Effect of molecular structure on spin-dependent electron transport in biferrocene-based molecular junctions: a first-principles study

The spin-dependent electronic transport properties in the molecular junctions based on ethynyl-connected biferrocene are investigated by density functional theory combined with nonequilibrium Green’s function. The calculated results show that the transport properties can be affected by the molecular structure of ethynyl-connected biferrocene. The linkage mode of the terminal ethynyl groups plays an important role on the spin-dependent currents, the spin polarization, and the total currents of the molecular junctions. The spin polarization value of the two molecules in this work is dependent on the applied bias. The difference of electron transport properties between the molecular junctions is related to the different electronic structures of the two isolated molecules.     

The degree of intelligence of the law of Biot-Savart

The author shows that the law of Biot-Savart is capable of giving the magnetic flux density due to DC currents in complete circuits or segments of circuits. The charge accumulations at the ends of such segments are automatically accounted for in this law. In such cases, the law of Biot-Savart is equivalent to the complete form of Maxwell's equation with the displacement current included. It is concluded that the law of Biot-Savart is more generation and contains more information than is generally assumed     

ICTS Measurements of Single Grain Boundaries in ZnO:rare-earth Varistor

Properties of single grain boundaries in ZnO:rare-earth varistors were examined by the isothermal capacitance transient spectroscopy (ICTS) method. Micro-electrodes prepared by photolithography were used for measuring the behavior of single junctions in ZnO varistors. From current-voltage measurements, it was found that the non-linear exponents of single junctions varied from 3 to 14. Interface state levels existed at 0.9 eV below the conduction band edge for every junction. On the other hand, the interface state density varied from junction to junction and the non-linearity was shown to increase with increasing interface state density.     

自然积污绝缘子表面污秽分布特性研究

绝缘子的污秽分布对绝缘子的污闪电压具有一定的影响,文章通过实验方法,重点研究分析了玻璃,瓷及复合三种类型绝缘子在自然积污条件下的污秽分布规律,所选择的这三种绝缘子为河北保定地区输电线路上已运行多年的悬垂绝缘子,利用盐密(Equivalent Salt Deposit Density,ESDD)与灰密(Non Soluble Deposit Density,NSDD)实验测量了绝缘子串中不同片的绝缘子及同一片绝缘子的不同表面的盐密、灰密,将数据以表、图的形式进行了处理分析,得到该地区绝缘子的污秽分布规律,最后以玻璃绝缘子及瓷绝缘子为研究对象,运用ANSYS对其在不同盐密分布下的电场及电流密度进行了仿真分析。     

Some Useful Analogies in the Teaching of Electromagnetic Theory

A chronology of teaching electromagnetic theory which pairs E and B, and defers introducing D and H until materials are considered, is advocated because of the richness of analogies available when this is done. The pairing of the "reduced" charge density distribution p/?0 with the "reduced" current density distribution J/?0-1 is shown to heighten recognition of the analogies. After the full basic theory is established, natural pairing is recommended, such as E and H for Poynting's theorem, E and B for the Lorentz force law, and D and B for electromagnetic momentum density.     

Comparison of Ampere's circuital law (ACL) and the law ofBiot-Savart (LBS)

Ampere's circuital law (ACL) and the law of Biot-Savart (LBS) are applied to the computation of the magnetic flux density at the center of the square loop. The results are compared and some common misunderstandings of students are clarified. It is seen that LBS is general and applies to closed or open circuits. In the case of open circuits, care must be exercised in applying the ACL. Here, two choices present themselves; one is to use a modified form of ACL appropriate to the problem at hand and the second is to use ACL in its generalized form, which takes into account the inevitable charge accumulations     

高压直流输电线下空间电荷与电场的关系研究

为了分析高压直流输电线下空间电荷与电场两个电磁环境指标之间的联系,采用Sarma法对单极直流线路下的空间电荷密度和标称场强分布进行数值计算。利用空间电荷密度测量设备,在高压直流输电线路下方进行实际测试,分析一些线路参数对空间电荷密度与标称场强分布规律的影响。结果表明:高压直流输电线路周围的空间电荷密度和标称电场具有相似的变化规律,提高导线高度能明显减小空间电荷密度和电场强度,导线极间距和分裂数的小范围变化对空间电荷密度与地面电场影响不大,大气质量是影响空间电荷密度和电场强度大小的重要因素。     

Experimental verification of the physics and structure of thebipolar junction transistor

The authors present an electrical characterization of discrete bipolar junction transistor (BJT) devices, with nonuniform doped emitter and base zones. The measurement of the I-V and C-V characteristics of the emitter-base and the collector-base junctions and the common emitter current gain allows to determine relevant parameters of the device. These are the built-in voltage of both junctions, the impurity gradient profiles, the electrical area of both junctions, the base and the emitter Gummel numbers and the collector doping. The whole experiment can be conducted in a laboratory session of 3-4 hour length and it is specifically addressed to students taking lectures in semiconductor device physics. The results obtained give a deep insight into both the physical structure and the physical processes involved in the transistor behavior     

