Analysis of Thermal Shock during Rapid Crystal Extraction from Melts

A time-dependent stress field in LiNbO3 and SrLaAlO4 single crystals immediately after their extraction from melts has been found analytically. Numerical calculations show that during rapid extraction from melts the crystals are subjected to thermal shock which can be high enough to produce considerable dislocation density if there exists a plasticity zone or to induce cracks. During LiNbO3 crystal growth by the Czochralski method in a low temperature gradient high thermal stresses occur near the interface at the center and at the crystal periphery. After rapid extraction from the melt the thermal shock results in an increase in thermal stress over an order of magnitude. This can lead to the generation of high dislocation density at the bottom part of LiNbO3 crystal causing residual stress distribution after its cooling to room temperature. Times for the relaxation of stresses caused by the thermal shock are about 30 minutes and 30 seconds for 55 mm diameter LiNbO3 crystal and for 20 mm diameter SrLaAlO4 crystal, respectively. The magnitude and relaxation time of the stress caused by the thermal shock depend mainly on the crystal diameter, its thermal conductivity and the axial temperature gradient in the growth system. From analytical investigations a suitable method of crystal extraction from the melt may be obtained.