有限元法计算磁流体在极靴齿隙中的磁场强度分布

磁流体往复轴动密封装置的磁场分布决定着磁流体的密封能力。本文用有限元方法 对这一边界形状较为复杂的磁场区域进行了计算。在计算中将磁流体的磁饱和效应考虑 在内，能够直观精确地求出场域中任一点上的磁场强度值。     

纳米磁流体磁性能与微结构特性关系研究

用水热法制备了Fe3O4纳米磁流体样品,测试了样品对磁场的响应度,利用原子力显微镜表征分析磁流体的微结构特征,观测了磁流体材料颗粒粒径大小及形貌,且重点分析了磁特性与微结构的关系。分析结果表明,制备的Fe3O4纳米磁流体粒径大小约为5.00nm;不同质量分数的配比影响着磁流体对磁场的响应度,质量分数为10%时的响应度最高;磁流体所表现的磁特性与链状微结构变化紧密相关,纳米颗粒在Fe3O4磁流体中的分布情况决定了其链状微结构的变化。     

纳米磁流体磁性能与一阶磁浮力的研究

磁流体是一种超顺磁性的液态磁性功能材料,其特殊结构使得它区别于普通液体的悬浮性能,各层表观密度随着外磁场改变,导致磁流体内不同高度处的磁压强改变从而产生磁浮力。不同的基载液及磁性微粒含量使得磁流体饱和磁化强度差异很大,在相同的外加磁场作用下磁性能表现有所差异。利用振动样品磁强计测出磁流体的磁化曲线并用有限元软件仿真得磁流体内部磁场分布,以磁压强公式计算不同磁场下的磁压差与磁浮力的理论值,设计磁浮实验装置进行测试。结果表明,磁流体的一阶磁浮力与饱和磁化强度呈正相关,在磁化曲线线性区的弱磁场环境下磁浮力正比于浸没的非磁性物体表面磁场的平方差,达饱和磁场后磁浮力正比于其表面磁场差。研究对于磁流体选矿、油水分离等浮选性能的应用有一定的参考价值。     

磁流体浸没物磁场力分析及磁浮特性

物体浸没于磁流体中表现出磁浮特性，对其受力状态分析是准确描述其悬浮状态的前提和基础。基于非线性磁流体应力张量模型和稳态Bernoulli方程，建立磁流体中浸没物受力模型。借助非磁性体受力模型的简化计算方法以及磁性体与磁流体之间的多场效应关系，分别对非磁性体、磁性体两类浸没物在磁流体中所受磁浮力进行分析，结果表明非磁性浸没物在磁流体中仅受到外加磁场贡献的一次磁浮力，而磁性浸没物除受到一次磁浮力外，还受到其自身激发磁场贡献的二次磁浮力．永磁体在磁流体中位置决定的磁浮力满足一定条件时，永磁体能够自悬浮于磁流体中。     

超磁致伸缩材料ΔE效应理论建模与试验研究

为研究超磁致伸缩材料的ΔE效应,采用Jiles-Atherton模型和磁弹性效应相结合的方法,提出了一种考虑动态应力影响及系数变化的磁化模型。分别对磁场和应力作用下超磁致伸缩材料的磁场-磁化关系和应力-磁化关系进行建模,并根据胡克定律以及二次畴转模型计算出材料在应力和磁场综合作用下的总应变,得到不同外加磁场下材料的应变-应力环。通过对应变-应力曲线斜率的计算,得到超磁致伸缩材料在不同外加磁场下,弹性模量随应力变化规律。搭建了应力测试装置,对不同磁场作用下Terfenol-D的磁化-应力响应和应变-应力响应测试,试验结果与模型计算结果基本一致,结果表明,ΔE效应是磁场能量和应力各向异性平衡的结果,Terfenol-D最大弹性模量变化达到513%。研究成果为新型机电系统变刚度设计提供了理论基础和调控手段。     

磁流体材料在肿瘤靶向药物传递的应用研究

磁流体以其独有特性在磁靶向药物传递系统中具有较大优势,通过分析磁流体靶向药物传递载体、传递机理研究现状,指出磁流体靶向药物传递主要表现为磁场力与血液流动剪切力之间的竞争.鉴于实验研究中使用的磁场大大超过临床上使用的安全磁场并会对病人造成伤害,提出采用包括恒定磁场和梯度磁场的组合磁场来降低最大磁场强度的思路,并指出磁流体传递过程中需考虑磁流体磁粘特性,以最终降低磁场强度,减少对人体危害,推动临床应用的发展.     

