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排序方式: 共有330条查询结果,搜索用时 62 毫秒
1.
2.
Fbio Gomes Costa Elias Andrade Braga Soraia Teixeira Brando Alexandre de Freitas Espeleta Zênis Novais da Rocha Lílian Maria Tosta Simplício Emerson Andrade Sales 《Applied Catalysis A: General》2005,290(1-2):221-226
Infrared and UV–vis studies of metallocene immobilization on silica are reported here. The results have indicated changes in the Zr coordination sphere of metallocene depending on the immobilization route used. The reaction of [(Cp)2ZrCl2] with silica formed [(Cp)2ZrCl]+[SiO]− species. The same metallocene, reacting with TMA modified silica, formed monomethylated and dimethylated species by the substitution of chloro for methyl ligands, stabilized on the surface by interaction with “MAO-like” species (methylaluminoxane, MAO). These monomethylated and dimethylated cationic zirconium species are the active centers for the polymerization reaction. Different order of TMA addition in the silica modification step generated surface species of a similar nature, differing in their relative quantities. The highest amount of these active species was obtained when the support was added to the TMA solution rather than adding the TMA solution to the silica support. This was the most significant parameter affecting catalytic activity in ethylene polymerization. 相似文献
3.
María Pinto Dora Sales Anne-Vinciane Doucet Andrés Fernández-Ramos David Guerrero 《Scientometrics》2007,72(2):345-370
The purpose of the study proposed in this paper is to evaluate the Spanish public university websites dedicated to the European
Higher Education Area (EHEA). To do so, the quality of these resources has been analysed in the light of data provided by
a series of indicators grouped in seven criteria, most of which were used to determine what information is made available
and in what way. The criteria used in our analysis are: visibility, authority, updatedness, accesibility, correctness and
completeness, quality assessment and navigability.
All in all, the results allow us to carry out an overall diagnosis of the situation and also provide us with information about
the situation at each university, thus revealing their main strengths, namely authority and navegability, and also their chief
shortcomings: updatedness, accessibility and quality assessment. In this way it is possible to detect the best practices in
each of the aspects evaluated so that they can serve as an example and guide for universities with greater deficiencies and
thus help them to improve their EHEA websites. 相似文献
4.
Analytical and numerical analysis of phase and amplitude errors in the performance of arrayed waveguide gratings 总被引:1,自引:0,他引:1
Munoz P. Pastor D. Capmany J. Sales S. 《IEEE journal of selected topics in quantum electronics》2002,8(6):1130-1141
The closed expression of the mean power transfer function for an arrayed waveguide grating (AWG), taking into account fabrication errors, is presented. Comparison with numerical simulations using a previous Fourier optics modeling show good agreement. The procedure to predict the AWG floor level using the numerical modeling is presented and checked against the analytical derived expressions. 相似文献
5.
A.G. Gurgel W.F. Sales C.S. de Barcellos J. Bonney E.O. Ezugwu 《International Journal of Machine Tools and Manufacture》2006,46(12-13):1637-1642
The main objective of this study is to implement a parameter sensitivity analysis method to be used in the search of optimal machining conditions with respect to surface quality. Presently, the element-free Galerkin (EFGM) approximating functions are used to evaluate the properties of machined surfaces with cutting parameters when turning AISI 4140 steel using arbitrary sets of experimental values and the EFGM approximation functions, based on the moving least-squares method, in order to obtain the sensitivities through proper local derivations. This method shows the sensitivity of each surface parameter for each input variable. The variables investigated were cutting speed (vc), depth of cut (ap), feed rate (f) and the surface roughness (Ra). The sensitivity results showed that the feed rate has the highest influence on surface roughness when turning AISI 4140 steel followed by cutting speed and depth of cut. 相似文献
6.
HA Mook DM Paul BC Sales LA Boatner L Cussen 《Canadian Metallurgical Quarterly》1988,38(16):12008-12010
7.
