Shock compression of condensed matter

Shock compression of condensed matter is a fascinating phenomenon that has the unique feature of a gigantic release of energy in a short period of time caused by the shock waves sent through the material. Our research deals with covalently bonded materials subjected to shock compression. The major objective of this research project is to understand the atomic-scale mechanisms of shock wave propagation in covalent materials. The fundamental issues we address:

• The coupling of the anisotropic response of material with split-shock wave structure in covalent solids
• Mechanisms of shock-induced deformations and phase transitions, the critical role of the shear stresses in dynamics of plastic deformations during shock wave loading, and the dependence of shock-wave physics and chemistry on crystalline direction of shock wave propagation.

Fundamental properties of shock compressed covalent crystals

We have performed MD simulations of shock wave propagation in diamond using a REactive Bond Order (REBO) potential and discovered a rich variety of materials response. As the shock wave intensity increased, four different regimes of shock wave propagation were observed: (i) pure elastic wave, (ii) shock wave splitting into elastic and plastic waves, (iii) anomalous elastic regime, and (iv) overdriven plastic wave with carbon activated chemistry. Different regimes of material response were classified using the calculated Hugoniot, see Fig.

We observed the appearance of a split shock structure into elastic and plastic deformation waves in the [110] and the [111] crystallographic directions above piston velocity thresholds of 1.8 and 2.5 km/s, respectively. The shear deformation of the crystal lattice in the second wave is developed through the movement of (111) planes and has different structural character for [110] and [111] shock waves.

The most important and puzzling discovery was the observation of the anomalous elastic response. The anomalous elastic response is characterized by the absence of plastic deformations: the material remains uniaxially compressed in spite of the substantial amount of shear stress present behind the shock wave front. However, shear stresses developed at smaller piston velocities were substantially relieved by developing plastic deformations in the crystal. We found that the effective freezing of plastic deformations is related to non-monotonic behavior of shear stresses upon uniaxial compression of diamond along both the [110] and [111] directions.

Selected publications:

• S.Z. Zybin, I.I. Oleynik, M.L. Elert and C.T. White, "Atomistic Study of Diamond Deformation Under Shock Compression", submitted to Physical Review B.
• I.I. Oleynik, S.V. Zybin, M. L. Elert, and C. T. White, "Nanoscale molecular dynamics simulaton of shock compression of silicon", AIP Conference Proceedings 846, 413 (2006).
• I.I. Oleynik, S.V. Zybin, M. L. Elert, and C. T. White, "Shear stresses in shock-compressed covalent solids", AIP Conference Proceedings 846, 417 (2006).