非磁性微球掺杂对纳米磁流体场诱导双折射特性的影响

利用二氧化硅和聚苯乙烯非磁性微球掺入铁氧体纯磁流体中,制得了复合磁流体.定性地研究了复合磁流体在外磁场作用下的双折射与非磁性微球的种类、掺杂浓度以及纯磁流体自身浓度的关系.研究表明,不同浓度的纯磁流体掺杂等量的聚苯乙烯微球对其双折射随磁场的变化趋势影响不同;同一浓度的纯磁流体掺杂不同种类的非磁性微球,对其双折射的影响也...     

环境温度对纳米磁流体场诱导光学双折射的影响

在20~80 ℃温度范围内,研究了两种浓度的铁氧体(主要成分为Fe3O4)磁流体在一系列固定磁场强度(场强范围为0~200 mT)下的双折射与温度的关系.结果表明不同浓度磁流体的双折射具有类似的磁场和温度依赖性.固定磁场强度时,磁流体的双折射值与温度成反比|而温度恒定时,磁流体的双折射值与外磁场的强度成正比|在相同磁场强度、恒定温度下,高浓度磁流体的双折射值比低浓度磁流体的大.详细分析了实验结果,并深入讨论了磁流体双折射的温度、场强和浓度依赖性的物理机理.     

磁性液体复合体在外磁场中的赝双折射效应和二向色性

研究了由非磁性聚苯乙烯颗粒弥散于煤油基Fe_3O_4磁性液体中制备而成的磁性液体复合体.该复合体双折射效应和线二向色性随外磁场变化.在相同的磁场条件下,复合体的双折射效应较纯磁性液体有减弱而二向色性较后者有所增强.文中采用一简化模型对结果给出了解释.     

环境温度对纳米磁流体场诱导光学双折射的影响

在20～80℃温度范围内,研究了两种浓度的铁氧体(主要成分为Fe3O4)磁流体在一系列固定磁场强度(场强范围为0～200mT)下的双折射与温度的关系.结果表明不同浓度磁流体的双折射具有类似的磁场和温度依赖性.固定磁场强度时,磁流体的双折射值与温度成反比;而温度恒定时,磁流体的双折射值与外磁场的强度成正比;在相同磁场强度、恒定温度下,高浓度磁流体的双折射值比低浓度磁流体的大.详细分析了实验结果,并深入讨论了磁流体双折射的温度、场强和浓度依赖性的物理机理.     

两个非磁性颗粒在磁流体中的沉降现象研究

在磁场作用下,在磁流体里添加非磁性颗粒(non-magnetic particles,NPs),可以使得NPs形成不同的结构,操控NPs的运动从而影响磁流体的特性,这种应用逐渐受到了研究者的关注.为了更好地操控磁流体里NPs的运动,本文采用一种多物理模型研究在外加磁场作用下,磁流体中两个NPs沉降的运动过程.其中,用格子玻尔兹曼方法模拟磁流体的运动,外加磁场对磁流体的影响用一种自修正方法求解泊松方程,这个自修正方法可以使欧姆定律满足守恒定律.NPs之间的偶极干扰力采用偶极力模型,同时采用一种相对过渡平滑的共轭边界条件处理NPs与磁流体交界面的流固干扰以避免磁场密度过渡的突变.本文主要探究两个NPs在磁流体中的沉降,揭示磁场作用下NPs的相互干扰原理;同时,对控制NPs运动时的参数进行调节,得到NPs不同的运动轨迹,达到操控颗粒运动的目的.本研究可对NPs在磁流体中的应用提供定量的分析结果,对NPs在工业上的应用提供有力的理论支撑.     

毛细微模塑法制作聚合物微球的有序结构

用毛细微模塑法在玻璃基片上组装了聚苯乙烯微球紧密的有序阵列 .扫描电镜观察了组装后的微球排列 .结果表明 ,在毛细通道的出口端 ,聚苯乙烯的微球堆积得紧密有序 .毛细通道的尺寸 ,环境温度和聚合物微球乳液的浓度是毛细微模塑法的主要影响因素 .     

