Thermal plasma crystal growth

A process has been developed for growing crystals of AlN, TiN, and Si₃N₄ by reaction from the gaseous phase. In this process aluminum nitride has been prepared by evaporation of the aluminum anode of a free-burning nitrogen arc (P = 1 atm, I = 100 A). Forty percent of the aluminum is deposited in the form of highly oriented single crystals of AlN with very low oxygen content (0.34%). The rest is synthesized as an ultrafine powder with an average particle diameter of 300 angstrom. AlN crystals nucleate initially on both electrodes in the vicinity of the plasma attachments. The crystals continue to grow parallel to the arc axis, in the shape of a tube, until the entire plasma area is covered. Crystal growth seems to take place only in a narrow temperature range (1400-2000 K) in the vicinity of the plasma. The powder collected from the chamber walls is synthesized from 60% of the evaporated aluminum. This percentage can be increased up to 100% by setting higher arc currents and flow rates. The purity of the AlN powder is less than 80%. By introducing ammonia in the reactor, a purity greater than 95% has been obtained.