SciELO Brasil - Scientific Electronic Library Online
(53.052 recursos)
SciELO (Scientific Electronic Library Online) is an electronic library covering a selected collection of Brazilian scientific journals. The objective of the site is to implement an electronic virtual library, providing full access to a collection of serial titles, a collection of issues from individual serial titles, as well as to the full text of articles. The project envisages the development of a common methodology for the preparation, storage, dissemination and evaluation of scientific literature in electronic format.
Mostrando recursos 1 - 20 de 1.436
1.
Defeitos superficiais em 2H-WS2 observados por microscopia de tunelamento - Wypych,F.; Weber,Th.; Prins,R.
Scanning tunnelling microscopy (STM) was used to characterise the basal surface of fresh cleaved crystals of 2H-WS2. Although no impurity or stacking faults could be detected by X-ray diffraction, STM images obtained with negative bias voltage showed two kinds of defects. These defects were attributed to an iodine derivative used as transport agent. In a flat surface free of defects, an image with atomic resolution was achieved with sulphur distances and angles as expected for hexagonal symmetry of 2H-WS2.
2.
Efeito do campo magnético na precipitação de parafinas - Rocha,Nelson de Oliveira; González,Gaspar; Vaitsman,Delmo Santiago
In the oil industry, the paraffination phenomenon is a major problem during oil production, displacement and treatment. Paraffin deposition in subsea flowlines, surface equipment, production string or even in the reservoir, can cause significant and increasing oil losses. To minimize paraffin precipitation, the application of magnetic field in the petroleum path has been suggested based on empiric studies. In this study, we assembled a labscale magnetic conditioner to determine the influence of magnetic field on the physical-chemical properties of two fluids : oil and a paraffin mixture. We observed that magnetic field reduce sample aparent viscosity due to crystal morphology...
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Modificação por via enzimática da composição triglicerídica do óleo de piqui (Caryocar brasiliense Camb) - Facioli,Nara Lúcia; Gonçalves,Lireny A.G.
Piqui (Caryocar brasiliense Camb) oil was transformed into a cocoa butter-like fat through an enzymatic interesterification reaction using Lipozyme in the presence of stearic acid that was incorporated in the sn 1,3-3,1 position into triglyceride. Stearic acid incorporation was determined by HPLC, based on the quantification of the principal triglycerides (POP, POS e SOS) found in cocoa butter. The proposed process was feasible with a reaction time of 240 minutes with 10% of Lipozyme at 70ºC and substract weight ratio of 0,33 (stearic acid:piqui oil).
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Eletrólise de resíduos poluidores: I - Efluente de uma indústria liofilizadora de condimentos - Angelis,Dejanira F. de; Corso,Carlos R.; Bidoia,Ederio D.; Moraes,Peterson B.; Domingos,Roberto N.; Rocha-Filho,Romeu C.
Wastewater from a seasoning freeze-drying industry was electrolysed to increase its biodegradability. Stainless-steel electrodes were used at 9.09 A/m², for up to 80 min. Conductivity, pH, biochemical (BOD) and chemical (COD) oxygen demands, Daphnia similis acute toxicity bioassays, and bacteria counting through the plate count agar method were determined after different times of electrolysis. The results (e.g. higher BOD and lower COD) showed that the biodegradability of the wastewater was significantly increased; furthermore, Fe2+ ions liberated by the electrodes cause microorganisms to die and, when oxidised to Fe3+, contribute for the flocculation and sedimentation of solid residues.
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Modelagem dos processos químicos em plasmas de misturas gasosas usadas na corrosão de silício. Parte 1: CF4 / O2 - Bauerfeldt,G. F.; Arbilla,G.
The plasma etching of semiconductor surfaces with fluorine-containing compounds has technological interest. Presently, considerable effort is being devoted to understand the chemistry involved. In this work, a numerical modeling analysis of the gas-phase decomposition of CF4/O2 mixtures, in the presence of silicon, was performed. The relative importance of individual processes was determined as well as the effect of the parameters' uncertainties. The results were compared with experimental data. The main etching agent in the system is the fluorine atom. The concentration of the main species, SiF4, CO, CO2 and COF2 depend on the composition of the mixture.
6.
Modelagem dos processos químicos em plasmas de misturas gasosas usadas na corrosão de silício. Parte 2: SF6 / O2 - Bauerfeldt,G. F.; Arbilla,G.