掺杂下铁磁/反铁磁双层膜中交换偏置的增强

采用了Monte-Carlo方法，讨论了反铁磁层中不同非磁性掺杂浓度下，铁磁/反铁磁双层膜中交换偏置的温度特性. 模拟结果显示：反铁磁层中非磁性掺杂能导致铁磁/反铁磁双层膜中交换偏置的增强. 同时，交换偏置随非磁性掺杂浓度的变化存在极大值，即同一温度下交换偏置随掺杂浓度的变化是非单调的. 并且，随着温度的升高交换偏置的最大值所对应的掺杂浓度向浓度低的方向移动. 它和Hong Jung-Il等人的实验结果完全一致. 究其原因在于反铁磁层相应的自旋排布、磁畴结构等随掺杂浓度的改变发生大的变化，当其正向磁畴和负向磁畴都形成连通的网络结构时，系统的交换偏置达最大. 比较了随机掺杂与规则掺杂的模拟结果. 模拟结果表明规则掺杂能够获得比随机掺杂更大的交换偏置，进一步表明了铁磁/反铁磁双层膜中交换偏置的特性与铁磁/反铁磁界面磁畴结构密切相关.     

磁性液体中非磁性小球与磁性纳米颗粒的相互作用及磁组装

利用磁性液体与聚苯乙烯小球溶液混合得到的复合磁性液体, 研究了聚苯乙烯小球和磁性纳米颗粒在外加磁场作用下的动力学过程. 实验结果表明, 当外加磁场的方向平行于样品平面时, 聚苯乙烯小球在沿着磁场的方向上表现出相互吸引而形成链状结构, 其动力学过程可分为聚苯乙烯小球被反磁化产生相互吸引而形成短链的快过程以及短链间相互吸引形成长链的慢过程; 当外加磁场的方向垂直于样品平面时, 相邻聚苯乙烯小球表现出排斥的相互作用而形成短程有序的二维结构, 当磁场强度增加到一定的阈值时, 聚苯乙烯小球和磁性纳米颗粒形成的团簇会产生相互吸引而组装成复合式的花瓣结构. 关键词： 磁性液体 磁组装 非磁性颗粒     

掺杂单晶硅纳米薄膜杨氏模量的多尺度理论模型

硅纳米材料物理性能的研究对其在半导体技术中的应用是十分重要的. 而掺杂有利于改善硅纳米材料的物理特性, 提高应用价值, 所以本文基于半连续体模型运用Keating形变势, 通过模型计算, 研究了不同位置及不同掺杂浓度的单晶硅纳米薄膜[100]方向的杨氏模量, 分析了掺杂浓度及掺杂位置不同时硅膜杨氏模量与膜厚关系, 结果表明, 与纯硅膜杨氏模量相比, 不同位置的掺杂对硅膜杨氏模量的影响并不明显, 不同浓度的掺杂对硅膜杨氏模量的影响较小. 而随着硅膜厚度的不断增加, 掺杂硅膜杨氏模量与纯硅膜杨氏模量的变化趋势一致, 特别是较小尺寸时的硅膜杨氏模量变化较大. 说明影响硅膜杨氏模量的主要因素是硅膜厚度. 该计算结果对研究硅纳米材料的其他力学特性有一定的参考价值, 也为进一步研究掺杂对纳米硅材料力学性能的影响提供一种全新思路.     

聚苯乙烯相对分子量对薄壁聚苯乙烯空心微球质量的影响北大核心CSCD

激光惯性约束聚变(ICF)作为探索受控核聚变的有效途径,有望获得清洁无污染的能源,而薄壁聚苯乙烯(PS)空心微球是ICF物理实验中亟需的一类微球。针对薄壁空心微球因径厚比(直径/壁厚)增大导致其在干燥、使用中易开裂的问题,研究了PS原料对薄壁微球质量的影响,探讨了其影响机制。结果表明:当油相PS质量分数为4%时,随着油相粘度增加,W1/O/W2复合乳粒稳定性逐渐提高;当油相质量分数不低于8%时,复合乳粒稳定性良好。PS原料对微球表面粗糙度影响较小,微球球形度和壁厚均匀性随初始油相粘度的增大而降低,在干燥过程中微球开裂率随原料力学性能提高而减小。在外水相中引入氟苯(FB)液滴,延缓固化速率,可减小油相粘度增加对微球球形度和壁厚均匀性的不利影响。     

