GB697804A - Improvements in and relating to the manufacture of synthetic corundum containing titanium - Google Patents

Abstract

697,804. Gems. UNION CARBIDE & CARBON CORPORATION. Dec. 13, 1950 [Dec. 21, 1949], No. 30424/50. Class 43. A synthetic corundum monocrystal containing a titanium compound comprises a series of superposed convex layers wherein the distribution of the compound in alternate layers differs from that in the intermediate layers. Alumina powder mixed with 0.1 to 0.3 per cent titania, with or without a small amount of colouring material, e.g. chromium oxide or ferric oxide, is fused in an oxy-hydrogen flame and then progressively crystallized on a support in the usual manner, but the thermal conditions are made to fluctuate, e.g. by alternately increasing and decreasing at frequent intervals the rate of oxygen flow to the name, with the result that the layers deposited during periods of low oxygen flow have titania across their full width, whereas those deposited during high oxygen flow have titania in annular peripheral zones only. The boule so formed is either annealed or split vertically to relieve internal stresses, and the whole boule or the two parts are treated so that they have the appearance of being asteriated homogeneously. This is achieved by heating them for more than two hours at 1100‹ to 1500‹ C., whereupon a titanium compound precipitates across the full width of the layers 49 deposited during periods of low oxygen flow, but adjacent the periphery only of the intermediate layers 51 due to the above-described titania distribution. By cutting a cabochon 53, Fig. 4, from a whole asteriated boule so that the c-axis is parallel to, and the crown 55 is symmetric with respect to, the geometric axis, the curvature of the crown being approximately the same as that of the layers, all six rays 57, Fig. 6, of the star will extend equally down the sides of the stone to the base. In addition, although the colour in blue sapphire boules is confined to the parts containing the precipitate, the colour distribution in sapphire cabochons cut as above will be ostensibly continuous; a ruby boule is red throughout. The usual seed crystal on which the fused material accumulates is so mounted that its caxis makes the same angle with the longitudinal growth axis as is desired in the boule, i.e. an angle of zero in the case described above; satisfactory cabochons may, however, be cut from whole or half boules having a c-axis at right angles to the longitudinal growth axis as shown in Fig. 5. Alternative methods of achieving fluctuating thermal conditions round the growing boule comprise fluctuating the hydrogen flow rate, or both the oxygen and hydrogen flow rates, or the powder feed rate, or alternately raising and lowering the crystal within the furnace.