Study of structural,dielectric, electric,magnetic and magnetoelectric properties of K0.5Na0.5NbO3 − Ni0.2Co0.8Fe2O4 composites

Magnetoelectric composites with general formula (x) Ni_0.2Co_0.8Fe₂O₄- (1- x) (K_0.5Na_0.5)NbO₃ (x = 0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) have been synthesized by solid state reaction method. X-ray diffraction pattern asserts the existence of both the constituent phases in the synthesized composites. FESEM micrographs are used to investigate the microstructure and for calculation of average grain size of the composites. Temperature dependent dielectric properties are investigated as a function of temperature and found to enhance with addition of ferrite in the composites. P-E hysteresis loops obtained for individual (K_0.5Na_0.5)NbO₃ (KNN) phase and composites indicate the ferroelectric ordering in the composites. Saturation and remnant magnetization show increasing trend with increase in NCFO concentration. FC-ZFC magnetization curves indicate charge ordering, metal-insulator transition (Verwey transition) in NCFO and composites. Impedance spectroscopy shows that bulk resistance reduces with increase in temperature, thereby indicating negative temperature coefficient of resistance (NTCR) behaviour of the composites. The magnetoelectric effect is confirmed by measuring magnetoelectric voltage coefficient, α_ME and the maximum value of α_ME is 5.389 mV/cm-Oe for 20% NCFO-80% KNN composite.     

具有Fe_4N结构的混自旋最近邻的Ising模型磁性质

在晶场作用下对具有Fe4N结构的混自旋Ising模型,采用平均场理论,推导出考虑最近邻相互作用下系统的磁矩和自由能的理论公式,重点考察了最近邻格点的相互作用及晶场对系统磁性质的影响,得到了系统的基态相图和有限温度的磁化曲线,观察到系统的相变特征:重入现象及二级相变。研究发现,二级相变是普遍现象,当DA/|J1|、DB/|J1|取得足够大时,系统一定存在二级相变。基态相图中D1与O1及D2与O1、O2的边界线上会发生重入现象。     

爬壁机器人履带多体磁化结构吸盘的设计及优化

介绍了基于稀土永磁均匀磁化特性而研究设计的有别于传统普通吸盘结构的爬壁机器人履带多体磁化结构吸盘。根据履带吸盘工作状况特点和稀土永磁均匀磁化的特性,首先建立了履带吸盘与金属壁面间隙工作空间的镜像简化模型,推导出间隙工作空间y方向的磁场强度的计算表达式。通过仿真结果和对比试验都证明,该履带吸盘的平均磁场强度和吸力比普通结构吸盘大很多。为得到间隙工作空间磁场强度的分布规律,在忽略永磁体末端效应的假设条件下,建立其多次谐波方程,由此提出了该结构的履带吸盘的优化设计原则。并通过实际应用证明,该结构吸盘完全满足爬壁机器人爬行性能的要求。     

Crystallization and magnetic behavior of nanosized nickel ferrite prepared by citrate precursor method

NiFe₂O₄ nanoparticles have been synthesized by citrate precursor gel formation with subsequent heat treatment. Differential thermal and thermogravimetric (DTA/TG) analyses show that the metal citrates decomposed around 230 °C followed by crystallization of the ferrite. X-ray diffraction (XRD) patterns reveal the formation of the cubic spinel phase in the samples after sintering the gel at 350 °C, 500 °C and 700 °C. For the samples annealed at 350 °C and 500 °C a small amount of α-Fe₂O₃ was detected whereas single phase was obtained for the sample annealed at 700 °C. The lattice constant a for all the samples is comparable to the value of the bulk material. The mean crystallite size D_XRD of the samples determined from XRD line broadening is 26.2-28.5 nm. Transmission electron microscope (TEM) analysis shows that the single-phase particles form clusters with the particle size in the range of 21-82.5 nm and the most probable value D_TEM of 55.4 nm. Magnetic measurements show that its Curie temperature T_C is close to the bulk value while the spontaneous magnetization M_s at 5 K is lower than that of the bulk. The thermal variation of M_s in the temperature range from 5 to 300 K can be best fitted to a modified Bloch T^α law with the exponent value α ≈ 2. The magnetization data are explained with reference to the disordered surface spins and the finite size effects. In this investigated temperature range, the coercive force H_c decreases linearly with increasing temperature. The coercivity mechanism in the nanoparticle sample with broad particle size distribution is expected to be complex and different factors which affect the H_c value were proposed.     

