HEAT TRANSFER IN THE BRIDGMAN-STOCKBARGER SOLIDIFICATION TECHNIQUE

Heat transfer analyses of the Bridgman-Stockbarger solidification system are performed in order to provide insight of the factors influencing the solid/liquid interface shape and the temperature field inside the crystal growth ampoule. Here a quantitative investigation was made to study the effects of inserting a layer of insulation between the heater and the cooler on the sensitivity of the interface shape and the interface position to variation in operating parameters. It was found that the isotherms in the insulated region are relatively planar. The sensitivity of the interface shape to system perturbations decrease dramatically with increasing thickness of the insulation, while the temperature gradient at the solid/liquid interface is only diminished slightly for high thermal conductivity materials.;For low (< 0.2) Biot number systems, the heat transfer is predominantly along the axis of the growth sample, such that a one-dimensional (along the axis) thermal analysis is useful. The one-dimensional thermal model was used to study the transient behavior of interface movement after a sudden change of the ampoule moving rate (relative to the furnace). An explicit finite difference numerical scheme was developed to compute the solid/liquid interface position vs. time after the rate change. It was found that the transient behavior of the freezing rate strongly depended on the Biot number of the system, the dimensionless latent heat, and the thickness of the insulation in between the heater and the cooler. The numerical results were correlated by an equation, which can be used to describe the relation between the freezing rate and time. Based on this correlating equation for a sudden change of the ampoule moving rate, relations between the freezing rate and time were also derived for a gradual change of the ampoule moving rate and for a sinusoidally fluctuating ampoule moving rate.;The Peltier Interface Demarcation (PID) technique was demonstrated to be a potential technique to mark the solid/liquid interface during the solidification of MnBi/Bi eutectic. The solid/liquid interface movement after a sudden increase of the ampoule moving rate was studied experimentally using PID technique. Reasonably good agreement was obtained with the theoretical predictions mentioned above.