Verneuil Growth of TiO2 (rutile) Crystals of Large Size and Low Dislocation Density at Low Gas Flow Rate

Large (30 mm in diameter) TiO2 (rutile) single crystals with low dislocation density were grown by the Verneuil method using a single crystal furnace designed and improved by the authors. The structure of the burner was optimized by numerical simulation analysis so that the crystal could be grown at a low gas flow rate. The investigation of the growth process parameters (i.e., growth rate, outer flow rate of O-2, inner flow rate of O-2, and the increment of H-2 flow rate) shows that the inner flow rate of O-2 and the increment of H-2 flow rate have the strongest influence on the crystal growth process. On this basis, other growth parameters (growth rate, outer flow rate of O-2) were also optimized. Compared with the conventional Verneuil method, the crystal can be grown at a considerably low gas flow rate (40-50% lower) with the method in this work, which reduces the melt turbulence impacted by gas flow, enabling a steady and clear solid-liquid interface and improving the crystal quality. The optimum growth conditions are for the growth rate of 6 mm/h, O-2 outer flow rate of 3.5 L/min, O-2 inner flow rate of 5.5 L/min, and increment of H-2 flow rate of 0.1 L/4 min. The etch pit density of the rutile crystals is 3.29 x 10(4) cm(-2), an order of magnitude lower than that of the crystals grown by the conventional Verneuil method. The optical properties of the crystal are comparable to those grown by the floating zone method. Especially, it is easier to obtain a larger crystal size with lower production costs. Our results provide a possible route for industrializing the Verneuil production of large, high-quality and low-cost rutile single crystals.