Spectral characterization of diffusion with chemical shift resolution: Highly concentrated water-in-oil emulsion

We present a modulated gradient spin-echo method, which uses a train of sinusoidally shaped gradient pulses separated by 180° radio-frequency (RF) pulses. The RF pulses efficiently refocus chemical shifts and de-phasing due to susceptibility differences, resulting in undistorted, high-resolution diffusion weighted spectra. This allows for the simultaneous spectral characterization of the diffusion of several molecular species with different chemical shifts. The technique is robust against susceptibility artifacts, field inhomogeneity and imperfections in the gradient generating equipment. The feasibility of the technique is demonstrated by measuring the diffusion of water, oil, and water-soluble salt in a highly concentrated water-in-oil emulsion. The diffusion of water and salt reveal precise information about the droplet size distribution below the μm-range. Common droplet size distribution explains both the data for water with finite long-range diffusion and the data for salt with negligible long-range diffusion. The results of water diffusion show that the technique is efficient in deconvolving the effects of molecular exchange between droplets and restricted diffusion within droplets. The effects of water exchange suggest that droplets of different sizes are uniformly distributed within the sample.     

Extremely fast ambipolar diffusion in n-i-p-i doping superlattices investigated by an all-optical pump-and-probe technique

We report on investigations of the ambipolar diffusion process in n-i-p diodes and n-i-p-i doping superlattices performed by a new all-optical pump-and-probe technique. This new technique allows not only the determination of the ambipolar diffusion coefficient but also the spatially resolved investigation of the stationary distribution of the optically-induced excess carriers. The n-i-p-i doping superlattice exhibited an extremely large ambipolar diffusion coefficient in the range of 10⁴cm²s⁻¹. The ambipolar diffusion coefficient of the n-i-p and n-i-p-i structure was demonstrated not to be a constant but a function of the charge carrier density. This strongly affects the spatial distribution of the excess carriers especially in the large signal regime (density of photo-generated carriers much larger than dark carrier concentration). The spatial distribution of the carriers in the small as well as in the large signal case can be understood theoretically if the real dependence of the ambipolar diffusion coefficient on the carrier density is taken into account.     

激光惯性约束聚变靶靶丸制备与表征

激光惯性约束聚变的核心思想是利用球形内爆技术对聚变燃料进行增压,使热核燃料达到高温、高密度的等离子体状态,进而实现聚变点火。基于对称压缩、流体界面不稳定性和实验诊断的考虑,ICF实验对作为热核燃料容器的空心微球的品质在球形度、壁厚均匀性、表面粗糙度以及掺杂水平等方面提出了严格的要求。为满足这些要求,陆续发展了乳液微封装技术、降解芯轴技术、低压等离子体聚合/掺杂技术、干凝胶玻璃微球制备技术等用于多层塑料微球和空心玻璃微球的研制。另一方面,针对ICF靶丸量小、质轻以及表面要求高的特点,发展了相应的非破坏性靶丸参数表征技术,如X光照相技术、4π形貌表征技术、微球掺杂水平测量技术以及微球内燃料负载水平快速测试技术。基于这些制备与表征技术,初步实现了多层塑料微球、玻璃微球、聚-!-甲基苯乙烯芯轴微球、梯度掺杂CH微球的研制,满足了"神光Ⅱ"、"神光Ⅲ原型"及"神光Ⅲ主机"上开展的一系列内爆物理实验的要求,同时为未来点火物理实验用靶丸的研制提供了技术支撑。     

The Influence of the Magnetic Impurity Content on the Pore Size Distribution Determination via the DDIF Technique

The pore size distribution of porous media can be determined in a completely non-invasive manner using a new nuclear magnetic resonance (NMR) technique which monitors the magnetization decay due to diffusion in internal fields (DDIF). However, using of the DDIF technique is restricted to the low-phase encoding limit when only the relaxation mode and the first-order diffusion mode are excited. In the present work the fulfillment of such a limit is verified for a progressive increase of the magnetic impurity content of the porous media. If the higher order diffusion modes are excited they lead both to a stronger attenuation of the echo signal and to the appearance of ripples in the DDIF spectra which cannot be related to a pore size distribution. The samples used in this study are porous ceramics prepared using the replication technique and the magnetic impurity is iron (III) oxide which is introduced in an increasing concentration inside the porous matrix. All NMR experiments were done on water filling such porous ceramics using a low-field instrument operating at a proton resonance frequency of 20 MHz. The average pore dimension obtained with the DDIF technique in the weak encoding limit indicates a satisfactory agreement with that observed in optical microscopy images.     

