Investigation of multicomponent diffusion in cat brain using a combined MTC-DWI approach

In this study, multiple-component water diffusion in the cat brain is investigated using an approach that combines diffusion-weighted images using multiple b values with magnetization transfer contrast (MTC). The MTC allows filter of signal originating from water molecules that rapidly exchange with binding sites on large macromolecular structures, and in brain white matter, it is assumed that a significant portion of the MTC is due to the interaction of water with the extraaxonal myelin sheath. Henceforth, multicomponent analysis of diffusion curves with and without MTC may shed light on the contribution of the extraaxonal water to the diffusion signal and on the relationship between diffusion components and tissue compartments in the brain. When a biexponential model was applied to the data, the volume fractions of the two diffusion components changed significantly in white matter with the application of the MTC. These changes are then discussed in the frame of tissue components and the possible interaction with the myelin sheath.     

A theoretical study of the modal dispersion characteristics of an annular circular waveguide with helical winding as the outer cladding

In this present communication, we introduce a new type of annular optical waveguide whose outer cladding is made of sheath helix. The sheath helix is made up of the dielectric material of lower refractive index than the core materials. Using vector approach, the general characteristic equation for the proposed waveguide has been derived. The modal dispersion characteristics for the lowest-order modes for different pitch angles are determined and analyzed. We observe that there is no effect of pitch angles on dispersion curve nearly up to 89° but drastic change is observed at 90° and above pitch angle. On comparison of helically inner cladded annular circular waveguide, we have found that there is not the existence of the negative dispersion curve and photonic band gap in the helically outer cladded annular circular waveguide. It means only the inner cladding is responsible for the negative dispersion as well as photonic band gap. There is no effect of the outer cladding.     

柱面内爆磁通量压缩实验中的磁扩散过程

利用一维磁流体力学程序MC11D,对套筒材料为不锈钢的柱面内爆磁通量压缩实验进行了数值模拟,研究了其中的磁扩散过程。计算结果表明:当套筒空腔中的磁场被压缩到350 T左右时,峭面磁扩散波开始形成,磁扩散波的波前从套筒内壁开始以0.75 km/s的平均速度向外快速推进,给磁场的压缩带来了不利影响;随着套筒内壁温度迅速升高,内壁附近会形成一个电阻率仅有0.3 mcm左右的等离子体保护层,又极大地减缓了空腔磁场向套筒中扩散的速度。在磁压缩过程中,峭面磁扩散波和等离子体层对于空腔磁场的扩散起着相反的作用,两者在发展的过程中相互竞争,在不同的阶段分别起着主导作用。     

柱面内爆磁通量压缩实验中的磁扩散过程

利用一维磁流体力学程序MC11D,对套筒材料为不锈钢的柱面内爆磁通量压缩实验进行了数值模拟,研究了其中的磁扩散过程。计算结果表明:当套筒空腔中的磁场被压缩到350 T左右时,峭面磁扩散波开始形成,磁扩散波的波前从套筒内壁开始以0.75 km/s的平均速度向外快速推进,给磁场的压缩带来了不利影响;随着套筒内壁温度迅速升高,内壁附近会形成一个电阻率仅有0.3 mcm左右的等离子体保护层,又极大地减缓了空腔磁场向套筒中扩散的速度。在磁压缩过程中,峭面磁扩散波和等离子体层对于空腔磁场的扩散起着相反的作用,两者在发展的过程中相互竞争,在不同的阶段分别起着主导作用。     

Interaction between concentric tubes in DWCNTs

A detailed investigation of the Raman response of the inner tube radial breathing modes (RBMs) in double-wall carbon nanotubes is reported. It revealed that the number of observed RBMs is two to three times larger than the number of possible tubes in the studied frequency range. This unexpected increase in Raman lines is attributed to a splitting of the inner tube response. It originates from the possibility that one type of inner tubes may form in different types of outer tubes. In this case, a splitting of lines results since the inner tube RBM frequency depends on the diameter of the outer tube. Finally, a comparison of the inner tube RBMs and the RBMs of tubes in bundles gave clear evidence for a stronger interaction between tubes in a bundle as compared to the interaction between inner and outer tubes.Received: 15 September 2004, Published online: 23 December 2004PACS: 81.07.De Nanotubes - 81.05.Tp Fullerenes and related materials - 78.30.Na Fullerenes and related materials     

Structural models and undamped torsional natural frequencies of a superconducting turbogenerator rotor

Superconducting turbogenerators with a “double rotor structure” have a torsional natural frequency within the generator, the outer rotor moving in opposition to the inner rotor. For large machines this natural frequency may approach 100 Hz. In this paper a finite element model and simple lumped mass and spring models of the rotor, for the calculation of the undamped torsional natural frequencies, are described and compared. A method by which equivalent spring stiffnesses for both the inner and outer rotors can be derived is described, allowing one to use a rotor model with one lumped mass and equivalent spring stiffness for each of the inner and outer rotors. Such a rotor model can be readily used for studying electromagnetic interaction effects and assessing fault torques in the outer rotor and inner rotor torque tubes.     

