EXPLOSIVE GASIFICATION OF ALUMINUM NITRIDE POWDER AND RAPID CONDENSATION OF ULTRASUPERSATURATED GAS IN THE RAREFACTION WAVE-FRONT

The fast phenomena occurring in the Mach disc front and the rarefactio n wave front behind the disc formed by a cylindrical shock technique h ave been investigated using AlN powders with low densities, 17-26% of the theoretical bulk value. Electron microscopy on the microstructures of the recovered samples proved that a dense medium, shock compressed in the Mach disc front, was produced, accompanied by a drastic gasifi cation in the rarefaction wave front. The resulting high-temperature h igh-pressure gas contained a large quantity of plasma (molecular gas c loud), that had rapidly condensed under the steep pressure, temperatur e and density gradients. The gas density was calculated to be about 76 -84% of the theoretical density of AlN, and it was presumed that the g asification was terminated in the order of a nanosecond, while the con densation of ultrasupersaturated gas proceeded over the time range nan oseconds to microseconds. The distribution of the component particles developed from the gas suggested the following condensation processes. At first, numerous liquid droplets developed in the molecular gas clo ud. The liquid droplets increased their sizes by collision and coalesc ence. The agglomeration of spherical particles formed by solidificatio n of the liquid droplets led to a great number of clusters of differen t sizes. Subsequently to the formation of cluster, shells with several surface structures were produced by deposition of the intercluster re sidual gas on the cluster surfaces. Finally, fine particles, having py ramidal, needle-like, filamental and dendritic shapes, grew via gas-so lid processes. All particles obtained, except for part of shells havin g the cubic structure, were single crystals of AlN with the wurtzite c rystal structure.