HMX Cocrystal Research Articles

A comprehensive experimental and theoretical study of thermal response mechanisms of TKX-50 and HMX

TKX-50 and HMX, as two highly promising energetic materials, are widely used in rocket propellants. TKX-50/HMX cocrystal can also improve the defects of TKX-50 and HMX and vastly expand their application scope. Their response mechanisms to the same thermal stimulation, including reactivity and decomposition and combustion behavior, are important in understanding their combustion characteristics and engine performance. This study systematically investigates the thermal response mechanisms of TKX-50 and HMX under the same thermal stimulations based on experimental measurements and theoretical analysis. Ignition delay times and burn times of TKX-50 and HMX with different particle sizes at various temperatures and pressures are measured in shock tube experiments. ReaxFF MD simulations are performed to analyze the thermal decomposition of TKX-50 and HMX in terms of potential energy evolution, intermediate and final products, and possible reaction pathways. Ab initio kinetics of hydrogen atom abstraction reactions of TKX-50 and HMX with three highly active species, Ḣ, ȮH, and NO2, are studied. The barrier heights, reaction enthalpies and rate coefficients for all reactions are further analyzed. The results show that: (1) the ignition delay time and burn time of TKX-50 (128, 138, and 55 µm) are measured to be 200–600 µs and 40–400 µs respectively, which are almost the same as those of HMX (6 and 0.8 µm) in the same temperature range; (2) the burn time of TKX-50 is less affected by pressure but more affected by particle size, whereas both pressure and particle size have a significant effect on the burn time of HMX; (3) TKX-50 is shown to be readily decomposed and less affected by temperature changes than HMX under the same conditions; (4) ȮH is generated more rapidly in the TKX-50 system, but more persistently in the HMX system; (5) H atom abstraction reactions from TKX-50 by all species are less endothermic but more exothermic than those from HMX.

Isothermal structural evolution of CL-20/HMX cocrystals under slow roasting at 190 °C.

As a new type of energetic material, cocrystal explosives demonstrate many excellent properties, such as high energy density and low sensitivity, due to the interaction between the molecules of the two components. The known decomposition temperature is 235 °C for CL-20/HMX cocrystals at a faster heating rate. CL-20 molecules could separate from the cocrystal matrix and decompose at a higher temperature, much lower than the decomposition temperature. The current work provided deep insight into the isothermal structural evolution of CL-20/HMX cocrystals with slow roasting at 190 °C. We found that the initial decomposition originates from separating CL-20 molecules from the surface along the (010) plane of the cocrystals. The gas products, such as NO2 and NO, escape from the largest exposed surface of the (010) plane and generates microbubbles and microholes. At the same time, the residual HMX molecules form δ-phase HMX crystals and shrink the volume by 72%. By increasing the time held at 190 °C, the decomposition of CL-20 molecules and recrystallization of the residual HMX molecules form a gully-like structure on the (010) plane of the CL-20/HMX cocrystal. After a long time at 190 °C, the CL-20 component completely decomposes, and all HMX molecules recrystallize in the δ-HMX form. The interaction between HMX and CL-20 molecules makes the decomposition rate of the CL-20/HMX cocrystal much slower than that of the CL-20 pure crystal with a similar decomposition activation energy during isothermal heating. This work can help to deeply understand the safety of CL-20/HMX cocrystal explosives at a temperature lower than the recognized decomposition temperature.

Solubility measurement, correlation, and dissolution thermodynamics of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane · 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (2HNIW·HMX) co-crystal in two binary solvent mixtures

The solubility of 2HNIW·HMX co-crystal in two binary solvent mixtures (ethanol + methyl acetate, and 1,2-dichloroethane + methyl acetate) was measured by gravimetric method at various temperatures (283.15, 288.15, 293.15, 298.15, 303.15, 308.15, 313.15 and 318.15 K) under atmospheric pressure (0.1 MPa). The experimental results revealed that the solubility for co-crystal in binary solvent mixtures is a simple function of ethanol (or 1,2-dichloroethane) mass fraction and temperature, which decreased with increasing temperature and mass fraction of ethanol (or 1,2-dichloroethane) in the binary solvent mixtures. The solubility of 2HNIW·HMX co-crystal was correlated with the modified Apelblat equation, CNIBS/R-K model, Jouyban-Acree-van't Hoff model, and Jouyban-Acree-Apelblat model. The fitting results demonstrate that the modified Apelblat model and CNIBS/R-K model provided a better correlation with the experimental results than the other models. Furthermore, the van’t Hoff equation was used to study the thermodynamic properties of the 2HNIW·HMX co-crystal dissolution process, including the apparent molar enthalpy change (ΔdisH°), the apparent molar entropy change (ΔdisS°), and the apparent molar Gibbs free energy change (ΔdisG°). The results show that the dissolution of the co-crystal in the selected binary solvent mixture is an exothermic process. The experimental and calculated solubility data and thermodynamic properties of the 2HNIW·HMX co-crystal would be helpful for selecting the appropriate solvent system for the reaction, crystallization, and purification of the 2HNIW·HMX co-crystal.

