金属表面镀高分子膜对真空击穿阈值的影响

探索提高金属表面真空击穿阈值的方法,对脉冲功率技术的发展和应用具有重要意义。在金属表面电子发射理论分析的基础上,采用有限元法计算阴极杆表面电场随二极管电压的变化规律,设计了实验系统,并开展了实验研究。实验对比了在脉宽约30 ns、阴极杆与阳极筒间隙12 mm时,钛合金TC4阴极杆在不同种类高分子膜（膜厚30～60 μm）下真空击穿阈值的变化情况。在表面粗糙度R_z（轮廓最大高度）为0.8 μm的TC4阴极杆表面分别镀环氧树脂膜和丙烯酸膜,实验结果表明,镀丙烯酸膜阴极杆的击穿阈值约505 kV/cm,相对于不镀膜阴极杆,击穿场强提高了约20.6%;在表面粗糙度R_z为0.2 μm的TC4阴极杆表面分别镀聚酰亚胺膜和聚醚醚酮膜,实验结果表明,镀聚酰亚胺膜阴极杆的击穿阈值为584 kV/cm,相对于不镀膜阴极杆,击穿场强提高了约28.1%。因此,在金属表面镀丙烯酸膜、聚酰亚胺膜可以有效提高金属表面的真空击穿阈值。     

一维NaYF4∶Er/Yb微米棒中上转换荧光的输运特性

稀土掺杂上转换(upconversion,UC)发光材料是一种把两个或多个长波长、低能量的近红外光子转换成一个短波长、高能量的可见或紫外光子的发光材料。由于其锐线发射、长寿命和无背底荧光等特性,在生物成像、检测与治疗、太阳能电池、药物输送和光催化等诸多领域具有极大的应用价值。目前,NaYF4∶Yb^3+/Er^3+微/纳晶体是公认的最重要的上转换发光材料之一,其相关的上转换发光机理、制备方法、光谱调制以及实际应用方面均已被科学家们进行了大量的研究。然而,荧光效率低这一严重问题一直制约着上转换发光材料实际应用的进程。另外,关于单颗粒上转换NaYF4∶Yb^3+/Er^3+微/纳晶体中荧光的产生、输运和耦合特性还不清楚。一维微/纳材料作为一种二维受限体系,为研究电子和光子的输运特性提供了一个理想模型。通过柠檬酸钠辅助的水热法合成了长径比可控的一维NaYF4∶Yb^3+/Er^3+微米棒和一维NaYF4∶Yb^3+/Er^3+@NaYF4∶Yb^3+/Tm^3+核-壳棒结构。采用激光共聚焦激发系统,在单颗粒一维NaYF4∶Yb^3+/Er^3+微米棒中,通过控制棒的长径比、激发方式和制备特殊的核-壳结构等方式,研究了荧光的产生和同步荧光图案,揭示了在一维棒结构中荧光光线的输运模式为:垂直于棒轴方向的荧光沿棒横截面的戒指形谐振腔趋肤传播和近似沿棒轴方向的荧光在棒介质中反复全反射以波导方式输运至棒端后靶向输出;在NaYF4∶Yb^3+/Er^3+@NaYF4∶Yb^3+/Tm^3+核-壳一维棒结构中,在单一波长激发下,获得了源自单颗粒的Tm^3+的蓝色和Er^3+绿色特征发射。采用点激发方式激发一维核-壳微米棒样品一端时,呈强烈蓝色荧光空间不对称的哑铃状分布,且中间出现了微弱绿色荧光,构筑出单颗粒单一波长激发下的局域色彩调制发射,并为微量稀土发光中心局域掺杂的检测提供了一种途径。在波导激发和点激发方式下,在对单颗粒一维NaYF4∶Yb^3+/Er^3+和NaYF4∶Yb^3+/Er^3+@NaYF4∶Yb^3+/Tm^3+核-壳微米棒的荧光产生和输运的调制中,实现了对离子荧光发射颜色的控制,并揭示了荧光在一维棒中传输耦合过程中,沿棒长模式荧光比沿径向模式荧光更容易传输耦合。该研究的一维棒荧光的输运和耦合特性,暗示了其在光子耦合器件、上转换波导激光以及荧光成像方面的潜在应用。     

Numerical simulations of thunderstorm-induced corona processes near lightning rods installed on grounded structures

