基于ANSYS的超导磁体机械结构的应力应变分析

为合理设计超导磁体机械结构,保证磁体正常运行,机械结构的应力分析在磁体设计中起到非常重要的作用.借助于ANSYS有限元分析软件,给出一了个5T超导磁体机械结构的受力分析,讨论了超导磁体内部电磁力对不同材料的机械结构所产生的应力和应变.结果表明,由磁体内部电磁力引起的机械结构的最大应力和应变,出现在磁体芯筒的中间部位,最...     

Effect of thermomagnetic and thermomechanical treatments on the magnetic properties and structure of the nanocrystalline soft magnetic alloy Fe81Si6Nb3B9Cu1

The structural and magnetic states of ribbon samples of the soft magnetic alloy Fe-Si-Nb-B-Cu (6 at % Si) have been investigated after the nanocrystallization at a temperature of 550°C in a constant magnetic field (thermomagnetic treatment), in a field of mechanical tensile stresses (thermomechanical treatment), and without external effects. It has been shown that exposure to a constant magnetic field or a field of mechanical tensile stresses gives rise to a longitudinal anisotropy of magnetic properties. The mag- netic hysteresis loop transforms and becomes close to rectangular. This is accompanied by a significant increase in the residual magnetic induction, which approaches the saturation magnetic induction. While the time required to complete the processes of nanocrystallization is as short as 20 min and, under thermome- chanical treatment, the magnetic anisotropy is induced for 20 min, the time it takes to decrease significantly the coercive force of the alloys under thermomagnetic treatment is substantially longer (up to 60 min). After the thermomagnetic treatment, no lattice strains of α-FeSi nanocrystals have been found. Either they do not exist at all, or their values are within the error of the X-ray diffraction experiment. In the samples subjected to annealing under tensile loading, anisotropic lattice strains of nanocrystals with the values increasing pro- portionally to the applied stress have been revealed. The highest strains reaching 1% have been observed after the annealing under a stress of 860 MPa.     

In situ TEM observation of synergistic electronic-excitation-effects of phase stability in III-V binary compound nanoparticles

Electronic-excitation-effects of phase stability in III-V binary compound nanoparticles have been studied by TEM. When GaSb particles were excited by 75 keV electrons, the compound transforms to a two-phase consisting of an antimony core and a gallium shell or an amorphous phase, or remains the original crystalline phase, depending on particle size and/or temperature. It is suggested that such nonlinear responses of the phase stability may arise from synergistic effects of bond instability under excited states, formation of high density of excited states, chemical equilibrium under excited states and temperature dependence of defects mobility.     

Transitions between the states with uniform and vortex distributions of magnetization in ferromagnetic nanoparticles under the action of an inhomogeneous magnetic field

Specific features of magnetization distribution in elliptical Co nanoparticles have been investigated by magnetic force microscopy. Reversible transitions (induced by the microscope magnetic probe) between the uniform and vortex magnetization states have been found. The possibility of controlling the vorticity direction in such particles is shown.     

Disorder and suppression of quantum confinement effects in Pd nanoparticles

Size-selectable ligand-passivated crystalline and amorphous Pd nanoparticles (<4 nm) are synthesized by a novel two-phase process and verified by high-resolution transmission electron microscopy. Scanning tunneling spectroscopy preformed at 5 K on these two types of nanoparticles exhibits clear Coulomb blockade and Coulomb staircases. Size dependent multipeak spectral features in the differential conductance curve are observed for the crystalline Pd particles but not for the amorphous particles. Theoretical analysis shows that these spectral features are related to the quantized electronic states in the crystalline Pd particle. The suppression of the quantum confinement effect in the amorphous particle arises from the reduction of the degeneracy of the eigenstates and the level broadening due to the reduced lifetime of the electronic states.     

