Void structure

Gies FI, Marler B and Werthmann U 1998 Synthesis of porosils crystalline nanoporous silicas with cage- and channel-like void structures Moiecuiar Sieves Science and Technoiogy vo 1, ed FI G Karge and J Weitkamp (Berlin Springer) pp 35-64... 

Because of the three-dimensional void structure of a packed column, the exact form of the velocity profile is not clearly defined as in the microtubular... 

Void structure (content of voids, specific interface, void size)... 

U. (1998) Synthesis of porosils Crystalline nanoporous silicas with cage-and channel-like void structures in Molecular Sieves Science and Technology, vol. 1 (eds H.G.Karge and ). Weitkamp), Springer, Heidelberg, pp. 35-64. 

Scanning electron microscopy and other experimental methods indicate that the void spaces in a typical catalyst particle are not uniform in size, shape, or length. Moreover, they are often highly interconnected. Because of the complexities of most common pore structures, detailed mathematical descriptions of the void structure are not available. Moreover, because of other uncertainties involved in the design of catalytic reactors, the use of elaborate quantitative models of catalyst pore structures is not warranted. What is required, however, is a model that allows one to take into account the rates of diffusion of reactant and product species through the void spaces. Many of the models in common use simulate the void regions as cylindrical pores for such models a knowledge of the distribution of pore radii and the volumes associated therewith is required. 

In this work positron annihilation lifetime spectroscopy (PALS) was used to investigate structural diversity inside zeolite precursor matrix caused by the presence of alkali cations Na, K, Rb and Cs. PALS is an established and well-proven method for structural investigations of various materials, extensively used for metals and alloys, semiconductors and porous materials [3, 4]. In the investigations of zeolites PALS has been mostly used for their void structure and size study [5, 6, 7, 8], also in correlation to... 

The third family of research grade materials is less well defined and encompasses aerogels of carbon [81,82] designed mesoscopic void structures in C3 with nanostruc-tured fillers [51,83], composites with nanocarbon fillers [24,82,84 88] and carbon-heterostructure [54,89-94] compounds. The references stated here are only examples for a wide range of activities stemming from the efforts to synthesize novel nanostruc-tured composites. These materials often exhibit unusual surface properties and are used in electrochemical and catalytic applications rather in the domain of traditional C3 compounds where mechanical properties dominate the application profile. 

This preliminary void structure does not develop much during the first 5h of heating only a homogenization of the sizes of the cavities or an increase in their degree of connection is observed. 

These observations suggest that the trans-azo ammonium can stabilize the supramolecular channel structure, which is formed by assembling relatively hydrophilic oligoether units based on the molecular recognition in the membrane phase. Compared to the extended molecular form of the trans-azo compound, which is appropriate for covering the hydrophilic component from the outside, the cis compound with a bulky structure cannot stabilize the structure and hence prohibits the assembly formation because it requites a large void structure in the membrane. Therefore, only leaky currents are observable (Figure 26). 

A microporous membrane is very similar in structure and function to a conventional filter. It has a rigid, highly voided structure with randomly distributed, interconnected pores. However, these pores differ from those in a conventional filter by being extremely small, on the order of 0.01 to 10 un in diameter. All particles larger than the largest pores are completely rejected by the membrane. 

INFLUENCE OF SOLUBLE ALKALIS ON THE STABILITY OF THE AIR-VOID STRUCTURE... 

Water-soluble alkalis in Portland cement or fly ash can have a harmful effect on the characteristics of the air-void structure (AVS), particularly the stability of the spacing factor. Because of the more restrictive environmental regulations affecting... 

Distribution of monomer was found to be good only at high loading which resulted in the polymer being mostly in the homopolymer form in the void structure (63, 64, 65, 66, 67, 68). 

With the development of powder metallurgy a vast new field has recently been opened in which the value of particle-size measurements and packing behavior will play an important role. Selection of the proper combination of sizes and their relation to void structure will be fundamental considerations. A knowledge of the electrical and gas adsorption properties of fine powders should also prove of inestimable value in this field. 

In this work the potential of biphasic synthesis mixtures for the synthesis of microporous silicoalumino-phosphates is investigated. The influence of emulsification of the synthesis mixture, agitation and temperature on the crystallization is studied. The structural identity of MCM-1 and A1P0.-H3, which is an aluminum phosphate hydrate that crystallizes in absence of template, is demonstrated. Special attention is paid to the mechanism of Si substitution in MCM-1, to its catalytic activity and its intracrystalline void structure. 

Void Structure of MCM-l Determined with the Decane Test. The results from the decane test for determining the void structure are presented in Table VI. The test could not be applied to MCM-l synthesized with Pr4N-OH as this material is too weakly acidic. 

Theoretical Explanation. The major factors of the thermal expansion coefficient of calcined coke are the degree of preferred orientation of the crystallites and void structure (12-14). For example, the thermal expansion coefficient is low for needle coke because it is strongly affected by the preferred orientation of its crystallites. 

A systematic study of the adsorption of nitrogen by packed assemblages of spheroidal particles was undertaken by Adkins and Davis (1986, 1987). After the consideration of various pore filling models, it was concluded that the desorption process can be adequately described by the instability of a Kelvin, hemispherical meniscus in the neck (i.e. the window) of the structure and the adsorption process can be viewed as a delayed Kelvin condensation in the largest dimension of the void structure. This reasoning is consistent with the network-percolation theory of hysteresis, which is discussed in Section 7.5. 

These results suggest that sulfur-bonded composites may have uses in civil enginering where thermal insulation is required. General principles indicate that increased voids content or a stable retention of a noncrystalline form by the sulfur would reduce the thermal conductivities of the composites. A composite incorporating potters flint did have a fine void structure, but nonetheless its thermal conductivity was higher than that of sulfur (24) presumably a still larger void content is required. Use of moist fillers such as damp soil produced foaming, but a stable, well distributed system of voids was not produced the bubbles which formed collapsed under the conditions used. 

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