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941.
Helder Fleury Pinheiro Attílio José Giarola Carlos Le?nidas da Silva Souza Sobrinho 《International Journal of Infrared and Millimeter Waves》1995,16(11):1965-1975
The formulation is developed in the frequency domain and the finite difference method is used for the numerical solution of the scalar wave equation, written in terms of the transverse components of the magnetic field. As a result a conventional eigenvalue problem is obtained without the presence of spurious modes due to the implicit inclusion of the divergence of the magnetic field equal to zero. The formulation is developed to include biaxial anisotropic dielectrics with an index profile varying arbitrarily in the cross section of the waveguide under analysis. This formulation is then applied to the analysis of the influence on the dispersion characteristics of the dimensions of asymmetric coupled rectangular uniaxial anisotropic dielectric waveguides. As expected, the reduction of the height or the width of one of the rectangular dielectric waveguides causes the dispersion curves to move towards higher frequencies. 相似文献
942.
M. L. A. Tavares N. Queiroz I. M. G. Santos A. L. Souza E. H. S. Cavalcanti A. K. D. Barros R. Rosenhaim L. E. B. Soledade A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2011,106(2):575-579
The higher is the degree of unsaturation in ester chain of a biodiesel, the smaller is its oxidation stability. Sunflower
biodiesel obtained by the ethyl route possesses a high amount of unsaturated fatty acids, mainly oleic acid (C18:1) and linoleic
acid (C18:2), thus being more prone to the oxidation process. In Brazil, with the purpose of meeting the specifications of
the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP), antioxidant additives, from synthetic and natural
origins, have been added to the biofuel. Antioxidants are an alternative to prevent the oxidative deterioration of the fatty
acid derivatives, as they are substances able to reduce the oxidation rate. In this study, the oxidative stability of sunflower
biodiesel, obtained by the ethyl route and additivated with different concentrations of the antioxidants butylated hydroxytoluene
(BHT) and t-butylhydroquinone (TBHQ), was evaluated by means of Pressure differential scanning calorimetry (P-DSC) and the
Accelerated oxidative stability test (Rancimat, Method EN 14112). The results obtained by the two techniques showed the same
oxidation tendency. Thus, P-DSC can be used as an alternative to determine the oxidative stability of biodiesel. The antioxidant
TBHQ, added to biodiesel at the concentrations of 2000 and 2500 mg kg−1, raised the oxidation induction time to a value higher than 6 h, the limit established by the Resolution ANP number 7/2008,
thus being the best alternative among the studied antioxidants. 相似文献
943.
E. F. S. M. Ramalho I. M. G. Santos A. S. Maia A. L. Souza A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2011,106(3):825-829
Chemical composition of oils and fats used in the biodiesel synthesis can influence in processing and storage conditions,
due to the presence of unsaturated fatty acids. An important point is the study of the biodiesel thermal stability to evaluate
its quality using thermal analysis methods. In this study the thermal stabilities of the poultry fat and of their ethyl (BEF)
and methyl (BMF) biodiesels were determined with the use of thermogravimetry (TG/DTG), differential thermal analysis (DTA)
and differential scanning calorimetry (DSC), in different atmospheres. The TG/DTG curves of the poultry fat in synthetic air
presented three decomposition steps while only one step was observed in nitrogen (N2) atmosphere. The DSC results indicated four exothermic enthalpic transitions in synthetic air and an endothermic transitions
in N2 atmosphere attributed to the combustion process and to the volatilization and/or decomposition of the fatty acids, respectively.
For both biodiesels the TG/DTG curves in air indicated two mass loss steps. In the DSC curves four exothermic transitions
were observed in synthetic air besides an endothermic one in N2 atmosphere. 相似文献
944.
Anne G. D. Santos Vinícius P. S. Caldeira Mirna F. Farias Antonio S. Araújo Luiz D. Souza Allan K. Barros 《Journal of Thermal Analysis and Calorimetry》2011,106(3):747-751
In this study, the physico-chemistry characterization and kinetic study of the thermal decomposition of sunflower oil and
its biodiesel were carried out. Sunflower biodiesel was synthesized by the methanol route and basic homogeneous catalysis.