磁流体的光学特性及其在光电信息传感领域中的应用

详细介绍了磁流体的光学性质,包括磁流体的热透镜效应、磁光效应.磁流体折射率的可控性以及磁致分色效应.由于磁流体的光学性质具有可调谐性,这为磁流体在新型光子器件与光纤传感器的设计和研究提供了新原理.新材料.论述了应用磁流体设计的多路复用器、光开关、光纤调制器、可调谐磁流体光栅与可调谐滤波器,以及基于磁流体的光纤电流传感器原理和结构.     

水基磁流体的制备及Fe-13Cr-2.5Mo阻尼合金磁畴的观察

本研究利用化学共沉积法制备了一种改进的Fe3O4磁流体,利用该磁流体在光学显微镜下成功地观察到Fe-13Cr-3Mo阻尼合金的磁畴及在磁场下磁畴的变化过程.实验表明,在外界磁场作用下,磁畴因畴界的移动而明显粗化,但同一晶粒中磁畴的排列几乎是有序的.不同晶粒中磁畴的走向截然不同,宽窄也不同,局部区域的内应力会导致磁畴的细小和堆积.参与移动的磁畴的数量和畴界移动的幅度是内耗随应变变化的本质原因.     

铁磁流体互易性磁致旋光效应的传输特性及数值模拟

对水基铁磁流体在外加磁场作用下的磁致旋光效应的传输特性进行实验和数值模拟研究.在构建铁磁流体纵向磁光效应测试平台的基础上,对铁磁流体在恒定磁场、单个脉冲磁场以及连续方渡脉冲磁场作用下的磁致旋光效应进行了研究.结果表明,铁磁流体对偏振光振动方向的旋转与纵向调制磁场的方向无关;铁磁流体对于快脉冲磁场的响应是一个双指数弛豫过...     

小型磁偏转质谱计磁场的分析计算

磁偏转质谱计是根据离子在磁分析器中运动时,不同质荷比的离子有不同的偏转半径原理来实现质量分离的。磁场大小和分布对质谱计的性能影响较大,因而设计时需要对质谱计磁场分布进行精确计算。应用有限元法建立了计算质谱计磁分析器磁场的物理模型,并利用这一模型计算了磁分析器磁感应强度在空间的分布情况。结果表明,在半径分别为20 mm和50 mm的1/4圆弧轨道上,其磁场分布规律类似。由于边缘磁场效应,在磁铁边界区域约3 mm范围内,磁感应强度基本呈直线下降,这一结果为磁分析器的结构优化和边缘场补偿提供了理论依据。     

Optical bistability and multistability driven by external magnetic field in a dielectric slab doped with nanodiamond nitrogen vacancy centres

Abstract

The theoretical investigation of controlling the optical bistability (OB) and optical multistability (OM) in a dielectric medium doped with nanodiamond nitrogen vacancy centres under optical excitation are reported. The shape of the OB curve from dielectric slab can be tuned by changing the external magnetic field and polarization of the control beam. The effect of the intensity of the control laser field and the frequency detuning of probe laser field on the OB and OM behaviour are also discussed in this paper. The results obtained can be used for realizing an all-optical bistable switching or development of nanoelectronic devices.     

Effect of aggregates on the magnetization property of ferrofluids: A model of gaslike compression

The effect of field-induced aggregation of particles on the magnetization property of ferrofluids is investigated. From the viewpoint of energy, magnetizability of ferrofluids is more complicated than predicted by Langevin theory because the aggregation, i.e., the transition of ferrofluid microstructure, would consume the energy of the applied magnetic field. For calculating the effect of aggregates on the magnetization of ferrofluids, a model of gaslike compression (MGC) is proposed to simulate the evolution of the aggregate structure. In this model, the field-induced colloidal particles aggregating in ferrofluids is equivalent to the “gas of the particles” being compressed by the applied magnetic field. The entropy change of the ferrofluid microstructure is proportional to the particle volume fraction in field-induced aggregates ø_H. On the basis of the known behavior of ferrofluid magnetization and the aggregate structure determined from the present experiments, ø_H is obtained and found to depend on the aggregating characteristic parameter of ferrofluid particles γ in addition to the particle volume fraction in ferrofluids ø and the strength of applied magnetic field H. The effect of the nonmagnetic surface layer of ferrofluid particles is also studied. The theory of MGC conforms to our experimental results better than Langevin theory.     

