Boron carbide, B₆C

Boron carbide, B₆C was isolated in small quantities by Joly in 1883 from among the products of the interaction, in a carbon crucible, of boron sesqui-oxide and aluminium. Later it was prepared in quantity by Moissan, by heating to 3000° in the electric furnace a mixture of sugar carbon and amorphous boron contained in a carbon crucible. A cheaper method of preparation consists in employing boron sesqui-oxide and petroleum coke as starting materials, the former being fed through a hollow graphite electrode on to a layer of the latter contained in a graphite crucible which forms the other electrode. The product is purified from graphite by many treatments with boiling nitric acid and potassium chlorate, followed by treatment with boiling sulphuric acid.

Boron carbide forms brilliant black crystals, of density 2.51, and melts at 2350°. The carbide conducts electricity. It is extremely hard (harder than silicon carbide) and can be used in diamond polishing. It is not affected by mineral acids at the boiling temperature, sulphur at 500°, nitrogen, phosphorus, bromine, or iodine at a bright red heat. It is slowly attacked by oxygen at 1000°, and by chlorine at a somewhat lower temperature. When fused with potassium hydroxide it is decomposed with the evolution of carbon monoxide.

The so-called carbide BC is a mixture of the preceding carbide and graphite (Tucker and Bliss).