Elucidation of the role of gravity and gravity driven convection in the formation of silicon carbide/silicon nitride whiskers

Microgravity experiments on the role of gravity in the growth of silicon carbide and silicon nitride whiskers are to be conducted in a sounding rocket. A rocket qualified high temperature furnace has been designed and developed for these experiments. The vapor solid liquid mechanism of crystal growth is explained and the scientific issues to be addressed in the microgravity experiments are discussed. These are the role of droplet size in determining the whisker diameter, effects of flow field fluctuations caused by gravity-driven and Marangoni convection, and the role of multiple nuclei. The furnace consists of a graphite insulation module surrounding a hot zone 3 inches in diameter and 4 inches long, all enclosed in a vacuum tight shell. The furnace is designed to reach an operating temperature of over 1700 C in under a minute, to sustain this temperature for the 5-6 minute duration of the microgravity period, and to cool down to below reaction temperature before the microgravity period is over. The furnace is battery powered and is energized by off-board batteries to an intermediate temperature below the reaction temperature in order to minimize the power requirement from on-board batteries. Ground tests have demonstrated the satisfactory capabilities of the furnace system.