Porosity types, classification

FIGURE 3.1 Classification of porosity types. (After Choquette and Pray, 1970.)... 

Figure 2.2 Classification of different types of porous materials, (a) A purely microporous zeolite is considered as a non-hierarchical system according to the single level of porosity, (b) Fragmentation of the zeolite into nanocrystals engenders a network of mesopores constituting the intercrystalline space, leading to an interconnected hierarchical system. Intraconnected...

Of the five isotherm classifications depicted in Fig. 3.1 the types I, IV and V isotherms are associated with porosity. The type I isotherm usually corresponds to microporosity, that is, pores of diameters only slightly larger than adsorbate molecules. The types IV and V isotherms are associated with pores ranging in radius from about fifteen to several hundred angstroms. The type IV isotherm is more frequently encountered with porous adsorbents. 

Adsorption equilibria determine the thermodynamic limits of the specific amounts of adsorption (mol/g) of a pure gas or the components of a fluid mixture (gas or liquid) under a given set of conditions [pressure (P), temperature (T), and mole function (y or Xi) of component /] of the bulk fluid phase. The simplest way to describe adsorption equilibria of pure gas i is in the form of adsorption isotherms where the amount adsorbed (n ) is plotted as a function of gas pressure (P) at a constant temperature (P). The pure gas adsorption isotherms can have various shapes (Types I-V) by Brunauer classification depending on the porosity of the adsorbent (microporous, mesoporous, or nonpo-rous) and the system temperature (below or above the critical temperature of the adsorbate). However, the most common isotherm shape is Type I, which is depicted by most microporous adsorbents of practical use. These isotherms exhibit a linear section in the very low-pressure region (Henry s law region) where the amount adsorbed is proportional to the gas pressure [ n ) = KiP]. The proportionality constant is called... 

Photograph 7-14 Randomly scattered silicates in well-made clinker. Small, angular, brown alite Relatively large, round, multicolored Type A belite well-differentiated matrix of aluminate (CjA) and ferrite (C AF). Moderate porosity not shown. See proposed belite classification on p. 35. (S A6634)... 

For water-rinsed and acid-leached rice husks. Fig. 13.11a, b shows lower nitrogen capacity and no apparent desorption hysteresis loop, indicating that the porosity of the two raw materials is relatively lower than that of heat-treated rice husks samples. For carbonized in nitrogen and burned in air atmosphere husk samples. Fig. 13.11c, d shows that the isotherms are of type 1 according lUPAC classification. The hysteresis loops (associated with capillary condensation) found in both samples are of various shapes. According to these observations, BRHA is mainly microporous with narrow pore size distribution while WRHA contains both micro- and mesopores. 

Inspection of the overall shape of a nitrogen isotherm can provide a first useful indication of the nature of the adsorbent porosity and hence of the most appropriate procedure to be adopted for the analysis of the isotherm data. The hypothetical isotherms in Fig. 1 are representative of nitrogen isotherms obtained with a wide variety of adsorbents. The general features of these isotherms are consistent with Types I, II and IV, in the classification already proposed by the International Union of Pure and Applied Chemistry. ... 

The classification of C/C composites, and ceramic matrix composites, is generally based on the number of directions of the reinforcement architecture. Sometimes it is also completed by the nature of the fiber and/or of the matrix. Occasionally it can also take into account a physical characteristic (high or low density high or low thermal conductivity). A more sophisticated classification is under development, based on the type of fiber, reinforcement architecture, matrix type, fiber fraction volume, density, porosity, tensile strength, and modulus at room temperature (ASTM WK49676). 

The presence of adsorption hysteresis is the special feature of all adsorbents with a mesopore structure. The adsorption and desorption isotherms differ appreciably from one another and form a closed hysteresis loop. According to the lUPAC classification four main types of hysteresis loops can be distinguished HI, H2, H3 and H4 (ref. l). Experimental adsorption and desorption isotherms in the hysteresis region provide information for calculating the structural characteristics of porous materials-porosity, surface area and pore size distribution. Traditional methods for such calculations are based on the assumption of an unrelated system of pores of simple form, as a rule, cylindrical capillaries. The calculations are based on either the adsorption or the desorption isotherm, ignoring the existence of hysteresis in the adsorption process. This leads to two different pore size distributions. The question of which of these is to be preferred has been the subject of unending discussion. In this report a statistical theory of capillary hysteresis phenomena in porous media has been developed. The analysis is based on percolation theory and pore space networks models, which are widely used for the modeling of such processes by many authors (refs. 2-10). The new percolation methods for porous structure parameters computation are also proposed. 

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