STRUCTURAL METALLIC MATERIALS BY INFILTRATION

A lot of research has also been devoted to the infiltration of macroporous templates by reactive components. Porous polymeric material was synthesized by either infiltration of a monomer-initiator mixture with subsequent polymerization [29], or by infiltration of a prepolymer solution, which can be UV-cured afterwards [27]. A quite common route to fabricate metal oxide networks is to infiltrate the precursor structure with its corresponding sol-gel solution, which eventually hydrolyzes and solidifies in the desired porous shape. This technique has been shown for a great variety of materials (compare Table 2 in [10], Table 1 in [37], and Table 1 in [38]), such as silica [52], titania [30,50], zirconia [30] or aliunina [30], just to mention a few. Another pathway to metal oxide structures was introduced by Park et al., who precipitated acetate salt solutions of the desired material in the free voids. After addition of oxalic acid the porous metal oxide was formed during the combustion of the latex template [73]. 

Tantalum, which is used for a number of applications, was recently made into a porous material that could be used for bone reconstruction. The porous structure is made by depositing the metal onto a vitreous carbon scaffold using chemical vapor deposition/infiltration techniques. Its low stmctural density means that its stiffness (2.5-4 GPa) is closer to that of natural bone than the solid metal, and the porosity means that bone can fully integrate into the stmcture. forming an excellent bond. " ... 

The ageing mode essentially consists of the infiltration of the material by zinc, and of possible changes in the structure of the intergranular oxide phase. In the upper, colder part of the column, the structure of the material seems not significantly affected by ageing. Only metallic zinc can be found, and it probably migrated through the porosity of the material. In this case, only the cristobalite form of silica was detected in the upper part of the column. 

One strategy is to fabricate a template structure using polymeric material (thus, using the same chemistry as described in Sects. 5.2 and 5.3) and back-fill or coat this structure with inorganic materials. For example, surface modification, followed by electroless deposition of Ag [217-219] or Cu [220], or by chemical reduction of Au solutions by surface functionalities [220], has been used to obtain metallized structures, while infiltration of polymeric photonic bandgap-type structures with Ti(0 Pr)4 solution, followed by hydrolysis and calcination, has been used to obtain highly refractive inverted Xi02 structures [221]. Au has also been deposited onto multiphoton-patterned matrices of biomaterials [194]. 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

Composite materials can be formed by numerous methods. Two modes in which incorporation of the inorganic material in the template can be achieved will be discussed sol-gel processes or nanoparticle infiltration. They are both solution methods that can be processed at low temperatures, hence allowing the use of polymeric templates. In the first method the sol-gel chemistry is performed after the incorporation of a metal oxide precursor in the polymer matrix or around the template entities. The second method makes use of preformed metal oxide nanoparticles, which are infiltrated into the organic scaffold or suspended in solution with the individual structures for controlled adhesion. 

In another approach Subramanian et al. [32] infiltrated the polymeric opal template directly with ultrafine particles instead of employing metal alkoxides or salts. In this approach infiltration with a nanocrystalhne material of known crystal phase is possible, and therefore materials with a predetermined crystal structure of the walls can be obtained even at mild processing conditions. For instance, Ti02 frameworks with a rutile phase of the wall material could be obtained without the necessity to resort to sintering at high temperatures. Additionally, shrinkage, which commonly occurs in the case of condensation of metal alkoxides or conversion of metal salts, is largely reduced. Finally, this route opens a pathway to obtain porous metal oxide materials, which are barely accessible by wet chemistry approaches. 

Similar to PANI, the carbon-related materials, activated carbon, carbon nanotube, and GO, were also doped with PPy to fabricate PPy/carbon-related material composites for wastewater treatment. PPy/impregnated porous carbon was prepared by vapor infiltration polymerization technique to obtain a mesoporous structure [72], The as-prepared composite exhibited an improved adsorption ability to remove heavy metal ions, such as Hg(II), Pb(II), and Ag(I) due to amino groups of PPy. PPy/carbon nanotube composites can be prepared by grafting from technique either chemically or electrochemically. The chemical fabricated PPy/carbon nanotube composite can effectively remove heavy metals, anions and chemical oxygen demand from paper mill waste [73]. The electrochemical synthesized PPy/carbon nanotube provided a simple and highly effective method for ClO removal via electrically switched ion exchange technique (Figure 11.15) [74]. 

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