For the purpose of estimating the efficacy of a lightning rod, the current of a corona was numerically calculated from a rod placed (i) centrally on the roof of a vertical grounded cylindrical structure, a model of a man-made object, and (ii) at the top of a grounded hemispherical structure of large radius, such as a hill or mountain. The calculations were carried out for a slowly varying thundercloud electric field and when this field was enhanced by the charge of an approaching downward leader. In case (i), variation of the ratio between the height and radius of the cylindrical structure leads to a variation in the (a) corona current from the tip of the rod over a wide range and (b) distance from which lightning is attached to the rod. In case (ii), it is shown that, contrary to the case of rods installed on the ground surface, a slowly rising thundercloud electric field can be sufficient to initiate streamers and upward leaders from rods tens of meters in height installed on the top of tall, grounded, hemispherical structures. When the thundercloud electric field is enhanced by the charge of an approaching downward leader, the discharge processes near the rod depend on its length and the height of the tip of the downward leader relative to the top of the hemispherical structure, but are almost independent of the hemisphere radius.     

On the optimum rod geometry for practical lightning protection systems

The effectiveness of a lightning rod as a strike receptor has been the subject of a number of studies recently, particularly in relation to the influence of the tip radius of curvature. The evidence suggests that sharp-tipped rods are not as effective as those with a more blunt geometry. The overall aim of the present paper, as a major extension of previously published work, is to develop and implement a method for computing the “optimum” tip radius of any lightning rod installed on a structure of any given dimensions. Numerical modelling techniques were used to determine the optimum tip radius as a function of the rod length and to account for the difference in electric field intensification created by the placement of the rod in various positions on structures. The results show that the optimum tip radius has a significant dependence on the: (i) rod length, (ii) dimensions of the structure on which the rod is installed, and (iii) physical criterion used for the calculations. In general, the additional electric field intensification created by placing rods on structures, particularly when they are positioned near edges and corners, means that the tip radii required for optimum effectiveness are larger than their counterparts on the ground surface.     

A novel approach for compensation of birefringence in cylindrical Nd: YAG rods

A new concept for compensation of birefringence in solid-state rod lasers is presented. The basic idea of our approach is the simultaneous use of an in-cavity polarization rotator and an imaging optical system between the two passes through the laser rod. The theoretical analysis based on matrix optics shows that perfect compensation for the birefringence and bifocusing in laser rods is possible for a large pumping range. Experimental results with two Nd:YAG rods are also presented. The measured depolarization loss per transit for a test beam at 0.63 m is less than 5% for pumping power up to 6kW per rod. This would allow efficient generation of high-power TEM₀₀ mode and linearly polarized multimode output power from Nd:YAG oscillators in rod geometry.     

Transverse polarizability of an aligned assembly of charged rods

The low-field transverse polarizability of the counterions condensed on an isolated charged rod is small. We show that it can be much larger if the rod is a member of an assembly of aligned rods. The polarization free energy of the assembly of rods in a transverse field is then similar to its polarization free energy in a field parallel to the rods. The polarization free energy of the assembly in a transverse field becomes lower than in a parallel field if the extent of the assembly (as measured, for example, by the diameter of a cylindrical assembly) is larger than the length of the individual rods. We suggest that this model may provide a reasonable explanation for the occurrence of “anomalous” birefringence in systems of interacting charged rodlike particles.     

A model for phase transitions in a system of rod-like colloidal suspensions

The present paper describes an extension of the Maier-Saupe theory of liquid crystals to a system of charged interacting rods and shows that nematic to isotropic transitions can also be induced by increasing the charge on the rods. The effects of length of rod, concentration of salt and the volume fraction on the phase diagram are described. The concept of the charge-induced isotropic phase is described, and it is argued that the structure is expected to be glassy. Evidence to support these conclusions from the existing literature is presented.     

Acoustic measurements and computational results on material specimens with harmonic variation of the cross section

The present work represents both an experimental and theoretical investigation of the behavior of finite cylindrical rods with harmonic variation of the cross section. The matrix method was used to compute the transfer power spectra of elastic rods with uniform circular cross section and of rods with harmonic variation of the cross section with distance. Theoretical and experimental results show that for a rod with periodical variation of the cross section, a new set of supplementary frequencies appear for which the transfer power coefficient has significant values, which are in relation with the space period of the inhomogeneity. Also, due to the radial component of the displacement certain modes are enhanced which satisfy boundary conditions on the surface and are obtained from the zeroes of Bessel functions.     

