Structural transformations in thermally modified porous glasses

The changes in the structural parameters of porous glasses (pore radius, pore volume, specific surface of pores, structural resistance coefficients) upon heating are investigated as a function of the composition of initial two-phase alkali borosilicate glasses with the use of a number of independent methods, such as the adsorption techniques (water vapor adsorption, mercury porosimetry, thermal desorption of nitrogen), transmission electron microscopy, small-angle X-ray scattering, and membrane conductivity measurements. It is demonstrated that the structural transformations in thermally modified porous glasses are associated with the processes of overcondensation of pores and viscous flow in the silica network.     

Atomic and micro-structure features of nanoporous aluminosilicate glasses from reactive molecular dynamics simulations

Long-term chemical durability of borosilicate glasses that makes them a widely accepted form of nuclear waste disposal is achieved through the formation of a porous aluminosilicate gel layer that provides passivity and limits the transport of water to the reaction front. Detailed understanding of the porous silicate gel layer is thus critical in elucidating the corrosion mechanism of these glasses and to design of new glass composition for waste immobilization and other applications. In this paper, we use the diffuse charge reactive potential to generate porous aluminosilicate glass structures with compositions equivalent to the gel layers formed at the glass-water interface with an aim to understand the processing condition on the microstructure and atomic structure of these systems. We demonstrate the use of the charge scaling techniques is an effective approach to generate these porous structures with controllable pore mophologies. After initial validation of the potentials and calcium aluminosilicate glass structures using neutron diffraction, we created gel structures with compositions similar to well-known model nuclear waste borosilicate glasses. The porosities and the pore size distribution bear a strong correlation to the processing temperature, as well as to the local atomic structure. Thus, by controlling the processing parameters, the generated porous structures can be customized to closely resemble gel structures due to borosilicate glass corrosion. These results provide insights of the micro- and atomic structure features of the porous aluminosilicate glasses and on the optimal procedure to generate porous structures that can be comparable to experimentally observed gel layer structures thus to elaborate on the correlations between the structure and phenomena in glass-water interactions.     

Radiation Resistance of Porous Glasses

The sintering and evaporation of porous glasses under exposure to CO₂ laser radiation are investigated. It is demonstrated that the resistance of porous glasses to middle-IR laser radiation depends on the conditions of their preparation. The threshold radiation power densities necessary for sintering porous glasses of different compositions are obtained. The optical breakdown in porous glasses is studied. The resistance to near-IR laser radiation is determined for porous glasses prepared under different conditions.     

Simulation of Membrane Distillation for Purifying Water Containing 1,1,1‐Trichloroethane

Vacuum membrane distillation is modeled for the purification of water containing organic matter. The separation medium is a hollow‐fiber membrane contactor that is simplified to a two‐dimensional structure with a single porous membrane wall. The model considers the transport phenomena of a vacuum membrane distillation system in porous media, in which the aqueous volatile organic solution was considered as an incompressible and steady fluid. The numerical simulation of the two‐dimensional model of vacuum membrane distillation for an aqueous solution of 1,1,1‐trichloroethane was established under steady state. The effects of the bulk feed temperature and the feed flow rate on the percentage of 1,1,1‐trichloroethane removal from an aqueous solution are discussed.     

Permeability of heterogeneous membranes based on methacrylic acid

The permeability of H⁺, Na⁺, insulin, and hemoglobin through porous membranes made of crosslinked poly(methacrylic acid) was investigated at different pH values on both sides of the membrane. It was shown that a change in the charge of the protein during its transport through the membrane might raise the driving force of the process so much that the permeability coefficient of the protein through the membrane would be higher than in solution. In the case reported here, the flow of the protein may be regarded as partial electrophoresis in a porous medium.     

Microporosity of Porous Glasses: New Investigation Techniques

Two new methods are developed for investigating the microporosity of porous materials. The first method, namely, diffusion diagnostics, is based on a computer simulation of the kinetics of gas desorption from a porous material into high vacuum. In this method, the kinetics of gas desorption is measured using mass spectrometry. Another method is based on the analysis of the equilibrium isotherms of gas desorption at low pressures. The microporous and mesoporous substructures of the porous glasses prepared by leaching of two-phase alkali borosilicate glasses in a 3 M HCl solution at 100°C are investigated using both the new methods and the classical adsorption methods. A correlation between the porous structure of glasses and their synthesis and phase separation conditions is revealed. It is demonstrated that porous glasses have a through mesoporous substructure with mean pore diameters ranging from 4 to 15 nm and a polymodal microporous substructure with pore sizes ranging from 0.35 to 2 nm. This microporous substructure is formed by ultramicropores of molecular sizes and also medium- and large-sized micropores. It is found that porous glasses contain bottle-shaped pores with sizes corresponding to the micropore range. An increase in the temperature of heat treatment of glasses from 550 to 700°C and an increase in the boron oxide content in the silica phase lead to an increase in the volume of micropores in porous glasses.     

