BUOYANCY-DRIVEN HEAT-TRANSFER DURING APPLICATION OF A THERMAL-GRADIENT FOR THE STUDY OF VAPOR-DEPOSITION AT LOW-PRESSURE USING AN IDEAL-GAS

A mathematical model has been developed to determine heat transfer dur ing vapor deposition of source materials under a variety of orientatio ns relative to gravitational accelerations. The model demonstrates tha t convection can occur at total pressures as low as 10(-2) mm Hg. Thro ugh numerical computation, using physical material parameters of air, a series of time steps demonstrates the development of flow and temper ature profiles during the course of vapor deposition. These computatio ns show that in unit gravity vapor deposition occurs by transport thro ugh a fairly complicated circulating flow pattern when applying heat t o the bottom of the vessel with parallel orientation with respect to t he gravity vector. The model material parameters for air predict the e ffect of kinematic viscosity to be of the same order as thermal diffus ivity, which is the case for Prandtl number similar to 1 fluids. Quali tative agreement between experiment and the model indicates that 6-(2- methyl-4-nitroanilino)-2,4-hexadiyn-1-ol (DAMNA) at these pressures in deed approximates an ideal gas at the experiment temperatures, and may validate the use of air physical constants. It is apparent that compl icated nonuniform temperature distribution in the vapor could dramatic ally affect the homogeneity, orientation, and quality of deposited fil ms. The experimental test is a qualitative comparison of film thicknes s using ultraviolet-visible spectroscopy on films generated in appropr iately oriented vapor deposition cells. In the case where heating of t he reaction vessel occurs from the top, deposition of vapor does not n ormally occur by convection due to a stable stratified medium. When va por deposition occurs in vessels heated at the bottom, but oriented re lative to the gravity vector between these two extremes, horizontal th ermal gradients induce a complex flow pattern. In the plane parallel t o the tilt axis, the flow pattern is symmetrical and opposite in direc tion from that where the vessel is positioned vertically. The ground-b ased experiments are sufficient preliminary tests of theory and should be of significant interest regarding vapor deposited films in microgr avity.