Forced convection of a temperature-sensitive ferrofluid in presence of magnetic field of electrical current-carrying wire: A two-phase approach

This research examines laminar forced convection of a temperature-sensitive magnetic nanofluid flowing within a horizontal tube through the two-phase mixture model. The ferrofluid flowing in the tube is exposed to the magnetic field generated by electrical current-carrying wire(s) along the tube, and the effect of such magnetic field is studied on heat and mass transfer phenomena. It is observed that due to the dependency of magnetization on temperature, the cold fluid flowing at the central regions of the tube is attracted more significantly towards the source of the magnetic field, which results in creation of secondary flow. Such mixing in the flow, subsequently, disturbs the thermal and hydrodynamic boundary layers, especially at the vicinity of the magnetic field source, leading to better heat transfer rate and also higher pressure drop. Furthermore, increasing the strength of the magnetic field leads to greater enhancement in heat transfer, while increasing the Reynolds number decreases the effectiveness of the magnetic field on the ferrofluid flow and heat transfer. Moreover, placing two wires above and under the tube can enhance the heat transfer even more significantly, such that the average convective heat transfer coefficient in this case is about 34.5% higher than that of the case without magnetic field.     

Effect of aggregates on the magnetization property of ferrofluids: A model of gaslike compression

The effect of field-induced aggregation of particles on the magnetization property of ferrofluids is investigated. From the viewpoint of energy, magnetizability of ferrofluids is more complicated than predicted by Langevin theory because the aggregation, i.e., the transition of ferrofluid microstructure, would consume the energy of the applied magnetic field. For calculating the effect of aggregates on the magnetization of ferrofluids, a model of gaslike compression (MGC) is proposed to simulate the evolution of the aggregate structure. In this model, the field-induced colloidal particles aggregating in ferrofluids is equivalent to the “gas of the particles” being compressed by the applied magnetic field. The entropy change of the ferrofluid microstructure is proportional to the particle volume fraction in field-induced aggregates φ_H. On the basis of the known behavior of ferrofluid magnetization and the aggregate structure determined from the present experiments, φ_H is obtained and found to depend on the aggregating characteristic parameter of ferrofluid particles γ in addition to the particle volume fraction in ferrofluids φ and the strength of applied magnetic field H. The effect of the nonmagnetic surface layer of ferrofluid particles is also studied. The theory of MGC conforms to our experimental results better than Langevin theory.     

Magnetic field and concentration dependence of light scattering from a ferrofluid

Measurements of light scattering from a ferrofluid were undertaken as functions of both magnetic field and particle concentration. The results show that the distribution pattern of light intensity in space is a continuous banding perpendicular to the field direction. The light intensity weakens with increasing scattering angle. The experiments also indicate that the scattering coefficient increases both with the magnetic field and with the particle concentration and tends to saturate at higher field strengths. Finally, the experimental results are discussed in terms of an expanded theory of light scattering established by considering the widths of the chains formed in the ferrofluid as functions of both the magnetic field and the particle concentration.     

Betatron contributions to the angular gain spectrum of a free-electron laser with an imperfect beam trajectory

The effect of betatron oscillations on the angular gain spectrum of a free-electron laser is examined in the case of an imperfect beam trajectory. The use of an axial magnetic field is suggested to improve the beam–mode coupling and gain of the laser.     

Numerical calculations for ferrofluid seals

The magnetic field and the seal capacity of ferrofluid seals are calculated and analyzed by numerical methods. Based on the magnetic filed calculations, the isobars in the ferrofluid and the cross sections of the fluid sealing ring are described. The relations of the seal capacity to the ferrofluid amount and the magnetic circuit parameters are analyzed. The action of the centrifugal force on the seal is indicated     

Ferrofluids and magnetically guided superparamagnetic particles in flows: a review of simulations and modeling

Ferrofluids are typically suspensions of magnetite nanoparticles, and behave as a homogeneous continuum. The ability of the ferrofluid to respond to an external magnetic field in a controllable manner has made it emerge as a smart material in a variety of applications, such as seals, lubricants, electronics cooling, shock absorbers and adaptive optics. Magnetic nanoparticle suspensions have also gained attraction recently in a range of biomedical applications, such as cell separation, hyperthermia, MRI, drug targeting and cancer diagnosis. In this review, we provide an introduction to mathematical modeling of three problems: motion of superparamagnetic nanoparticles in magnetic drug targeting, the motion of a ferrofluid drop consisting of chemically bound nanoparticles without a carrier fluid, and the breakage of a thin film of a ferrofluid.     

Calculation of the radiation trapping force for laser tweezers by use of generalized Lorenz-Mie theory. I. Localized model description of an on-axis tightly focused laser beam with spherical aberration

Calculation of the radiation trapping force in laser tweezers by use of generalized Lorenz-Mie theory requires knowledge of the shape coefficients of the incident laser beam. The localized version of these coefficients has been developed and justified only for a moderately focused Gaussian beam polarized in the x direction and traveling in the positive z direction. Here the localized model is extended to a beam tightly focused and truncated by a high-numerical-aperture lens, aberrated by its transmission through the wall of the sample cell, and incident upon a spherical particle whose center is on the beam axis. We also consider polarization of the beam in the y direction and propagation in the negative z direction to be able to describe circularly polarized beams and reflected beams.