共查询到20条相似文献,搜索用时 31 毫秒
1.
Microelectronic mechanical (MEM) beam resonators with high quality factors are always preferred in practical applications. As one of the damping sources, thermoelastic damping (TED) caused by irreversible heat flows is usually considered as an upper limit of the overall damping effect. A new method is proposed in this work to compensate TED by taking advantage of the piezoresistive effect. Such a method is implemented by applying an electrostatic field along the beam length with a negative piezoresistive coefficient. During a resonance, the stretched part of the beam generates a higher electrical power density and thus a higher temperature, while the compressed region leads to a lower temperature. Such a temperature distribution is opposite to the temperature change caused by the thermoelastic effect. The working principle is described by a set of coupled differential equations, which are subsequently solved by the finite element method. The result indicates that the TED in the beam resonators can be completely compensated when the strength of electrical field is tuned to a critical value, namely CEF. The value of the CEF is further analyzed by a series of parametric studies on various material properties and geometric factors. 相似文献
2.
The electron-ion instability is excited in counterstreaming ion beams and plasma system. The instability is a new type of standing oscillations whose wavelength is given by D/n, where D is the distance between boundaries and n is an integer. The amplitude is controlled by a difference between the velocities of the beams, which changes the phase of a feedback loop. The internal feedback is caused by a reflected wave and by a coupling between the boundaries. Temporal evolution of the instability is measured and is found to agree with numerical solutions of the Van der Pol equation including a feedback term. A measured growth rate is proportional to the square of the oscillation amplitude. 相似文献
3.
基于米氏散射理论解释了激光照明下玻璃微珠的二次彩虹精细结构的成因,发现折射率的差异将直接影响二次彩虹精细结构的位置.对于实验中玻璃微珠半径变化引起二次彩虹精细结构间距变化的现象亦用米氏散射理论进行了模拟分析和实验研究.利用米氏散射的近似理论——艾里理论对玻璃微珠的折射率进行了测量.在对玻璃微珠二次彩虹精细结构所计算得到的折射率的统计分析基础上,通过校正测量误差后得到了玻璃微珠折射率的准确数据. 相似文献
4.
A collection of static atoms is fixed in a crystal at a low temperature and prepared by a pulse of incident radiation of wave vector . The atoms are well described by an entangled Dicke-like state, in which each atom carries a characteristic phase factor exp(ik0.r(j)), where is the atomic position in the crystal. It is shown that a single photon absorbed by the N atoms will be followed by spontaneous emission in the same direction. Furthermore, phase matched emission is found when one photon is absorbed by N atoms followed by two-photon down-conversion. 相似文献
5.
A mathematical model of a diode system based on a sharp-edged field cathode is presented. The surface of the sharp-edged cathode
is simulated by two infinitely thin spherical segments, while the anode is modeled by a single infinitely thin spherical segment.
The region occupied by the cathode is a lens (lune-shaped region). The problem of the electrostatic potential distribution
in the entire domain occupied by the system is solved. 相似文献
6.
An experiment is described in which a wave scattered by a rough water surface is the factor by virtue of which the signal diffracted by a moving scatterer is received with confidence. Experimental corroboration of this fact is presented, and the origin of this phenomenon is revealed. 相似文献
7.
Fran?ois Hild Stéphane Roux Renaud Gras Néstor Guerrero Maria Eugenia Marante Julio Flórez-López 《Optics and Lasers in Engineering》2009,47(3-4):495-503
It is proposed to develop a digital image correlation procedure that is suitable for beams whose kinematics is described by an Euler–Bernoulli hypothesis. As a direct output, the degrees of freedom corresponding to flexural and axial loads are directly measured. The performance of the correlation algorithm is evaluated by using a picture of a cantilever beam experiment. One load level is analyzed with the present algorithm. The latter is validated by comparing the displacement field with that given by a finite element based correlation algorithm. It is also shown that a locally buckled zone is detectable with the present procedure. 相似文献
8.
We observe the dynamics of a single magnetic vortex pinned by a defect in a ferromagnetic film. At low excitation amplitudes, the vortex core gyrates about its equilibrium position with a frequency that is characteristic of a single pinning site. At high amplitudes, the frequency of gyration is determined by the magnetostatic energy of the entire vortex, which is confined in a micron-scale disk. We observe a sharp transition between these two amplitude regimes that is due to depinning of the vortex core from a local defect. The distribution of pinning sites is determined by mapping fluctuations in the frequency as the vortex core is displaced by a static in-plane magnetic field. 相似文献
9.
Railway wheel vibrations are caused by a number of mechanisms. Two of these are considered: (a) gravitational load reaction acting on different points of the wheel rim, as the wheel rolls on, and (b) random fluctuating forces generated at the contact patch by roughness on the mating surfaces of the wheel and rail. The wheel is idealized as a thin ring, and the analysis is limited to a single wheel rolling on a rail. It is shown that the first mechanism results in a stationary pattern of vibration, which would not radiate any sound. The acceleration caused by roughness-excited forces is much higher at higher frequencies, but is of the same order as that caused by load reaction at lower frequencies. The computed acceleration level (and hence the radiated SPL) caused by roughness is comparable with the observed values, and is seen to increase by about 10 dB for a doubling of the wagon speed. The driving point impedance of the periodic rail-sleeper system at the contact patch, which is used in the analysis, is derived in a companion paper. 相似文献
10.
