Tailoring-specific porosity

Taking advantage of the advances in materials science, it is possible to prepare ceramic-based membranes with improved properties with regard to the specifications of the intended final applications. The choice of materials, the tailoring of porosity, and the shape of the membrane are the main variables at the disposal of designers of these membranes. 

A very good review article based on a panel study of status, future research needs, and opportunities for porous sorbent materials was published several years ago. It was pointed out that very significant advances have been made in tailoring the porosity of porous sorbent materials in terms of size and shape selectivity. Relatively little progress has been achieved in terms of chemoselectivity of sorbents based on specific interactions between adsorbate molecules and functional groups in the sorbents. Incorporation of active sites into sorbents is of high priority in the development of sorbents. 

Carbon is inert in nature and has a high surface area, making it highly suitable as a support for catalysts. The surface characteristics and porosity of carbon may be easily tailored for different applications. Acid treatment is often applied to modify its surface chemistry for specific applications. Typically, active metal species are immobilized on carbon for catalytic applications. 

Being able to vary porosity at will is a very desirable property since one can select a porosity tailored to the needs of specific repair sites. 

Due to the variety in porous structure, particle size and surface area, pure silica gels and powders find a very wide range of applications. Variation in preparation methods and parameters allows the tailoring of the substrate properties for specific application needs. The main features in the silica applications are its porosity, active surface, hardness, particle size and the viscous and thixotropic properties. Although most applications are based on a combination of those, a classification according to the main properties of interest may be set up. For references, the reader is referred to the works of Iler6 and Unger7 and to the references cited in chapter 8. 

Both EDLCs and pseudocapacitors benefit from tailored, high surface area architectures because they each store charge on the surface by electrostatic or faradaic reactions, respectively. There are numerous examples in the hterature which show that materials possessing such features as nanodimensional crystallite size and mesoscale porosity exhibit significantly higher specific capacitance as compared to nonpotous materials or materials composed of micron-sized powders. The assembly of nanoscale materials is also important. One structure envisioned to be of interest is an array of vertically aligned carbon nanotubes where the spacing between the tubes is matched to the diameters of the solvated electrolyte ions (3). 

Impregnated catalysts have many advantages compared to precipitated catalysts. Their pore strucmre and specific surface area are largely determined by the support. Since support materials are available in all desired ranges of surface area, porosity, shape, size, and mechanical stability, impregnated catalysts can be tailor-made with respect to mass transport properties [9]. 

The oxidation and reduction (redox) processes in electroactive polymers (EAPs) make it possible to use these polymer materials as charge storage devices, either as battery electrodes or as supercapacitors. The potential for reduced cost, weight, and enviromnental impact of EAP electrodes relative to the metals and metal oxides that are traditionally used in such devices makes these polymers attractive alternatives. While inorganic options are limited, EAPs can be tailored to provide specific properties, such as conductivity, voltage window, storage capacity, porosity, reversibility, and chemical and environmental stability. 

Sol-gel techniques are being employed to fabricate components not only for mainstream applications such as photonics, thermal insulation, electronics and microfluidics, but also for more exotic applications such as space dust and radiation collectors [1]. Methods have been developed to tailor the physical properties of sol-gel materials to the requirements of a specific application. For example, porosity and pore size distribution can be controlled by forming micelles in a sol [2-4-] gels can be made hydrophobic by derivatizing the otherwise hydrophilic pore walls with hydrophobic moieties [5] superhydrophilicity can be obtained by ultraviolet irradiation [6, 7] mechanical strength can be increased by cross-linking the oxide nanoparticles that make up the gel [1, 8, 9], and optical properties can be controlled by adding chromophores and nanoparticles to control index of refraction, absorption and luminescence [10-12]. 

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