Pore structures, graded

Improved characterization of the morphological/microstructural properties of porous solids, and the associated transport properties of fluids imbibed into these materials, is crucial to the development of new porous materials, such as ceramics. Of particular interest is the fabrication of so-called functionalized ceramics, which contain a pore structure tailored to a specific biomedical or industrial application (e.g., molecular filters, catalysts, gas storage cells, drug delivery devices, tissue scaffolds) [1-3]. Functionalization of ceramics can involve the use of graded or layered pore microstructure, morphology or chemical composition. 

The same approach was applied to enhance and extend resid catalyst technology. We gave considerable attention to specific grades of Kaolin to assure a very good pore structure and to keep costs down For a combination of low cost, good open pore structure, low iron content, and low attrition, we preferred the smaller, spherical equivalent Kaolin crystals with a high diameter-to-thickness ratio. 

The activated carbon produced is graded, screened, and de-dusted. Activated carbons produced by steam activation generally exhibit a fine pore structure, ideal for the adsorption of compounds from both the liquid and vapor phases. 

When no water was added (/f-value of co) and hydrothermal conditions were applied, the material totally lost its pore structure. With increasing amount of water (at constant total volume) and decreasing ratio R, we could create graded porosity starting fiom a parent silica (material 1 in Tab. 2). The pore size distributions of these MCMoidal beads are displayed in Fig. 4. 

If a membrane has a graded pore structure but is made in one processing step, frequently from the same material across its thickness, it is called an asymmetric membrane. If, on the other hand, the membrane has two or more distinctively different layers made at different steps, the resulting structure is called a composite membrane. Almost invariably in the case of a composite membrane, a predominantly thick layer provides the necessary mechanical strength to other layers and the flow paths for the permeate and is called the support layer or bulk support. Composite membranes have the advantage that the separating layer and the support layer(s) can be tailored made with different materials. Permselective and permeation properties of the membrane material are critically important while the material for the support layer(s) is chosen for mechanical strength and other consideration such as chemical inertness. The composite membranes can have... 

Four main types of porous silica adsorbents have been identified compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature cf Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties. 

Werner, J., Linner-Krcmar, B., Friessc, W., and Greil, P. (2002) Mechanical properties and in vitro cell compatibility of hydroxyapatite ceramics with graded pore structure, Biomaterials 23, 4285. 

A comparison of the pore structural properties of MCM-41 containing titanium, prepared at room temperature, with those of aluminosilicate grades is presented. The influence on the structural characteristics of using different metal sources and metal content is also considered. Additionally, the stability of Al-MCM-41 and Ti-MCM-41 samples, with Si/M=30, towards prolonged exposure to pure water vapour at 298K was investigated. 

Amongst the most important features of ordered mesoporous materials for flieir use in catalysis are the extremely high surface area, which potentially allows an efficient dispersion of active sites, and the large pore diameters that favour the diffusion of bulky molecules. However, the introduction of the heteroatoms into ordered mesoporous silicas to make them catalytically active can lead to a decrease in the quality of the pore structure as compared with the pure silica grades, the extent of which may depend on the preparation conditions. Therefore, the control of the quality of the pore structure is a relevant aspect to consider. 

In the present study we have used other metal sources and higher metal contents and present a comparison of the effects of introduction of aluminium and titanium on the pore structural properties. Additionally, considering that a severe limitation of ordered mesoporous materials, either pure silica grades [5] as well as some hydrothermally synthesised MCM-41 containing aluminium [6] or titanium [7], is that structural changes usually occur in the presence of water vapour, a study of the stability towards water vapour at 298K is also presented. 

Radjy and SeUevold [54] developed a phenomenological theoretical theory for the f-method of pore structure analysis for slit-shaped and cylindrical pores. A comparison [55] of adsoiption and desoiption methods for pore size distribution, with transmission electron microscopy using closely graded cylindrical pores in alumina, closed at one end, confirmed the superiority of the Broeckhoff-de Boer equations over the Kelvin equation. Lamond [56] found Lippen s t silica values unsuitable for carbon black and proposed a new set based on adsorption data for fluffy carbon blacks. 

A similar, but highly porous, vitreous carbon material—reticulated vitreous carbon (RVC)—has found widespread application for flow analysis and spectro-electrochemistry (25). As shown in Figure 4-10, RVC is an open-pore ( spongelike ) material such a network combines the electrochemical properties of glassy carbon with many structural and hydrodynamic advantages. These include a very high surface area ( 66 cm2 cm-3 for the 100-ppi grade), 90-97% void volume, and a low resistance to fluid flow. 

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