Fabrication and characterization of a 3D-structured field emitter using carbon nanotube

Carbon nanotube has good electrical properties and a high aspect ratio, which enable it to obtain a high current at a low voltage due to its high field. Due to the life and uniformity of their emission tips, carbon nanotube field emitters are hard to commercialize. A field emitter with a three-dimensional (3D) structure was fabricated in this study to overcome such problems. In the 3D-structured field emitter, the field emission tips are located only at the vertical plane, where an enlarged field emission area can be attained. To fabricate the tip of the 3D-structured field emitter, carbon nanotube/silver nanocomposite powders were fabricated via molecular-level mixing and were sprayed at a substrate with good attachment and homogeneous dispersion between the CNT tips and the silver. The field emission properties of the 3D-structured field emitter were then determined and compared with those of a flat field emitter. The field emission area of the 3D-structured field emitter was found to be 4.5 times larger than that of the flat field emitter, with six times higher current density. Moreover, the 3D-structured field emitter had better stability than the flat field emitter. At a high gate field, the emission images of the 3D-structured field emitter showed light spots expanded towards the gate direction.     

Build Axial Gradient Field by Using Axial Magnetized Permanent Rings

Axial magnetic field produced by an axial magnetized permanent ring was studied. For two permanent rings, if they are magnetized in the same directions, a nearly uniform axial field can be produced. If they are magnetized in opposite direction, an axial gradient magnetic field can be generated, with the field range changing from-Bo to Bo- A permanent magnet with a high axial gradient field was fabricated, the measured results agree with the PANDIRA calculation very well. For wider usage, it is desirable for the field gradient to be changed. Some methods to produce the variable gradient field are presented. These kinds of axial gradient magnetic field can also be used as a beam focusing for linear accelerator if the periodic field can be produced along the beam trajectory. The axial magnetic field is something like a solenoid, large stray field will leak to the outside environment if no method is taken to control them. In this paper, one method is illustrated to shield off the outside leakage field.     

Build Axial Graident Field by Using Axial Magnetized Permanent Rings

Axial magnetic field produced by an axial magnetized permanent ring was studied.For two permanent rings,if they are magnetized in the same directions,a nearly uniform axial field can be produced.If they are magnetized in opposite direction,an axial gradient magnetic field can be generated,with the field range changing from-B0 to B0.Apermanent magnet with a high axial gradient field was fabricated,the measured results agree with the PANDIRA calculation very well.For wider usage,it is desirable for the field gradient to be changed.Some methods to produce the variable gradient field are presented.These kinds of axial gradient magnetic field can also be used as a beam focusing for linear accelerator if the periodic field can be produced along the beam trajectory.The axial magnetic field is something like a solenoid,large stray field will leak to the outside environment if no method is taken to control them.In this paper,one method is illustrated to shield off the outside leakage field.     

Effects of remanent field on an elliptical flaw and a crack in a poled piezoelectric ceramic

Commonly used piezoelectric ceramics such as PZT and PLZT are polarized ferroelectric polycrystals. After poling, remanent strains and a remanent polarization exist in a ceramic material. Remanent field can affect the electroelastic field and consequently plays a critical role in fracture of poled ceramics. Based on a linear constitutive law, the electroelastic field and the energy release rate of an elliptical cavity (or a crack) in a poled piezoelectric are re-examined in this study by including the effects of remanent field. It is noted that the remanent field generally has a minor effect on the stress field and a pronounced effect on the electric field at the apex of the major axis of an elliptical flaw. When the permittivity of the cavity is small, the effect of remanent polarization is similar to that of a very strong electric field applied along the poling direction. However, for the case of a conducting flaw, the remanent field does not influence the electroelastic field and energy release rate. Energy release rate of a flaw in a poled ferroelectric ceramic with and without the remanent polarization is generally different.     

