Voids, interconnected

Pressure testing of the finned oil cooler in Fig. 15.29 revealed leaks. Examination of the interior of the cooler after sectioning in the vicinity of the leaks revealed a small cavity in the weld zone in the corner of some fins (Fig. 15.14) and porous areas inside the channel in the welded zone in other fins. Microstructural examinations of specimens cut through the sites revealed interconnected voids resulting from either shrinkage during solidification of the weld or lack of fusion of the base metal and weld metal. 

Compared with microporous and mesoporous materials, the larger, interconnected voids in macroporous materials potentially provide easier molecule transportation through the materials. This is of particular interest for the transport of large biomolecules (e.g., proteins and cells). The pore sizes in macroporous materials are usually from tens to hundreds of nanometers, and the pores are typically... 

Macroporous substrates with interconnected voids can be used as platforms for biomacromolecule separation and enzyme immobilization. These assemblies are likely to find application in biocatalysis and bioassays. The inorganic framework can provide a robust substrate, while their large and abundant pores allow the transportation of biomolecules. The availability of various morphologies for macroporous materials provides another level of control over the function of the hybrids. 

The porous membrane templates described above do exhibit three-dimensionality, but with limited interconnectedness between the discrete tubelike structures. Porous structures with more integrated pore—solid architectures can be designed using templates assembled from discrete solid objects or su-pramolecular structures. One class of such structures are three-dimensionally ordered macroporous (or 3-DOM) solids, which are a class of inverse opal structures. The design of 3-DOM structures is based on the initial formation of a colloidal crystal composed of monodisperse polymer or silica spheres assembled in a close-packed arrangement. The interconnected void spaces of the template, 26 vol % for a face-centered-cubic array, are subsequently infiltrated with the desired material. 

A general three-step procedure for the formation of macroporous materials by colloidal crystal templating is illustrated in Figure 6. In the first step, monodispersed colloidal spheres assemble into ordered 3D or sometimes 2D arrays to serve as templates. Secondly, the voids of colloidal crystals are filled by precursors that subsequently solidify to form composites. Finally, the original spheres are removed, creating a solid framework with interconnected voids, which faithfully replicate the template arrays. 

Microporous polymer systems consisting of essentially spherical, interconnected voids with a narrow range of pore- and cell-size distribution have been produced from a variety of thermoplastic resins by phase separation [94]. If a polyolefin or polystyrene is insoluble in a solvent at low temperatures but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solution containing 10-70% polymer is cooled to ambient temperature, the polymer separates as a second phase. The remainder can be extracted. These microporous polymers may be used in microfiltration or as controlled-release carriers for chemicals. 

A powdered active carbon electrode consists of a continuous matrix of electrically conducting solid that is interspersed with interconnecting voids or pores whose characteristic dimensions are small compared to the size of the electrode. The electrochemical reactions in such electrodes occur predominantly in the pores, which represent the major fraction of the total surface area. The external surface area is relatively small with respect to the area of the pore walls. It is the high interfacial surface area available for electrochemical reaction that provides the major advantage of porous electrodes over smooth electrodes (e.g., glasslike car-... 

Pore systems found in useful membranes are characterised by interconnected voids (pores). Spongy microstructures are observed by phase decomposition and leaching [1] or in porous carbon, obtained by controlled pyrolysis of polymeric precursors [1]. 

Scheme 2. Formation of colloidal crystals and their use as templates. A colloidal dispersion containing monodisperse particles undergoes controlled filtration, centrifugation, dip coating, sedimentation, or physical confinement, which results in ordered packing of the particles with void spaces between them. By infiltrating these spaces with precm-sor solution or preformed nemoparticles the hybrid material is formed. Removal of the polymer template (using solvent (toluene) orheatingtechniques) gives an inverse replica with air-fiUed, interconnected voids of monodisperse size, which is dependent on the initial particle size...

In each of the above analyses, the pores were considered as parallel sets of large and small pores without interconnection between the separate sets. However, most void structures comprise a network in interconnected void spaces and "network effects" will diaate the potential implications of changes in pore structure. The generic influence of pore networks were analyzed by Beeckman and Froment22 based on modified Bethe tree two-dimensional networks. Based on this simulated analyses, the authors concluded that the nature of the deactivation does depend on the nature of the network structure. Sahami and Tsotsis employed percolation theory to analyze a three-dimensional network of interconnected pores and concluded that the void interconnectivity is crucial in determining the influence of network structure on the deactivation phenomena. 

A porous material consists of at least two immiscible phases of which one is usually a continuous sohd material, the matrix, which surrounds the second phase of finely dispersed voids, the pores, containing a liquid, gas, or vacuum. If the void phase is discontinuous and comprises individually separated cavities filled with gas (bubbles), the material represents a foam structure. On the other hand, if both phases form two interpenetrating continua with the matrix as well as the pores being continuous, the material represents a sponge structure or a so-called porous network. Such porous networks with interconnected voids are the focus of this article as they may have funda-... 

A comparative plot between pore sizes determined by the BJH model and NLDFT is shown in Figure 9.4. It can be seen that the deviation is most noticeable for small pore sizes. However, it should be remembered that both models make the assumption that pores are rigid and of well-defined shape, but in reality the pores could be quite irregular or composed of complex interconnected voids of various sizes. Also the choice of the isotherm branch used in the calculations can affect the extracted values of pore size. 

The bulk specific gravity of a dense compacted bituminous mixture is determined from the mass of the specimen in the air and the mass of the volume of water for the volume of the specimen. The latter, when the specimen has no open or interconnecting voids or does not absorb more than 2% of water by volume, is determined by submerging the specimen into the water bath without any other treatment of its surface. The procedure analytically is described in ASTM D 2726 (2011) or AASHTO T 166 (2013). 

When the specimen has open or interconnecting voids and absorbs more than 2% of water by volume, the bulk specific weight is determined after coating or sealing the specimen. 

PA is a bituminous mixture with a high content of interconnecting voids that allow passage of water and air in order to provide the compacted mixture with drain and noise-reducing features. It is used for surface courses and it can be laid in one or more than one layer. 

The typical property of PA is the high air void content (usually >18%), as well as the high number of interconnecting voids. Because of the above two characteristics, there is quick drainage of surface rainwater and a reduction of the tyre/pavement. 

The reduction of air voids results in a mixture that is less pervious to air and water and increases the stiffness of the asphalt. The reduction of permeability is mainly due to the fact that the number of interconnected voids decreases. In case of under-compaction, the number of interconnected voids is relatively large. However, the number of interconnected voids may also increase in case of over-compaction. This is more predominant in dense-graded asphalts owing to the development of internal hydraulic pressures related to the excessive reduction of available space (voids). Considering the above, it is obvious that an optimum void content, not the minimum possible, is required to be achieved during compaction. 

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