Theoretical research on performances of CL-20/HMX cocrystal explosive and its based polymer bonded explosives (PBXs) by molecular dynamics method.

In this article, the CL-20/HMX cocrystal model was established and its based polymer bonded explosives (PBXs) were designed. The static performances, including mechanical properties, stability and detonation performance of CL-20/HMX cocrystal model and PBXs models, were predicted by molecular dynamics (MD) method. The mechanical parameters, binding energy, and detonation parameters of PBXs models were calculated and compared with that of pure CL-20/HMX cocrystal model. The influence of polymer binders on performances of CL-20/HMX cocrystal explosive was evaluated. Results show that the polymer binders make the engineering moduli (tensile modulus, shear modulus, and bulk modulus) of PBXs declined and Cauchy pressure increased, meaning that the polymer binder can obviously improve mechanical properties of CL-20/HMX cocrystal explosive, and the PBXs model with fluorine rubber (F2311) has the best mechanical properties. In different PBXs models, the binding energy between CL-20, HMX molecules and F2311 is higher than other polymer binders, indicating that the CL-20/HMX/F2311 model is more stable. The PBXs models have lower value of crystal density and detonation parameters compared with pure CL-20/HMX cocrystal and the energetic performance of PBXs is weakened. The PBXs model with fluorine resin (F2314) has the highest energetic performance and it is higher than pure HMX. Therefore, the CL-20/HMX/F2311 and CL-20/HMX/F2314 models have more favorable comprehensive properties, proving that F2311 and F2314 are more preferable and promising to design CL-20/HMX cocrystal based PBXs.

Study on the Cocrystallization Mechanism of CL-20/HMX in a Propellant Aging Process through Theoretical Calculations and Experiments.

Energetic materials undergo physical and chemical aging due to environmental effects, resulting in the degradation of safety and detonation performances. Therefore, studying the aging performance of energetic materials is of great importance for the efficient application of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20)-based solid propellants. In this paper, XRD and FTIR of the CL-20-based propellant and CL-20/1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX)-based propellant samples showed CL-20/HMX cocrystal formation according to appearance of new peaks. SEM and EDS analyses showed that pores and dehumidification in the propellant occurred with the cocrystallization of CL-20 and HMX during the aging process. Furthermore, molecular dynamics simulation was used to predict the crystal transformation of the CL-20- and HMX-based propellant under a long-term storage process. The stability of ε-CL-20 was obtained by analyzing the crystal transformation rate. The binding energy, radial distribution function between CL-20 and HMX, as well as mechanical properties of the CL-20/HMX cocrystal and the mixture were calculated to reveal the stronger binding between CL-20 and HMX in the cocrystal. Meanwhile, the inducer effect of a nitrate ester during the cocrystallization process was analyzed. The theoretical calculation shows that during aging, ε-CL-20 tends to exist stably, while CL-20/HMX tends to form cocrystals because of the strong bond. The present work on the transformation and cocrystallization of CL-20 and HMX during long-term storage is beneficial for understanding the degradation mechanism of the propellant performances, facilitating safe storage and life evaluation of propellants.

Microchannel-confined crystallization: shape-controlled continuous preparation of a high-quality CL-20/HMX cocrystal

Shape-controlled continuous preparation of a high-quality CL-20/HMX cocrystal has been realized through a microchannel-confined crystallization strategy.

The role of electric field on decomposition of CL-20/HMX cocrystal: A reactive molecular dynamics study.

