Caution to Apply Magnetic Barkhausen Noise Method to Nondestructive Evaluation of Plastic Deformation in Some Ferromagnetic Materials

Magnetic Barkhausen Noise(MBN) method is known as an effective nondestructive evaluation(NDE) method for evaluation of residual stress in ferromagnetic materials. Some studies on the feasibility of the MBN method for NDE of residual strains were also conducted and found applicable. However, these studies are mainly focused on the state of residual strains which were introduced through a one-cycle-loading process. In practice, however, structures may suffer from an unpredictable and complicated loading history, i.e., the final state of plastic strain may be induced by several times of large loads. Whether the loading history has influences on MBN signals or not is of great importance for the practical application of the MBN method. In this paper, several ferromagnetic specimens with the same final state of residual strain but of different loading history were fabricated and inspected by using a MBN testing system. The experimental results reveal that the loading history has a significant influence on the detected MBN signals especially for a residual strain in range less than 1%, which doubts the feasibility to apply the MBN method simply in the practical environment. In addition, micro-observations on the magnetic domain structures of the plastic damaged specimens were also carried out to clarify the influence mechanism of loading history on the MBN signals.

Ferromagnetism in High-Surface-Area ZnO Nanosheets Prepared by a Template-Assisted Hydrothermal Method

High-surface-area ZnO nanosheets are prepared using a template-assisted hydrothermal method.A saturation moment as high as 0.02 emu/g is obtained for the ZnO nanosheets.Both photoluminescence spectroscopy and x-ray photoelectron spectroscopy demonstrate the existence of abundant oxygen vacancies on the surfaces of the nanosheets.In addition,the oxygen vacancy concentration increases with an increasing nanosheet surface area.The results show that the origin of the room-temperature ferromagnetism is closely related with a large surface area and oxygen vacancies of the nanosheets.This finding suggests that the high-surface-area ZnO nanosheets are promising to be applied to spintronic devices.

Magnetoelastic combined resonance and stability analysis of a ferromagnetic circular plate in alternating magnetic field

The nonlinear combined resonance problem of a ferromagnetic circular plate in a transverse alternating magnetic field is investigated. On the basis of the deformation potential energy, the strain potential energy, and the kinetic energy of the circular plate, the Hamilton principle is used to induce the magnetoelastic coupling transverse vibration dynamical equation of the ferromagnetic circular plate. Based on the basic electromagnetic theory, the expressions of the magnet force and the Lorenz force of the circular plate are presented. A displacement function satisfying clamped-edge combined with the Galerkin method is used to derive the Duffing vibration differential equation of the circular plate. The amplitude-frequency response equations of the system under various combined resonance forms are obtained by means of the multi-scale method, and the stability of the steady-state solutions is analyzed according to the Lyapunov theory. Through examples, the amplitude-frequency characteristic curves with different parameters, the amplitude of resonance varying with magnetic field intensity and excitation force, and the time-course response diagram, phase diagram, Poincar′e diagram of the system vibration are plotted, respectively. The effects of different parameters on the amplitude and stability of the system are discussed. The results show that the electromagnetic parameters have a significant effect on the multi-valued attribute and stability of the resonance solutions, and the system may exhibit complex nonlinear dynamical behavior including multi-period and quasi-periodic motion.

Enhancement of Ferromagnetic Ordering Curie Temperature in N-Doped MgO under Hydrostatic Pressure

We have explored the magnetic properties of Nitrogen doped cubic MgO using the full potential linearized augmented plane wave (FP-LAPW) method. The unit cell has 128 atoms, and two Nitrogen atoms are placed in the positions of oxygen sites. This corresponds to 3.125% doping concentration. Our calculations predict that the ferromagnetic state, with a magnetic moment of about 1.0 μB per Nitrogen-dopant, is more favorable in energy than the nonmagnetic state, and the ferromagnetic correlations are influenced by the impurity bound state. The magnetic moment mainly arises from p orbital of Nitrogen which substitutes the Oxygen atom, with a little contribution from the Oxygen atoms surrounding Nitrogen atom. The resulting band structure and densities of states agree well with the recent theoretical works. The ferromagnetic ordering temperatures obtained from DFT simulations have been given in detail. Our results show that the pressure enhances the temperature in MgO:N.

