逆体积排阻层析法测定层析介质的孔径分布

针对层析介质的孔径分析,基于刚性球状分子进入柱状孔的假设,分别采用高斯正态分布和对数正态分布描述孔径分布,利用简化的单孔分配因子模型,建立了孔径分布函数和分配因子Kd的关联,通过体积排阻层析实验测定系列标准物的Kd,从而拟合得到孔径分布信息,建立了逆体积排阻层析法。以葡聚糖作为分子大小的标准物,测定了5种典型层析介质(SP Sepharose FAST FLOW、Q Sepharose FAST FLOW、ToyopearlDEAE-650M、Streamline DEAE和Sephadex G-150)的Kd,计算和比较了不同介质的孔径分布,分析了介质的可吸附孔表面积等结构参数,证实了逆体积排阻层析法分析层析介质孔径分布的可行性和实用性。     

Determination of oxygen effective diffusivity in porous gas diffusion layer using a three-dimensional pore network model

In proton exchange membrane fuel cell (PEMFC) models, oxygen effective diffusivity is the most important parameter to characterize the oxygen transport in the gas diffusion layer (GDL). However, its determination is a challenge due to its complex dependency on GDL structure. In the present study, a three-dimensional network consisting of spherical pores and cylindrical throats is developed and used to investigate the effects of GDL structural parameters on oxygen effective diffusivity under the condition with/without water invasion process. Oxygen transport in the throat is described by Fick's law and water invasion process in the network is simulated using the invasion percolation with trapping algorithm. The simulation results reveal that oxygen effective diffusivity is slightly affected by network size but increases with decreasing the network heterogeneity and with increasing the pore connectivity. Impacts of network anisotropy on oxygen transport are also investigated in this paper. The anisotropic network is constructed by constricting the throats in the through-plane direction with a constriction factor. It is found that water invasion has a more severe negative influence on oxygen transport in an anisotropic network. Finally, two new correlations are introduced to determine the oxygen effective diffusivity for the Toray carbon paper GDLs.     

基于逾渗理论的多孔过滤介质孔径分布估计方法

结合逾渗理论和网格模型,建立了过滤系数与过滤介质孔径分布的联系方程。利用此方程结合过滤实验数据获得过滤介质孔径分布的统计参数,同时与其他方法获得孔径分布参数以及其他文献中的数据进行比较,并利用计算机模拟过滤过程验证了获得的孔径分布参数,结果表明基于逾渗理论的过滤模型用于孔径分布参数的估计是比较准确和有效的。     

Application of F NMR relaxometry to the determination of porosity and pore size distribution in hydrated cements and other porous materials

The potential to determine the porosity of cements and other porous materials by employing ¹⁹F NMR relaxometry was explored for samples of hydrated Portland cement, white cement and calcium aluminate, filled with Freon 11. The dependence of ¹⁹F signal amplitudes on the content of Freon in samples with completely filled pores was linear with a small (< 6%) intercept thus allowing a direct determination of total porosity. Additionally it was found that magnetic susceptibility (hence the content of paramagnetic compounds) of the solid can be evaluated from ¹⁹F NMR spectra of samples containing exterior liquid. Information concerning pore size distribution can be obtained from the analysis of multiexponential relaxation of ¹⁹F nuclei in samples with completely filled pores. Mathematical models employing sets of distributions of relaxation rates have been developed for this purpose. Distributions were assumed to be of a (fixed) square/triangular type allowing for the calculation of moments of any order from the estimated initial value, width of distribution and asymmetry factor. Longitudinal and transverse relaxation were characterised by either single or several continuous distributions of relaxation rates. One of the relaxating phases (assigned to fluid occupying throats connecting the pores) was found to selectively disappear when Freon was evaporated from samples. Another approach is to analyse the dependencies of means and variances of relaxation rates on Freon content. This has been done by employing the two-site relaxation model with a permanent adsorption layer and pore-dependent relaxation enhancement. Simultaneous fitting of dependencies of means and variances of relaxation rates on Freon content yields parameters of pore size distribution and of the dependence of relaxation enhancement on the pore radius. Analysis of experimental data shows agreement between these two approaches.     