Self-diffusion and sedimentation of tracer spheres in (semi)dilute dispersions of rigid colloidal rods

Long-time self-diffusion and sedimentation of fluorescent tracer spheres in electrostatically stabilized dispersions of rigid colloidal host rods have been measured in situ with fluorescence recovery after photobleaching, and gravitational and ultracentrifugal sedimentation. The dynamics of silica tracer spheres of 39 and 370 nm radius was monitored in dispersions of host rods with aspect ratios 9.6 and 25.7 at various rod volume fractions. The translational and rotational diffusion coefficient of the host rods was obtained independently with dynamic light scattering and birefringence decay measurements. Our results indicate that sedimentation and long-time self-diffusion are determined by the same friction factor. Furthermore we find that, as long as the host rods are relatively mobile, tracer sphere sedimentation and long-time self-diffusion are governed by the macroscopic solution viscosity, regardless of the tracer and host rod size. However, when the host rods are immobilized, due to rod entanglements at higher volume fractions, tracer sphere dynamics depends strongly on the tracer size relative to the pore size of the host rod network. The large tracers are completely trapped in the network whereas the small tracer spheres remain mobile. Current models for tracer sphere motion in rod assemblies do not satisfactorily explain the complete dynamic regime covered by our experimental model system because the effect of host rod mobility is not properly taken into account.     

Size-dependent tunneling and optical spectroscopy of CdSe quantum rods

Photoluminescence excitation spectroscopy and scanning-tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero-dimensional to a one-dimensional quantum-confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length. With increasing diameter, the band gap and the excited state level spacings shift to the red. The level structure was assigned using a multiband effective-mass model, showing a similar dependence on rod dimensions.     

基于环形微腔的多频段三角晶格光子晶体耦合腔波导光学传输特性

本文理论研究了近红外波段硅基三角晶格光子晶体环形微腔的光场局域特性,通过将微腔在空间周期性排列组成耦合腔光波导,研究了多个导带区域内光束传输时的群速度,最大和最小值分别为0.0028c和0.00028c.将环形微腔在垂直于光传输方向上进行交错排列,通过改变相邻微腔之间的耦合区域,可以大幅降低多频段范围内光束在耦合腔波导中传输时群速度之间的差异,并提高部分频段的透过率数值.在不改变介质柱半径条件下,通过去掉三角晶格光子晶体中距中心介质柱距离分别为2a和√3a的六个介质柱构成了两种微腔,研究了两种微腔所支持的谐振波长之间的差异,在此基础上构造了两种耦合腔波导,进而将这两种耦合腔光波导与W1型输入/输出波导相连,最终实现了在多个不同频率范围内降低群速度的同时实现频段选择和频段分束功能,其导模群速度可降低到0.00047c.     

Calculation of the luminescence of a finite volume with population inversion

Amplified luminescence is examined for a polished cylindrical rod. Nonlinear transport equations are used to examine the distribution of the emission along the rod and the mean density of the amplified luminescence. It is found that the probability of luminescence-induced transitions is of the same order as that for spontaneous transitions for a ruby rod. Calculations are presented for storage in a metastable level and for oscillation. Estimates are presented for the amplified luminescence from a cylinder with nonreflecting surfaces and also for a sphere.I am indebted to B. I. Stepanov for interest and valuable advice.     

Well-aligned ZnO rod arrays grown on glass substrate from aqueous solution

Well-aligned ZnO rod arrays have been successfully synthesized on glass substrate from the aqueous solution of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ (HMT). Some critical issues such as seed layers, concentration and reaction time were investigated. The results show that ZnO seed layers were pre-requisite for the aligned growth of ZnO rod arrays. The length of rods is tunable in a range from 2 μm to 3 μm by varying the solution concentration and reaction time. X-ray diffraction results demonstrate that ZnO rods are wurtzite crystal structures preferentially orienting in the direction of the c-axis. Microstructure observation by scanning electron microscope confirms that ZnO rods grew up perpendicular to the substrate. Room-temperature photoluminescence (PL) spectrum of rod arrays shows a strong emission band at about 396 nm.     

Review of the formula of thermal focal length in side-pumped laser rods

The expressions of focal length in side-pumped rod lasers are studied. Good agreement is obtained between simulation and experiments reported previously, using Koechner's theory. The results reveal that underestimation of thermal lens accrues from overestimating the material's thermal conduction and without considering the nonuniform heat deposit. In the case of Nd:YAG rods, temperature-dependent variation of the refractive index constitutes the major contribution of thermal lensing, whereas the temperature- and stress-dependent variations of the refractive index make the major contributions of thermal lensing for Nd:Glass rod lasers.     

