A. I. Danilenko

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.

Structure of polycrystals produced by sintering nanocrystalline powders of cubic and wurtzitic boron nitrides

The structure and some properties of polycrystals produced by sintering nanocrystalline powders of the dense modifications of shock—wave-synthesized BN have been studied. The sintering was conducted at a static pressure of 7.7 GPa and temperatures from 1100 to 1800° C. The highest density (3 g/cm3) and microhardness (up to 20 GPa) have been exhibited by polycrystals produced by sintering the powder containing wurtzitic and cubic modifications in amounts that are approximately equal. In the temperature range from 1100 to 1300° C the wurtzitic phase transformed into the cubic one. In this temperature range the average size of cBN grains changed from 20 to 50 nm. The structure of compacts is characterized by the presence of grain (grain-boundary) interlayers 2–5 nm in thickness.

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.

Effect of Shock Compression Conditions on the Production of Cubic Si3N4

The yield of cubic γ-Si3N4 phase depending on the shock compression pressure and temperature and on the type of starting modification (α or β) is studied. Pressure and temperature depend on the explosive power, shock-wave loading pattern, KCl content of the charge, and charge density. The yield of the γ-phase was determined by quantitative X-ray diffraction using calculated intensities of lines for each phase. The optimum conditions of shock compression to reach the maximum yield of the cubic phase were found (for specific explosives). It is concluded that the cubic phase forms from hexagonal modifications by the diffusion-controlled mechanism and that the α-phase is metastable.

Effect of Preliminary Treatment on the Structure and Phase Transformations of Graphite Under High-Temperature Shock Compression

The effect of preliminary shock-wave treatment of graphite on its subsequent transformations into dense modifications of carbon under high-temperature shock compression is studied. It is shown that this treatment leads to higher concentration of turbostratic stacking defects, increases the extent of lattice microdeformation along the c axis, and induces the formation of twins with three-dimensional configuration of C— bonds on twin boundaries. These defects significantly but differently influence the phase transformations. Turbostratic defects inhibit phase transformations whereas microdeformation and twins accelerate them.