Aluminium borocarbides

The researches of Wohler and Deville and of Hampe were concerned mainly with the interaction of boron oxide and aluminium in either clay or graphite crucibles. Joly showed that when a graphite crucible was employed, the following products were obtained, the relative quantities varying with the conditions of the experiment: (i.) aluminium boride AlB₂; (ii.) aluminium boride AlB₁₂; (iii.) yellow crystals containing boron, aluminium, and carbon; and (iv.) boron carbide B₆C. Hampe's experiments led him to believe in the existence of a definite borocarbide Al₃C₂B₄₈. Later, Biltz obtained crystals of definite composition, corresponding to the formula Al₃C₂B₄₄. The method of preparation was similar to that described for the preparation of the boride AlB₁₂, except that soot (2 grams) was added to the mixture. The product, after treatment with water, concentrated hydrochloric acid, and then the warm, dilute acid for some days, consists of sparkling yellow crystals mixed with a few black crystals of AlB₁₂.

The crystals have a density at 18° of 2.590 ± 0.006, are harder than corundum and softer than diamond. Towards reagents they resemble the boride AlB₁₂, but are more resistant towards mineral acids.

It is difficult to believe that the formulae given by Hampe and Biltz represent definite chemical compounds. Probably each chemist obtained products of definite composition simply because the conditions of experiment were not sufficiently varied. Crystals of quite different composition have been prepared by Binet du Jassonneix, by heating aluminium with excess of boron in a graphite crucible in an electric arc furnace. They are yellow, transparent, and occur in six-sided plates; mixed with them are crystals of boron carbide, from which they cannot be completely separated. The crystals are attacked slowly by dilute hydrochloric acid, quickly by nitric acid. After allowing for mechanically admixed boron carbide, the average composition was Al, 64 per cent.; B, 15 per cent.; C, 21 per cent.; but different preparations varied considerably in composition.