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Growth of lithium triborate crystals Parfeniuk, Christopher Luke
Abstract
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.
Item Metadata
Title |
Growth of lithium triborate crystals
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
1994
|
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.
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Extent |
8882497 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2009-04-10
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0078456
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
1994-05
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.