A cathode with end screens for a relativistic magnetron

The results of application of a cathode with end screens in relativistic magnetron microwave generators are presented. A 40?C50% increase in the microwave power from that of microwave generators using cathodes without screens is obtained.     

Fibrillar structure of superdrawn polyoxymethylene fibres

A fibre structure of superdrawn polyoxymethylene fibres was examined by scanning electron microscopy. The fibre had a skin-core structure and was characterized by a double fibrillar network which was formed from a frame network of trunk fibril screens parallel to the fibre axis, a sub-network of branch fibrils inside the frame, thin cross-fibrils connected to the network and longitudinal void-chains connected to the cross-fibrils between the fibril screens. The frame fibrillar network originated in the undrawn spherulite network, trunk and branch fibrils originated in a bundle of lamellar crystallites and cross-fibrils probably resulted from the interlamellar amorphous phase. The fibrillar network was also varied in the radius direction, possibly due to the diversities of the undrawn spherulitic morphology and the deformation mode at necking.     

On the performance of copper foaming metal in the heat exchangers of pulse tube refrigerator

As key components in pulse tube refrigerators (PTRs), heat exchangers have great influence on the performance of the PTRs, especially the cold end heat exchangers which dominate the cooling effect between the cold gas and heat load. Filling copper screens are widely used to improve the performance of heat exchange and laminar flow. Whereas, the heat transfer rate of copper screens is still not good enough for the actual requirements of PTRs. Furthermore, the flow resistance of the copper screen is growing up quickly with the increase of screen mesh. In this paper, we propose a new type of copper foaming metal with high heat transfer area and low flow resistance in the heat exchanger instead of the copper screens. The heat transfer performances of the copper screens and the copper foaming metal are firstly compared by theoretical calculation, which shows that the performance of the copper foaming metal with 600 μm pore size is better than that of 20 and 80 mesh copper screens, verified by experimental results. A four valve pulse tube refrigerator (FVPTR) with copper foaming metal of 600 μm pore size as filling material of the heat exchanger achieved 69.5 K, 2.5 K lower than that of using 20 mesh copper screens, 1.7 K lower than that of using 80 mesh copper screens.     

多穿孔管板阻性消声器的声学特性研究

应用包含高阶模态声波的二维解析法研究多穿孔管板阻性消声器的声学性能,消声器传递损失的计算结果和实验结果吻合良好.分析了穿孔率和吸声材料的流阻率对阻性消声器声学性能的影响,并且研究了具有组合式阻抗及组合流阻率吸声材料的多穿孔管板消声器的声学特性.结果表明:不同类型的组合式阻抗及组合流阻率的吸声材料对消声器在不同频段的消声性能产生一定影响.     

Control of the characteristics of a long-period grating by cladding etching

We have analyzed the effects of the fiber cladding radius on the characteristics of long-period fiber gratings. By etching the cladding of a common single-mode fiber, we verified the characteristics experimentally. When by etching we reduce the cladding radius of a common single-mode fiber on which a long-period refractive-index modulation has been imposed, the coupling strength of the core and the cladding modes increases. In addition, the difference in the propagation constants (for a fixed wavelength) between the core mode and the cladding modes increases; hence the resonant transmission dip wavelengths shift to longer wavelengths. The proposed method can be useful in making and detuning long-period fiber grating filters.     

Incoherent superposition of off-axis polychromatic Hermite-Gaussian modes

We study a statistical ensemble of multimode laser beams. Each beam is made up of an incoherent superposition of off-axis polychromatic Hermite-Gaussian modes. We obtain analytic expressions for the squared beam radius, the waist position, the Rayleigh range, the skewness parameter, the kurtosis parameter, and the squared beam-propagation factor. We demonstrate that the squared beam radius has a quadratic dependence on the distance from the waist plane. The skewness parameter may be different from zero in the near-field zone, but it tends to zero in the far-field zone. The kurtosis parameter in the far-field zone coincides with the kurtosis parameter of the incoherent superposition of on-axis modes.     

