磁矩翻转动力学及研究进展

如何控制单畴磁纳米粒子的磁矩翻转对于磁性数据的存取以及自旋电子学都具有十分重要的意义。本文介绍了斯通纳-沃尔法特极限、斯通纳粒子的概念以及斯通纳粒子磁矩翻转的相关理论及研究进展。     

低浓度纳米颗粒在剪切流中扩散、吸附的简单模型

剪切流场中粒子扩散、吸附的微观机理探索为提高分离、传递等典型化工过程的效率提供理论依据。针对旋流分离过程中低浓度污染物纳米颗粒在剪切流场中被螯合剂大颗粒(又称吸附大颗粒)吸附及扩散过程构建近似理论模型, 研究了污染物颗粒扩散及螯合剂捕集吸附与剪切流速度的关系。结果表明, 剪切流的存在对于颗粒的吸附过程起到强化的效果, 剪切流速度越大效果越强, 而吸附的方向(朝向或远离吸附大颗粒)只取决于吸附大颗粒及溶液的特性, 此外低浓度污染物纳米颗粒被捕集吸附的过程主要与颗粒的扩散有关。推导了扩散时间的简洁解析表达式, 扩散时间与吸附大颗粒浓度的4/3次方呈反比, 与粒子的体积大小呈正比。剪切流对吸附的影响及捕集吸附时间的估算, 为物理化学吸附结合旋流分离处理污染物等技术开发提供理论指导。     

声场强化微小流化床流动特性研究

在内径4.3mm微小流化床中,考察了声场对FCC及石英砂颗粒流化质量的影响。重点讨论了声压级与频率对微小流化床最小流化速度的影响。结果表明,声场能改善微小流化床流化质量。尤其对于51μm石英砂颗粒,声场可以使其消除沟流,实现稳定流化。声压越大,声场对微小流化床流化质量改善越明显。最小流化速度随声压增高呈单调下降趋势。相同声场条件下,声波对微小流化床最小流化速度数值降低幅度大于大尺度流化床。声场对微小流化床最小流化速度的影响存在最佳频率。但不同颗粒的最佳频率不同。内径4.3mm流化床,51,67,83μm石英砂颗粒与83μmFCC颗粒对应的最佳频率分别为90,90,140和140Hz。在一定的声压与频率下,声场可以降低最小流化速度约9%～21%。对于微小流化床,床径越小,则床层空隙率越大,越有利于实现外场强化,最小流化速度的降低幅度也逐渐增大。     

流体混合物纳米管膜体系传递的分子动力学模拟

本文使用分子动力学模拟研究了三种不同尺寸粒子的二元混合流体在三种管径纳米管膜体系中的传递现象。研究表明纳米管径与流体尺寸相当时流体仅能单列通过,流体粒子尺寸越大的体系,所需驱动压力越大。纳米管径较大时,混合流体粒子尺寸对驱动压力影响较小。混合流体中大粒子尺寸越大,进入纳米管的流量越小。大粒子流体在上游数密度比下游的大,小粒子流体则相反。两种粒子尺寸差距越大,浓度差异越大。     

均匀沉淀法制备不同粒径的纳米Cu（OH）2

以无水硫酸铜和氢氧化钡为原料,采用均匀沉淀法研究了不同粒径纳米Cu（OH）2的制备,考察了搅拌温度、硫酸铜溶液浓度等反应条件对纳米Cu（OH）2粒径的影响。实验结果表明,用均匀沉淀法可以制备出不同粒度的纳米Cu（OH）2颗粒。反应条件对纳米Cu（OH）2的粒径有显著影响,反应温度越高,纳米粒子的粒径越小;硫酸铜溶液的浓度越高,纳米粒子的粒径越大。     

油水两相间界面张力系数对胶凝原油单颗粒运动状态影响的数值模拟研究

利用相场法对胶凝原油单个颗粒的运动状态进行数值模拟,分析了不同油水两相间的界面张力系数对胶凝原油原油颗粒运动的影响。结果表明,油水两相间界面张力影响胶凝原油颗粒在运动中的形变,界面张力越大胶凝原油颗粒变形越不明显。胶凝原油颗粒的形变影响颗粒上各点的速度,形变越剧烈胶凝原油颗粒上点的速度变化幅度越大。模拟结果为进一步开展胶凝原油颗粒流动特性的研究奠定了理论基础。     

