Preforming, directed metal oxidation

Fig. 1. A schematic illustration of CMC growth to net-shape using a directed metal oxidation process where the preform is formed by cold-pressing.

Fig. 3. A schematic illustration showing the various steps employed to form a tubular component by the directed metal oxidation process. The preform is formed by slip casting.

Typically, the directed metal oxidation process involves the simultaneous reaction of molten metal, e.g., A1 with Oz, and infiltration of the reaction product and metal into a porous preform of the desired reinforcement. The directed metal oxidation process can also form composites in the absence of a reinforcement phase, termed matrix-only growth. Although the former process is more interesting commercially because of the ability to tailor the composite properties and because the product does not show significant preferred orientation, the latter case is simpler conceptually and theoretically. Thus, the thermodynamic discussion will begin with growth in the absence of reinforcements and then cover the additional complications that arise from their presence. 

The generic process for fabrication of fiber-reinforced aluminum oxide matrix composites by directed metal oxidation includes preforming, fiber-matrix interface coating, matrix growth and removal of residual aluminum. A flow chart with the various processing steps is shown in Fig, 1. 

A useful variation of the Williamson synthesis involves silver oxide, Ag20, as a mild base rather than NaH. Under these conditions, the free alcohol reacts directly with alkyl halide, so there is no need to preform the metal alkoxide intermediate. Sugars react particularly well glucose, for example, reacts with excess iodomethane in the presence of Ag20 to generate a pentaether in 85% yield. 

First, the p-type material needs to have electronic levels into which holes can be injected from the oxidized or excited state of the dye. The redox levels of the dye and the p-type material therefore have to be adapted carefully. An intimate contact between the sensitized metal oxide and the p-type material is vital to assure fast injection and regeneration processes (Fig. 1). This implies either the growth or deposition of one semiconductor inside a preformed, sensitized porous film of its counterpart or the in situ formation of the sensitized composite. Direct formation of the sensitized junction would be appreciable however, charge collection within the two independent semiconductor networks, in which at least one semiconductor is formed from nanometer-sized inorganic semiconductor particles, demands intimate contact between the particles. Reduced... 

Figure 7.1 Synthetic strategies to obtain tnetal ceria core-shell structures, (a) Co-precipitation of either preformed metal particles or metal particle precursors and the metal-oxide precursor, (b) Microemulsion, (c) Direct functionalization of preformed metal particles.

FIGURE 6.11 Diagram of the processing technique used to prepare Cu-Ce02-YSZ anodes for direct oxidation of hydrocarbon fuels by preparing a porous preform of YSZ and then infiltrating it with cerium nitrates to form ceria and then with copper nitrates to form metallic copper [84]. Reprinted from [84] with permission from Elsevier. 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

Complexation/decomplexation of metal ions or of neutral organic molecules, protonation/deprotonation reactions, and oxidation/reduction processes can all be exploited to alter reversibly the stereoelectronic properties of one of the two recognition sites, thus affecting its ability to sustain noncovalent bonds [30-34, 41]. These kinds of switchable [2 catenanes can be prepared following the template-directed synthetic strategy illustrated in Figure 5, wherein one of the two macrocyclic components is preformed and then the other one is clipped around it with the help of noncovalent bonding interactions. 

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