导电颗粒属性对各向异性导电粘合剂电导的影响

利用差分法对导电粘合剂中导电颗粒的属性(包括导电颗粒的形状、大小等)对导电粘合剂电导的影响进行了计算.在交流信号下，导电粘合剂的金属填充颗粒越小，导电粘合剂的电导越大.通过理论计算发现，导电粘合剂的导电颗粒若为球形(正方体)，导电粘合剂的电导与金属颗粒的半径(边长)成反比例变化；若为长方体或圆柱体颗粒，则电导随颗粒高度的增加而减小，而基本不随颗粒底面积的改变而变化. 关键词： 导电粘合剂 电导 导电颗粒 差分法     

Mass spectrometry of molecules and radicals in glow discharge plasma

The article represents a method and equipment developed for mass spectrometric analysis of plasma, that is, for measurement of concentration of atoms and molecules, and their fragments, including free radicals. A compact and inexpensive mass spectrometer is based on a quadrupole residual gas analyzer (RGA-200, Stanford Research Systems). The design of the two-section differential pumping chamber makes it possible to bring the mass-spectrometer analyzer to the entrance diaphragm to a distance of 40 mm in order to measure quick reacting and easily condensed particles. The equipment was used for analyzing the composition of spherical glow discharge plasma in methanol vapor and acetone-nitrogen mixture. A procedure for mass spectrum processing is proposed. Time-varying concentrations of all observed neutral particles are measured. Presently available data on sections of complete and dissociative ionization of molecules and their fragments, which are necessary for reconstructing concentrations of particles in plasma from measured mass spectra, are presented.     

Adiabatic time-dependent Hartree-Fock calculation of fusion cross sections: Application to 40Ca-40Ca

Using the adiabatic time-dependent Hartree-Fock method we calculate the interaction and inertial mass parameter relevant for a heavy-ion collision. In this work, which extends an earlier study for lighter systems, we investigate the case of ⁴⁰Ca-⁴⁰Ca for which detailed experimental information is available. As already found for lighter colliding ions, the mass parameter exhibits a large peak for interdistances slightly smaller than the barrier radius. We use our results to evaluate the fusion cross section and we find that the structure in the inertial mass brings the theoretical cross section in close agreement with data.     

Nano-laser on silicon quantum dots

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantum dots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.     

强流脉冲电弧作用下石墨电极蒸发特性

气体开关电弧的热侵蚀作用是电极损耗的主要成因。石墨电极在电弧作用下发生蒸发并在多次放电后有明显的质量损耗,改变了开关内的气体环境和电极间距,导致开关动作可靠性降低。为研究石墨电极在脉冲电弧冲击下的侵蚀特征,基于开关电弧瞬态扩散特征和石墨材料参数,在弧根区域建立了电弧-电极能量耦合模型,得到了等离子体-固体区域的传热特性。考虑石墨电极的相变特征,计算瞬态热作用下石墨电极的加热范围以及临界相变点,研究瞬态电弧热冲击作用下的石墨电极相变机制。研究结果表明,电弧-电极界面热流主要集中在电弧接触面中心,电弧沉积的能量密度最高可达109 W/m2,石墨在电流上升初期基本处于加热状态,在能量积聚作用下,石墨转变为升华状态,传热强度随半径急剧衰减,蒸发区域略小于电弧半径。通过实验记录了5种开关工况下石墨电极烧蚀形貌和质量损失情况,结果表明,电极质量损失与电弧沉积在电极表面的能量线性相关,近似为0.015 mg/J。研究了电弧关键参数对电极质量损失速率的影响,为延缓电极损耗提供数据支撑。     

DNS and LES of spark ignition with an automotive coil

The cycle to cycle combustion variability which is observed in spark-ignition engines is often caused by fluctuations of the early flame development. LES can be exploited for a better understanding and mastering of their origins. For that purpose appropriate models taking into account energy deposition, mixture ignition and transition to propagation are necessary requirements. This paper presents first DNS and LES of spark ignition with a real automotive coil and simplified pin-pin electrodes. The electrical circuit characteristics are provided by ISSIM while the energy deposition is modelled by Lagrangian particles. The ignition model is first evaluated in terms of initial spark radius on a pin-pin ignition experiment in pure air performed at CORIA and EM2C laboratories, showing that it pilots the radius of the torus formed by the initial shock wave. DNS of a quiescent lean propane/air mixture are then performed with this ignition system and a two-step mechanism. The impact of the modelled transferred energy during glow phase as well as the initial arc radius on the minimum ignition energy (MIE) are examined and compared to experimental values. Replacing the two-step chemistry by an analytically reduced mechanism leads to similar MIE but shows a different ignition kernel shape. Finally, LES of turbulent ignition using a Lagrangian arc model show a realistic prediction of the arc shape and its important role on the energy transfer location and thus on the flame kernel shape.     

