金属柱平板慢波系统高频特性研究

对于离散的金属柱结构构建的周期性平板慢波系统,本文利用3维FDTD方法结合HFSS仿真软件深入分析了该慢波系统的高频特性.研究了金属柱高度、周期长度对色散特性的影响,计算了耦合阻抗,并与传统光栅慢波系统特性进行了对比分析.分析表明金属柱慢波系统既有与传统光栅慢波系统相似的高频特性,又具有独自的特点,位于离散的金属柱周期间隙中的电子注互作用耦合阻抗具有对称性;金属柱结构用作真空电子器件的高频系统可增加发生有效互作用的电子注厚度,降低起振电流密度,提高器件效率.本文的分析结果为设计低电流密度工作的多电子注短毫 关键词： 金属柱平板慢波系统 3维FDTD算法 色散特性 耦合阻抗     

Studies on nano suspensions of silver sol and redispersed nanocrystallite silver in aqueous and alcoholic media

Nanocrystallite silver and silver sol were prepared and characterized by UV-visible spectra, XRD, TEM and HRTEM. The crystallite silver is re-dispersed in two different media, namely, water and alcohol and sonicated before ultrasonic investigation. The silver sol was used as such. Three different models for the propagation of ultrasound through two phase media are compared in these three different types of nano suspensions. Effect of particle size and medium on ultrasonic velocity (U), compressibility (κ) impedance (Z) and viscous relaxation time (τ) is studied. The particle concentration range was 0.2-1 v/v. Density and viscosity of the dispersion and sol are measured at different particle volume fractions. Effective density and ultrasonic velocity are computed by Urick, Kuster and Toksöz and Urick and Ament models and compared with experimental velocities. Values of effective density obtained by using Urick and Urick and Ament equations closely agree with experimental results of density while Urick's equation prediction of velocity is in close agreement with the experimental velocities. This comes as a surprise in view of the large density difference between the medium and suspended particle and suggests the possibility of the balancing effect of the inertial and viscous forces operating in the suspension.     

Quantitative nanoscopic impedance measurements on silver-ion conducting glasses using atomic force microscopy combined with impedance spectroscopy

Nanoscopic impedance measurements were carried out on silver ion conducting glasses by coupling an impedance spectrometer with an atomic force microscope. When ac voltages were applied to a conducting AFM tip being in contact with the glass surface, silver nanoparticles were formed during the cathodic half cycle, which were not completely reoxidized in the anodic half cycle. We describe two protocols allowing for a controlled particle growth. The electrochemical oxidation/reduction processes led to low tip/sample interfacial impedances, and the formed silver particles acted as nanoelectrodes sensing the spreading resistance of the glass below the particles. We made a quantitative check of the spreading resistance formula under the assumption that spreading of the electric field is governed by the lateral diameter of the particles and found good agreement between the mean value of the local conductivities obtained at different tip positions and the macroscopic conductivity.     

Mathematical and physical modelling of the dynamic fluidic impedance of arteries using electrical impedance equivalents

In this work, a detailed mathematical and physical model for calculating the fluidic impedance of an artery is developed. Initially, the model calculates the impedance of a healthy artery, and then, it compares this with the alteration in the impedance of an artery with atherosclerosis. Copyright © 2013 John Wiley & Sons, Ltd.     

Impedance of faradeic process with partial charge transfer

The electrochemical impedance of charge-transfer reaction involving the stages of adsorption and desorption of intermediate partially charged substance is analyzed theoretically in the frequency-potential plane in order to study the possibility for measuring the partial charge transfer. The simplest case of adsorption two-electron transfer, namely, the Langmuir adsorption with liner dependence of reaction rate on the coverage, is considered. It is assumed that the partial charge transfer is independent of the electrode potential.     

Distinguishability of equivalent circuits containing CPEs: Part I. Theoretical part

Constant phase elements (CPEs) with impedance Z_CPE=1/(Qs^α_f) are widely used to explain electrochemical impedance spectroscopy (EIS) data. It has long been known that different circuits can explain EIS data in an equivalent manner because they exhibit identical impedance for all frequencies (two-terminal non-distinguishable (TTND) circuit). Equivalent circuits containing two CPEs are considered here. TTND circuits containing two CPEs become distinguishable, at least theoretically, if the CPE exponents are different. The experimental distinguishability of these circuits is discussed. The 12 transformation formulae between the four circuits containing two CPEs and two resistors are given for CPEs with equal exponents.     

Asymptotics of optimal control in the problem of harmonic wave scattering by an obstacle

Optimal impedance control for the Helmholtz equation in an unbounded domain is studied. Asymptotics of the optimal control with respect to a regularization parameter are constructed.     

Characteristic Study of the Periodically Iris-Loaded Elliptical Waveguide for Slow-Wave Structures

The slow-wave characteristics taking no account of space harmonics for periodically iris-loaded elliptical waveguides is presented. By using the field-matching method, the dispersion equation and the mean interaction impedance for odd and even hybrid modes of this structure are derived respectively. It is indicated from the numerical calculation results that changing the eccentricity can improve the dispersion characteristics and the interaction impedance for _oHEM₀₁ mode.     

Rigorous Analysis of the Disk-Loaded Waveguide Slow-Wave Structures

Recently disk-loaded waveguide has been widely used in high-power traveling wave tubes (HPTWT). Although TM ₀₁ mode is it's main mode, the asymmetry mode may be excited by slight asymmetry of the structure, misalignment of the beam, or the asymmetry associated with the input or output structure. So research about asymmetry mode of disk-loaded waveguide is necessary. The general dispersion equation and interaction impedance expression of disk-loaded waveguide is obtained with accurate field theory for the first time. Based on these results, a broadband and a narrowband disk-loaded waveguides are designed which can be used in the Ka band HPTWT. Moreover, the theoretical calculated results are compared with the simulated results from the HFSS (High Frequency Structure Simulator) code using finite element method. It is found that frequency domain overlapped by these two modes of the narrowband structure is very narrow, so the HEM ₁₁ mode may be ignored when calculating Beam-wave interaction. In the other hand, the interaction impedance of HEM ₁₁ mode is very low generally.     

W-Mo系复合材料的制备及其特性波阻抗

 采用放电等离子烧结技术，在相同的温度（1 473 K）及压力（30 MPa）下，制备出不同配比的致密的W-Mo系复合材料样品。采用高精度超声波脉冲回波重合方法，精确测量了超声波在样品中传播的横、纵波声速，并由此得到样品的特性波阻抗值。对样品的相组成分析及电子探针分析的结果表明，W-Mo系复合材料主要是以W、Mo机械混合的形式通过粘结相获得致密化的。因此，选用混合物模型对其特性波阻抗值进行了理论预测，与实测值的比较表明该模型能对W-Mo系复合材料的特性波阻抗作出比较准确的预测。