Study on the melt/crystal interface in large-size yttrium aluminum garnet crystal growth

It is crucial to control the melt/crystal (m/c) interface and ensure that it is convex to achieve high-quality and large-size yttrium aluminum garnet (YAG) laser crystal growth using the Czochralski method. The enthalpy-porosity method is used to describe the m/c interface, and the reasons for the formation of the W-shaped m/c interface during large-size YAG laser crystal growth are studied. The results show that the W-shaped m/c interface is caused by the inflection point in the variation of the temperature gradient along the crystal radius. The interface formation can be avoided by reducing the thermal insulation lid aperture, crucible lid aperture, crystal rotation rate, and crystal diameter. An enclosed vortex is formed near the W-shaped m/c interface, which hinders impurity transportation, causes inclusion defects, and increases thermal stresses in the crystal. A convex m/c interface is beneficial for reducing thermal stress in crystals. Using an iridium crucible with a diameter of 400 mm, the W-shaped m/c interface observed during large-size YAG crystal growth by the Czochralski method closely matches the results of numerical simulations. We successfully grew YAG crystals with a diameter of 140 mm using the improved technique. These results provide guidance for large-size YAG laser crystal growth.