Effective resistance of current-carrying superconducting wire in oscillating magnetic fields 1: Single core composite conductor

The influence of oscillating magnetic fields on the dc transport current in a single core superconducting wire has been studied by observing the terminal voltage of the wire. The voltage versus oscillating field characteristics can be explained quantitatively by a theoretical analysis based on the critical state model. The analysis shows that the voltage measurement gives detailed information on the change of the distribution of the magnetic induction inside the superconductor. An effective resistance appears in the transport current loop when the amplitude of the oscillating field exceeds a certain threshold value. The dependence of the effective resistance on the amplitude and frequency of the oscillating field and on the conductor diameter is discussed.     

Micromagnetic Study of Forced Oscillation of Magnetic Domain Wall in Ferromagnetic Nanowires with Variation of Damping Constant

We have investigated a DW motion driven by AC external field with a variation of damping parameter ?? in Landau?CLifshitz?CGilbert equation as well as a variation of an oscillating force frequency by means of a micromagnetic simulation. Straight ferromagnetic wires having a DW at the wire center is considered, where the DW is assumed to behave like a free quasiparticle. With variation of strength and frequency of the external AC field, we have observed that relative phase delays between the forced DW motion and the AC field depends on the damping constant. The overall behavior is explainable with a single-degree-of-freedom model of a damped forced oscillation without a restoring force.     

Internal friction of foamed aluminium in the range of acoustic frequencies

The internal friction of aluminium foams with various porosities was measured in the range of acoustic frequencies over a wide strain-amplitude range by the bending-vibration method. The measured internal friction shows that aluminium foams have a damping capacity which is enhanced in comparison with bulk aluminium, increases with increasing porosity, decreases with increasing frequency and increases with increasing strain amplitude. In order to explain the behaviour of the internal friction, a mechanism of internal dissipation energy was presented, and an approximate expression for internal friction is derived which is based on the equations of plane waves in elastic material with voids. This expression can account for the dependence of the internal friction on porosity, pore size and frequency. To gain further insight into the dependence of the internal friction on amplitude, the non-linear characteristics of oscillations were observed, and it was found that the resonance curves are asymmetric and the resonant frequencies are proportional to the square of amplitude with a negative slope. On the basis of the equations of the motion and the experimental results, the non-linearity of oscillations was ascribed to a non-linear damping term and an approximate expression for the damping coefficient with respect to amplitude was obtained. © 1998 Chapman & Hall     

Current-induced domain-wall motion in U-shaped permalloy wire

The current-induced domain-wall motion has been observed by using a U-shaped permalloy wire. We observed two magnetic states in a U-shaped pattern. One is the vortex domain wall at the center of semicircular arc region of the U-shaped pattern, and the other is the continuous magnetic state without the domain wall in between. In general, the current density of the order of /spl sim/10/sup 7/ A/cm/sup 2/ is needed to drive a magnetic domain wall. In this paper, the critical current for domain-wall motion increases as the bias field increases. The bias field means the field deviated from the switching field of the wire.     

Magnetic field shielding concepts for power transmission lines

High current carrying cables used in power transmission lines create strong magnetic fields in their vicinity. For ac lines at 50-60 Hz, the magnetic field is quasi-static and hence uncoupled from the electric field. Shielding of such low frequency magnetic fields is a challenge. In this paper, finite element modeling is used to study the effect of various shield geometries and shield materials around a current carrying wire. Two-dimensional analysis is sufficient for this problem because the cables are very long compared to the wire diameters. Shielding effectiveness, defined as the difference in the magnetic field intensity with and without the shield, is presented. It is concluded that a partial shield is the optimal design for shielding the region below the cable. To achieve this, the shield gap must be oriented above the high current wire     

Effects of thermal martensite content and deformation on damping capacity of a Co–32 wt.% Ni alloy

The damping capacity of Co–32 wt.% Ni alloy was investigated as a function of the amount of thermal and strain-induced martensite under non-magnetic and 900 Oe magnetic fields, respectively. The damping capacity of the Co–32 wt.% Ni alloy containing martensite without magnetic field consists of the magneto-mechanical damping capacity of mainly α phase, damping capacities of α and phases without magneto-mechanical damping effect. Under a magnetic field of 900 Oe, the more the thermal martensite mass fraction the higher the damping capacity. However, the damping capacity of the deformed Co–32 wt.% Ni alloy with the strain-induced martensite decreases with increasing deformation degree despite the increase in total martensite fraction, because the lattice defects like dislocations introduced during deformation act as barriers to movement of damping sources such as magnetic domain walls, stacking faults boundaries in both α and phases, and α/ interfaces.     