The physicochemical characterization of the sunflower oil and biodiesel were performed according to standards set out in the
ANP resolution, and both are in accordance to the specifications. The chromatographic analysis was obtained by GC-FID. The
yield of conversion of 97.4 wt% of sunflower oil in methyl esters confirms the efficiency of the conversion of the fatty acids
into esters. The thermal analysis was performed on a thermobalance, using heating rates of 5, 10, and 20 °C min−1. In these three rates, we observed a single well-defined step of mass loss that describes the volatilization and decomposition
of the sunflower oil and the biodiesel. The kinetic study was performed using equations of approximation and integration methods
such as Coats–Redfern, Van Krevelen, and Horowitz–Metzger. The kinetic parameters reaction order (n) and apparent activation energy (E
a), obtained by applying these method were correlated. 相似文献
945.
Lima Ade F de Moura GF de Lima MA de Souza PM da Silva CA Takaki GM do Nascimento AE 《Molecules (Basel, Switzerland)》2011,16(3):2486-2500
This study concerns the metabolism of polyphosphate in Trichoderma harzianum, a biocontrol agent with innate resistance against most chemicals used in agriculture, including metals, when grown in the presence of different concentrations of cadmium. The biomass production was affected by the concentration of metal used. Control cultures were able to accumulate polyphosphate under the conditions used. Moreover, the presence of cadmium induced a reduction in polyphosphate content related to the concentration used. The morphological/ultrastructural aspects were characterized by using optical and scanning electron microscopy, and were affected by the heavy metal presence and concentration. The efficiency of cadmium removal revealed the potential of the microorganism for use in remediation. The data indicate the potential for polyphosphate accumulation by the fungus, as well as its degradation related to tolerance/survival in the presence of cadmium ions. 相似文献
946.
Souza AB de Souza MG Moreira MA Moreira MR Furtado NA Martins CH Bastos JK dos Santos RA Heleno VC Ambrosio SR Veneziani RC 《Molecules (Basel, Switzerland)》2011,16(11):9611-9619
The antimicrobial activity of four labdane-type diterpenes isolated from the oleoresin of Copaifera langsdorffii as well as of two commercially available diterpenes (sclareol and manool) was investigated against a representative panel of microorganisms responsible for periodontitis. Among all the evaluated compounds, (-)-copalic acid (CA) was the most active, displaying a very promising MIC value (3.1 μg mL-1; 10.2 μM) against the key pathogen (Porphyromonas gingivalis) involved in this infectious disease. Moreover, CA did not exhibit cytotoxicity when tested in human fibroblasts. Time-kill curve assays performed with CA against P. gingivalis revealed that this compound only inhibited the growth of the inoculums in the first 12 h (bacteriostatic effect). However, its bactericidal effect was clearly noted thereafter (between 12 and 24 h). It was also possible to verify an additive effect when CA and chlorhexidine dihydrochloride (CHD, positive control) were associated at their MBC values. The time curve profile resulting from this combination showed that this association needed only six hours for the bactericidal effect to be noted. In summary, CA has shown to be an important metabolite for the control of periodontal diseases. Moreover, the use of standardized extracts based on copaiba oleoresin with high CA contents can be an important strategy in the development of novel oral care products. 相似文献
947.
Lago JH Souza ED Mariane B Pascon R Vallim MA Martins RC Baroli AA Carvalho BA Soares MG dos Santos RT Sartorelli P 《Molecules (Basel, Switzerland)》2011,16(12):9827-9837
The chemical composition and antimicrobial activity of essential oils obtained from leaves of two Myrtaceae species-Eugenia uniflora L. and Plinia trunciflora (O. Berg) Kausel-were determined. Analysis by GC/MS as well as determination of Kovatz indexes indicated atractylone (26.78%) and curzerene (17.96%) as major constituents of E. uniflora oil and α-cadinol (19.15%), apiole (11.15%) and cubenol (5.43%) as main components in P. trunciflora oil. Both essential oils were tested for antimicrobial activity against yeasts and bacteria. E. uniflora and P. trunciflora essential oils were active towards two Gram-positive bacteria, Streptococcus equi and Staphylococcus epidermis. In addition, biological activity of both essential oils was detected for pathogenic yeasts of the genus Candida and Cryptococcus. E. uniflora was active towards all yeast tested and exhibited interesting minimal inhibitory concentrations (0.11 to 3.75 mg/mL) across a broad spectrum of activity. 相似文献
948.
