Determination of the pore size distribution of correlated mesoporous networks

In the present work we study how the adsorption desorption hysteresis loop of a mesoporous disordered medium represented by a 3-dimensional Dual Site-Bond Model (DSBM) is affected by percolation. Site and bond distributions are assumed to be gaussians. The behavior of the threshold pressure for the evaporation processe suggests a method to determine the site and bond distributions from experimental adsorption-desorption hysteresis curves. Traditional methods developed for non-correlated networks are tested and evaluated against our simulation results showing the discrepancy mainly for highly correlated networks. Results of the prediction capability of our method are shown. Received: 30 March 2000     

Theoretical relationship between maximum pore size and toughness in experimental inflammatory arthritis

Rheumatoid arthritis increases the risk of fracture. In an animal model of inflammatory arthritis, femoral diaphysis had a decreased toughness as well as increased cortical porosity, when compared to normal bone. Based on the hypothesis that stress concentration from the large porous defects reduces the ability of the cortical bone to resist failure, this work determined if the changes observed in porosity could explain the changes observed in toughness. Using theoretical relationships of the stress concentration and stress states, a model of the observed conditions was considered. A relationship was developed that indicated the relative difference in toughness between normal and arthritic specimens as a function of pore size. Results indicated that the increase in cortical pore size could theoretically reduce toughness by 55%. This decrease compares with the experimentally observed drop in toughness of 61%. Furthermore, the critical parameter for fracture in this situation is the ratio of pore diameter to cortical thickness. Efforts to reduce cortical porosity seen in inflammatory arthritis would be effective in enhancing the toughness of bone and may reduce morbidity in a human population. ©2000 Kluwer Academic Publishers     

The effect of carbon precursor on the pore size distribution of mesoporous carbon during templating synthesis process

The starch and cyclodextrin were selected as the precursors and the mixture of surfactant and tetraethyl orthosilicate (TEOS) as template to prepare mesoporous carbon. The result showed that a bimodal pore size distribution in mesoporous carbon derived from starch appeared; one was around 3.4 nm and the other ranged from 3.8 to 16.2 nm. However, there existed a concentrated pore size distribution from 3.2 to 4.2 nm in mesoporous carbon derived from cyclodextrin. The different molecular structure of starch and cyclodextrin and their polymerization process in the presence of sulfur acid were responsible for the resulted mesoporous carbon structure; the starch could polymerize by head to head or side by side, but the cycleodextrin was only polymerized by head to head.     

From pore size distribution to an equivalent pore size of cement mortar

An experimental and theoretical study has been performed with the aim of quantifying pore size distribution curves and correlating them with water and oxygen permeability. Twenty mortars have been investigated which contained Portland cement, blast-furnace slag cement and silica fume as a binder. Admixtures have been used as well. The water-cement ratio varied between 0.4 and 0.75 and two curing conditions were applied. By the use of mercury intrusion porosimetry, the pore size distribution was determined. Water and oxygen permeability have been measured in the steady state. Equivalent pore sizes have been calculated which quantify the pore size distribution by a single number. This number is not a constant but depends on the physical transport mechanism. It is shown that equivalent pore size and porosity together are sufficient to predict the physical properties with an acceptable accuracy.     

Frost resistance and pore size distribution in bricks

Correlation between frost resistance, porosity and pore size distribution was examined. Different test methods were used to evaluate the frost resistance. Porosity and pore size distribution were examined in mercury intrusion porosimeter (MIP). Scanning electron microscopy gave a visual view of the pore geometry, pore size and porosity. A linear correlation was found between frost resistance and the inverse value of the intruded pore volume. A linear correlation was also found between frost resistance and percentage of pores with diameter bigger than 3 μm. The test results were analysed using statistical methods. From the MIP-test a frost resistance number can be calculated which indicates the frost resistance of the brick.     

六方相介孔SiO2孔径及壁厚调节

以表面活性剂聚环氧乙烷-聚环氧丙烷-聚环氧乙烷三嵌段共聚物(P123)和十六烷基三甲基溴化铵(CTAB)为共模板剂并通过控制其物质的量比合成了具有不同孔径和壁厚的六方相介孔SiO2材料,以XRD,N2吸附方法进行了表征.结果显示,随CTAB与P123物质的量比的增加介孔材料孔径减小而壁厚呈增大趋势,但介孔规整度有所降低.初步研究表明,相比杂原子如钒、铁修饰以单一P123作为模板剂合成的SBA-15,所合成材料在苯乙烯过氧化氢选择氧化反应中催化性能有较大改进.苯甲醛选择性保持95％左右不变,而苯乙烯转化率分别由20.1％(Fe-SBA-15)和25.1％(V-SBA-15)提高到36.8％ (F-S)和44.6％(V-S).     

