Abstract
Ultrafine powders of AlN and (AlN+Al) with particle sizes of less than 0.5μm were produced by arc melting of Al in atmospheres of N2+Ar, N2+H2, N2+NH3 at 0.1MPa pressure. The size, surface area, chemical composition, and crystal structure of obtained powders were determined by electron microscope, BET method, chemical analysis and X-ray diffraction analysis respectively. The following results were obtained.
(1) AlN ratio in (AlN+Al) powders obtained in (N2+Ar) atmospheres decreases from 0.3 to zero with decreasing nitrogen partial pressure.
(2) AlN ratio in (AlN+Al) powders obtained in (N2+NH3) atmospheres increases from 0.3 to 0.95 with increasing NH3 partial pressure.
(3) Projections were formed on molten Al surface by evolved nitrogen gas from the melt and by reaction between nitrogen and molten Al.
The reaction between nitrogen plasma and molten Al plays an important role in the formation of ultrafine particles. The driving force for the formation of ultrafine particles can be explained as follows; In arc melting of Al in a nitrogen atmosphere, molten Al simultaneously contacts with two gas phases, one is atomic nitrogen in the arc gas phase and another is the molecular nitrogen in the non-arc gas phase. The extent of dissolution of atomic nitrogen in the molten Al is much larger than that of molecular one. Then, the dissolved nitrogen through the arc gives higher nitrogen content for the non-arc gas phase. That is, super-saturated nitrogen will evolve in non-arc gas phase carrying with Al vapor. A kind of enhanced evaporation of Al is induced. This Al vapor reacts with surrounding gas species near the arc such as N2, N, NH3, NH2, NH in high temperature ranges (above 1600K) to form AlN or (AlN+Al) vapor which condense to ultrafine particles.