The effect of interparticle forces on fluidization regimes in the magnetized fluidized beds

This paper investigated the influence of interparticle forces on the quality of fluidization in a magnetically stabilized fluidized bed (MSFB), where we can “artificially” create interparticle forces (F_attr) of any magnitude by applying an external magnetic field to ferromagnetic particles. A theoretical model was proposed which predicts the transition point from a homogeneous to a heterogeneous fluidization as a function of the magnitude of the interparticle force and other physical characteristics of both particles and fluids that are usually observed in fluidizationρ _p, ρ_f,μ, d_p, ε). The concept of the elastic wave velocity, U_e, and the continuity wave velocity, U_ε, was introduced. In particular, the interparticle force manipulated by an externally applied magnetic field was taken into account in addition to a general consideration of a conventional fluidized bed. Bubbles form in a bed when the continuity wave velocity becomes faster than the elastic wave velocity. The simulation demonstrated the proposed model could predict the transition point of fluidization regime with reasonable accuracy.     

法向漏磁通随外磁化场和矩形槽尺寸的变化

用磁偶极子模型讨论法向漏磁通随外磁化场和矩形槽尺寸的变化.法向漏磁通的蜂-峰值随外磁化场强度或有效磁感应强度的变化有相似的趋势;法向漏磁通的峰-峰值随矩形槽深度的变化受铁磁材料磁化状态影响;在不同的提离高度时,法向漏磁通的峰-峰值随矩形槽宽度的变化趋势不同;法向漏磁通的峰-峰间隔受提离高度以及矩形槽深度和宽度影响,提离高度＞1mm时,法向漏磁通的峰-峰间隔随提离高度线性增加,矩形槽宽度的影响已不明显.     

大型电力变压器操作冲击试验的一些改进

分析了５００ｋＶ变压器操作冲击试验的现状，提出用直流励磁法代替传统的操作冲击励磁法。     

Improvement of electrical and magnetic properties in La0.67Ca0.33Mn0.97Co0.03O3 ceramic by Ag doping

For the perovskite manganite La_1-xCa_xMnO₃, achieving high temperature coefficient of resistance (TCR) and magnetoresistance (MR) is the key to realize its potential applications. In this study, high-quality La_0.67Ca_0.33Mn_0.97Co_0.03O₃:Ag_x polycrystalline ceramics were prepared by the sol-gel method. The results show that Ag doping has important impact on metal-insulator and ferromagnetic-paramagnetic transitions. A Ag doping amount increases, the grain size of the samples increases at x = 0.05 and then decreases. The doping of Ag can improve the crystalline quality of the samples and enhance the connectivity between grains, thereby improving the metallicity of the system. Additionally, with Ag doping amount increases, the resistivity of the samples gradually decreased, while the Curie temperature and the metal-insulator transition temperature gradually increased. Especially after Ag doping, both the TCR peak (TCR_peak) and the MR peak (MR_peak) values are significantly improved. The TCR_peak reaches 65.2%·K^?1 at x = 0.1, while the MR_peak is as high as 82.6% at x = 0.05 under 1 T magnetic field. Doping perovskite manganite ceramics with Co and Ag can greatly optimize their TCR and MR, favoring the potential applications of these materials.     

Electrical properties of cold-pressing welded NbTi persistent joints

The cold-pressing welding method is employed to fabricate persistent joints between NbTi multifilamentary conductors, and a series of persistent joints are thus made at different mechanical pressures. The electrical properties of these persistent joints are tested by a joint resistance measurement device based on the current decay measurement method. Test results show that the cold-pressing welding method is reliable and applicable to NMR and MRI applications. Experiment results also shows that the joint resistances and current-carrying capacities of the joints seem to have no apparent relevance to the mechanical pressure in a wide range of 4–20 MPa, but the current decay ratio due to flux creep has obvious relevance to the mechanical pressure. Further research is needed to advance this joint manufacturing technology. Besides, measured current increase under 1 T background magnetic field is observed and explained, which has a reference meaning for testing persistent joint resistances.     

电磁力平衡传感器的永磁体性能影响与改善方法

永磁体磁性能的稳定性直接影响电磁力平衡传感器的精度，其自动补偿具有重点意义。文中分析了永磁全磁性能的时间、温度、外磁场、腐蚀、冲击、振动等因素影响，介绍了改善２电磁力平衡传感器永磁体性能、克服上述因素影响的老化方法与补偿方法，实了应用表明这些方法是切实可行的。