First principles molecular dynamics simulation and thermal decomposition kinetics study of CL-20.

CONTEXT: 2,4,6,8,10, 12-hexanitro-2,4,6,8,10, 12-hexazepane (CL-20) is a new energetic material with high performance and low sensitivity. In-depth study of the thermal decomposition mechanism of CL-20 is a necessary condition to improve its performance, ensure its safety, and optimize its application. On the basis of a large number of empirical force fields used in molecular dynamics simulation in the past, the machine learning augmented first-principles molecular dynamics method was used for the first time to simulate the thermal decomposition reaction of CL-20 at 2200 K, 2500 K, 2800 K, and 3000 K isothermal temperature. The main stable resulting compounds are N2, CO2, CO, H2O, andH2, where CO2 and H2O continue to decompose at higher temperatures. The initial decomposition pathways are denitration by N-N fracture, ring-opening by C-N bond fracture, and redox reaction involving NO2 and CL-20. After ring opening, two main compounds, fused tricyclic pyrazine and azadicyclic, were formed, which were decomposed continuously to form monocyclic pyrazine and pyrazole ring structures. The most common fragments formed during decomposition are those containing two, three, four, and six carbons. The formation rule and quantity of main small molecule intermediates and resulting stable products under different simulated temperatures were analyzed. METHODS: Based on ab initio Bayesian active learning algorithm, efficient and accurate prediction of CL-20 is made using the dynamic machine learning function of Vienna Ab-Initio Simulation Package (VASP), which constructs the energy potential surface by learning a large number of data based on AIMD calculations. The result is a machine learning force field (MLFF). Then the molecular dynamics of CL-20 was simulated using the trained MLFF model. PAW pseudopotentials and generalized gradient approximation (GGA), namely, Perdew-Burke-Ernzerhof (PBE) functional, are used in the calculation. The plane wave truncation energy (ENCUT) is set to 550 eV, and using the Gaussian broadening, the thermal broadening size of the single-electron orbital is 0.05 eV. A van der Waals revision of the system with Grimme Version 3. The energy convergence accuracy (EDIFF) of electron self-consistent iteration is set to 1E-5 eV and 1E-6 eV, respectively. The two-step structure optimization is carried out using 1'1'1 k point grid and conjugate gradient method. The ENCUT was changed to 500 eV and EDIFF to 1E-5 eV, and NVT integration (ISIF = 2) of Langevin thermostat was used for machine learning force field training and AIMD simulation of the system.

Smart Host-Guest Energetic Material Constructed by Stabilizing Energetic Fuel Hydroxylamine in Lattice Cavity of 2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane Significantly Enhanced the Detonation, Safety, Propulsion, and Combustion Performances.

The host-guest inclusion strategy has become a promising method for developing novel high-energy density materials (HEDMs). The selection of functional guest molecules was a strategic project, as it can not only enhance the detonation performance of host explosives but can also modify some of their suboptimal performances. Here, to improve the propulsion and combustion performances of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW), a novel energetic-energetic host-guest inclusion explosive was obtained by incorporating energetic rocket fuel, hydroxylamine (HA), into the lattice cavities of HNIW. Based on their perfect space matching, the crystallographic density of HNIW-HA was determined to be 2.00 g/cm3 at 296 K, which has reached the gold standard regarding the density of HEDMs. HNIW-HA also showed higher thermal stability (Td = 245.9 °C) and safety (H50 = 16.8 cm) and superior detonation velocity (DV = 9674 m/s) than the ε-HNIW. Additionally, because of the excellent combustion performance of HA, HNIW-HA possessed higher propulsion performances, including combustion speed (SC = 39.5 mg/s), combustion heat (QC = 8661 J/g), and specific impulse (Isp = 276.4 s), than ε-HNIW. Thus, the host-guest inclusion strategy has potential to surpass the limitations of energy density and suboptimal performances of single explosives and become a strategy for developing multipurpose intermolecular explosives.

Pressure-Induced In Situ Construction of P-CO/HNIW Explosive Composites with Excellent Laser Initiation and Detonation Performance.