ANALYSES ON NONLINEAR COUPLING OF MAGNETO-THERMO-ELASTICITY OF FERROMAGNETIC THIN SHELL—II:FINITE ELEMENT MODELING AND APPLICATION

Based on the generalized vaxiational principle of magneto-thermo-elasticity of a ferromagnetic thin shell established （see, Analyses on nonlinear coupling of magneto-thermo- elasticity of ferromagnetic thin shell--I）, the present paper developed a finite element modeling for the mechanical-magneto-thermal multi-field coupling of a ferromagnetic thin shell. The numerical modeling composes of finite element equations for three sub-systems of magnetic, thermal and deformation fields, as well as iterative methods for nonlinearities of the geometrical large-deflection and the multi-field coupling of the ferromagnetic shell. As examples, the numerical simulations on magneto-elastic behaviors of a ferromagnetic cylindrical shell in an applied magnetic field, and magneto-thermo-elastic behaviors of the shell in applied magnetic and thermal fields are carried out. The results are in good agreement with the experimental ones.

The influence of the Dresselhaus spin-orbit coupling on the tunnelling magnetoresistance in ferromagnet/ insulator /semiconductor/ insulator /ferromagnet tunnel junctions

This paper investigates the effect of Dresselhaus spin orbit coupling on the spin-transport properties of ferromagnet/insulator/semiconductor/insulator/ferromagnet double-barrier structures. The influence of the thickness of the insulator between the ferromagnet and the semiconductor on the polarization is also considered. The obtained results indicate that （i） the polarization can be enhanced by reducing the insulator layers at zero temperature, and （ii） the tunnelling magnetoresistance inversion can be illustrated by the influence of the Dresselhaus spin-orbit coupling effect in the double-barrier structure. Due to the Dresselhaus spin-orbit coupling effect, the tunnelling magnetoresistance inversion occurs when the energy of a localized state in the barrier matches the Fermi energy EF of the ferromagnetic electrodes.

Changes in the natural frequency of a ferromagnetic rod in a magnetic field due to magnetoelastic interaction

Based on the magnetoelastic generalized variational principle and Hamilton＇s principle, a dynamic theoretical model characterizing the magnetoelastic interaction of a soft ferromagnetic medium in an applied magnetic field is developed in this paper. From the variational manipulation of magnetic scale potential and elastic displacement, all the fundamental equations for the magnetic field and mechanical deformation, as well as the magnetic body force and magnetic traction for describing magnetoelastic interaction are derived. The theoretical model is applied to a ferromagnetic rod vibrating in an applied magnetic field using a perturbation technique and the Galerkin method. The results show that the magnetic field will change the natural frequencies of the ferromagnetic rod by causing a decrease with the bending motion along the applied magnetic field where the magnetoelastic buckling will take place, and by causing an increase when the bending motion of the rod is perpendicular to the field. The prediction by the mode presented in this paper qualitatively agrees with the natural frequency changes of the ferromagnetic rod observed in the experiment.

线圈放置方式对脉冲涡流检测的影响分析

[目的]核电厂汽水管线一般在管道外壁加装保温层,从而提高换热效率。目前对于铁磁性管道的检测手段主要为常规超声及超声导波,检测前需要将管道外壁保温层拆除,导致检测工期延长,人力成本增加,无法达到核电厂高质量发展的要求。核电厂脉冲涡流技术的应用可以省去保温层的拆装,实现不停机在线筛查。检测线圈的放置方式对缺陷的检出能力是脉冲涡流技术重要指标。[方法]文章利用ANSYS中的Maxwell模块进行管件建模及仿真,分别设计同轴式与垂直式检测线圈,保持提离距离、材料一致及其他条件一致下,模拟脉冲涡流对平底缺陷的检测能力。选取核电厂样管进行同轴式与垂直式脉冲涡流测试,将脉冲涡流(Pulsed Eddy Current Testing,PECT)测试结果与超声测厚进行复核,对比两种线圈放置方式对脉冲涡流检测的影响。[结果]研究表明:垂直式线圈相对于同轴式线圈对缺陷检出效果更佳。[结论]核电厂脉冲涡流技术的应用对脉冲涡流技术在核电领域实施具有重要意义。

