Abstract
The fast phenomena occurring in the Mach disc front and the rarefaction wave front behind the disc formed by a cylindrical shock technique have been investigated using AIN 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 gasification in the rarefaction wave front. The resulting high-temperature high-pressure gas contained a large quantity of plasma (molecular gas cloud), that had rapidly condensed under the steep pressure, temperature and density gradients. The gas density was calculated to be about 76–84% of the theoretical density of AIN, and it was presumed that the gasification was terminated in the order of a nanosecond, while the condensation of ultra-supersaturated gas proceeded over the time range nanoseconds 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 cloud. The liquid droplets increased their sizes by collision and coalescence. The agglomeration of spherical particles formed by solidification of the liquid droplets led to a great number of clusters of different sizes. Subsequently to the formation of cluster, shells with several surface structures were produced by deposition of the intercluster residual gas on the cluster surfaces. Finally, fine particles, having pyramidal, needle-like, filamental and dendritic shapes, grew via gas-solid processes. All particles obtained, except for part of shells having the cubic structure, were single crystals of AIN with the wurtzite crystal structure.
