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81.
Grettel Valle-Bourrouet Martín Gómez Ignacio González Carlos Frontana 《Electrochimica acta》2010,55(28):9042-9050
Changes in electrochemical reactivity for lawsone anions (lawsone, 2-hydroxy-1,4-naphthoquinone, HLw) being coordinated to a series of metallic ions in dimethylsulfoxide solution were evaluated. Upon performing cyclic voltammetry experiments for metal complexes of this quinone with pyridine (Py) - structural formula M(II)(Lw−)2(Py)2; M: Co(II), Ni(II), Zn(II) - it was found that the reduction of coordinated Lw− units occurs during the first and second electron uptake in the analyzed compounds. The stability of the electrogenerated intermediates for each complex depends on the d electron configuration in each metal center and is determined by magnetic interactions with the available spins considering an octahedral conformation for all the compounds. This was evidenced by in situ spectroelectrochemical-ESR measurements in the Zn(II) complex in which due to the lack of magnetic interaction owing to its electron configuration, the structure of the coordinated anion radical species was determined. Successive reduction of the associated Lw− units leads to partial dissociation of the complex, determined by the identification of free radical dianion structures in solution. These results show some insights on how metal-lawsone complexation can modify the solution reactivity and stability of the electrogenerated radical species. 相似文献
82.
83.
The modeling of catalytic hydrotreatment reactors for petroleum fractions have been classified in several ways, for example steady‐state and dynamic, pseudohomogeneous, and heterogeneous, and so on. Depending on the system to be modeled, operating conditions and type of feedstock, these approaches could exhibit some advantages and disadvantages, wide scopes and limitations. In this review, the discussion about those modeling aspects, already published, is used to develop a generalized reactor model, which can be simplified in order to derive each single model previously reported. Some guides to estimate model parameters are also given. 相似文献
84.
R. Ignacio Madrid Herre Van Oostendorp Mari Carmen Puerta Melguizo 《Computers in human behavior》2009
Problems in learning with hypertext systems have been claimed to be caused by high levels of disorientation and cognitive load. This was recognized by DeStefano and LeFevre [DeStefano, D., & LeFevre, J. -A., (2007). Cognitive load in hypertext reading: A review. Computers in Human Behavior, 23(3), 1616–1641.] who predicted an increase of cognitive load and impairment of learning for hypertexts with a higher number of links per page. From a practical perspective, several navigation support techniques, such as providing link suggestions, have been proposed for guiding learners and reducing cognitive overload. In an experiment, we tested DeStefano and LeFevre’s predictions as well as the usefulness of link suggestions. Participants used different versions of a hypertext, either with 3-links or 8-links per page, presenting link suggestions or not. We tested their cognitive load and learning outcomes. Results showed that there was a benefit of using link suggestions for learning, but no effect of number of links on learning was found. Moreover, the effects of our manipulations on cognitive load were mediated by the reading order that participants selected. Implications for research and the design of navigation support systems are discussed. 相似文献
85.
Pedro J. Sánchez-Soto María del Carmen Jiménez de Haro Luis A. Pérez-Maqueda Ignacio Varona José L. Pérez-Rodríguez 《Journal of the American Ceramic Society》2000,83(7):1649-1657
The present study examined the effects of dry grinding, using ball-milling, on the structure of reference well-crystallized (KGa-1) and poorly crystallized (KGa-2) kaolinite powders from Georgia. Grinding produced a strong structural alteration, mainly along the c axis, resulting in disorder and total degradation of the crystal structure of the kaolinite and the formation of an amorphous product. The surface area increased with grinding time, mainly in KGa-2 (maximum value 50.27 m2 /g), a result associated with particle-size reduction. These particles became more agglomerated with grinding, and the surface area decreased after 30 min, as confirmed by scanning electron microscopy and particle-size-distribution analysis. There was a limit to particle-size reduction with grinding time. When grinding time was increased, the original endothermic differential thermal analysis (DTA) effects of dehydroxylation in both samples shifted to lower temperatures, decreased in intensity, then disappeared completely after 120 min of grinding. The temperature of the characteristic first exothermic effect shifted slightly to lower temperatures with grinding, although the DTA effects did not increase with grinding time in either kaolinite sample, at least up to 325 min. The amorphous, mechanically activated kaolinite converted into low-crystalline mullite nuclei at a lower temperature than did the unground samples, as deduced by thermal and X-ray observations. This effect was especially important for the KGa-2 sample. Grinding did not seem to influence the formation of silicon-aluminum spinel from kaolinite. The present results may explain why ground kaolinite samples prepared via different routes—e.g., with differences in grinding—behave differently during high-temperature transformations, as reported in the related literature. 相似文献
86.
