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Theoretical and experimental investigations of flexural wave propagation in periodic grid structures designed with the idea of phononic crystals
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The propagation characteristics of flexural waves in periodic grid
structures designed with the idea of phononic crystals are
investigated by combining the Bloch theorem with the finite element
method. This combined analysis yields phase constant surfaces, which
predict the location and the extension of band gaps, as well as the
directions and the regions of wave propagation at assigned
frequencies. The predictions are validated by computation and
experimental analysis of the harmonic responses of a finite
structure with 11× 11 unit cells. The flexural wave is
localized at the point of excitation in band gaps, while the
directional behaviour occurs at particular frequencies in pass
bands. These studies provide guidelines to designing periodic
structures for vibration attenuation. 相似文献
43.
Analysis of underwater decoupling properties of a locally resonant acoustic metamaterial coating
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This paper presents a semi-analytical solution for the vibration and sound radiation of a semi-infinite plate covered by a decoupling layer consisting of locally resonant acoustic metamaterial. Formulations are derived based on a combination use of effective medium theory and the theory of elasticity for the decoupling material. Theoretical results show good agreements between the method developed in this paper and the conventional finite element method(FEM), but the method of this paper is more efficient than FEM. Numerical results also show that system with acoustic metamaterial decoupling layer exhibits significant noise reduction performance at the local resonance frequency of the acoustic metamaterial, and such performance can be ascribed to the vibration suppression of the base plate. It is demonstrated that the effective density of acoustic metamaterial decoupling layer has a great influence on the mechanical impedance of the system. Furthermore, the resonance frequency of locally resonant structure can be effectively predicted by a simple model, and it can be significantly affected by the material properties of the locally resonant structure. 相似文献
44.
通过水热共沉淀方法制备了亚微米长余辉发光材料Sr2MgSi2O7:Eu2+,Dy3+,使用X射线衍射、扫描电镜、光致发光以及热释发光光谱对所合成的材料进行了研究。结果表明:在1 000℃下烧结4 h的样品为四方晶系单相,结晶度随着烧结温度的升高而进一步改善,所制备的材料分散性良好,尺寸在200~300 nm之间。在399 nm激发下,样品呈现出主峰位于467 nm的蓝光发射,来自于Eu2+的5d-4f跃迁。在1 100℃烧结获得的样品的长余辉发光优于1 000℃烧结的样品。样品的热释光谱为一个不对称分布的宽带,主峰位于330~340 K之间,热释光谱对应的峰值范围有利于产生长余辉发光。 相似文献
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Mechanism of micro-waviness induced KH<sub>2</sub>PO<sub>4</sub> crystal laser damage and the corresponding vibration source
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The low laser induced damage threshold of the KH2PO4 crystal seriously restricts the output power of inertial confinement fusion.The micro-waviness on the KH2PO4 surface processed by single point diamond turning has a significant influence on the damage threshold.In this paper,the influence of micro-waviness on the damage threshold of the KH2PO4 crystal and the chief sources introducing the micro-waviness are analysed based on the combination of the Fourier modal theory and the power spectrum density method.Research results indicate that among the sub-wavinesses with different characteristic spatial frequencies there exists the most dangerous frequency which greatly reduces the damage threshold,although it may not occupy the largest proportion in the original surface.The experimental damage threshold is basically consistent with the theoretical calculation.For the processing parameters used,the leading frequency of micro-waviness which causes the damage threshold to decrease is between 350-1 μm-1 and 30-1 μm-1,especially between 90-1 μm-1 and 200-1 μm-1.Based on the classification study of the time frequencies of microwaviness,we find that the axial vibration of the spindle is the chief source introducing the micro-waviness,nearly all the leading frequencies are related to the practical spindle frequency(about 6.68 Hz,400 r/min) and a special middle frequency(between 1.029 Hz and 1.143 Hz). 相似文献
47.
Study on incident laser modulation using surface micro-defects on KH<sub>2</sub>PO<sub>4</sub> crystal
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KH2PO4 crystal is a crucial optical component of inertial confinement fusion.Modulation of an incident laser by surface micro-defects will induce the growth of surface damage,which largely restricts the enhancement of the laser induced damage threshold.The modulation of an incident laser by using different kinds of surface defects are simulated by employing the three-dimensional finite-difference time-domain method.The results indicate that after the modulation of surface defects,the light intensity distribution inside the crystal is badly distorted,with the light intensity enhanced symmetrically.The relations between modulation properties and defect geometries(e.g.,width,morphology,and depth of defects) are quite different for different defects.The modulation action is most obvious when the width of surface defects reaches 1.064 μm.For defects with smooth morphology,such as spherical pits,the degree of modulation is the smallest and the light intensity distribution seems relatively uniform.The degree of modulation increases rapidly with the increase of the depth of surface defects and becomes stable when the depth reaches a critical value.The critical depth is 1.064 μm for cuboid pits and radial cracks,while for ellipsoidal pits the value depends on both the width and the length of the defects. 相似文献
48.
采用程序升温脱附、在线质谱和原位漫反射红外光谱等手段, 比较了 NO 和 NO2 在 V2O5 及 V2O5/AC 催化剂表面的选择催化还原 (SCR) 反应行为. 结果表明, 氨以质子态 NH4+和共价态 NH3 分子两种形态吸附于纯 V2O5 表面, V=O 为氨的主要吸附活性位. 无氧状态下, NO 和 NO2 皆可与吸附于 V2O5 表面的 NH3 反应, 并且 NO2 与吸附态 NH3 的反应活性高于 NO. 但在 V2O5/AC 催化剂表面, 同样在无氧条件下, NO 几乎不与吸附态 NH3 反应, 而 NO2 却可以反应并生成 N2. 在 V2O5/AC 表面, NO 很容易被气相 O2 氧化为 NO2, 然后参与 SCR 反应. 可见, NO2 是 NO 在 V2O5/AC 表面发生 SCR 反应的中间体. 相似文献
49.
溶剂热制备条件对镍晶体结构的影响 总被引:1,自引:0,他引:1
Nickel powders were obtained after chemical reduction of nickel chloride by KBH4 using absolute ethylenediamine as solvent in a stainless autoclave. The product was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD results show that the as-prepared sample have different crystal structures at different reaction temperatures. It was face-centered cubic (fcc) nickel under 200 ℃, and there was only hexagonal close-packed (hcp) Ni above 300 ℃. At 250 ℃, the patterns indicate that both fcc and hcp phases of Ni coexist in the as-prepared sample. When the ethylenediamine was respectively substituted by benzene and toluene, only fcc phase nickel was obtained. SEM and TEM indicate that the fcc Ni is nano-particles with about 100 nm in diameter, and hcp nickel is nano-crystalline clusters. 相似文献
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