同轴电缆结构复合丝巨磁阻抗效应的研究

采用脉冲电沉积工艺在直径为200μm的铜丝表面沉积铁镍合金镀层,形成具有同轴电缆结构的巨磁阻抗复合丝材料。分别改变复合丝磁性外壳厚度与铜丝直径,研究复合丝结构对巨磁阻抗效应的影响。发现巨磁阻抗比值随磁性外壳厚度的增大及铜丝直径的增大而增大,特征频率则向低频端移动。本文还发现在外加直流磁场低于5．57kA／m时,驱动交流电幅值增大,巨磁阻抗效应增大;高于5．57kA／m时,驱动交流电幅值的大小几乎不再影响复合丝的巨磁阻抗效应。在驱动电流上叠加20mA以下的直流偏置对巨磁阻抗效应没有影响。     

Analysis of Forced Flow of Granular Materials in Vertical Pipes without and with Air Permeation

The problems associated with grain elevation and conveying under forced flow in vertical pipes are discussed. Based on experimental results, a theory is presented to describe forced flow with varying degrees of air permeation up to and just beyond the fluidization point. The theory takes into account the boundary and internal frictional properties, the degree of consolidation of the bulk granular material, and the stress fields that occur during forced flow. The force to elevate grain in a vertical tube is shown to be composed of two components, one to overcome Coulomb friction and initiate motion, and the other a time-dependent component that depends on the stiffness and damping characteristics of the granular material. The Coulomb friction component increases approximately exponentially with column height due to the positive feedback effect of the shear stresses at the pipe wall opposing the motion. Air permeation is shown to significantly reduce this component of the conveying force by reducing both the internal friction and the apparent bulk specific weight, the latter being the actual bulk specific weight less the air pressure gradient. Air permeation has a very significant effect on reducing both the bulk stiffness and, particularly, the damping characteristics, thereby reducing the time-dependent component of the conveying force.     

Temperature dependent damping studies of Ni-Mn-Ga polymer composites

Mechanical properties of ferromagnetic shape memory alloy (FSMA) polyurethane (PU) polymer composites are studied using dynamic mechanical analysis (DMA) at different temperatures. The sample used in the study is Ni-Mn-Ga/PU polymer composite. DMA studies reveal that the mechanical modulus decreases, when the temperature increases. The peak in internal friction (tanδ) appears twice due to martensite transformation of the reinforced particles and glass transition of the polymer matrix. The strain is due to the motion of twin boundaries rather than the phase transformations. Twin boundary motion is found to be the root cause for the martensite-austenite transformation and vice versa, and leads to enhanced damping mechanism. Speculations on twin boundaries of this sample to show concrete evidence on change in internal friction (tanδ) are still underway. This paper proposes that Ni-Mn-Ga/PU polymer composites can be used as efficient dampers.     

新型竖向电涡流-磁力混合阻尼装置试验研究

针对大跨轻质结构的减振需要,研制了一种新型竖向电涡流-磁力混合阻尼器样机.介绍混合阻尼器的减振原理及振动方程;接着介绍了3种不同的磁路设计和制作过程;随后对样机进行试验,测量其动力参数和减振性能.结果 表明:在铜板后方和底板上方安装磁铁均能改变阻尼器的附加刚度;特定磁路设计减少电涡流阻尼系数,增大等效磁力阻尼系数和负刚...     

The one-dimensional excitation gas in superfluid3He-A at very low temperatures

By setting up an experiment with a static A-B boundary stabilised by a magnetic field, we have been able to cool the liquid to the regime where the A-phase excitation gas is almost one-dimensional. We have measured the damping of a vibrating wire resonator in the A-phase while simultaneous measuring the damping in the B-phase. The response in the A-phase is found to follow an exponential temperature dependence and not a power law as might be naively expected. This effect is a result of the bending of the texture by the wire which excludes a large fraction of the bulk low energy excitations from the surface.     

热处理对喷射共沉积SiCP/6061Al复合材料内耗峰的影响

采用喷射共沉积方法制备了SiC_P/6061Al复合材料,研究了五种热处理工艺对其阻尼性能和内耗峰的影响。结果表明:各种热处理状态试样,在150~200℃范围内均出现温度内耗峰,且随频率增加该峰峰位向高温移动,峰高增加。通过Arrhenius方程测得内耗峰的激活能均高于1eV;另外,不同的淬火处理对该内耗峰有明显影响,当淬火冷却速度高于水淬时,随冷却速度的加大,内耗峰峰位向高温移动。     

Effect of loading on recovery of internal friction of Al-16 Wt % Ag and Al-16 Wt % Ag-0.28 Wt % Fe alloys

The phenomenon of internal friction recovery of Al-16 wt %Ag and Al-16 wt %Ag-0.28 wt % Fe alloys is studied after cold-working the samples in uniaxial tension using the free decay method. The wire samples are investigated for internal friction recovery while they were loaded-within the elastic limit-by different loads at different resting temperatures. It is found that by increasing the temporary loading the rate of recovery decreases. TEM investigations confirmed that the addition of Fe to Al-Ag alloy accelerates the formation and coarsening of G.P.zones. The results are explained on the basis of dislocation damping adopting the Granato-Lücke model for recovery of internal friction.     