沉淀转化法制备不同粒径的纳米氧化镁

以氧化镁和碳酸铵为原料,采用沉淀转化法研究了不同粒径纳米氧化镁的制备,讨论了加料方式、反应物配比、反应温度、煅烧温度等条件对其粒径的影响。实验结果表明:用沉淀转化法制备出不同粒径的纳米氧化镁颗粒;加料方式对纳米氧化镁的平均粒径有影响,滴加加料法比一次加料法制备的氧化镁粒径大;反应条件对粒径也有显著影响,反应物配比越大,纳米氧化镁粒径越大;反应温度越高,其粒径越小;煅烧温度越高,其粒径越大。     

超声场中蔗糖酯-纳米氧化银制备研究

以硝酸银和氢氧化钠为原料,蔗糖酯(SE)为表面活性剂,利用化学法制备纳米氧化银材料。考察不同功率和作用时间的超声场对纳米氧化银材料形貌和粒径的影响。运用场发射扫描电镜/能谱仪(FESEM/EDS)及X-射线衍射仪(XRD)对样品进行表征。结果表明,在氢氧化钠浓度为0.008 mol/L、硝酸银浓度为0.008 mol/L、蔗糖酯质量浓度为1.6 g/L、反应温度为70.0℃、超声场功率为300 W、超声场作用时间为90 min时,制得最小粒径为48.41 nm且分散性良好的纳米氧化银材料。说明超声场能够有效减小纳米氧化银粒子粒径,提高纳米氧化银颗粒分散性并抑制团聚现象。     

不同粒径纳米碱式碳酸锌的制备

以碳酸钠和氯化锌为原料,研究了直接沉淀法制备不同粒径的纳米碱式碳酸锌的工艺条件,讨论了工艺条件对粒径的影响规律。实验结果表明：采用直接沉淀法可以制备出平均粒径为7-16nm的纳米碱式碳酸锌;反应条件对纳米碱式碳酸锌的粒径有显著影响,反应物浓度越大,纳米粒子的粒径越大;反应温度越高,纳米粒子的粒径越小。     

三氧化二铝纳米流体粘滞度的实验研究

使用直接混合法制备三氧化二铝纳米流体,探讨三氧化二铝纳米流体在不同浓度(0%,0.1%,0.5%,1.0%,ω)和温度(10～40 ℃)下粘滞度的变化规律.使用Brookfield DVⅢ 流变仪进行测量,发现添加入纳米粒子于去离子水中所得三氧化二铝纳米流体符合牛顿粘性定律,为牛顿流体.任一纳米流体的粘滞系数随着温度的上升而下降,呈反比关系,而随浓度上升而增加,呈正比关系,在温度40 ℃和浓度为1.0%(ω)时粘滞度增大比率可达到25.2%.而压降在任何浓度的纳米流体均与去离子水相差大约5 MPa,显示加入纳米粒子对压降的影响不明显.当温度愈高时,压降也随着降低,代表未来可将纳米流体应用层面推向更高温的领域.     

The damping behavior of magnetorheological gel based on polyurethane matrix

As a kind of new MR material, Magnetorheological Gel (MRG) can be considered to be the intermediate system between the MR fluids (MRF) and MR elastomers (MRE). And damping performance plays a significant role in its application system (especially in the vibration and noise control). However, the damping mechanism and source of MRG have not been investigated comprehensively. In this study, several samples of MRGs with different iron particle contents (0, 20, 40, 60, and 80 wt%) were prepared. The nonmagnetic damping (without magnetic field) and magneto‐induced damping (with several constant magnetic fields) of MRG were systematically studied by using an advanced commercial rheometer under oscillatory shear modes. The influence of time history, strain amplitude and frequency in the zero‐field and several constant magnetic fields on the nonmagnetic damping and magneto‐induced damping of MRG was also systematically studied. The experimental results show that, just the opposite to MRE, the loss factor of MRG decrease with the increase of carbonyl iron powder particles (CIP) content. The reasons are known through theoretical analysis combined with microstructure observation that the polyurethane molecular chain will become shorter with the increase of CIP content is a very important factor, and the formation and thickening of the chain structure of CIP is the reason for the loss factor of MRG decline with the magnetic field. POLYM. COMPOS., 38:1248–1258, 2017. © 2015 Society of Plastics Engineers     

