Enhanced Thermal Decomposition and Safety of Spherical CL-20@MOF-199 Composites via Micro-Nanostructured Self-Assembly Regulation

The characteristics of high burning rate, high energy output, and low pressure exponent have always been the focus of development in the field of composite solid rocket propellants. In this paper, a metal-organic framework (MOF-199) compound is introduced to prepare micro-nanospherical CL-20@MOF-199 composites via the spray-drying self-assembly technique to reach the above goals. MOF-199, which acts as an attractive combustion catalyst and a safety regulator, is uniformly coated on the surface of CL-20 with close interface contact between particles, effectively accelerating the thermal decomposition of CL-20 and ensuring safety performance. The average noncovalent interaction (aNCI) analysis illustrates that there are strong C-H···O hydrogen bonds and van der Waals interaction between CL-20 and MOF-199 molecules, greatly enhancing the effect of interparticle assembly. The effects of different contents of MOF-199 on the thermal, safety, and energy properties of CL-20 were discussed. The thermal analysis demonstrates that MOF-199 has a significant thermal catalytic effect on CL-20, with an advanced peak temperature of thermal decomposition of 14.2 °C and a reduced activation energy barrier of 34.2 kJ·mol-1, mainly benefitting from more exposed catalytic active sites and close interface contact. In addition, CL-20@MOF-199 composites exhibit decreased mechanical sensitivity (IS: 21-40 cm, FS: 80-240 N) and excellent energy performance. This work clearly demonstrates that MOF-199 is both a superior combustion catalyst and a good safety buffer for CL-20, and it opens new potential for further applications of CL-20 in composite solid propellants.

Towards Computational Screening for New Energetic Molecules: Calculation of Heat of Formation and Determination of Bond Strengths by Local Mode Analysis

The reliable determination of gas-phase and solid-state heats of formation are important considerations in energetic materials research. Herein, the ability of PM7 to calculate the gas-phase heats of formation for CNHO-only and inorganic compounds has been critically evaluated, and for the former, comparisons drawn with isodesmic equations and atom equivalence methods. Routes to obtain solid-state heats of formation for a range of single-component molecular solids, salts, and co-crystals were also evaluated. Finally, local vibrational mode analysis has been used to calculate bond length/force constant curves for seven different chemical bonds occurring in CHNO-containing molecules, which allow for rapid identification of the weakest bond, opening up great potential to rationalise decomposition pathways. Both metrics are important tools in rationalising the design of new energetic materials through computational screening processes.

Enhancing chemical stability of tetranitro biimidazole-based energetic materials through co-crystallization

Co-crystallization technology was employed as a way of solving two problems hampering the usefulness of 4,4′,5,5′-tetranitro-2,2′biimidazole (TNBI) as a viable energetic material, namely hygroscopicity and corrosiveness (high acidity). Co-crystal screening was carried out with 15 co-formers containing nitrogen or oxygen as the primary hydrogen-bond acceptor site. Formation of co-crystals was confirmed by IR spectroscopy and DSC, and suitable co-crystals were then analysed via single-crystal X-ray diffraction. In each case, the formation of a co-crystal was driven by the formation of multiple N–H···N or N–H···O hydrogen bonds between TNBI and the co-former. The N-oxide based acceptors produce better energetic materials due to a more optimal oxygen balance. Hygroscopicity evaluations and corrosion tests revealed that the unavailability of N–H protons in the co-crystals of TNBI reduce hygroscopicity and suppress the chemical acidity of the free parent compound thereby making it substantially easier to handle, store, and transport.

Properties and Promise of Catenated Nitrogen Systems As High-Energy-Density Materials

The properties of catenated nitrogen molecules, molecules containing internal chains of bonded nitrogen atoms, is of fundamental scientific interest in chemical structure and bonding, as nitrogen is uniquely situated in the periodic table to form kinetically stable compounds often with chemically stable N-N bonds but which are thermodynamically unstable in that the formation of stable multiply bonded N₂ is usually thermodynamically preferable. This unique placement in the periodic table makes catenated nitrogen compounds of interest for development of high-energy-density materials, including explosives for defense and construction purposes, as well as propellants for missile propulsion and for space exploration. This review, designed for a chemical audience, describes foundational subjects, methods, and metrics relevant to the energetic materials community and provides an overview of important classes of catenated nitrogen compounds ranging from theoretical investigation of hypothetical molecules to the practical application of real-world energetic materials. The review is intended to provide detailed chemical insight into the synthesis and decomposition of such materials as well as foundational knowledge of energetic science new to most chemists.

