指数-6流体的自扩散系数的分子动力学模拟

Self-diffusion coefficients of exponential-six fluids are studied using equilibrium molecular dynamics simulation technique. Mean-square displacements and velocity autocorrelation functions are used to calculate self-diffusion coefficients through Einstein equation and Green-Kubo formula. It has been found that simulation results are in good agreement with experimental data for liquid argon which is taken as exponential-six fluid. The effects of density, temperature and steepness factor for repulsive part of exponential-six potential on self-diffusion coefficients are also investigated. The simulation results indicate that the self-diffusion coefficient of exponential-six fluid increases as temperature increases and density decreases. In addition, the larger self-diffusion coefficients are obtained as the steepness factor increases at the same temperature and density condition.     

Tracer diffusion of cadmium and mercury in mercury over the temperature range 283–333 K

Calculations associated with the technique of amalgam polarography have demonstrated that there is an urgent need for accurate diffusion coefficient for metals diffusing in mercury. A set of data for trace cadmium diffusing in mercury amalgams has been produced over the temperature range 283–333 K and for zinc at 298 K. As a procedural check, self-diffusion coefficients in liquid mercury have been remeasured over the same temperature range. The classical open-ended capillary technique has been used for both sets of measurements.     

Prediction of the self-diffusion coefficients in aqueous KCl solution using molecular dynamics: A comparative study of two force fields

Molecular dynamic simulation was used to calculate the self-diffusion coefficients of ions in aqueous KCl solution. The simulations were performed for enough time (12 ns) in the form of all-atom to determine the accurate values of the self-diffusion coefficients. The values of the self-diffusion coefficients were calculated by Einstein equation. Two different force fields of Dang and Deublein were employed in the simulations, and we found that at low ion concentration (equal or less than 3mol/(kg of H₂O)), the Dang force field is more accurate for prediction of the selfdiffusion coefficient of K⁺ ions and Deublein force field is more accurate for Cl^? ions. An Arrhenius type equation was used to model the temperature dependence of the self-diffusion coefficients and the diffusion activation energies at different ion concentrations were reported.     

高压下长链分子的自扩散系数研究

在Lennard-Jones流体自扩散系数方程的基础上,结合链状硬球流体自扩散系数的分子模拟数据,提出了链状Lennard-Jones流体的自扩散系数方程。将所提出的方程应用于计算长链烷烃和硅油在0.1-600MPa下的自扩散系数,其平均相对偏差在10％以内,计算结果优于自扩散系数的粗糙Lennard-Jones模型。     

Determination of the Oxygen Self-Diffusion Coefficients in Y2O3-Containing Tetragonal Zirconia Polycrystals by Raman Spectrometric Monitoring of the 16O-18O Exchange Reaction

A new method of determining the oxygen self-diffusion coefficients (D) in oxides has been developed. The method is based on Raman spectroscopy combined with the ¹⁶O–¹⁸O exchange technique. By using the time dependent ¹⁸O2 concentration measured in a quasi in situ manner by Raman spectroscopy, the oxygen self-diffusion coefficients are calculated. The calculation takes into account the influence of the surface exchange reaction and the limited gas volume. The result obtained in 2.8 mo1% Y₂O₃-containing tetragonal zirconia polycrystals is D = 1.55 (^+0.07_-0.07) x 10^-2 exp[(-120.0 0.4) (kJ/mol) / RT ] cm²/sec. It was demonstrated that Raman spectroscopy is useful tool for determining the oxygen self-diffusion coefficients in oxides.     

Thermal sampling in polymers with distributed relaxations: PMMA

The thermal sampling (t.s.) technique, a modification of thermally stimulated discharge (t.s.d.), has been applied to investigate molecular motions in poly(methyl methacrylate) and to compare the results with the theory of t.s. It was found that the distribution of relaxation times is represented by a distribution in activation energies and the temperature dependence of relaxation times is governed by the compensation law. A statistical test confirmed the validity of the compensation effect in the case of glass transition in PMMA. A theory based on self-diffusion has been applied to explain this effect and good agreement of the calculated expansion coefficient with the dilatometric data was obtained.     

