Magnetic properties and magnetic domain structure of bulk glass forming Nd60Al10Fe20Co10 alloy

The transition from hard to soft magnetic behaviour with increasing quenching rate is shown for Nd60WAl10Fe20Co10 melt-spun ribbons with different thickness. Microstructure and magnetic domain structure of ribbons were studied by magnetic force microscopy (MFM). Particle sizes < 5 nm decreasing gradually with increasing quenching rate were deduced from topographic images which differ from large-scale magnetic domains with a periodicity of about 350 nm in all ribbons irrespective the coercivity. This indicates that the magnetic properties of the alloy are governed by interaction of small magnetic particles. It is concluded that the presence of short-range-ordered structures with a local ordering similar to the Al metastable Nd-Fe binary phase is responsible for the hard magnetic properties in samples subjected to relatively low quenching rate.

Streamline topology and dilute particle dynamics in a Karman vortex street flow

Three types of streamline topology in a Karman vortex street flow are shown under the variation of spatial parameters. For the motion of dilute particles in the Karman vortex street flow, there exist a route of bifurcation to a chaotic orbit and more attractors in a bifurcation diagram for the proportion of particle density to fluid density. Along with the increase of spatial parameters in the flow field, the bifurcation process is suspended, as well as more and more attractors emerge. In the motion of dilute particles, a drag term and gravity term dominate and result in the bifurcation phenomenon.

Generalization of the JTZ model to open plane wakes

The JTZ model [C. Jung, T. T¶el and E. Ziemniak, Chaos 3, (1993) 555], as a theoretical model of a plane wake behind a circular cylinder in a narrow channel at a moderate Reynolds number, has previously been employed to analyze phenomena of chaotic scattering. It is ex- tended here to describe an open plane wake without the con¯ned nar- row channel by incorporating a double row of shedding vortices into the intermediate and far wake. The extended JTZ model is found in qualitative agreement with both direct numerical simulations and ex- perimental results in describing streamlines and vorticity contours. To further validate its applications to particle transport processes, the in- teraction between small spherical particles and vortices in an extended JTZ model °ow is studied. It is shown that the particle size has signif- icant in°uences on the features of particle trajectories, which have two characteristic patterns: one is rotating around the vortex centers and the other accumulating in the exterior of vortices. Numerical results based on the extended JTZ model are found in qualitative agreement with experimental ones in the normal range of particle sizes.

Defect-induced failure of optical coating filters

Optical filters composed of Ag, Al2O3, and ZnSe films were prepared on BK7 substrates by evaporation. By employing spectrophotometer, microscope, scanning electron microscope (SEM), and energy dispersive spectrum (EDS) analysis, the moisture-dependent stability of the samples was tested. The experimental results revealed that filter failure often occurs initially at defect sites. Small sputtering particles and pinhole are found to be two types of defects that induced the optical coating filter failure. The mechanisms of the defect-induced failure of the filters also are discussed in the article. (C) 2005 Elsevier Ltd. All rights reserved.

Tunneling magnetoresistance in devices based on epitaxial NiMnSb with uniaxial anisotropy

We demonstrate tunnel magnetoresistance junctions based on a trilayer system consisting of an epitaxial NiMnSb, an aluminum oxide, and a CoFe trilayer. The junctions show a tunneling magnetoresistance of Delta R/R of 8.7% at room temperature which increases to 14.7% at 4.2 K. The layers show a clear separate switching and a small ferromagnetic coupling. A uniaxial in-plane anisotropy in the NiMnSb layer leads to different switching characteristics depending on the direction in which the magnetic field is applied, an effect which can be used for sensor applications. (c) 2006 American Institute of Physics.

Fine structure of the plasmon resonance absorption peak of Ag nanoparticles embedded in partially oxidized Si matrix

Ag/Si nanocomposite films were prepared by the radio-frequency magnetron cosputtering method. The fine structure of the plasmon resonance absorption peak was found in film samples. X-ray photoelectron spectroscopy analysis indicated that the samples were composed of a two-layer structure, which accounted for the structure of the optical absorption spectra. The peak located near 445 nm is the plasmon resonance absorption peak of Ag nanoparticles embedded in a partially oxidized Si matrix. Its intensity decreases with decreasing film thickness and disappears in a very thin sample. The peak located near 380 nm originates from the plasmon resonance absorption of the thoroughly oxidized surface layer of the sample. Its intensity does not change with increasing thickness, but it cannot be observed in the very thick sample. (C) 2001 American Institute of Physics.