Electric field can initiate decomposition or detonation of explosives, but underlying mechanism is unclear. Here, we performed ReaxFF molecular dynamics simulation for decomposition of a cocrystal, formed by 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), solely induced by electric field. A new analytical method was proposed to obtain detailed decomposition mechanism. Results show that electric fields play important roles in decomposition of CL-20/HMX cocrystal, such as heating the system and causing the explosive to decompose. Strong constant field makes CL-20 molecules in the cocrystal decompose at significantly lower temperature, which greatly increases sensitivity. This is ascribed to the distinct decomposition mechanism that CN bond rupture dominates the initial step of CL-20's decomposition. Contrarily, oscillating field has a stronger heating effect but weaker influence on sensitivity. Moreover, HMX exhibits desensitizing effect in CL-20/HMX cocrystal under electric field. These results enhance our understanding of sensitivity mechanism beyond mechanical stimuli in explosives.

Quality Assessment of the CL‐20/HMX Cocrystal Utilising Digital Image Processing

AbstractThe reduction of liquid and gas inclusions in the crystals is an effective strategy for the production of reduced sensitivity high explosives. Assessment of the achieved crystal quality is paramount for the improvement of the crystallisation techniques. Here we present the quality evaluation of the CL‐20/HMX cocrystal obtained from semibatch reaction cocrystallisation (SBRC), batch reaction cocrystallization (BRC), and antisolvent crystallisation by comparison of the results of digital image processing of photomicrographs of crystals immersed in an optically equally dense liquid, 1H NMR, HPLC, GC, helium pycnometry, SEM, pXRD, and DSC. It was found that digital image processing is capable of differentiation between crystal qualities of batches that are indistinguishable by all the other listed methods. It presents itself as a very promising tool for crystal quality assessment and improvement. The analysis shows that SBRC can produce coarse crystals of the same quality on a pilot plant scale as antisolvent crystallisation on a laboratory scale.

Semibatch Reaction Crystallization for Scaled-Up Production of High-Quality CL-20/HMX Cocrystal: Efficient Because of Solid Dosing

CL-20/HMX cocrystal is one of the most promising energetic cocrystals, but scaled-up production was prevented by the large inherent solubility difference of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-he...

High‐Precision Density Measurements of Energetic Materials for Quality Assessment

AbstractEven though helium pycnometry is considered a very precise method of density determination, its use to analyse factors influencing crystal quality (e. g. solvent inclusions, amorphous states, crystal defects…) is limited due to the unknown inherent accuracy of the method. Previous studies have attempted to identify the best possible accuracy of helium pycnometry. However, no robust method has been proposed to validate even the precision obtained. This means that the often occurring, seemingly random density deviations have never been satisfactorily explained. In this study the influencing factors on density measurements of the CL‐20/HMX cocrystal and phase stabilised ammonium nitrate carried out with an AccuPyc 1340 TEC were quantified and a robust method of data analysis was developed to judge with certainty the quality and significance of the data obtained. This study shows that statistical tests such as the t‐test and ANOVA cannot be utilised to differentiate between samples or repeat measurements, even though the density data points exhibit a normal distribution. By utilising confidence intervals and quantifying the three major sources of error (sampling error, change in barometric pressure during the measurement, and weight in error), a repeatability of 0.015 % was achieved and a method was developed that enables differentiation between samples with a certainty as good as 0.05 %.

Theoretical insight on effect of DMSO-acetonitrile co-solvent on the formation of CL-20/HMX cocrystal explosive.

CL-20/HMX-solvent interface models were established to understand the effect of DMSO-acetonitrile co-solvent on the formation of CL-20/HMX cocrystal. The molecular dynamics simulations were applied to theoretically investigate the interactions of CL-20/HMX cocrystal surfaces and dimethyl sulfoxide/acetonitrile co-solvents. The binding energies were calculated, and the interaction between solvent molecules and CL-20/HMX cocrystal faces was analyzed. The results show that molecular interactions would be affected by the mole ratios of solvent, and the comparison of the binding energies with different mole ratios revealed that dimethyl sulfoxide/acetonitrile with mole ratio of 1:3 favors the formation of CL-20/HMX cocrystal.

Investigation of the solid-liquid ternary phase diagrams of 2HNIW·HMX cocrystal.