Choice of input fields in stochastic finite elements

The problem of the arbitrary choice of variables for random field modelling in structural mechanics or in soil mechanics is treated. For example, it is relevant to ask the question of whether it is best to choose a stiffness field along a beam element or to choose its reciprocal field, the flexibility field, as the input to the stochastic finite element model. To answer this question the focus should be on the error of the output of the mechanical model rather than on the input field itself when discretizing the field through replacing it by a field defined in terms of a finite number of random variables. Several reported discretization methods define these random variables as integrals of the product of the field and some suitable weight functions. In particular, the weight functions can be Dirac delta functions whereby the random variables become the field values at a finite set of given points. The replacement field is often defined as the linear regression of the original field on the considered vector of the weighted integrals of the field. For example, this holds for discretizations obtained by truncation of the Karhunen–Loéve expansion of the field, but only approximately so for truncations of expansions given in terms of any other complete orthogonal function basis. Solely discretizations based on the linear regression method are considered herein. The solution to the problem of best choice of the vector of weight functions is not universal but depends on the mechanical problem under consideration as well as on the choice of the input field. Obviously it makes a difference whether it is the stiffness field or its reciprocal field that is chosen to be represented by a vector of weighted averages of the field. As a test example a lognormal stiffness field along the axis of a linear-elastic Bernoulli–Euler column is considered. Then the exact one-to-one conversion from the stiffness field to the flexibility field is directly obtained. From the form of two functionals that have similarity to the potential energy functional and the complementary energy functional, respectively, both derived from the differential equation of the column displacement and the relevant boundary conditions, it can be expected that the discretization of the flexibility field is preferable over the discretization of the stiffness field. Direct mechanical considerations support this expectation.     

Energy flow lines for the radiation emitted by a dipole

An oscillating electric dipole emits radiation, and the flow of energy in the electromagnetic field is represented by the field lines of the Poynting vector. In the most general state of oscillation the dipole moment vector traces out an ellipse. We have evaluated analytically the field lines of the Poynting vector for the emitted light, and it appears that each field line lies on a cone, which has its axis perpendicular to the plane of the ellipse. The field lines exhibit a vortex structure near the location of the dipole, and they approach a straight line in the far field. It is shown that due to the spiraling of the field lines near the source, the asymptotic limit of a field line is displaced as compared to a ray which would come directly out of the source. Both the spatial extent of the vortex in the near field and the magnitude of the displacement of the image in the far field are of nanoscale dimension.     

洛仑兹力的应力应变效应

阐述了在非定常电磁场和机械场的作用下,洛仑兹力对非线性载流薄板薄壳应力应变状态的影响。导出了在电磁场中洛仑兹力的具体表达式。通过对环形薄板在电磁场和机械场联合作用下应力应变状态的计算,指出变化洛仑兹力的作用可以得到最佳的应力应变状态。本文可供在强电磁场中工作的载流元件、防护元件及设备在设计及实际应用时作参考。     

The effect of DC in plane field on the operation of field access bubble memory devices

The effect of a small dc in-plane field on the start-stop operation of field access bubble devices has been studied. Experimental results show that the bias margin in this mode is very sensitive to the magnitude of the field and its orientation relative to the start-stop direction of the drive field. In a T-I circuit a complete margin loss was observed for an in-plane field of 3 Oe oriented antiparallel to the start-stop direction. For parallel orientations of the in-plane field the start-stop margin improved and approached that of the continuous propagation margin at an in-plane field of approximately 6 Oe. Dependence of the start-stop margin on the orientation of the start-stop direction relative to the pattern was also observed. Measurements of the bubble collapse field at various points in the pattern show a very strong dependence on the in-plane field and the permalloy geometry. The collapse-field results and magnetostatic energy considerations which take into account local field variations and bubble-bubble interactions provide a basis for understanding the experimentally observed start-stop margins. These results show that a small tilt (2 to 3°) should be introduced in the bias field to overcome normal alignment tolerances and ensure that a favorable in-plane field is always present. This assures reliable start-stop operation.     

