corundum, synthetic corundum.7, gems

Single crystals of corundum attaining 5 cm across were synthesized from cryolite (Na₃AlF₆) flux by the seed rotation method. The effect of growth conditions such as growth temperature, temperature difference, baffle aperture, and rotation rate upon crystal perfection is discussed to clarify the optimum conditions required to obtain crystals of high quality, containing the minimum amount of flux inclusions. The external shape of the corundum crystals did not show a noticeable change, except that the edges became more rounded with an increase in rotation rate. Experiments to simulate convective flow in solution were carried out on two systems—water and water–saccharose—under conditions simulating those of flux growth. When a baffle plate with 20% aperture was used, the flow velocity reached a maximum.