Alkoxide infiltration

Besides the common features of most metal oxides as chemically stable and mechanically robust materials, this class of compounds can also provide unique functions for ceramics, high refractive index and magnetic materials, catalysis, and biocompatibility. A wide range of porous metal oxide materials have been synthesized over the last decade. In general three distinct pathways have been undertaken a sol-gel synthesis of the metal alkoxide infiltration with an appropriate metal salt and subsequent conversion or infiltration or codeposition with preformed metal oxide nanocrystals. 

Fabes BD, Berry GD. (1990) Infiltration of glass flaws by alkoxide coatings. / Non-Crystalline Solids 121 357-364. 

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. 

Figure 12.10 shows a schematic of the two-step infiltration process. The first step consists of the preparation of a powder compact within the fiber preform by a routine slip-cast infiltration, the body being dried, and then strengthened by heating at a temperature close to the onset of shrinkage. The second step consists of the infiltration of the strengthened body by a liquid (polymeric) preousor that is first converted into a solid (a gel by reaction with water or diols if alkoxides are used ), or by the action of oxygen if polyvinylsilane (PVS) is used to... 

FIGURE 12.10 Schematic of the oxide (alkoxides) or nonoxide (polymers) matrix precursors infiltration for fabricating three-dimensional reinforced composites. (After Reference 87.)... 

In 1999, Brasseur-Tilmant [56] presented a work dealing with modification of macroporous alumina media by TiOi particles deposition using supercritical isopropanol. The aim was to prepare inorganic membranes for cross-fiow filtration. Anatase particles were deposited on plane alumina support after thermal decomposition of titanium alkoxide precursors. A slight infiltrated zone was observed and a pore size reduction was achieved from 110 to 5 nm, leading to obtain fine ultrafiltration membranes. The main problem was to control the reaction at the membrane interface and not in the porosity, and moreover, this process was suitable for mbular membrane preparation. 

Closely related to the salt infiltration method discussed above is the utilization of metal alkoxide precursors in a sol-gel process to fill the interstitial sites in an opal template. The template is first infiltrated with an alkoxide (neat or as solution), then the hydrolysis and condensation reaction following the infiltration transforms the alkoxides into an extended network of the corresponding metal oxide. By this method Ti02 networks besides other oxides were initially prepared with a strong focus on their optical properties as photonic crystals [30,50]. For this purpose a high refractive index material is desired, which would be the rutile modification of titania with the highest refractive index. Nevertheless, the sol-gel procedure provides an amorphous or anatase modification with lower refractive index. By calcination of the anatase... 

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. 

The most successful metal precursors are alkoxides of Ti and Zr, because their reactivity is easily controllable. They are liquids at room temperature and can be used without a solvent, and the metal concentration is high. After infiltration into the voids of templates, they can be solidified via a sol-gel transformation. Therefore, well-ordered macroporous crystalline TiOi and ZrOi can often be prepared. 

Three-dimensionally ordered macroporous crystalline Ti02 could be obtained by using titanium alkoxides as a metal source. Various titanium alkoxides [Ti(OEt)4, Ti(0 Pr)4, Ti(OBu)4] can be used in alcoholic solutions or neat.f " After the titanium alkoxides have infiltrated the voids of the template, they can be solidified by drying in air (sometimes heated at ca 50 °C) by a sol-gel process. Another titanium source, ammonium lactate titanium dihydroxide, can be used. A gaseous Ti source, TiCE, can be used. Gaseous TiCU was deposited on a SiOi template, and then TiCU was hydrolysed with vaporous H2O. This two-step deposition of Ti02 on the template was repeated to deposit Ti02 in the voids of the template. ... 

Tungsten alkoxide, W lOEtls, is commercially available and is soluble in alcoholic solution. Tungsten alkoxide solution infiltrates the voids and is solidified in the voids by a sol-gel method as described by Holland Ethanol solution of WClg can infiltrate the voids of... 

Alkoxides of late transition metals are not easily available. Some chemical companies produce alkoxides of late transition metals, but they are expensive and too reactive to infiltrate into the template. They react quickly on the surface of the template, and solids that are produced prevent complete infiltration. An example using iron alkoxide, Fe(OEt)3, has been reported, but in this case, some nonporous bulk material existed in the sample. 

If mixed-metal alkoxide precursors can be prepared and their reactivity can be controlled, ordered macroporous mixed metal oxides can be produced by a sol-gel method using a colloidal crystal template. Alkoxides of Ti and Zr with other metals are easily prepared by mixing titanium alkoxide and zirconium alkoxide with alkoxides of other metals or metal salts, and the formed mixed metal alkoxides can infiltrate the voids of templates. Therefore, production of mixed metal oxides with Ti and Zr is straightforward. 

Zhang et al [184] infiltrated a carbon fiber preform with a metal alkoxide followed by gelling to give a stable dispersion (sol) of small partieles (<0.1 pm). Partial evaporation, or the addition of a suitable accelerator, gives a three dimensional gelatinous network. The gel is dried to remove water and heat treated to form a eeramie powder. 

Metal—organic decomposition Polymeric precursors/sol-gel method alkoxide precursor, halide precursors, nitrate and acetate precursors Impregnation/infiltration Slurry coating... 

In the case of Ti-based perovskite mixed metal oxides, precursor solutions of Ti alkoxide such as Ti(OiPr)4 and Ti(OBu)4 mixed with other metal sources are used. The Ti-based alkoxide solution is so mild that the solution infiltrates into the voids, and the solution is so reactive that it solidifies in the voids by heating to less than 90 °C. When metal concentration is not high enough, repeated infiltration and solidification is possible to enhance the amount of metal in the voids. 

Nafion -Si02 nanocomposite membranes are used to increase the proton conductivity and water retention at higher temperatures as well as improve the thermomechanical properties. Mauritz [53] investigated the preparation of Nafion/SiO composites by infiltration of Nafion membranes with silicon alkoxides. They exploited the fact that the pendant group clusters catalyze the sol-gel reac-... 

Most of 3DOM metal oxides are fabricated by an alkoxide-based sol-gel process. The typical procedures include (i) preparation of a colloidal crystal template by ordering monodispersive microspheres (e.g., PS, PMMA or silica) into a face-centered close-packed array (ii) infiltration of a metal alkoxide solution into voids in the colloidal crystals, and in-situ solidification of the precimsor via a sol-gel process and (iii) template removal via a calcination or extraction pathway [99,217-219], Using the alkoxide-based sol-gel method, 3DOM oxides of Si, Ti, Zr, Al, Sb, W, Fe, and 3DOM mixed oxides of some of them can synthesized due to the moderate reactivity of their alkoxide preciuTsors [99], However, most of the other metal alkoxides exhibit high reactivity and their reactions are too quick to be... 

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