GB935390A - Improvements in or relating to the manufacture of corundum crystals - Google Patents

Abstract

Corundum crystals are made by forming a mixture of alumina and a molten flux, the proportion of flux being sufficient to dissolve all the alumina but being such that the resulting solution is supersaturated at a temperature above the melting point of the flux, heating the mixture at a temperature above the said temperature at which supersaturation occurs until a homogeneous solution is obtained, cooling the solution to room temperature, and separating, e.g. by washing or chemically dissolving the flux, the alumina crystals so formed. The flux is a material with a melting-point below that of alumina and in which alumina dissolves without chemical action taking place. Suitable flux materials are alkali metal halides, lead and boric oxides, lead chloride and, preferably lead fluoride. A minor proportion of an additive may be incorporated into the mixture, e.g. a compound yielding paramagnetic or colouring ions when the mixture of alumina and flux is heated. The starting mixture may be formed by adding solid alumina to molten flux or a solid mixture of alumina and flux may be heated to the melting-point of the flux. The proportion of flux is dependent upon the nature of the flux material and the maximum temperature to be attained, e.g. if lead fluoride is used as flux and the temperature is 1250 DEG C., the starting mixture contains at least 85 mol. per cent of lead fluoride. The mixture is placed in a non-reactive vessel, e.g. platinum, preferably sealed and evacuated, and is conveniently heated by means of an electric furnace. Heating is continued for several hours, e.g. at 1150 DEG to 1300 DEG C., using lead fluoride as flux. The melt is then cooled very slowly, e.g. 1 DEG to 2 DEG C. per hour until the flux solidifies and thereafter may be cooled more rapidly. Hexagonal plates of corundum are formed. In a modification, seed crystals of corundum, e.g. crystals cut from boules grown by flame fusion, are placed on top of the mixture of alumina and flux, oriented with their major surfaces parallel to the surface of the mixture. In an example, a mixture of gamma alumina powder, lead fluoride powder and ammonium dichromate powder is heated at 1280 DEG C. for 10 hours, cooled to 990 DEG C. at the rate of 15 DEG C. per hour, and then allowed to cool to room temperature. The lead fluoride was then removed by boiling the mass in hydrochloric acid, leaving hexagonal plates 3-4 cms. x 1 cm. of "ruby" corundum.