液相法制备金属纳米粒子

液相法是在均相溶液中，利用各种途径引发化学反应，通过均相或异相成核及随后的扩散生长而制备出粒径分布窄且表面功能化的纳米尺度材料．介绍了液-液两相法、反相胶束、高温液相法等制备单分散金属纳米粒子的方法和高温液相法制备金属纳米粒子的影响因素，以及近年来在金属纳米粒子的制备和性能研究上的进展，尤其是Co等多种磁性纳米粒子的制备、磁性研究．     

Fabrication and characterization of iron oxide nanoparticles filled polypyrrole nanocomposites

The effect of iron oxide nanoparticle addition on the physicochemical properties of the polypyrrole (PPy) was investigated. In the presence of iron oxide nanoparticles, PPy was observed in the form of discrete nanoparticles, not the usual network structure. PPy showed crystalline structure in the nanocomposites and pure PPy formed without iron oxide nanoparticles. PPy exhibited amorphous structure and nanoparticles were completely etched away in the nanocomposites formed with mechanical stirring over a 7-h reaction. The thermal stability of the PPy in the nanocomposites was enhanced under the thermo-gravimetric analysis (TGA). The electrical conductivity of the nanocomposites increased greatly upon the initial addition (20 wt%) of iron oxide nanoparticles. However, a higher nanoparticle loading (50 wt%) decreased the conductivity as a result of the dominance of the insulating iron oxide nanoparticles. Standard four-probe measurements indicated a three-dimensional variable-range-hopping conductivity mechanism. The magnetic properties of the fabricated nanocomposites were dependent on the particle loading. Ultrasonic stirring was observed to have a favorable effect on the protection of iron oxide nanoparticles from dissolution in acid. A tight polymer structure surrounds the magnetic nanoparticles, as compared to a complete loss of the magnetic iron oxide nanoparticles during conventional mechanical stirring for the micron-sized iron oxide particles filled PPy composite fabrication.     

Chemical synthesis of self-assembled Ni nanoparticles and understanding of nonmagnetic layer inside their surface

To chemically synthesize mono-dispersed and self-assembled Ni nanoparticles, it was important to find the best combination of a Ni precursor and a ligand. Our Ni nanoparticles exhibited a face-centered cubic structure and superparamagnetism at room temperature. The value of saturation magnetization for our Ni nanoparticles was largely different from that of bulk Ni. Because of the relationship between the diameter and saturation magnetization per volume, the number of atoms composing the Ni nanoparticle was correlated with magnetization. This result indicated that a magnetic core/shell structure inside a Ni nanoparticle was produced. The nonmagnetic layer, as a magnetic shell of the core/shell structure, was created due to the low crystallinity of Ni nanoparticles and was composed of amorphous Ni‒O states. As a result, antiferromagnetic spins arrayed in the Ni‒O states were broken. Disordered spins were generated, which eventually decreased the total magnetization of the Ni nanoparticles.     

Giant nonhysteretic responses of two-phase nanostructured alloys

A new class of functional materials with giant nonhysteretic strain responses to applied fields is considered. They are decomposed two-phase systems consisting of single-domain nanoprecipitates of a low-symmetry phase. Their strain response is caused by the field-induced change of structural orientation of the domain states of these precipitates. The superresponse follows from the novel concept of structural anisotropy that is analogous to the magnetic anisotropy. Its vanishing produces a new glasslike structural state. The developed phase field theory and modeling allow us to formulate criteria for searching superresponsive two-phase nanostructured alloys.     

The magnetic and hyperthermia studies of bare and silica-coated La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> nanoparticles