Effect of magnetic field on electrical properties of nanocrystalline poly(vinylidene fluoride) samples

BACKGROUND: The electrical properties of nanocrystalline poly(vinylidene fluoride) (PVDF) samples of 20 µm in thickness were measured in terms of thermally stimulated current (TSC), conduction current and dielectric constant after application of a magnetic field. RESULTS: TSC shows the release of trapped charges inside the material that enhances the current with magnetic field. The reason for the polarity reversal of the current with reversal of the magnetic field polarity is due to the change in spin of electrons depending upon the direction of the magnetic field. CONCLUSION: The magnetic field causes trapping of charge carriers in different traps, as the reason for the increase of activation energy with increasing field. The flow of conduction current at constant temperature in magnetically polarized PVDF is governed by Poole–Frenkel and Schottky–Richardson mechanisms. The decrease in dielectric constant at a certain alternating current (AC) frequency and magnetic field with temperature is caused by magnetic polarization in addition to the AC field. Copyright © 2009 Society of Chemical Industry     

The magnetization reversal and high temperature dielectric response in Y1−xHoxFe0.5Cr0.5O3

The structural, magnetic, and dielectric properties of the Y_1−xHo_xFe_0.5Cr_0.5O₃ (x=0, 0.05, 0.1, 0.3, and 0.5) compounds have been investigated. Rietveld refinement of the XRD patterns shows that the compounds possess orthorhombic perovskite structure. The dual magnetization reversal is observed in the samples with x=0.05 and 0.1, and it vanishes when x≥0.3. Ferromagnetic-like behavior with large coercive fields is observed in all Ho³⁺ doped YFe_0.5Cr_0.5O₃ samples, indicating a doping induced metamagnetic behavior. This abnormal magnetization behavior can be explained by the antiparallel magnetic coupling between the Ho³⁺ and the canted Cr³⁺/Fe³⁺ moments, as well as the Ho–O–Ho magnetic interaction. The dielectric behavior in the frequency range from 100 Hz to 10 MHz is investigated. The low doped samples (x=0, 0.05, and 0.1) exhibit relaxation-like dielectric behavior and colossal dielectric constant in a wide temperature and frequency range. The dual magnetization reversal under low magnetic field makes these materials attractive candidates for the magnetic dual sensor devices.     

Magnetization reversal in asymmetric trilayer dots: effect of the interlayer magnetostatic coupling

The spin structure and magnetization reversal in Co/insulator/Fe trilayer nanodots are investigated by micromagnetic simulations. The vortex and C-state are found and the magnetization reversal is dominated by the shape asymmetry of the dots, which is produced by cutting off a fraction of the circular dot. The vortex chirality is thus controlled by the magnetic field direction. On the other hand, the magnetostatic interaction between the top and bottom magnetic layers has interesting influence on the dot reversal process, where the magnetocrystalline anisotropy direction of the Co layer is allowed to vary within the layer plane. The combined effect of these two aspects is discussed on the base of dot coercivity, remanent magnetization, vortex nucleation and annihilation, and the bias of the Fe layer hysteresis loop. While leading to a new S-state in circle dots, the interlayer interaction facilitates the formation of C-state in asymmetric dots, which reduces the vortex nucleation field. The bias effect of all dots is decreased with the deviation of the Co layer easy axis from the field direction. Unlike the circle and semicircle dots, the field range of the vortex state in other asymmetric dots increases with the angle between the cutting direction and the Co layer anisotropy. Additionally, vortex ranges in less asymmetric dots even larger than that in the circle dots are evidenced unexpectedly. Therefore, the control of the vortex chirality and enhancement of the vortex range are found simultaneously.     

