Mixed carbide powders

Another way of production is the coating of c-BN by electro-less plating with Ni-P, Ni-B, Ni-Fe-P, Ni-Cr-P, Ni-Cu-P, or Ni-W-P alloys, and mixing these powders with >1% of various carbides, borides, nitrides, silicides, and/or oxides. These powders are compacted and pre-sintered at 700-900 °C. Finally, hot-isostatic pressing at 1000-1400 °C and 1000-2000 bar is performed to reduce porosity [264]. 

The mixed carbides are prepared either by arc melting the elements or carbides or by heating the mixed, compressed powders for a time sufficient to insure a uniform product. Since metal atom dilTusion is slow, the homogenization T must be high if excessive times are to be avoided. The conditions depend on the system and whether a binder is used, which accelerates the reaction. 

These carbides are prepared by arc melting the elements or by sintering the mixed, compressed-carbide powders. The arc-melted material is pulverized and hot pressed. This two-step process insures a more uniform material. ... 

Uranium carbide UC (UC2 and U2C also exist) has a melting point of about 2300 °C and is an important nuclear fuel for high-temperature reactors. It is prepared by reduction of UO2 with carbon, followed by pressing and sintering. It can also be made by hot pressing of mixtures of uranium metal powder with graphite at 1000 to 1100 °C. A mixed carbide with ThC is manufactured in the form of spheroids by melting. As the product is hydrolyzed on exposure to air, it is coated with a protective carbon layer. 

Carbide powder is manufaetured by carburization of the metal, metal oxide or metal hydride with nuclear-pure graphite at ca. 2000°C. Carbide powder pellets (see Section 5.5.5.1.5) or beads up to 0.6 mm in diameter are formed and sintered using ceramic process technology. Reaction sintering is commonly used for the manufacture of uranium earbide fuel beads, in whieh uranium oxide and carbon are first mixed then annealed to form the carbide and then are sintered to 90 to 95% of the theoretical density. An... 

The cold pressing is carried out in hardened steel dies. The carbide powder is first mixed with 0.5-5% of binder lubricants, such as paraffin, camphor, Carbowax, cetyl alcohol, or beeswax dissolved in nonaqueous inert solvents such as benzene, isopropyl alcohol, or methyl alcohol. The bonded carbide powder is granulated by forcing through a screen and then loaded into the dies. The density of the cold-pressed pellets may be as high as 80% of theoretical, and the sintered pellets may be densified to 90% of theoretical. The optimum sintering condition is 1800°C for 4 h. 

WC or WC + mixed carbides can be recycled to hardmetal grade powder preparation. 

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... 

Carbides of metals and other elements have been produced by this approach, including carbides of boron, silicon, titanium, zirconium, hafnium, vanadium, niobium, molybdenum, tungsten, tantalnm, and thorium (Funke, Klementiev, Kosukhin, 1969 Sheppard Wilson, 1972 MacKinnon Wickens, 1973 Chase, 1974 Steiger Wilson, 1974 Swaney, 1974 MacKinnon Renben, 1975). The produced caibide particles are very small their diameter is usually about 20-200 mu. Halides are mixed with hydrocarbons, usually in a ratio H2 Me = 2-30. The gas mixture is heated up in plasma to temperatures of 1300-4000 K time of synthesis exceeds 50 ms. As an example, production of submicron boron carbide powder from gaseous boron trichloride and methane occurs in the strongly endothermic process ... 

Noncombustible gas. Sulfur dioxide reacts violently with alkali metals at their melting points. Reactions with finely divided metals produce incandescence. Explosion occurs when it is mixed with fluorine or interhalogen compounds of fluorine. Incandescence occurs when carbides of alkali metals are placed in a sulfur dioxide atmosphere or by heating metal oxides with sulfur dioxide. An alcoholic or ethereal solution of sulfur dioxide explodes when mixed with powdered potassium chlorate (Mellor 1946). The dry gas reacts with chlorates to form chlorine dioxide, which ignites and explodes on heating. 

Carbonitride powders for industrial applications are prepared by mixing carbide and nitride powders intimately together and heating the mixtures to high temperatures,... 

Tungsten and carbon (soot) is mixed in exact proportions. The mixture is heated at high temperature in hydrogen under formation of Cemented carbide powder (WC). 

During these last few years, synthesis methods assisted by the use of plasma or a lazer beam have been developed. They offer several advantages compared to conventional methods in particular, they lead to ultrafine multi-element powders (mixed carbides) of uniform size (a few dozen nanometers). 

Flame plating (D-gun) employs oxygen and fuel gas. In this method, developed by the Union Carbide Corporation, the gas mixture is detonated by an electric spark at four detonations per second. The powders, mixed with the gas, are fed under control into a chamber from which they are ejected when detonation occurs. The molten, 14—16-pm particles are sprayed at a velocity of 732 m/s at distances of 5.1—10.2 cm from the surface. The substrate is moved past the stationary gun. 

Pressing is carried out within a cemented carbide die between two steel or cemented carbide punches. In order to impart enough mechanical strength to the blank to permit further manipulation without risk, removable organic binders (camphor, natural or synthetic waxes, latex or synthetic rubber, methyl polymethacrylate, polyvinyl alcohol, carboxymethylcellulose, ammonium alginate) are mixed into the powder, dissolved in a convenient volatile solvent. Some of these also act as lubricants thus minimizing the wear on the die. 

More recently, Stanicioiu, Chinta Hartner (1959) attempted to reinforce the cement with glass fibres, but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. 

In the presence of air, the carbide combusts spontaneously. With water this substance detonates. Another accident involved powdered amorphous carbon mixed with potassium and heated to a high temperature. Ignition followed by detonation was attributed to the presence of the superoxide on the metal surface. If the suggested interpretation is correct, this accident illustrates the comment made previously. 

In practice, a mixture of actinide dioxide and graphite powder is first pelletized and then heated to 2275 K in vacuum in a graphite crucible until a drop in the system pressure indicates the end of CO evolution. The resulting actinide carbide is then mixed with tantalum powder, and the mixture is pressed into pellets. The reduction occurs in a tantalum crucible under vacuum. At the reduction temperature, the actinide metal is vaporized and deposited on a tantalum or water-cooled copper condenser. 

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