Magnetic structure of molecular magnet Fe[Fe(CN)<Subscript>6</Subscript>]·4H<Subscript>2</Subscript>O

We have studied the magnetic structure of Fe[Fe(CN)₆]·4H₂O, prepared by precipitation method, using neutron diffraction technique. Temperature dependent DC magnetization study down to 4.2 K shows that the compound undergoes from a high temperature disordered (paramagnetic) to an ordered magnetic phase transition at 22.6 K. Rietveld analysis of neutron diffraction pattern at 60 K (in its paramagnetic phase) revealed a face centred cubic structure with space group Fm3m. The structure contains three-dimensional network of straight Fe³⁺-C≡N-Fe³⁺ chains along the edges of the unit cell cube. Fe³⁺ ions occupy 4a (0, 0, 0) and 4b (1/2, 1/2, 1/2) positions. Fe³⁺(0, 0, 0) is surrounded octahedrally by six nitrogen atoms and Fe³⁺ (1/2, 1/2, 1/2) is surrounded octahedrally by six carbon atoms. Magnetic Rietveld refinement of neutron diffraction data at 11 K shows a ferromagnetic coupling between the two inequivalent Fe³⁺ sites. Refinement yielded an ordered moment of 4.4(6) and 0.8(6) μ_B per Fe ion located at (0, 0, 0) and (1/2, 1/2, 1/2), respectively. Ordered moments are found to align along the face diagonal. The observed net moment from low temperature neutron diffraction study is consistent with DC magnetization results.     

永磁同步直线电机空载反电势的分层有限元分析

在利用有限元分析永磁同步直线电机空载反电势波形时,二维方法不能反映磁钢的形状而三维方法又过于复杂费时.该文提出一种分层有限元分析方法,很好地克服了这种矛盾.将电机沿横向分层,对每层进行二维有限元分析,最后把每层结果相加得到总体结果.利用该方法计算了一台具有六边形磁钢形状的样机的空载反电势波形.结果表明,该方法与三维有限元分析相比可大大缩短分析时间,得到的波形与实验结果吻合较好.     

使用永磁透镜减小永磁微波离子源引出束流发散的研究

介绍了北京大学重离子物理研究所2.45GHz袖珍永磁微波离子源引出系统的一项改进,即在距该离子源永磁环9cm处加一个长为10cm的永磁透镜。通过计算机模拟和实际测量,对增加此透镜前后比较了磁场分布的变化。模拟计算和实验结果表明,永磁透镜产生的磁场可以有效地聚焦引出的束流,减小引出束流的束斑尺寸。与电磁螺线管透镜相比,该项改进简化了束流传输系统,节省了电功率,并有利于改善离子源区的真空。     

30MeV医用回旋加速器束流输运线上旋转扫描磁铁的研制

加速器引出束流分布一般都是高斯分布, 而在束流应用中需要更多的是均匀分布的束流. 目前国内研究已经实现了束流的均匀分布, 但事实上这些束流的均匀度不够理想. 因此, 重点介绍了能够使束流实现高均匀分布的旋转扫描磁铁的研制过程, 在理论推敲和实践检测的基础上充分证实: 通过旋转扫描磁铁的作用, 束流可以在目标靶上完全实现高均匀分布. 这项技术在国内实属首创, 其性能已经达到国际同类磁铁的先进水平.     

用DSP开发的单自由度磁浮主轴数字控制系统

讨论了以TMS320C542型数字信号处理器（DSP）为核心建立的单自由度磁浮主轴数字控制系统，着重叙述了模数转换（A／D）、PWM产生电路以及DSP与8031的主从连接。实验结果表明，这些电路都是可行的。     

烧结NdFeB磁体的显微组织和磁性的研究

采用能谱、扫描电镜等手段 ,研究烧结NdFeB磁体的显微组织及磁性能。结果表明 ,加入Nb可细化晶粒 ,并有大量的二次晶粒析出 ,使矫顽力提高。Tb的加入量不能太多 ,太多会使矫顽力降低     

基于DSP的PMSM模糊位置伺服系统的研究

在对高精度交流伺服系统的研究中 ,提出了一种基于数字信号处理器 (DSP)的全数字永磁同步电机位置伺服系统 ,采用美国TI公司的TMS32 0F2 4 0系列DSP作为数字控制器 ,充分利用其高速运算能力和面向电机控制的丰富的外围设备 ,设计了一套全数字永磁同步电机位置伺服系统 ,并在永磁同步电机矢量控制的前提下利用模糊控制策略对位置伺服系统进行了控制 计算机仿真和实验结果表明 ,整个控制系统设计简单、可行、有效 ,同时可以取得较好的控制效果     

Nd_2Fe_(14)C/α-Fe系稀土永磁材料微观组织及磁性能

为了提高纳米双相稀土永磁材料Nd2Fe14B/α-Fe的性能,研究了一种新型合金Nd9.0Fe85.5Nb1.0B4.0C0.5。在合金中添加碳可提高矫顽力,添加钕可细化晶粒;合金的淬态微观组织显著影响其磁性能,合金中的部分预析出微晶相有助于在随后的热处理中获得均匀的微观组织;在热处理工艺中,晶化退火温度和时间对合金微观组织结构具有显著影响,并影响合金的磁性能。使用原子力/磁力显微镜观察Nd-Fe-(BC)/α-Fe纳米复合磁体的微观组织及磁畴结构,并由此对纳米双相稀土永磁材料中的交换耦合作用进行了解释。结果表明,最佳热处理工艺为:700℃保温15min,其性能为:剩磁1.381Wb.m-2,矫顽力518.05kA.m-1,剩磁比0.74,最大磁能积137.75kJ.m-3。     

永磁无刷直流电机转矩波动及其抑制的探讨

研究了永磁无刷直流电机的转矩特性．根据定子电流和转子反电势推导出了转矩表达式，根据反电势和电流的各次谐波分量之值可以计算出转矩的各次谐波分量．对于一台给定的电机，只要其反电势一定，通过优化定子电流就可以消除主要的转矩谐波分量，从而达到减小转矩波动的目的     

Unusually Strong Dipole–Dipole and Dipole–Quadrupole Interactions in a Nanoporous Solid. Crystal Structure and Related Properties of (VO)3[M(CN)6]2·nH2O (M = Fe,Co)

In porous Prussian blue (PB) analogues, the partially naked central metal atoms found at the cavities surface are responsible for many of their physical properties, among them the adsorption potentials. In the as‐synthesized PB analogues, such metal sites stabilize water molecules inside the cavity through coordination bond formation. The filling of the cavity volume is completed with water molecules linked to the coordinated ones through hydrogen bonds formation. Vanadyl‐based PB analogue shows quite different features. The metal(V) at the cavities surface has saturated its coordination sphere with the O atom of the vanadyl ion (V=O). In this material, the V=O group preserves enough strong dipole moment to stabilize adsorbed species at the cavity through dipole–dipole and dipole–quadrupole interactions. This contribution reports the preparation, crystal structure and properties for (VO)₃[M(CN)₆]₂ · nH₂O (M = Fe, Co). According to the refined crystal structure, IR spectra and TG data, six water molecules remain stabilized inside the cavities through a strong dipole–dipole coupling with the vanadyl group. The cavity contains additional water molecules interacting through hydrogen bond bridges with the water molecules coupled to the V=O group. The vanadyl ion is free of hydrogen bonding interactions with the water molecules. The recorded adsorption isotherms for N₂, CO₂ and H₂, three molecules with only quadrupole moment, reveal presence of relative strong adsorption forces due to dipole‐quadrupole interactions.