超临界CO_2在岩石孔隙内流动状态的分子模拟

本文采用分子模拟的方法研究了地质封存条件下超临界CO_2在镁橄榄石孔隙内的流动,分析了孔隙尺寸、温度、压强对超临界CO_2的密度分布和流动速度的影响.研究结果表明,只有当孔隙尺寸大于5.0 nm时,超临界CO_2分子在岩石孔隙内的流动才符合Poiseuille流动;同时,超临界CO_2分子在岩石近壁面存在1.5 nm的密度震荡,当孔隙尺寸小于15 nm时,密度振荡现象会影响CO_2分子在岩石孔隙内的平均密度;升高温度、降低压强的方法可以减小密度震荡的第一峰值,减弱岩石壁面与CO_2的相互作用,使得CO_2的流动速度增加.     

Structure and dynamics of water: from ambient to supercritical

Our recent works on supercritical water are reviewed. In order to elucidate the hydrogen bonding state of supercritical water, the proton chemical shift of the water proton is measured at temperatures up to 400 °C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60 g/cm³. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is then related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule at the densities larger than the critical. The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is also estimated from the number of hydrogen bonds. The dynamical counterpart of our structural study of supercritical water has been performed by NMR relaxation measurements. Using D₂O, we measured the spin-lattice relaxation time and determined the reorientational relaxation time as a function of the water density and temperature. It is then found that while the reorientational relaxation time decreases rapidly with the temperature in the subcritical condition, it is a weak function of the density in the supercritical conditions.     

Distinctive features of random local motions in critical and supercritical fluids

Equations for large-scale local fluctuations in fluids, from an ideal gas to an incompressible fluid, including the critical and supercritical state are derived for the first time based on the first principles. The modern phenomenological representation of the critical state of fluids is confirmed and essentially refined; in particular, it is demonstrated that that local density fluctuations in a compressible fluid are accompanied by nonthermodynamic fluctuations in the collective velocity and temperature of the fluid. Distinctive features of the development of these fluctuations near the critical point determine the specific behavior of fluids in the critical and supercritical states.     

Performance Enhancement of Al Ga N/Ga N MIS-HEMTs Realized via Supercritical Nitridation Technology

This paper proposes a method of repairing interface defects by supercritical nitridation technology,in order to suppress the threshold voltage shift of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors(MIS-HEMTs).We find that supercritical NH_3 fluid has the characteristics of both liquid NH_3 and gaseous NH_3 simultaneously,i.e.,high penetration and high solubility,which penetrate the packaging of MIS-HEMTs.In addition,NH_2~- produced via the auto coupling ionization of NH_3 has strong nucleophilic ability,and is able to fill nitrogen vacancies near the GaN surface created by high temperature processes.After supercritical fluid treatment,the threshold voltage shift is reduced from 1 V to 0 V,and the interface trap density is reduced by two orders of magnitude.The results show that the threshold voltage shift of MIS-HEMTs can be effectively suppressed by means of supercritical nitridation technology.     

Hydrogen bonding and clusters in supercritical methanol–water mixture by neutron diffraction with H/D substitution combined with empirical potential structure refinement modelling

ABSTRACT

Neutron diffraction measurements of H/D isotopic substitution have been performed for seven H/D substituted methanol-water mixtures of 0.3?mol fraction of methanol (x_M) under the supercritical (618?K, 100?MPa) and ambient (298?K, 0.1?MPa) conditions. The seven structure factors obtained were subjected to an empirical potential structure refinement (EPSR) modelling to derive all site-site pair correlation functions, coordination number distributions, spatial density functions, and cluster distributions. Water has a four coordinated structure in the first coordination shell under both ambient and supercritical conditions; however, the spatial density distribution of water molecules in the second coordination shell is delocalised under the supercritical condition. The mean coordination number of all atomic pairs with hydrophilic interactions decreases in the supercritical state. On the other hand, the mean coordination number of interactions between the hydrophobic part of methanol and water molecule is less sensitive to temperature. In the supercritical condition, water clusters with a wide size distribution are generated in a methanol-water mixture as well as in pure water. Since the critical temperature of a methanol-water mixture is lower than that of pure water, it can be concluded that the addition of methanol can generate fragment water clusters at a lower temperature.     

