mix designed to represent a dredged sand, the more traditional, bulk hydraulic construction material. The results of the erosion study suggest that both minestone and CSM were more resistant to erosion and abrasion than equivalently treated sand mixes. The greater resistance of minestone to the agents of erosion and abrasion is attributed to several factors including the size of the particles, a greater degree of cement bonding and the ability of the minestone aggregate to absorb, rather than transmit shock waves produced by impacting abrasive particles. Although minestone is shown to be highly unstable when subjected to cyclic changes in its moisture content, the study suggests that even in an intertidal regime where cyclic immersion does takes place, minestone will retain sufficient moisture within its fabric to prevent slaking from taking place. The slaking study reveals a close relationship between slaking susceptibility and total pore surface area as revealed by porosimetry. The immersion study shows that although the fabric of CSM is rapidly attacked in sea water, a high degree of the disruption is associated with the edges and corners of samples (ie. free surface) while the integrity of the internal fabric remains relatively intact. CSM samples were shown to be resilient when subjected to immersion in distilled water. An overall assessment of minestone and CSM would suggest that with the implementation of judicious selection and appropriate quality control they could be used as alternative materials for flood and sea defences. It is believed, that even in the harsh regime of a marine environment, CSM could be employed for temporary and sacrificial schemes.

Couette Gas Flow Between Cylinders with Different Temperature

Петър Господинов, Добри Данков, Владимир Русинов, Стефан Стефанов - Изследвано е стационарно течение на Кует на разреден газ в случая на въртене на вътрешния цилиндър и неподвижен външен цилиндър чрез използване на DSMC метод и числено решение на уравненията на Навие–Стокс за относително малка (дозвукова) скорост на въртене. Изследвани са различни случаи при промяна на температурата на въртящият се цилиндър и числото на Кнудсен. Целта на изследването е да се установи влиянието на малки скорости на въртене върху макрохарактеристиките – плътността, скоростта и температурата на газа. Установено е добро съвпадение на резултатите получени по двата метода за Kn = 0.02. Получените резултати са важни при решаването на неравнинни, задачи от микрофлуидиката с отчитане на ефектите на кривината. Ключови думи: механика на флуидите, кинетична теория, разреден газ, DSMC.

XFEL resonant photo-pumping of dense plasmas and dynamic evolution of autoionizing core hole states

Similarly to the case of LIF (Laser-Induced Fluorescence), an equally revolutionary impact to science is expected from resonant X-ray photo-pumping. It will particularly contribute to a progress in high energy density science: pumped core hole states create X-ray transitions that can escape dense matter on a 10 fs-time scale without essential photoabsorption, thus providing a unique possibility to study matter under extreme conditions. In the first proof of principle experiment at the X-ray Free Electron Laser LCLS at SCLAC [Seely, J., Rosmej, F.B., Shepherd, R., Riley, D., Lee, R.W. Proposal to Perform the 1^st High Energy Density Plasma Spectroscopic Pump/Probe Experiment", approved LCLS proposal L332 (2010)] we have successfully pumped inner-shell X-ray transitions in dense plasmas. The plasma was generated with a YAG laser irradiating solid Al and Mg targets attached to a rotating cylinder. In parallel to the optical laser beam, the XFEL was focused into the plasma plume at different delay times and pump energies. Pumped X-ray transitions have been observed with a spherically bent crystal spectrometer coupled to a Princeton CCD. By using this experimental configuration, we have simultaneously achieved extremely high spectral (λ/δλ ≈ 5000) and spatial resolution (δx≈70 μm) while maintaining high luminosity and a large spectral range covered (6.90 - 8.35 Å). By precisely measuring the variations in spectra emitted from plasma under action of XFEL radiation, we have successfully demonstrated transient X- ray pumping in a dense plasma.

Hydrometallurgical process modeling using advanced mathematical tools

Hydrometallurgical process modeling is the main objective of this Master’s thesis work. Three different leaching processes namely, high pressure pyrite oxidation, direct oxidation zinc concentrate (sphalerite) leaching and gold chloride leaching using rotating disc electrode (RDE) are modeled and simulated using gPROMS process simulation program in order to evaluate its model building capabilities. The leaching mechanism in each case is described in terms of a shrinking core model. The mathematical modeling carried out included process model development based on available literature, estimation of reaction kinetic parameters and assessment of the model reliability by checking the goodness fit and checking the cross correlation between the estimated parameters through the use of correlation matrices. The estimated parameter values in each case were compared with those obtained using the Modest simulation program. Further, based on the estimated reaction kinetic parameters, reactor simulation and modeling for direct oxidation zinc concentrate (sphalerite) leaching is carried out in Aspen Plus V8.6. The zinc leaching autoclave is based on Cominco reactor configuration and is modeled as a series of continuous stirred reactors (CSTRs). The sphalerite conversion is calculated and a sensitivity analysis is carried out so to determine the optimum reactor operation temperature and optimum oxygen mass flow rate. In this way, the implementation of reaction kinetic models into the process flowsheet simulation environment has been demonstrated.

