Growth of lithium triborate crystals
Creators: name: Parfeniuk, Christopher Luke
Publisher: name: University of British Columbia
Year: 2009

description: Lithium Triborate (LiB₃O₅)is a nonlinear optical crystal used to produce short wave length radiation from a longer wavelength source. Large lithium triborate crystals of good quality are difficult to grow. The present investigation was undertaken to examine the parameters influencing the growth process, and the growth process itself. for crystals pulled from an LiB₃O₅ melt containing the flux MoO₃. The pseudo phase diagram of the LiB₃O₅ - MoO₃ system was established and the eutectic concentration shown to be 61.5 weight percent MoO₃. The viscosity of the LiB₃O₅ system was measured as a function of temperature and MoO₃ concentration. It was shown that the viscosity decreases with increasing MoO₃ content. As the crystal grows MoO₃ is rejected by the crystal at the interface. A major factor in growing larger crystals is the movement of the rejected flux away from the interface, which depends on the fluid flow in the melt. The fluid flow in turn is dependent on buoyancy forces due to temperature gradients, as well as crystal and crucible rotations. Calculations were carried out using a mathematical model for heat and fluid flow in the melt to establish the temperature distribution, fluid flow velocity and flow direction in the melt as a function of crystal and crucible rotation. Temperature measurements were then made in the melt in a crystal grower with a simulated crystal over a range of crucible rotation rotation rates, and the results compared to the mathematical model predictions. The boundary conditions used in the model were determined from temperature measurements in the melt. Comparing the calculated radial and axial temperature gradients iii the melt with the measured values showed good agreement between the calculated and measured temperatures. The flow patterns in the melt predicted by the model were also compared to the observed flow in a physical, model using glycerine as the melt and ink as a tracer, for the same size crucible and crystal used in crystal growth. The observed flow pattern was consistent with the model predictions. The results of both the mathematical and physical models clearly showed that most of the mixing in the liquid is associated with the crucible rotation and very little from buoyancy forces. From these results it was concluded that maximum crucible rotation should be used during growth to move the concentrated MoO₃ away from the advancing interface as rapidly as possible. Maximum MoO₃ concentrations should also be used, consistent with other constraints, since the viscosity of the liquid decreases with increasing MoO₃ concentration. It was shown that increasing the size of the crucible increased the flow velocity in the melt. As a result larger crucibles were used in the crystal growth experiments. The length of good quality crystal which can be grown, is limited by the formation of inclusions in the crystal at the interface. The inclusions are shown to he primarily MoO₃ and are considered to form when the concentration of the MoO₃ reaches the eutectic concentration of 61.5 wt% at the interface. Calculations of the diffusion of MoO₃ through the boundary layer away from the advancing interface, show that growth must he slow with strong liquid mixing below the interface, to produce a crystal 1 cm in length. A series of crystals of LBO were grown in a commercial crystal grower selecting the MoO₃ concentration, crystal and crucible rotations, and the pulling rate from the optimum values of the growth parameters given by the model predictions. Using these growth parameters, larger and better quality crystals were produced. Facets were observed on the crystal surfaces for the [001] growth direction which resulted in stagnant areas on the interface and the earlier appearance of MoO₃ inclusions. It was also observed that the crystal cracked readily under thermal stresses during cooling. To prevent cracking. crystals have been cooled slowly after growth in a uniform thermal gradient.