Magnetic resonance lineshape in the presence of slow motion

The effect on the magnetic resonance lineshape of slow molecular motion is considered. The molecules are assumed to undergo diffusive jumps of angle ε described by the distribution function W(ε)=(N/τ) exp (λ cos ε), where τ is the mean lifetime between jumps and λ a parameter describing the width of the distribution. As λ is varied from zero to infinity this distribution describes diffusion models that change continuously from the strong collision limit to Brownian rotational diffusion. Magnetic resonance lineshapes are calculated by Freed's method using the asymptotic expansion technique, and results are presented for an axially symmetric (secular) Zeeman hamiltonian.

An exact expression for the magnetic resonance lineshape is derived in the strong collision limit for any secular hamiltonian and a few examples are presented. It is also shown how this technique can be used as a basis for lineshape calculations in the presence of moderately large jumps.     

用四探针及阳极氧化方法测量硅中扩散层杂质分布

本文讨论了:(1)用阳极氧化法在硅片表面去层的技术;(2)用四探针测量扩散层面电导的方法;(3)用阳极氧化去层及四探针测量面电导方法求得扩散层精细杂质分布。文中着重讨论了实验技术中的实际问题,如如何在阳极氧化过程中取得精细而均匀的去层(300—1500?);如何控制及测量去层厚度;测量面电导及杂质分布时的误差来源及减小误差的措施。以典型的磷在硅中扩散的杂质分布测量为例:扩散深度为4.9μm,测量间距为400—1600?,面电导测量误差估计小于3％,杂质分布误差估计小于20％。简单地提出了一些测量中尚待进一步解决的问题。     

Geometry of intensive scalar dissipation events in turbulence

The maxima of the scalar dissipation rate in turbulence appear in the form of sheets and correspond to the potentially most intensive scalar mixing events. Their cross section extension determines a locally varying diffusion scale of the mixing process and extends the classical Batchelor picture of one mean diffusion scale. The distribution of the local diffusion scales is analyzed for different Reynolds and Schmidt numbers with a fast multiscale technique applied to very high-resolution simulation data. The scales always take values across the whole Batchelor range and beyond. Furthermore, their distribution is traced back to the distribution of the contractive short-time Lyapunov exponent of the flow.     

Effect of different analyte diffusion/adsorption protocols on SERS signals

The effect of different analyte diffusion/adsorption protocols was studied which is often overlooked in surface-enhanced Raman scattering (SERS) technique. Three protocols: highly concentrated dilution (HCD) protocol, half–half dilution (HHD) protocol and layered adsorption (LA) protocol were studied and the SERS substrates were monolayer films of 80 nm Ag nanoparticles (NPs) which were modified by polyvinylpyrrolidone. The diffusion/adsorption mechanisms were modelled using the diffusion equation and the electromagnetic field distribution of two adjacent Ag NPs was simulated by the finite-different time-domain method. All experimental data and theoretical analysis suggest that different diffusion/adsorption behaviour of analytes will cause different SERS signal enhancements. HHD protocol could produce the most uniform and reproducible samples, and the corresponding signal intensity of the analyte is the strongest. This study will help to understand and promote the use of SERS technique in quantitative analysis.     

Photon migration at multiple frequencies in YLiF4:Nd crystal

A spatially resolved microluminescence technique was used to measure the spatial distribution of emitted light and photon propagation in Nd³⁺-doped YLiF₄ crystal excited at 514 nm. The spatial distribution of the luminescence was studied and the energy transfer processes among neodymium ions were discussed. We measured the photon diffusion length for specific wavelengths. It was found that re-absorption by ground and excited states plays a crucial role to the propagation. With this information, the microluminescence technique has played a useful role in the investigation of energy transfer processes in rare-earth doped systems.     