常规流场和交指型流场的质子交换膜燃料电池传热传质三维模拟

针对常规流场和交指型流场的质子交换膜燃料电池提出了三维非等温数学模型。模型详细考虑了电池内部的传热、传质和电化学反应，重点考察了多孔介质内的组分传递和膜内水的电渗和扩散作用，对氧气传递限制和膜内水迁移对电池性能的影响进行了分析和讨论。结果表明，流道的交指型设计加强了气体在多孔介质内的质量传递，提高了电池的输出性能，但相应地，阴极催化层界面水分的减少也使得膜的水合程度降低，这就需要更有效的水管理来防止膜脱水。     

Electrical characterization of NF membranes. A modified model with charge variation along the pores

Nanofiltration rejection measurements have been performed for KCl solutions to characterize a non-commercial NF membrane. Membrane Potential (MP) and Tangential Streaming Potential (TSP) measurements have also been carried out for a wide range of concentrations. The charge of the membrane and the pore dielectric constant has been obtained as functions of the salt concentration from these three ensembles of experiments, showing a fair agreement.For the data treatment, two modifications of the Steric Electric and Dielectric Exclusion (SEDE) model have been proposed to allow the electrical characterization of the NF membrane: (1) the volumetric charge density is considered to change along the pores by means of the inclusion of a charge isotherm (which correlates the membrane charge with the ion concentration) in the transport equations, and (2) the porous support is considered by the inclusion of the adequate transport equations similarly to how the active layer is considered.     

Microporous Substructure of Porous Glasses: Dynamic and Equilibrium Approaches

Porous glasses produced by the leaching of two-phase alkali borosilicate glasses have been studied by dynamic and equilibrium methods for diagnostics of pore morphology. It has been detected the availability of microporous substructure with some kinds of adsorbing micropores of diameter 0.3–2 nm including ultramicropores of molecular size in porous glasses with transporting pores of mean diameter from 4 to 15 nm. The multimodal nanoporous structure of porous glasses detected by kinetic mass spectral method of gas diffusion diagnostics (DD-method) at low temperatures is consistent with the results obtained from analyzing equilibrium desorption isotherms of nitrogen, oxygen and argon at 77.5 K by different calculation techniques including an equilibrium method of gas desorption at low partial pressures (LPED-method). Micropore volume in porous glasses is equal to 6–18% from total pore volume. The dependence of nanoporous morphology of porous glasses on conditions of their production and composition has been established. The diffusion and equilibrium characteristics of different molecules (nitrogen, oxygen, argon) varying in molecule size and quadrupole moment value have been determined for primary and secondary porous substructures of porous glasses at liquid nitrogen temperature at the first time.     

CFD-based optimization and design of multi-channel inorganic membrane tubes☆

As a major configuration of membrane elements,multi-channel porous inorganic membrane tubes were studied by means of theoretical analysis and simulation.Configuration optimization of a cylindrical 37-channel porous inorganic membrane tube was studied by increasing membrane filtration area and increasing permeation efficiency of inner channels.An optimal ratio of the channel diameter to the inter-channel distance was proposed so as to increase the total membrane filtration area of the membrane tube.The three-dimensional computational fluid dynamics(CFD) simulation was conducted to study the cross-flow permeation flow of pure water in the 37-channel ceramic membrane tube.A model combining Navier–Stokes equation with Darcy's law and the porous jump boundary conditions was applied.The relationship between permeation efficiency and channel locations,and the method for increasing the permeation efficiency of inner channels were proposed.Some novel multichannel membrane configurations with more permeate side channels were put forward and evaluated.     

A new Navier-Stokes and Darcy's law combined model for fluid flow in crossflow filtration tubular membranes

Numerical simulations were performed for laminar fluid flow in a porous tube with variable wall suction, a model of a crossflow filtration tubular membrane. The variable wall suction is described by Darcy's law, which relates the pressure gradient within a flow stream to the flow rates through the permeable wall. The feed stream in the tube, which flows mainly tangentially to the porous wall, is modelled by the Navier-Stokes equations. A method of coupling the Navier-Stokes and the Darcy equations in a solution scheme was considered to develop a fluid dynamic model of crossflow filtration. The governing equations were solved numerically using a finite difference scheme. The solution depends on both the Reynolds axial and filtration number. Some assumptions adopted in simplified models are discussed.     