Effect of particle size distribution of raw powders on pore size distribution and bending strength of Al2O3 microfiltration membrane supports

To lower the sintering temperature of Al₂O₃ microfiltration membrane support, Al₂O₃ powders with particle size distribution of tri-modal are chosen. The results show that the function of fine Al₂O₃ grains depends on their agglomeration state: if fine Al₂O₃ grains distribute discretely, the bending strength of the support increases along with a slight increase in porosity; however, the aggregated fine grains are harmful to both bending strength and pore size distribution of the support. The bridging of medium Al₂O₃ grains between coarse grains contributes to increase the bending strength, but has less effect on porosity. The addition of medium (and/or fine) Al₂O₃ powder has less effect on the pore size distribution of the support if only coarse Al₂O₃ grain forms the support's framework, which suggests a new way to prepare the support with both high bending strength and high porosity at low temperature.     

Relationship between pore fluid velocity distribution and pore size distribution

This study investigated the normalized velocity magnitude distribution and normalized pore volume distribution in different porous media with porosity between 13.5% and 85%, including sandstones, carbonates, synthetic silica, sphere packings and fiber scaffolds. It was found that both velocity magnitude and pore size follow the same distribution. These results allow the prediction of the velocity distribution in a porous medium when its pore structure is known or tuning the velocity by controlling the pore structure.     

Modelling of membrane nanofiltration—pore size distribution effects

The effects of log-normal pore size distributions on the rejection of uncharged solutes and NaCl at hypothetical nanofiltration membranes have been assessed theoretically. The importance of pore radius-dependent properties such as solvent viscosity and dielectric constant is increased by the introduction of a pore size distribution in calculations. However, the effect of porewise variation in viscosity is less apparent when considered at a defined applied pressure rather than at a defined flux, showing a further advantage of basing theoretical analysis of nanofiltration in terms of applied pressure.Truncated pore size distributions gave better agreement than full distributions with experimental rejection data for a Desal-DK nanofiltration membrane. Such truncation is in agreement with the findings of atomic force microscopy (AFM). Analysis of uncharged solute rejection data alone could not give useful information about membrane pore size distribution. Neither could such a distribution be obtained quantitatively directly from AFM images. However, use of the shape of the distribution obtained by AFM in conjunction with experimental rejection data for an uncharged solute allows calculation of corrected distributions. Importantly, incorporation of such a corrected pore size distribution in calculations of NaCl rejection gave better agreement with experimental data, compared to calculations assuming uniform pores, at high pressure, the region of industrial interest.     

Strategy for predicting effective transport properties of complex porous structures

Measurements of effective transport properties of porous media, such as effective diffusivity and permeability, are well established by several experimental techniques. Effective transport properties can be also calculated from the spatially 3D reconstructed porous media, where the morphology characteristics required for the reconstruction are obtained from electron microscopy images. Here we demonstrate the reconstruction of porous alumina catalyst carrier with bimodal pore size distribution. Multi-scale concept is employed for the computation of effective diffusivity and permeability of reconstructed porous media and calculated effective transport properties are compared with transport parameters experimentally determined in Graham diffusion and simple permeation cell. The limitations of current state-of-the-art reconstruction techniques for porous media with broad pore size distribution are discussed. We show that the contribution of nano-pores towards the total diffusion flux is significant and cannot be neglected, but it is reasonable to neglect the contribution of nano-pores towards the sample permeability.     

A new analysis method for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements

A new analysis method has been developed for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements. The method is based on a molecular model for the adsorption of nitrogen in porous carbon. It allows, for the first time, the distribution of pore sizes to be determined over both the micropore and mesopore size ranges using a single analysis method. In addition to carbons, this method is also applicable to a range of adsorbents, such as silicas and aluminas.     

Porosity, pore size distribution and in situ strength of concrete

In this study, in situ strength of concrete was determined through compression test of cores drilled out from laboratory cast beams. The apparent porosity and pore size distribution of the same concrete were determined through mercury intrusion porosimetry, performed on small-drilled cores. The normal-strength concrete mixes used in the experimental investigation were designed to exhibit a wide variation in their strengths. To ensure further variation in porosity, pore size distribution and strength, two modes of compaction, two varieties of coarse aggregates, different levels of age, curing period and exposure condition of concrete were also introduced in experimental scheme. With the data so generated, an appraisal of the most frequently referred relationships involving strength, porosity and pore size of cement-based materials was carried out. Finally, a new empirical model relating the in situ strength of concrete with porosity, pore size characteristics, cement content, aggregate type, exposure conditions, etc., is presented.     

