传导薄板的非线性磁弹性振动问题

研究了磁场环境中传导薄板的非线性磁弹性振动问题。由虚功原理,给出了磁场中薄板的磁弹性耦合运动方程。并根据薄板薄壳的磁弹性基本假设及麦克斯威尔方程,得到了方程中电磁力及力矩的表达式。作为具体问题,采用多尺度法求出了横向磁场中条形板非线性振动的近似解析解。通过算例,分析了磁场环境对振动周期和幅值的影响。从而证实了,通过改变磁场因素,可达到控制该磁场环境中传导薄板振动的目的。所得的结论,对工程实际将有较大的应用参考价值。     

Influence of the electric current waveform on the dynamic response of the electrified composites

The influence of the electric current waveform (DC, AC and pulsed currents) on the dynamic electromagnetic, thermal, and impact response of the composite plate is studied. The analysis includes solving Maxwell’s equations in the electrified composite plate to determine an electric-current induced magnetic field and heat transfer equation to estimate the electric-current-induced heating. In addition, the dynamic mechanical response of the electrified composite plate subjected to impact and various electromagnetic loads (DC, AC, pulsed electric currents and a constant magnetic field) is analyzed by solving a coupled system of equations of motion and Maxwell’s equations in the composite plate. The results show that the dynamic response of the plate is highly dependent on the characteristics of the electromagnetic field, and the pulsed electromagnetic fields are most effective in reducing vibrations caused by the application of dynamic mechanical loads.     

Scalar potentials in multiply connected regions

An algorithm, for use on a digital computer, is explained for cutting multiply connected regions in three-dimensional space. The algorithm is suitable for complex electromagnetic field problems where a magnetic scalar potential, affected by current loops, is to be calculated in a multiply connected region. Problems in fluid dynamics with vorticity affecting a scalar potential, could also be attacked with this algorithm. Since finite element methods are ubiquitous in field calculations, the cuts of the space constructed by the algorithm should ideally correspond to the given discretization; this can be arranged by an application of the Lebesgue Covering Theorem, for which a second algorithm is provided.     

P型半导体金刚石膜的磁阻

研究了p-型异质外延和同质金刚石膜的在不同温度和磁场下的磁阻,磁阻器件的结构为条形和圆盘形,实验结果表明磁阻强烈依赖于磁场、温度和样品的几何形状,圆盘结构的磁阻大于条形结构,条形结构的磁阻还取决于不同的长-宽比。利用F-S薄膜理论,计算磁场为5T时条形和圆盘结构的磁阻分别为0.38和0.74,讨论了霍耳效应对磁阻的影响。给出了形状效应的可能机制。     

Electromagnetic field formulation for eddy current calculations in nondestructive testing systems

The authors present the most practical configuration for detecting cracks in material, by applying an electromagnetic field along the largest dimension of the crack. An electromagnetic field formulation of the system equation is proposed using Maxwell's relations and separating the magnetic field into externally applied field and reaction field. The system equation is solved by using two classical methods : finite element technic for spatial problem and finite difference for time discretisation. So two dimensional eddy currents can be calculated immediately and related to the excitation characteristics for impedance calculation. Eddy currents lines in material for different cracks and sensor positions are presented. Theoretical results show important impedance changes.     

A generalized variational model of magneto-thermo- elasticity for nonlinearly magnetized ferroelastic bodies

Based on a generalized variational principle of total energy functional, this paper presents a theoretical model to describe the magneto-thermo-elastic interaction of soft ferroelastic bodies with nonlinear magnetization under stationary thermal and magnetic fields. The energy functional of the magneto-thermo-elastic system is established by the summation of energy of sub-systems of nonlinearly magnetized magnetic field, thermal field, and mechanical deformation. By means of the manipulation of the mixed variational principle with independent variations of magnetic scalar potential, displacement vector, and temperature, all governing equations, which are nonlinear and coupling among magnetic, elastic and thermal fields, together with the expressions of magnetic forces are obtained from the variational approach. In order to valuate the obtained model, some existing models of the magneto-elasticity and the thermo-elasticity, which are validly demonstrated in literature, as special cases of the problem considered here are deduced out from the general case. Finally, an analytical analysis of magneto-thermo-elastic instability is conducted to a simply supported ferromagnetic rectangular thin plate under both a uniform distribution of temperature and a uniform transverse magnetic field by means of the linearized theory and the perturbation technique.     

