Powder route

The electrochemical generation of Cr(II) produces yields comparable to the conventional zinc powder route. Advantages of the electrochemical method include reduced revenue costs and a more environmentally acceptable process with respect to the heavy metal effluents. 

Although powder routes have been successfully developed for synthesis of catalysts used in low temperature fuel cells, electrodeposition offers the highest noble metal utilization and is preferred over the alternative chemical methods. Electrodeposition enables the formation of catalyst particles on specific sites where they can be essentially utilized, i.e., the triple phase boundary where the membrane (ionic conductor), electrode (electronic conductor), and reactants meet. Powder methods do not guarantee that all catalyst particles are in contact with both electrode and membrane materials, and therefore, a portion of catalyst particles may remain inactive. 

As already mentioned, synthesis of multinary silicon boron nitrides or carbon nitrides cannot be achieved via the well-known powder route, including mixing, milling, and sintering of binary nitride/carbide powders, because the interdiffiision of the covalent nitrides, and carbides, proceeds... 

This is simply an extension of the powder route for producing monolithic ceramics. A powder of the matrix constituent is mixed with the toughening constituent, which is in particulate or whisker form, together with a binder. The mixture is then pressed and fired or hot pressed. 

C. B. Von Schweitzer, R. D. Rawlings and P. S. Rogers, Processing and Properties of Silceram Glass-ceramic Matrix Composites Prepared by the Powder Route, in Third Euroceamics Vol. 2, P. Duran and J. F. Fernandez eds., Faenza Editrice Iberica, 1139-1144 (1993). 

Basically, processing can be divided into the powder and chemical routes. The powder route includes sintering, slurry impregnation and reaction bonding, whilst the chemical route includes CVI, direct oxidation, sol gel and polymer pyrolysis. Final consolidation and densification must be achieved at high temperature. The powder route produces higher density ceramic matrix composites and the chemical route produces better quality. 

Figure 1.27 Flow chart for the production of Ti02 by (a) a conventional powder route and (b) the powder route used by Barringer and Bowen (89).

Several kinds of electro- and/or magneto-ceramics have been formed into thin fibers by the sol-gel method with the aim at the application to sensing elements. In the preparation of strictly stoichiometric double or multicomponent oxides such as LiNbOs, PbTi03 and so on, the intimate mixing of raw materials at molecular level in sols or gels would permit the direct formation of desired compounds without diffusion process of constituent atoms or ions, which is indispensable in the conventional powder route. It is often observed. 

Trends in the field of economics are the centralization of the powder fabrication to enable production on a large scale and the manufacture of low quahty anisotropic materials by a much less expensive technology. An example of the latter is the introduction of alignment during pressing of the raw material mixture in the fabrication route of isotropic materials. 

The resulting titanium nitride forms a sintered mass, which must be subsequently milled to form a powder having a wide size distribution. The powders produced by these routes ate typically 0.5—10 )Tm, with a wide size distribution. Very fine powders (0.005—0.5 -lm) have been prepared at pilot-plant scale... 

Powder Preparation. There are several routes to preparing SiC powders having variable purity levels, crystal stmcture, particle size, shape, and distribution. Methods that have been examined include growth by sublimation from the vapor phase, carbothermic reduction, and crystallization from a melt. 

G s-Ph se Synthesis. A gas-phase synthesis route to making fine, pure SiC having controllable properties has been described (78,79). Methane was used as a carbon source if required, and the plasma decomposition of three feedstocks, siUcon tetrachloride [10026-04-7] SiCl, dimethyl dichi orosilane, and methyltrichlorosilane [75-79-6] CH Cl Si, into fine SiC powders was investigated. 

Water-Soluble Trivalent Chromium Compounds. Most water-soluble Cr(III) compounds are produced from the reduction of sodium dichromate or chromic acid solutions. This route is less expensive than dissolving pure chromium metal, it uses high quaHty raw materials that are readily available, and there is more processing fiexibiHty. Finished products from this manufacturing method are marketed as crystals, powders, and Hquid concentrates. 

Other metal chlorides and salts can also be used lea ding to formation of numerous other metal dichloroisocyanurates (34). Other routes to SDCC iuclude reaction of a moistened powdered mixture of TCCA, CA, and NaHCO (35), reaction of cmde CA with NaOCl (36), and reaction of NaH2 with HOCl (37) or CI2O (38). 

There is a great deal of flexibility in the choice of laser radiation in the production of thin Aims by photochemical decomposition, and many routes for achieving the same objective can be explored. In most reactions of indusuial interest the reaction path is via tire formation of free radicals as intermediates, and the complete details of the reaction patlrs are not adequately defined. However, it may be anticipated that the success of the photochemical production of new materials in tlrin fllms and in fine powder form will lead to considerably greater effort in the elucidation of these kinetics. 

Increasing use is now being made of alternative processing routes. In powder metallurgy the liquid metal is atomised into small droplets which solidify to a fine powder. The powder is then hot pressed to shape (as we shall see in Chapter 19, hot-pressing is... 

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