介孔材料孔径调节的最新研究进展

可调孔径的介孔材料在多个领域具有广泛的应用,许多因素引起材料孔径发生变化。综述了实验材料及反应条件对介孔材料孔径的调节,阐述了添加疏水性扩孔剂和改变温度调节孔径的相关机理。     

Computer simulation of effects of the pore size distribution on the kinetics of pressure-assisted final-stage densification

Most theoretical treatments of pressure-assisted densification of porous solids assume a single size for all pores. We remove this assumption and consider a distribution of pore sizes. Dissolution of intragranular pores by volume diffusion and dissolution of intergranular pores by grain-boundary diffusion are both treated. The evolution with time of pore size distributions is calculated for distributions that are initially described by log-normal and Weibull functions, and differences in predicted behaviours are discussed. The pore size distribution is then related to two important quantities: porosity and number of pores per unit volume. The assumption of a distribution of pore sizes is found to avoid certain unrealistic predictions obtained from models with a single pore size, such as abrupt disappearance of all pores and rapid approach to full density.     

SiC电子输运的蒙特卡罗模拟

采用了Monte Carlo方法研究了2H-、4H-和6H-SiC的电子输运特性.在模拟中,采用动量弛豫率近似的方法确定散射角,显著压缩散射次数,并用高效的新查表法确定自由飞行时间,相对于阶梯值的自散射方法,完全消除了自散射,大量节省cpu时间.     

煤基活性炭孔径分布的调控

提出并研究了一种煤基活性炭孔径分布的调控方法及调控机理.将煤样与不同质量的KOH混合后炭化,分别对炭化料进行酸洗,以控制其中的钾含量,然后对酸洗料进行蒸汽活化,制成活性炭.通过对所制活性炭进行氮气吸附实验、扫描电镜及能谱分析和吸附能力表征实验后发现:改变KOH加入量和采用质量浓度为5%的盐酸对炭化料进行酸洗,能够改变炭化料中的钾含量;随着KOH含量的提高,活性炭的口发附能力逐渐增强,平均孔径从2.379 nm逐渐增大到2.636nm,同时孔径分布由以微孔为主逐渐向以中孔为主转移,其中孔含量由30.9%提高到46.1%.     

Correlation between the capacitor performance and pore structure of ordered mesoporous carbons

Mesoporous carbons were synthesized by organic–organic self-assembly of triblock copolymer F127 and phenolic resin oligomers composed of resorcinol–formaldehyde or resorcinol–furfural. The mesostructure control was performed by using different polymerization catalysts, ammonia and acetic acid. The effects of the aldehyde and polymerization catalyst on pore architecture of mesoporous carbons were investigated. Disordered mesostructure with poorly disconnected mesopores was formed using furfural. In contrast, when formaldehyde was used, ordered structure with mesochannels was formed. In addition, changes in mesochannel length and the degree of long-range order are found to depend on polymerization catalyst. The porous carbons with different structure were used as a model material to investigate the ion storage/transfer behavior in electrical double-layer capacitor. Electrochemical cyclic voltammetry measurements were conducted in 1 M sulfuric acid electrolyte solution. The ordered mesoporous carbons show superior capacitances and rate performance over the disordered carbons. Electrochemical impedance spectroscopy was used to assess the transport properties. The impedance data clearly demonstrated that the degree of long-range order and channel length can influence the ion transport, resulting in superior capacitive performances.     

Determination of pore size distribution by suction prosimetry

The theory and practice of suction porosimetry methods (suction plate, pressure membrane and centrifuge) are reviewed together with related methods based on the rate of penetration of wetting liquids, and on static and dynamic fluid/fluid displacement measurements.

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Résumé La porosimétrie par succion repose sur la mesure de la dépression capillaire que suscite un liquide mouillant retenu dans un milieu poreux. C’est donc une méthode complémentaire à celle de la porosimétrie par pression (par ex. pénétration de mercure) où l’on opère avec des liquides mouillants. On expose la théorie et la pratique des méthodes par succion, entre autres les méthodes de la plaque, de la membrane et de la centrifugation. Un groupe de méthodes voisines découle de la vitesse de pénétration des liquides mouillants sous l’action des forces capillaires (par ex. la méthode d’Astbury), et de l’observation dans les conditions statique et dynamique des mouvements gaz/liquide et liquide/liquide.

     

Porous hydroxyapatite ceramics of bi-modal pore size distribution

A route for the fabrication of porous hydroxyapatite ceramics having two populations of open pores is reported. The bodies are prepared by sintering the spherical gelatin/hydroxyapatite granules. As the result, ceramics containing intragranular small-size pores and intergranular large-size interconnecting pores are obtained. The pore size and content are dependent on the route. Ceramics can generally be applied as bone replacement materials where the interconnections in the intergranular pores are the pathway to conduct cells and vessels for the bone ingrowth, whereas the intragranular pores can be filled with a drug, e.g. to eliminate infections.     

