Spherical sapphire single-crystal synthesis by aerodynamic levitation with high growth rate

This article describes a technique for forming spherical ${\mathrm{Al}}_2{\mathrm{O}}_3$ single crystals by an aerodynamic levitator by applying the unidirectional solidification method. A levitation method intrinsically produces a spherical material. In addition, both the focused laser beam heating and the gas flows to the sample trapped in the levitator generate a steep temperature gradient in the molten sample, enabling the sample to grow with high growth rate. Spherical sapphire single-crystal production consists of two processes. First step is to solidify the molten sample unidirectionally from polycrystalline parts at the bottom of the sample. This sample was almost a single crystal with polycrystalline parts remaining just around the bottom. The crystal is then revolved before the second step. In the second step the revolved crystal was remelted from the polycrystalline part and solidified from the seed, resulting in complete single-crystal production. The x-ray Laue and crossed nicol experiment results for the ${\mathrm{Al}}_2{\mathrm{O}}_3$ spherical crystal with 2 mm diameter demonstrated that twin-free bulk single crystal had been produced. There were no macroscopic voids or cracks in the crystal at 60 mm/h growth rates in both steps. In addition, a hexagonal shape of the crystal seems to be the habit-plane of a natural corundum. This indicates the growth direction was along the c-axis of a sapphire.