改进条件对抗域适应网络的弱磁信号分类研究

针对小目标铁磁体弱磁场序列图像数据获取困难的小样本问题,采用基于稠密光流法与关联特征对齐的条件对抗域适应网络,解决缺乏标签数据的弱磁信号分类检测任务。算法引入稠密光流法学习磁场序列图像之间的动态特征,并将其与原始图像得到的静态特征相结合,提高分类器对纹理相似图像的鉴别能力;同时引入关联特征对齐模块,减少源域与目标域的特征分布差异。网络模型基于近场磁场分布图像与远场磁场分布图像进行验证评估,与原始算法相比,在有无添加熵条件两种情况下准确率分别提升了5.50%和4.50%。

J-A磁化建模理论的质疑与修正方法研究

J-A理论在磁建模领域应用广泛,但因存在多种版本,致使应用时对分析结果的正确性难以甄别。从无磁滞磁化曲线表达、采用的能量守恒方程等方面出发,对描述材料磁滞现象的原J-A理论的数学描述提出了质疑,指出了其中的不合理及错误之处。基于材料的磁化物理机制,通过分析给出了正确的能量守恒等式,并藉此推导出J-A理论的修正表征形式。由非线性寻优算法求得模型参数后,针对不同材料样本开展了仿真研究,并与测量数据进行比较,结果表明:相比于原J-A模型,本文提出的修正模型的仿真结果与材料的实测磁特性数据吻合较好,可更准确地模拟磁材料的磁化物理过程。

车用电涡流缓速器三维有限元分析

运用电磁位对A Φ A方法和库仑规范,实现了三维涡流场定解问题的完整表述,建立了1/8有限元模型;运用四面体单元,模拟并分析了电涡流缓速器的磁通量分布。由有限元程序求得的电涡流缓速器的制动力矩与试验测量值基本吻合,说明该模型和计算方法是可行的。

改进水热法制备BiFeO_3粉体及其磁性研究

以Bi(NO3)3.5H2O和Fe(NO3)3.9H2O为原料,采用改进的水热法制备了BiFeO3微晶。采用浓度为0.1mol/L的前驱物沉淀制备得到非常均匀的尺度为10μm的微球,经超声分散可以得到100 nm的均匀颗粒。XRD研究表明BiFeO3纳米颗粒具有斜方的钙钛矿结构。扫描电镜结果表明BiFeO3纳米粒子具有立方形貌。而TEM结构分析表明BiFeO3纳米颗粒具有完整的单畴结构;磁性分析表明纳米的BiFeO3颗粒具有弱的铁磁有序。制备的BiFeO3具有Fe-O键的伸缩振动和弯曲振动,证实了Fe-O八面体的存在以及BiFeO3具有钙钛矿结构。

一种消除电磁式电压互感器铁磁谐振方法的探讨与实践

通过对一般消谐方法的介绍与存在问题的分析,提出用测量接地电容器组相电流和零序电流获取系统三相对地电压和零序电压信号以取代PT肩负的绝缘监视功能,解除PT中性点接地,从根本上消除了PT的铁磁谐振;还介绍了该方法在某一个变电所的实施结果,证明其消谐振效果较理想。

基于六面体单元表面磁场积分法求解三维静磁场

三维静磁场计算分析在工程中具有广阔的应用背景。在磁场积分法求解三维静磁问题中,根据均匀磁化体磁化特征将系数矩阵体积分计算形式化简为面积分计算形式,简化了运算且消除了系数矩阵积分计算的奇异性。为适应任意复杂形体剖分,选择非规则六面体作为剖分单元,导出了其单元耦合系数的解析求解方法,从而消除了传统计算中采用数值积分近似计算带来的误差。基于上述两者分析,建立了基于六面体单元表面积分的三维静磁场计算模型。为验证该方法的正确性和有效性,将其应用于地磁场磁化下球壳和空心圆柱体的空间磁特征分析实例中。计算值和实际值的一致表明该方法完全可应用于工程实际的三维静磁场分析,有一定的工程实用价值。