The contribution of out-of-plane pore dimensions to the pore size distribution of paper and stochastic fibrous materials

Theory is presented for the distribution of the narrowest dimension of voids encountered in a path through a stochastic fibre network from one side of its plane to the other. Using expressions from the literature we show that the mean out-of-plane pore dimension is always less than half the mean in-plane dimension such that out-of-plane pore dimensions have a controlling influence on the measured pore size distribution. An approximate model is derived which predicts that the overall mean pore dimension of a network of fibres with given dimensions is inversely proportional to its areal density and proportional to a simple function of porosity. Good agreement is demonstrated between this model and experimental data. Regression analysis suggests that out-of-plane pore dimensions account for more than 70% of the measured pore size distribution.     

Porosity and pore size distribution influence on thermal conductivity of yttria-stabilized zirconia: Experimental findings and model predictions

Porous yittria-stabilized zirconia is an important advanced ceramic material for technological applications. One of the most important characteristics of this material is low thermal conductivity, which is greatly influenced by the presence of pores into the microstructure. In fact, air trapped in the pores represents a better thermal insulator. The role of the pore volume fraction on porous material characteristics has been extensively studied. On the other hand, the influence of the structure disorder, the pore size range and pore size distribution have been studied much less. In this study, an intermingled fractal model capable of relating thermal properties of ceramic materials and their pore microstructure has been proposed. Model predictions are found confirming the experimental data fairly well, even better than the others models available in the literature.     

RO and NF membrane fouling and cleaning and pore size distribution variations

The variations of porosity parameters of some reverse osmosis (RO) and nanofiltration (NF) polyamide thin-film composite membranes were determined in order to explain the changes of membranes' performances caused by membrane fouling and chemical cleaning of the fouled membranes. The pore size distribution curves and the effective number of pores in the membrane surface indicated plugging of the tight network pores in the membrane surface and even their disappearance during fouling. The enlargement of the wider aggregate pores was responsible for the noticed reduction in salt rejection. The initial pore structure of the fouled RO membrane was restored by immediate chemical cleaning. A delay of chemical cleaning of the fouled membranes led to irreversible changes in the porous structure of both the RO and NF membranes, which were caused by a microbial activity.     

Permeability of the porous Al2O3 ceramic with bimodal pore size distribution

Porous Al₂O₃ ceramics with bimodal pore size distribution were fabricated by partial sintering with monodispersed PMMA micro balls as pore agent. The porosity of the fabricated porous Al₂O₃ is increased with content of the pore agent increase, the bulk density and bending strength are decreased, accordingly. Relations between pressure drop and flow velocity of the air through the porous Al₂O₃ fit the Forchheimer's equation well for compressible fluid. Due to pores introduced by the pore agent, the Darcy permeability and inertial permeability of the porous Al₂O₃ are increased obviously. For given flow velocity, with increase of the PMMA content, the Forchheimer's number of the fluid through the porous Al₂O₃ is decreased, which results in decrease of the inertial resistance ratio to the total pressure drop. The porous Al₂O₃ ceramics with pores introduced by the monodispersed PMMA micro balls show higher permeability while the filtration selectivity is not deteriorated.     

Prediction of the hydraulic diffusivity from pore size distribution of concrete

A model is presented in this work through which variation of hydraulic diffusivity of concrete with relative water content can be obtained from pore size distribution as an input. The specific water capacity and hydraulic conductivity of concrete are expressed in terms of pore size characteristics, considering laminar flow due to capillary suction through tortuous elliptic tubes, oriented equally in three orthogonal directions. Hydraulic diffusivity being the ratio of hydraulic conductivity and specific water capacity is thus expressed in terms of pore size characteristics. The input pore size distributions have been determined experimentally for normal strength concrete mixes through mercury intrusion porosimetry. Using the model the variation of hydraulic diffusivity with relative water content is determined for three cases viz. 1) ideal continuous wetting, 2) ideal continuous drying and 3) random access of pores by water. These results are then compared with an experimentally obtained variation.     

