Synthesis and modification of materials by shock waves: Real-time measurements and mechanisms of reaction

The formation of AB-type compounds under isothermal conditions is accompanied by a volume decrease. However, as a result of the heat release, there is a net increase in volume, when the reaction exothermicity is high. Exothermic reactions under high pressure lead to a volume increase of approximately 80% (alkali halogenides) to 20% (other compounds). The kinematic measurements allow the determination of the volume changes for the latter cases if the conversion degree is more than 30% because the experimental error of the volume measurements is usually 3-6%. In this article, a review of the kinematic (shock and particle velocities) shock-wave measurements of reactions in the condensed state is presented. The temperature measurements have an accuracy and time resolution of one order better than the kinematic ones. The mechanism of superfast diffusion based on a difference of the particle velocities of compounds is discussed and it is suggested that the penetration of particles through each other follows their crushing. Taking into consideration that spalling in shock waves leads to domains of 10 nm in size and assuming that chemical reactions occur on surfaces of such domains, a degree of conversion is obtained that correlates well with the experiment.