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Abstract:Cyclotrimethylenetrinitramine (RDX), Cyclotetramethylenetetranitramine (HMX) and Hexanitrohexaazaisowurtzitane (CL-20) own abundant phase transition behavior and characteristics under high pressure/high-temperature and high-pressure. The phase transition routes of three high explosives under different conditions, part of the phase structures under high pressure and p-T phase diagrams were summarized. These researches have provided effective references for the study of detonation behavior and theoretical analysis of energetic materials. Based on current works, there still exist some problems in this area. For example, some divergences still exist about the study of the complex phase transitions, most of the new structures under high pressure are not confirmed and the p-T diagrams are incomplete. In addition, the profound theoretical mechanism of the phase transition has not been sufficiently revealed. Therefore, it will serve as a main tendency to explore the mechanism of transformation between different crystal phases and obtain more information of phase structures in the future research.

Abstract:If the molecules in an energetic crystal are regarded as the bricks of the crystal building, the intermolecular interactions will be the adhesive among the bricks. Thus, the intermolecular interactions are one of directions and bases for understanding and designing energetic crystal. This article reviews the intermolecular hydrogen bonding, halogen bonding and π-π stacking in energetic crystals, and their influences on molecular stacking pattern, impact sensitivity and thermal stability. Some characteristics and enlightenment from the intermolecular interactions can be summarized as follows: (1) the hydrogen bonding in low impact sensitive crystals is stronger than that in highly sensitive ones; (2) the face-to-face π-π stacking is prone to low impact sensitivity; (3) it serves as one of the crystal engineering strategies to enhance intermolecular interactions and their anisotropy to reduce impact sensitivity; and (4) it will worsen thermal stability when the intermolecular hydrogen bonding is blindly strengthened. Besides, we should focus upon the accurate description of intermolecular interactions and their evolution rules against thermomechanical stimuli.

Abstract:Explosive crystal characteristics essentially influence physic-chemical properties, functions and applications, including safety, thermal stability and compatibility with other chemical substances, mechanical properties, initiation and detonation performances. Based on the properties and shortages of current high energy explosives, regulation and control of the explosive crystal characteristics to improve the performances of explosives and expand their applications has achieved increasing research attention and become a significant branch of energetic materials. This review summarized the current development of the control principle and process methods of explosive crystal characteristics including polymorph, crystal quality, particle shape and aggregated structure, and focused on the analysis and discussion of crystal characteristics, property and performance, and application and prospects of high quality reduced sensitivity explosive, explosive spherulite and explosive with hierarchical micro-nanostructure. Additionally, considering the current major problems, several developing suggestions based on explosive crystal engineering are proposed. All these investigations may guide the production, process and application of explosive.

Abstract:In order to study the effect of formic acid/water mixed solvent on the growth morphology of energetic ionic salt dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50), interaction energy between TKX-50 faces and formic acid/water mixed solvent was calculated using molecular dynamics method. The growth morphology of TKX-50 in formic acid/water mixed solvent with different volume ratios (1/4, 1/3, 1/2, 1/1 and 2/1) was predicted by modified attachment energy model, and the effect of temperature on the crystal morphology of TKX-50 was simulated. The results show that changing the volume ratio of formic acid/water in the mixed solvent can significantly change the crystal morphology of TKX-50. When the volume ratio of formic acid/water is 1/2 and the temperature is 298 K, the crystal morphology of TKX-50 is relatively closer to spherical shape. Radial distribution function analysis shows that hydrogen bonds, van der Waals forces and electrostatic forces exist between the (1 1 0) crystal face of TKX-50 and the mixed solvent molecules.

Abstract:In order to study the formation mechanism of 2,4,6-trinitrotoluene (TNT)/ 1-nitronaphthalene (NNAP) cocrystal, powder XRD, FTIR and DSC were used to study the TNT / NNAP samples after grinding at 0 s, 10 s, 20 s, 30 s, 40 s, 50 s and 2 mins. The XRD patterns showed that the new diffraction peak, corresponded to (2 -1 1) plane of TNT/NNAP cocrystal, appeared at 2θ=25.8° and gradually increased its intensity. FTIR spectrum illuminated that the C—N—O bending vibration peak (716 cm^-1) of TNT had a blue-shift and became sharp. At the same time, due to π-π stacking, the C—C bending vibration peak (734 cm^-1) of TNT benzene ring exhibited a red-shift. The DSC curves indicated there were three endotherm peaks during the formation of cocrystal. These results showed that cocrystal packed at (2 -1 1)) plane firstly. H-bond and π-π stacking played important roles in the formation of TNT/NNAP cocrystal. TNT and NNAP firstly generated two eutectics, then transferred into TNT/NNAP cocrystal. The melting point of this cocrystal is 65 ℃.

Abstract:The calculations of the crystal structures, in-crystal intermolecular interactions, physicochemical properties, crystal stability and detonation performance for 16 reported cocrystal explosives were carried out to explore their effect on crystal stability and detonation performance of cocrystal explosives. We show that the crystal stability of the cocrystal explosives is mainly determined by the hydrogen bonding (HB) amount when the HB strength is less than 21 kJ·mol^-1. When the HB strength is more than 21 kJ·mol^-1, the crystal stability of the cocrystal explosives is mainly determined by the HB strength. Compared to traditional single-component explosives, the reported 16 cocrystals exhibit better nitrogen content and oxygen balance, but their material densities and detonation performance are less competitive. Through the analysis of CL-20 cocrystal explosives, it is theoretically suggested that enhancing HB strength, instead of introducing more hydrogen atoms to increase HB amount, could be useful to improve crystal stability of cocrystal explosives. This strategy can simultaneously meet the requirement of oxygen balance and nitrogen content in resulting satisfactory detonation performance of cocrystal explosives.