Comparative investigation on the thermostability, sensitivity, and mechanical performance of RDX/HMX energetic cocrystal and its mixture

Molecular mechanics (MM) and molecular dynamics (MD) simulation method were applied to explore the impact of temperature (220-380 K) on the thermostability, sensitivity, and mechanical performance of RDX (1,3,5-trinitro-1,3,5-triazacyco-hexane)/HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) energetic cocrystal and mixture models. The mechanical property, the maximum trigger bond length ([Formula: see text]), binding energy, and cohesive energy density (CED) of the pure RDX, β-HMX crystal, the cocrystal, and mixture models were acquired and compared. The results manifest that temperature has an important impact on the binding capacity between the components of the cocrystal and mixture. The binding energies decrease as the temperature rises, and the cocrystal has larger values than those of mixture. For all the models, the [Formula: see text] increases and the CEDs decrease with the rising temperature, implying that the sensitivity of the explosives increases, while the [Formula: see text] values of the cocrystal are smaller than those of HMX and the CED values are between those of RDX and β-HMX, indicating that the sensitivity has been enhanced through co-crystallization. As the temperature increases, the shear modulus (G), bulk modulus (K), and tensile modulus (E) values of all models have an evident downtrend. Simultaneously, G, K, and E values of the cocrystal model are less than those of RDX and β-HMX, while the K/G ratio and Cauchy pressure (C₁₂-C₄₄) are larger, signifying that co-crystallization can weaken the brittleness and enhance the ductility of the pure crystals. Compared with the mixture, the cocrystal has better ductility and stability.

Theoretical calculation into the effect of molar ratio on the structures, stability, mechanical properties and detonation performance of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane/ 1,3,5-trinitro-1,3,5-triazacyco-hexane cocrystal

Molecular dynamics (MD) simulation was conducted to research the effect of molar ratio on the thermal stability, mechanical properties, and detonation performance of HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane)/RDX (1,3,5-trinitro-1,3,5-triazacyco-hexane) cocrystal explosive at ambient condition. The binding energy, mechanical properties, and the detonation parameters of the pure β-HMX, RDX crystal, and the cocrystal models were got and contrasted. The results demonstrate that molar ratio has a great influence on the properties of the cocrystal system. The binding energy of the cocrystals has the maximum values at the 1:1 molar ratio, indicating that the stability of HMX/RDX(1:1) cocrystal is the best and HMX and RDX may prefer to cocrystallizing at 1:1 molar ratio. What's more, the tensile modulus (E) and shear modulus (G) of the HMX/RDX(1:1) cocrystals have the minimum value, while the C₁₂-C₄₄ and K/G have the maximum value, implying that the cocrystal at 1:1 molar ratio has the best mechanical properties. Simultaneously, the E, K, and G of the cocrystals are all smaller than those of β-HMX's and generally larger than those RDX's, while the Cauchy pressure (C₁₂-C₄₄) and K/G ratio were greater, demonstrating that cocrystallizing can improve the brittleness and enhance the ductility. The detonation velocity (D) and detonation pressure (P) decrease with the rising RDX content, while the properties are still superior to the pure RDX crystal; thus, the energy properties of the cocrystal are still excellent. In a word, HMX/RDX cocrystal at 1:1 molar ratio has the best thermal stability, mechanical properties, and the excellent energetic performance.

High pressure behavior of crystal [2,2'-bi(1,3,4-oxadiazole)]-5,5'-dinitramide: A DFT investigation

Density functional theory (DFT) computation was carried out to investigate the crystal, molecular and electronic structures of high energy crystal [2,2'-bi(1,3,4-oxadiazole)]-5,5'-dinitramide (BODN) with the pressure 0-120 GPa. The relaxed crystal structure by the GGA/PBE-TS functional matches well with the experimental data at ambient pressure condition. With the intensifying of pressure, the lattice parameters, volumes, bond lengths, H-bond energies, atomic charges, bond populations, band gaps and density of states of crystal BODN change gently. Under the pressure of 48, 104, and 107 GPa, three pressure-induced transformations occurred. The intramolecular six membered rings pose strong affect in stabilizing systems in the pressure range 0-120 GPa. Between O1 and H2 atoms, the H-bond interaction transforms into covalent interaction under the circumstance of 48 GPa. At 104 GPa, structural transformation occurs with the distortion of the intramolecular six membered ring. In addition, O1⋅⋅⋅H2 and O2⋅⋅⋅H1 have the largest H-bond energies in comparison with the others. When the pressure reaches 107 GPa, the H-bond O1⋅⋅⋅H2 is formed again with the deformation and non-coplanarity of two oxadiazoles in crystal BODN. The electrons can be moved easily based on the density of states and energy bands under high pressure. Helpful information will be conveyed by this work in the field of further analysis connected the pressure effect on molecular transformations.

From BTO2− to HBTO− insensitive energetic salt: a route to boost energy

A promising strategy was utilized to boost the detonation performance of insensitive energetic salts.

