Ultraviolet Laser-induced ignition of RDX single crystal
The RDX single crystals are ignited by ultraviolet laser (355 nm, 6.4 ns) pulses. The laser-induced damage morphology consisted of two distinct regions: a core region of layered fracture and a peripheral region of stripped material surrounding the core. As laser fluence increases, the area of the whole crack region increases all the way, while both the area and depth of the core region increase firstly, and then stay stable over the laser fluence of 12 J/cm2. The experimental details indicate the dynamics during laser ignition process. Plasma fireball of high temperature and pressure occurs firstly, followed by the micro-explosions on the (210) surface, and finally shock waves propagate through the materials to further strip materials outside and yield in-depth cracks in larger surrounding region. The plasma fireball evolves from isotropic to anisotropic under higher laser fluence resulting in the damage expansion only in lateral direction while maintaining the fixed depth. The primary insights into the interaction dynamics between laser and energetic materials can help developing the superior laser ignition technique.
Growth and Characterization of β-RDX Single-Crystal Particles
1,3,5-Trinitrohexahydro-s-triazine (RDX) single-crystal particles crystallized in the β phase have been successfully grown by sublimation–recrystallization. Synchrotron X-ray diffraction results confirm that the recrystallized products are β-RDX single crystals. Compared to the traditional solution deposition methods, the sublimation method used in this work breaks through the mass limitations during the growth of β-RDX. The distribution of α- and β-RDX on different substrates reveals that hydrophilic substrates are prone to grow β-RDX single crystals and that hydrophobic substrates prefer α-RDX single crystals. The sublimation rate dominates the growth of pure β-RDX. The mechanisms of the phase transition from β-RDX to α-RDX during the process of sublimation–recrystallization have been studied. The current results can promote the potential application of β-RDX in the propellant, explosives, and military fields.
A comparison of the decomposition of electronically excited nitro-containing molecules with energetic moieties C-NO2, N-NO2, and O-NO2.
Decomposition of electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) moieties has been intensively investigated over the past decades; however, their decomposition behavior has not previously been compared and contrasted. Comparison of their unimolecular decomposition behavior is important for the understanding of the reactivity differences among electronically excited nitro-containing molecules with different X-NO(2) (X = C, N, O) bond connections. Nitromethane (NM), dimethylnitramine (DMNA), and isopropylnitrate (IPN) are used as model molecules for C-NO(2), N-NO(2), and O-NO(2) active moieties, respectively. Ultraviolet lasers at different wavelengths, such as 226, 236, and 193 nm, have been employed to prepare the excited states of these molecules. The decomposition products are then detected by resonance enhanced two photon ionization (R2PI), laser induced fluorescence (LIF) techniques, or single photon ionization at 10.5 eV. NO molecules are observed to be the major decomposition product from electronically excited NM, DMNA, IPN using R2PI techniques. The NO products from decomposition of electronically excited (226 and 236 nm) NM and IPN display similar rotational (600 K) and vibrational distributions [both (0-0) and (0-1) bands of the NO molecule are observed]. The NO product from DMNA shows rotational (120 K) and vibrational distributions (only (0-0) transition is observed) colder than those of NM and IPN. At the 193 nm excitation, electronically excited NO(2) products are observed from NM and IPN via fluorescence detection, while no electronically excited NO(2) products are observed from DMNA. Additionally, the OH radical is observed as a minor dissociation product from all three compounds. The major decomposition pathway of electronically excited NM and IPN involves fission of the X-NO(2) bond to form electronically excited NO(2) product, which further dissociates to generate NO. The production of NO molecules from electronically excited DMNA is proposed to go through a nitro-nitrite isomerization pathway. Theoretical calculations show that a nitro-nitrite isomerization for DMNA occurs on the S(1) surface following a (S(2)/S(1))(CI) conical intersection (CI), whereas NO(2) elimination occurs on the S(1) surface following the (S(2)/S(1))(CI) conical intersection for NM and IPN. The present work provides insights for the understanding of the initiation of the decomposition of electronically excited X-NO(2) energetic systems. The presence of conical intersections along the reaction coordinate plays an important role in the detailed mechanism for the decomposition of these energetic systems.