磁性液体纳米磁性颗粒磁场诱导链状结构研究

研究了磁性液体在有、无外加磁场作用时磁性液体中纳米磁性颗粒的微观排列结构, 发现无外加磁场作用时磁性颗粒随机均匀分布在载液中, 有外加均匀磁场作用时, 磁性颗粒沿磁场方向排列成均匀链状结构。当外加均匀磁场强度为11.1 kA/m, TEM照片清晰显示磁性颗粒排列成一条链状结构, 随着外加均匀磁场强度逐渐增强到28.6 kA/m, 更多磁性颗粒沿磁场方向紧密排列成链状结构。在外加梯度磁场作用下, 铁芯中心处磁场强度为28.7 kA/m时, 大量磁性颗粒聚集于线圈中心轴附近形成复杂链状团簇结构; 沿r轴方向磁场强度较弱的地方, 较少磁性颗粒排列成链状结构。当磁场梯度从1.73 kA/m²逐渐增加到5.11 kA/m²时, 磁场梯度轴线上的磁性颗粒团簇结构由稀疏逐渐变成为密集。磁场诱导磁性颗粒链状排列结构的研究对磁性液体在机械工程、生物工程、热力工程等领域的应用有重要意义。     

Micromagnetic investigation by a simplified approach on the demagnetization field of permanent magnets with nonmagnetic phase inside

A simplified analysis method based on micromagnetic simulation is proposed to investigate effects of nonmagnetic particles on the demagnetizing field of a permanent magnet. By applying the additivity law of the demagnetizing field, the complicated demagnetizing field of the real magnet could be analyzed by only focusing on the stray field of the reserved magnet. For a magnet with nonmagnetic particles inside, the particle size has no significant effect on the maximum value of the demagnetization field, but the area of the affected region by the particle is proportional to the particle size. A large particle produces a large affected area overlapped with those influenced by other particles, which leads to the large demagnetization field. With increasing the length of the particle along the magnetization direction, the demagnetization field on the pole surface increases. The pole surface with a convex shape will increase the demagnetization field. The demagnetizing field near the nonmagnetic particle will be further increased by the large macroscopic demagnetizing field near the pole surface. This work suggests some practical approaches to optimize the microstructure of permanent magnets.     

Modeling of two-dimensional random fields

This paper presents a method of conditional stochastic modeling of two-dimensional fields which can be used to predict values at certain field points at a given time, based on field values at other locations at the same time and on data about second order field moments at given points. For computer simulations, the Gaussian truncated distributions are used. The aim of this work is also to present a derivation of a formula for the probability density of an n-dimensional random variable with the Gaussian conditional truncated distribution. As a numerical example, a soil contamination field described by correlation functions corresponding to the white noise field, the Shinozuka field and the Markov field is analyzed. The acceptance-rejection method is applied to generate covariance matrices and vectors of field values. Then, conditional expected field values for adequate correlation functions are calculated.     

Phase shift caused by microwave field based on light storage

We have theoretically investigated the phase shift of a probe field for a four-level atomic system interacting successively with two fields tuned near an EIT resonance of an atom, a microwave field, and a coupling field. It has been found that the phase of retrieved signal has been shifted due to the cross-phase modulation when the stored spin wave was disturbed by a microwave. Because of the low relaxation rates of the ground hyperfine state, our proposed technique can impart a large phase rotation onto the probe field with low absorption of retrieved field and very low intensity of the microwave field.     

纳米管场发射的电场计算

利用电磁场理论计算出单根纳米管场发射时其尖端附近的电势和电场分布。结果表明 ,纳米管尖端表面电场非常强 ,随着距离尖端表面的距离的增加 ,电场迅速下降 ;尖端附近的场强与纳米管场发射有低的阈值电压相符合。计算还给出在保持极板间距离和电压不变的情况下 ,纳米管长径比越大 ,尖端电场越强 ,因此 ,具有更低的阈值电压     

Development of a 4‐node hybrid stress tetrahedral element using a node‐based smoothed finite element method

In this paper, a new 4‐node hybrid stress element is proposed using a node‐based smoothing technique of tetrahedral mesh. The conditions for hybrid stress field required are summarized, and the field should be continuous for better performance of a constant‐strain tetrahedral element. Nodal stress is approximated by the node‐based smoothing technique, and the stress field is interpolated with standard shape functions. This stress field is linear within each element and continuous across elements. The stress field is expressed by nodal displacements and no additional variables. The element stiffness matrix is calculated using the Hellinger‐Reissner functional, which guarantees the strain field from displacement field to be equal to that from the stress field in a weak sense. The performance of the proposed element is verified by through several numerical examples.     