Bifurcations and stability of internal solitary waves

We study both supercritical and subcritical bifurcations of internal solitary waves propagating along the interface between two deep ideal fluids. We derive a generalized nonlinear Schrödinger equation to describe solitons near the critical density ratio corresponding to transition from subcritical to supercritical bifurcation. This equation takes into account gradient terms for the four-wave interactions (the so-called Lifshitz term and a nonlocal term analogous to that first found by Dysthe for pure gravity waves), as well as the six-wave nonlinear interaction term. Within this model, we find two branches of solitons and analyze their Lyapunov stability.     

Experimental Observation of Simultaneously Existing Moving and Standing Patterns in a Gas-Discharge System

In a quasi 1-dimensional lateral gas-discharge system we observe experimentally sub- and supercritical bifurcations from a spatially homogeneous to an inhomogeneous distribution of the current density. Among other results we find for the first time the simultaneous existence of a nontrivial nonstationary pattern (travelling waves) with a nontrivial stationary pattern (periodic filaments) in a laterally extended dc-driven gas-discharge system.     

Topology and hydrogen connectivity in supercritical water

The experimental density dependences of relative fractions of the “liquid-like” and “gas-like” components of supercritical water have been adequately described in terms of the two-component structural dynamic model proposed earlier for interpreting the experimental results on neutron scattering by supercritical water (on the isotherm at 400°C, the density range 0.005 < ρ < 0.32 g/cm³) using the data on the hydrogen connectivity in supercritical water obtained by the molecular dynamics method.     

超临界二氧化碳类液-类气区边界线数值分析

超临界二氧化碳(S-CO₂)因在萃取、沉淀、热力循环及化学反应等方面有着十分广阔的应用前景,逐渐成为学术界的重要研究课题.由于在近临界区,可以观察到随温度或压力变化出现大量的物性异变现象,使得各国学者对流体临界点附近区域的研究产生了浓厚兴趣.随着分子动力学模拟技术的快速发展,该技术可辅助传统实验方法用于研究近临界流体的相关物性.为确定S-CO₂在近临界区Widom线范围及类液-类气区的分子结构特征,本文通过分子动力学模拟技术结合聚类分析,研究了温度和压力范围分别在300—350 K和5.5—18.5 MPa下,CO₂密度时间序列变异系数及偏度同Widom线和类液-类气区间的关系.结果表明:S-CO₂在近临界区Widom线的确定可通过连接密度时间序列曲线变异系数极大值点来确定,Widom线沿着临界点开始延伸直到350 K时停止;S-CO₂类液区和类气区的分子分布结构可以用数密度分布的偏度来区分,偏度在类气态时为正值,在类液态时为负值,而在Widom线上达到最大值.     

超临界二氧化碳类液-类气区边界线数值分析

超临界二氧化碳(S-CO₂)因在萃取、沉淀、热力循环及化学反应等方面有着十分广阔的应用前景,逐渐成为学术界的重要研究课题.由于在近临界区,可以观察到随温度或压力变化出现大量的物性异变现象,使得各国学者对流体临界点附近区域的研究产生了浓厚兴趣.随着分子动力学模拟技术的快速发展,该技术可辅助传统实验方法用于研究近临界流体的相关物性.为确定S-CO₂在近临界区Widom线范围及类液-类气区的分子结构特征,本文通过分子动力学模拟技术结合聚类分析,研究了温度和压力范围分别在300—350 K和5.5—18.5 MPa下,CO₂密度时间序列变异系数及偏度同Widom线和类液-类气区间的关系.结果表明:S-CO₂在近临界区Widom线的确定可通过连接密度时间序列曲线变异系数极大值点来确定,Widom线沿着临界点开始延伸直到350 K时停止;S-CO₂类液区和类气区的分子分布结构可以用数密度分布的偏度来区分,偏度在类气态时为正值,在类液态时为负值,而在Widom线上达到最大值.     

