Modelling of TGS Growth in Space
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MODELLING OF TGS GROWTH IN SPACE L. C. LIU* AND W. R. WILCOX Department of Chemical Engineering, Clarkson College of Technology, Potsdam, New York 13676, USA R. KROES Space Science Laboratory, NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA R. LAL Department of Physics, University of Alabama A & M, Normal, Alabama
35762, USA
ABSTRACT To maintain a constant growth rate in the absence of convection it is necessary to program down the temperature. Trial and error numerical computations were performed to find the first approximation for the required isothermal dissolution period, linear ramp; and polynomial temperature variation period. The linear ramp rate was limited by the specified maximum temperature gradient. The isothermal dissolution turned out to be unnecessary. The linear ramp had to be stopped before the polynomial period was begun to avoid overshooting the specified maximum growth rate. After a few hours the temperature profile approached steady state behavior. INTRODUCTION As described elsewhere in this volume, we are planning to grow triglycine sulfate (TGS) crystals from aqueous solutions in Spacelab. In the absence of convection, the rate of growth of a crystal from an isothermal solution decreases rapidly, due to the depletion of solute from the solution adjacent to the crystal. A constant growth rate can be maintained only be slowly lowering the temperature of the solution. The purpose of the work described here was to determine a proper temperature programming cycle for TGS. The growth sequence will consist of insertion into an undersaturated (superheated) solution, permit dissolution to occur, linear ramp the temperature down to initiate growth, and then program to maintain a constant growth rate. The polyhedral crystal will be mounted on thermoelectric sting, so that both the sting temperature and the cell wall temperature can be varied. The geometry is very complex and so, for a first approximation, was simplified to the spherical symmetry shown in Figure 1. Even with this simplification, the simultaneous heat and mass transfer are too complex for analytical solution. Consequently a finite difference numerical technique was employed, subject to the following conditions: 1. Isothermal dissolution (soak) to yield amount of dissolution found necessary to remove spurious nuclei on earth, - 16.7 pm. , Present address:
Halcon R
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Maximum growth rate of 1 mm/day, which has been found to be the upper limit of high quality growth on earth. Maximum amount of growth. Maximum temperature gradient of 6eC/cm in crystal, to avoid formation of defects due to thermal stress. 0 Maximum supercooling of 10 C in solution, to avoid spurious nucleation. Maximum cooling rate of 1lC/hr on wall, to maintain temperature unifo
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