Crystallographic Orientation Dependence of the Schottky Properties of Au/SrTiO3 Junctions

Crystallographic orientation dependence of the Schottky properties of Au/Nb-doped SrTiO₃ (STO:Nb) junctions has been investigated using single crystals of STO:Nb (1 0 0) and (1 1 1). It is found from electrical properties that the Schottky barrier height (SBH) of the Au/STO:Nb junctions estimated from current density (J)-voltage (V) characteristics shows crystallographic orientation dependence, while the flat band voltage estimated from capacitance (C)-voltage (V) characteristics is independent of the orientation. Displacement currents originated from the junction capacitance have been clearly observed at reverse bias voltage even in a condition of dV/dt 8.75 × 10^–3 [V/s] because of large electrostatic permittivity of the STO, and the displacement currents also showed crystallographic orientation dependence. The different response in the electrical properties of the Schottky junctions suggests that electric properties of intrinsic low permittivity layers, which exist at Au/STO:Nb interfaces, have the crystallographic orientation dependence.     

计及元胞发展程度的空间负荷预测方法

针对分类负荷在不同元胞内发展程度不同导致元胞负荷分布不均衡,从而影响空间负荷预测结果精度的问题,提出一种计及元胞发展程度的空间负荷预测方法。首先建立电力地理信息系统(GIS),在电力GIS中生成元胞,并整合基础信息,其中包括用地信息、10kV馈线的供电范围及分类负荷数据。其次求出总分类负荷密度的饱和值,再结合生长曲线揭示总分类负荷密度的发展规律。然后找到当前年各元胞内分类负荷密度在总分类负荷发展规律曲线上的位置,即为各元胞内分类负荷密度的发展程度。最后根据当前年元胞内各分类负荷密度的发展程度,结合总分类负荷密度发展规律曲线,确定目标年各元胞内分类负荷密度,再乘以元胞中每类负荷所对应的面积实现对元胞负荷值的预测。实例分析表明了该方法的正确性和有效性。     

Solution of static electric and magnetic problems using theintegral definitions of the curl and divergence operators

Numerical methods are described which use a basis consisting of the values of the electric field intensity along each edge of a cell and the values of the magnetic flux density normal to each face of a cell. Definitions of the curl and divergence operators in terms of line and surface integrals respectively, as well as Gauss' law and Ampere's law, are used to define mesh and nodal equations for determining the field values within the grid from values specified on the outer boundary. Static electric and magnetic problems are solved as examples     

Passive intermodulation analysis of single contact junctions of wire mesh

Because the nonlinearity of contact junctions in a wire mesh is the main source of passive intermodulation (PIM) generation, this work aims to analyze the PIM problem of single contact junctions. This work first derives the formulas of micro-contact effects, which reveals the mathematical relationship between the contact force and the micro-contact effects for a single contact junction of a wire mesh. Then, an equivalent circuit model describing contact nonlinearity is constructed to evaluate the nonlinear current through a single contact junction. Finally, an experimental platform is built to test the current through a single contact junction of two metal wires. The PIM power levels of a single contact junction are determined by the finite difference time domain method. The comparison of PIM power levels between the theoretical and experimental results is illustrated to verify the validity of the proposed method. The research results indicate PIM is influenced greatly by the contact forces of junctions. The appropriate contact forces on junctions of wire mesh are beneficial to reduce PIM distortion.     

考虑热进口段时管内对流换热过程的热力学分析

利用热力学第二定律，分别在壁面热流恒定和壁面温度恒定两种情况下，在考虑热进口段时对管内对流换热过程进行了热力学分析。引入了熵产强度和无因次熵产强度两个概念，并给出了有关计算式；讨论了壁面热流、壁面温度及流动阻力对无因次熵产强度的影响。本文的有关结果可为工程上换热器的优化设计提供理论依据     

Circularly polarized spin current assisted fast resonant switching in magnetic tunnel junctions with perpendicular anisotropy

We propose a mechanism to substantially enhance spin transfer torque induced switching in perpendicular anisotropy magnetic tunnel junctions. Our method is based on injecting an additional assisting DC current with circular spin polarization into a magnetic free layer at frequencies that are resonant with its normal ferromagnetic resonance frequencies. We observe 80 % reduction in switching delays at constant switching currents and 2× improvement in critical switching current density. Spin current polarization chirality and spin polarization efficiency dependencies are investigated. Further, a device structure to experimentally realize the mechanism proposed in this letter is presented.     