Abstract:The microstructure of HMX (Octogen) crystal particles with average sizes of 5 and 20 μm was studied by in situ variable-temperature small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The WAXS results showed that the initial β→δ phase transition temperature of HMX (5 μm) is 194 ℃, which is 8 ℃ higher than that of the HMX (20 μm). Guinier law modelling analysis of SAXS measurements revealed the presence of defects with gyration radii between 0.6~0.9 nm will occur in HMX when above 150 ℃. The volume ratio of the defects irreversibly increases with temperature. The number of defects in HMX (20 μm) was found to be higher than that of HMX (5 μm). SAXS, WAXS and scanning electron microscopy all indicate that the thermal stability of HMX (5 μm) is better than that of HMX (20 μm). Finally, the mechanism of defects formation and their effect on the structural integrity and sensitivity of HMX were discussed.

Abstract:In order to study the crystal nucleation process of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), the CrystlScan multi-channel crystallizer was used to measure the crystallization nucleation induction period of LLM-105 in antisolvent system. The influence of supersaturation on the nucleation of LLM-105 was studied by using the induction period measurement method, and the important nucleation parameters were calculated by the classic nucleation theory. The results show that the induction period of anti-solvent crystallization of LLM-105 decreases with increasing supersaturation. When the supersaturation ratio S<2.53, heterogeneous nucleation was mainly observed, the interfacial tension γ value is 6.67717×10^-23 J·m^-2, and the surface entropy factor f value is 1.49; when S>2.53, the homogeneous nucleation was mainly achieved, the interfacial tension γ value is 9.89842×10^-23 J·m^-2 and the surface entropy factor f value is 2.23. The surface entropy factor values were all less than 3, indicating that the growth mode of LLM-105 in ethyl acetate as an anti-solvent crystal is a continuous growth mechanism, the nucleation rate increases as the supersaturation ratio increases, and the critical nucleus radius and the number of critical nucleation decrease as the supersaturation ratio increases.

Abstract:The crystal data of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50), dihydroxylammonium azotetrazole-1,1′-diolate (ATZO-1), and sodium 5,5′-azotetrazole-5-oxide pentahydrate (ATO-Na) were analyzed and compared. The effects of the introduction of ─N􀰗N─ and ─N􀰗N(O)─ into the structure of bis-tetrazolium molecules was summarized from the micro-level. Results show that the bistetrazole linked from two tetrazolium rings directly has the densest structure among three compounds, the symmetrical and compact structure further make it own the highest crystal density. Based on the Born-Haber cycle and the Hess law, the heat of formation for TKX-50, ATZO-1 and dihydroxylammonium 5,5′-azotetrazole-5-axide (ATO-1) were calculated. According to the Kamlet Jacobs formula (K-J equation), detonation parameters of these three compounds were also obtained. By comparison, it could be found that both the introduction of ─N􀰗N─ and ─N􀰗N(O)─ bonds can increase the enthalpy of formation for compounds to a certain extent. However, due to the apparent density difference in three crystals, TKX-50 still holds the highest detonation value.

Abstract:To facilitate the crystallization of spherical FOX-7, a laser dynamic method was performed to determine the solubility of FOX-7 in the solvent mixtures of DMSO-EAC with different volume ratio in the temperature range of 298.15~333.15 K. The solubility equation was established and the crystallization thermodynamic parameters were estimated. The cooling crystallization experiment was carried out in DMSO-EAC mixed solvent. The results show that the solubility of FOX-7 in mixed solvents increases with increasing temperature and DMSO content. All models fit well with experimental data, of which the CNIBS/R-K model has the best correlation. The crystals of FOX-7 obtained by cooling crystallization in V_DMSO∶V_EAC=1∶3 system are regular, ellipsoidal and uniform in particle size.

Abstract:In order to provide the thermodynamic criterion for the cocrystal formation and give theoretical basis for the screening and optimization of crystallization solvents and parameters, the thermodynamic of hexanitrohexaazaisowurtzitane (CL-20)/1,4-dinitroimidazole (1,4-DNI) cocrystal formation has been investigated. Taking acetone, ethyl acetate and methanol as solvents, the solubility data of CL-20, 1,4-DNI in pure solvents and CL-20 in 1,4-DNI solutions with different concentrations (0.04, 0.06, 0.08, 0.10, 0.12, 0.16, 0.20, 0.24 g·mL^-1) were achieved with the help of high performance liquid chromatography. The ternary phase diagrams of CL-20/1,4-DNI solvents were built. The thermodynamic parameters of the solubility product K_sp, the complexation constant K₁₁, and the reaction free energy ΔG⁰ were obtained through fitting the solubility data based on the mathematical models of solution chemistry theory. The results show that if the solubilities of CL-20 and 1,4-DNI are larger and their discrepency is less in a solvent, the cocrystal region is larger and its shape is more symmetrical. The order of the cocrystal regions in the three solvents from large to small is acetone > ethyl acetate > methanol, and the shape of the cocrystal region in the acetone and ethyl acetate is more symmetrical. The calculated thermodynamic parameters indicate that the acetone is the most beneficial to the formation of CL-20/1,4-DNI cocrystal among the three solvents, followed by ethyl acetate. The ternary phase diagram and thermodynamic parameters in acetone solvent at different temperatures show that lowering the crystallization temperature is beneficial to the precipitation of the CL-20/1,4-DNI cocrystal.

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