Effects of Thermal Annealing and Pulsed Photon Processing on the Magnetomechanical Properties of the Amorphous Fe<Subscript>79.3</Subscript>P<Subscript>18.2</Subscript>V<Subscript>2.5</Subscript> Alloy

The effects of thermal annealing and pulsed photon processing on the structural transformations, ΔE effect, and internal friction in the amorphous Fe_79.3P_18.2V_2.5 alloy are studied. The results demonstrate that annealing and pulsed irradiation in a magnetic field have a significant influence on the magnetomechanical properties of the alloy, whereas pulsed irradiation in zero magnetic field has little effect on these properties. The irradiation effect is shown to depend on the structural transformations of the alloy at temperatures far below its crystallization temperature.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 823–827.Original Russian Text Copyright © 2005 by Braun, Vavilova, Ievlev, Kalinin, Kovneristyi, Palii, Perov, Serbin.     

Effects of macroscopic graphite particulates on the damping behavior of CuAlMn shape memory alloy

A novel composite was fabricated using a Cu–11.9Al–2.5Mn (wt%) shape memory alloy as the matrix and macroscopic graphite particulates as the second phase. The damping behavior of the resultant composite was examined in the present study through internal friction measurements. It was found that the net height of the internal friction peak relating to phase transformation decreased in the composite due to the constraining effect of the particulates, but the internal friction background significantly increased with increasing the volume fraction or decreasing the diameter of the particulates, giving rise to an improved damping capacity particularly at low temperatures. It is proposed that the interface damping, dislocation damping and the intrinsic damping of the matrix and the constraint are predominant in the composite.     

永磁式电涡流调谐质量阻尼器的研制与性能试验

为了克服传统的调谐质量阻尼器 (TMD) 阻尼单元存在的易漏油等耐久性问题,研制了一种基于电涡流阻尼耗能的竖向TMD装置。其特点在于：电涡流阻尼无需与结构接触,没有任何摩擦;产生磁场的元件为永磁体,无需外界供电;所有构件都由金属材料制成,不存在老化等现象。此外,通过调整永磁体与导体板的距离,很容易实现TMD阻尼参数的后期调节,且不会影响TMD刚度参数。理论与试验结果研究表明,研制的新型TMD具有优良的阻尼特性,而且当磁场间隙较小时电涡流阻尼理论预测值与试验结果吻合较好。     

Fluid Flow and Solidification Under Combined Action of Magnetic Fields and Microgravity

Mathematical models, both 2-D and 3-D, are developed to represent g-jitter induced fluid flows and their effects on solidification under combined action of magnetic fields and microgravity. The numerical model development is based on the finite element solution of governing equations describing the transient g-jitter driven fluid flows, heat transfer, and solutal transport during crystal growth with and without an applied magnetic field in space vehicles. To validate the model predictions, a ground-based g-jitter simulator is developed using the oscillating wall temperatures where timely oscillating fluid flows are measured using a laser PIV (Particle Image Velocimetry) system. The measurements are compared well with numerical results obtained from the numerical models. Results show that a combined action derived from magnetic damping and microgravity can be an effective means to control the melt flow and solutal transport of single crystal growth in space environment.     

An Equivalent Circuit Modeling of an Equispaced Metallic Nanoparticles (MNPs) Plasmon Wire

Based on the electric dipole moment (EDM) model of free oscillating electrons inside a single metallic nanoparticle (MNP), a comprehensive methodology is presented in the paper for calculating the equivalent circuit elements associated with an MNP. To find out the passive circuit elements for the MNP, the electromagnetic (EM) power flows are calculated by deriving the relaxation damping, radiation outflow, host matrix EM coupling, and applied signal interaction. The law of conservation of energy is then used to compute the extended oscillatory equation motion of a spherical MNP. The resonant behavior of a single MNP is represented by a lumped resonant circuit model, where the circuit parameters RLC are derived from the equation of motion of the EDM and EM near-field energy outside the MNP. Finally, equivalent circuit of a linearly equispaced MNPs plasmon wire is modeled as a voltage-controlled voltage source by using the nearest surface plasmon interactions.      

Anomalous Damping of a Low Frequency Vibrating Wire in Superfluid 3He-B due to Vortex Shielding

We have investigated the behaviour of a large vibrating wire resonator in the B-phase of superfluid ³He at zero pressure and at temperatures below 200 μK. The vibrating wire has a low resonant frequency of around 60 Hz. At low velocities the motion of the wire is impeded by its intrinsic (vacuum) damping and by the scattering of thermal quasiparticle excitations. At higher velocities we would normally expect the motion to be further damped by the creation of quasiparticles from pair-breaking. However, for a range of temperatures, as we increase the driving force we observe a sudden decrease in the damping of the wire. This results from a reduction in the thermal damping arising from the presence of quantum vortex lines generated by the wire. These vortex lines Andreev-reflect low energy excitations and thus partially shield the wire from incident thermal quasiparticles.