Analysis of ultrasonic attenuation in particle-reinforced plastics by a differential scheme

Attenuation of ultrasonic longitudinal waves in some particle-reinforced polymer composites is studied theoretically by a micromechanical model based on a differential (incremental) scheme. A set of differential equations is established by which the attenuation spectrum of the composite can be computed from the known properties of viscoelastic matrix and elastic particles. For a composite reinforced with glass particles with radius 0.15 mm, the proposed scheme is shown to predict the attenuation in better agreement with the foregoing experimental results than the previous simplistic independent scattering model. Based on this scheme, the dependence of the longitudinal attenuation spectrum of a particulate polymer composite on the wavelength-to-particle radius ratio and the particle volume fraction is examined in detail. It is then shown theoretically that the attenuation of the composite decreases monotonically with the particle volume fraction when the particle radius is sufficiently small compared to the incident wavelength, while it shows non-monotonic particle-fraction dependence when the ratio of the particle radius to the wavelength is larger. To examine this theoretical finding from an experimental point of view, the longitudinal attenuation in a glass-particle-reinforced polyester composite with particle radius 0.0225 mm is measured for different particle volume fractions. The measured attenuation characteristics are shown to support the qualitative features of the theoretical prediction.     

Analysis of ultrasonic attenuation in particle-reinforced plastics by a differential scheme

Attenuation of ultrasonic longitudinal waves in some particle-reinforced polymer composites is studied theoretically by a micromechanical model based on a differential (incremental) scheme. A set of differential equations is established by which the attenuation spectrum of the composite can be computed from the known properties of viscoelastic matrix and elastic particles. For a composite reinforced with glass particles with radius 0.15 mm, the proposed scheme is shown to predict the attenuation in better agreement with the foregoing experimental results than the previous simplistic independent scattering model. Based on this scheme, the dependence of the longitudinal attenuation spectrum of a particulate polymer composite on the wavelength-to-particle radius ratio and the particle volume fraction is examined in detail. It is then shown theoretically that the attenuation of the composite decreases monotonically with the particle volume fraction when the particle radius is sufficiently small compared to the incident wavelength, while it shows non-monotonic particle-fraction dependence when the ratio of the particle radius to the wavelength is larger. To examine this theoretical finding from an experimental point of view, the longitudinal attenuation in a glass-particle-reinforced polyester composite with particle radius 0.0225 mm is measured for different particle volume fractions. The measured attenuation characteristics are shown to support the qualitative features of the theoretical prediction.     

Single-Parameter Quantum Pumping in Graphene Nanoribbons with Staggered Sublattice Potential

We present a theoretical study of quantum charge pumping in metallic armchair graphene nanoribbons using the Floquet-Green function method. A central part of the ribbon acting as the scattering region is supposed to have staggered sublattice potential to open a finite band gap. A single ac gate is asymmetrically applied to a part of the scattering region to drive the pumping. Corresponding to the gap edges, there are two pumped current peaks with opposite current directions, which can be reversed by changing the position of the ac gate relative to the scattering region. The effects of the parameters, such as the staggered sublattice potential, the driving frequency and the geometric parameters of the structure, on the pumping are discussed.     

Effects of Pumping Sizes on THz Radiation Based on Ultrashort Light Pulse Optical Rectification for High Spatial Resolution T-Ray Imaging

We present our experimental studies on the effects of the pumping sizes on THz radiation based on ultrashort light pulse optical rectification for high spatial resolution T-Ray imaging. Our experiments show that high spatial resolution T-ray imaging requires both thin THz emitter and sample, and rigorous tolerance of the gap between the sample and the emitter, as well as small pumping size which usually much smaller compared with THz wavelength. Such a small pumping size results in dramatic decrease of the THz wave power, which originates from strong diffraction of THz wave, the depolarization of the focused tightly pumping beam, the spatial filtering of the emitter exit-surface, and the strong phase-mismatching between the pumping and the high spatial Fourier components of the THz signal, rather than two-photon absorption.     

