壳聚糖与聚乙烯醇磁性水凝胶的制备和性能研究

采用反复冷冻-融化技术(冻融)制备了Fe3O4/PVA/CS磁性水凝胶,并以相同方法制备了不交联、不掺磁的PVA/CS共混水凝胶和交联、不掺磁的PVA/CS交联水凝胶,研究对比了三类水凝胶的溶胀性能、再溶胀性能和力学性能随组分配比及冻融次数的变化行为,并研究了Fe3O4/PVA/CS磁性水凝胶的磁性能.结果表明:随着交联剂和Fe3O4的加入和冻融次数的增加,水凝胶的溶胀率减小,再溶胀速率减缓,但力学性能却呈现出增大趋势.Fe3O4/PVA/CS磁性水凝胶各个样品的磁滞回线皆剩磁很少,矫顽力很小,且剩磁和矫顽力的大小取决于Fe3O4掺量的大小,与冻融次数基本无关,随着Fe3O4掺量的增加剩磁和矫顽力都增大.     

Ce、Sr共掺杂BiFeO_3的铁电性及Raman谱研究

采用溶胶凝胶法成功制备了Ce_(0.05)Bi_(0.95-x)Sr_xFeO_3(x=0,0.05,0.10,0.15)多铁性样品,并且对样品的XRD图谱、电滞回线、漏电流曲线及Raman光谱进行测量与研究。XRD结果表明,随着Sr离子掺杂浓度的增加,样品由钙钛矿结构转变为四方晶格结构。电滞回线显示随着Sr离子掺杂浓度的增加,样品的室温铁电性得到了改善。漏电流曲线表明Sr离子掺杂浓度的增加并不能很好地抑制漏电流的产生。通过Raman谱分析得出,Sr离子的掺杂导致Bi-O共价键结构发生变化,并且Sr离子浓度的增加使氧八面体扭曲程度加剧,进而引起了漏电机制的改变,使得样品的铁电性能发生了变化。     

振动样品磁强计在磁记录介质中的应用

采用7407型振动样品磁强计(VibratingSampleMagnetometer)对(FePt/C)10磁记录多层膜的磁学性质进行了测量,包括磁滞回线、直流退磁曲线(DCD)、等温剩磁曲线(IRM)及磁化强度与时间(M-t)的关系。通过磁滞回线可直接得到Hc、Mr、Ms、S*、Mr/Ms、SFD等宏观磁学性能参数;由DCD和IRM得到的δM曲线可用来研究晶粒间交换耦合作用;磁反转体积(Va)的计算通过DCD、M-t实验可实现,由此可以确定磁记录介质的热稳定性。     

关于密实化γ-Fe_2O_3的流度学特性和磁性参数

本文分析了几种γ-Fe_2O_3样品的磁特性(矫顽力、剩余矫顽力、剩磁与饱和剩磁之比、矫顽力系数)和粒子的流变学特性(轻敲密度、表现密度和可湿性)。样品有如下三种:1.从密度时间不同的α-Fe_2O_3制得的γ-Fe_2O_3;2.从密度实器中取出来的密实化时间不同的γ-Fe_2O_3;3.市场上的密实化和非密实化磁粉。作为密实化程度函数(也就是与表现密度的关系曲线)的磁性数据,证实了粒子间的相互作用增加长松散粒子重新聚集成单颗粒、颗粒束和环形状态分布。样品Ⅰ与样品Ⅱ的可湿性大不相同。假设密实化使粒子产生较为“清洁”的表面 ,从而产生了易于湿润的活性表面。样品Ⅱ恰好证实了这一设想。     

振动样品磁强计在磁记录介质中的应用

采用 74 0 7型振动样品磁强计 (VibratingSampleMagnetometer)对 (FePt/C) 1 0 磁记录多层膜的磁学性质进行了测量 ,包括磁滞回线、直流退磁曲线 (DCD)、等温剩磁曲线 (IRM)及磁化强度与时间 (M -t)的关系。通过磁滞回线可直接得到Hc、Mr、Ms、S 、Mr/Ms、SFD等宏观磁学性能参数 ;由DCD和IRM得到的δM曲线可用来研究晶粒间交换耦合作用 ;磁反转体积 (Va)的计算通过DCD、M -t实验可实现 ,由此可以确定磁记录介质的热稳定性。     

