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951.
A new global circuital model is proposed taking into account some piezoelectric and electronic non-ideal aspects of the broadband ultrasonic transceivers used in NDT and imaging applications. A quadratic approach, alternative to previous linear Spice implementations, is proposed for the frequency dependence of the mechanical losses into the piezoelectric sections. Non-ideal frequency-dependent electrical effects influencing excitation circuitry performance, and attenuations in the propagation medium varying with frequency, are also considered. Results calculated, by using PSpice implementations, from conventional and the new modelling approaches show how the quadratic option to calculate losses clearly reduces the rippled and sharpened waveforms typically originated in many echo-signals simulations, and which are not observed in real measured responses. Experimental waveforms for driving spikes and for echo-signals, in very good agreement with responses simulated from the new modelling here proposed, are also shown. 相似文献
952.
An improved method for ultrasonic location of multiple flaws using perpendicular ultrasonic arrays, working in near-field conditions, is presented. The method uses the pulse-echo technique and is based on the processing of the information acquired by both the elemental emitting transducers and their adjacent ones. This type of processing improves the performance of a previous method that sometimes presents limitations for the detection of multiples flaws. This new method is applied to a set of ultrasonic traces, obtained with an ultrasonic prototype specifically designed for testing this method, showing the improvement obtained in imaging results. 相似文献
953.
Absorption and fluorescence properties of methylene blue (MB), a well-known singlet molecular oxygen photosensitizer, and its mixtures with pheophorbide-a (Pheo) sorbed on microgranular cellulose are studied, with emphasis on radiative and nonradiative energy transfer from Pheo to MB. Although pure MB builds up dimeric species on cellulose even at 2 x 10(-8) mol g(-1), addition of 2.05 x 10(-7) mol g(-1) Pheo largely inhibits aggregation up to nearly 10(-6) mol g(-1) MB. At the same time, the absorption spectrum of monomeric MB in the presence of Pheo differs from the spectrum in pure cellulose. Both effects reveal a strong influence of Pheo on the medium properties. A model relying entirely on experimental data is developed, through which energy transfer efficiencies can be calculated for thin and thick layers of dye-loaded cellulose. At the largest concentration of MB assuring no dye aggregation, nonradiative energy transfer efficiencies reach a maximum value of nearly 40%. This value is quite high, taking into account the low fluorescence quantum yield of Pheo, Phi = 0.21, and results from the existence of high local concentrations of the acceptor within the supporting material. These results show that large energy transfer rates can exist in a system devoid of any special molecular organization. 相似文献
954.
Simó C Pérez P Neusüss C Pelzing M San Román J Gallardo A Cifuentes A 《Electrophoresis》2006,27(11):2250-2258
Analytical characterization of dimethacrylate-tyrosine-lysine-tyrosine (DMTLT, a new biodegradable acrylic cross-linker synthesized at our laboratory) is carried out using CE-MS. DMTLT is a pseudopeptide composed by tyrosine-lysine-tyrosine amino acids linked through urea bonds with two methacrylic groups, one at each end of the molecule, making this compound an excellent cross-linker for polymerization reactions and for obtaining new biodegradable materials. A new CE-MS method is developed for the characterization of DMTLT and its products of degradation after basic hydrolysis. In order to carry out an exhaustive examination of such degradation products methods based on CE coupled to IT and TOF-MS are employed. Based on CE-IT-MS results and the elemental composition of the degradation products obtained by CE-TOF-MS, conclusions on the mechanism and kinetic of hydrolysis of DMTLT are obtained confirming both the usefulness of CE-MS to characterize new biomaterials and the applicability of DMTLT for preparing new biodegradable polymers. These results are corroborated through the CE-MS detection of the identified products of degradation in a dimethyl acrylamide polymer cross-linked with DMTLT. 相似文献
955.
This paper describes the equilibrium established between a phosphonium dienolate zwitterion and a vinylogous phosphorus ylide, and their reactions with aldehydes. The reactions between ethyl 2-methyl-2,3-butadienoate and various aldehydes occur through either a phosphonium dienolate or a vinylogous ylide intermediate, depending on the presence/absence of a Lewis acid and the nature of the phosphine. We observed a rare vinylogous Wittig olefination from the reaction between ethyl 2-methyl-2,3-butadienoate and an electron-deficient aromatic aldehyde in the presence of a stoichiometric amount of an electron-deficient triarylphosphine and a catalytic amount of a Lewis acid (e.g., BF3·Et2O). On the other hand, the use of triphenylphosphine, in the absence of a Lewis acid, facilitated vinylogous aldol addition, accompanied by a rare 1,2-aryl phosphorus-to-carbon migration. 相似文献
956.
