Our group conducts cutting-edge theoretical and computational studies of several fundamental aspects of the behavior of energetic materials under strong shock compression. We are particularly interested in processes at the atomic scale where an accurate quantum-mechanical description of interatomic interactions is critically important. Typical energetic materials, such as HMX, RDX, PETN, CL-20, and TATB, consist of complicated polyatomic molecules, and the only method that satisfies both the requirements of computational efficiency and accuracy needed to describe these systems is first-principles density functional theory (DFT).
Our research effort in EM consists of several research topics. We perform first-principles DFT modeling of PETN, HMX, RDX, TATB and other EM crystals in order to obtain an accurate description of their constitutive relationships at extreme pressures and temperatures. Our recently established thrust in first-principles molecular reactive dynamics of energetic materials addresses the fundamental question of the initial chemical response of energetic molecular crystals to the passage of a strong shock wave through the material. The unifying theme of these two efforts is the issue of sensitivity of EM: we aim at elucidating the main factors determining the initiation sensitivity of detonation in EM upon application of impact, heat and radiation stimuli.
Anisotropic constitutive relationships in EMs at extreme conditions
This project aims to determine fundamental properties of EM crystals with and without defects under static pressures and elevated temperatures in order to understand how defects contribute to initiation and affect the rates of decomposition reactions. We perform a systematic and thorough investigation of constitutive relationships of PETN, HMX, RDX, TATB and other EMs including anisotropic, stress dependent equations of state (EOS) using first-principles DFT. The first step in our investigation is to obtain the isotropic EOS at 0K by performing full optimization of the EM crystal under the constraint of a constant hydrostatic stress tensor. Fig. 1 shows the pressure, as well as the lattice parameters, for PETN-I as a function of compression ratio V/V0 compared to experiment. The isotropic EOS has then been extended to include a description of the materials response upon uniaxial compression. The uniaxially compressed state of the crystal is directly related to the state that the crystal experiences upon shock loading: the lattice rapidly transforms at the shock wave front to a uniaxially compressed state. The shear stresses are of our particular interest because they are usually considered to be the driving forces of plastic deformations in crystals.
First-principles molecular reactive dynamics of energetic materials
The atomic-scale understanding of initiation chemistry of shock-compressed energetic materials is of fundamental importance for developing a predictive theory of initiation of detonation. In spite of several decades of intensive experimental and theoretical efforts in the field, we still do not know the answers to the following questions: what is the first chemical event that triggers the chemistry behind the shock wave front, what are the mechanisms of subsequent decomposition and eventually self-sustained propagation of chemical reactions?
We have recently started his effort to study shock-induced reactivity of EMs using first-principles DFT molecular dynamics. The important fundamental questions of molecular reaction dynamics that we address include investigation of the fundamental mechanisms responsible for the transformation of mechanical energy from the shock wave into molecular degrees of freedom. Dynamics of the energy flow including translation-to- vibration (T-V) energy transfer; subsequent redistribution of energy between different vibrational modes and excitation of a reaction mode under strongly non-equilibrium shock wave conditions; unimolecular, bi-molecular or multi-molecular nature of initial reactive events; stereochemistry of initial reaction events: dependence of the energy flow and transformations on crystalline environment and direction of shock wave propagation; the importance of quantum effects in T-V energy transfer.
- I.I. Oleynik, M. Conroy, S.V. Zybin, and C. T. White, "Energetic materials at high compression: first-principles density functional theory studies", to be published in Proceedings of 13th International Detonation Symposium.
- I.I. Oleynik, M. Conroy, S.V. Zybin, L. Zhang, A.C. van Duin, W.A. Goddard III and C. T. White, "Energetic materials at high compression: first-principles density functional theory and reactive force field studies", AIP Conference Proceedings 846, 573 (2006).