| 摘要 |
1,241,206. Boron nitride. A. P. GREEN REFRACTORIES CO. 22 Aug., 1968 [5 Sept., 1967], No. 40295/68. Heading C1A. [Also in Divisions B3 and C5] A boron nitride consists of particles composed of primary crystallites having stacked graphite type lamellµ with the layers arranged at random with respect to each other, the average crystallite thickness on the " C " direction perpendicular to the layers being 14 to 25 A and the " C " spacing (for the 002 planes) being 3.36 to 3À64 A. The crystallites may be 45 to 100 A broad. The particles may be equiaxed and less than 1000 A, and may be partly present as agglomerates. The boron nitride which may contain boron oxide as impurity is greater than 50% pure, preferably above 90% pure and has a bulk density of 0.015 to 0.031 g/ml. A product of substantially 100% purity and composed of platelets several microns broad and 1000 A thick may be formed by nitriding the boron nitride with ammonia. The boron nitride is prepared by introducing an inert gas into a bed 12 of a porous medium, introducing boric oxide, orthoboric acid, metaboric pyroboric acid, condensed boric acid or mixtures thereof into the bed which is maintained at 1500‹ C. to 2400‹ C. to produce boric oxide vapour and reacting the boric oxide vapour in an adjoining reaction zone 10 at 600‹ C. to 2400‹ C. with anhydrous ammonia. The inert gas may be nitrogen, helium or argon, and the porous medium of the bed may be boron nitride, carbon or graphite, the last two of which become coated with boron nitride under the prevailing reaction conditions. The porous bed may be heated by induction, plasma or electrical resistance heating. Alternatively the bed may be dispensed with and the boron oxide vaporized directly by a plasma jet (Fig. 4, not shown). Vacuum may be applied to the reactor outlet to speed up the gas flow, and the product may be nitrided with further anhydrous ammonia in a further reactor to convert boron oxide impurity into boron nitride. |
