Electromagnetic properties of manganese-zinc ferrite with lithium substitution

Polycrystalline manganese-zinc ferrite with lithium substitution of composition Li_0.5xMn_0.4Zn_0.6−xFe_2+0.5xO₄ (0.0≤x≤0.4) was prepared by the usual ceramic method. X-ray diffraction analysis confirmed that the samples have a spinel structure and are of single phase for some values of Li content. Lithium doping considerably modifies saturation magnetization since its value increases from 57.5 emu/g for x=0.0 to 82.9 emu/g for x=0.4. Lithium inclusion increases the real permeability (over 1 MHz) while the natural resonance frequency shifts to lower values as the fraction of Li increases. These ferrites show good electromagnetic properties as absorbers in the microwave range of 1 MHz - 1 GHz.     

特异材料吸收体的透射线模型与场分析:理想的电磁波浓缩器(英文)

Arising from the proposed Transmission Line(TL) model for ERR and wire structure, a TL model for a metamaterial absorber is proposed. The S-parameters obtained by this TL model demonstrate the same shapes as the simulation. An investigation of the TL model and average absorption power densities shows that the metamaterial absorber does not simply convert the electromagnetic wave into thermal energy, but concentrate the electromagnetic wave into a small space where it is finally absorbed. This suggests that ...     

A theoretical analysis of the effects of erbium ion pair on the dynamics of an optical gain stabilized fiber amplifier

In this paper, the effects of ion-pair formation on the gain dynamics of multi cavity optical automatic gain control erbium doped fiber amplifier is modelled. The inhomogeneous Cabezas simple model is used to write the rate and propagation equations for the active medium. The solution of the governing equations shows that in high concentration of the erbium ions, depending on the pumping rate, the relaxation oscillations are converted to nT-Periodic or even to chaotic behavior. Although the high concentration erbium ion in the optical amplifiers decreases the conversion efficiency and increases the threshold pump power, amplitude of the transient effects is reduced in the multi feedback-loop WDM amplifiers relative to the low concentration erbium ions multi feedback-loop optical amplifiers and ion-pairs, which act as a saturable absorber to compensate the gain tilt caused by the OAGC laser spectral hole burning. The theoretical results are in complete agreement with the previous published experimental results.     

金属丝网——油液介质耦合减振器抗冲击动态力学特性研究

基于金属丝网－油液介质耦合减振器原理样机的抗冲击动态力学特性进行研究，样机的多参数匹配冲击试验研究表明该减振器具有强非线性动态特征，并能实现抗大冲击；在此基础上结合流体理论和库伦摩擦理论建立了原理样机非线性冲击动态特性模型，研究减振器抗冲击力学特性的设计可控性，该建模方法直接引入结构参数，为直接设计具体的器件建立了理论基础。     

Experimental evidence of energy pumping in acousticsMise en évidence expérimentale de pompage énergétique en acoustique

This Note presents an experimental vibro-acoustic set-up that aims to reproduce the energy pumping phenomenon between an acoustic medium and an essentially nonlinear oscillator. It shows a one-way irreversible transfer of energy between the first acoustic mode in a tube and a thin visco-elastic membrane. To cite this article: B. Cochelin et al., C. R. Mecanique 334 (2006).     

Steady-State dynamics of a linear structure weakly coupled to an essentially nonlinear oscillator

We present a theoretical study of the dynamics of the coupled system of Jiang, McFarland, Bergman, and Vakakis. It comprises a harmonically excited linear subsystem weakly coupled to an essentially nonlinear oscillator. We explored the rich dynamics exhibited by this coupled system by determining its periodic responses and their bifurcations. Not surprisingly, we found a lot of interesting dynamics over a broad frequency range: cyclic-fold, Hopf, symmetry-breaking, and period-doubling bifurcations; phase-locked motions; regions with multiple coexisting solutions; hysteresis; and chaos. We did not find any occurrence of energy transfer via modulation (also known as zero-to-one internal resonance); theoretically, the possibility of its occurrence was ruled out for systems with weak nonlinearity and damping. Finally, we investigated the ef fectiveness of the so-called nonlinear energy sink (NES) in vibration attenuation of forced linear structures. We found that the NES results in an increase in the vibration amplitude of the linear subsystem, especially when the damping is low, contrary to the claim made by Jiang et al. Also, we did not find any indication of nonlinear energy pumping or localization of energy in the NES, away from the directly forced linear subsystem, indicating that the NES is not ef fective for controlling the vibrations of forced linear structures.     

主动移频式动力吸振器及其动力特性的研究

动力吸振器是振动控制中比较有效的减振装置,只要吸振器(子系统)的振动固有频率与振动物体(主系统)的振动频率相同,即可有效地消除主系统的振动。但传统动力吸振器的控制频率带宽较窄,限制了其稳定性和减振效果的提高。本文通过独特的机械设计,研制了一种可以通过调节自身的几何参数,使得其固有频率随几何参数线性变化的主动移频的新型动力吸振器,并初步设定了相应的控制方法。文中还对其动力学特性进行了理论分析和实验测试,分析了它的机理,评估了它的实际减振效果。研究结果表明该吸振器可以大范围调节自身固有频率,有效拓宽吸振频带,具有良好的减振性能和稳定性。     

Suppression of super-harmonic resonance response using a linear vibration absorber

Super-harmonic resonances may appear in the forced response of a weakly nonlinear oscillator having cubic nonlinearity, when the forcing frequency is approximately equal to one-third of the linearized natural frequency. Under super-harmonic resonance conditions, the frequency-response curve of the amplitude of the free-oscillation terms may exhibit saddle-node bifurcations, jump and hysteresis phenomena. A linear vibration absorber is used to suppress the super-harmonic resonance response of a cubically nonlinear oscillator with external excitation. The absorber can be considered as a small mass-spring-damper oscillator and thus does not adversely affect the dynamic performance of the nonlinear primary oscillator. It is shown that such a vibration absorber is effective in suppressing the super-harmonic resonance response and eliminating saddle-node bifurcations and jump phenomena of the nonlinear oscillator. Numerical examples are given to illustrate the effectiveness of the absorber in attenuating the super-harmonic resonance response.     

泡沫铝冲击吸能器被动冲击隔离技术研究

对隔振系统中引入泡沫铝冲击吸能器进行探讨.首先,在综合考虑最大加速度和最大相对位移这2个影响舰船设备防护的重要指标情况下,对舰船单层隔振系统中引入泡沫铝冲击吸能器进行了理论分析,在此基础上进行了实验验证.通过实验结果可知,泡沫铝冲击吸能器能有效消除舰船设备的二次冲击.实验结果与理论结果吻合较好,可为泡沫铝冲击吸能器的应...     

Feasibility of applying micro-perforated absorbers in acoustic window systems

The paper examines the feasibility of using transparent micro-perforated absorbers (MPA) in a window system to allow noise attenuation whilst at the same time maintaining high levels of comfort ventilation and daylighting. The underlying theory for micro-perforated panels and membranes and the application in silencers is presented. Experiments have been performed between a semi-anechoic and a reverberant chamber using a standard window mock-up, and the effectiveness of MPA has been demonstrated. With a constant air backing, MPA are more effective with a wider ventilation path. With an air flow of up to 2 m/s the performance of the MPA remains unchanged. Current results are based on readily available materials and relatively simple configurations, but the theoretical analysis suggests possibilities for increasing the noise reduction and widening the frequency range by using more strategically designed materials and configurations.