直流接地极及杆塔接地网附近电磁场的有限元分析

高压直流输电系统处于单极大地回线运行方式时,有很大的直流电流通过直流接地极流入大地,这将造成接地极本身及附近输电杆塔接地网的腐蚀。在理论分析接地极和杆塔接地网电磁场的基础上,应用有限元分析软件COMSOL Multiphysics,以德宝直流输电工程千阳接地极为例,建立了多层大地土壤结构下的直流接地极和杆塔接地网数值模型,添加相关边界条件,进行网格划分处理,计算分析了接地极地表电位分布规律,并对杆塔接地网附近电位及泄漏电流密度进行了研究,结果发现:接地极地表电位沿径向距离逐渐降低;杆塔接地网本体上的电位最高,接地网的射线末端泄漏电流密度最大,射线首端的泄漏电流密度最小,接地网矩形与射线的连接处电流密度有突变。该研究对掌握直流接地极及杆塔接地网周围电场分布情况和腐蚀规律,具有重要的意义。     

磁极分段式高速轴向磁通永磁电机转子强度研究

为了满足机械强度要求,高速永磁电机通常采用径向磁通结构。随着非晶合金等新型超薄软磁材料的发展,高速高频轴向磁通永磁电机逐步引起关注。为此,针对一种适合于高速运行的磁极分段式轴向磁通永磁电机转子结构进行研究。建立了该转子结构强度解析计算模型,分别利用解析法和有限元法计算了不同极弧因数、转子轮缘宽度以及转子磁极分段数对转子机械强度的影响规律。同时研究了磁极分段式结构对轴向磁通永磁电机气隙磁密、空载反电动势、齿槽转矩和转矩密度等电磁性能的影响。结果证明采用磁极分段式结构能有效提高转子强度,相关研究工作为高速轴向磁通永磁电机的设计提供参考。     

基于蒙特卡罗法的测量不确定度评定

当采用不确定度传递律进行测量不确定度评定（GUM方法）有困难或不方便时,蒙特卡罗法是实用的替代方法。在建立数学模型、设定输入量概率密度函数的基础上,通过对概率分布随机采样进行分布传递,确定输出量的概率密度函数,从而得到输出量的估计值、标准不确定度以及在指定包含概率下的包含区间,实现对测量不确定度的评定。通过小功率座校准因子测量不确定度评定示例,说明了采用蒙特卡罗法进行测量不确定度评定的实现方法,并与GUM方法进行了比较。     

Effect of Silica Doping on the Electrical Conductivity of 3 mol % Yttria-Stabilized Tetragonal Zirconia Prepared by Colloidal Processing

Silica-doped (SiO₂ = 0 - 1.0 wt%) 3Y-TZP (3 mol % yttria-doped tetragonal zirconia polycrystal) ceramics are prepared from hetero-coagulated aqueous suspension by colloidal processing. Consolidation of the suspension was carried out by pressure filtration at 10 MPa followed by cold isostatic pressing (CIP) at 400 MPa. Consolidated compacts are densified to a relative density over 99% by sintering at 1573 K for 2 h. The formation of glass pockets at grain boundary multiple junctions was observed by SEM for ≥0.5 wt % silica-doped samples. Electrical conductivity measurements were performed to evaluate the modification of grain-boundaries by silica. The apparent grain boundary conductivity decreased with an increase in silica content and became nearly constant above 0.3 wt % of silica, while the bulk conductivity was constant with silica content.     

POS工作参量定标关系的数值模拟

利用通用的２－１／２维铨电磁网格粒子模拟程序对等离子体断路开关工作电压与等离子体密度的关系作了数值模拟研究。提出了临界工作电压的概念,并获得了等离子体密度在１０＾１１至１０＾１３ｃｍ＾－３范围内变化、负载为强流电子束二级管时临界工作电压与等离子体密度的定标关系。该定标关系在较低和较高等离子体密度区解析的融蚀模型的预期有显著差别,而在１０＾１２ｃｎｍ＾－３附近区域则基本一致。     

滤波电容器心子中介质薄膜表面残余电荷对其振动影响的分析

高压直流输电换流站滤波电容器噪声问题日益凸显,亟须深入探索其内部振动规律,为提出更有效的减振降噪措施奠定基础。本文为了研究电容器心子内部介质薄膜表面残余电荷密度的大小,及其对电容器心子振动带来的影响,以静电探头和电荷反演为手段实测了电容器心子内部介质薄膜表面残余电荷量,计算分析了电荷随机分布下极板受力的概率统计规律。然后通过电场-机械力-振动耦合仿真了电容器心子表面振动加速度,并与实验结果对比验证了仿真结果的有效性。研究发现：电容器心子内部介质薄膜表面残余电荷密度主要分布在?10^?4~10^?4C/m²,当施加单频激励时,电容器心子受力以及振动频谱不仅包含了传统分析中认为存在的二倍频成分,也存在由残余电荷引起的与外施激励同频的分量,该分量加剧了电容器内部极板受力的不平衡。