In this work, a numerical modeling analysis of the gas-phase decomposition of SF6 / O2 mixtures, in the presence of silicon, was performed. The relative importance of individual processes and the effect of the parameters' uncertainties were determined. The model was compared with experimental data for the plasma etching of silicon and with the calculated results for the CF4 / O2 system. In both systems the main etching agent is the fluorine atom and the concentration of the major species depends on the composition of the mixture. The etching rate is greater for SF6 / O2.
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Excitação eletrônica das moléculas de metacrilato de metila e estireno na região do ultravioleta de vácuo - Rocco,M. L. M.; Souza,G. G. B. de; Lopes,M. C. A.; Lucas,C. A.
Angle-resolved electron energy-loss spectra have been measured for the methyl methacrylate (MMA) and styrene molecules in the 0 - 50 eV energy range. The spectra have been obtained at 1 keV incident energy, with an energy resolution of 0.8 eV and covering an angular range of 2.0 to 7.0 degrees. Within our knowledge, this is the first gas-phase excitation spectrum for MMA and styrene in this energy range. The spectra of MMA at small scattering angles are dominated by an intense peak at 6.7 eV followed by a broad band centered at about 16 eV. In the case of styrene,...
8.
Determinação espectrofotométrica de ácido ascórbico em fármacos empregando amostragem binária em fluxo - Paim,Ana Paula S.; Kronka,Eloisa Ap. M.; Reis,Boaventura F.; Korn,Mauro
A flow system procedure for spectrophotometric determination of ascorbic acid in drugs based on [Fe3+(SCN-)n]+3-n complex decomposition by reduction is described. The flow network was constituted by a set of three-way solenoid valves, controlled by a microcomputer running a software wrote in QuickBasic 4.5 language. The feasibility of the procedure was ascertained by determining ascorbic acid in drug samples with mass ranging from 0.0018 up to 0.0180 g. The results showed an agreement of about 7% when compared with recommended method. Other profitable features such as a standard deviation of 1.5% (n = 7) and a throughput of 120 determinations...
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Uma metodologia para o projeto teórico de conversores moleculares de luz - Andrade,Antônio V. M. de; Costa Jr.,Nivan B. da; Simas,Alfredo M.; Longo,Ricardo L.; Malta,Oscar L.; Sá,Gilberto F. de
Recentlly, we have proposed the representation of lanthanides within AM1 as sparkles for the purpose of obtaing ground state geometries of their complexes. We tested our quantum chemical sparkle model (SMLC/AM1) for the prediction of the crystallographic structure of complexes with coordination number nine, eight and seven. A technique is introduced for the theoretical prediction of eletronic spectra of the organic part of lanthanide complexes by replacing the metal ion by a point charge with the ligands held in their positions as determined by the SMLC/AM1, and by computing the theoretical spectra via the intermediate neglect of differential overlap/spectroscopic-configuration interaction...
10.
Extração líquido-líquido em sistemas de fluxo - Facchin,Ileana; Pasquini,Celio
This review deals with principles of the liquid-liquid extraction, when performed in flow systems. This approach is frequently used for sample treatment to improve the selectivity and/or sensitivity in analytical measurements. The advances in this area are reported, including the use of monosegmented flow systems to perform metal extraction through both two-phase and single phase processes.
11.
A fotocatálise heterogênea e sua aplicação ambiental - Nogueira,Raquel F. P.; Jardim,Wilson F.
This article gives some basic principles of heterogeneous photocatalysis using titanium dioxide as photocatalyst and the state of art of its applications to the abatement of aqueous and atmospheric pollutants.
12.
A contaminação dos oceanos por radionuclídeos antropogênicos - Figueira,Rubens C. L.; Cunha,Ieda I. L.
Several hundreds of artificial radionuclides are produced as the result of human activities, such as the applications of nuclear reactors and particle accelerators, testing of nuclear weapons and nuclear accidents. Many of these radionuclides are short-lived and decay quickly after their production, but some of them are longer-lived and are released into the environment. From the radiological point of view the most important radionuclides are cesium-137, strontium-90 and plutonium-239, due to their chemical and nuclear characteristics. The two first radioisotopes present long half life (30 and 28 years), high fission yields and chemical behaviour similar to potassium and calcium, respectively....