J. Llorca A. Barbier G. A. Martin J. Sales P. Ramírez de la Piscina N. Homs 《Catalysis Letters》1996,42(1-2):87-91
Hydroformylation of ethylene and CO hydrogenation were studied over cobalt-based catalysts derived from reaction of Co2(CO)8 with ZnO, MgO and La2O3 supports. At 433 K a similar activity sequence was reached for both reactions: Co/ ZnO > Co/La2O3 > Co/MgO. This confirms the deep analogy between hydroformylation and CO hydrogenation into alcohols. In the CO hydrogenation the selectivity towards alcohol mixture (C1-C3) was found to be near 100% at 433 K for a conversion of 6% over the Co/ZnO catalyst; this catalyst showed oxo selectivity higher than 98% in the hydroformylation of ethylene. Magnetic experiments showed that no metallic cobalt particles were formed at 433 K. It is suggested that the active site for the step that is common to both reactions is related to the surface homonuclear Co2+/[Co(CO)4]– ion-pairing species. 相似文献
8.
Luciana Gampert Miranda Carolina Pereira Peglow Victória Cardoso Deorristt Geórgia Campos Sales Neto Ariela Milbrath Cardoso Suzana Frighetto Ferrarini Marçal José Rodrigues Pires 《应用聚合物科学杂志》2021,138(7):49842
The use of polymeric films incorporated with zeolite-TiO2 composites associated with UV radiation can be an alternative in the removal of volatile organic compounds (VOCs) through the adsorption and photodegradation processes. This study produced poly(lactic acid) (PLA) films incorporated with 13× zeolite, TiO2, and 13×-TiO2 zeolite composite to remove n-butanol and evaluate the by-products generated in the process. The results showed that 13× zeolite and TiO2 added individually or as a composite to PLA, gave the polymer matrix a significant increase in the removal capacity of n-butanol. The best performance was presented by the zeolite-TiO2, composite, confirming a synergistic effect. However, the formation of CO and CO2 exceeded the expected values, with the verification that the polymeric matrix underwent photodegradation action by TiO2. The polymeric film only containing zeolite is the most suitable for the removal of VOCs, as it did not present degradation of the PLA, generating a lower concentration of by-products. 相似文献
9.
Paulo H. da Rocha Henrique C. Ferreira Michael C. Porsch Roberto M. Sales 《Control Engineering Practice》2009,17(10):1148-1156
Electromagnetic suspension systems are inherently nonlinear and often face hardware limitation when digitally controlled. The main contributions of this paper are: the design of a nonlinear H∞ controller, including dynamic weighting functions, applied to a large gap electromagnetic suspension system and the presentation of a procedure to implement this controller on a fixed-point DSP, through a methodology able to translate a floating-point algorithm into a fixed-point algorithm by using l∞ norm minimization due to conversion error. Experimental results are also presented, in which the performance of the nonlinear controller is evaluated specifically in the initial suspension phase. 相似文献
10.
P. Ramírez de la Piscina J. L. G. Fierro G. Muller J. Sales N. Homs 《Catalysis Letters》1992,14(1):45-49
New heterogenized Pt/Sn catalysts selective for the hydroformylation of 1-pentene have been synthesized. The complex cis-[PtCl2(PPh3)2] and the SnCl2.2H2O or SnC2O4 precursors have been anchored on silica-, magnesia- and alumina-carriers. X-ray photoelectron spectroscopy was used to determine the surface composition and the nature of the anchored species. The hydroformylation activity was found to depend on the type of support and tin precursor used. Only the silica supported catalysts were active in the hydroformylation reaction. Samples prepared from SnCl2-2H2O were 200-fold more active than those prepared from SnC2O4. Selectivity ton-hexanal of the silica-supported catalyst prepared from SnCl2-2H2O was as high as 94.4% at 39.2% conversion of 1-pentene. 相似文献