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421.
Teixeira JM Ventura J Araujo JP Sousa JB Wisniowski P Cardoso S Freitas PP 《Physical review letters》2011,106(19):196601
We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown. 相似文献
422.
Luzia Patricia Fernandes Carvalho Galvão Anne Gabriella Dias Santos Amanda Duarte Gondim Marcela Nascimento Barbosa Antonio Souza Araujo Luiz Di Souza Valter José Fernandes Junior 《Journal of Thermal Analysis and Calorimetry》2011,106(2):625-629
Biodiesel can contain unsaturated fatty acids, which are susceptible to oxidation, being able to change into polymerized compounds.
Oxidative stability is very important in the quality control of oils and biodiesel. In this study, biodiesel samples were
produced through the methyl route, using a homogeneous catalyst. The determination of methyl esters was performed by gas chromatography
in order to confirm the conversion of the carboxylic acids present in the raw material for the methyl esters. Also proved
the presence of methyl linoleate and methyl oleate to the major constituent of biodiesel. The thermal and oxidative stability
of sunflower and cotton oils and their biodiesel, using TG and P-DSC techniques were investigated. The use of P-DSC to measure
the oxidative induction time was very important. These measurements were used to evaluate the cotton and sunflower oils, and
their respective biodiesel. It was found that the thermal-oxidative stability of vegetable oils and their biodiesel were similar,
due to the fact that both presented chemical composition and percentages of fatty acids similar. 相似文献
423.
Késia K. V. Castro Ana A. D. Paulino Edjane F. B. Silva Thiago Chellappa Maria B. D. L. Lago Valter J. FernandesJr. Antonio S. Araujo 《Journal of Thermal Analysis and Calorimetry》2011,106(3):759-762
Thermogravimetry (TG) was used in this study to evaluate thermal and catalytic pyrolysis of Atmospheric Petroleum Residue
(ATR) which can be found in the state of Rio Grande do Norte/Brazil, after a process of atmospheric distillation of petroleum.
The utilized sample in the process of catalytic pyrolysis was Al-MCM-41, a mesoporous material. The procedures for obtaining
the thermogravimetric curves were performed in a thermobalance with heating rates of 5, 10, and 20 °C min−1. From TG, the activation energy was determined using the Flynn–Wall kinetic method, which decreased from 161 kJ mol−1, for the pure ATR, to 71 kJ mol−1, in the presence of the Al-MCM-41, showing the efficiency of the catalyst in the pyrolysis of Atmospheric Petroleum Residue. 相似文献
424.
425.
Flavia C. De Sousa Fonseca Flavia M. Araujo Tanus J. Nagem Tânia T. de Oliveira Carlos Roque D. Correia 《合成通讯》2013,43(5):768-773
N-Boc-allenylaniline undergoes cyclization in the presence of a catalytic amount of a cationic gold complex to exclusively form 2-oxazolidinones, whereas the N-Moc-allenylaniline affords dihydroquinolines. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
426.
María Goncalves José Daniel Martínez Mary Lorena Araujo Felipe Brito Giuseppe Lubes Mildred Rodríguez Vito Lubes 《Journal of solution chemistry》2011,40(9):1517-1527
The complex species formed in aqueous solutions (25 °C, I=3.0 mol⋅dm−3 KCl ionic medium) between the V(III) cation and the ligands 6-methylpicolinic acid (MePic, HL), salicylic acid (H2Sal, H2L) and phthalic acid (H2Phtha, H2L) have been studied by potentiometric and spectrophotometric measurements. Application of the least-squares computer program
LETAGROP to the experimental emf(H) data, taking into account the hydrolytic species and hydrolysis constants of V(III), indicates
that under the employed experimental conditions the complexes [VL]2+, [V(OH)L]+, [V(OH)2L], [V(OH)3L]−, [VL2]+, [VL3] and [V2OL4] form in the vanadium(III)–MePic system. Were observed the complexes [VL]+, [VL2]−, [V(OH)L2]2− and [VL3]3− in the vanadium(III)–H2Sal system, and the species [VHL]2+, [VL]+, [V(OH)L], [VHL2], [VL2]−, [V(OH)L2]2−, [V(OH)2L2]3− and [VL3]3− in the vanadium(III)–H2Phtha system. The stability constants of these complexes were determined by potentiometric measurements, and spectrophotometric
measurements were made in order to perform a qualitative characterization of the complexes formed in aqueous solution. 相似文献
427.
