SILICON CARBIDE is one of the small class of solid com pounds containing only non-metallic elements. It does not occur in nature, but was discovered accidentally in 1871 by Acheson, who gave it the name carborundum (q.v.). On account of its extreme hardness (next to diamond on Mohs's scale), its abrasive value was soon recognized, and it remains the first in importance, as it was the first in time, of commercially produced artificial abra sives (q.v.). It often appears as greenish, bluish, or brownish translucent or opaque crystals, but is generally seen as a greyish brown powder, being of most industrial use in this form. Chemi cally it has the composition SiC (hardness, 9; sp.gr. 3-2). Its crystal structure is the same as that of the diamond, only with alternate carbon atoms replaced by silicon atoms. This fact is doubtless connected with its extreme hardness.
Silicon carbide is made by heating a mixture of 35% coke (C), 53% sand (Si02), o% sawdust and 2% salt (NaCl). The furnace is rectangular, about 3o ft. long, 1 o ft. wide, and io ft. high; and the electrodes are bundles of thick carbon rods passing through thick brick walls at the ends of the furnace. The space between the carbons of the electrodes is packed tightly with graphite, the charge of coke, sand, sawdust and salt is filled in up to the level of the centre of the electrodes, and a trench is made in it to hold small coke running longitudinally from one pole to the other. A further charge is then added and heaped up above the level of the top of the furnace. The brick walls are separated from the charge by a thickness of fine coke. An alter nating current of about 6,000 amperes at 23o volts is switched on, and as the resistance diminishes, the current is regulated so as to be kept at 20,000 amperes and 75 volts. After about 48 hours the
current is stopped, and after a day's cooling the furnace is care-. fully unpacked, and a layer of some 18 in. of nearly pure crys tallized silicon carbide is found, together with layers of graphite and of partially converted material. Special precautions are re quired in starting and stopping the current (some thousands of kilowatts). The substance produced by this process resists the action of even the strongest acids, and the action of air or of sulphur at high temperatures. The crude product can therefore be treated with hot sulphuric acid to purify it.
Silicon carbide forms a more or less porous crystalline mass, which shows some well-defined hexagonal plates. Although harder than artificial corundum, it is less tough and has a different fracture. The properties of silicon carbide make it the most efficient abrasive for working on materials of low tensile strength such as cast iron, copper, aluminium, marble, granite and pearl. Carborundum paper, made like emery paper, is now largely used in place of garnet paper in American shoe factories, and finds a market in other directions as well. It is also used in the form of grinding wheels, and coated cloth, and as loose grains. Silicon carbide is also extensively used in the manufacture of high tem perature refractories.
The amorphous carbide, which was at first a waste product, has been tried, it is reported with success, as a lining for steel furnaces, since it is said not to be affected by iron or iron oxide at a white heat.