Nano-Ag on vanadium dioxide. II. Thermal tuning of surface plasmon resonance

Thermal tuning of the localized surface plasmon resonance (LSPR) of Ag nanoparticles on a thermochromic thin film of VO2 was studied experimentally. The tuning is strongly temperature dependent and thermally reversible. The LSPR wavelength lambda(SPR) shifts to the blue with increasing temperature from 30 to 80 degrees C, and shifts back to the red as temperature decreases. A smart tuning is achievable on condition that the temperature is controlled in a stepwise manner. The tunable wavelength range depends on the particle size or the mass thickness of the metal nanoparticle film. Further, the tunability was found to be enhanced significantly when a layer of TiO2 was introduced to overcoat the Ag nanoparticles, yielding a marked sensitivity factor Delta lambda(SPR)/Delta n, of as large as 480 nm per refractive index unit (n) at the semiconductor phase of VO2.

Time-dependent density-functional calculations for optical spectra of Na-2 and Na-4 clusters

With the frame of the time-dependent local density approximation, an efficient description of the optical response of clusters has been used to study the photo-absorption cross section of Na-2 and Na-4 clusters. It is shown that our calculated results are in good agreement with the experiment. In addition, our calculated spectrum for the Na-4 cluster is in better agreement with experiment than the GW absorption spectrum.

高性能小型带电粒子探测阵列的研制

随着核物理理论研究和各种先进探测设备研制工作的不断深入,世界各大实验室相继建立了各具特色的放射性束装置,放射性束物理得到了长足的发展。在放射性束引起的核反应实验上,要得到更加精确的非稳定原子核的结构信息和反应机制，需要对反应产物实现完全运动学测量,这就要求位置分辨和能量分辨更精确的探测器装置。高性能小型带电粒子探测阵列是基于△E-E粒子鉴别方法,采用位置分辨精确的双面硅微条作为△E探测器,用阻止本领大的BGO晶体阵列作为E探测器,来测量反应产物的能量分辨和角分布,从而进行带电粒子的鉴别。 Geant4是由欧洲核子中心(CERN)开发的一个大型高能物理探测器模拟程序，是采用当代先进的面向对象程序设计技术利用C++语言编写的。我们基于该模拟工具,对探测器分辨随能量的变化情况、粒子斜入射对探测器分辨的影响情况、晶体厚度变化对分辨的影响情况以及与阻止相同能量的CsI晶体分辨情况等进行了一系列的模拟对照。 本论文工作的主要内容有:首先,介绍了探测器的研制及测试过程。该探测器用一块双面硅微条作为△E探测器,用64块BGO晶体阵列作E探测器。晶体阵列采用一块64单元位置灵敏的光倍管读出信号。该探测器具有体积小，死区小,位置分辨好等优点。其次,根据探测器在实验中可能遇到的问题,我们利用Geant4系统的模拟了晶体尺寸变化对分辨的影响情况；晶体分辨随能量升高的变化情况及粒子斜入射对晶体分辨造成的影响等，从而为探测器选用合适尺寸的晶体提供了很好的参考。模拟结果显示，晶体分辨随着能量升高而变好,这为选用BGO晶体作为带电粒子探测阵列提供了很好的参考；粒子斜入射会对分辨造成一定的影响，并从三维图上显示了粒子的踪迹，这为今后实验数据处理提供了很好的参考；与CsI晶体位置分辨和能量分辨作了对照模拟，模拟结果显示，随着带电粒子能量的增加BGO分辨逐渐与CsI晶体接近，并且角分辨会比CsI晶体好，从而得出BGO晶体用来作带电粒子探测阵列的独特优势。最后，针对以后的工作方向作了简单的讨论

Synthesis of carbon nanotube bundles with mesoporous structure by a self-assembly solvothermal route

A kind of carbon nanotube bundle has been synthesized by a simple one-step solvothermal reaction between Na and hexachlorobenzene (HCB) using NiCl2 as catalyst precursor. Before the reaction, NiCl2 was initially dispersed ultrasonically in cyclohexane then prereduced by Na at 230degreesC to produce small Ni particles in reduced state. The tubes thus-produced have a uniform outer diameter of about 20 nm, an inner diameter of 4 nm, and are highly ordered assembled as bundles which have a 2D hexagonal arrangement as proven by SAXS and TEM experiments.