Analysis on thermal characteristics of annular beam pumped vertical-external-cavity surface-emitting lasers by finite element method

The annular beam as a pump light of VECSELs is presented for the first time in this study. The thermal characteristics of VECSELs were calculated and simulated by finite element method, and the influence of the annular beam's annulus radius, the annulus width, and the pumping power to the VECSELs’ thermal characteristics was analyzed in detail. The results indicated that the annular beam can improve the VECSELs’ thermal characteristics efficiently, and provide a favorable term to generate laser in the central part of the annular. The theoretical results provided theoretical reference and experimental study for the design of high-power optically pumped VECSELs.     

Brownian pump induced by the phase difference between the potential and the entropic barrier

Transport of Brownian particles in a finite channel is investigated in the presence of asymmetric potential and an unbiased external force. It is found that the phase differencebetween the potential (energetic barriers) and the entropic barrier can break the symmetryof the system and control the transport of Brownian particles. Especially, the particlescan be pumped through the channel from a reservoir at low concentration to one at the sameor higher concentration. There exist optimized values of the parameters (the temperatureand the amplitude of the external force) at which the pumping capacity takes its maximumvalue. The pumping capacity decreases with increasing the radius at the bottleneck of thechannel.     

Pumping Power Minimization in Staggered Finned Circular and Elliptic-Tube Heat Exchangers in Turbulent Flow

An experimental study aimed at minimizing pumping power required to supply air through a finned tube bundle configuration is presented in this article. Results were obtained for the Reynolds number based on the smaller ellipse axis (RE_2b) ranging from 2,650 to 10,600, i.e., in turbulent flow. In the turbulent regime, pressure drops are expected to vary with eccentricity. Experimental optimization results for finned circular and elliptic tubes show that pumping power can be minimized with respect to tube spacing and eccentricity. In comparison with values obtained for circular tubes, the optimal elliptical arrangements show pumping power reductions from 5% to 10%.     

混合物状态方程的计算

多介质流体动力学过程的数值模拟往往涉及混合物状态方程的计算. 做图法和Newton 法是混合物状态方程计算常采用的方法, 前者虽直观精度却差, 后者计算效率高却只具有局部收敛性, 当解与其初始猜测值相差较远时Newton法不一定能够获得收敛解. 为此, 本文给出一种具有大范围收敛性的嵌入算法(imbedding method)求解混合物状态方程, 其基本思想是通过引入嵌入参数, 将待解的混合物状态方程和易解的混合物状态方程线性组合, 构成嵌入方程组, 当嵌入参数从0连续地变化到1 时, 嵌入方程组的解由易解的混合物状态方程的解连续地变化为待解的混合物状态方程的解. 嵌入方程组可由Newton法迭代求解, 也可转化为以嵌入参数为自变量的常微分方程组, 从而易于由成熟的计算方法如梯形法等进行求解. 进一步利用热力学基本关系, Maxwell形式的微分方程描述了压力和温度随嵌入参数的演化速率与应变速率和组分质量分数演化速率的关系. 对铅锡混合物热力学量的计算表明了本文算法的有效性.     

纤维膜在膜蒸馏过程中的膜内毛细冷凝现象

膜蒸馏过程中的膜内冷凝现象是该技术在实际应用中不可忽视的一个重要问题。本文以疏水化改性处理的纤维膜作为研究对象,采用实验现象观察和理论分析并用的方法,研究了纤维膜在膜蒸馏过程发生膜内毛细管冷凝的影响因素包括膜蒸馏模块结构、蒸馏膜参数、热力学参数,探讨了毛细冷凝对膜蒸馏的影响。实验表明毛细冷凝可以在膜蒸馏过程中发生,但并不终止膜蒸馏过程,而是减小了传质系数。毛细冷凝容易发生在蒸馏膜厚度小、热料液温度高、冷却液温度低或气隙小的情况下。实验结果和理论分析表明,传质系数实验值的降低可作为膜孔内发生毛细冷凝的判定依据。通过简化蒸气在疏水多孔介质的传递过程,建立了膜蒸馏过程发生毛细冷凝时的传质平衡模型。     