Fe、Ni掺杂ZnO纳米棒的水热法制备与磁性能研究

采用水热法制备稀磁半导体材料,样品Zn0.95Fe0.05O和Zn0.95Fe0.03Ni0.02O的XRD图谱和TEM图谱发现,样品具有纤锌矿结构,形貌为纳米棒状结构。XEDS分析显示,掺杂的Fe和Ni元素进入到了ZnO晶体中。RAMAN光谱表明,Zn2+被Fe2+、Ni2+替换,晶体发生拉曼光谱红移。PL光谱分析发现,在室温条件下,随着Fe2+和Ni2+的掺杂,出现了猝灭现象。VSM测量显示,掺杂样品在室温条件下存在明显的铁磁性,且这种铁磁性属于稀磁半导体的内禀属性。实验结果表明在水热法条件下,获得了ZnO基稀磁半导体材料样品,且样品具有良好的光学和磁学特性,为进一步研究稀磁半导体材料提供了一定的参考。

热处理对高钒高速钢中残余奥氏体量的影响

采用铁磁性法测定了高钒高速钢中残余奥氏体量。研究了淬火温度和回火温度对高钒高速钢残余奥氏体量的影响。结果表明:淬火加热温度升高,残余奥氏体量增加;回火温度升高,残余奥氏体量降低。在试验淬火温度范围(900～1100℃)内450℃以下回火,奥氏体含量变化不明显;回火温度达到550℃时,残余奥氏体含量迅速降低。

Mn和Fe掺杂SnO_2的水热合成与磁结构研究

采用水热法合成了Sn1-x-yMnxFeyO2(0≤x≤0.10,0≤y≤0.10)稀磁半导体.通过XRD,Raman,TEM,SQUID和Mssbeaur等技术对化合物进行了结构和性能的表征.结果表明,XRD中没有出现第二相的沉积,Raman光谱中出现了Mn位于SnO2晶格中的局域模式.磁性测试结果表明,当x=0.10,y=0时,样品在低温下具有较强的磁化强度,但室温下其磁化强度急剧降低.而y=0.10,x=0时,样品的磁化强度和矫顽力都比较小,但随温度的改变变化不大,Mssbeaur谱测试结果表明,其中的Fe一部分是铁磁耦合的,拟合得到超精细场和同质异能移等参数表明,铁磁性来源于Fe替代SnO2本征性能.Mn和Fe共同掺杂的样品的磁化强度随x的减少和y的增加而减少,矫顽力却相对于单一元素掺杂的样品大大增加.

不同掺杂量对水热合成Zn1-xNixO稀磁半导体粉体的影响

本文主要考虑不同掺杂量对水热合成Zn1-xNixO稀磁半导体粉体的影响。采用水热法,以3 mol/L NaOH作为矿化剂,在240℃下,保温24 h左右,进行Ni掺杂(x=0.05,0.1,0.2),合成Zn1-xNixO稀磁半导体晶体。XRD、FE-SEM测试表征晶体的物相组成和晶体形貌,XRD表明所制备的Zn0.95Ni0.05O稀磁半导体晶体发育比较完整。通过UV-vis测试进一步说明掺杂的效果。VSM测试表明,所制备的样品在室温下有良好的磁滞回线,表现出铁磁性。

共沉淀法制备BiFeO_3粉体

以Bi(NO3)3.5H2O和Fe(NO3)3.9H2O为原料,冰醋酸做溶剂,柠檬酸为络合剂,采用共沉淀法制备前驱物,对前驱物沉淀进行退火处理,制备出BiFeO3纳米粉体。研究了煅烧温度和冰醋酸的加入量对BiFeO3纳米粉体的晶粒尺寸及形貌的影响。采用X射线衍射仪(XRD)、场发射扫描电子显微镜(FE-SEM)对所制备的BiFeO3纳米粉体进行表征,用EDS确定了样品的表面成分组成。结果表明:退火温度为450℃时出现BiFeO3衍射峰,但存在杂相;退火温度为600℃时制备出了BiFeO3纯相粉体,粉体为立方体结构。冰醋酸加入量由1mL增大到1.8mL时,粉体的晶粒尺寸由14.21nm减小到11.65nm。通过EDS能谱分析得出BiFeO3粉体由Bi,Fe和O三种元素组成。

激光惯性约束聚变(ICF)玻璃靶丸研究及进展

空心玻璃微球(HGM)在常温常压下对DT气体的渗透率低,机械强度高,在惯性约束聚变靶的制备工艺中是一种常用的DT燃料容器。已经应用于神光I和II系列实验的薄壁玻璃微球,壁厚降到1μm以下,提高了中子产额,并获得内爆特征信息。简要综述了国内外有关惯性约束玻璃靶丸的研究及进展。