The interaction of silicon with carbon in various structural modifications under shock compression in cylindrical recovery fixtures was studied. X-ray diffraction was employed to determine the contents of phases in the shock-compressed samples. The pressure and temperature of the process were calculated. The interaction of silicon with graphite and carbon black was found to depend on the composition of the starting mixture. The interaction is most intensive at 50% Si because it depends on the contact area between the reagents, which decreases when molten silicon droplets coalesce. The interaction between silicon and diamond decreases with higher silicon content since the contact area of the components depends on their composition. The shock compression experiments for diamond–silicon mixtures show that diamond powders can be subjected to liquid-phase sintering.

The yield of cubic γ-Si 3 N 4 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.

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).

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.

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.

A comparative study of phase transformations in carbon-base materials at shock compression has been carried out according to two different schemes: in cylindrical ampoules with a central rod at a direct contact of explosives with the ampoule and in ampoules without the central rod with throwable outer shells. It has been found that in the shock compression on the second scheme a higher absolute output of diamond is achieved, but as opposed to the first scheme, the shock compression products do not virtually contain lonsdaleite or dense amorphous phase. The analysis shows that in the second case the states with lower pressures but higher temperatures, which lead to a transformation of metastable phases into stable diamond, are realized.

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.

The effect of structural disordering of graphitic materials on their phase transformations into dense modifications of carbon under shock compression conditions (P (shock) = 30 GPa, T (shock) = 3000 K) is studied. It is shown that a lower degree of three-dimensional ordering of initial structure (P-3) first decreases and then increases the overall yield of dense phases, reaching the maximum at P-3 = 0. The content of lonsdaleite simultaneously decreases and that of the dense amorphous phase (C (am)) 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 C (am)) that form at the initial transformation stage partly transform into a stable diamond phase.

Shock compression-induced phase transformations of graphite into carbon dense modifications in cylindrical recovery containers ( p shc = 20–36 GPa, T shc = 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.

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.

The carbon-black transformations into diamond and amorphous carbon phase having an intermediate density of 2.9 g/cm 3 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.

This article reviews works of the Institute for Problems of Materials Science evoted to the application of explosion energy for the synthesis of superhard phases (SHPs) based on phase transformations of layered structures of carbon and boron nitride at high pressures. Major attention is given to the development and application of the high-temperature shock compression method, which made it possible to synthesize a new structural form of carbon and to attain, for the first time, a 70% yield of cubic BN under shock compression. The diagram of BN phase transformations by different mechanisms is discussed, which demonstrates the dependence of the phase composition of shock compression products on the ordering degree of the starting graphite-like structure.

The formation of diamond-like modifications of BN under high-temperature shock compression (P=33 GPa and T up to 3500 K) has been studied. The powders of graphite-like BN with different degree of three-dimensional ordering of structure (D3) were used as starting materials. To increase shock temperature and preserve dense phase formed, boron nitride powder mixed with alkali halide salt was compressed in cylindrical ampoule. The recovered samples were studied by X-ray diffraction and electron microscopy. It was revealed that the phase composition of shock compression products depends on the degree of three-dimensional ordering of initial graphite-like structure. Turbostratic BN with disordered structure (D3=0) transformed mainly into cubic cBN by diffusion mechanism, whereas highly ordered graphite-like BN (D3=0.95) transformed only into wurtzite wBN by martensitic mechanism. Both cubic and wurtzite dense modifications are formed from partly ordered initial structures (for which D3 was 0.45÷0.7). The both diamond-like phases have nanocrystalline structure.

We have studied the structure of the products of high-temperature shock compression of wood charcoal by x-ray diffractometry and transmission electron microscopy. We have shown that as a result of phase transformations at high pressures (30 GPa) and temperatures (above 2500 K), besides nanocrystalline diamond we see formation of an amorphous carbon phase having a density intermediate between the densities of the original charcoal and diamond. Based on comparison of the observed diffraction patterns with the patterns calculated for different models for amorphous carbon, and also considering the intermediate value of the density for the amorphous phase, we have concluded that this phase has the short-range order of the hypothetical H-6 structure, characterized by a three-dimensional framework of sp 2 bonds. The amorphism of the intermediate phase is due to both the crystallographic nature of the H-6 structure and the disorder of the original structures, the regular rearrangement of which to form crystalline structures is not possible.

We used shock compression of a mixture of boron nitride powder with an alkali halide salt as an additive to realize the phase transformation of turbostratic BN to the cubic modification. We have experimentally studied the effect of the type and amount of the additive and also the initial density of the mixture on the yield of the cubic phase of BN. We have roughly calculated the pressure and temperature arising upon shock compression of powder mixtures in an annular cylindrical storage ampul. We have shown that melting of the additive has a substantial effect on development of the phase transformation in boron nitride under shock loading conditions.

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.

Features of crystal-orientated transformations of the rhombohedral modification of boron nitride (rBN) into dense modifications of the wurtzite (wBN) and sphalerite (spBN) types with shock compression in the range from 15 to 50 GPa are studied. Three compression schemes are used: in annular ampules (∼15 GPa); in the ampules of K. M. Rabinin (25–35 GPa); in a plane shock wave (∼50 GPa). With an increase in pressure from 15 to 50 GPa and a corresponding change in the loading scheme the degree of rBN transformation into dense modifications increases from 0 to 40%. There is a change in the rBN transformation mechanism: from longitudinal bending of layers (as a result of which wBN forms, and spBN is formed from it) to corrugation leading to spBN formation directly from rBN.

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.

Sintering of pure wurzitic boron nitride [99% (wt.) BNw] was performed under high static pressure at high temperature (P=8–9 GPa, T=1700–2000 K). The “double toroid” equipment is used at high pressure. Compact single- and two-layer plates 14–15 mm in diameter are fabricated. It is shown that sintering of BNw is accompanied by the phase transformation BNw→BNsp (cubic boron nitride). The phase composition, microstructure, density, Young’s modulus, and wear resistance during cutting are also studied. It is found that sintered materials have the same physical and chemical properties as the well-known superhard material Hexanite-R but due to the larger blank dimensions there are greater cutting possibilities.

The methods of quantitative phase analysis of low-absorbing powders by X-ray diffraction have been developed. The approach used accounts for the dependence of line intensity absorption factor on the Bragg angle and sample porosity and thickness. It has been shown that the use of traditional methods without reference to this dependence results in considerable errors in quantitative determination of phase content and in phase structure investigation. The application of proposed methods is illustrated by phase analysis of products of boron nitride shock compression.