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排序方式: 共有413条查询结果,搜索用时 31 毫秒
31.
Diligent O Grahn T Boström A Cawley P Lowe MJ 《The Journal of the Acoustical Society of America》2002,112(6):2589-2601
A study of the interaction of the S0 Lamb wave with a circular through-thickness hole in a plate is presented. The study is limited to the nondispersive frequency range of this wave, in which the distributions of stress and displacement are simple. This allows a Finite Element analysis to be undertaken using a two-dimensional membrane discretization. Predictions of the direct reflection of the S0 mode and the lateral scattering of the SH0 mode are made for a range of diameters of the hole. At the same time, an analytical solution based on modal superposition is developed, and this is also used to predict the reflection and scattering coefficients. Both sets of predictions are validated by experimental measurements. It is found that the trends of the reflection coefficients for different hole diameters, frequencies and distances from the hole satisfy a simple normalization. On a detailed scale, the functions exhibit undulations which are shown to result from the interference of the direct reflection with secondary reflections which arrive slightly later. 相似文献
32.
Atkinson IM Chartres JD Groth AM Lindoy LF Lowe MP Meehan GV 《Chemical communications (Cambridge, England)》2002,(20):2428-2429
A new second generation dendrimer incorporating nine S2N2-donor macrocyclic units that bind nine Pd(II) cations is reported. 相似文献
33.
From a measurement of the cross section for 12C(α, γ)16O at the Eα = 4.253±0.014 MeV(E = 10.352±0.011 MeV) resonance, the radiative width for the 10.35 MeV (Jπ = 4+) to 6.92 MeV (Jπ = 2+) transition was determined to be Γγ = 58±7 meV. This is somewhat higher than the value predicted by large j-j shell-model calculations, but agrees well with predictions based on the rotational and the SU(3) models for a 4p-4h band, and the deformed- basis calculation of Green and Brown. 相似文献
34.
We describe simulations of an elastic filament immersed in a fluid and subjected to a body force. The coupling between the fluid flow and the friction that the filament experiences induces bending and alignment perpendicular to the force. With increasing force there are four shape regimes, ranging from slight distortion to an unsteady tumbling motion. We also find marginally stable structures. The instability of these shapes and the alignment are explained by induced bending and nonlocal hydrodynamic interactions. These effects are experimentally relevant for stiff microfilaments. 相似文献
35.
The magnetization under the spin-lattice relaxation and the nuclear magnetic resonance radiofrequency (RF) pulses is calculated for a signal RF pulse train and for a sequence of multiple RF pulse-trains. It is assumed that the transverse magnetization is zero when each RF pulse is applied. The result expressions can be grouped into two terms: a decay term, which is proportional to the initial magnetization M0, and a recovery term, which has no M0 dependence but strongly depends on the spin-lattice relaxation and the equilibrium magnetization Meq. In magnetic resonance pulse sequences using magnetization in transient state, the recovery term produces artifacts and can seriously degrade the function of the preparation sequence for slice selection, contrast weighting, phase encoding, etc. This work shows that the detrimental effect can be removed by signal averaging in an eliminative fashion. A novel fast data acquisition method for constructing the spin-lattice relaxation (T1) map is introduced. The method has two features: (i) By using eliminative averaging, the curve to fit the T1 value is a decay exponential function rather than a recovery one as in conventional techniques; therefore, the measurement of Meq is not required and the result is less susceptible to the accuracy of the inversion RF pulse. (ii) The decay exponential curve is sampled by using a sequence of multiple pulse-trains. An image is reconstructed from each train and represents a sample point of the curve. Hence a single imaging sequence can yield multiple sample points needed for fitting the T1 value in contrast to conventional techniques that require repeating the imaging sequence for various delay values but obtain only one sample point from each repetition. 相似文献
36.
Recent studies have shown that in certain cases, cardiac and respiratory rate fluctuations in BOLD-weighted MRI time courses may be an artifact unique to rapid sampled acquisitions and may not be present in longer repetition-time acquisitions. The implication of this is that, in these cases, cardiac and respiratory rate fluctuations are not aliased into data that undersample these effects and do not affect the resulting time course measurements. In this study, we show that these cases are specific to regions of large cerebrospinal fluid content and are not generally true for gray matter regions of the brain. We demonstrate that in many brain regions of interest, these fluctuations are directly observed as BOLD fluctuations and thus will affect measurements that undersample these effects. 相似文献
37.
Lowe LB Brewer SH Krämer S Fuierer RR Qian G Agbasi-Porter CO Moses S Franzen S Feldheim DL 《Journal of the American Chemical Society》2003,125(47):14258-14259
Laser-induced temperature jumps (LITJs) at gold nanoparticle-coated indium tin oxide (ITO) electrodes in contact with electrolyte solutions have been measured using temperature-sensitive redox probes and an infrared charge-coupled device. Upon irradiation with 532 nm light, interfacial temperature changes of ca. 20 degrees C were recorded for particle coverages of ca. 1 x 1010 cm-2. In the presence of a redox molecule, LITJ yields open-circuit photovoltages and photocurrents that are proportional to the number of particles on the surface. When ssDNA was used to chemisorb nanoparticles to the ITO surface, solution concentrations as low as 100 fM of target ssDNA-modified nanoparticles could be detected at the electrode surface. 相似文献
38.
A multiple focused probe approach for high intensity focused ultrasound based surgery 总被引:2,自引:0,他引:2
To produce deep-seated surgical lesions using ultrasound requires high power and intensity levels at the target sites. Conventionally, large transducers are used which require big apertures and so are generally suitable only for extra-corporeal access. Such transducers also result in strong off-focus maxima, where the resulting hot spots can destroy normal tissue whilst deep-seated large tissue volumes are being ablated. This paper describes a new approach in which it is proposed that multiple probes are used simultaneously, each at a relatively low power, to result in an overlapping focused region of high temperature without strong off-focus hot spots. Robotic techniques could be used to move the individual probes in coordination to sweep out the desired region of tissue ablation. Simulation and planning are the key to quality prediction for high intensity focused ultrasound (HIFU). Simplified two-dimensional and three-dimensional models of HIFU probes have been developed to study the characteristics of various multi-probe configurations. Preliminary simulation results show that the multi-probe system can be arranged successfully to minimise off-focus hot spots. Experimental results are presented which validate the modality and confirm this positive finding. Further prediction studies and planning will be necessary to achieve the most appropriate desired treatment, by varying the pulse duration and spacing. 相似文献
39.
40.
We develop an approximation scheme for the quantum mechanics of N D0-branes at finite temperature in the 't Hooft large- N limit. The entropy of the quantum mechanics calculated using this approximation agrees well with the Bekenstein-Hawking entropy of a ten-dimensional nonextremal black hole with 0-brane charge. This result is in accordance with the duality conjectured by Itzhaki, Maldacena, Sonnenschein, and Yankielowicz [Phys. Rev. D 58, 046004 (1998)]. Our approximation scheme provides a model for the density matrix which describes a black hole in the strongly coupled quantum mechanics. 相似文献