Stable microwave-assisted magnetization switching for nanoscale exchange-coupled composite grain

Magnetization mechanisms of nanoscale magnetic grains greatly differ from well-known magnetization mechanisms of micrometer- or millimeter-sized magnetic grains or particles. Magnetization switching mechanisms of nanoscale exchange-coupled composite (ECC) grain in a microwave field was studied using micromagnetic simulation. Magnetization switching involving a strongly damped or precessional oscillation was studied using various strengths of external direct current and microwave fields. These studies imply that the switching behavior of microwave-assisted magnetization switching of the ECC grain can be divided into two groups: stable and unstable regions, similar to the case of the Stoner-Wahlfarth grain. A significant reduction in the switching field was observed in the ECC grain when the magnetization switching involved precessional oscillations similar to the case of the Stoner-Wohlfarth grain. This switching behavior is preferred for the practical applications of microwave-assisted magnetization switching.     

Phase reversal during water dispersion of polyurethane anionomer

A series of polyurethane anionomers (PUas) was synthesized from isophorone diisocyanate, polyneopentanediol adipate (P756), and hydrophilic dimethanol propionic acid (DMPA). The variation of electric conductivity and viscosity of such anionomers during water dispersion was followed. The results showed that the average diameter and polyindex of particles were reduced, the viscosity of dispersion was lowered, the time necessary for phase reversal became shorter, if the amount of DMPA or the temperature, at which the water dispersion was carried out, increased. DSC test revealed that the order of hard segments was disturbed after water dispersion. IR spectra demonstrated that, after the chain extender ethyl diamine was added into the PUa dispersion, the hydrogen bonding of NH with carbonyls was stronger, if the dispersion temperature and the mass fraction of DMPA increased. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1234–1239, 2006     

Influence of magnetic field on jet impingement heat transfer with molten salt

Experimental study of jet impingement heat transfer with molten salt under the influence of external constant magnetic field was generated by permanent magnets. Both stagnation correlation and radial distribution of Nusselt number under magnetic field were obtained. The results showed that the Nusselt number with magnetic field became higher than that without magnetic field at stagnation region and jet impingement heat transfer was comparatively enhanced, while in wall jet region, the enhancement of heat transfer was gradually weakened. In addition, when the Reynolds number was constant, the Nusselt number of molten salt increased with increasing of the intensity of magnetic field, and the most enhanced heat transfer existed at the stagnation point. Under the conditions of Reynolds number Re=6400 and the intensity of magnetic field B=2800 Gs, the stagnation Nusselt number of molten salt was about 6 % higher than that without magnetic field. It can be seen that the magnetic field may promote the jet impingement heat transfer of molten salt.     

磁场对熔盐射流冲击传热的影响

以永磁铁构建定磁场，进行外加磁场作用下熔盐射流冲击传热的实验研究，并得到Nusselt数Nu驻点关联式和径向分布。结果表明，在驻点区范围内，Nusselt数较无磁场作用时增大，传热得到比较明显的增强，而在壁面射流区，这种强化传热效果逐渐减弱。此外，当Reynolds数Re一定时，熔盐Nusselt数随着磁场强度的增加而增大，且驻点处强化传热效果最为显著。在Reynolds数Re=6400与磁场强度B=2800 Gs条件下，熔盐驻点Nusselt数Nu₀提高约6％，可见磁场作用对熔盐射流冲击传热具有一定的强化效果。     

Magnetization reversal of Co/Pd multilayers on nanoporous templates

By making use of an e-beam deposition system, the [Co(2 Å)/Pd(10 Å)]₁₅ multilayers were prepared on a Si(100) substrate and anodized aluminum oxide [AAO] templates with average pore diameters of around 185, 95, and 40 nm. The mechanism of magnetization reversal of the Co/Pd multilayers was investigated. Wall motion was observed on the Co/Pd multilayers grown on the Si substrate. A combination of wall motion and domain rotation was found in the sample grown on the AAO template with a 185-nm pore diameter. For the samples grown on the AAO templates with pore diameters of around 95 and 40 nm, the reversal mechanism was dominated by domain rotation. The rotational reversal was mainly contributed from the underlying nanoporous AAO templates that provided an additional pinning effect.PACS: 75.30.Gw, magnetic anisotropy; 78.67.Rb, nanoporous materials; 75.60.Jk, magnetization reversal mechanisms.