Leaf-cutting ants cause large losses in several crops around the world. In the ant species Atta sexdens, each colony comprises up to 5 million individuals. In order to keep a close connection among such a large number of individuals, an efficient chemical communication system is necessary. Among other different substances, these animals use alkylpyrazines to mark their trails and to guide ant workers from the nest to their sources of food. In this study, CE-UV was used to apply a method for qualitative analysis of venom gland components of leaf-cutting ants. Mobility of these compounds proved to be a function of the ionization capability of these bases as well as their volumes. Migration order was thoroughly explained in terms of such parameters. The best analysis conditions were achieved with a BGE composed by 0.8% formic acid plus 20% methanol in water, hydrodynamic injection, and application of external pressure. Such analysis conditions may be easily applied in CE-MS analyses as well. CE-UV analyses proved to be as adequate as GC to analyze such compounds due to system detectability (LD ? 0.005?mmol/L), separation efficiency (from 5.07×10(4) to 1.23×10(5) theoretical plates), and resolution (minimum of 2.35). In addition, analysis time was ca. 15 min, which shows another advantage of CE analysis when compared with GC. Although the analytes are found in concentrations as low as 50 ng/venom glands, four putative pyrazine ring moiety substances could be detected in real samples, due to sample stacking and use of a capillary with extended detection cell. 相似文献
428.
Barboza AP Gomes AP Archanjo BS Araujo PT Jorio A Ferlauto AS Mazzoni MS Chacham H Neves BR 《Physical review letters》2008,100(25):256804
We report the direct experimental observation of the semiconductor-metal transition in single-wall carbon nanotubes (SWNTs) induced by compression with the tip of an atomic force microscope. This transition is probed via electric force microscopy by monitoring SWNT charge storage. Experimental data show that such charge storage is different for metallic and semiconducting SWNTs, with the latter presenting a strong dependence on the tip-SWNT force during injection. Ab initio calculations corroborate experimental observations and their interpretation. 相似文献
429.
H.L. Sullasi H.J. Khoury V. Barros R. Libonati P.L. Guzzo V. Asfora R.E. De Araujo M. Capriles J. Reyes 《Radiation measurements》2010,45(3-6):540-542
The aim of this paper is to evaluate the thermoluminescent (TL) response of a larimar, a variety of pectolite (NaCa2Si3O8[OH]). It is a blue pectolite only found on the terrestrial crust of the province of Barahona, in the south-western region of the Dominican Republic. The larimar rock used in this study, presented a light-blue color with brown areas. X-ray powder diffractometry (XRD), showed that the light-blue portion of the rock is composed essentially by pectolite and the brown portion is composed by a mixture of minerals. Aliquots of 5 mg of light-blue portions were irradiated with gamma rays with doses from 10 to 104 Gy and the TL glow curves were obtained from 50 to 400 °C. The glow curve showed a broad peak around 150 °C and a peak near 280 °C. Different pre-heat condition were used to remove the broad peak. The TL emission of the main peak appeared in the region of 580 nm. The TL response of the main peak showed a linear behavior up to 2500 Gy and a sub-linear behavior above this dose value. Studies of the 30 day fading effects in TL signal using a pre-heating temperature around 180 °C during 15 min were carried out and fading near 25% was observed. 相似文献
430.
We present an index that measures the nestedness pattern of bipartite networks, a problem that arises in theoretical ecology. Our measure is derived using the sum of distances of the occupied elements in the incidence matrix of the network. This index quantifies directly the deviation of a given matrix from the nested pattern. In the simplest case the distance of the matrix element ai,j is di,j=i+j, the Manhattan distance. A generic distance is obtained as di,j=(iχ+jχ)1/χ. The nestedness index is defined by ν=1−τ, where τ is the “temperature” of the matrix. We construct the temperature index using two benchmarks: the distance of the complete nested matrix that corresponds to zero temperature and the distance of the average random matrix where the temperature is defined as one. We discuss an important feature of the problem: matrix occupancy ρ. We address this question using a metric index χ that adjusts for matrix occupancy. 相似文献