Fe/Pt/Cu纳米颗粒膜晶粒间的相互作用

本文对不同厚度的Fe/Pt/Cu纳米颗粒膜进行了等温剩磁与直流退磁剩磁曲线分析,并将其与Stoner-Wohlfarth模型对比研究。结果表明,Fe/Pt/Cu多层膜的磁性颗粒间的相互作用随着厚度的增加而增强。在剩磁曲线中,Ir(15000Oe)亦随着膜厚增加而增大;从而可以推断,晶粒间相互作用的增强能减小多层膜的剩余磁化强度,提高矫顽力。     

Fe_(0.97)Pt_(0.03)纳米线的制备与磁性研究

以FeSO4与H2PtCl6的混合溶液作为沉积液,采用电化学沉积法成功地制备了Fe0.97Pt0.03合金纳米线.磁性测量表明,样品的矫顽力为1925Oe,剩磁比为0.95,磁各向异性显著提高。δM曲线,起始磁化曲线及回复曲线的分析表明,纳米线之间以及纳米线内部的主要相互作用可归结为静磁相互作用;同时发现,回复曲线的面积随磁场增大而增大。     

化学镀CoWP薄膜的制备及磁性研究

利用化学镀的方法制备了CoWP磁性薄膜。研究了施镀时间对化学镀CoWP薄膜矫顽力和饱和磁化强度的影响,并利用场发射扫描电子显微镜(FSEM)、X射线衍射(XRD)和振动样品磁强计(VSM)研究了CoWP磁性薄膜的表面形貌、成分、微结构及磁学性能。结果表明,化学镀CoWP薄膜主要为hcp结构Co相,晶粒大小为2~3μm;随着施镀时间的增加,CoWP薄膜的饱和磁化强度也随之增大,最高达到122emu/g;CoWP薄膜的矫顽力都在250Oe以下。通过计算Kelly-Hankel(δM)曲线,证实了CoWP磁性薄膜中存在交换耦合作用,产生了剩磁增强效应。     

烧结SrRuO_3的加场冷却(FC)与零场冷却(ZFC)磁化强度的研究

在5K到磁有序温度范围内,测量了SrRuO3样品在不同外场下的ZFC和FC磁化强度曲线。同时测量了样品在不同温度下的磁滞回线,并由此得到各温度下的矫顽力。通过分析ZFC和FC磁化过程的磁矩状态,提出一个经验公式,借助各温度下的矫顽力来计算或分析ZFC磁化强度。计算得到的ZFC磁矩与实验测量得到ZFC很好地符合。守析结果显示,不同ZFC变化规律起源于FC和矫顽力随温度变化之间的竞争结果。     

水热法合成单磁畴M型BaFe12O19超细粉体及其磁性能表征

采用水热法分别在不同条件下制备单磁畴M型BaFe12O19磁性颗粒．用X射线衍射仪、透射电镜、振动试样磁强计和网络分析仪对产物的组成、微观形貌和电磁性能进行了表征分析．结果表明,230℃水热反应在n（Fe）／n（Sa）和n（OH）／n（NO3^-）分别为8和3时得到平均晶粒为52nm的单磁畴BaFe12O19磁性颗粒;产物具有6．1×10^-3T饱和磁化强度和8．75×10^4A／m矫顽力;试样在3～15GHz宽频范围内具有至少-5dB的反射率,因此水热合成的BaFe12O19铁氧体具有宽频微波吸收能力．     

