This paper summarizes the results of a series of studies dealing with crystal growth from the melt and in particular, the growth of the narrow bandgap semiconductor PbSnTe by the Bridgman technique. Theoretical calculations have shown that for low Prandtl number fluids such as semiconductors, convection has no effect on the temperature field in the melt but has a significant effect on the compositional field. They also show that convective flow will exist in a Bridgman growth system on Earth, even in the “thermally stable” configuration. Measurements of the specific heat have been made up to temperatures of 722° C. The thermal diffusivity has been measured up to 1010° C which includes values for the melt. The interdiffusion coefficients for PbTe and SnTe in the melt have been determined. A technique has been developed and tested which provides a vacuum tight quartz ampoule with electrical feedthroughs for interface demarcation studies during the Bridgman growth. Analysis of crystals grown has indicated that the PbSnTe system is solutally unstable and the distribution of SnTe in the crystal, after an anomalous first to freeze region, follows that predicted by Pfann for convection controlled growth. Studies of the amount of supercooling in PbSnTe indicates that this may explain an anomalous composition distribution in the first to freeze region of an unseeded growth.
A numerical modeling study of the effects of gravity on methane diffusion flames has been performed to complement an experimental study conducted at NASA's Lewis Research Center. This study has been undertaken to see if the experimental results could be matched in a simplified model, and to improve understanding of the mechanisms by which buoyancy affects the size and shape of a flame.
The combined effect of Grashof and Reynolds numbers on the flow and heat transfer in a Metal Organic Chemical Vapor Deposition (MOCVD) reactor is investigated both experimentally and numerically. Experimental data for pure hydrogen, helium, and nitrogen with induction heating are obtained at the Chemical Vapor Deposition Facility for Reactor Characterization at NASA Langley Research Center (LaRC). The test facility measures the velocity field inside the reactor using a three dimensional laser velocimeter. Temperatures of the fused silica walls are recorded using an infrared camera. Each gas is tested over a range of flow rates. These experimental runs are repeated using a three-dimensional computational fluid dynamics code which models the flow and heat transport throughout the reactor. The model accounts for the mechanisms of conjugate heat conduction, convection, and radiation. The analytical results are compared with the experimental data and used to assess the heat and mass transfer in the system as a function of the Richardson number, Ri = GrIRe2.
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