Cantilever arrays with self-aligned nanotips of uniform height

Cantilever arrays are employed to increase the throughput of imaging and manipulation at the nanoscale. We present a fabrication process to construct cantilever arrays with nanotips that show a uniform tip-sample distance. Such uniformity is crucial, because in many applications the cantilevers do not feature individual tip-sample spacing control. Uniform cantilever arrays lead to very similar tip-sample interaction within an array, enable non-contact modes for arrays and give better control over the load force in contact modes. The developed process flow uses a single mask to define both tips and cantilevers. An additional mask is required for the back side etch. The tips are self-aligned in the convex corner at the free end of each cantilever. Although we use standard optical contact lithography, we show that the convex corner can be sharpened to a nanometre scale radius by an isotropic underetch step. The process is robust and wafer-scale. The resonance frequencies of the cantilevers within an array are shown to be highly uniform with a relative standard error of 0.26% or lower. The tip-sample distance within an array of up to ten cantilevers is measured to have a standard error around 10 nm. An imaging demonstration using the AFM shows that all cantilevers in the array have a sharp tip with a radius below 10 nm. The process flow for the cantilever arrays finds application in probe-based nanolithography, probe-based data storage, nanomanufacturing and parallel scanning probe microscopy.     

Characterization of supported cylinder-planar germanium waveguide sensors with synchrotron infrared radiation

Cylinder-planar Ge waveguides are being developed as evanescent-wave sensors for chemical microanalysis. The only non-planar surface is a cylinder section having a 300-mm radius of curvature. This confers a symmetric taper, allowing for direct coupling into and out of the waveguide's 1-mm(2) end faces while obtaining multiple reflections at the central <30-microm-thick sensing region. Ray-optic calculations indicate that the propagation angle at the central minimum has a strong nonlinear dependence on both angle and vertical position of the input ray. This results in rather inefficient coupling of input light into the off-axis modes that are most useful for evanescent-wave absorption spectroscopy. Mode-specific performance of the cylinder-planar waveguides has also been investigated experimentally. As compared to a blackbody source, the much greater brightness of synchrotron-generated infrared (IR) radiation allows a similar total energy throughput, but restricted to a smaller fraction of the allowed waveguide modes. However, such angle-selective excitation results in a strong oscillatory interference pattern in the transmission spectra. These spectral oscillations are the principal technical limitation on using synchrotron radiation to measure evanescent-wave absorption spectra with the thin waveguides.     

Parameters of deformation mixed modes with account of crack tip curvature

A method for calculating the fields of parameters of the stress-strain state at the crack tip for mixed modes of loading is developed and implemented taking into account the higher-order terms. The influence of the final crack tip curvature radius on the parameters characterizing the mixed failure modes under plane strain is shown. The nature of influence of the mixed modes of loading and the crack geometry on the behavior of dimensionless angular characteristics of the second term in the expansion of the stresses in the plastic crack tip region is established from the calculation results. The relationships between the mixing, constraint and triaxiality parameters are derived in a full range of mixed deformation modes.     

基于模态理论的叶片动频率计算方法

旋转状态下叶片动频率分布情况是叶片设计以及机组故障诊断必须考虑的重要因素。提出了一种基于模态分析理论,利用实测静态叶片模态参数重构旋转叶片动力方程,从而获得叶片在不同转速下的动频率。该方法可以考虑叶片径高比、叶片安装角、仰角的影响,可以考虑叶片切向和法向振动之间以及多模态之间的耦合,计算结果表明该方法具有很强的实用性。还提出了基于模态理论的叶片动频系数计算公式。最后结合实例进行了计算,并分析了误差来源。     

Flexible,Semitransparent, and Inorganic Resistive Memory based on BaTi0.95Co0.05O3 Film

Perovskite ceramics and single crystals are commonly hard and brittle due to their small maximum elastic strain. Here, large‐scale BaTi_0.95Co_0.05O₃ (BTCO) film with a SrRuO₃ (SRO) buffered layer on a 10 µm thick mica substrate is flexible with a small bending radius of 1.4 mm and semitransparent for visible light at wavelengths of 500–800 nm. Mica/SRO/BTCO/Au cells show bipolar resistive switching and the high/low resistance ratio is up to 50. The resistive‐switching properties show no obvious changes after the 2.2 mm radius memory being written/erased for 360 000 cycles nor after the memory being bent to 3 mm radius for 10 000 times. Most importantly, the memory works properly at 25–180 °C or after being annealed at 500 °C. The flexible and transparent oxide resistive memory has good prospects for application in smart wearable devices and flexible display screens.     