Tuning the pore size and structure of mesoporous carbons synthesized using an evaporation-induced self-assembly method

Mesoporous carbons (MCs) have been prepared by the carbonization of reverse triblock copolymer-phloroglucinol/formaldehyde resin composites, which were themselves formed by hydrogen-bonding interaction through the evaporation-induced self-assembly method. The microstructure of MCs was analyzed by nitrogen adsorption isotherms and transmission electron microscope. The results showed that the pore size could be readily tailored from 4.7 to 17.8 nm by simply varying the weight ratios of formaldehyde and surfactants while the amounts of the other reactants were kept constant. The highest surface area and pore volume of MCs are 410 m²/g and 0.77 cm³/g, respectively. Moreover, the pore structure also can be controlled as ink-bottle or cylinder type.     

Crystallization of amorphous silicon thin films: comparison between experimental and computer simulation results

Polycrystalline silicon obtained by the crystallization of thin amorphous silicon films has been an important material for microelectronics technology during the last decades. Many properties are improved in crystallized amorphous silicon compared to the as-deposited polysilicon such as larger grain size, smoother surface, and higher-carrier mobility. In this work, the crystallization of amorphous silicon is investigated by combining transmission electron microscopy (TEM) observations and molecular dynamics calculations. TEM observations on a series of specimens have shown that the majority of the silicon grains are oriented with a zone axis normal to the surface. In order to understand the crystallization mechanism molecular dynamic simulations were performed. It is found that the c/amorphous interface exhibits the lowest reduced interfacial energy density while the c/amorphous has the lowest reduced energy differences per unit interfacial area. The most energetically unfavorable interface is c/amorphous.     

Effect of media pore size distribution on deep-bed filtration

One important parameter of filtration, the media pore size distribution, has not been explicitly considered in modeling deep-bed filtration models by previous investigators. In this study, we propose, based on the hypothesis of O'Melia-Ali and the use of the effective medium approximation theory, a new transient-state model and examine the effect of the media pore size distribution on filter performance. From the simulation results based on six different pore size distribution functions, it is clear that the pore size distribution plays an important role in determining the removal efficiency as well as headloss development.     

Characterization of silica-based monoliths with bimodal pore size distribution

Band dispersion was studied and the retention thermodynamics addressed for insulin and angiotensin II on C18 silica monoliths with a bimodal pore size distribution, covering linear mobile-phase velocities up to 1 cm/s and different temperatures. These data suggest that the influence of average column pressure on retention (between 0 and 10 MPa) is not negligible. Plate height curves were interpreted with the van Deemter equation by assuming an independent contribution from mechanical and non-mechanical dispersion mechanisms. This analysis revealed diffusion-limited mass transfer in the mesoporous silica skeleton which, in turn, allowed us to calculate an equivalent dispersion particle diameter (d(disp) = 3 microm) using the C-term parameter of the van Deemter equation. The resulting superposition of reduced plate height curves for monolithic and particulate beds confirmed that this view presents an adequate analogy. The macroporous interskeleton network responsible for the hydraulic permeability of a monolith was translated to the interparticle pore space of particulate beds, and an equivalent permeability particle diameter (d(perm) = 15 microm) was obtained by scaling based on the Kozeny-Carman equation.     

聚醚砜中空纤维超滤膜的孔径大小及分布

对测定中空纤维超滤膜孔径大小及其分布的物质迁移方程法进行了改进，并考察了浓度对聚醚砜（ＰＥＳ）中空纤维膜孔径大小及其分布的影响．随纺丝液中ＰＥＳ质量浓度的增大，膜的孔径变小，对聚乙二醇的表观截留率增大，膜的纯水透过率减小，在大约２４％的ＰＥＳ质量浓度时出现临界值，大于该临界值，膜的结构和性能均发生急剧变化．     

Surface area, pore size distribution and microstructure of vacuum getter

An effective getter is necessary in vacuum technology. In order to obtain the adsorption mechanism of the getter, its microstructure information must be studied. Surface area, pore size distribution and microstructure of vacuum getter were studied with XRD, SEM and N₂ adsorption technique.Vacuum getter is composed of the different proportions of PdO and Ag₂O. The crystalline size of 3^# vacuum getter that includes W₂AgO = 22% and W_PdO = 78% is the maximum among all the vacuum getters. SEM images showed that vacuum getters expose a large number of nanometer-size pores. The adsorption isotherms of the vacuum getters are typical of type Ⅳ, characteristic of mesoporous material, and a type H₂ hysteresis loop is observed. Langmuir model describes N₂ adsorption at low pressure region. At the medium pressure, N₂ adsorption can be modeled by BET model. The pore size distributions of the vacuum getters are calculated by applying BJH method to the adsorption branch of N₂ isotherms at 77 K. With percentage of Ag₂O elevation, Langmuir surface area, BET specific surface area, the adsorption capacity and the pore volume become larger. But the average pore diameter becomes smaller. However, above Ag₂O content of 22%, an inverse behavior is observed. 3^# vacuum getter has the highest surface area and pore volume among all the vacuum getters. The experimental results and related analysis can be adopted in the later design of the vacuum tank.