87.
A.?de?Lucas J.?Rincón I.?GraciaEmail author 《Journal of the American Oil Chemists' Society》2003,80(2):181-188
Supercritical CO2 extraction is a viable alternative process for the extraction of high-quality oil from olive husk (also known as olive pomace),
a residue obtained in the production of olive oil. We analyzed the effect of pressure (100–300 bar), temperature (40–60°C),
solvent flow (1–1.5 L/min), and particle size (0.30–0.55 mm) on four important quality parameters of the oil extracted with
CO2: tocopherol concentration, extinction coefficients at 232 and 270 nm, and saponification value. Response surface methodology
was used to obtain mathematical expressions related to the operating variables and parameters studied. Results from these
experiments were also used to design a three-step sequential CO2 extraction procedure to obtain a higher-quality extract. The optimal operational sequence consisted of a first extraction
step at 75 bar for 1 h using 1% (vol/vol) ethanol modifier, followed by a second extraction stage at 350 bar for 2.5 h without
ethanol and a third step, also at 350 bar, for 2.5 h but using ethanol. These extraction conditions obtained an intermediate
fraction of oil with 64% yield and all normal parameters according to European Commission food legislation. This fraction
is suitable without any further refining. On the contrary, the oils obtained by hexane extraction and by conventional supercritical
CO2 extraction at optimal conditions are suitable for human consumption after further refining. This last finding may result
in improved economics of the sequential CO2 extraction process compared to the conventional extraction method with hexane. 相似文献
88.
Roberto Martínez Javier Calvo Francisco Arriaga Ignacio Bobadilla 《Holz als Roh- und Werkstoff》2018,76(2):509-515
Density values are essential for the characterization of wood elements in existing structures. A new method for the in-situ density estimation of timber pieces is proposed based on weighting the residue generated by conventional drilling. This research includes the design and development of a residue collector device coupled to a conventional drill and testing on four Spanish coniferous species: Laricio pine, Scots pine, Radiata pine and Maritime pine (Pinus nigra Arn., Pinus sylvestris L., Pinus radiata D. Don. and Pinus pinaster Ait., respectively). For each species, 44 specimens (except Scots pine, with 42) 60?×?90 mm cross-section and 150 mm length (half radial and half tangential orientation) were tested. Specimen density was obtained by mass/volume ratio. Three 8 mm diameter 46.5 mm deep holes were drilled in each specimen using a brad point bit. The drilling residue was weighed and density was calculated as a ratio to bore volume. Although only slightly smaller, residue density statistically differs from average specimen density. But the specimen density can be inferred by the correlation, statistically significant, with residuals drilling density (R2?=?80.7%) and even better with residual drilling mass (R2?=?84.4%). 相似文献
89.
José Luis Manzanares Japón Ignacio Hinojosa Sánchez-Barbudo 《Journal of Constructional Steel Research》2011,67(12):1850-1863
Curved graphs are proposed that relate the shape of the box cross-section of an arch to the real ultimate lateral strength for various values of length, support conditions and of height of the cross-section.The real ultimate lateral strength of a series of arches is determined and the results obtained are then compared to those provided by Sakimoto and Komatsu [1].Elastic–plastic steel material and geometric non-linearity hypotheses, residual stresses, and initial lateral deflection are all taken into consideration with respect to the real ultimate lateral strength. 相似文献
90.
Luis Ignacio Rizzi 《Networks and Spatial Economics》2008,8(2-3):125-140
Assuming that both road safety and travel time are a function of traffic density and care from a driver’s standpoint, a theoretical framework is developed. First, an expected-utility framework is developed to study the optimal choice of care by a rational driver. Second, the subjective value of traffic density is derived as a measure of drivers’ willingness to pay for simultaneous road safety improvements and travel time savings. Third, the above results are revisited when car insurance is available. Fourth, a simple cost-benefit analysis rule of road capacity expansion is derived when moral hazard is present. 相似文献