A physical interpretation of elastic guided-wave reflection from normal ends of a waveguide

Studied in this paper are two-dimensional guided wave reflections from normal boundaries in an isotropic elastic media. By making use of the transverse resonance concept, the reflections of the waveguide modes from normal interfaces are interrogated. A general condition is obtained under which the guided waves in an isotropic medium will undergo no mode conversion when interaction occurs with a normal traction free or fixed end. Under some circumstances, similarities are obtained between waveguide modes and bulk-wave modes, for example, doubling of the displacement field at a free end and doubling of the stress field at a fixed end. The results obtained are applicable to all two-dimensional, guided-wave modes, along one waveguide direction with lossless boundaries on the surface(s) parallel to the waveguide direction, including all possible guided-wave modes, propagating and nonpropagating, in plates, one half space, interface of two different half spaces, layers on a half space, multilayer structures, and all axisymmetric modes in cylindrical structures. In addition, the function of displacement potentials is analyzed in the course of guided-wave mode conversion at a normal end.     

Propagation of polychromatic Gaussian beams through thin lenses

The transformation by a lens of a polychromatic laser beam composed of on-axis superposed monochromatic TEM00 Gaussian modes in the paraxial approximation is studied. The chromatic aberrations are described by allowing the waist position on the z axis and the Rayleigh range to depend on wavelength. The beam radius, the far-field divergence, the Rayleigh range, the beam product, the beam propagation factor, and the kurtosis parameter are calculated. The relationship between the fourth-order and the second-order moments of Hermite-Gaussian and Laguerre-Gaussian modes is obtained and is used for calculating kurtosis parameter. The results are generalized to polychromatic modes of higher orders. It is shown that the on-axis superposition of monochromatic TEM00 modes with no chromatic aberration is leptokurtic.     

Experimental investigation on a thermoacoustic engine having a looped tube and resonator

The purpose of this paper is to study the impact of regenerator hydraulic radius, resonator length, and mean pressure on the characteristics of the tested thermoacoustic engine, which has a looped tube and resonator. Two different acoustic oscillations are observed in the tested engine [Yu ZB, Li Q, Chen X, Guo FZ, Xie XJ, Wu JH. Investigation on the oscillation modes in a thermoacoustic stirling prime mover: mode stability and mode transition. Cryogenics 2003;43(12):687-91]. In this paper, they are called two acoustic modes, high frequency mode (with a frequency independent of the resonator length) and low frequency mode (with a frequency depending on the resonator length). Experimental results indicate that the relative penetration depth (the ratio of penetration depth over hydraulic radius) plays an important role in the excitation and pressure amplitude of the two acoustic modes. For each tested regenerator hydraulic radius, there is a measured optimal relative penetration depth, which leads to the lowest onset temperature difference. Note that, in the tested engine, the measured optimal relative viscous penetration depths are in the range 3-5 (for low frequency mode). Furthermore, experimental results also show that the resonator length affects the presence of high frequency mode in this engine.     

Dispersion analysis of hollow-core modes in ultralarge-bandwidth all-silica Bragg fibers with nanosupports

Dispersion of the fundamental confined modes in hollow-core all-silica Bragg fibers with nanosupports is analyzed. The transfer-matrix formalism is applied. Anomalies in the group-velocity dispersion are evidenced at long wavelengths, toward the upper limit of the bandgap. The results confirm that, as in microstructured photonic crystal fibers, this anomalous dispersion is due to prevention of the confined hollow-core modes from crossing the surface modes, the avoided crossings are more apparent in the variation of group velocity with wavelength. The dependence of these avoided crossings on the hollow-core radius and the layer thicknesses is briefly analyzed.     

