Reducing melt inclusion by submerged heater or baffle for optical crystal growth

Optical and laser crystals grown by the Czochralski technique from a solute-rich melt usually suffer defects of melt inclusion or bubble core, which severely affects optical, thermal, and mechanical properties of the material. The main purpose of this paper is to study the inclusion mechanism and reduce such defects. Two types of mechanism possibly responsible for inclusion are presented. Both of them show that solute pileup in front of the interface is the prerequisite for inclusion. The method to reduce inclusion is to prevent solute from pileup by controlling melt flow. Czochralski-grown optical crystals are examined to recognize the effects of crystal rotation and natural convection on the melt flow pattern and solidification interface shape. It is established that increasing the rotation rate of crystal or reducing natural convection in the melt will cause the solid-liquid interface change from the convex to concave shape and high concentration of species may be pushed away from the solidification interface. Simulations are performed to establish the relationships among the nondimensional parameter Gr/Re(2), the change of growth interface shape, and the stagnant point location. A disk baffle or heater submerged into the melt is used to reduce natural convection. The effect of submerged baffle on enhancement of crystal rotation is demonstrated. Simulation results also demonstrate that melt flow near the solidification interface depends strongly on the baffle location. When the submerged heater is used, its temperature should be carefully designed. The value strongly depends on the ratio between crystal and crucible diameters, and a constant temperature may be not the best choice. An optimized temperature profile can be achieved by numerical simulations.