Metal carbides particle size

Thermal reduction processes have been apphed successfully in making the metal from salts. In one such process, potassium fluotantalate is reduced with sodium metal at high temperatures to form tantalum powder of high purity and small particle size. Also, tantalum oxide can be reduced at high temperatures in vacuum with aluminum, silicon, or tantalum carbide. When the oxide is reduced by tantalum carbide, a metal sponge is obtained which can be embrittled with hydrogen to form powder metal. 

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

NaX and NaY zeolites (Union Carbide) were exchanged by lithium, potassium, and caesium in a 1 M solution of the corresponding metal chloride at 80°C for 60 min, using a liquid to solid ratio of 10. The samples were then filtered and washed free of chlorides. After drying, the zeolite was pelletized, crushed, and sieved to different particle sizes. 

FIGURE 9.4. SEM images of three different WC powders (right), and of the respective W powders (left), from which they were made. Note that the particle size of the carbide corresponds well with the particle size of the metal. 

Depending on the operative conditions (i.e., temperature, carbon source, presence of hydrogen, etc.), the nature of the catalyst, and the metal particle size, one of these two reaction pathways would predominate, thus leading to the formation of different carbon materials. Decomposition of the surface metal carbide species would induce (1) the process of carbon emichment of a metal particle... 

Chase, ID. (1974), Method of Making Sub-Micron Particles of Metal Carbides of Controlled Size, American Cyanamid Corp., U.S. Patent no. 3,839,542. 

Reasons for use abrasion resistance, cost reduction, electric conductivity (metal fibers, carbon fibers, carbon black), EMI shielding (metal and carbon fibers), electric resistivity (mica), flame retarding properties (aluminum hydroxide, antimony trioxide, magnesium hydroxide), impact resistance improvement (small particle size calcium carbonate), improvement of radiation stability (zeolite), increase of density, increase of flexural modulus, impact strength, and stiffness (talc), nucleating agent for bubble formation, permeability (mica), smoke suppression (magnesium hydroxide), thermal stabilization (calcium carbonate), wear resistance (aluminum oxide, silica carbide, wollastonite)... 

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