The concentration fluctuations and the mutual diffusion coefficient in the supercritical solution

Molecular dynamics simulations on Lennard-Jones mixture in the supercritical region are performed. The number of the molecules in the unit cell is 23328 and the mole fraction is 0.5. The concentration fluctuations near the critical point are large compared to the ideal mixing. The mutual diffusion coefficient normalized by the self-diffusion coefficient has a small value near the critical density because of the large concentration fluctuation. The heat capacity, partial molar volume and correlation lengths of density and concentration in the supercritical region are compared with those at the normal density.     

Vacuum polarization and screening of supercritical impurities in graphene

Screening of charge impurities in graphene is analyzed using the exact solution for vacuum polarization obtained from the massless Dirac-Kepler problem. For the impurity charge below a certain critical value, no density perturbation is found away from the impurity, in agreement with perturbation theory. For the supercritical charge, however, the polarization distribution is shown to have a power law profile, leading to screening of the excess charge at large distances. The Dirac-Kepler scattering states give rise to standing wave oscillations in the local density of states which are prominent in the supercritical regime.     

超临界水冷堆中子能谱计算及安全性分析

超临界水堆是国际第Ⅳ代核能系统论坛推荐的六种第Ⅳ代核电反应堆堆型之一,与现有的轻水堆相比,具有热效率高、系统结构简单、造价低等优点。建立了MCNP程序下的超临界水堆堆芯物理计算模型,解决了燃料组件几何结构过于复杂精细难以建模的技术难题;考虑了堆芯轴向冷却剂密度的不均匀分布,计算并分析各区域的中子能谱分布;对失水事故下的超临界水冷堆安全性进行了分析,研究了不同区域冷却剂丢失程度对反应性及有效增殖系数的影响,表明所设计堆型具有较高的安全性;分析处理失水事故的应对措施,验证了使用注入硼水措施处理超临界水冷堆失水事故的可行性。     

Free energies of supercritical solvation from molecular dynamics simulation and integral equation studies

We present here molecular dynamics (MD) simulation and integral equation (IE) studies on free energies of solvation of a non-polar solute in a dilute supercritical solvent to estimate the contribution of inhomogeneities in solvent density to the free energy of solvation. The solvation of a Xe-like solute in an Ne-like solvent as well as that of naphthalene in CO₂ have been investigated. At state points in the compressible region in the neighborhood of the solvent critical point, we have utilized the IE estimates of free energies to model the ideal situation where local density inhomogeneities would be absent. The difference between the free energies in the presence (as derived from MD simulation) and in the absence (from IE) of local density inhomogeneities was studied as a function of density along an isotherm close to the critical point. Although for low density supercritical solvents, a marked difference is observed, a study of the density dependence of this difference across the critical density does not directly reveal any signature of local density enhancement on the thermodynamics of solvation.     

Hydrodynamics,electroosmosis, and electrokinetic instability in imperfect electric membranes

For the first time within the framework of the Nernst?Planck?Poisson?Stokes set, the behavior of an electrolytic solution in imperfect electric membranes is investigated numerically. The distributions of the electric potential, fluid velocities, charge density, and other values in the zones of the depleted and enriched solution and a porous membrane for all current modes are obtained: prelimit, limit, and superlimit. The dependence of selectivity of a membrane on the density of its space charge is obtained. The boundaries of superlimit modes, when the one-dimensional solution loses stability, are found and replaced by the twodimensional solution with the formation of microvortices in both the desalination and enriched-solution zones. The basic types of current arising at supercritical currents are investigated.     

Hydrothermal Synthesis of Metal Oxide Nanoparticles at Supercritical Conditions

Hydrothermal synthesis of CeO₂ and AlO(OH) were conducted using a flow type apparatus over the range of temperature from 523 to 673 K at 30 MPa. Nanosize crystals were formed at supercritical conditions. The mechanism of nanoparticle formation at supercritical conditions was discussed based on the metal oxide solubility and kinetics of the hydrothermal synthesis reaction. The reaction rate of Ce(NO₃)₃ and Al(NO₃)₃ was evaluated using a flow type reactor. The Arrhenius plot of the first order rate constant fell on a straight line in the subcritical region, while it deviated from the straight line to the higher values above the critical point. The solubility of Ce(OH)₃ and AlO(OH) was estimated by using a modified HKF model in a wide range of pH and temperature. In acidic conditions, where hydrothermal synthesis reaction is concerned, solubility gradually decreased with increasing temperature and then drastically dropped above the critical point. The trend of the solubility and the kinetics around the critical point could be explained by taking account of the dielectric constant effect on the reactions. There are two reasons why nanoparticle are formed at supercritical conditions. Larger particles are produced at subcritical conditions due to Ostwald ripening; that could not be observed in supercritical water because of the extremely low solubility. Second reason is the faster nucleation rate in supercritical water because of the lower solubility and the extremely fast reaction rate.     

