Transformations of graphite and boron nitride in shock waves

Abstract.

Peculiarities of shock adiabat of graphite are attributed to the graphite–diamond transformation. However only a very small amount of diamond can be recovered from pure shocked graphite with a density approaching the theoretical value. In order to interpret this fact, accessible data concerning the behaviour of graphite under static and dynamic load have been analysed. An additional peculiarity of the shock adiabat of graphite has been found at 12 GPa by analysing compressibility data. It has been attributed to shearing in the basal planes that paves the way for deformation of the planes. An isotherm of cold compression of graphite can be constructed on the basis of the results from theoretical modelling published in the literature. Another isotherm, fitting experimental data, has been proposed. An isotherm for graphitic boron nitride has been also proposed. The isotherms have been used in the interpretation of the peculiarities of shock adiabats. It has been shown that the so-called “mixed-phase” region is an apparent compressibility curve. Energy evaluations based on the isotherms have proved that the peculiarities of the shock adiabat of graphite correspond to the formation of hexagonal instead of cubic diamond. Similarly the formation of the wurtzite modification of BN is responsible for the peculiarities of the shock adiabat of $\alpha$BN. Literature data concerning the mechanism of the polymorphous transformations of graphite and $\alpha$BN in shock waves have been reviewed. On the basis of proposed isotherms of cold compression, the activation energy has been appraised and an equation of kinetics proposed. The equation has been analysed by comparing results of theoretical modelling and accessible experimental data.