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51.
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
52.
53.
In this paper, we propose a stable neurovisual servoing algorithm for set-point control of planar robot manipulators in a fixed-camera configuration an show that all the closed-loop signals are uniformly ultimately bounded (UUB) and converge exponentially to a small compact set. We assume that the gravity term and Jacobian matrix are unknown. Radial basis function neural networks (RBFNNs) with online real-time learning are proposed for compensating both gravitational forces and errors in the robot Jacobian matrix. The learning rule for updating the neural network weights, similar to a back propagation algorithm, is obtained from a Lyapunov stability analysis. Experimental results on a two degrees of freedom manipulator are presented to evaluate the proposed controller. 相似文献
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We present the design and realization of cylindrical dielectric resonators operating in the 40–60 GHz frequency range, designed
for the measurement of the surface resistance and of the surface reactance shift in High-T
c Superconductors (HTS) thin films in a dc magnetic field. The resonators are single tone, based on the TE
011 mode, and multiple tone, the latter allowing in principle to exploit the simultaneous determination of the surface impedance
at different frequencies. As an application example, we report the temperature and field dependencies of the effective surface
impedance of some cuprate superconductors thin films. The results are compared with those obtained through the use of a standard
metal cavity with a similar Q-factor and operating in the same frequency range. The comparison highlights a superior stability and a higher sensitivity,
resulting in an increase of about two orders of magnitude in the resolution of the surface impedance measurement. By contrast,
the dissipative part of the superconducting transition (above T/T
c = 0.97) is better studied with the metal cavity. We also present measurements in the vortex state in YBaCa2Cu3O7−δ and Tl2Ba2CaCu2O8+x
that show significantly different physics.
PACS: 74.25 Nf. 相似文献
58.
In this paper, some biparametric models for an elastic foundation are proposed. It is assumed that the foundation has the shape of the wedge. The modeling procedure starts from the linear elasticity equations into which we introduce some simplifying assumptions based on the conceptions of decay functions. The simplified models are described by the ordinary differential equations. Stationary and nonstationary Green’s functions for the foundation considered are obtained by applying the Hankel and Laplace transform methods. An example of the interaction between the rigid plate and the elastic wedge foundation is studied. 相似文献
59.
This paper presents a robustness analysis of the optimal solution of the step tracking problem in a one-degree of freedom control architecture, for the discrete-time multivariable (MIMO) case. It is shown that the resulting closed loop becomes arbitrarily fragile, i.e. the sensitivity peak becomes arbitrarily large, for certain reference directions. The cause of this problem is that the optimal controller depends on the reference direction. The paper shows that this dependence can be eliminated with an alternative formulation of the optimal problem where the cost is averaged over all reference directions. 相似文献
60.
da Costa D.B. Yacoub M.D. Filho J.C.S.S. Fraidenraich G. Mendes J.R. 《Communications Letters, IEEE》2006,10(1):13-15
This paper provides simple, exact, new closed-form expressions for the generalized phase crossing rate of Nakagami-m fading channels. Sample numerical results obtained by simulation are presented that validate the formulations developed here. A special case of this formulation is the Rayleigh case, whose result agrees with that obtained elsewhere in the literature. In passing, several new closed-form results concerning the statistics of the envelope, its in-phase and quadrature components, phase, and their time derivatives are obtained. 相似文献