Controlled high-power heat release as a tool to selecting working pressure for supercritical water

The specificity of nonsteady heat conduction in water in the course of its rapid isobaric transition from the state of compressed liquid to supercritical state was studied by the method of controlled pulse heating of a thin wire probe immersed in the substance under investigation. The characteristic pulse length was of the order of milliseconds, the thickness of heating layer appeared to be of the order of micrometers, the density of heat flux from the probe into the substance was 10MW/m² by the order of magnitude. Based on the results of our experiments, we have developed a phenomenological approach to selecting the way for further research of heat transfer in supercritical fluids, in order to reach guaranteed removal of the heat fluxes with high densities. Namely, taking into account the revealed phenomenon of threshold decrease in heat transfer intensity, which is higher, the closer the value of the pressure is to the critical one, there is a reason for selecting much higher values of working pressure. The corresponding choice should be made in the range of 2.0... 3.0 p _c. We can point out such benefits of this choice (in terms of thermophysics) as the decrease in thermal resistance of the boundary layer, the enhancement in water thermal effusivity and its relatively weak dependence on pressure.     

Direct and indirect correlations in low density supercritical Lennard-Jones fluids

We have carried out a detailed comparison of the direct and indirect correlation functions obtained from canonical molecular dynamics (NVT-MD) simulation of supercritical Lennard-Jones fluids to the results obtained by solving the Ornstein-Zernike equation with the approximate Duh-Henderson (DH) closure. The variations of equilibrium correlations are studied as functions of density at two supercritical isotherms near and away from the critical point. The direct and indirect correlations predicted using the DH closure provides a very good agreement with our simulation results at low densities. However, a marked deviation is observed at higher densities. These results are correlated to the discrepancies between the density and temperature dependence of the underlying bridge function. The implication of these results on the calculation of chemical potential and the Krichevskii parameter is also presented.     

~4He超流密度分布研究

一种密度泛函理论用来研究当4He超流体流速u超过朗道临界流速vc时的不稳定性。发现有稳定的周期性密度波出现,其振幅正比于(u-vc)1/2,其波矢与旋子波矢一致。同时对纳米管道内氦流体进行了计算。     

The low-temperature behavior of disordered magnets

We map out the low-temperature phase diagrams of dilute Ising ferromagnets and predominantly ferromagnetic ferrites, obtaining nonperturbative and essentially optimal conditions on the density of ferromagnetic couplings required to maintain long-range order. We also study mappings of dilute antiferromagnets in a uniform field onto random field ferromagnets.For the randomly dilute systems, we prove that ferromagnetically ordered states exist at low temperature if the density of ferromagnetic couplings exceeds the (appropriately defined) percolation threshold, thereby extending the result of Georgii to three or more dimensions. We also show that, for these systems, as the temperature tends to zero, the magnetization approaches the percolation probability of the corresponding Bernoulli system. In two dimensions, we prove that low-temperature ordering persists in the presence of antiferromagnetic impurities if the ferromagnetic couplings percolate and if the density of antiferromagnetic couplings is bounded above by the order of the inverse square of the corresponding percolation correlation length. For these systems, we rigorously compute the first order decrease in the zero-temperature nominal spontaneous magnetization, in terms of derivatives of the percolation probability, thereby establishing the existence of ferrimagnetically ordered states. Finally, we introduce a model of a random ferrite which exhibits spontaneous magnetization anticorrelated with the boundary conditions.National Science Foundation Postdoctoral Research Fellows. Work supported in part by the National Science Foundation under Grant No. PHY-8203669Work supported in part by the National Science Foundation under Grant No. MCS-8108814 (A03)