Effects of nano-alumina content on the formation of interconnected pores in porous purging plug materials

The physical properties and microstructure of porous purging plug materials added with different nano-alumina contents and firing temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, air permeability, pore size distribution, mean pore size, apparent porosity, bulk density, and cold crushing strength (CCS) tests. The results showed that the addition of nano-alumina had a great effect on the physical properties and microstructure of the porous purging plug materials. With increasing nano-alumina content in the composition, the main phase was α-Al₂O₃ in all compositions and the mean pore size, apparent porosity and air permeability all increased due to the increased number of pores and pore size of the specimens which facilitated the formation of interconnected pores. When the sintering temperature was changed from 1550 °C to 1650 °C, some of the smaller pores vanished due to solid phase sintering, which reduced the apparent porosity, and some open pores connected to form interconnected pores, which promoted increased air permeability. In addition, the strength and porosity were found to follow the relationship σ = σ₀ exp (-b P). When the apparent porosity increased, the CCS decreased, and vice versa.     

High porous yttria-stabilized zirconia with aligned pore channels: Morphology directionality influence on heat transfer

High porous yttria stabilized zirconia with unidirectionally aligned channels is used in engineering applications with extremely low thermal conductivity. This property is strongly influenced by microstructure features such as pore volume fraction, pore size distribution, random porous microstructure and pore morphology directionality. Although several models are reported in the available literature, but their analytical formulas are formalised for homogeneous structures or they are based on proportion between solid and fluid phases. These differences from real microstructures cause significant computational errors especially when thermal conductivity changes as the function of the measurement direction (parallel or perpendicular). In this context, the application of an intermingled fractal unit's procedure capable of reproducing porous microstructure as well as predicting thermal conductivity has been proposed. The results are in agreement with experimental ones measured for parallel and perpendicular directions and suggest improving the formalisation of fractal modelling in order to obtain an instrument of microstructure design.     

Using the high pressure methane isotherm for determination of pore size distribution of carbon adsorbents

A method of determining pore size distribution, PSD, of carbon adsorbents based on the high pressure methane isotherm is presented. A generic software package, and an IBM compatible PC, have been used to search for a PSD in the form of a histogram. The method relies on a known local isotherm, in this case, assuming a simplified model of infinite slit shaped carbon pores.Three carbons, having very different pore structures: BPL, PX-21, and PVDC, were analyzed using the new method and the results compared with those obtained from subcritical Ar, and N₂ isotherms. The analysis from the high pressure methane isotherm gave results which are different than those from the low pressure low temperature isotherms but not significantly enough to be unrealistic.     

氙常温吸附性能与多孔碳材料孔径分布关联分析

以多孔碳材料为研究对象,测量分析了材料孔径分布和氙吸附性能,讨论了氙吸附性能与材料孔径分布之间的关联关系。结果表明,多孔材料的微孔孔容与其比表面积呈正相关;大气氙常温吸附集中于多孔材料的微孔孔道内,氙吸附能力与材料中孔无显著关联;氙常温吸附能力与碳材料5.0～7.0?的微孔孔体积相关性较显著,大于10.0?微孔孔容关联较小。     

基于阻滞扩散模型估算柴油脱硫吸附剂的适宜孔径分布

通过GC-PFPD色谱分析国Ⅱ、国Ⅲ、国Ⅳ标准0^#柴油中的主要含硫组分,发现苯并噻吩(BT)、二苯并噻吩(DBT)及其烷基取代衍生物是0^#柴油中的主要含硫化合物,C₂-DBT、C₃-DBT是国Ⅳ柴油中的主要含硫化合物。以BT、DBT、4,6-DMDBT为模型化合物,计算含硫化合物在不同大小孔道内的扩散阻滞因子,结合氧化铝堆积孔模型,估算氧化铝基柴油脱硫吸附剂的适宜孔径分布。结果表明,当氧化铝的平均孔径为4～10 nm时,含硫化合物的扩散阻滞因子为0.24～0.65,氧化铝的比表面积为100～250 m²·g^-1,可同时满足较低的扩散阻力和足够大的比表面积。对比分析不同氧化铝的孔分布及其吸附脱硫性能,结果表明氧化铝中4～10 nm范围内的孔面积占总孔面积的百分比与其吸附脱硫性能存在显著的正相关关系,初步推测氧化铝基脱硫吸附剂的适宜孔径分布范围为4～10 nm。