Eddy current losses at cryogenic temperatures

The effect of thermal processes on eddy-current losses in the structural elements of cryogenic and superconducting devices are analyzed. Maxwell's equations coupled with the heat-conduction equation are solved, taking into account the dependence of resistivity, heat capacity, and heat-transfer coefficient on temperature. Losses are analyzed as a function of magnetic field, frequency, and geometry for a thin strip in a uniform magnetic field. It is shown that losses calculated taking the thermal processes into account may differ from those obtained at constant temperature     

铝薄板热顶—电磁铸造成型工艺研究

依据铝薄板的特点，提出了热顶－电磁铸造的基本思想，设计了一套完整的制取铝薄板的热顶－电磁成型系统，测量了电磁场分布，研究了铝薄板热顶－电磁铸造成型条件，经过大量工艺试验确定了液固界面位置，浇注温度，液柱高度和冷却水量等参数，获得了较佳的铸速０－时间曲线，成功地制取了厚２０ｍｍ，高８５０ｍｍ，宽４８０ｍｍ的铝薄板。     

Current‐Induced Nucleation and Annihilation of Magnetic Skyrmions at Room Temperature in a Chiral Magnet

A magnetic skyrmion is a nanometer‐scale magnetic vortex carrying an integer topological charge. Skyrmions show a promise for potential application in low‐power‐consumption and high‐density memory devices. To promote their use in applications, it is attempted to control the existence of skyrmions using low electric currents at room temperature (RT). This study presents real‐space observations for the current‐induced formation and annihilation of a skyrmion lattice (SkL) as well as isolated skyrmions in a microdevice composed of a thin chiral magnet Co₈Zn₉Mn₃ with a Curie temperature, T _C ≈ 325 K, above RT. It is found that the critical current for the manipulation of Bloch‐type skyrmions is on the order of 10⁸ A m^?2, approximately three orders of magnitude lower than that needed for the creation and drive of ferromagnetic (FM) domain walls in thin FM films. The in situ real‐space imaging also demonstrates the dynamical topological transition from a helical or conical structure to a SkL induced by the flow of DC current, thus paving the way for the electrical control of magnetic skyrmions.     

The Spin Wave Gap and Switching Field in Thin Films with In-Plane Anisotropy

In this paper, we have calculated the spin wave gap and the angular dependence of magnetization reversal in a single-layer thin magnetic film that includes the strong perpendicular magnetic anisotropy and in-plane anisotropy. The film is assumed to be under the influence of the out-of-plane direction of the applied magnetic field at zero temperature. Using the quantum model, it is shown that the calculated equations present a nonzero spin wave gap at zero magnetic field which is strongly affected by anisotropies. The effects of the in-plane anisotropy and the role of the applied field were examined. We also discussed a simple theoretical model for the angular variation of switching field by using a quasi-classical argument. We used some constants in connection with experimental data which are reported for chromium telluride thin films grown by molecular beam epitaxy.     

Alternative approaches to the numerical calculation of impedance

In many practical applications of numerical analysis applied to low-frequency electromagnetic problems, the desired output is often in terms of coil impedance. The mesh variable calculated by the more commonly used numerical formulations is the magnetic vector potential from which quantities like field intensities, eddy currents and others are calculated. In nondestructive testing applications, the quantity of interest is often the impedance of a coil or an array of coils. This paper discusses the calculation of impedance as a post-processing computation and introduces a new method of calculation of impedances and inductances based on computation of energy in the finite element mesh. The results presented clearly show the advantage of using direct integration methods for 2D and axisymmetric geometries. The energy approach, while valid regardless of dimensionality, should be restricted to 3D applications. Multiple-coil configurations in 3D applications present a special problem in analysis. The total impedance or inductance can be easily calculated but not independent coil or differential impedances. A method for calculation of these quantities in 3D computations is also presented.     

磁流变液阻尼器响应时间的试验研究及其动态磁场有限元分析

摘 要：首先,试验测试了不同速度和电流变化下,大吨位磁流变液阻尼器的响应时间;然后,对激励电流变化时阻尼器的磁场变化进行了有限元模拟,基于阻尼器间隙内磁流变液剪切屈服强度的变化考察了阻尼器的响应时间,并与试验数据做了比较。最后,研究了涡流和阻尼器电磁回路中电流响应时间对阻尼力响应时间的影响。结果表明,可以用有限元模拟得到的间隙内磁流变液的平均有效剪切屈服强度的时程曲线来研究磁流变液阻尼器的响应时间;电磁响应时间是阻尼力响应时间的决定因素,减小阻尼器中的涡流是缩短磁流变液阻尼器响应时间的重要途径;电流下降时涡流对阻尼器磁路的影响要大于电流上升的情况;无论是上升还是下降,电流初值越小,涡流对阻尼器磁路的影响越大,阻尼力响应时间也越长。研究还表明,缩短电流的响应时间,会带来更大的涡流,并不一定能缩短阻尼力的响应时间。     

Reduction of the classical electromagnetism to a two-dimensional curved surface

The reduction of the three-dimensional classical electromagnetism to a two-dimensional curved surface is performed in a twofold way. In the first case, the ordinary two-dimensional electromagnetism is obtained with sources in the form of conserved electric currents flowing along the surface. The electric field is a two-vector tangent to the surface and magnetic field is a scalar quantity. In the second approach, the reduced theory is that of the two-vector magnetic field and a scalar electric one. The only source coupled to the fields is now a scalar, subject to no conservation law. In the redefined theory, this scalar source may be converted into an eddy magnetic current flowing in the surface. No magnetic monopoles appear. Our results can find some applications in the electrodynamics of thin layers and of metal-dielectric interfaces.     