Tape Analysis of an Inhomogeneously-Loaded Helical Slow-Wave Structure for Broad-Band Traveling-Wave Tubes

Theoretical simulation of the dispersion and interaction impedance characteristics of an inhomogeneously-loaded helical slow-wave structure is validated. The structure is supported by double-curve-shaped rods which are smoothed out into a number of dielectric tubes with their respective effective permittivity values. The effects of the helix thickness are taken into account by considering a free-space gap equal to the difference between the mean helix radius and the outer helix radius. Moreover, the helix tape model is used instead of the simpler sheath-helix model. The theoretical predictions are compared with those of MAFIA simulation. The dispersion error is found to be within 3–6 percent and the impedance characteristic is in great agreement with that of MAFIA simulation. At last, for the sake of comparison, the cold-test characteristics under sheath-helix model are also provided.     

Combustion of bimodal nano/micron-sized aluminum particle dust in air

The combustion of bimodal nano/micron-sized aluminum particles with air is studied both analytically and experimentally in a well-characterized laminar particle-laden flow. Experimentally, an apparatus capable of producing Bunsen-type premixed flames was constructed to investigate the flame characteristics of bimodal-particle/air mixtures. The flame speed is positively affected by increasing the mass fraction of nano particles in the fuel formulation despite the lower flame luminosity and thicker flame zone. Theoretically, the flames are assumed to consist of several different regimes for fuel-lean mixture, including the preheat, flame, and post flame zones. The flame speed and temperature distribution are derived by solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries. The analysis allows for the investigation of the effects of particle composition and equivalence ratio on the burning characteristics of aluminum-particle/air mixtures. Reasonable agreement between theoretical results and experimental data was obtained in terms of flame speed. The flame structure of a bimodal particle dust cloud may display either an overlapping or a separated configuration, depending on the combustion properties of aluminum particles at different scales. At low percentages of nano particles in the fuel formulation, the flame exhibits a separated spatial structure with a wider flame regime. At higher nano-particle loadings, overlapping flame configurations are observed.     

Visualization of behaviors of a propagating flame quenching for hydrogen–air gas mixture

Abstract  

In this research, a flame arrester consisting of a slit structure was experimentally investigated. Experimental data show adequate maximum experimental safe gap (MESG) value for the flame arrester. The flow rate characteristics of the flame arrester were measured and compared with theoretical results. It was made clear that the flow impedance of convergent flow is 20% less than that of divergent flow. The experimental data and theoretical data show good agreement. The performance test by an EN12874 as ‘in-line stable detonation’ flame arrester was examined for a hydrogen–air gas mixture. The experimental data show that the gap was 0.2 times the MESG value in bi-directions for the flame arrester. The quenching and extinguishing processes were visualized by high-speed cameras.     

二维空心散射体声子晶体板的低频带隙特性及其形成机理

利用有限元方法,对设计的涂有硅橡胶包裹层的空心铅柱体嵌入到4个环氧树脂短连接板中构成的声子晶体板的低频带隙特性进行了研究,分析了其能带结构、传输损失及位移场。与正方连接板粘连结构、嵌入结构和细连接短板粘连结构这3种传统声子晶体板的带隙特性作对比,说明具有包裹层、短连接板结构的声子晶体板更容易产生低频宽带;观察位移矢量场的振动模态,并结合弹簧质量模型,解释了带隙形成的机理;通过讨论连接板的宽度、散射体的内外半径及高度对第一完全带隙的影响,说明连接板宽度越窄,厚度越小,散射体内半径越小,外半径越大,高度越高,越有利于带隙的扩展。      

Cellular instabilities of expanding hydrogen/propane spherical flames at elevated pressures: theory and experiment

An experimental and theoretical investigation of the onset of cellular instabilities on spherically expanding flames in mixtures of hydrogen and propane in air at elevated pressures was conducted. Critical conditions for the onset of instability were measured and mapped out over a range of pressures and mixture compositions. An asymptotic theory of hydrodynamic and diffusional-thermal cell development on flames in mixtures comprised of two scarce fuels burning in air was also formulated. Predicted values of Peclet number, defined as the flame radius at the onset of instability normalized by the flame thickness, were shown to compare favorably with the experimentally measured values.