Pores, porosity porous compacts

The density of bread is somewhat vanable and is related to the volume of the pores, being small in very porous loaves and higher in those which are more compact, in general the quality and digestibility of a bread increase with the lightness and porosity... 

In an attempt to reconcile various chondritic IDP classifications, Rietmeijer (1994) proposed to classify IDPs on the basis of properties of identifiable textural entities in these particles and their porosity as aggregate IDPs and collapsed aggregate IDPs. In highly porous aggregates the constituents are loosely bound (CP IDPs). Most aggregates are more compact with little pore space (CF IDPs). The collapsed aggregate particles typically have a smooth surface (CS IDPs). 

Hot isostatic pressing (HIP) is often applied to densify sintered materials and to correct casting defects. Recently a new HIP process for the production of porous materials was developed. Partially densified compacts are sintered in a high pressure atmosphere. The pressurized gas in the open pores delays densification and porosity remains largely intact. 

In this study we summarize the recent developments in catalyst development in which nano-porous catalytic sites are accessible through a network of arterial micro-pores. These catalysts are obtained through a solution deposition of metals on a micro-porous polymeric template which is subsequently heat-treated to obtain porous metallic structures where the size of the pores ranged from tens of micrometers to tens of nanometers thus eliminating the problems of accessibility and rapid pore fouling and closure. The technique differs fundamentally from the compression-based systems where the porosity is reduced as a result of compaction. It also differs from the well-known wash-coating or chemical vapor deposition techniques. Furthermore, the mechanisms of metal deposition within micro-pores and nano-structure formation are novel. The importance and current fabrication techniques of porous metallic systems can be found in Refs. l... 

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. 

The third essential requirement for a commercial accumulation of petroleum is a cap rock or seal. This is a sedimentary stratum that immediately overlies the reservoir and inhibits further upward movement. A cap rock need have only one property It must be impermeable. It can have porosity, and may indeed even contain petroleum, but it must not permit fluid to move through it. Theoretically atty impermeable rock may serve as a seal. In practice it is the shales and evaporites that provide most examples. Shales are probably the commonest, but evaporites are the more effective. We saw earlier how mud is compacted during burial into mudstone, or shale. These rocks are commonly porous, but because of the narrow diameter of the pore tluoats, they have negligible permeability. Thus shales generally make excellent seals to stop petroleum migration. When strata are folded or faulted, however, brittle shales may fracture. As described earlier, iiactures enhance permeability most dramatically. In such instances, petroleum may leak from an underlying reservoir and ultimately escape to the surface of the earth. 

Trabecular or cancellous bone is spongy in nature and occupies about 20% of the total bone. Cancellous bone is lighter, less dense, has higher porosity (pores diameter varies from a few micrometers to millimeters), and a higher concentration of blood vessels than compact bone (also called cortical or dense bone) (Fig. 2). The porous architecture of cancellous bone is easily visible under the microscope or even with the naked eye because it contain very large pores. Cortical bone, which has less porosity and thus a lower concentration of blood vessels, occupies about 80% of the total bone. Due to its lower porosity, its porous architecture is not visible to the naked eye. The diameters of pores are 10-20 pm and mostly separated by 200-300 pm intervals. Spongy bone acts mainly in compression, whereas compact bone acts mechanically in torsion, tension, and compression. 

Controlled Porosity, Chemical Valve . Environmentally controlled change in macromolecular size from a compact hydrophobic globule to an expanded hydrophilic coil is exploited when smart pol5nners are used in systems of environmentally controlled porosity, so-called chemical valves. When a smart polymer is grafted to the surface of the pores in a porous membrane or chromatographic matrix, the transition in the macromolecule affects the total free volume of the pores available for the solvent and hence presents a means to regulate the porosity of the system (see Membrane Technology). 

Permeability. The rate of flow of a fluid (usually air) through a porous ceramic material per unit area and unit pressure gradient. This property gives some idea of the size of the pores in a body -whereas the measurement of porosity (q.v.) evaluates only the total pore volume. From the permeability of a compacted powder the specific surface (q.v.) of the powder can be deduced. B.S. 1902 Pt. 3.9 describes a gas permeability test applicable to refractory materials, as does ASTM C577. The ASTM C866 test for the filtration rate of whiteware clays depends on their water permeability. See blaine test carman equation lea and nurse... 

All the three PI, P2 and P3 particles were found to be porous spherical agglomerates. The low tap density of the powders P2 and P3 compared to PI is due to their smaller agglomerate size. Isostatically pressed compacts (400 MPa) of PI, P2 and P3 showed pore distribution radii in the range of 0.01-10 ptm. Since the crystallite sizes of these powders were much smaller than the mean pore diameter, they exhibited inter-crystalline porosity. 

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