V. V. Yarosh

The influence of the shock compression conditions on the graphite transformations into lonsdaleite and diamond

Shock compression-induced phase transformations of graphite into carbon dense modifications in cylindrical recovery containers (pshc = 20–36 GPa, Tshc = 1800–3500 K) have been studied. The dependences of the lonsdaleite and diamond yields on the compression conditions have been established. The results obtained have been analyzed taking into account the formation mechanisms of the dense phases and special features of their shock-wave synthesis.

Phase transformations of carbon-black in high-temperature shock compression

The carbon-black transformations into diamond and amorphous carbon phase having an intermediate density of 2.9 g/cm3 in high-temperature shock compression at 20–32 GPa and 2500–3500 K have been studied. The conditions of compression that ensure the maximum yield of these phases have been defined. The transformation regularities have been analyzed under the assumption that the amorphous phase is an intermediate structure on the way to the transformation of turbostratic carbon into diamond.

Physical principles of shock-wave synthesis of superhard phases and their structure

This paper reviews work carried out at the Institute for Problems of Materials Science, Ukraine National Academy of Sciences, in developing physical bases for shock-wave synthesis of superhard phases of carbon (diamond, lonsdaleite) and boron nitride (wurtzite and sphalerite modifications). The effect of phase transformation mechanisms on structure features of the phases that are obtained under shock compression conditions is considered.

Nature of the real structure of graphite-like BN and its transformation into a wurtsite modification under impact compression

The dependence of the interlayer distance d on the degree of unidimensional disorder γ in graphite-like BN was determined. It was established that d=0.33273±0.00005 nm at 20°C in the perfectly ordered structure (γ=0); the value of d increases uniformly with increase of γ, and at γ=1 it equals 0.343±0.001 nm. It was also established that the yield of wurtsite phase produced by impact compression of the graphite-like modification sharply decreases with increase of γ, and for γ>0.2 it becomes equal to zero. This result is explained in terms of the nature of martensitic transformations in layered structures.

Effect of the Structural State of Graphitic Materials on Their Phase Transformations under Shock Compression

The effect of structural disordering of graphitic materials on their phase transformations into dense modifications of carbon under shock compression conditions (Pshock = 30 GPa, Tshock = 3000 K) is studied. It is shown that a lower degree of three-dimensional ordering of initial structure (P3) first decreases and then increases the overall yield of dense phases, reaching the maximum at P3 = 0. The content of lonsdaleite simultaneously decreases and that of the dense amorphous phase (Cam) increases. The results obtained are attributed to gradual change of the predominant martensite transformation mechanism to predominant diffusion-controlled mechanism, and also to the fact that the metastable phases (lonsdaleite and Cam) that form at the initial transformation stage partly transform into a stable diamond phase.

Shock-wave synthesis of diamond nanofibers and their structure

Diamond nanofibers produced by high-temperature shock compression of graphite nanofibers in the presence of KCl at pressures of 25–35 GPa and temperatures of 3000–3500 K have been considered. The synthesized fibers have been shown to consist of randomly oriented nanograins of diamond with the amorphous phase impurity, whose content decreases as the impact compression pressure increases.

Effect of Shock Compaction on the Synthesis of Silicon Carbide

The process of SiC synthesis during shock compaction of silicon and carbon black powdered mixtures is investigated. Shock wave treatment of mixtures is carried out in cylindrical container and the shock wave is generated by projecting the outer shell. The variation in the fraction of Si reacted with C and SiC with the composition and density of the charge is investigated. Thermal effects of the shock compaction and exothermic reaction of silicon with carbon and the temperature of shock wave synthesis are determined taking into account the dependence of the melting parameters of Si and the formation enthalpy of SiC on pressure. It is established that, during shock compaction, the melting of silicon is accompanied by coalescence of Si particles that inhibits the interaction of Si with C and reduces the SiC yield at high concentrations of Si in the mixture. The optimal composition of the mixture providing the highest SiC yield is found.

Phase Transformations in High-Temperature Shock Compression of Carbon Black in Various Recovery Capsules

Cylindrical recovery capsules without a central rod are used for the first time to study the phase transformations in carbon black under shock compression. Substantial differences in the regularities of transformations under shock compression in such capsules and in annular capsules (with a central rod) are revealed (the latter we used earlier to study the phase transformations in carbon materials).

Structurization of Materials in the Si–C System Under Shock Compression

Experiments on shock compression of Si + C powder mixtures were performed in annular recovery capsules at pressures of 20 and 30 GPa. The phase composition and structure of the compressed products were examined by X-ray diffraction and transmission electron microscopy. The results demonstrate the importance of silicon melting in the structurization of SiC + Si mixtures that occurs at high shock pressures.

Phase transformations of disordered structures of graphite-like Boron Nitride under high-temperature shock compression

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of high-pressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.