Zirconium directed metal oxidation

Because the details of processing in each class of CMCs (e.g., oxide, carbide, or nitride matrix) are slightly different, the appropriate thermochemical approach for each class may also be different. For example, in the formation of alumina matrix materials by directed metal oxidation, the alumina product grows from a molten aluminum alloy by reaction with an oxygen-containing gas phase. On the other hand, in the formation of platlet-reinforced zirconium carbide, the gas phase is not involved in the reaction at all, being inert to the reactants and products. Thus, a general approach to deal with the myriad of possible products formed by the... 

Fig. 4. A schematic illustration of the processing used to produce zirconium diboride reinforced zirconium carbide materials by directed metal oxidation.

Fig. 5. The microstructure of zirconium diboride-reinforced zirconium carbide composites produced by directed metal oxidation, (a) A composite prepared with 22 vol % metal, (b) A composite produced with less than 2 vol % metal. 

The ability of siloxane surfactants to stabilize different chemical reactions allows the preparation of different nano-materials. For example, gemini surfactants containing a siloxane moiety have been used as templates for the preparation of mesopoious metal oxides sueh as zirconium, titanium, and vanadium oxides. The siloxane segment seems to play an important nano-propping role during the surfactant removal by direct calcination [105]. 

Other preparation methods have recently been developed. Sulfated metal oxides have been prepared by a sol-gel method [42,57,58], which involves the formation of a zirconium-sulfate co-gel by adding sulfuric acid to zirconium n-propoxide in isopropyl alcohol. This one step method appears to be simpler than the two step preparation procedures and allows a better control of the variables. It also allows the direct formation of biiunctional catalysts by the addition of chloroplatinic acid to the gel mixture. A new preparation method, named rapid thermal decomposition of precursors in solution (RTDS), which involves the use of hot pressurized water at hydrotheimal conditions to force metal ion precursors to go into phases of oxyhydroxides and oxyhydrosulfates, has been used to produce sulfated zirconia with crystallite sizes below 100 A [59]. 

Another propylene ammoxidation catalyst that was used commercially was U-Sb-0. This catalyst system was discovered and patented by SOHIO in the mid-1960s (26,27). Optimum yield of acrylonitrile from propylene required sufficient antimony in the formulation in order to ensure the presence of the USbaOio phase rather than the alternative uranium antimonate compound USbOs (28-30). The need for high antimony content was understood to stem from the necessity to isolate the uranium cations on the surface, which were presumed to be the sites for partial oxidation of propylene. Isolation by the relatively inactive antimony cation prevented complete oxidation of propylene to CO2. Later publications and patents showed that the activity of the U-Sb-0 catalyst is increased by more than an order of magnitude by the substitution of a tetravalent cation, tin, titanium, and zirconium (31). Titanium was found to be especially effective. The promoting effect results in the formation of a solid solution by isomorphous substitution of the tetravalent cation for Sb + within the catalytically active USbaOio- phase. This substitution produces o gen vacancies in the lattice and thus increases the facility for diffusion of lattice o gen in the solid structure. As is discussed below, the enhanced diffusion of o gen is directly linked to increased activity of selective (amm)oxidation catalysts based on mixed metal oxides. 

Insolubilization. Insolubilization of compounds within textiles parallels the history of humanity the direct dyeing techniques for cotton were highly advanced in the Bronze Age. With the exception of fiber-reactive dyes discussed earlier, other cotton dyes, ie, vat and sulfur, are insolubilized within the fiber after an oxidization step. Insoluble metal oxides have been used to flameproof cotton, and zirconium compounds have been insolubilized on cotton to render the fabric microbial resistant (135) or mildew resistant (136) via a mineral dyeing process (see Textile Finishing). 

The primary drawbacks of the Nafion membranes are poor conductivity at low relative humidities (and consequently at temperatures >100 C and ambient pressure) and large crossover of methanol in direct methanol fuel ceU (DMFC) applications. As a result, considerable efforts have been made in recent years to overcome these drawbacks. Peihaps the most widely employed approach is the addition of inorganic additives to Nafion m branes to yield organic/inorganic composite membranes. Three major types of inorganic additives that have been studied (zirconium phosphates, heteropolyadds, metal hydrogen sulfates and metal oxides) are reviewed in the following. 

Zirconium(IV) and hafnium(IV) chlorides and bromides form 1 2 adducts of the type [ZrX4(RCN)2] (R = Me, Et, Pr or Ph X = Cl or Br) and [HfX4(MeCN)2] (X = Cl or Br).11SM24 These complexes may be prepared by (i) direct reaction of the metal tetrahalide with an excess of the nitrile120 123 or (ii) electrochemical oxidation of zirconium or hafnium metal in the presence of a solution of chlorine or bromine in acetonitrile.118 The adducts are moisture-sensitive, white solids, insoluble in nonpolar solvents, but soluble in acetonitrile. In the later solvent, [ZrBr4(MeCN)2 behaves as a nonelectrolyte.122... 

The activation of aluminum with ultrasound or dispersion of liquid aluminum. The suspension of powder aluminum in petrol or n-geptane without oxygen is subjected to ultrasound the tough oxide film on the surface of aluminum is removed and aluminum becomes reactive. The second activation technique is the dispersion of liquid aluminum with argon or purified nitrogen flow into a finely dispersed state. It should be noted, however, that the most reactive aluminum powder for direct synthesis is the powder alloyed with transition metals (titanium, zirconium, niobium, tantalum) with the size of particles from 10 to 125 pm. 

---