The influence of temperature and solvent on the solid–liquid ternary phase diagrams of the 2HNIW·HMX cocrystal has been investigated. Ternary phase diagrams were constructed for the 2HNIW·HMX cocrystal in acetonitrile and ethyl acetate at 15 °C and 25 °C. HMX and HNIW showed inconsistent dissolution behavior and congruent dissolution behavior in acetonitrile and ethyl acetate, respectively. In the HMX–HNIW–acetonitrile system, the 2HNIW·HMX cocrystal has a narrow thermodynamically stable region at both temperatures. The cocrystal exhibits a wider thermodynamically stable region in the HMX–HNIW–ethyl acetate system. The results show that the choice of solvent has a crucial influence on the dissolution behavior of the cocrystal and the size and position of each region in the phase diagram, while the temperature has no apparent effect on the overall appearance of the phase diagram. By properly selecting the ratios, the 2HNIW·HMX cocrystal could be prepared by the isothermal slurry conversion crystallization method.

External electric field induced conformational changes as a buffer to increase the stability of CL-20/HMX cocrystal and its pure components

As a significant stimulus, the external electric field (EEF) plays a vital role in affecting the behaviors of energetic materials (EMs). We investigated the effects of the EEF on CL-20/HMX cocrystal and its pure components by molecular dynamics simulations. The COMPASS force field, atom-based summation method for van der Waals interactions, and Ewald summation method for electrostatic forces were applied. The results show that there exists sudden conformational changes at the EEF intensity of 0.3 and 0.9 V/Å for β-HMX and ε-CL-20, respectively, while the EEF-induced conformational change in CL-20/HMX is a gradual process. The analysis of the N-NO2 bond length distributions, radial distribution functions (RDFs) of the intermolecular O⋯H atom pair, and cohesive energy density (CED) reveals that the EEF can remarkably decrease the N-NO2 bond length and enhance hydrogen bonding interactions in the systems. A comprehensive analysis of the structural stability based on the maximum bond length of the trigger bond (Lmax), the interaction energy between two N atoms of the NN trigger bond (EN-N), and mechanical properties indicates that EEF can effectively improve the stability of the systems. These findings provide basic insights into the comprehensive understanding of the effects of the EEF on the EMs.

Study on the isothermal decomposition of CL-20/HMX co-crystal by microcalorimetry

The thermal decomposition behavior and kinetics of CL-20/HMX co-crystal were studied by microcalorimetry at the temperature interval of 448.15–468.15 K. The thermal stability of CL-20/HMX co-crystal is mediated by the individual thermal stabilities of pure CL-20 and HMX. CL-20 has a catalytic effect on HMX decomposition. The heat flow curve of CL-20/HMX co-crystal is not a single exothermic peak, and there is an obvious inflection point when the degree of decomposition α = 50 %. The mechanism function and kinetic parameters of CL-20/HMX co-crystal are obtained. Before the inflection point, the model conforms to the Avrami-Erofeev equation, and the activation energy is 166.80 kJ mol−1. After the inflection point, the model conforms to the Jander model, and the activation energy is 174.43 kJ mol−1. The thermal decomposition process of CL-20/HMX co-crystal is not a simple superposition process, but has coupling feature. The decomposition residues are analyzed and identified by high-performance liquid chromatography, Fourier-transform infrared spectroscopy, and scanning electron microscopy, and the isothermal decomposition of CL-20/HMX co-crystal is extensively discussed.

Low-temperature heat capacities, standard molar enthalpies of formation and detonation performance of two CL-20 cocrystal energetic materials

Two cocrystals of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) with 2,4,6-trinitrotoluene (TNT) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) were prepared by a solvent/non-solvent (acetonitrile/distilled water) method at room temperature. The low-temperature heat capacities of CL-20∙TNT cocrystal and 2CL-20·HMX cocrystal were measured in the temperature range from 1.9 to 300 K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). By fitting the heat capacity data to a series of theoretical and empirical models, the thermodynamic functions were derived. In addition, the standard molar heat capacities, entropies and enthalpies of cocrystal samples at 298.15 K and 0.1 MPa were also obtained. The standard molar energies of combustion of CL-20, TNT, HMX, CL-20∙TNT cocrystal, and 2CL-20·HMX cocrystal were determined. From the combustion results, the standard molar enthalpies of combustion and formation for the compounds at T = 298.15 K were derived. The detonation velocity (D) and detonation pressure (P) were obtained by Kamlet-Jacobs equations. 2CL-20·HMX cocrystal shows superior detonation velocity and detonation pressure, which are higher than those of CL-20∙TNT cocrystal.

The influence of temperature and component proportion on stability, sensitivity, and mechanical properties of LLM-105/HMX co-crystals via molecular dynamics simulation.