铁电纳米材料BaTiO_3的激光拉曼光谱分析

铁电纳米材料BaTiO_3具有尺寸均匀,形貌统一的单晶纳米立方体结构。以LRS-Ⅲ型激光拉曼/荧光光谱仪为测量仪器研究其在固体纳米材料BaTiO_3分析中的应用。用碱热法制作得到4种无掺杂铁电纳米材料BaTiO_3,分别标记为纯1、纯2、纯3、纯4,将四种固体纳米材料作为标样待测;另取两个不同浓度Eu(铕)元素掺杂的材料分别标记为掺杂1、掺杂2待测。本实验中,不仅对比分析了不同烧结温度和时间下的固体纳米材料BaTiO_3激光拉曼光谱,并做出了相应的解释说明。与此同时,对掺杂BaTiO_3的也做了拉曼光谱对比分析,研究掺杂对固体材料拉曼谱线的影响。研究结果表明,含有相同成分的固体材料其拉曼谱线相似,且不同掺杂引起拉曼谱线的变化不同。最后用相关系数对掺杂材料和无掺杂材料做了进一步分析。从总体来看类内相关系数较高而类间相关系数较低,但即便掺杂的量比较少,对相关系数也有明显的影响。结合相关系数可以明确地判断出两种物质是否同类,如果已知某一种没有掺杂,那么,就可以很容易地判断出另一种材料中是否有掺杂。     

Distribution of micrometer-size particles in magnetic fluids in the presence of steady uniform magnetic field

In this paper, distribution of suspended micrometer-size particles in magnetic fluids is investigated. Microstructure formation of particles in magnetic fluids is simulated by using the discrete particle method based on the simplified Stokes dynamics. Not only magnetic particles but also nonmagnetic particles are rearranged in the field direction and form chain-like clusters due to the apparent magnetization in magnetic fluids in the presence of magnetic field. When the diameter of nonmagnetic particles is smaller than that of magnetic particles, nonmagnetic particles move into the empty space of microstructure of magnetic particles, and they are rearranged in the field direction. Uniformity of distribution of particles on the plane perpendicular to the field direction is maintained even after microstructure formation.     

The study on microstructure and microwave-absorbing properties of lithium zinc ferrites doped with magnesium and copper

Lithium zinc ferrites doped with magnesium and copper were prepared by means of a combination of sol-gel method and subsequent calcination. The crystalline phase and microstructure of different doped lithium zinc ferrites were measured by X-ray powder diffraction and scanning electronic microscopy analysis. The results indicate that there are no remarkable differences in phase composition between pure lithium zinc ferrite and the as-doped lithium zinc ferrites. The effects of magnesium and copper dopants on microwave absorption in low-frequency region were investigated by the transmission/reflection coaxial line method. It was found from the present work that doping with copper improved microwave-absorbing properties, while doping with magnesium had little effect on microwave absorption of pure lithium zinc ferrite.     

Magnetization of beryllium oxide in the presence of nonmagnetic impurities: Boron,carbon, and nitrogen

The electronic and magnetic states of a nonmagnetic insulator, namely, beryllium oxide, doped with nonmagnetic 2p elements (boron, carbon, and nitrogen) are studied using the density functional theory. The spin polarization of the 2p impurity states, as well as the transition of the doped BeO:(B,C,N) systems to the states of semiconducting or half-metallic magnets, is observed. The prospects for creating new magnetic materials by doping nonmagnetic insulators with nonmagnetic p impurities are discussed.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Zn1-xMnxS(001)稀磁半导体薄膜的能量稳定性、磁性和电子结构

