Growth of Triglycine Sulfate (TGS) Crystals by Solution Technique
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GROWTH OF TRIGLYCINE SULFATE (TGS)
399
CRYSTALS BY SOLUTION TECHNIQUE*
R. B. LAL, Alabama A & 4 University, Huntsville, Alabama, R. L. NASA/MSFC, and W. R. WILCOX, Clarkson College of Technology
KROES,
ABSTRACT The growth of crystals from solution is greatly influenced by buoyancy driven convection. In a low-g environment, convection is greatly suppressed and diffusion becomes the predominant mechanism for thermal and mass transport. An experiment to grow TGS crystals by solution technique during the orbital Spacelab III mission has been designed. Crystals are grown by a new and unique technique of extracting heat from the crystal through a sting. The cooling at the sting tip is responsible for the desired supersaturation near the growing crystal. Calculations indicate that the cooled sting technique for solution crystal growth is necessary in low-g to maintain a maximum growth rate of 1 mm/day. Results of groundbased work in support of the flight experiment are discussed.
INTRODUCTION As part of the NASA materials processing in space program, a study is being carried out of solution crystal growth in the low-g environment of an orbiting spacecraft [1]. This paper describes the approach taken for the development of a flight experiment. Results of the growth of TGS crystals by the cooled sting technique using oriented polyhedral and platelet seed crystals are discussed. Some aspects of the flight design are also discussed. The objectives of this experiment are: (a) to develop a technique for solution crystal growth in a low-g environment; (b) to characterize the growth environment provided by the orbiting spacecraft, and to determine its influence on the growth behavior; and (c) to determine how growth in a low-g environment influences the properties of the resulting TGS crystal. In order to achieve these objectives, a unique growth technique has been developed in which a thermally controlled sting is used to cool the seed crystal in such a way as to produce a constant growth rate. Optical holographic techniques are employed to measure crystal growth and to characterize the growth environment. The quality of the grown crystals will be determined and comparison made to earth grown crystals. TGS has been chosen for this experiment because of its technological importance as an infrared detector material [2, 3, 4]. In solution crystal growth there are two principal mechanisms which are responsible for the growth process, namely, solute mass transport and surface reaction kinetics. There is no reason to expect surface reaction kinetics to be altered by a low-g environment. However, the solute mass transport of TGS (NH CH2 COOH) 3 H2 SO4, through the bulk solution to the crystals liquid interface will be significantly altered by this environment. It is reasonable to
*Work supported by NASA Contract NAS8-32945.
400 anticipate that in a low-g environment buoyancy driven convection will be greatly suppressed and mass transport will take place by diffusion.
EXPERIMENTAL In a one-g environment, most solution growth t
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