Effect of a Hydrogen-Bonding Layer on a Hollow Capsule's Permeability

Hollow capsules formed by layer-by-layer self-assembly polymers have been intensively studied for their potential applications in medicine and biotechnology. We have controlled such capsules’ permeability with a hydrogen-bonding layer. Scanning electron microscopy and Fourier transform infrared results showed the fabrication of multilayer polyelectrolyte films. Z-potential results were employed to record the deposition of the polyelectrolyte. The diffusion coefficients for 6-carboxyfluorescein (6-CF) and fluorescently labeled goat anti-human IgG-FITC molecules passing through the wall were calculated based on the fluorescence recovery after photobleaching experiment. The results show that 6-CF molecules can pass through the capsule wall more easily than IgG-FITC molecules. By decreasing the solution's pH, the capsule's permeability to macromolecular dyes or small molecular dyes can be controlled, i.e., the capsule's permeability can be tuned by the hydrogen-bonding interaction.     

Radial anomalous diffusion in an annulus

In this study, time fractional radial diffusion has been modeled in cylindrical coordinates in order to analyze the anomalous diffusion in an annulus. By using an integral transform technique, the analytical solution of the concentration distribution formula is obtained. The establishing of the concentration distribution is found to be controlled by the fractional derivative α, and the influences of α on the concentration field, the total amount diffused and the quantity of mass passing through the inner wall are presented graphically and studied in detail. Asymptotic expressions for the exact solutions are developed in order to explain the numerical results at small and large time, respectively, and the physical mechanism explanation for the paradoxical behavior shown in the numerical results is given.     

利福平与人血清白蛋白作用的荧光光谱

蛋白质是一类重要的生物大分子,它不仅是构成细胞内原生质的主要成分,也是生命现象的物质基础。对于蛋白质的探索是最复杂的课题之一。 以荧光光谱为手段,文章研究了药物利福平(Rifampicin capsules)与人血清白蛋白(HSA)的作用,测量发现利福平和人血清白蛋白的色氨酸残基的结合位置为R=2.567 nm, 临界距离R₀为2.433 nm。利福平-HSA的Lineweaver-Burk猝灭曲线的离解常数K_d=19.42×10-6 mol·L-1且结合常数K_s=5.149×104 L·mol-1。     

New model for tracer-diffusion in amorphous solid

The tracer-diffusion and structure of polymorphic states of amorphous solid is studied by mean of the statistic relaxation technique and simplex analysis. Several different metastable states of amorphous iron have been constructed based on the model containing 2 × 10⁵ atoms. All models have almost the same pair radial distribution functions, but they differ in the potential energy per atom and the density. We found a large number of vacancy-simplexes which varies according to the relaxation and serves as a diffusion vehicle. New diffusion mechanism for tracer-diffusion is found of which the elementary diffusion process likes a collapse of “microscopic bubble” in amorphous matrix. This includes a jump of diffusing atom and the collective movement of a large number of neighboring atoms. The diffusion constant D determined in accordance with considered diffusion mechanism is in reasonable agreement with experimental data. The decrease in diffusion constant D upon thermal annealing is explained by the reducing vacancy-simplex concentration which is caused by both the local atomic rearrangement and the elimination of excess free volume.     

Self-Diffusion Measurements by Pulsed-Gradient Multiple-Spin-Echo Imaging

A new method is described which allows the spatial distribution of the self-diffusion coefficient over a sample to be determined in a single NMR imaging experiment. This technique combines NMR imaging principles with the pulsed-field-gradient multiple-spin-echo (PGMSE) method. Two basic forms of the pulse sequence for PGMSE imaging have been devised and image intensity as a function of the diffusion-gradient strength is given. The effects of the imaging gradients on the additional diffusion attenuation of image intensities are considered. Finally, the preliminary experimental verification of the PGMSE imaging technique is illustrated by measuring the diffusion coefficient for doped water using the version of the pulse sequence in which all of the 180° radiofrequency pulses are not slice-selective.     

Tracer measurements of atomic diffusion inside shear bands of a bulk metallic glass

Atomic diffusion in deformed Pd(40)Ni(40)P(20) bulk metallic glass containing a single family of deformation-induced shear bands was measured by the radiotracer technique. The significant, by orders of magnitude, enhancement of the diffusion rate with respect to that in the untransformed matrix suggests that the shear bands represent short-circuit diffusion paths. Correlations between diffusivity, viscosity, and the excess free volume distribution inside of shear bands are discussed.     

