具有表面弛豫的液体自扩散的M0nte Carlo模拟

研究多孔介质中液体的自扩散行为能获得介质的微观结构信息,有助于了解介质中液体的传输性质.以具有不同孔隙大小的无限长圆柱体模型中的液体为对象,采用Monte Cado随机行走方法,模拟存在表面弛豫时液体的自扩散系数和核自旋磁化强度随时间的变化,导出将NMR弛豫参数和随机行走参数联系在一起的表达式.结果表明:在快扩散区,液体的核自旋磁化强度随弛豫时间呈单指数衰减,且自扩散系数在短时情况下独立于表面弛豫率;在慢扩散区,液体的核自旋磁化强度衰减和自扩散系数在短时情况下均与表面弛豫率无关.模拟结果与理论分析相吻合,可用于求解介质的表面积与体积之比及孔径大小等结构信息.     

岩心孔隙介质中流体的核磁共振弛豫

弛豫时间是核磁共振研究中的一个重要参数,岩心孔隙介质流体的弛豫过程是自由流体弛豫机制、表面弛豫机制和流体的扩散弛豫机制共同作用的结果,它包含了丰富的孔隙和流体本身的信息. 弛豫时间和自扩散系数的测量及对弛豫时间的分析是核磁共振技术应用于岩心分析和石油勘测的重要内容.     

表面活性剂在水溶液中聚集状态的1H NMR研究

表面活性剂在水溶液中的自扩散系数,¹H化学位移和自旋晶格弛豫的NMR测量表明,上述参数随表面活性剂浓度而变化,并在某一浓度开始有转折点.这一转折点恰好对应着各自的临界胶团浓度.表面活性剂分子的不同基团的质子化学位移随其浓度变化幅度的不同提供胶团形成的分子水平描述.     

核磁共振中多量子相干扩散行为的理论表述和计算机模拟

改进了Warren所提出的CRAZED脉冲序列以研究分子间多量子相干的扩散过程 ,讨论了利用核磁共振测量分子内和分子间多量子相干表观自扩散系数的理论表述 ,采用粒子的随机行走模型模拟其扩散行为 .在短脉冲近似和长脉冲梯度场两种实验条件下 ,分别获得了因扩散引起的不同相干阶数的相对信号衰减强度随梯度场脉冲间隔时间的变化曲线 ,由此得到分子内多量子相干和分子间多量子相干的表观扩散率与溶液分子扩散系数的关系 .还将计算机模拟结果与理论预测进行分析和比较 ,发现二者能很好地吻合 .研究结果表明 ,分子间多量子相干的表观扩散率与常规的分子内多量子相干的表观扩散率明显不同 ,因此 ,分子间多量子相干的表观扩散率可能提供一种新的核磁共振成像的对比度机理     

从常温常压到超临界乙醇的分子动力学模拟

采用分子动力学方法系统地研究了从常温常压到超临界状态乙醇的热力学性质、结构性质和动力学性质.模拟发现随着温度的升高,体系焓值增大,乙醇分子间的氢键作用减弱,自扩散系数增大;随着压强的增大,乙醇分子间的氢键作用增强,自扩散系数减小;乙醇自扩散系数在液相区随温度变化不明显,在气相区随压强增大很快减小,超临界区乙醇的自扩散系数比液相区大十几倍.温度和压强对乙醇自扩散系数的影响可通过密度来体现.与常温常压相比,超临界条件下的乙醇体系因密度涨落存在分子聚集现象,且在低密度区域更显著;乙醇分子间的氢键作用明显减弱,结     

纳米铜团簇扩散性质研究

本文通过分子动力学模拟研究了纳米铜团簇的自扩散性质,结果表明Nc8949铜团簇自扩散系数随温度的升高而增大,在温度为1000 K时纳米铜团簇的扩散系数随团簇半径的倒数基本呈线性增加.同时指出在常温下团簇几乎无扩散行为,而某些文献中关于常温下晶粒扩散分子动力学模拟结果是模拟体系宏观转动造成的虚假现象.?     

饱和砂岩的滞弹性弛豫衰减特征及微观机理的探索

应用Metravib热机械分析仪,以饱和岩石进行正弦波加载的方式, 分别对三种不同孔隙度的泵油饱和彭山砂岩、泵油和甘油饱和自贡长石砂岩进行了衰减实验研究,得到衰减的热弛豫规律.据此规律求得它们的激活能和原子振动频率, 其激活能和原子的振动频率比间隙原子的激活能和振动的频率低, 此现象用饱和砂岩中产生的缺陷原子簇的整体振动比单个或孤立的原子的振动频率低做出了解释. 在饱和岩石的晶粒间界缺陷处参与扩散的是固体原子、液体及气体原子. 并得出随孔隙度和黏滞系数增大,衰减强度和激活能增大,原子的振动频率增高,弛豫时间减小的结论. 在正弦波应力作用下,由多种矿物晶体胶结而成的饱和砂岩是一种多晶、多相的固体, 由于内部结构复杂、缺陷广布,产生弛豫衰减是普遍现象.砂岩中存在点缺陷、位错、 晶界及孪晶界面等许多缺陷及缺陷间的相互作用都可以产生弛豫型衰减峰. 用饱和砂岩中特有的饱和液体及砂岩内部结构的复杂性解释了饱和砂岩的衰减机理, 很自然地将其宏观衰减特征与微观结构紧密连在一起.饱和砂岩中的各种缺陷、 相界等会导致多重弛豫,使它们的弛豫衰减峰变宽,分布参数增大.这项研究既具有理论意义,也具有实用价值.     

