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Numerical modeling of physical vapor transport under microgravity conditions: Effect of thermal creep and stressOne of the most promising applications of microgravity (micro-g) environments is the manufacture of exotic and high-quality crystals in closed cylindrical ampoules using physical vapor transport (PVT) processes. The quality enhancements are believed to be due to the absence of buoyant convection in the weightless environment - resulting in diffusion-limited transport of the vapor. In a typical experiment, solid-phase sample material is initially contained at one end of the ampoule. The sample is made to sublime into the vapor phase and deposit onto the opposite end by maintaining the source at an elevated temperature with respect to the deposit. Identification of the physical factors governing both the rates and uniformity of crystal growth, and the optimization of the micro-g technology, will require an accurate modeling of the vapor transport within the ampoule. Previous micro-g modeling efforts have approached the problem from a 'classical' convective/diffusion formulation, in which convection is driven by the action of buoyancy on thermal and solutal density differences. The general conclusion of these works have been that in low gravity environments the effect of buoyancy on vapor transport is negligible, and vapor transport occurs in a diffusion-limited mode. However, it has been recently recognized than in the non-isothermal (and often low total pressure) conditions encountered in ampoules, the commonly-assumed no-slip boundary condition to the differential equations governing fluid motion can be grossly unrepresentative of the actual situation. Specifically, the temperature gradients can give rise to thermal creep flows at the ampoule side walls. In addition, temperature gradients in the vapor itself can, through the action of thermal stress, lead to bulk fluid convection.
Document ID
19940019867
Acquisition Source
Legacy CDMS
Document Type
Conference Paper
Authors
Mackowski, Daniel W.
(Auburn Univ. AL, United States)
Knight, Roy W.
(Auburn Univ. AL, United States)
Date Acquired
September 6, 2013
Publication Date
December 1, 1993
Publication Information
Publication: NASA. Marshall Space Flight Center, Microgravity Studies of Organic and Polymeric Materials
Subject Category
Materials Processing
Accession Number
94N24340
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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