α-Fe/BaFe12O19复合粉体的有机凝胶–热还原法制备及磁性能

以柠檬酸和金属盐为原料,采用有机凝胶–热还原法制备了α-Fe/BaFe12O19软硬磁复合粉体。通过X射线衍射、扫描电子显微镜和振动样品磁强计对还原产物的物相、形貌及磁性能进行了表征。结果表明:复合粉体的磁性能与两相的组成及还原工艺有关。随着还原温度升高和时间延长,复合粉体的饱和磁化强度逐渐升高,矫顽力却先下降后上升。在氢气–氮气中经375℃还原1h后,制备的α-Fe/BaFe12O19复合粉体的比饱和磁化强度为58.04A.m2/kg,矫顽力为32.54 kA/m,比剩余磁化强度为24.6 Am2/kg。     

Magnetic interactions and hysteresis loops study of Co/CoFe2O4 nanoparticles

Co/CoFe₂O₄ nanoparticles with the mean size of 8.8, 10.8 and 16.9 nm were prepared by thermal decomposition of metal salts in the presence of citric acid. The X-ray diffraction patterns and Rietveld refinements confirmed coexistence of Co-ferrite and metallic cobalt phases in the nano-powders. Scanning electron microscope images showed an increase in particles aggregates mean diameter with increasing the annealing temperature. Magnetic hysteresis loops showed a demagnetization jump at low fields, which was attributed to different reversal fields of ferrite and the cobalt phases. Field-dependent behavior of maximum magnetization (M_max), remanence (M_r), squareness (S) and coercivity (H_c) were studied through minor loops measurements. The calculated S value of the loops showed a maximum, between anisotropy and coercive fields. A sharp increase in H_c of larger particles was observed with increasing the applied field when compared to smaller particles. Henkel plots showed that the samples are interacting. Negative deviation of Henkel plots from linear behavior and negative δm plots revealed the dominant role of dipole–dipole interactions in the nano-aggregates.     

A method for identifying the interface shear stress of unidirectional ceramic matrix composites

A method for identifying the interface shear stress of unidirectional ceramic matrix composites (CMCs) under fatigue loading was proposed. A functional relationship between the interface shear stress and the area of the hysteresis loop, the slip strain of the hysteresis loop was established based on the shear lag model by the method of symbolic-graphic combination. A fatigue test of SiC/SiC mini-composites was performed and the hysteresis curves were obtained. Then the interface shear stress under a different number of cycles was identified by inserting the experimental data of the area and slip strain of the hysteresis curves into the functional relationship. The identification results using previously introduced methods based on the area or the secant modulus of the hysteresis loop were also examined. To verify these methods, the interface shear stress identified by the three different methods was entered into the shear lag model to simulate the hysteresis curves for the different number of cycles. A comparison between the simulated results and experimental data shows that the proposed method is more appropriate for the composites used in this paper.     

Enhancement of Interparticle Exchange Coupling in CoFe2O4/CoFe2 Composite Nanoceramics Via Spark Plasma Sintering Technology

Powders and nanoceramics composed of composites of CoFe₂O₄, CoFe₂, and a small amount of FeO were prepared by heating CoFe₂O₄ powder in reducing atmosphere and by sintering the product of reducing reaction at 350°C via spark plasma sintering technology. In the powders, increase in the molar ratios of CoFe₂:CoFe₂O₄ and a great change in magnetic parameters were observed with the change in heating temperature from 300°C to 400°C, and the dominance of dipole interaction over exchange coupling in the interparticle interactions was confirmed by the steps in magnetic hysteresis loops and the negative Henkel plots. However, in the nanoceramics, significant enhancement in exchange coupling was found when the sintering temperature was raised to 500°C and 650°C, which was confirmed by both the positivity of Henkel plot and the single‐phase style of the magnetic hysteresis loop.     

Dielectric Properties of (Pb1–xXx) (Zr0.7Ti0.3)O3 (X = Ca, Sr, Ba) Ceramics

The dielectric properties of (Pb_{1– x} X_x) (Zr_0.7Ti_0.3)O₃ (X = Ca, Sr, Ba) ceramics (abbreviated PXZT) were investigated for applications to multilayer ceramic capacitors (MLCs) with dielectric layers thinner than 10 μm. The dissipation factors for MLCs with 5-μm-thick dielectric layers were estimated from those for 100-μm-thick disk specimens measured at an oscillation voltage of 20 V_rms. Those for PCZT and PSZT were less than 1.0% when the oscillation voltage was 20 V_rms, while those for conventional BaTiO₃-based dielectric ceramics were greater than 2.5% at 20 V_rms. According to polarization–electric field hysteresis measurements, PCZT and PSZT revealed linear and double hysteresis loops, respectively, while PBZT and BaTiO₃ indicated typical ferroelectric hysteresis loops. The differences in the dissipation factors for the dielectric compositions are attributed to hysteresis in the polarization–electric field loops. These results suggest that PCZT and PSZT are promising dielectric ceramics for MLCs with dielectric layers thinner than 10 μm.     