液压冲击器流场分析

通过流体力学的相关理论分析了压力反馈式液压冲击器的整体流场,建立了液压冲击器流场的基本方程;对冲击器的力学模型进行了简化,并求解了流场中的速度分布和流动损失情况。为了形象地得出冲击器流场的参数变化规律,文章利用Catia软件和Fluent的前处理器Gambit对冲击器的流场进行了建模,并利用Fluent软件对液压冲击器的瞬态流场进行了模拟求解。对比CFD模拟的结果和求解基本方程的结果,结果比较吻合。对瞬态流场的分析表明,在进油口截面突然变化的部位流速变化较大,远离这一区域后开始趋于平稳,而冲击过程中的中腔出油口存在速度较大的区域。另外,随着进油速度增大,流场的紊流情况加剧,流动的损失量呈抛物线增加。     

非均匀磁场螺旋波等离子体研究进展

螺旋波等离子体具有高电离率、低工作气压、低外加磁场、无内电极、静态均匀性好等特点,广泛应用于聚变研究、等离子体推进、等离子体材料处理等领域。均匀磁场和非均匀磁场下螺旋波等离子体的耦合效率有所不同,在螺旋波等离子体源中应用非均匀磁场可显著改善等离子体参数和射频能量吸收,提高等离子体电离效率。本文主要对非均匀磁场下的螺旋波等离子体的磁场设计及分布、放电特性、应用进行综述,并展望了非均匀磁场螺旋波等离子体的发展方向。     

用FEM/FIM研究单壁碳纳米管束

利用范氏力将单壁碳纳米管样品组装到钨针尖上 ,用FEM/FIM对同一碳纳米管样品用热处理方法和场脱附方法进行了研究。场离子显微镜是具有原子级分辨能力的尖端表面分析工具 ,由场离子像推测这次组装的样品是由三根单壁碳纳米管突起组成的碳纳米管束。清洁碳纳米管束样品的场发射像和场离子像有极好的对应关系。场脱附后的碳纳米管束的场发射特性较好地符合Fowler Nordheim场发射模型。通过比较碳纳米管束吸附态和热处理后以及场脱附后的Fowler Nordheim曲线的斜率变化 ,得出碳纳米管束样品逸出功的变化 ,再结合场发射像的变化推断出场脱附与热处理结合是一种较理想的获得清洁碳纳米管表面的方法     

行波磁场在晶体生长过程中的研究进展

针对行波磁场的应用评述了其近几年理论和实验的研究进展,包括行波磁场原理、行波磁场发生器的设计、行波磁场在晶体生长过程中的效应。论述了在晶体生长过程中行波磁场引起的子午线流对导电熔体的稳定性、生长界面形貌和溶质分布的影响,指出了该领域当前研究存在的问题并展望了其发展趋势。     

Isolated attosecond pulse generation with the chirped two-color laser field

We propose a scheme to generate isolated attosecond pulse using a linearly chirped two-color laser field, which includes a fundamental laser field and a weak infrared control laser field in the multicycle regime. The fundamental laser field consists of one linearly up-chirped and one linearly down-chirped pulses. The control pulse is chirped free. We compare the attosecond pulse generated in the chirped two-color field and the chirp-free field. It is found that an IAP can be generated even without carrier envelop phase stabilization in the chirped two-color laser field with a duration of 40 fs. We also discuss the influence of the relative intensity, relative phase, time delay, and chirping parameters on the generation of IAPs.     

Undercooling behavior of glass-fluxed Sb melts under gradient magnetic fields

The influence of gradient magnetic field on the undercooling behavior of glass-fluxed pure Sb melts was investigated using a superconducting magnet. It was found that under a positive gradient magnetic field, the mean undercooling of pure Sb melts increased with increasing magnetic field intensity. However, under a negative gradient magnetic field, the mean undercooling showed a decreasing tendency following an initial increase with increasing magnetic field intensity. The results were discussed by considering the Lorentz force and the magnetization force imposed by the gradient magnetic field.