Based on molecular dynamics (MD) simulation, the binding energy, cohesive energy density (CED), bond length, and mechanical parameters were calculated for 2,6-diamino-3,5-dinitropyrazine-l-oxide (LLM-105) crystal, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystal, and their co-crystals under different temperatures. Three LLM-105/HMX patterns were constructed to investigate the influence of component proportion on structures and properties of co-crystals, in which the mole ratios of LLM-105 and HMX are 1:1, 1:2, and 2:1. The effect of temperature and components on the stability and sensitivity were investigated as well. The results show that the binding energies, CED and mechanical parameters of all the co-crystals, decrease when the temperature increases from 248 to 398K, while their maximum N-NO2 bond length (Lmax) increases with rising temperature, indicating that the sensitivities increase and stabilities decrease when temperature rises. At all temperatures, co-crystals exhibit larger CED and shorter bond length than that of single explosive, demonstrating that they are more stable and less sensitive than single crystal, where the stability of co-crystals was ordered as 2:1>1:1>1:2. Moreover, the bulk modulus (K) and shear modulus (G) of co-crystals are lower than that of HMX, conversely, the Cauchy pressure and K/G are higher than that of HMX, implying co-crystals have better ductility. Finally, the 2:1 ratio of LLM-105/HMX co-crystal was identified as the excellent one, owning to the highest binding energy, highest CED, shortest Lmax, and greatest ductility. Graphical Abstract Models of LLM-105/HMX and one of the properties.

The isothermal decomposition of a CL-20/HMX co-crystal explosive

The isothermal decomposition process of CL-20/HMX co-crystals was studied through an isothermal decomposition dynamics research method.

The catalytic effects of nano-Fe2O3 and rGO–Fe2O3 on the thermal decomposition properties of CL-20/HMX cocrystals

Adding nano-Fe2O3 to a CL-20/HMX cocrystal affects its thermal decomposition peak temperature and products.

Application and Properties of CL‐20/HMX Cocrystal in Composite Modified Double Base Propellants

AbstractThe composite modified double base (CMDB) propellants containing 2,4,6,8,10,12‐hexanitro‐2,4,6,8,10,12‐hexaazatetracyclo[5.5.0.05,9.03,11] dodecane (CL‐20), 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (HMX) or CL‐20/HMX cocrystal were prepared by the solventless extrusion formation technique. The performances of CMDB propellants were evaluated by means of stability, sensitivity, combustion and mechanical experiments. Raman spectra analysis indicated the stability of CL‐20/HMX cocrystal is partially destroyed in the rolling process under 90 °C, 10 MPa. Methyl violet tests revealed that the colored time of CMDB propellants containing CL‐20/HMX cocrystal, CL‐20 or HMX are 77, 60, and 75 min, respectively. Burning rate tests shown that the burning rates of CMDB propellants containing CL‐20/HMX cocrystal, CL‐20 or HMX in the pressure range of 8 to 18 MPa, increased 1.82, 4.67, and 4.67 mm/s, with the pressure exponents 0.095, 0.306, and 0.346, respectively. Mechanical property tests revealed that the mechanical properties of CMDB propellant containing CL‐20/HMX cocrystal are inconsistent, but the tensile strength has improved compare with CMDB propellant containing CL‐20. These results suggest that CL‐20/HMX cocrystal is a potential material to solve the application problem of the safety performance and stability of the propellants containing CL‐20.

Investigation Of Crystallisation Conditions to Produce CL‐20/HMX Cocrystal for Polymer‐bonded Explosives

AbstractSince its discovery in 2012, multiple techniques to generate the CL‐20/HMX cocrystal have been published. However, as yet no assessment or trial has been reported of crystallisation methods capable of producing the cocrystal in a size region and production scale suitable for its use in polymer bonded explosives (PBX). This paper provides insight into the selection of suitable crystallisation methods, solvent selection and process optimisation with a focus on the efficient production of high‐quality cocrystals for use in PBX. Through extensive solvent screening, acetonitrile was identified as the best solvent for solution‐based crystallisation, due to its capability to produce compact parallelepipedic crystals and its comparably wide cocrystal phase region. Crystallisation conducted at 60 °C was found to increase the conversion rate and the material efficiency compared to room temperature. By application of an advanced seeding procedure, high‐quality cocrystals in the size region of 180–250 μm were produced in laboratory‐scale antisolvent and cooling crystallisations. By pilot‐plant‐scale batch reaction cocrystallization, cocrystals with a volume‐weighted mean diameter of 33 μm were produced in quantities of 250 g per day.