Formation and destruction of polyelectrolyte microcapsules modified by rhodamine 6G

Polyelectrolyte capsules with Rhodamine 6G molecules included into the shell are obtained in this work. The inclusion of dye molecules into the shell can provide the destruction of capsules by photosensitization. Measurements of the dye fluorescence intensity in the shell show that the energy of photoexcited molecules is effectively dissipated by the surrounding organic matrix. The capsule suspension is laser irradiated in the absorption band of Rhodamine 6G molecules. Measurements of the size distribution of capsules before and after laser irradiation reveal that the capsules are destroyed by laser illumination.     

Water transbilayer diffusion in macroscopically oriented lipid bilayers as studied by pulsed field gradient NMR

Water self-diffusion in lipid bilayers macroscopically oriented on glass plates was studied by pulsed field gradient¹H nuclear magnetic resonance technique. Diffusion decays were multicomponent with a distribution of diffusion coefficients ranging from about 10⁻¹⁰ to about 10⁻¹³ m²/s. A number of measurements with variations of the sample orientation, diffusion time and the distance between the glass plates showed that the “fast” component of diffusion corresponds to water in the bilayer “cracks”. The “slow” component of diffusion corresponds to transbilayer water diffusion in the long-diffusion-time regime. For a more reliable separation of parts corresponding to fast and slow diffusion of water, a “component-resolved spectroscopy” method for the global analysis of correlated spectral data (P. Stilbs, K. Paulsen, P.C. Griffiths: J. Phys. Chem. 100, 8180, 1996) was applied.     

Analysis of lithium and hydrogen movement in functional solid materials by neutron radiography

The principle and availability of Neutron Radiography are demonstrated. The technique is actually applied to obtain the images as to lithium in the oxide lithium ion conductors and/or hydrogen in the hydrogen storage alloys. From analysis of the lithium or hydrogen distribution, the ionic transport number and the diffusion coefficient can be calculated and the hydrogenation process is discussed. In the case imaging the special cross-section in the sample, Neutron Computed Tomography is more favorable. Here, it will be demonstrated using our actual experimental results that the NCT technique is also useful in the field of Solid State Ionics to clarify the ion distribution and movement. Paper presented at the Patras Conference on Solid State Ionics, Patras, Greece, Sept. 14–18, 2004.     

Ag-Na离子交换玻璃波导的折射率分布研究

用Ag-Na离子交换技术制备了玻璃平面波导.通过棱镜耦合技术测量了波导的模折射率,用反WKB方法拟合得到了平面波导的折射率分布为高斯分布.发现Ag-Na离子交换的扩散系数与交换时间有关,并且随着交换时间的增加而减小.使用随离子浓度变化的扩散系数求解扩散方程得到了玻璃内部Ag离子浓度分布,并通过SEM谱证实了求解的正确性;使用一次多项式模拟的方法求解折射率变化与Ag离子浓度变化之间的关系,得到了平面波导的折射率分布.与反WKB法只能获得多模离子交换平面波导的折射率分布相比,这种方法可以得到任意扩散时间下折射率变化与Ag离子浓度,可以获得单模平面波导的折射率分布.     

The role of dose distribution gradient in the observed ferric ion diffusion time scale in MRI-Fricke-infused gel dosimetry

Ferric ion diffusion is a detrimental factor in MRI-Fricke-infused gel dosimetry. In this study, a novel approach involving MR image subtraction and a fast image-based dosimetry technique to study ferric ion diffusion effects is presented. The fast image-based approach allows studying dose profile degradation within minutes post-irradiation. The relationship between the rate of dose profile deterioration and dose distribution gradients can be elucidated with the improved imaging temporal resolution also. Our results showed that for a dose distribution with gradient 4 Gy/mm or higher, ferric ion diffusion causes apparent dose profile degradation in 0.5-1 h post-irradiation. For a gradual dose gradient change of 2.1 Gy/mm or smaller, dose profile degradation appears insignificant for a two-hour elapsed diffusion time. These observations agree well with the theoretical analysis of a square dependence between dose profile degradation and dose distribution gradient. Because all stereotactic radiosurgery procedures produce steep dose distributions and because the ideal "snapshot" of MR scanning cannot be achieved, knowledge of the ferric ion diffusion time scale is important in experimental designs in order to avoid potential measurement errors in MRI-Fricke-agarose gel dosimetry.