非均匀交换各向异性铁磁介质的非线性表面自旋波

利用Landau-Lifshitz 方程，研究了具有非均匀交换各向异性的半无限大铁磁体的非线性表 面自旋波理论。导出了部分钉扎纯交换铁磁介质的磁化强度所满足的边界条件和非线性表面 自旋波的色散关系，并获得了自旋波振幅沿z方向驻波的一维非线性Schrdinger方程和包 络振幅沿平面传播的二维非线性Schrdinger方程，结果表明铁磁体磁化强度的包络振幅随时空变化的性质是由二维非线性Schrdinger方程决定的。因此预言铁磁介质的表面非线性激发应是二维孤波的形式。对于弱非线性表面自旋波，对非线性Schrdinger方程存在孤子形式解的可能性作了讨论. 关键词： 表面自旋波 Landau-Lifshitz方程 非线性Schrdinger方程 孤子     

正十二烷基硫酸钠在聚丙烯酰胺溶液中聚集的1H NMR研究

利用核磁共振自旋-晶格弛豫时间（T₁）、自旋-自旋弛豫时间（T₂）、自扩散系数（D）以及二维核Overhause增强谱（2D NOESY）技术研究了表面活性剂正十二烷基硫酸钠（SDS）在聚丙烯酰胺（PAM）浓度固定为10 g/L水溶液中的聚集. 结合与SDS水溶液体系核磁共振实验数据比较,得到了如下信息：（1） 当溶液中有PAM存在时,SDS分子的运动性下降,临界聚集浓度提前;（2） 随着SDS浓度的增加,PAM分子的自扩散性能下降,同时分子链的柔软性也下降了;（3） 2D NOESY谱结果表明,PAM与SDS分子间未发生直接的缔合作用.     

Ultrasound Attenuation in s-wave Two-band Superconductors北大核心CSCD

s~±-波电子配对被认为很可能用于解释铁基超导体的配对机制.本文中,我们计算了s~±-波二带超导体的超声衰减系数,并利用T-矩阵近似研究了杂质的影响.我们的计算表明,当两带的能隙大小相当时,超声衰减-温度曲线中出现一个Hebel-Slichter峰.我们同时验证了核自旋弛豫率的研究中发现的低温下的共振束缚态效应.     

Zur Deutung der Protonenspinrelaxation in Glycerin

In contrast with the usual assumption that proton spin relaxation in glycerol originates from rotational molecular diffusion, quantitative evaluation of the temperature and Larmor frequency dependence in the ranges ?30 °C to+70 °C and 10 kHz to 120 MHz, respectively, gives extensive agreement with Fick's translational random walk model. This observation is supported by direct measurements of the self-diffusion constant by means of the pulse gradient method, which reveals the same activation barrier as relaxation spectroscopy.     

Nonstochastic behavior of atomic surface diffusion on Cu(111) down to low temperatures

Atomic diffusion is usually understood as a succession of random, independent displacements of an adatom over the surface's potential energy landscape. Nevertheless, an analysis of molecular dynamics simulations of self-diffusion on Cu(111) demonstrates the existence of different types of correlations in the atomic jumps at all temperatures. Thus, the atomic displacements cannot be correctly described in terms of a random walk model. This fact has a profound impact on the determination and interpretation of diffusion coefficients and activation barriers.     

Ground states versus low-temperature equilibria in random field Ising chains

Random walk arguments and exact numerical computations are used to study one-dimensional random field chains. The ground state structure is described with absorbing and non-absorbing random walk excursions. At low temperatures, the local magnetization follows the ground state except at regions where a local random field fluctuation makes thermal excitations easier. This is explained by the random walk picture, implying that the magnetization lengthscale is a product of the domain size and the thermal excitation scale. Received 16 October 2000 and Received in final form 7 June 2001     

Unified analysis of diffusion and relaxation processes in amorphous metallic alloys

Abstract

Self-diffusion data on amorphous metallic alloys determined in long-time radio-tracer experiments and activation-enthalpy spectra deduced from short-time structural relaxation studies on such materials are reviewed and analyzed in terms of current random transition rate models. It is shown that the seeming discrepancy between the self-diffusion enthalpies and the comparatively small enthalpies obtained from measurements of the magnetic after-effect arises from the existence of activation-enthalpy spectra. Whereas the short-time experiments reveal the small-activation-enthalpy parts of these spectra, the long-time self-diffusion experiments are controlled by the larger activation enthalpies. Assuming that the activation-enthalpy distributions are Gaussian, their characteristic parameters have been determined by comparing the two types of measurements. It is shown that the Arrhenius-type diffusion coefficients found by experiment are compatible with half-widths of the activation-enthalpy spectra of about 0.3 eV. Based on an analysis of the pre-exponential factors and other typical properties of the diffusion coefficients, potential mechanisms of the diffusion in amorphous alloys are proposed.     