Pirólise termoquímica de pós da fibra de coco seco em um reator de cilindro rotativo para produção de bio-óleo

This master thesis aims at developing a new methodology for thermochemical degradation of dry coconut fiber (dp = 0.25mm) using laboratory rotating cylinder reactor with the goal of producing bio-oil. The biomass was characterized by infrared spectroscopy with Fourier transform FTIR, thermogravimetric analysis TG, with evaluation of activation energy the in non-isothermal regime with heating rates of 5 and 10 °C/min, differential themogravimetric analysis DTG, sweeping electron microscopy SEM, higher heating value - HHV, immediate analysis such as evaluated all the amounts of its main constituents, i.e., lignin, cellulose and hemicelluloses. In the process, it was evaluated: reaction temperature (450, 500 and 550oC), carrier gas flow rate (50 and 100 cm³/min) and spin speed (20 and 25 Hz) to condensate the bio-oil. The feed rate of biomass (540 g/h), the rotation of the rotating cylinder (33.7 rpm) and reaction time (30 33 min) were constant. The phases obtained from the process of pyrolysis of dry coconut fiber were bio-oil, char and the gas phase non-condensed. A macroscopic mass balance was applied based on the weight of each phase to evaluate their yield. The highest yield of 20% was obtained from the following conditions: temperature of 500oC, inert gas flow of 100 cm³/min and spin speed of 20 Hz. In that condition, the yield in char was 24.3%, non-condensable gas phase was 37.6% and losses of approximately 22.6%. The following physicochemical properties: density, viscosity, pH, higher heating value, char content, FTIR and CHN analysis were evaluated. The sample obtained in the best operational condition was subjected to a qualitative chromatographic analysis aiming to know the constituents of the produced bio-oil, which were: phenol followed by sirigol, acetovanilona and vinyl guaiacol. The solid phase (char) was characterized through an immediate analysis (evaluation of moisture, volatiles, ashes and fixed carbon), higher heating value and FTIR. The non-condensing gas phase presented as main constituents CO2, CO and H2. The results were compared to the ones mentioned by the literature.

Void fraction measurement and signal analysis from multiple-electrode impedance sensors

Void fraction sensors are important instruments not only for monitoring two-phase flow, but for furnishing an important parameter for obtaining flow map pattern and two-phase flow heat transfer coefficient as well. This work presents the experimental results obtained with the analysis of two axially spaced multiple-electrode impedance sensors tested in an upward air-water two-phase flow in a vertical tube for void fraction measurements. An electronic circuit was developed for signal generation and post-treatment of each sensor signal. By phase shifting the electrodes supplying the signal, it was possible to establish a rotating electric field sweeping across the test section. The fundamental principle of using a multiple-electrode configuration is based on reducing signal sensitivity to the non-uniform cross-section void fraction distribution problem. Static calibration curves were obtained for both sensors, and dynamic signal analyses for bubbly, slug, and turbulent churn flows were carried out. Flow parameters such as Taylor bubble velocity and length were obtained by using cross-correlation techniques. As an application of the void fraction tested, vertical flow pattern identification could be established by using the probability density function technique for void fractions ranging from 0% to nearly 70%.

Optimizing electrode sites for segmental bioimpedance measurements

Recent advances in the application of bioelectrical impedance analysis (BIA) have indicated that a more accurate approach to the estimation of total body water is to consider the impedance of the various body segments rather than simply that of the whole body. The segmental approach necessitates defining and locating the physical demarcation between both the trunk and leg and the trunk and arm. Despite the use of anatomical markers, these points of demarcation are difficult to locate with precision between subjects. There are also technical problems associated with the regional dispersion of the current distribution from one segment (cylinder) to another of different cross-sectional area. The concept of equipotentials in line with the proximal aspects of the upper land lower) limbs along the contralateral limbs was investigated and, in particular, the utility of this concept in the measurement of segmental bioimpedance. The variation of measured segmental impedance using electrode sites along these equipotentials was less than 2.0% for all of the commonly used impedance parameters. This variation is approximately equal to that expected from biological variation over the measurement time. It is recommended that the electrode sites, for the measurement of segmental bioelectrical impedance in humans, described herein are adopted in accordance with the proposals of the NM Technology Assessment Conference Statement.

The processes involved in the Se electrodeposition and dissolution on Au electrode: the H2Se formation

The processes involved in the Se electrodeposition, mainly the one related to the formation of H2Se species on Au electrode in perchloric acid solutions, have been investigated through cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM), rotating ring-disc electrode (RRDE), and atomic force microscopy (AFM) techniques. In the experiments performed with the EQCM, with the potential sweep in the negative direction, the responses for the mass variation were divided in three well-defined potential regions: A (from 1.55 to 0.35 V), B (from 0.35 to -0.37 V), and C (from -0.37 to -0.49 V). It was verified that the following processes can occur, respectively: the species (AuO)(2)H2SeO3 was desorbed during the AuO reduction, the reduction of Se(IV) to Se(0), and the formation of H2Se. When the potential was swept in the positive direction, the responses for the mass variation were divided in four well-defined potential regions: D (from -0.49 to 0.66 V), E (from 0.66 to 0.99 V), F (from 0.99 to 1.26 V), and G (from 1.26 to 1.55 V), and the described processes in these regions were, respectively: the Se deposition and adsorption of water molecules and/or perchlorate ions, the Se dissolution, the Se incorporating mass in the form of HO-Se, and the Au oxidation (all potentials are referred to the Ag/AgCl electrode). Making use of the RRDE, using the collection technique, the formation of H2Se species during the Se electrodeposition was investigated. Therefore, it was confirmed that this species is formed on the disc electrode between -0.3 and -0.55 V vs the Ag/AgCl potential range (collecting the oxidized compound onto the ring electrode). AFM images also indicated that the surface topography of the Se-massive deposit on Au is different from the images registered after the formation of H2Se species, confirming the cathodic stripping of Se.

Scanned stripping chronopotentiometry at bismuth film rotating disc electrodes: a method for quantitative dynamic metal speciation

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)