TE and TM beam decomposition of time-harmonic electromagnetic waves

The present contribution is concerned with applying beam-type expansion to planar aperture time-harmonic electromagnetic field distribution in which the propagating elements, the electromagnetic beam-type wave objects, are decomposed into transverse electric (TE) and transverse magnetic (TM) field constituents. This procedure is essential for applying Maxwell's boundary conditions for solving different scattering problems. The propagating field is described as a discrete superposition of tilted and shifted TE and TM electromagnetic beams over the frame-based spatial-directional expansion lattice. These vector wave objects are evaluated either by applying differential operators to scalar beam propagators, or by using plane-wave spectral representations. Explicit asymptotic expressions for scalar, as well as for electromagnetic, Gaussian beam propagators are presented as well.     

Pulse distortion due to eddy currents induced in a multiply excitedcylindrical shell of finite length and thickness

The electromagnetic field due to pulse excitation of two crossed pairs of coils surrounded by a cylindrical conducting shell is determined by accounting for the eddy-current effect. Each coil is modeled as a pair of straight, parallel wires of infinite length; the surrounding conducting shell is a cylindrical tube of finite length and thickness. The problem is formulated in the frequency domain, and the resulting expressions are used to calculate the transient magnetic flux density which originates from the quadruple coil and the eddy currents induced within the metallic shell. The part due to the eddy currents represents an unwanted distortion, which is investigated through a numerical example. The analytical and numerical results are complemented by a brief discussion on compensation techniques and potential applications in magnetic resonance imaging     

A perturbational approach to magneto-thermal problems of a deformed sphere levitated in a magnetic field

This paper presents a perturbation method for the solution of the electromagnetic and thermal problems of a deformed sphere levitated in an alternating magnetic field. The analytical solutions of the electromagnetic field distribution, the Joule heat generation, the magnetic lifting force and the temperature field are obtained based on a linear perturbation theory. The Maxwell equations are first simplified in terms of the vector potential and then solved by the method of separation of variables. The time-averaged Joule-heat source is calculated and coupled to the Fourier heat-conduction equation. The coupled equation is solved for temperature distributions within the deformed sphere by a combined approach of series expansion and variation of parameters. Both asymptotic and numerical analyses are provided. The total power absorption and temperature field for both single and multiple coils are also discussed.     

Computation of 3-D magnetostatic fields using a reduced scalarpotential

Some improvements to the finite element computation of static magnetic fields in three dimensions using a reduced magnetic scalar potential are presented. Methods are described for obtaining an edge element representation of the rotational part of the magnetic field from a given source current distribution. When the current distribution is not known in advance, a boundary value problem is set up in terms of a current vector potential. An edge element representation of the solution can be directly used in the subsequent magnetostatic calculation. The magnetic field in a DC arc furnace is calculated by first determining the current distribution in terms of a current vector potential. A 3-D problem involving a permanent magnet as well as a coil is solved, and the magnetic field in some points is compared with measurement results     

Analysis of a novel laminated coil using eddy currents for AC highmagnetic field

The analysis and performance of an eddy current type laminated coil for a high AC magnetic fields are described. It is usually difficult to obtain a high AC magnetic field using an air-gap coil because of eddy currents. The present coil circumvents this limitation by making use of the magnetic shielding effect of eddy currents. Two different realizations of the coil are proposed to provide design flexibility. The field distributions are analyzed by the two-dimensional finite-element method. The coils can also be applied to an induction electromagnetic pump     

An integral equation method for the solution of 3-dimensional,non-linear,magnetostatic problems

A finite element method for computing the resultant magnetic field arising from a given source field in the presence of a magnetic material of variable permeability is described; in this method finite element approximations to the scalar potential of the resulting field and the magnetic susceptibility, in the region occupied by the magnetic material, are determined from the non-linear integral equation for the scalar potential and the constitutive susceptibility relation, using a collocation scheme. The method is used to compute the shielding effect of a thin rectangular plate of variable permeability on a given source field. The plate is subdivided uniformly into brick elements; the resulting translational invariance of the integrals required in the calculations is exploited to achieve major computational savings. A consequence of the thinness of the plate is that the calculation of the requisite integrals by analytic methods leads to considerable loss of accuracy by differencing; this difficulty is overcome by using a scheme which combines both analytic and quadrature techniques. The resulting system of non-linear algebraic equations is solved by Powell's hybrid method; an efficient scheme for calculating an initial approximation to the Jacobian, which utilizes the structure of the equations, is presented. The results of the calculations are discussed.     

Spatial Harmonic Expansion for Use With Magnetic Sensor Arrays

In order to reduce the effects of external magnetic fields on the accuracy of magnetic sensor measurements used for the reconstruction of ac electric currents flowing in massive parallel conductors, we use a spatial circular harmonic expansion of the magnetic scalar potential. Thanks to the linearity of the magnetic field problem with respect to the sources, we can then apply the least squares inversion and obtain the set of currents from the knowledge of the magnetic field data collected by the sensor array in the vicinity of the current carrying conductors. Furthermore, we can optimize the positions and the orientations of the magnetic sensors using D-optimality theory and particle swarm optimization.