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  • 1
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    Flight Experiment to Study Double-Diffusive Instabilities in Silver-Dopped Lead Bromide Crystals (2001)
    In:  CASI
    Details
    Publication Date: 2017-09-27
    Description: The main objective of the present program is to understand thermosolutal convection during crystal growth of PbBr2-AgBr alloys. This involves identification of the growth conditions for microgravity experiments delineating the microsegregation, observation of convecto-diffusive instabilities and comparison with theoretical models. The overall objectives can be summarized as follows: 1. Observe and study the double diffusive and morphological instabilities in controlled conditions and to compare with theoretically predicted convective and morphological instability curves. 2. Study the three-dimensional morphological instabilities and resulting cellular growth that occur near the onset of morphological instability in the bulk samples under purely diffusive conditions. 3. Understand the micro-and macro-segregation of silver dopant in lead bromide crystals in microgravity. 4. Provide basic data on convective behavior in alloy crystals grown by the commercially important Bridgman crystal growth process.
    Keywords: Solid-State Physics
    Type: Microgravity Materials Science Conference 2000; Volume 3; 548-550
    Format: text
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  • 2
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    Crystal Growth Control (1997)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: We present an innovative design of a vertical transparent multizone furnace which can operate in the temperature range of 25 C to 750 C and deliver thermal gradients of 2 C/cm to 45 C/cm for the commercial applications to crystal growth. The operation of the eight zone furnace is based on a self-tuning temperature control system with a DC power supply for optimal thermal stability. We show that the desired thermal profile over the entire length of the furnace consists of a functional combination of the fundamental thermal profiles for each individual zone obtained by setting the set-point temperature for that zone. The self-tuning system accounts for the zone to zone thermal interactions. The control system operates such that the thermal profile is maintained under thermal load, thus boundary conditions on crystal growth ampoules can be predetermined prior to crystal growth. Temperature profiles for the growth of crystals via directional solidification, vapor transport techniques, and multiple gradient applications are shown to be easily implemented. The unique feature of its transparency and ease of programming thermal profiles make the furnace useful for scientific and commercial applications for the determination of process parameters to optimize crystal growth conditions.
    Keywords: Solid-State Physics
    Type: NASA/CR-97-206655 , NAS 1.26:206655
    Format: application/pdf
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  • 3
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    Convection in the Physical Vapor Transport Process (1994)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: We consider the effect of an inert gas on the diffusive-convective physical vapor transport process. We investigate the case when the temperature gradient is stabilizing and the concentration gradient is destabilizing for a wide parametric range. When an inert gas is present, the thermal and solutal convection oppose each other. The solutal field is destabilizing while the thermal field and the advective-diffusive flux stabilize the flow field. When the pressure of the inert component is increased, the stabilizing effect of the advective-diffusive flux is decreased. The intensity of convection as well as the oscillatory transient time increases. Below, the critical Rayleigh number, the nonlinear dynamics of the flow field show an oscillatory approach to steady state. For parametric values in the neighborhood of the critical Rayleigh number, the flow field undergoes a chaotic transient which settles to a periodic state. The asymptotic state of the flow field shows that growth and amalgamation of cells yields an overturning motion which results in an asymmetric cellular structure. The low gravity environment yields the stabilizing advective-diffusive flow which results in uniform temperature and concentration gradients near the crystal interface.
    Keywords: Fluid Mechanics and Heat Transfer
    Type: NASA-TM-111848 , NAS 1.15:111848 , Chemical Vapor Deposition (ISSN 1056-7860); 2; 282-310
    Format: text
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  • 4
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    Convection in the Physical Vapor Transport Process-I: Thermal (1994)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The effects of convection on diffusive-convective physical vapor transport process are examined computationally. We analyze conditions ranging from typical laboratory conditions to conditions achievable only in a low gravity environment. This corresponds to thermal Rayleigh numbers Ra, ranging from 1.80 x 10 to 1.92 x 10(exp 6). Our results indicate that the effect of the sublimation and condensation fluxes at the boundaries is to increase the threshold of instability. For typical ground based conditions, time dependent oscillatory convection can occur. This results in unsteady transport, and non- uniform temperature and concentration gradients at the crystal interface. Spectral analysis of the flow field shows parametric regions exhibiting both an oscillatory approach to steady state and a chaotic transient to a periodic state. Low gravity conditions stabilize the flow field. Convective effects are effectively reduced, thus resulting in uniform temperature and concentration gradients at the interface, a desirable condition for crystal growth.
    Keywords: Fluid Mechanics and Heat Transfer
    Type: NASA-TM-111849 , NAS 1.15:111849 , Journal of Chemical Vapor Deposition (ISSN 1056-7860); 2; 188-218
    Format: text
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  • 5
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    Convection in the Physical Vapor Transport Process (1994)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The effects of convection on diffusive-convective physical vapor transport process are examined computationally. We analyze conditions ranging from typical laboratory conditions to conditions achievable only in a low gravity environment. This corresponds to thermal Rayleigh numbers Ra(sub tau) ranging from 1.80 x 10 to 1.92 x 10(exp 6). Our results indicate that the effect of the sublimation and condensation fluxes at the boundaries is to increase the threshold of instability. For typical ground based conditions, time dependent oscillatory convection can occur. This results in unsteady transport, and non-uniform temperature and concentration gradients at the crystal interface. Spectral analysis of the flow field shows parametric regions exhibiting both an oscillatory approach to steady state and a chaotic transient to a periodic state. Low gravity conditions stabilize the flow field. Convective effects are effectively reduced, thus resulting in uniform temperature and concentration gradients at the interface, a desirable condition for crystal growth.
    Keywords: Fluid Mechanics and Heat Transfer
    Type: NASA-TM-112143 , NAS 1.15:112143 , Journal of Chemical Vapor Deposition (ISSN 1056-7860); 2
    Format: text
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