Effect of cerium on microstructure and electrochemical performance of Ti-V-Cr-Ni electrode alloy

Effect of cerium on the microstructure and electrochemical performance of the Ti0.25V0.35-xCexCr0.1Ni0.3 (x = 0, 0.005) electrode alloy was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy/energy dispersive X-ray spectrometry (FESEM-EDS), and electrochemical impedance spectroscopy (EIS) measurements. On the basis of XRD and FESEM-EDS analysis, the alloy was mainly composed of V-based solid solution with body-centered-cubic structure and TiNi-based secondary phase. Ce did not exist in two phases, instead, it existed as Ce-rich small white particles, with irregular edges, distributed near the grain boundaries of the V-based solid solution phase. Discharge capacity, cycle stability, and high-rate discharge ability of the alloy electrode were effectively improved with the addition of Ce at 293 K. It was very surprising that the charge retention was abnormal with larger discharge capacity after standing at the open circuit for 24 h. EIS indicated that addition of Ce improved the dynamic performance, which caused the charge transfer resistance (R-T) to decrease and exchange current density (I-0) to increase markedly. The exchange current density of the electrochemical reaction on the alloy surface with Ce addition was about 2.07 and 3.10 times larger than that of the alloy without Ce at 303 and 343 K, respectively.

Preparation of a carbon supported Pt catalyst using an improved organic sol method and its electrocatalytic activity for methanol oxidation

The organic sol method for preparing ultrafine transition metal colloid particles reported for the first time by Bonnemann et al. [H. Bonnemann, W Brijoux, R. Brinkmann, E. Dinjus, T. Jou beta en, B. Korall, Angew. Chem. Int. Ed. Engl., 30 (1991) 1312] has been improved in this paper. The improved organic sol method uses SnCl2 as the reductant and methanol as the organic solvent. Thus, this method is very simple and inexpensive. It was found that the average size of the Pt particles in the Pt/C catalysts can be controlled by adjusting the evaporating temperature of the solvent. Therefore, the Pt/C catalysts prepared by the same method are suitable for evaluating the size effect of the Pt particles on electrocatalytic performance for methanol oxidation. The results of the X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that when the evaporating temperatures of the solvent are 65, 60, 50, 40, and 30 degrees C, the average sizes of the Pt particles in the Pt/C catalysts prepared are: 2.2, 3.2, 3.8, 4.3, and 4.8 nm, respectively. The X-ray photoelectron spectroscopic (XPS) results demonstrated that the small Pt particles are easily oxidized and the decomposition/adsorption of methanol cannot proceed on the surfaces of Pt oxides.

Preparation of macroporous functionalized polymer beads by a multistep polymerization and their application in zirconocene catalysts for ethylene polymerization

Macroporous functionalized. polymer beads of poly(4-vinylpyridine-co-1,4-divinylbenzene) [P(VPy-co-DVB)] were prepared by a multistep polymerization, including a polystyrene (PS) shape template by emulsifier-free emulsion polymerization, linear PS seeds by staged template suspension polymerization, and macroporous functionalized polymer beads of P(VPy-co-DVB) by multistep seeded polymerization. The polymer beads, having a cellular texture, were made of many small, spherical particles. The bead size was 10-50 mum, and the pore size was 0.1-1.5 mum. The polymer beads were used as supports for zirconocene catalysts in ethylene polymerization. They were very different from traditional polymer supports. The polymer beads could be exfoliated to yield many spherical particles dispersed in the resulting polyethylene particles during ethylene polymerization. The influence of the polymer beads on the catalytic behavior of the supported catalyst and morphology of the resulting polyethylene was investigated.

High-frequency ferromagnetic resonance on ultrafine cobalt particles

We report on high-frequency (300-700 GHz) ferromagnetic resonance (HF-FMR) measurements on cobalt superparamagnetic particles with strong uniaxial effective anisotropy. We derive the dynamical susceptibility of the system on the basis of an independent-grain model by using a rectangular approach. Numerical simulations give typical line shapes depending on the anisotropy, the gyromagnetic ratio, and the damping constant. HF-FMR experiments have been performed on two systems of ultrafine cobalt particles of different sizes with a mean number of atoms per particles of 150 +/- 20 and 310 +/- 20. In both systems, the magnetic anisotropy is found to be enhanced compared to the bulk value, and increases as the particle size decreases, in accordance with previous determinations from magnetization measurements. Although no size effect has been observed on the gyromagnetic ratio, the transverse relaxation time is two orders of magnitude smaller than the bulk value indicating strong damping effects, possibly originating from surface spin disorders.

High-field ferromagnetic resonance in fine particles

We investigate high-field ferromagnetic resonance of superparamagnetic particles with uniaxial anisotropy, In this case, since the field is large enough to saturate the magnetization, the thermal orientational fluctuations of the magnetic moment of the particle are negligible. Thus, we derive the dynamic susceptibility of the system on the basis of an independent particle model. High-field ferromagnetic resonance has been performed on fine cobalt particles, The analysis of the spectra obtained at different frequencies allows us to estimate the effective magnetic anisotropy, the gyromagnetic ratio, and the transverse relaxation time. (C) 1998 Elsevier Science B.V. All rights reserved.