(460g) Existence, Stability, and Nonlinear Dynamics of Growth States in Detached Bridgman Crystal Growth

Detached Bridgman growth originated from the serendipitous observation of unusual behavior exhibited by early, space-based melt crystal growth experiments, where the melt dewetted from the wall, allowing the crystal to pull away and grow without ampoule contact. The detached growth mode eliminated deleterious interactions between the growing crystal and the ampoule wall, producing crystals of dramatically improved quality.  However, the promising early results of microgravity experiments have been difficult to advance in terrestrial growth systems.

As a basis for better understanding the physics underpinning this process, we develop and apply a thermocapillary model to study the existence, stability, and nonlinear dynamics of detached melt crystal growth in a vertical Bridgman system under zero gravity conditions.  The model incorporates time-dependent heat, mass, and momentum transport, and accounts for temperature-dependent surface tension effects at the meniscuses bounding the melt.  The positions of the meniscuses and phase-change boundary are computed to satisfy the conservation laws rigorously.  A rich bifurcation structure in gap width versus pressure difference is uncovered, demarcating conditions under which growth with a stable gap is feasible.  Thermal effects shift the bifurcation diagram to a slightly different pressure range, but do not alter its general structure.  Necking and freeze-off are shown to be two different manifestations of the same instability mechanism.  Supercooling of melt at the meniscus and low thermal gradients in the melt ahead of the crystal-melt-gas trijunction, either of which may be destabilizing, are both observed under some conditions.  The role of wetting and growth angles in dynamic stability is clarified.

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This work has been supported in part by the Department of Energy, National Nuclear Security Administration, under Award Numbers DE-FG52-06NA27498 and DE-FG52-08NA28768, the content of which does not necessarily reflect the position or policy of the United States Government, and no official endorsement should be inferred.