Electrical carbides

The original method for the manufacture of ethyne, the action of water on calcium carbide, is still of very great importance, but newer methods include the pyrolysis of the lower paraffins in the presence of steam, the partial oxidation of natural gas (methane) and the cracking of hydrocarbons in an electric arc. 

The covalent carbides These include boron carbide B4C and silicon carbide SiC the latter is made by heating a mixture of silica and coke in an electric furnace to about 2000 K ... 

Acetylene was discovered m 1836 by Edmund Davy and characterized by the French chemist P E M Berthelot m 1862 It did not command much attention until its large scale preparation from calcium carbide m the last decade of the nineteenth century stim ulated interest m industrial applications In the first stage of that synthesis limestone and coke a material rich m elemental carbon obtained from coal are heated m an electric furnace to form calcium carbide... 

A = Aristech Chemical B = BP Chemicals Ce = Celanese Cy = CYRO Industries Do = Dow Chemical Du = Du Pont E = Eastman Chemical G = General Electric R = Rohm Haas S = Shell Chemical U = Union Carbide... 

Union Carbide Corp., Kabelitems Wire and Cables No. 157, A. Critical Comparison ofXEPE andEPR for use as Electrical Insulation on Underground Power Cables, Danbury, Conn. 

Calcium Carbide. Until the 1940s, calcium carbide, which is made by interacting quicklime and coke in an electric furnace, was the only source of acetylene. Although much more acetylene is now derived from natural gas, calcium carbide is stiH being produced, using 0.9—1.0 t of quicklime to make 11 of carbide... 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Carbon, Carbides, and Nitrides. Carbon (graphite) is a good thermal and electrical conductor. It is not easily wetted by chemical action, which is an important consideration for corrosion resistance. As an important stmctural material at high temperature, pyrolytic graphite has shown a strength of 280 MPa (40,600 psi). It tends to oxidize at high temperatures, but can be used up to 2760°C for short periods in neutral or reducing conditions. The use of new composite materials made of carbon fibers is expected, especially in the field of aerospace stmcture. When heated under... 

Silicon carbide has very high thermal conductivity and can withstand thermal shock cycling without damage. It also is an electrical conductor and is used for electrical heating elements. Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential usehilness as technical ceramics in aerospace appHcation, eg, the carbides (qv) of B, Nb, Hf, Ta, Zr, Ti, V, Mo, and Cr. Ba, Be, Ca, and Sr carbides are hydrolyzed by water vapor. 

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. 

Sihca is reduced to siUcon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous siUcon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum haUdes, siUca can be converted to silane in high yields by reaction with hydrogen (15). SiUcon itself is not hydrogenated under these conditions. The formation of siUcon by reduction of siUca with carbon is important in the technical preparation of the element and its alloys and in the preparation of siUcon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and siUcate. At 800—900°C, siUca is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce siUca to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Pig iron and iron and steel scrap are the sources of iron for steelmaking in basic-oxygen furnaces. Electric furnaces have rehed on iron and steel scrap, although newer iron sources such as direct-reduced iron (DRI), iron carbide, and even pig iron are becoming both desirable and available (see Iron bydirectreduction). In basic-oxygen furnaces, the pig iron is used in the molten state as obtained from the blast furnace in this form, pig iron is referred to as hot metal. 

Flaws in the anodic oxide film are usually the primary source of electronic conduction. These flaws are either stmctural or chemical in nature. The stmctural flaws include thermal crystalline oxide, nitrides, carbides, inclusion of foreign phases, and oxide recrystaUi2ed by an appHed electric field. The roughness of the tantalum surface affects the electronic conduction and should be classified as a stmctural flaw (58) the correlation between electronic conduction and roughness, however, was not observed (59). Chemical impurities arise from metals alloyed with the tantalum, inclusions in the oxide of material from the formation electrolyte, and impurities on the surface of the tantalum substrate that are incorporated in the oxide during formation. 

Research-grade material may be prepared by reaction of pelleted mixtures of titanium dioxide and boron at 1700°C in a vacuum furnace. Under these conditions, the oxygen is eliminated as a volatile boron oxide (17). Technical grade (purity > 98%) material may be made by the carbothermal reduction of titanium dioxide in the presence of boron or boron carbide. The endothermic reaction is carried out by heating briquettes made from a mixture of the reactants in electric furnaces at 2000°C (11,18,19). 

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