Experimental and calculated research on a large band gap constituting of tubes with periodic narrow slits

A new band gap structure composed of a square array of parallel steel tubes with narrow slits is presented. The propagation of acoustic waves in a two-dimensional composite medium constituted of slit tubes in air is investigated both theoretically and experimentally. The band gap is calculated with the finite element method in which the acoustic-solid coupling is taken into account. The transmissions of the band system with both different single-width narrow slits and multi-width narrow slits are analyzed. Experimental measurements show that the transmission through an array of slit tubes with periodic narrow slits drops to noise level throughout frequency interval in good agreement with the calculated forbidden band. The large band gap and low starting frequency is obtained by arranging different width of slits embedded in the tubes. The experiments and theoretical results show that this new band gap structure has an especial character based on the resonant cavity playing an important role on the band gap besides the traditional Bragg interference.     

二维开缝金属圆管带隙结构禁带特性中缝参数的研究

对二维组合开缝金属圆管的禁带特性进行了数值模拟和实验验证,着重研究了缝参数对禁带的影响,并分析了不同缝参数开缝圆管组合结构具有的独特禁带性质.模拟计算和实验结果符合较好,结果表明这种微缝带隙结构具有起始频率低的特点.同单一缝参数开缝圆管带隙结构相比,组合缝参数带隙结构具有连续的大禁带. 关键词： 微缝散射体 禁带     

双锥型五模材料低频声波调控及参数设计

为有效控制噪声并进行声波调控,构造了双锥区域为TC4钛合金、节点区域为硫化橡胶的六边形排列双锥五模材料,进行能带分析发现其具有较窄的低频声子带隙和单模传输区域。为提高五模材料的低频声波调控性能,设计了正方形和三角形排列构型,结果表明三角形排列的双锥五模材料带隙的频率更低,带宽更宽,且具有单模传输性能和较好的五模特性。此外分别探究了五模材料构型的材料参数(包括双锥区和节点区的密度、泊松比和杨氏模量)以及几何参数(包括双锥宽和节点半径的变化)对带隙及单模传输区域的影响,得到带隙和单模传输区的变化规律,选择密度较轻的填充材料、较小的双锥宽和较大的节点半径不仅可以提高低频声波调控性能,而且可以降低结构质量,提高结构的稳定性。该文结果对用于低频声波传播调控的五模材料的构型和参数的设计具有一定的借鉴意义。     

Boron-nitride and boron-carbonitride nanotubes: synthesis,characterization and theory

We present in this review a joint experimental and theoretical overview of the synthesis techniques and properties of boron-nitride (BN) and boron-carbonitride (BCN) nanotubes. While their tubular structure is similar to that of their carbon analogues, we show that their electronic properties are significantly different. BN tubes are wide band gap insulators while BCN systems can be semiconductors with a band gap in the visible range.     

第一性原理研究电场对BN纳米管电子结构的影响

采用基于密度泛函理论的第一性原理计算研究了电场对BN纳米管的电子结构的影响.首先对在不同电场强度下的纳米管几何结构进行了优化,可以看出纳米管沿轴方向层间距出现了不规则的变化.电子能带结构显示,在电场作用下,zigzag型和armchair型两种结构纳米管的能带向低能方向移动,并且导致纳米管的带隙有显著的减小.电场使得armchair型纳米管的带隙发生了从间接带隙向直接带隙的转变.在电场作用下,纳米管的两端态密度呈现出明显的差异,正负电荷沿轴向出现了沿轴向的空间分离,Mulliken电荷分布图揭示出最高占据轨道和最低未占据轨道分居在纳米管的两端.     

内螺旋肋管流动与传热特性的实验研究

对六种内螺旋肋管进行了流动与传热的实验研究,实验管内径为16．25-16．69 mm,内螺旋肋高为0．28-0．44 mm,螺旋肋牙数为40-45,螺旋角为43°-45°．研究表明,内螺旋肋管可以有效地强化传热,本文所研究的管型的传热强化倍率为1．67-2．99．比较了两种评价内螺旋肋管性能的方法．用Webb模型及Ravigururajan模型对内螺旋肋管进行了性能预测并与实验值进行了比较．两个模型的预测值与本试验结果有较大偏差,相对而言,传热模型稍优．     

基于开槽单矩形栅和圆形电子注的W波段返波振荡器

提出将开槽单矩形栅和圆形电子注作为 W波段返波振荡器的注波互作用回路. 使用 3维电磁场仿真软件CST-MWS对开槽单矩形栅的高频特性进行了仿真分析, 研究结果表明: 相对于传统单矩形栅, 新结构的基模带宽有所展宽; 基模与高次模发生模式竞争的可能性很小; 在采用圆形电子注时新结构能获得大得多的耦合阻抗; 新结构的趋肤损耗略有改善. 将该慢波结构应用于设计一支以94 GHz为频带中心的W 波段返波振荡器: 设计了简洁的慢波过渡部分、输出耦合器和终端匹配衰减器, 优化参数后获得了良好的信号传输特性; 利用粒子模拟软件CST-PS对返波振荡器模型进行了三维大信号注波互作用计算, 设定合适的电子注电流等参数后, 调整工作电压在较宽的频带内获得了瓦级的功率输出, 电子效率在整个频带范围内优于1%. 关键词： 开槽单矩形栅 圆形电子注 返波振荡器 W波段     