13.
Ultramicroeletrodos. Parte II: construção e aplicações - Silva,S. M.; Alves,C. R.; Correia,A. N.; Martins,R. M.; Nobre,A. L. R.; Machado,S. A. S.; Mazo,L. H.; Avaca,L. A.
This work describes the techniques of construction and several applications of ultramicroelectrodes in electrochemistry and electroanalytical chemistry. Disc shaped UME are produced by embedding metal wires on insulating materials such as glass or epoxy resin. In the field of electrochemistry, UME have been applied in studies of the hydrogen evolution reaction and the electrocrystallization of metals. The negligible values of sensibility for ohmic drop and the enhanced mass transport rate by spherical diffusion are the main advantages of UME in these applications. New important conclusions regarding the phenomena under study were drawn from the experimental results. The applications in electroanalytical...
14.
Anticorpos catalíticos: expandindo o alcance da catálise enzimática - Justo,Giselle Zenker
The vast binding repertoire of the immune system has been exploited for the generation of tailor-made selective catalysts. Since the first reports of chemical reactions catalyzed by antibodies were published, research in this field, which borders chemistry and biology, has been rapidly established and a number of catalytic antibodies that carry out a wide range of reactions, have been developed. Recent advances have led to antibodies that catalyse complex, multi-step reactions and difficult chemical transformations, as well as reactions that do not have an organic equivalent at all. Current research in this field has been devoted to practical applications of...
16.
Porta-amostras polimérico para espectroscopia infravermelha de sistemas líquidos aquosos ou orgânicos - Wassermann,Adriana I.; Gorga,Carlos J. P.; Baumhardt-Neto,Ricardo
Infrared spectroscopy laboratories works under permanent care with respect to water contamination, mainly in liquid samples. In this case crystal plates destruction or damage are frequent, increasing the operational expenses. On the other hand, the laboratory which produces such samples must be very careful in drying liquid samples. In this work we develop a simple and inexpensive way to operate in such conditions using polypropylene and HDPE films which were thermally soldered resulting little containers or sample holders. The spectra of sample/sample holder is achieved having the sample holder as background.
17.
Modificação de um micro-extrator de vidro para pré-enriquecimento de traços de pesticidas organoclorados de água para análise por cromatografia gasosa - Rocha,Eduardo Carasek da; Pini,Glaucia Falleiros; Augusto,Fabio; Valente,Antonio Luiz Pires
Two models of a glass micro-extractor were constructed after modification of a model proposed in the literature. The two models were used for the simultaneous extraction and enrichment of organochlorinated pesticides from aqueous matrixes to an organic solution appropriate for the gas chromatographic analysis of the pesticide. It was established that the performance of one of the modified micro-extractors permits the pre-concentration of the pesticides to a level that allow their quantitation at the trace level with electron capture detection. It is thus concluded that the glass micro-extractor is a suitable tool for the sample preparation step in the gas...
18.
Simulação do processo Solvay no laboratório didático - Araújo,Adriano L. de; Neves,Carlos A.; Ferreira,Ana Maria C.; Koiti,Araki
The Solvay Process is an important route for the industrial preparation of sodium carbonate from brine and CO2. However, experiments illustrating such process are performed only in a few chemistry courses, mainly because of the difficulties to precipitate out the intermediate product NaHCO3 within the 3 or 4 hours generally available for laboratory classes. In this article we describe a method that properly simulates the Solvay process using glass apparatus that can be found in any chemistry laboratory. The good yields and purity attainable, associated with the short time (~3 hours) required to get the sodium bicarbonate, account for the...
20.
Troca iônica no estado sólido de európio3+ em zeólita Y: influência do tempo de reação - Nassar,Eduardo J.; Serra,Osvaldo A.; Souza-Aguiar,Eduardo F.
Rare earth elements supported in zeolites are the most important catalysts in the fluid cracking of petroleum. The solid state ion exchange of Eu3+ in Y zeolite was investigated. First of all, the hydrated EuCl3 was well mixed in a ball mill and was then heated at 300ºC for different times. The quantitative determination of Eu3+ showed that the degree of ion exchange depends on the reaction time at constant temperature, being ~95% in 4 h. The X-ray study showed that the crystallinity of the zeolite is little affected by the exchange procedure. The study of spectroscopic properties of Eu3+,...
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