Nitrogen Diffusion in Uranium Nitride as Measured by Alpha Particle Activation of 15N

A new technique, based on the nuclear activation reaction ¹⁵N(α, n)¹⁸F, was used to measure nitrogen self-diffusion coefficients in UN. Temperatures were 1700° to 2000°C and N₂ pressures 7 to 603 torr. The diffusion coefficients were in fair agreement with those obtained by the gaseous exchange method in an independent study. No fundamental significance was attached to the activation energy because UN is nonstoichiometric under these conditions, and the relation between composition and N₂ pressure is unknown. The results are compatible with nitrogen interstitial diffusion.     

Molecular weight and temperature dependence of self-diffusion coefficients in polyethylene and polystyrene melts investigated using a modified n.m.r. field-gradient technique

Self-difusion in polyethylene and polystyrene melts has been investigated using a modified n.m.r. field-gradient technique. In both polymer examples, self-diffusion coefficients were found to be proportional to M^?2.0±0.1_w above and below the critical molecular weight M_c. The observation of undeuterated chains in deuterated matrices of varying chain length indicates only relatively weak matrix effects. The influence of internal field gradients, which have previously been reported for polyethylene melts, is demonstrated and discussed. Certain peculiarities have been observed in the temperature dependences of the diffusion coefficients.     

Self-Diffusion of 60Co in Co1-xO Single Crystals of Controlled Stoichiometry

Cobalt tracer self-diffusion coefficients were determined in single crystals of Co_1-x O at 1037° to 1350°C and for x=0.002 to 0.00s. In crystals with x fixed at 0.005, the results were    
The activation energy in this constant-stoichiometry case was identified as the enthalpy of motion for Co ions moving via a vacancy jump mechanism. Based on known stoichiometry rela- tions, a published activation energy for Co self-diffusion mea- surements in air was corrected to yield an enthalpy of motion in excellent agreement with the number given above. At a constant temperature of 115O0°±2°C, the P_o2 dependence of Co self- diffusion was    
The P_o2 expynent suggests that the predominant mobile ion defects are singly ionized Co vacancies. Simple defect reactions with no modifications for defect clustering were adequate to explain all results obtained.     

N-甲基甲酰胺的自扩散和结构性质的分子动力学模拟

采用分子动力学模拟方法系统研究了宽广的温度和压力范围内N-甲基甲酰胺的密度、自扩散系数和结构性质。结果表明,自扩散系数的模拟值与文献实验值吻合得很好,平均相对误差为7.3%。N-甲基甲酰胺中的氧原子与氨基上的氢原子形成强烈的氢键,而氧原子与羰基上的氢原子的相互作用很弱。自扩散系数和径向分布函数都是受温度的影响比受压力的影响更明显。     

氨的自扩散系数的分子动力学模拟研究

采用分子动力学模拟方法研究了氨在较宽温度和压力范围的分子自扩散系数。从常温到高温,自扩散系数的模拟值与实验值吻合得很好,这表明可以采用分子动力学模拟来代替实验,获得高温高压条件下实验难以测量的自扩散系数。     

Positron Emission Profiling Study of the Diffusion of 3-Methylpentane in Silicalite-1

The influence of the loading on the self-diffusion of 3-methylpentane within silicalite-1 has been studied with Positron Emission Profiling technique. A monotonous decrease in the self-diffusivities of alkane was observed with zeolite loading, which could be approximated by a stretched exponential. As a consequence, the apparent activation energy increases with partial pressure.     

Self-Diffusion of Er in Pure and HfO2-Doped Polycrystalline Er2O3

Using a tracer sectioning technique, the self-diffusion of Er in pure and HfO₂-doped polycrystalline Er₂O₃ was measured at 1614° to 1900°C. Up to ≊ 10 mol% HfO₂ dopant level, the Er self-diffusion coefficients followed a relation based on cation vacancies as the principal mobile defects present and available for cation diffusion. Above 10 mol% HfO₂, deviation from this relation occurred, apparently due to clustering of cation vacancies and oxygen interstitials around the dopant hafnium ions. The activation energy for the self-diffusion of Er in pure Er₂O₃ was 82.2 kcal/mol and increased with the HfO₂ dopant level present.     