Pore-Size Distributions and Tortuosity in Heterogeneous Porous Media

The diffusion coefficient, measured at long observation times by pulsed-held-gradient NMR, can in principle be used to estimate the tortuosity of a porous medium. This method is useful for glass-sphere packs, but we find that it does not generally work for porous sedimentary rock. Natural sedimentary rocks are characterized by complex microgeometries and broad distributions of pore sizes, which cannot be adequately sampled by diffusing molecules in experimentally accessible observation times. The time-dependent diffusion coefficient D(t) can be distinctly irregular for rocks with very large pores. In heterogeneous porous media, determination of pore-size distribution by relaxation-time measurements and tortuosity by PFG diffusion measurements are mutually exclusive.     

双层丝阵Z箍缩电流分配实验研究

负载内外层电流分配是决定双层丝阵Z箍缩内爆动力学模式的关键因素. 在"强光一号"装置上, 利用微型磁探针系统定量测量了双层钨丝阵三个重要径向位置点的电流, 获得了其在驱动电流开始上升至驱动电流达到峰值之前20ns这一阶段内的时间演化行为. 实验使用的双层钨丝阵负载高度为20mm、单丝直径为3.8μm、内/外层丝阵直径分别为8mm/16mm. 对比了内/外层丝根数分别为42/21和21/42时电流分配的差异. 结果表明: 驱动电流上升过程中, 内外层丝阵的电流均逐步增大, 外层丝阵电流份额逐渐减小, 而内层丝阵电流份额逐渐增大; 内层丝阵最大电流份额未超过50%; 内/外层丝根数为21/42的负载外层电流较大. 关键词： Z箍缩 双层丝阵 电流分配     

双层钨丝阵的软X射线辐射参数

在俄罗斯新能源研究所的Angara-5-1装置上对双层钨丝阵的辐射参数进行了实验研究,将不同结构的双层钨丝阵的实验结果及双层钨丝阵与单层钨丝阵的实验结果进行了比较。根据理论模拟提出的4种双层丝阵内爆动力学模式对3次中俄联合实验双层丝阵辐射功率结果进行了分析。双层丝阵的外层与内层同步内爆后,内层相对均匀等离子的分布将有效改善外层等离子体时空分布的均匀性,与单层丝阵相比,X射线辐射功率增加了32%。     

管内两层介质入口段扩散混合的耦合换热研究

对圆管内辐射物性不同的两层介质层流入口段,采用SIMPLEC算法与蒙特卡罗法数值模拟了二维稳态流动与扩散混合时的辐射-对流耦合换热。通过计算,分析了介质层几何参数、介质物性与流动参数对组份分布与耦合换热的影响。结果表明,介质组分的扩散混合对耦合换热存在明显的影响区域,且该影响区大于组分的扩散混合区;外层介质的吸收系数、入口截面的相对厚度对耦合换热的影响基本一致;质扩散系数对耦合换热的影响很小,入口雷诺数的增加会抑制质扩散。     

Molecular simulation of the kinetic selectivity of a model silica system

Equilibrium and non-equilibrium molecular dynamics (MD) simulation results are presented for the transport of low pressure gases composed of both structureless (spherical) and linear molecules within and through membranes generated by randomly ‘etching’ silicon atoms and concomitant oxygens from an atomistic model of dense amorphous silica. The MD simulation results demonstrate that while an infinite medium possessing the morphological properties of the moderately dense etched glass is non-percolating, thin layers of this material are kinetically selective. In agreement with earlier results for carbon molecular sieves, it is shown that the permeabilities of individual gases within a layer of the etched glass depend exponentially on the layer (or membrane) thickness. The relationship between the finite length scale of the microvoids of the medium and the size and shape of the diffusing gas molecules is explored.     

双层钨丝阵的软X射线辐射参数

在俄罗斯新能源研究所的Angara-5-1装置上对双层钨丝阵的辐射参数进行了实验研究,将不同结构的双层钨丝阵的实验结果及双层钨丝阵与单层钨丝阵的实验结果进行了比较.根据理论模拟提出的4种双层丝阵内爆动力学模式对3次中俄联合实验双层丝阵辐射功率结果进行了分析.双层丝阵的外层与内层同步内爆后,内层相对均匀等离子的分布将有效...     

Diffusive diffraction observed with volume-selective STEAM MRS in 100microm water-filled capillaries

Diffusive diffraction patterns may be useful for probing the local environment of diffusing molecules in materials with geometrically ordered microstructure. A model system consisting in a bundle of water-filled 100microm glass capillaries was probed with volume-selective stimulated echo (STEAM) MRS on a 7T Bruker PharmaScan tomograph with variable diffusion times for both, parallel and perpendicular diffusion-weighting with respect to the capillaries' axes. The precise orientation of the capillaries was determined with image processing methods. Echo attenuation curves were numerically evaluated with respect to the inner radius of the capillaries R and the diffusion coefficient D using equations given in the literature. Good agreement was found between simulation and experiment. For perpendicular diffusion-weighting and diffusion times in the order of R(2)/D two diffraction minima were observed which were not present for shorter diffusion times and for parallel diffusion-weighting. In conclusion, volume-selective diffusive diffraction was observed with a standard small-animal tomograph.     