通过基于广义梯度近似的总能密度泛函理论研究不同Mn掺杂浓度的ZnS(001)薄膜的电学和磁学特性. 计算单个Mn原子和两个Mn原子处于各种掺杂位置及不同的磁耦合状态时的能量稳定性.计算了单个Mn原子掺杂和两个Mn原子掺杂的ZnS(001)薄膜的态密度. 不同掺杂组态的p-d杂化的程度不同. 不同掺杂组态,Mn原子所处的晶场环境不同,所以不同掺杂组态的Mn的3d分波态密度峰的劈裂有很大的不同. 掺杂两个Mn原子时,得到三种稳定组态的基态都是反铁磁态. 分析了以上三种能量稳定的组态中,两个Mn原子在不同磁耦合状态下的3d态密度图. 当两原子为铁磁耦合时,由于d-d电子相互作用,使反键态的态密度峰明显加宽. 随着Mn掺杂浓度的增加,Mn原子有相互靠近,并围绕S原子形成団簇的趋势. 对于这样的组态,Mn原子之间为反铁磁耦合能量更低.     

Ag-Cr共掺LiZnP新型稀磁半导体的光电性质

采用基于密度泛函理论的第一性原理平面波超软赝势法,对纯Li Zn P, Ag/Cr单掺和Ag-Cr共掺Li Zn P新型稀磁半导体进行了结构优化,计算并分析了掺杂体系的电子结构、磁性、形成能、差分电荷密度和光学性质.结果表明:非磁性元素Ag单掺后,材料表现为金属顺磁性;磁性元素Cr单掺后, sp-d杂化使态密度峰出现劈裂,体系变成金属铁磁性;而Ag-Cr共掺后,其性质与Ag和Cr单掺完全不同,变为半金属铁磁性,带隙值略微减小,导电能力增强,同时形成能降低,原子间的相互作用和键强度增强,晶胞的稳定性增强.通过比较光学性质发现,掺杂体系的介电函数虚部和光吸收谱在低能区均出现新的峰值,且当Ag-Cr共掺时介电峰峰值最高,同时复折射率函数在低能区发生明显变化,吸收边向低能方向延展,体系对低频电磁波吸收加强.     

Magnetic resonance study of Ce and Gd doped NiFe2O4 nanoparticles

Present work is a study of temperature dependent electron paramagnetic resonance spectra of Ce and Gd doped nickel ferrite nanoparticles. The samples, synthesised by chemical route were characterised by X-ray diffractometer, electron paramagnetic resonance spectroscopy (EPR) and vibrating sample magnetometer (VSM). The average crystallite size of pure nickel ferrite is ∼64 nm and for Gd and Ce doped samples it is ∼20 nm and ∼14 nm, respectively. The EPR spectra were recorded from 120 to 300 K. Doping with Gd and Ce reduces the line width and g-value in comparison to that of pure nickel ferrite. Ce doped samples have the lowest values of both these parameters at room temperature. This indicates that Ce doped samples show lowest loss and is suitable for high frequency devices. EPR spin numbers are reduced while the spin relaxation time is increased after doping with rare earth ions. Gd doped samples have higher values of relaxation time and lower spin numbers in comparison to that of Ce doped samples. VSM results show that the magnetisation and coercivity are reduced after doping with both Ce and Gd rare earth ions.     

Dynamic rheological properties of viscoelastic magnetic fluids in uniform magnetic fields

The dynamic rheological properties of viscoelastic magnetic fluids in externally applied uniform magnetic fields are investigated by a laboratory-made cone-plate rheometer in this study. In particular, the effects of the magnetic field on the viscoelastic properties (the complex dynamic modulus) of the viscoelastic magnetic fluids are studied. In the investigation, three viscoelastic magnetic fluids are made by mixing a magnetic fluid and a viscoelastic fluid with different mass ratios. As a supplementation to the experimental investigation, a theoretical analysis is also presented. The present study shows that the viscosity and elasticity of the viscoelastic magnetic fluids are significantly influenced by the magnetic field and the concentrations of the magnetic particles in the test fluids. Theoretical analysis qualitatively explains the present findings.