Shock Waves in High-Energy Materials: The Initial Chemical Events in Nitramine RDX

We extend the reactive force field ReaxFF to describe the high energy nitramine RDX and use it with molecular dynamics (MD) to study its shock-induced chemistry. We studied shock propagation via nonequilibrium MD simulations at various collision velocities. We find that for high impact velocities ($>6\mathrm{k}\mathrm{m}/\mathrm{s}$) the RDX molecules decompose and react to form a variety of small molecules in very short time scales ($<3\mathrm{p}\mathrm{s}$). These products are consistent with those found experimentally at longer times. For lower velocities only ${\mathrm{N}\mathrm{O}}_2$ is formed, also in agreement with experiments.

Figure 1

Energetics of unimolecular decomposition mechanisms in RDX obtained using the ReaxFF (full lines with filled symbols) and with QM (dashed lines with open symbols) [8]. Circles represent the sequential HONO elimination, triangles show the decomposition process following homolytic N-N bond breaking (${\mathrm{N}\mathrm{O}}_2$ elimination), and diamonds represent the concerted ring-opening pathway. Intermediates and products are described in 8.

Figure 2

The shock velocity ($v_{\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{c}\mathrm{k}}$) as a function of particle velocity ($v_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}$) obtained from nonequilibrium MD simulations using ReaxFF compared with the experimental (unreactive) results [12]. The sound velocity obtained from the ReaxFF results is $2.90\mathrm{n}\mathrm{m}/\mathrm{p}\mathrm{s}$, only $4.4\%$ larger than experiment [12].

Figure 3

The time evolution of various fragments during the shock process for impact velocities: $v_{\mathrm{i}\mathrm{m}\mathrm{p}}=4\mathrm{k}\mathrm{m}/\mathrm{s}$ (a); $6\mathrm{k}\mathrm{m}/\mathrm{s}$ (b); $8\mathrm{k}\mathrm{m}/\mathrm{s}$ (c); and $10\mathrm{k}\mathrm{m}/\mathrm{s}$ (d). The initial number of RDX molecules is 64.

Figure 4

Mass spectrum corresponding to $v_{\mathrm{i}\mathrm{m}\mathrm{p}}=8\mathrm{k}\mathrm{m}/\mathrm{s}$ at time $t=4\mathrm{p}\mathrm{s}$. Population as a function of mass for all the molecules found up to mass $50\mathrm{g}/\mathrm{m}\mathrm{o}\mathrm{l}$ (all species with population larger than 3 are labeled).