Akira Sawaoka

Structural changes of wurtzite-type and zincblende-type boron nitrides by shock treatments

Wurtzite-type boron nitride (w-BN) and zincblende-type boron nitride (z-BN) powders were shock-treated in the pressure range of 60 to 200 GPa to clarify their polymorphic transformations. The recovered BN powders revealed the effects of the shock wave and residual temperature on phase transition of BN. W-BN was partly transformed to z-BN by shock compression at a pressure of about 100 GPa. At pressures greater than 100 GPa, portions of the w-BN and z-BN powders changed into the BN having a turbostratic structure. Subsequently, this form was crystallized to graphite-like BN (g-BN) and a new form of BN due to high residual temperatures. This new BN modification, probably stabilized by the high surface energy associated with its fine crystallite size of less than 50 nm, was identified as fcc structure with a lattice parameter ofa0 = 0.8405 nm. The transformation of z-BN to w-BN was not detected in this post-shock study, as was observed in static high pressure studies.

Effects of multicompressions on Al2O3 powder

Abstract Effects of multiple shock compressions on Al 2 O 3 powder were studied. Crystallization of amorphous-like structure formed by the first shock compression was observed in the recovered material. Polygonized microstructure was produced by a second shock compression. X-ray diffraction analysis showed that the polygons with 1 μm consisted of 50 nm crystallites. A formation mechanism of such polygonized structure is proposed.

Dynamic consolidation of non-oxide ceramic powders

Abstract The possibilities of dynamic consolidation technology of ceramic powders as an industrial application are discussed. Well sintered compacts of non-oxide ceramics without any sintering aids can be produced by shock wave compaction techniques, however the size of crack free compacts is limited. Improved dynamic consolidation techniques might overcome some difficulties such as crack generation. However, cost reduction of this process is not easy and the lowest limit of the cost will be at the level of conventional hot pressing. As a typical non-oxide ceramic, study of the dynamic consolidation of silicon carbide is summarized.

The Role of Shock Wave Compression in Materials Science

Shock wave compression technology has contributed much to the progress of materials science. The present author has tried to discuss the role of shock wave techniques with respect to very high pressure materials science studies. Shock wave techniques also unique very high temperature methods. The important role of shock wave technology seems to be the attainment of pressures above those which can be obtained by static techniques. A typical example is equation of state studies done up to pressures of a few megabars. Recently, a static technique utilizing a small pair of diamond anvils has been established which is capable of attaining a few megabar pressures. This technique is performing the majority of equation of state studies. Shock wave compression is accompanied by high shock temperature. Recently, new dynamic compression techniques, which are isentropic-like, are being developed and applied to equation of state studies. On the other hand, several shock studies suggested that due to the very high shock temperatures, accompanying shock compression, dynamic compression techniques could be utilized as a unique high temperature method. High inhomogeneous shock temperatures can be produced in porous and/or compressible materials during strong shock compression. These temperatures induce local melting and ionization. It is expected that a high dense plasma-like state is formed in extremely strong shock wave compression. After passage of the shock wave, the materials cool rapidly during pressure release to a residual temperature and pressure. Occasionally the materials recovered after having been converted to a dense liquid or gas state showed very unique microstructures. The present author believes that more precise, compact and easily operative shock wave compression apparatus should be developed and made available to general material scientists and engineers.

Dynamic compaction of SiC powder

Dynamic compaction techniques have considerable potential in the consolidation and fabrication of hard-to-densify powder materials such as non-oxide ceramic materials. Consolidation of the powder materials by this technique involves the densification of the powder at the shock wave front followed by interparticle bonding which occurs during the shock-compression process. In the dynamic compaction of a porous material, most of the energy of shock compression has been assumed to be deposited at the surfaces of the particles (1, 2) due to the heterogeneous deformation of material during shock loading (3). This preferential energy deposition, in some cases, allows the particle surfaces to melt and results in interparticle bonding.