甲烷的自扩散系数的分子动力学模拟研究

运用分子动力学模拟方法研究了甲烷在较宽温度和压力范围的分子自扩散系数。结果表明,甲烷自扩散系数的模拟值与文献实验值吻合得较好。因此,可以采用分子动力学模拟来代替实验,获得高温高压条件下实验难以测量的甲烷自扩散系数。     

非电解质溶液扩散系数的理论研究评述

许多过程都涉及扩散控制的传质,相应的扩散速率计算对过程工程的精确量化具有重要意义.本文简述了扩散速率的理论表述模型,并着重介绍了非电解质溶液内扩散系数的理论分析计算途径.分子动力学模拟正处于发展中,并用于估算自扩散系数,但进入实用化阶段尚需时日,唯象模型仍为最重要的理论计算手段.作为最常用的方法,由无限稀释浓度下的扩散系数以及合适的插值方法可计算特定浓度下的扩散系数;自由体积理论是半经验性模型,可以计算自扩散系数,并借助混合规则计算相互扩散系数;最后,对扩散系数的理论研究作了展望.     

Kinetics of Initial Sintering of Vacuum-Reduced Titanium Dioxide

Observations of sphere-to-plate bonding of vacuum-reduced monocrystalline rutile have been carried out over the temperature range 1200° to 1275°C. The rate law governing the interfacial growth between sphere and plate indicates that the predominant mechanism of material transport in this sintering process is volume diffusion. In the light of the reasonably satisfactory agreement between the oxygen self-diffusion coefficients calculated from the present sintering data and those directly observed by Haul and Just utilizing an isotopic exchange technique, it is likely that oxygen ion diffusion is the rate-determining step in the sintering experiments described.     

Oxygen Diffusion in Magnesium Aluminate Spinel

Oxygen self-diffusion in MgAl₂O₄ single crystals was investigated by annealing the samples in ¹⁸O enriched gas at 98 kPa (1 atm) and measuring the tracer concentration profiles using a proton activation technique. The diffusion coefficients in the range 1625 to 1925 K for stoichiometric crystals are represented by:    
Oxygen diffusivity in alumina-rich nonstoichiometric samples is higher than in stoichiometric samples.     

Temperature dependence of thermal diffusion in lithium borate melts from light scattering data

It has been established that a constant temperature gradient increases the intensity of light scattering by lithium borate melts containing 1.2 and 1.6 mol % Li₂O. It has been shown that the change of the light scattering intensity is characterized by some general regularities that are in accord with the predictions of the theory of light scattering by liquids in an inhomogeneous temperature field. Using the obtained experimental data and the conclusions of this theory, the thermal diffusion coefficients have been determined and the temperature coefficients of the diffusion process have been calculated. It has been found that the mass thermal diffusion coefficients of the studied melts almost coincide in magnitude, and the thermal coefficients of the diffusion processes are close to the activation energy of the viscous flow of boron oxide. The obtained results indicate the applicability of the light scattering method for determination of the thermal diffusion coefficients in oxide melts.     

On predicting self-diffusion coefficients from viscosity in gases and liquids

The relations between self-diffusion and viscosity for compressed liquids and gases have been reviewed, and a new equation for correlating viscosities over wide ranges of temperature and pressure is proposed. This formula is inspired by the Lennard-Jones Chain model of Yu and Gao for self-diffusion, and represents the viscosities of 15 compounds (1046 data points) with an average absolute deviation of 6.95%. Moreover, as the presented equation and the Yu-Gao model require the same fitting parameters, the ability to calculate self-diffusion coefficients from the viscosity parameter is studied. Some of the classic reviewed relations, such as the Stokes-Einstein formula, are also contrasted with the available experimental data of both transport properties.     

Estimating liquid transport properties by a free volume method

A reduced free volume has been used to represent the viscosities as well as the self-diffusion coefficients of different hydrocarbon liquids with viscosity less than 1 × 10^?3 Pa.s at 298K by a master curve. Such correlations allow for the estimation of liquid viscosities as well as the self-diffusion coefficients of a binary liquid mixture at infinite dilution using only density and occupied volume data of the liquids.