Specific features of proton NMR relaxation of hydrocarbons and water in the pore space of silicates

The relaxation of the proton magnetization of water and hydrocarbons in a model medium of glass beads and quartz sand is studied by the NMR method. The spectrum of relaxation times of fluids is one-component in the model environment and three-component in quartz sand. The surface relaxivities measured in the model medium are used to determine the pore size distribution in quartz sand. Estimates of the specific surface area of sand based on the relaxation data are consistent with the values measured by the sorption method. The EPR method is used to determine the chemical nature of the active paramagnetic centers responsible for the surface relaxation of the proton magnetization. Differences in the relaxation behavior of aqueous and hydrocarbon fluids are interpreted within the framework of a simple model of surface relaxation.     

Magnetic relaxation in a random system of interacting rodlike nanoparticles

It is shown in the framework of the generalized mean-field approximation taking into account spatial fluctuations of the local magnetic field that the collective effect of dipole interaction in a random 3D system of identical (rodlike) magnetic nanoparticles with parallel easy magnetization axes shifts the relaxation magnetization curves towards shorter times (i.e., accelerates the relaxation process). In addition, the course of this process depends (via the demagnetizing field) on the sample shape. The interaction between nanograins affects the magnetization relaxation of a random 2D system only when the magnetic moments of the grains are perpendicular to the plane of the system.     

Asymptotic properties of multistate random walks. I. Theory

A calculation is presented of the long-time behavior of various random walk properties (moments, probability of return to the origin, expected number of distinct sites visited) formultistate random walks on periodic lattices. In particular, we consider inhomogeneous periodic lattices, consisting of a periodically repeated unit cell which contains a finite number of internal states (sites). The results are identical to those for perfect lattices except for a renormalization of coefficients. For walks without drift, it is found that all the asymptotic random walk properties are determined by the diffusion coefficients for the multistate random walk. The diffusion coefficients can be obtained by a simple matrix algorithm presented here. Both discrete and continuous time random walks are considered. The results are not restricted to nearest-neighbor random walks but apply as long as the single-step probability distributions associated with each of the internal states have finite means and variances.     

Application of random walk concept to the cyclic diffusion mechanisms for self-diffusion in intermetallic compounds

Huntington–Elcock–McCombie (HEM) mechanism involving six consecutive and correlated jumps, a triple-defect mechanism (TDM) involving three correlated jumps and an anti-structure bridge (ASB) mechanism invoking the migration of an anti-structure atom are the three mechanisms currently in vogue to explain the self- and solute diffusion in intermetallic compounds. Among them, HEM and TDM are cyclic in nature. The HEM and TDM constitute the theme of the present article. The concept of random walk is applied to them and appropriate expressions for the diffusion coefficient are derived. These equations are then employed to estimate activation energies for self-diffusion via HEM and TDM processes and compared with the available experimental data on activation energy for self-diffusion in intermetallic compounds. The resulting activation energies do not favour HEM and TDM for the self-diffusion in intermetallic compounds. A comparison of the sum of experimentally determined activation energies for vacancy formation and migration with the activation energies for self-diffusion determined from radioactive tracer method favours the conventional monovacancy-mediated process for self-diffusion in intermetallic compounds.     

Diffusion-weighted spatial information from1H relaxation in restricted geometries

Summary NMR relaxation of water¹H confined in restricted geometries, whatever is the nature of the system (porous media saturated by water as well as biological tissues), exhibits common characteristics. Artificial microporous media saturated by water have been chosen as model systems to study the longitudinal and transverse relaxation of¹H magnetization of water molecules diffusing in restricted geometries. These systems are very stable, easy to prepare, with well-characterized pore size distribution and connections, and with highly homogeneous surface properties. The response was compared with that from more complex natural porous media. Scanning Electron Microscopy techniques demonstrated spatial characteristics and surface properties of the samples. The information content of longitudinal relaxation curves associated with spatial structure and due to restricted diffusion is shown in these samples. The effect on transverse relaxation of self-diffusion in the presence of spatially varying magnetic fields due to susceptibility differences is shown. A simple linear relationship has been found in all samples between the transverse relaxation rate and the interpulse delay in CPMG experiments, in spite of the variety of pore shapes and sizes. In general, one can say that relaxation curves beardiffusion-weighted information on the pore space framework. The role of the investigated relaxation mechanisms is important also in the response of biological tissues, including in the presence of MR Imaging contrast agents inducing microscopic magnetic-field gradients. Work partially supported by CNR and MURST Grants.