Investigation of the hydrolysis of single DNA molecules using fluorescence video microscopy

We developed a method of studying the hydrolysis of single DNA molecules by exonuclease (exo) III using fluorescence video microscopy. A single DNA molecule immobilized at one end on a coverslip and labeled with a fluorescent bead at the other end confined the motion of a bead to a two-dimensional projected circular area determined by the contour length of DNA. The radius of this area decreased with time after the addition of exo III, Mg2+, or a single-stranded DNA-binding protein (SSB), which caused the single-stranded (ss) DNA to twist around itself. However, the radius was relatively constant over time in the absence of Mg2+, which is a cofactor in exo III activity, even when exo III and SSB were both present. These observations indicated that the decrease in the radius was due to hydrolysis of DNA by exo III. We then evaluated the rate of exo III hydrolysis of single DNA molecules by monitoring the decrease in the radius.     

规则排列圆形蜂窝共面对角线方向缓冲性能的研究

目的 研究冲击速度和结构参数对规则排列圆形蜂窝共面对角线方向缓冲性能的影响规律。方法 使用有限元分析软件ANSYS/LS−DYNA建立规则排列圆形蜂窝共面对角线方向动态冲击有限元模型,基于此模型进行参数化仿真模拟,得到不同冲击速度和结构参数下规则排列圆形蜂窝共面对角线方向的变形模式、密实化应变、平台应力和能量吸收特征,并以图表的形式呈现。结果 在共面对角线方向的不同冲击速度下,规则排列圆形蜂窝表现出不同的变形模式。密实化应变在低速和高速冲击下,只与壁厚半径比有关;在中速冲击下,密实化应变同时受冲击速度和壁厚半径比的影响。在给定壁厚半径比下,共面平台应力(或最佳单位体积能量吸收)与冲击速度的平方呈线性关系;在给定冲击速度下,共面平台应力(或最佳单位体积能量吸收)与壁厚半径比呈幂指函数关系。结论 并基于有限元计算结果,得到了动态密实化应变、平台应力和单位体积能量吸收的经验表达式。     

The use of porous screens as wave dampers in narrow wave tanks

Expressions are derived for the reflection of plane incident waves from the closed end of a narrow wave tank when a number of thin vertical porous screens are introduced to damp the waves. The results may have application to the design of wave tanks where a small amount of beach reflection is desirable.     

腹板开洞的工形截面拱的平面内特征值屈曲性能

用有限元模型分析了受径向均布荷载的腹板开洞的工形截面圆弧拱的平面内特征值屈曲性能。选用板壳单元构建了有限元模型,得出了较准确的特征值屈曲荷载和屈曲模态,获得了三种典型的特征值屈曲模态;通过选用一种代表开洞拱效率的特征值屈曲荷载参数,研究了腹板孔洞的半径和间距的优化尺寸。通过一系列拱的数值分析,给出了拱的矢跨比、长细比、孔洞的半径和间距对特征值屈曲荷载的影响,并把这些参数的影响归结为两个参数k和α,给出了这种腹板开洞拱的特征值屈曲荷载的简化计算公式。     

Planar photonic crystal structure with inherently single-mode waveguides

A planar photonic crystal that allows inherently gap-guided single-mode waveguides is proposed and discussed. This novel structure consists of a two-dimensional lattice of silicon rods embedded on a thin silica slab sandwiched between two silica claddings whose refractive indices are slightly lower than the index of the silica core. The physical parameters of the structure, i.e., rod radius and core thickness, are optimized to maximize the bandgap width for odd modes. Lossless guided modes inside the bandgap and below the claddings' light cone are obtained by reducing the radius of a row of rods. The waveguide bandwidth can be increased by inserting a thin silicon dielectric waveguide instead of the row of rods. The proposed approach may overcome many of the common drawbacks in conventional holes-on-dielectric planar photonic crystal waveguides.