Periodic DFT study of structural transformations of cocrystal NTO/TZTN under high pressure

Density functional theory (DFT) periodic calculations were performed to study the geometrical and electronic structures of energetic cocrystal NTO/TZTN under pressures ranging from 0 to 80 GPa. The optimized crystal structure by the GGA/PW91 (Perdew-Wang-91) and dispersion corrections corresponds well with the experimental values under ambient pressure. With the pressure increasing, the lattice constants, unit cell volumes, interatomic distances, H-bond energies, atomic charges, and bond populations of cocrystal NTO/TZTN change gradually. At pressures of 4, 8, and 23 GPa, three structural transformations occurred, shown by the results. The cyclization plays an important role in stabilizing the systems. The increasing pressure contributes to the increase of interaction force gradually. At 4 GPa, a new hydrogen bond O3⋯H5 is formed. At 8 GPa, the formation of eight membered rings is because of the existence of a covalent bond O1-H3 between two NTO molecules. In addition, a covalent interaction is formed between N2 and H4 atoms with the biggest H-bond energy compared to the others. As the pressure reaches 23 GPa, another new hydrogen bond forms between N8 and H5 atoms, which contributes to the formation of a five membered ring between NTO and TZTN. The electrons can move freely according to the results of the density of states between the valence and conduction bands when the pressure is high. This work will provide useful information in understanding the high-pressure effect on the structural transformation.

A comparative study of the structure, stability and energetic performance of 5,5′-bitetrazole-1,1′-diolate based energetic ionic salts: future high energy density materials

The structure–property–performance interrelationship of energetic ionic salts based on 5,5′-bitetrazole-1,1′-diolate was thoroughly investigated using ab initio calculations.

Preparation of CL-20/DNDAP cocrystals by a rapid and continuous spray drying method: an alternative to cocrystal formation

A CL-20/DNDAP cocrystal explosive prepared by a spray drying method exhibited a small particle size with a narrow size distribution and good comprehensive performance.

Mechanochemical fabrication and properties of CL-20/RDX nano co/mixed crystals

By milling 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) together, a nano CL-20/RDX co/mixed crystal explosive with a mean particle size of 141.6 nm is prepared from the raw materials, and the co/mixed crystals are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC) and thermal-infrared spectrometry online (DSC-IR) technology; furthermore, the impact, friction and thermal sensitivity of the samples are tested. The results show that after milling, the morphology of the co/mixed crystal explosive is near-spherical, and the particle size reveals a normal distribution. The milled sample showed the same molecular structure and surface elements as the raw materials, but the XRD test shows that CL-20/RDX has a new crystal phase and the Raman and IR spectra gave a supplementary confirmation for the existence of a cocrystal phase in the milled sample. The activation energy of the thermal decomposition of CL-20/RDX is 206.49 kJ mol⁻¹ higher than that of raw RDX. DSC-IR analysis showed that the thermolysis of CL-20/RDX produces a large amount of CO₂ and N₂O and a small amount of H₂O, NO₂ and NO. The mechanical sensitivity of CL-20/RDX is very low. In impact sensitivity tests with a 5 kg hammer, the special height (H₅₀) is 51.43 cm, which is higher than the values of 36.43 cm for raw CL-20 and 9.78 cm for raw RDX. In the friction sensitivity tests, the explosion probability (P) is 56%; however, the thermal sensitivity of CL-20/RDX is higher than that of the raw materials, with its 5 s burst point being only 243.51 °C.

A nano CL-20/RDX co/mixed crystal explosive with mean size of 141.6 nm is prepared by mechanical milling method. Its structure is characterized and thermal decomposition is investigated. Additionally, its impact, friction and thermal sensitivities are tested.

Novel insensitive energetic-cocrystal-based BTO with good comprehensive properties

Combining a layer construction strategy with cocrystallization techniques, we designed and prepared a structurally unusual 1H,1′H-5,5′-bistetrazole-1,1′-diolate (BTO) based energetic cocrystal, which we also confirmed by single-crystal X-ray diffraction and powder-crystal X-ray diffraction. The obtained cocrystal crystallizes in a triclinic system, P-1 space group, with a density of 1.72 g cm⁻³. The properties including the thermal stability, sensitivity and detonation performance of the cocrystal were analyzed in detail. In addition, the thermal decomposition behavior of the cocrystal was studied by differential calorimetry and thermogravimetry tandem infrared spectroscopy. The results indicated that the cocrystal exhibits strong resistance to thermal decomposition up to 535.6 K. The cocrystal also demonstrates a sensitivity of >50 J. Moreover, its formation enthalpy was estimated to be 2312.0 kJ mol⁻¹, whereas its detonation velocity and detonation pressure were predicted to be 8.213 km s⁻¹ and 29.1 GPa, respectively, by applying K–J equations. Therefore, as expected, the obtained cocrystal shows a good comprehensive performance, which proves that a high degree of layer-by-layer stacking is essential for the structural density, thermal stability and sensitivity.

Combining a layer construction strategy with cocrystallization techniques, we designed and prepared an unusual energetic cocrystal, which confirmed by single-crystal X-ray diffraction.

Direct insight into the formation driving force, sensitivity and detonation performance of the observed CL-20-based energetic cocrystals

This work elucidated the underlying mechanism of the dramatic and divergent physicochemical properties of CL-20-based energetic cocrystals.

Hydrazine 5,5′-bitetrazole-1,1′-diolate: a promising high density energetic salt with good properties

The salt HA·BTO has proved to be a good insensitive explosive alternative, and has promising application as an RDX replacement.