正负折射率材料组成的一维光子晶体的能带及电场

计算了由正负折射率材料交替排列组成的一维光子晶体的能带及电场，发现其能带不同于由普通正折射率材料组成的光子晶体的能带.当选择合适的参数时，由正负折射率材料组成的光子晶体的TE模或TM模有完全光子带隙出现，这在普通光子晶体中不出现.导带中的电场波函数与普通光子晶体相比具有很强的局域性.对于负折射率材料层为色散介质的情况，计算了在不同的具有正负折射率区域能带.     

Electro-optical properties of zigzag and armchair boron nitride nanotubes under a transverse electric field: Tight binding calculations

The electro-optical properties of zigzag and armchair BNNTs in a uniform transverse electric field are investigated within tight binding approximation. It is found that the electric field modifies the band structure and splits band degeneracy where these effects reflect in the DOS and JDOS spectra. A decrease in the band gap, as a function of the electric field, is observed. This gap reduction increases with the diameter and it is independent of chirality. An analytic function to estimate the electric field needed for band gap closing is proposed which is in good agreement with DFT results. In additional, we show that the larger diameter tubes are more sensitive than small ones. Number and position of peaks in DOS and JDOS spectra for armchair and zigzag tubes with similar radius are dependent on electric field strength.     

Photonic band gap of one-dimensional periodic structure containing dispersive left-handed metamaterials

Band structures of one-dimensional(1D)photonic crystals(PCs)containing dispersive left-handed metamaterials are studied theoretically.The results show that the structure possesses a type of photonic band gap originating from total internal reflection(TIR).In contrast to photonic band gaps corresponding to zero average refractive index and zero phase.the TIR gap exhibits sharp angular effect and has no polarization effect.It should also be noted that band structures of transverse electric(TE) and transverse magnetic(TM) mode waves are exactly the same in the PCs we studied.     

Wave propagation in hexagonal and re-entrant lattice structures with cell walls of non-uniform thickness

On the basis of the Bloch’s theorem, the in-plane wave propagation in hexagonal and re-entrant lattice structures with cell walls of non-uniform thickness is investigated using the dynamic stiffness matrix in conjunction with the Wittrick–Williams algorithm. Special attention is devoted to the effects of the internal angle, the slenderness ratio and the material distribution on the directional and band gap behaviors. Results show that the three considered parameters can significantly influence the band gap characteristics. For the wave propagation directionality, however, the internal angle is more prominent than the other two factors. The work expects to serve as a guide for the optimal design of directional mechanical filters.     

Enhanced acoustic localization in the two-dimensional phononic crystals with slit tube defect

In this paper, the acoustic localization characteristics of the two-dimensional phononic crystals with slit tube defect are investigated theoretically and experimentally. In contrast to the typical formation pattern of defect states, the proposed defect states are created by replacing a slotted tube in the center of perfect phononic crystal. Compared to perfect phononic crystal, the proposed structure can effectively localize waves of specific frequencies in the point defect and improve the acoustic pressure amplification. Then the effects of the geometric parameters of the slotted tube on the acoustic localization characteristics are studied. Numerical results show that the resonant frequency and acoustic pressure amplification amplitude could be effectively modulated by the geometric parameters of the slotted tube. Experimental results are in good agreement with the simulation results.     

用转移矩阵法数值研究二维正方介质柱光子晶体的传输特性

利用转移矩阵方法对二维正方介质柱光子晶体的传输特性进行了研究,数值计算研究了不同晶格、同一晶格柱体截面面积不同、放置方位角不同时光子晶体的传输特性。数值结果表明光子禁带的宽度与中心频率和晶格结构有很大关系,正方晶格更易形成平坦光子禁带,柱体截面面积大,则形成的禁带较宽,在其他因素相同的条件下柱体放置的方位角对光子禁带有重要影响。数值研究表明在正方介质柱下设计宽平坦光子禁带时,可以首先考虑正方晶格结构,其次设法使柱体截面尽量大一些,最后可通过柱体放置方位角来微调光子禁带的宽度与中心频率以达到设计要求。     

Common electronic band gaps and similar optical properties of ZnO nanotubes

Electronic and optical properties of single-walled zinc oxide （ZnO） nanotubes are investigated from the firstprinciples calculations. Electronic structure calculations show that ZnO nanotubes are all direct band gap semiconducting nanotubes and the band gaps are relatively insensitive to the diameter and chirality of tubes. The origin of the common electronic band gaps of ZnO nanotubes is explained in terms of band-folding from the two-dimensional band structure of graphite-like sheet. Moreover, the optical properties such as dielectric function and energy loss function spectra of different ZnO nanotubes are very similar, relatively independent of diameter and chirality of tubes. The calculated dielectric function and loss function spectra show a moderate optical anisotropy with respect to light polarization.     