Exchange-spring behavior in BaFe12O19-Ni0.5Zn0.5Fe2O4 nanocomposites synthesized by a combustion method

Exchange-spring systems offer various technological applications. In this study, BaFe₁₂O₁₉-Ni_0.5Zn_0.5Fe₂O₄ nanocomposite magnets with single-step hysteresis loops were synthesized through a simple combustion method. Their composition, microstructure, and magnetic properties were also investigated. It was found that the magnetic properties and the mechanisms governing the magnetization of these nanocomposite magnets are strongly influenced by the calcination temperature as well as the molar ratios of the hard and soft phases. The exchange-coupling between the hard and soft magnetic phases was confirmed by the study of Henkel plots and the variation in the magnetic properties could also be explained by the dominant role of exchange and dipolar interactions in the nanocomposites. The study provides a simple but efficient route for the fabrication of exchange-coupled nanocomposite magnets based on ferrites having controllable magnetic properties.     

TOMOGRAPHY OF MACRO-MESO-PORE STRUCTURE BASED ON MERCURY POROSIMETRY HYSTERESIS LOOP SCANNING Part II: MP Hysteresis Loop Scanning Along the Overall Retraction Line

The concept of macro-meso-pore structure tomography based on mercury porosimetry hysteresis loop scanning along the retraction curve (MP-SHL-R) is introduced. The random corrugaled pore structure (CPSM) model is used in the simulation of the present loop-scanning variant. The CPSM predicted, distribution (V_T) of trapped mercury, reported elsewhere, is employed in the definition of a retraction line'free of entrapment'(V_r) and hence, a 'nominal' MP hysteresis loop scanning (MP-SHL-R(A')) is constructed. The CPSM model have been used in MP-SHL-R(N)) scanning cycles applications to deduce the relevant intrinsic pore size distributions. These psds drawn on a 2-D and/or 3-D plots, that constitute the pore structure tomography, are compared graphically to those obtained by MP hysteresis loop scanning along the penetration line (Part I), Further the CPSM model, as applied in Part II, has been successfully used in regression trials of experimental MP hysteresis loop scanning data and generated the respective tomography spectra. It is concluded that given an MP overall penetration-retraction experimental data, the CPSM model can be best fitted over these data and hence achieve sensible predictions of pore structure tomography based on both MP scanning modes.     

Fatigue behaviour of multiblock thermoplastic elastomers. 1. Stepwise increasing load testing of poly(aliphatic/aromatic-ester) copolymers

Fatigue properties of poly(aliphatic/aromatic-ester) (PED) multiblock copolymers were evaluated based on the hysteresis measurement method. This method allows the digitalization of the hysteresis loop and beside the determination of stress and strain-related parameters, stored energy, lost energy, damping and dynamic creep behaviour can be determined. The correlation between hard/soft segment concentration of PED copolymers, and fundamental parameters, which derived during the cyclic loading (using stepwise increasing load testing, SILT), has indicated a good load-carrying performance of polymers containing a high amount of the hard phase. PED copolymers compare very well with commercially available poly(ester-ethers) and show a much better performance than poly(ester-urethanes) when samples are loaded at the same fatigue stress level relative to their ultimate tensile strength. Softer PEDs and poly(ether-urethane) copolymer show much higher values of dynamic modulus than chemically cross-linked silicone elastomer. Therefore, these multiblock copolymers can be considered as good candidates for applications where materials are subjected to oscillatory deformations (for example passive flexor tendon reconstruction).