The magnetic nanoparticles of La_0.75Sr_0.25MnO₃ perovskite manganite with a controlled size were prepared via sol–gel procedure, followed by thermal treatment and subsequent mechanical processing of the resulting raw product. The prepared materials were structurally studied by the XRD and TEM methods and probed by DC magnetic measurements. The nanoparticles of the mean crystallite sizes 11–40 nm exhibit T _C in the range of ≈310–347 K and the sample possessing 20-nm crystallites was identified as the most suitable for hyperthermia experiments. In order to obtain a colloidally stable suspension and prevent toxic effects, the selected magnetic cores were further encapsulated into silica shell using tetraethoxysilane. The detailed magnetic studies were focused on the comparison of the raw product, the bare nanoparticles after mechanical processing and the silica-coated nanoparticles, dealing also with effects of size distribution and magnetic interactions. The heating experiments were carried out in an AC field of frequencies 100 kHz–1 MHz and amplitude 3.0–8.9 kA m⁻¹ on water dispersions of the samples, and the generated heat was deduced from their warming rate taking into account experimentally determined thermal losses into surroundings. The experiments demonstrate that the heating efficiency of the coated nanoparticles is generally higher than that of the bare magnetic cores. It is also shown that the aggregation of the bare nanoparticles increases heating efficiency at least in a certain concentration range.     

磁纳米粒子介导的靶向剪切波弹性成像

组织力学特性与其生理病理变化过程密切相关.因此,对组织力学特性的分析有望为疾病诊断提供重要依据.超声弹性成像可以定量分析组织的剪切模量,但在检测的特异性和灵敏度等方面仍存在局限性.针对这一问题,该文发展一种磁纳米粒子介导的靶向剪切波弹性成像新方法.该方法是基于磁纳米粒子在脉冲磁场作用下产生磁致振动,从而导致周围组织的剪...     

Properties of MgFe2O4 Nanoparticles Synthesized by Ultrasonic Aerosol Pyrolysis for Biomedical Applications

Physics of the Solid State - We present the data of studies on the structure, phase states, and magnetic properties of magnetic nanoparticles (MNPs) of magnesium ferrite spinel (MgFe2O4),...     

Analysis of high gradient magnetic field effects on distribution of nanoparticles injected into pulsatile blood stream

Magnetic nanoparticles are widely used in a wide range of applications including data storage materials, pharmaceutical industries as magnetic separation tools, anti-cancer drug carriers and micro valve applications. The purpose of the current study is to investigate the effect of a non-uniform magnetic field on bio-fluid (blood) with magnetic nanoparticles. The effect of particles as well as mass fraction on flow field and volume concentration is investigated. The governing non-linear differential equations, concentration and Navier-stokes are coupled with the magnetic field. To solve these equations, a finite volume based code is developed and utilized. A real pulsatile velocity is utilized as inlet boundary condition. This velocity is extracted from an actual experimental data. Three percent nanoparticles volume concentration, as drug carrier, is steadily injected in an unsteady, pulsatile and non-Newtonian flow. A power law model is considered for the blood viscosity. The results show that during the systole section of the heartbeat when the blood velocity increases, the magnetic nanoparticles near the magnetic source are washed away. This is due to the sudden increase of the hydrodynamic force, which overcomes the magnetic force. The probability of vein blockage increases when the blood velocity reduces during the diastole time. As nanoparticles velocity injection decreases (longer injection time) the wall shear stress (especially near the injection area) decreases and the retention time of the magnetic nanoparticles in the blood flow increases.     

Quantum tunneling of the magnetic moment in a free nanoparticle

We study tunneling of the magnetic moment in a particle that has full rotational freedom. Exact energy levels are obtained and the ground-state magnetic moment is computed for a symmetric rotor. The effect of mechanical freedom on spin tunneling manifests itself in a strong dependence of the magnetic moment on the moments of inertia of the rotor. The energy of the particle exhibits quantum phase transitions between states with different values of the magnetic moment. Particles of various shapes are investigated and the quantum phase diagram is obtained.     

Fitting the flow curve of a plastically deformed silicon steel for the prediction of magnetic properties

We report measurements and modelling of magnetic effects due to plastic deformation in 2.2% Si steel, emphasizing new tensile deformation data. The modelling approach is to take the Ludwik law for the strain-hardening stress and use it to compute the dislocation density, which is then used in the computation of magnetic hysteresis. A nonlinear extrapolation is used across the discontinuous yield region to obtain the value of stress at the yield point that is used in fitting Ludwik's law to the mechanical data. The computed magnetic hysteresis exhibits sharp shearing of the loops at small deformation, in agreement with experimental behavior. Magnetic hysteresis loss is shown to follow a Ludwik-like dependence on the residual strain, but with a smaller Ludwik exponent than applies for the mechanical behavior.     

