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排序方式: 共有199条查询结果,搜索用时 15 毫秒
91.
Francisco Palacios Concepción Alonso Gloria Rubiales Maite Villegas 《Tetrahedron letters》2004,45(21):4031-4034
A method for the preparation of 3-fluoroalkyl substituted 2-aza-butadienes by aza-Wittig reaction of N-vinylic phosphazenes and aldehydes is reported. [4+2] Cycloaddition reaction with enamines affords fluoralkyl substituted pyridine derivatives in a regioselective fashion. 相似文献
92.
M.A. Villegas M.A. García J. Llopis J.M. Fernández Navarro 《Journal of Sol-Gel Science and Technology》1998,11(3):251-265
Sol-gel coatings in the xM (100-x) SiO2 system, (M = Cu, Ag and Au) x =0.1–10 mol%), are deposited on soda lime glass slides by using silicon tetramethoxide Si(OCH3)4) and methyltriethoxysilane (SiCH3[OCH2CH3]3) as silica precursors. Anhydrous CuCl, CuCl2 2H2O, Cu(NO3)2 3H2O, CuSO4 5H2O, AgNO3 and HAuCl4 3H2O are used as copper, silver and gold sources. Coatings with thicknesses ranging from 100 to 900 nm are deposited on the subs trates by dip-coating and subsequently densified at 500°C for 1 h in air. Spectroscopic studies of the coatings as a function of the thicknesses and the metal concentration are carried out by photoluminescence (PL) and optical absorption (OA). In addition, direct observations of some gold coatings were performed by transmission electron microscopy (TEM). Results indicate that for silver and copper containing coatings the excitation and emission spectra arise from electronic transitions in Ag+ and Cu+ ions and no significant absorption bands due to colloidal precipitation are observed. Gold containing coatings show purple coloration due to an absorption peaking in the 520–560 nm range, which is characteristic of gold colloids. The presence of these colloids is confirmed by TEM observations. 相似文献
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We analyse different error propagation mechanisms for conservativeand nonconservative time-integrators of nonlinear Schrödingerequations. We use a geometric approach based on interpretingwaves as relative equilibria. 相似文献
95.
Tamás Hausel Emmanuel Letellier Fernando Rodriguez Villegas 《Comptes Rendus Mathematique》2010,348(3-4):131-135
In Hausel et al. (2008) [10] we presented a conjecture generalizing the Cauchy formula for Macdonald polynomial. This conjecture encodes the mixed Hodge polynomials of the character varieties of representations of the fundamental group of a punctured Riemann surface of genus g. We proved several results which support this conjecture. Here we announce new results which are consequences of those in Hausel et al. (2008) [10]. 相似文献
96.
F. Velázquez Villegas S.D. Santillán Gutiérrez 《Revista Internacional de Métodos Numéricos para Cálculo y Dise?o en Ingeniería》2012,28(3):135-141
This paper presents a method for shape optimization of two-dimensional models subjected to simple or multiple load cases. The optimization is performed iteratively using evolutionary rules, based on the stress level. These rules determine regions on the boundary where the material is underused or overused; the objective is that the model evolves to a minimum weight model with a high and homogeneous stress level. The evolution is performed by modifying the boundary of the model slowly. Since the boundary is defined by parametric B-spline curves, generated changes result in a smooth boundary. The main proposal of the method is that boundary modifications are given by a set of displacements that, its magnitude and direction, take into account the geometrical information of the neighborhood, unlike other methods that simply generate displacement perpendicular to the boundary. Finally, the paper presents the design of a frame bike in order to show the good performance of the method. 相似文献
97.
García MA Ruiz-González ML Quesada A Costa-Krämer JL Fernández JF Khatib SJ Wennberg A Caballero AC Martín-González MS Villegas M Briones F González-Calbet JM Hernando A 《Physical review letters》2005,94(21):217206
In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of Mn(3+) and Mn(4+) via a double-exchange mechanism. The presence of the ZnO around MnO(2) modifies the kinetics of MnO(2)-->Mn(2)O(3) reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples. 相似文献
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