Effect of the dielectric background on dispersion characteristics of metallo-dielectric photonic crystals

We theoretically study the effect of the dielectric background in two-dimensional metallo-dielectric photonic crystals. The metallo-dielectric photonic crystal consists of a square lattice of circular metallic rods embedded into a dielectric background. We calculate the photonic band structure by means of the plane wave method and the frequency-dependent finite-difference time-domain method. The transfer matrix method is used to obtain the reflectivity characteristics. Results show that the band structures shift toward lower frequencies and become flatter when the background dielectric constant increases. In addition, degeneracy can be broken and new gaps can be created in function of the dielectric background. We also found that the relative band gap width Δω/ω_g grows with increasing background dielectric constant and widths as large as 42.3% and 13.8% for the second and third band gaps can be achieved for ε_b = 9. We have investigated the origin of the new gap in these structures by studying the electric-field distribution at the band edges for the first five modes.     

三维光子晶体典型结构完全禁带的最佳参数理论分析

基于平面波展开法，理论分析了晶格结构、填充率、介电常数比等因素对fcc，diamond，woodpile三种三维光子晶体典型结构完全禁带的影响.三种结构中，fcc结构由于高对称性导致的能级简并，只适用于密堆积排列的反蛋白石结构；diamond结构非常容易产生高带隙率的完全禁带，并且可以通过调节多项参数得到所需的完全禁带；woodpile结构参数调节范围比较宽，为实验制备带来方便.对于不同的三维光子晶体结构，随着介电常数比的增大，完全禁带的宽度和带隙率也会随着增大.还发现了一些以前未引起注意的现象. 关键词： 三维光子晶体 完全禁带 介电常数比 带隙率 平面波展开法     

含特异材料一维超导光子晶体的带隙特性研究

利用传输矩阵法研究了含特异材料的一维超导光子晶体的带隙特性. 研究表明, 这类超导光子晶体同样具有由传统的电介质材料构成的超导光子晶体一样的低频带隙, 且在一定的参数下该低频带隙可以相当宽. 但在一定的结构参数下, 这类超导光子晶体同完全由传统的电介质构成的光子晶体一样不存在低频带隙. 还就超导光子晶体的偏振特性、光子晶体结构参数及环境温度的变化对光子带隙结构的影响进行了研究. 关键词： 超导光子晶体 传输矩阵法 特异材料 光子带隙     

Flexural vibration band gaps in a thin plate containing a periodic array of hemmed discs

Using a periodic expansion by means of the Bloch theorem, the flexural vibration band gaps are studied in a thin plate with two-dimensional ternary locally resonant structures, i.e. a thin epoxy plate containing a periodic square array of lead discs hemmed around by rubber. The full band gaps of flexural vibration in the thin plate are obtained within which sound and vibration will be forbidden. The numerical results are used to show how the width of the first full band gap depends on the radius ratio of lead disc to hemmed disc, filling fraction, lattice constant (distance between the centers of the nearest lead discs) and thickness of the thin plate. It is observed that the gap width can be changed a lot by modulating these physical parameters.     

Electronic structure of collapsed C,BN, and BC3 nanotubes

The electrical properties of single-wall C, BN, and BC₃ nanotubes in ideally rolled-up forms show a wide spectrum from truly metals to large band gap semiconductors. In the presence of radial deformations that collapse tubes, the electrical properties are severely modified such that metals turn into semiconductors and vice versa. Based on first-principles pseudopotential calculations, we find that metallic C nanotubes have a finite band gap if radial deformations break all mirror symmetries of the tubes, and that original finite gaps (∼0.5 eV) of semiconducting C and BC₃ tubes are closed by collapsing deformations. In BN tubes, band gaps can be tuned in the range 2–5 eV. On the other hand, the band gaps of armchair BN and zigzag BC₃ nanotubes are found to be insensitive to radial deformations. These new findings can be applied to design new types of nanotube-based functional devices using radial deformations.     

Matryoshka locally resonant sonic crystal

The results of numerical modeling of sonic crystals with resonant array elements are reported. The investigated resonant elements include plain slotted cylinders as well as their various combinations, in particular, Russian doll or Matryoshka configurations. The acoustic band structure and transmission characteristics of such systems have been computed with the use of finite element methods. The general concept of a locally resonant sonic crystal is proposed that utilizes acoustic resonances to form additional band gaps that are decoupled from Bragg gaps. An existence of a separate attenuation mechanism associated with the resonant elements that increases performance in the lower frequency regime has been identified. The results show a formation of broad band gaps positioned significantly below the first Bragg frequency. For low frequency broadband attenuation, a most optimal configuration is the Matryoshka sonic crystal, where each scattering unit is composed of multiple concentric slotted cylinders. This system forms numerous gaps in the lower frequency regime, below Bragg bands, while maintaining a reduced crystal size viable for noise barrier technology. The finding opens alternative perspectives for the construction of sound barriers in the low frequency range usually inaccessible by traditional means including conventional sonic crystals.