Variable porosity

Soils and vadose zone information, including soil characteristics (type, holding capacity, temperature, biological activity, and engineering properties), soil chemical characteristics (solubility, ion specification, adsorption, leachability, cation exchange capacity, mineral partition coefficient, and chemical and sorptive properties), and vadose zone characteristics (permeability, variability, porosity, moisture content, chemical characteristics, and extent of contamination)... 

In another study, Chen and Zhao [55] demonstrated that by using a Ni-Cr alloy metal foam as the cathode DL (and current collector), instead of a CFP or CC, the performance of a DMFC can be enhanced significantly due to the improvement of the mass transfer of oxygen and overall water removal on the cathode side. Fly and Brady [56] designed a fuel cell stack in which the distribution layers were made out of metal foams (open cell foams). In addition, more than one foam (with different porosity) could be sandwiched together in order to form a DL with variable porosity. 

G. W. Fly and B. K. Brady. Fuel cell with variable porosity gas distribution layers. US Patent 6566004 (2003). 

Figure 4. Relation between sulfur loading and compressive strength of sulfur-infiltrated concrete. Wide scattering at higher loadings suggests variable porosity and/or inconsistent filling.

The qualitative nature of the variable porosity of pore size is best appreciated by computer-generated pictorializations. An example is shown in Fig. 5, which conveys the impression of large pores hidden amongst smaller ones. The pores in this illustrative 30 X 30 network obey a uniform distribution between 10 A and 2000 A and comprise a... 

A further complication is that the ideal dispersion hemisphere of a gas is prone to distortion. The source may not liberate gas uniformly over time, producing fluctuations in m. The rock and overburden column above the source may comprise lithologies of variable porosity, which may be cut by faults and fractures, and these various voids may be (partially) occupied by liquids, thus producing several different values of p in the column. The voids themselves may be occupied at different times by liquid (usually water) or by gas (usually soil air) of variable barometric pressure, with the result that the capacity of the voids to disperse gases from depth changes with time. 

Lin, S.H. 1977. Longitudinal dispersion in porous media with variable porosity. J. Hydrol. 34 13-19. 

The variable porosity must be included explicitly in the transport equations for mobile species. Values of porosity are related to volumes of reactive solids. 

Figure 3 shows the porosity at the boundary a = 0 as a function of time for the variable porosity case. The porosity declines monotonically with time, approaching the asymptotic value of zero this result was verified by plotting porosity against reciprocal time and extrapolating to = 0. 

Under the isothermal condition imposed in this example, gypsum precipitates only at the boundary a = 0 in both constant porosity and variable porosity cases. Significant changes of porosity occur at this location in the variable porosity case. Figure 5 shows the accumulation of gypsum at this location at times up to 10 seconds for the two cases. The results are qualitatively similar to the results of the previous example. In the case of constant porosity the volume of precipitated gypsum exceeds available pore space at times greater than about... 

X10 seconds. In this example, the critical concentration of solid is 0.67 mole/dm of porous medium. In the variable porosity case the solid concentration rises asymptotically toward the critical value, again as a result of decreasing influxes of reactants at the boundary caused by decreasing porosity there. 

An alternative approach concerns the preparation of multilayer films in which each layer has different refractive indexes. Li et al. [107] fabricated multilayers with an increasing refractive index from the top to the bottom of the film. They prepared blends of BCP/homopolymers (PS-b-PMMA/PMMA). They used an octadecyltri-chlorosilane-modified glass substrate, and a mixture of solvents made the PMMA domains form a gradient distribution in the vertical direction of the entire micro-phase-separated film. This unique behavior allows, upon removal of PMMA, to obtain porous PS-based structures with variable porosity in the vertical direction (Fig. 6.17). Later, the authors investigated further the parameters [total degree of polymerization (AO, the volume fraction of the PMMA block (/pmma). the weight percentage of the homopolymer, and the solution concentration] that direct the structure formation and related them with the broadband antireflection properties [108]. 

Aluminum nitrate proved to be the best Al precursor in the sol-gel process. Using it resulted in one of the greatest Al incorporations, its application was very easy, a washing step and catalysis were not needed, and the gel samples obtained aged very slowly. The new sol-gel method provides aluminosilicate materials with new structures (e.g. fractals) and new properties (e.g. piezoelectricity or a wide range of variable porosity). The disadvantage of the use of aluminum nitrate is the release of nitrous gases noxious to the environment. 

For investigation of physical and mechanical properties of the functionally graded alumina ceramics with variable porosity the set of samples was prepared. The values of tested properties are given in Table 4. 

Another method for fabricating polymer/HAp scaffolds with variable porosity and pore size utilizes an emulsion freeze-drying process. In this technique, control of... 

However, it did not offer all the models required to address complex multiphase proeesses in textile fabries. The authors propose a model in whieh each fabric layer is a porous membrane of variable porosity. There are fom components solid fibers, gas/vapor mixture, bound liquid and free liquid. 

For this wide range of products, fibers and textiles are used for three reasons their mechanical properties, such as tenacity, elongation, shrinkage, and E-modulus, can be adjusted, their unique high ratio of surface to mass, and their variable porosity. 

The stoichiometric HAP, characterized by an atomic ratio Ca/P = 1.67 and a hexagonal structure, is the nearest relative of biologic apatite ciystals. Moreover, the HAP is the least soluble and the least resorbable calciirm phosphate. When an HAP ceramic is implanted in a bone site, the bone tissue formation is observed on its contact (osteoconduction) (see Figure 12.1). Besides, in certain conditions, calcium phosphate ceramics can induce the formation of bone tissue in ectopic sites. HAP implants appear in the form of dense ceramics or with variable porosity or again, in the case of prostheses, as thin coatings deposited by plasma sprayed on a metal. 

Technical ceramics are composed of raw materials generally as powder and of natirral or synthetic chemical additives, favoring either compaction (hot, cold or isostatic), or setting (hydraulic or chemical) or accelerating sintering processes. According to the formulation of the bioceramic and the shaping process used, we can obtain ceramics, dense or with variable porosity, cements, ceramic depositions or ceramic composites. 

Despite the fact that boehmite is a naturally occurring mineral, the majority of commercial samples are man-made, often by the hydrolysis/thermolysis of aluminum salts. In addition, they commonly contain significant quantities of gibbsite, A1(0H)3, and are of variable porosity. Although we have shown die reactions discussed below to be applicable for samples from different commercial sources, to be self consistent we have chosen to discuss results obtained using a single source of low gibbsite (> 99 % boehmite) obtained from American Cyanamid. 

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