The RVB model is investigated by a mean field theory, in which the holon is expressed by a fermion operator, and the spinon by a boson operator. The effective holon-holon interaction by exchanging the spinons is derived for T≠0. It is found that the interaction is attractive. 相似文献
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Yasuhiko Arai Hiroyuki Hirai Shunsuke Yokozeki 《Optics and Lasers in Engineering》2008,46(10):733-738
The speckle interferometer based on multi-camera technologies using two cameras is applied to a dynamic measurement. The new speckle interferometer is constructed by a prism array and two cameras. A phenomenon, which a bearing-ball collides against a thin polymer film, is investigated by the proposed interferometer. Then, it is shown that the local maximum deformation of the thin film by the collision is about 1.0 μm. Such a deformation process can precisely be analyzed by this method without any troubles of optical dislocations. In the results, it is confirmed that a large deformation process can be analyzed by accumulating measured results of small deformation in every small continuous analysis. Furthermore, it is estimated that the measurement precision of this method is about 5 nm as experimental results. 相似文献
13.
Nanoindentation is simulated on the computer by means of a 2D discrete dislocation model under the conditions of a constrained geometry. First, an indentation test near a grain boundary is investigated by the arrangement of only one boundary and second, an indentation test into the center of the surface of a small grain (lamella) is mimicked by the arrangement of two boundaries. The effect of a limited number of dislocation sources is studied by the simulations of an indentation test in a plastically deformable film on an ideal elastic substrate and by such tests on an ideal elastic film on a plastically deformable substrate. The discrete nature of plasticity is shown to have a significant influence on the mechanical material behavior in all our investigations. 相似文献
14.
Yair Zarmi 《Physica D: Nonlinear Phenomena》2009,238(14):1274-1287
A new approach to the perturbative analysis of dynamical systems, which can be described approximately by soliton solutions of integrable non-linear wave equations, is employed in the case of small-amplitude solutions of the ion acoustic wave equations of plasma physics. Instead of pursuing the traditional derivation of a perturbed KdV equation, the ion velocity is written as a sum of two components: elastic and inelastic. In the single-soliton case, the elastic component is the full solution. In the multiple-soliton case, it is complemented by the inelastic component. The original system is transformed into two evolution equations: An asymptotically integrable Normal Form for ordinary KdV solitons, and an equation for the inelastic component. The zero-order term of the elastic component is a single-soliton or multiple-soliton solution of the Normal Form. The inelastic component asymptotes into a linear combination of single-soliton solutions of the Normal Form, with amplitudes determined by soliton interactions, plus a second-order decaying dispersive wave. Satisfaction of a conservation law by the inelastic component and of mass conservation by the disturbance to the ion density is determined solely by the initial data and/or boundary conditions imposed on the inelastic component. The electrostatic potential is a first-order quantity. It is affected by the inelastic component only in second order. The charge density displays a triple-layer structure. The analysis is carried out through the third order. 相似文献
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16.
Kanji Etori 《Letters in Mathematical Physics》1992,26(4):271-275
From both the Langevin equation, including a gravitational term, the Fokker-Planck equation based on the dynamical behavior of Brownian particles, and equation of smoke molecular diffusion due to a constant point source is introduced and is solved by applying the Laplace transformation with the convolution theorem.The solution is expressed by the complementary error function with a mean pathway of smoke molecules affected by gravity and is proved to be reduced to conventional forms by a certain restriction neglecting gravity without any forced term. 相似文献
17.
In graded-index polymer optical fiber (GI-POF), the refractive index profile is an important parameter in defining its bandwidth. However, direct determination of the refractive index profile of GI-POF is difficult due to its extreme thinness. In this study, the refractive index distribution of the GI-POF is indirectly determined by measuring the refractive index distribution of the GI-POF perform by applying the transverse ray tracing method to a simplified measurement system that we developed.In this system, a parallel tabular ray is irradiated transversely to a GI-POF preform. The transverse ray from the preform is then projected on a screen, and its digital image is processed to calculate the refractive index distribution. The calculation is based on a transverse ray simulation, a computer program that we developed in which the refractive index distribution of the preform is determined by comparing the displacement of the transverse ray projected on the screen with the actual measurement.The accuracy of this new measurement method is validated by comparing the refractive index distribution of a GI-POF preform with the refractive index distribution measured by the conventional method using an interferometer. We find that the refractive index distribution measured by this novel method agrees well with that measured by the conventional method. 相似文献
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The transfer of spin between photons and localized atomic levels, for a chain of two-level atoms, is studied. The Hamiltonian of the system is modelled by a radiation term and by atom-atom and radiation-atom interactions. Effects due to dissipation are accounted for by atom-photon exchange-interactions of complex strength. It is found that the spin-squeezing is suppressed by dissipation. Calculations are performed for arrays of Rb atoms excited by a GaAlAs laser. 相似文献