Mechanical impedance gauge based on measurement of strains on a vibrating rod

An impedance gauge based on measurement of strains at two different cross-sections of a vibrating rod is analyzed and tested. The gauge rod, which may have variable characteristic impedance, is in contact with the object at one end and is driven by a harmonic vibrator at the other end. For conical and cylindrical rods explicit relations between point impedance and measured strains are derived. For a cylindrical gauge rod of steel with length 800 mm, diameter 10 mm, and distance between strain gauges 250 mm a fair agreement was generally obtained between experimental and theoretical point impedances of cylindrical test objects in the frequency range 50 Hz to 1·7 kHz. Significant improvements in accuracy over that of the tested prototype are expected to be feasible.     

The phase transition in a one-dimensional lattice of axisymmetric bodies

The partition function is studied for an array of axisymmetric, hard bodies (capped cylinders, etc.) with each fixed at the base on a regular one-dimensional lattice. It is shown that if a phase transition occurs in a system ofn molecules, then it also occurs in a system of two molecules for the same value of the spacing parameter. With certain additional technical assumptions the converse is also true. Results are reported specifically for a system of thin, hard rods. Necessary and sufficient conditions are shown for a first-order transition to occur in the thermodynamic limit; there is only one transition and that happens when the spacing parameter is equal to the length of the rod. As expected, there is no phase transition when the rod is contracted to a point.     

Nonlinear Optical Processes in 3D-Quantized Spectrum of a Semiconductor

Two-photon and three-photon Raman scattering processes are considered. Analytical expressions are derived for the non-linear susceptibility, gain coefficient and generation threshold. It is shown that the resonant fluorescence accompanies the Raman scattering and the hot luminescence processes when the finite population of states is taken into account. Numerical evaluations show that the specification of the medium leads to low generation thresholds and high magnitudes of gain coefficients.     

Study on the prestressed sandwich piezoelectric ceramic ultrasonic transducer of torsional-flexural composite vibrational mode

Based on the classical torsional and flexural vibrational theory of a slender rod, the prestressed sandwich torsional-flexural composite mode piezoelectric ceramic ultrasonic transducer is studied. This type of transducer consists of the slender metal rods and the longitudinally and tangentially polarized piezoelectric ceramic rings. The resonance frequency equations for the torsional and flexural vibrations in the transducers are derived. The simultaneous resonance of the torsional and flexural vibrations in the transducer is acquired by correcting the length of the metal slender rods resulting from the piezoelectric ceramic elements. The experimental results show that the measured resonance frequencies of the transducers are in good agreement with the computed ones, and the measured resonance frequencies of the torsional and the flexural vibrations in the composite transducers are also in good agreement with each other.     

Prediction of the generation of acoustic waves due to the penetration of pulsed microwaves in multilayer media

The acoustic wave generation in a body irradiated by a pulsed microwave is predicted theoretically. The irradiated body is a viscoelastic multilayer rod inserted into a waveguide or an uniformly irradiated viscoelastic plate. The model is based on Maxwell's equations, the heat equation, and thermoviscoelasticity theory. It is validated experimentally by means of four tests performed on three different specimens. The two first specimens are homogeneous rods used to evaluate and verify the mechanical and electromagnetic characteristics of two different materials. The third specimen is a composite rod made up of these materials. Two tests are performed with this specimen. The comparison between the experimental results and the theoretical computations leads to the validation of the theoretical model.     

Thermal rectification in nonmetallic solid junctions: Effect of Kapitza resistance

Based on Fourier's law for heat conduction, we investigate the asymmetric heat flow in two segment rods of nonmetallic materials. Specifically, we study the effect of the Kapitza resistance at the boundary of the segments on the thermal rectification. To understand basic features of the rectification, we first develop analytical calculation for the heat currents in an ideal rod of a macroscopic length. Explicitly, this is made by assuming that the thermal conductivity of each constituent has a power-law dependence on temperature and also assuming the continuity of temperature at the boundary. Then, we introduce the temperature jump at the boundary due to the Kapitza resistance and show that this effect on the thermal rectification becomes significant as the length of the rod decreases typically to submillimeters. In particular, we find that the temperature jump yields a finite rectification even when no asymmetry is predicted in the heat currents from the continuity of temperature at the junction. The obtained results have an important implication for the analysis of the thermal rectification of a rod consisting of semiconductors Ge and Si.