A two-dimensional, two-phase mass transport model for liquid-feed DMFCs

A two-dimensional, isothermal two-phase mass transport model for a liquid-feed direct methanol fuel cell (DMFC) is presented in this paper. The two-phase mass transport in the anode and cathode porous regions is formulated based on the classical multiphase flow in porous media without invoking the assumption of constant gas pressure in the unsaturated porous medium flow theory. The two-phase flow behavior in the anode flow channel is modeled by utilizing the drift-flux model, while in the cathode flow channel the homogeneous mist-flow model is used. In addition, a micro-agglomerate model is developed for the cathode catalyst layer. The model also accounts for the effects of both methanol and water crossover through the membrane. The comprehensive model formed by integrating those in the different regions is solved numerically using a home-written computer code and validated against the experimental data in the literature. The model is then used to investigate the effects of various operating and structural parameters, such as methanol concentration, anode flow rate, porosities of both anode and cathode electrodes, the rate of methanol crossover, and the agglomerate size, on cell performance.     

Influence of the composition and temperature of heat treatment of porous glasses on their structure and light transmission in the visible spectral range

The pore structure and light transmission of high-silica porous glasses in the visible spectral range have been investigated as a function of the heat treatment temperature and the composition of the initial two-phase alkali borosilicate glass. The character of light transmission in porous glasses has been analyzed in the framework of the concepts of structural features of their pore space and the processes occurring in the porous glass during heating. It has been demonstrated that an increase in the temperature of heat treatment of porous glasses with different compositions leads to an increase in the pore size and a decrease in their specific surface area (with a nearly constant total porosity), which is associated with the processes of overcondensation of pores due to the rearrangement and the change in the packing density of secondary silica particles. It has been revealed that the introduction of phosphate and fluoride ions into the initial sodium borosilicate glass results in an increase in the light extinction coefficient of porous glasses due to the increase in the sizes of phase-separated inhomogeneity regions in the initial two-phase glasses, the formation of larger pores, and the presence of nanosized microcrystalline phases in porous glasses.     

Characterization and Sintering of a Porous Glass-Ceramic in the System Na2O-B2O3-Sc2O3

Clear glasses which included droplet-like microphases were produced when SiO₂ in sodium borosilicate glasses was replaced by Sc₂O₃. Phase separation and/or crystallization occurred after heat treatment. The porous skeleton of leached glasses consisted of hexagonal ScBO₃. The specific surface areas and pore radii are comparable to those of porous SiO₂ glass. The sintering temperature of porous Sc-based material is higher than that of porous SiO₂. Alumina contamination influenced the structure of the porous material.     

Local Structure of Alkaline-Earth Boroaluminate Crystals and Glasses: I, Crystal Chemical Concepts—Structural Predictions and Comparisons to Known Crystal Structures

This paper presents a model which predicts glass structures based on local charge balance considerations. The model is shown to be consistent with known structures of borate, aluminate, and boroaluminate crystals and predicts that boroaluminate glasses should contain 3- and 4-coordinate boron, 4-, 5-, and 6-coordinate aluminum, and oxygens coordinated to one, two, three, and perhaps even four network-forming cations. The predicted glass structures are more complex than previous models for boroaluminate glasses, explaining the solid-state NMR data for the boroaluminate glasses discussed in Part II.     

ANALYSIS OF PERMEABILITY FOR ELLIS FLUID FLOW IN FRACTAL POROUS MEDIA

This work studies the flow characteristics of Ellis fluid in saturated porous media. A fractal model is developed for the effective permeability of Ellis fluid flow in porous media based on the assumptions that porous media consist of a bundle of tortuous capillaries, whose size distribution and tortuosity follow the fractal scaling laws. The average flow velocity and the effective permeability for Ellis fluid flow in porous media are derived. The proposed fractal model does not contain any empirical constant, and every parameter in the model has clear physical meaning. The model predictions are compared with the measured data, and good agreement between them is obtained.     

甲醇/正己烷的渗透汽化分离的研究 Ⅱ.渗透汽化复合膜传质阻力模型

本文对复合膜各部分－表皮分离层、多孔底膜致密区、多孔底膜孔区的传质阻力进行了探讨，推导了表征分离层及底膜致密区阻力与进料浓度、膜厚度之间的半经验方程，较好地揭示了以粘性流为主的气体混合物在底膜孔区内的传质规律及影响因素，并在此基础上建立了复合膜类电阻传质阻力模型。理论预测所得的结果与实验规律基本一致。     

可溶铅酸液流电池模拟与仿真

可溶铅酸液流电池是一种使用单个容器存储电解液并且不需要微孔隔膜的氧化还原液流电池,这使得电池设计简单并降低了成本。建立二维暂态可溶铅酸液流电池模型,模型基于对质量、电荷以及能量的转移与守恒以及包含铅离子反应的宏观动力学模型为基础,研究了电极间隔、电极形状、电流密度、实验温度、入口电解液流速和电解质初始浓度对电池性能的影响。研究表明：与平板电极相比,弧形电极明显提高了充电时的电池电压。在影响铅酸液流电池性能的诸多条件中,电池温度和电流密度可能是优化电池性能的重要因素。