Simultaneous Measurement of Temperature and Refractive Index of Liquid Using Cladding-Etched Double-Core Fiber Bragg Grating

Abstract

This study proposed and demonstrated a fiber Bragg grating sensor for simultaneous detection of the temperature and refractive index of a liquid. The fiber Bragg grating is composed of three index layers: an inner core, an outer core, and a cladding. The cladding was etched for evanescent wave coupling between the outer core mode and an external liquid. The Bragg wavelength of the inner core mode is used for obtaining temperature data, while the refractive index for the liquid was determined by using the difference between the Bragg wavelength of the inner core mode and that of outer core mode.     

First study of manganese diffusion in Cr2O3 polycrystals and thin films by SIMS

Chromia layers are formed on many industrial alloys and act as a protective barrier against the corrosion of the materials by limiting the diffusion of oxygen and cations. Most of these alloys contain manganese as an impurity, and manganese oxides are often found at the outer surface of the oxide films. In order to clarify the oxidation mechanism and to check if chromia acts as a barrier, manganese diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni–30Cr alloy in the temperature range 700–1100°C at an oxygen pressure of 10^?4?atm. After deposition of Mn on the chromia surface and a diffusing treatment, the manganese penetration profiles were established by secondary ion mass spectrometry. In all cases, the diffusion profiles showed two domains. For the first domain, using the solution of Fick's law for diffusion from a thick film into a semi-infinite medium, bulk diffusion coefficients were determined. For the second domain, the Le Claire model allowed the grain boundary diffusion parameter (αD _gbδ) to be obtained. Manganese diffusion does not vary significantly according to the nature and microstructure of chromia. The activation energy of grain boundary diffusion is not far from that obtained for bulk diffusion, probably on account of segregation phenomena. Manganese diffusion was compared to cationic self-diffusion and iron diffusion, and related to the protective character of chromia.     

Interaction of a low-pressure mercury discharge with the discharge tube wall

The darkening of soda-lime glass envelopes of low-pressure mercury discharge tubes has nothing to do with the formation of sodium amalgam on the inner surface of the tube. It is, however, connected with the diffusion of mercury ions into the glass of the tube. By applying a high negative voltage to part of the outer surface of a burning tube a sodium depleted region is created at the corresponding part of the inner surface. After removal of the high-voltage electrode and continued normal operation the rate of darkening of the sodium-depleted part of the tube is about 50 times larger than the rate observed for the normal parts of the same tube.     

Precursor effect during water vapor diffusion in a porous medium

In water vapor diffusing through a granular adsorbent layer into vacuum, water molecules overcoming an adsorption column without an appreciable delay as pulses, i.e., precursors of the main front, were detected. This effect is explained by the sluggishness of water molecule diffusion into adsorbent grains and the dependence of the adsorption capacity of molecules on their rotational states via the Stark effect in a surface electric field of an adsorbent.     

Observation of molecular migration in porous media using 2D exchange spectroscopy in the inhomogeneous magnetic field

We present a new method for observing fluid diffusion in a porous medium. The method employs 2D exchange spectroscopy for molecules diffusing in the presence of local magnetic field inhomogeneities, in our case distilled water in various sized glass bead packs. Our experiment involves an acquisition and evolution time domain with the two Fourier domains corresponding to the spectral distribution of local fields. We show that exchange in the internal magnetic field can be seen in a 2D spectrum with a characteristic time on the order of that required to diffuse 0.15 sphere diameters with similar behavior found for computer simulations. The method is potentially useful for studying the internal migrations in more complicated systems such as sandstones or other porous media.     

Dipolar bosons in a planar array of one-dimensional tubes

We investigate bosonic atoms or molecules interacting via dipolar interactions in a planar array of one-dimensional tubes. We consider the situation in which the dipoles are oriented perpendicular to the tubes by an external field. We find various quantum phases reaching from a "sliding Luttinger liquid" phase to a two-dimensional charge density wave ordered phase. Two different kinds of charge density wave order occur: a stripe phase in which the bosons in different tubes are aligned and a checkerboard phase. We further point out how to distinguish the occurring phases experimentally.     

The evolution equation for the radius of a premixed flame ball in fractional diffusive media

The evolution equation for the radius of an isolated premixed flame ball is derived in the framework of a new method that strongly simplifies previous ones and highlights that they are based on Gaussian modelling of diffusion. The main idea is to split the flame ball in two components: the inner kernel, which is driven by a Poisson-type equation with a general polynomial forcing term, and the outer part, which is driven by a generalized diffusion process valid for fractional diffusive media. The evolution equation for the radius of the flame ball is finally determined as the evolution equation for the interface that matches the solution of the inner spherical kernel and the solution of the outer diffusive part and it emerges to be a nonlinear fractional differential equation. The effects of fractional diffusion on stability of solution are also picked out.