Laser initiation is a popular research topic in the energetic community. Particularly, the direct ignition of secondary explosives by laser ignitors is considered to be an advanced strategy for enhancing safety and promoting the miniaturization of weapons. Here, to improve the laser sensitivity of secondary explosives, P-CO synthesized under high pressure was employed as a coating for HNIW owing to its laser sensitivity and excellent energetic properties. In this operation, HNIW underwent an obvious isostructural phase transition from the ε-phase to the γ'-phase in the pressure range of 1.0-4.8 GPa. Subsequently, sub-nanoscale HNIW-based composites were formed when CO in situ polymerized to P-CO on the surfaces of HNIW at 5.1 GPa. This HNIW-based composite could be ignited at a much lower laser power (0.49-0.65 W) compared with pure HNIW (2.75-2.98 W) when excited by an Nd:YAG laser with a wavelength of 1064 nm. Additionally, the DFT calculations demonstrated that the arrangement density between HNIW and P-CO was significantly enhanced as the pressure increased. Thus, the introduction of advanced materials into explosive formulations through high-pressure technology is a novel and feasible strategy for developing multipurpose energetic materials.

Characterization of Crystal Microstructure Based on Small Angle X-ray Scattering (SAXS) Technique.

Small-angle X-ray scattering (SAXS) is an effective method to obtain microstructural information of materials. However, due to the influence of crystal surface effects, SAXS has a deviation in the characterization of the crystal microstructure. In order to solve the influence of crystal surface effect on the internal defect signal, the microstructure of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystal was characterized by soaking the sample in the matching solution. We found that the absolute scattering intensity, specific surface and volume fraction of the sample in the matching solution are significantly lower than the initial sample, which solves the influence of the crystal surface effect on the test results. Comparing the scattering results of the samples in different electron density matching solutions, it was found that the best result was obtained when using GPL-107 perfluoropolyether (PFPE) matching solution and the same law was obtained by controlling the experiment with 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20) crystal. The fitting density was calculated according to the theoretical density and void volume fraction of the sample, and the calculated results are close to the test results of Particle Density Distribution Analyzer (PDDA). Based on this paper, we provide a method to obtain the correct information of crystal microstructure.

Establishment and application of multiplexed CRISPR interference system in Bacillus licheniformis.

Bacillus licheniformis has been regarded as an outstanding microbial cell factory for the production of biochemicals and enzymes. Due to lack of genetic tools to repress gene expression, metabolic engineering and gene function elucidation are limited in this microbe. In this study, an integrated CRISPR interference (CRISPRi) system was constructed in B. licheniformis. Several endogenous genes, including yvmC, cypX, alsD, pta, ldh, and essential gene rpsC, were severed as the targets to test this CRISPRi system, and the repression efficiencies were ranged from 45.02 to 94.00%. Moreover, the multiple genes were simultaneously repressed with high efficiency using this CRISPRi system. As a case study, the genes involved in by-product synthetic and L-valine degradation pathways were selected as the silence targets to redivert metabolic flux toward L-valine synthesis. Repression of acetolactate decarboxylase (alsD) and leucine dehydrogenase (bcd) led to 90.48% and 80.09 % increases in L-valine titer, respectively. Compared with the control strain DW9i△leuA (1.47 g/L and 1.79 g/L), the L-valine titers of combinatorial strain DW9i△leuA/pHYi-alsD-bcd were increased by 1.27-fold and 2.89-fold, respectively, in flask and bioreactor. Collectively, this work provides a feasible approach for multiplex metabolic engineering and functional genome studies of B. licheniformis.

Design, preparation, characterization and formation mechanism of a novel kinetic CL-20-based cocrystal.

2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane (CL-20)-based cocrystals have gained increasing attention as a means of obtaining insensitive high explosives. However, the design of ideal candidates for these cocrystals remains difficult. This work compares the crystal energies of the CL-20-dinitrobenzene (DNB) and CL-20-2,4,6-trinitrotoluene (TNT) cocrystals with those of the respective pure coformers. The results indicate that the cocrystal formation is driven by the differences in the energies of the cocrystals and the coformers. Furthermore, analysis via Hirshfeld surfaces and two-dimensional fingerprint plots confirms that the O...O, O...H, O...N and C...O interactions were the main force for stabilizing the CL-20-based cocrystal structure. Based on these findings, a novel energetic-energetic cocrystal of CL-20-2,4,6-trinitrophenol (TNP) was designed and prepared by means of a rapid method for solvent removal. The crystal structure was investigated via powder X-ray diffraction methods, solid-state nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy. The results revealed that the O-H...O hydrogen bonding interaction between the phenolic hydroxyl group of TNP and nitro groups of CL-20, as well as nitro...π, nitro...nitro and ONO2...π(N)NO2 interactions, based on the benzene ring and nitro groups, are the main interactions occurring in the cocrystal.