Vladislav Yakushev

Formation of cubic silicon nitride from the low-pressure phase by high-temperature shock compression

The effect of high-temperature shock compression conditions on the degree of transformation of silicon nitride into a cubic γ-modification at pressures of 36 and 50 GPa in planar recovery ampoules is studied. The x-ray Rietveld method is used to determine the quantitative phase composition before and after shock compression. The temperature reached during compression and the residual temperature after unloading are calculated. It is shown that the use of high-temperature shock compression at 36 GPa provides a higher transformation level compared to the classical approach based on the addition of copper powder into the samples.

Porous silicon nitride under shock compression

The measurements of the Hugoniot and sound velocity pressure dependence in â-Si3N4 porous samples under shock loading up to 55 GPa are presented. The Hugoniot and experimental particle velocity profiles demonstrate no peculiarities arising from the phase transition into high density c-Si3N4. Nevertheless, it follows from the comparison of measured Hugoniot with the Hugoniot for monolithic samples that phase transition does take place, starting at a much lower pressure of approximately 25 GPa, where the two Hugoniots intersect. The analysis of the positional relationship of the Hugoniots for monolithic and porous samples allowed us to confirm that the transformation pressure threshold decreases with the temperature increase due to a greater heating of porous material under loading. Measurements of sound velocity in shocked samples showed that sound velocity pressure dependence has a kink near 23 GPa, which is very close to the presumable pressure of the transition to c-Si3N4 in porous samples.

Structure of detonation waves in PETN

A VISAR technique was used to study the structure of the reaction zone in PETN for various initial densities and dispersion of samples. The flow in the pressed charges corresponds to the classical denotation model. In the case of bulk density, the peculiarities corresponding to an explosive combustion model are recorded. In the vicinity of the initial density of 1.7 g/cm3, a kink is found on the curve showing detonation velocity versus density, and the shock-wave initiation of PETN above and below the kink point is studied.

Determination of P , T -conditions developed at high-temperature shock compression of silicon nitride in planar recovery ampoules

We performed experiments on shock compression up to pressures of 36 and 50 GPa of mixed samples of silicon nitride and potassium bromide, placed between copper plates that serve as walls of a recovery ampoule. For comparison, similar experiments were carried out by the conventional compression of a mixture of silicon nitride and copper powder. The loading of the samples was fulfilled by means of aluminum flyers accelerated by products of explosion to a few kilometers per second. The pressure profiles prior to a shock wave entering the sample and after its runout were measured with the use of manganin sensors. It is found that for the configurations of the experimental assembly used, the pressure in the samples, accumulated by circulating the shock wave, reaches the desired value before unloading. Based on estimates of the rate of heat transfer between the components, it is shown that thermal equilibrium can be set during the existence of high pressure in the mixed samples. Within the framework of the single-temperature medium model, the equations of state of the samples are derived, and the temperatures of their shock compression are calculated. Using these equations, we performed numerical simulations that showed good agreement with the experimental data.

SHOCK COMPRESSION AND EQUATION OF STATE OF C60 FULLERITE

Hugoniot of C60 fullerite and sound velocity in shock‐compressed fullerite were experimentally studied at the pressure range up to P∼60u2009GPa. The results of the shock‐wave measurements were used for the semi‐empirical description of thermodynamic properties of simple cubic phase of C60 fullerite.