Claude Dellacherie Servet Martinez Jaime San Martin 《Journal of Theoretical Probability》2009,22(2):311-347
In this article we study which infinite matrices are potential matrices. We tackle this problem in the ultrametric framework
by studying infinite tree matrices and ultrametric matrices. For each tree matrix, we show the existence of an associated
symmetric random walk and study its Green potential. We provide a representation theorem for harmonic functions that includes
simple expressions for any increasing harmonic function and the Martin kernel. For ultrametric matrices, we supply probabilistic
conditions to study its potential properties when immersed in its minimal tree matrix extension.
C. Dellacherie thanks support from Nucleus Millennium P04-069-F for his visit to CMM-DIM at Santiago.
The research of S. Martinez is supported by Nucleus Millennium Information and Randomness P04-069-F and by the BASAL CONICYT
Program.
The research of J. San Martin is supported by FONDAP and by the BASAL CONICYT Program. 相似文献
957.
We study the interplay between charge doping and intermolecular distance in the polymerization of C(60) fullerene chains by means of density functional theory-based first-principles calculations. The potential energy surface analysis shows that both the equilibrium intermolecular distance of the unpolymerized system and the polymerization energy barrier are inversely proportional to the electron doping of the system. We analyze the origin of this charge-induced polymerization effect by studying the behavior of the system's wavefunctions around the Fermi level and the structural modifications of the molecules as a function of two variables: the distance between the centers of the molecules and the number of electrons added to the system. 相似文献
958.
The effect of Dzialoshinski-Moriya (DM) interaction on the quantum discord and the thermal entanglement of the density matrix of a spin star model is investigated. Our results imply that the quantum correlation measured by quantum discord and thermal entanglement can be established between two surrounding parties both of which have no interaction with each other but interact with the central party independently. From the analysis, we find that the strong DM interaction can enhance the quantum discord and thermal entanglement while the external magnetic field with a large value and the high temperature can shrink them. Specially, the quantum discord is more robust than the thermal entanglement in the sense that the range of parameters in which the quantum discord takes a zero value is much smaller than that of the thermal entanglement. 相似文献
959.
Casaseca-de-la-Higuera P Tristán-Vega A Aja-Fernández S Alberola-López C Westin CF San José Estépar R 《Magnetic resonance imaging》2012,30(4):506-517
Diffusion tensor imaging (DTI) constitutes the most used paradigm among the diffusion-weighted magnetic resonance imaging (DW-MRI) techniques due to its simplicity and application potential. Recently, real-time estimation in DW-MRI has deserved special attention, with several proposals aiming at the estimation of meaningful diffusion parameters during the repetition time of the acquisition sequence. Specifically focusing on DTI, the underlying model of the noise present in the acquired data is not taken into account, leading to a suboptimal estimation of the diffusion tensor. In this paper, we propose an optimal real-time estimation framework for DTI reconstruction in single-coil acquisitions. By including an online estimation of the time-changing noise variance associated to the acquisition process, the proposed method achieves the sequential best linear unbiased estimator. Results on both synthetic and real data show that our method outperforms those so far proposed, reaching the best performance of the existing proposals by processing a substantially lower number of diffusion images. 相似文献
960.
We study the Hamiltonian dynamics and spectral theory of spin-oscillators. Because of their rich structure, spin-oscillators
display fairly general properties of integrable systems with two degrees of freedom. Spin-oscillators have infinitely many
transversally elliptic singularities, exactly one elliptic-elliptic singularity and one focus-focus singularity. The most
interesting dynamical features of integrable systems, and in particular of spin-oscillators, are encoded in their singularities.
In the first part of the paper we study the symplectic dynamics around the focus-focus singularity. In the second part of
the paper we quantize the coupled spin-oscillators systems and study their spectral theory. The paper combines techniques
from semiclassical analysis with differential geometric methods. 相似文献