Magnetostatic interaction effects in an ordering hexagonal array of ferromagnetic nanoparticles

The results from investigating magnetostatic interaction effects in ordered hexagonal arrays of anisotropic single-domain ferromagnetic nanoparticles are presented. It is demonstrated theoretically and experimentally that two stable states (with quasi-uniform configurations of magnetic moments and with zero averaged magnetic moment configurations) can be easily attained in such arrays. It is shown that the structure of an ferromagnetic resonance spectrum depends strongly on the extent of magnetostatic interaction and the spatial configuration of the magnetic moments in the array.     

High-temperature magnetization of nanoparticles in “static” and gamma-resonance measurements in a weak magnetic field

An alternative approach to describing the magnetic dynamics of an ensemble of nanoparticles in a magnetic field is proposed, in which the precession orbits of uniform magnetization are regarded as the stochastic states of each particle. Using this approach, one can describe the nonconventional features of the high-temperature magnetization of nanoparticles that are observed in low-frequency magnetization measurements and Mössbauer spectroscopy.     

Magnetorheology of colloidal dispersion containing Fe nanoparticles synthesized by the arc-plasma method

Spherical crystalline Fe nanoparticles, ∼100 nm in diameter, were synthesized under Ar-50% H₂ arc-plasma. These nanoparticles were dispersed in silicone oil after silane treatment on as-grown thin oxide layer (∼2 nm) to make their surfaces hydrophobic. The resulting Fe nanoparticles exhibited a high saturation magnetization of ∼190 emu/g at room temperature. The static magnetorheological behavior was measured for the colloidal dispersion (solid concentration: 15 vol%) at room temperature under magnetic flux densities of 0-0.3 T, using a parallel-plate-type commercial rheometer. The yield stress continuously increased with magnetic flux density, demonstrating the Bingham plastic behavior. Moreover, subjecting the sample to a magnetic flux density of 0.3 T increased the yield stress by ∼10². Additionally, the colloidal dispersion exhibited good stability against sedimentation.     

Magnetism in nanoparticles of semiconducting FeSi2

Iron disilicide has been found to exhibit superparamagnetism in nanoparticles, even though no magnetic ordering occurs in bulk. The unexpected behavior was attributed, based on magnetic studies, to chemical disorder. A lack of sextet-type signals in Mössbauer spectra supports that the observed magnetic order is confined to only a very small fraction of magnetic Fe ions. Moreover, quadrupole-splitting and isomer-shift parameters reveal a significant amount of Fe in a short-range α-FeSi₂ structure, while XRD suggests an overall β-FeSi₂ structure. Such a compositional heterogeneity is also reflected in a calorimetrically obtained spin-glass-like anomaly at low temperatures. Meanwhile, as the particle size decreases, specific heat and Sommerfeld constant are enhanced due to lattice softening and the emergence of surface charge density of states, respectively, in nanoparticles.     

Reorientation,multidomain states and domain walls in diluted magnetic semiconductors

The competition between surface/interface and intrinsic anisotropies yields a number of specific reorientation effects and strongly influences magnetization processes in diluted magnetic semiconductors as (Ga,Mn)As and (In,Mn)As. We develop a phenomenological theory to describe reorientation transitions and the accompanying multidomain states applicable to layers of these magnetic semiconductors. It is shown that the magnetic phase diagrams of such systems include a region of four-phase domain structure with four adjoining areas of two-phase domains as well as several regions with coexisting metastable states. We demonstrate that the parameters of isolated domain walls in (Ga,Mn)As nanolayers are extremely sensitive to applied magnetic field and can vary in a broad range. This can be used in microdevices of magnetic semiconductors with pinned domain walls. For (Ga,Mn)As epilayers with perpendicular anisotropy the geometrical parameters of domains have been calculated.