Valov I Luerssen B Mutoro E Gregoratti L De Souza RA Bredow T Günther S Barinov A Dudin P Martin M Janek J 《Physical chemistry chemical physics : PCCP》2011,13(8):3394-3410
Nitrogen is often used as an inert background atmosphere in solid state studies of electrode and reaction kinetics, of solid state studies of transport phenomena, and in applications e.g. solid oxide fuel cells (SOFC), sensors and membranes. Thus, chemical and electrochemical reactions of oxides related to or with dinitrogen are not supposed and in general not considered. We demonstrate by a steady state electrochemical polarisation experiments complemented with in situ photoelectron spectroscopy (XPS) that at a temperature of 450 °C dinitrogen can be electrochemically activated at the three phase boundary between N(2), a metal microelectrode and one of the most widely used solid oxide electrolytes--yttria stabilized zirconia (YSZ)--at potentials more negative than E = -1.25 V. The process is neither related to a reduction of the electrolyte nor to an adsorption process or a purely chemical reaction but is electrochemical in nature. Only at potentials more negative than E = -2 V did new components of Zr 3d and Y 3d signals with a lower formal charge appear, thus indicating electrochemical reduction of the electrolyte matrix. Theoretical model calculations suggest the presence of anionic intermediates with delocalized electrons at the electrode/electrolyte reaction interface. The ex situ SIMS analysis confirmed that nitrogen is incorporated and migrates into the electrolyte beneath the electrode. 相似文献
949.
Thermal Lens Spectrometry has traditionally been carried out in the single-beam and the mode-mismatched dual-beam configurations. Recently, a much more sensitive dual-beam TL setup was developed, where the probe beam is expanded and collimated. This feature optimizes Thermal Lens (TL) signal and allows the use of thicker samples, further improving the sensitivity. In this paper, we have made comparisons between the conventional and optimized TL configurations, and presented applications such as measurements of very low absorptions and concentrations in water and Cr(III) aqueous solution in the UV-vis range. For pure water we found linear absorption coefficients as low as the Raman scattering one due to the stretching vibrational modes of OH group. The detection limit was estimated 1 × 10−6 cm−1 with a 180-mW excitation power using a 100-mm cell length. This sensitivity is very high, considering that water has a photothermal enhancement factor ∼33 times smaller than CCl4, for example. For Cr(III) species in aqueous solution, the limit of detection (LOD) was estimated in ∼40 ng mL−1 at 514 nm, or ∼10 ng mL−1 at 405 nm, which is ∼30 times smaller than the LOD achieved with conventional transmission techniques. The more recent TL configuration is very attractive to obtain absorption spectra, since the result does not depend critically on the beam parameters, unlike the other configurations. The main drawbacks of this optimized TL configuration are the longer acquisition time and the need for larger samples. 相似文献
950.
Alan Patrick da SilvaGisele de Freitas Gauze Tiago Araújo MatiasVagner Roberto de Souza Thiago de Castro RozadaErnani Abicht Basso 《Tetrahedron letters》2011,52(39):5043-5046
Complexation between the Schiff base N4-tetradentate N,N′-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane (bpydip) and ion Ru(II) occurs only in trans geometry. However, it is known that the most complex of these ligands with ruthenium present predominantly cis geometry. In order to clarify the effects that drive the formation of the complex [Ru(bpydip)Cl2], we performed a new experimental study of the reaction and a complete theoretical investigation of their transition states. The results showed that nonformation of the cis isomer could be explained by the difference in relative stability of the reaction products and the transition states proposed. 相似文献