Decomposition Of CL-20 Research Articles

Nanochromates MCr2O4 (M = Co, Ni, Cu, Zn): Preparation, Characterization, and Catalytic Activity on the Thermal Decomposition of Fine AP and CL-20.

The chromate nanoparticles such as CoCr2O4, NiCr2O4, CuCr2O4, and ZnCr2O4 were prepared by a modified sol–gel method. The structural and morphological properties of the chromate nanoparticles were characterized by X-ray diffraction (XRD) and a scanning electron microscope (SEM). The catalytic effects of chromates on the thermal decomposition of fine ammonium perchlorate (FAP) and hexanitrohexaazaisowurtzitane (CL-20) were studied by the TG-DTG measurements. Results show that the addition of CoCr2O4, NiCr2O4, CuCr2O4, and ZnCr2O4 nanoparticles makes the activation energies for the thermal decomposition of FAP decrease by 93.9, 94.0, 83.1, and 67.0 kJ·mol–1, and the thermal decomposition temperatures decrease by 45.0, 24.9, 57.7, and 38.8 K respectively. On the other hand, a similar trend exists in the case of the thermal decomposition of CL-20. Therefore, the addition of nanochromates shows high catalytic efficiency on the thermal decomposition of both FAP and CL-20 components, which would be beneficial to promote the burning rate of propellants containing FAP and CL-20 components.

Differences of thermal decomposition behaviors and combustion properties between CL-20-based propellants and HMX-based solid propellants

Differences of thermal decomposition characteristics and combustion properties between CL-20-based propellants and HMX-based propellants were researched by combination of theory and practice. Burning rates and burning rate pressure exponents of CL-20-based propellants were much higher than those of HMX-based propellants, when contents and particle sizes of CL-20 and HMX were identical. The thermal decomposition of CL-20 and CL-20-based propellants was systematically studied by comparison of HMX and HMX-based propellants. HMX melted firstly at 198.53 °C and then decomposed violently at 284.3 °C, while CL-20 only decomposed violently at 246.9 °C without any melting peak, and the heat released from decomposition of CL-20 was much higher than that of HMX. Thermal decomposition of CL-20 or HMX could be greatly enhanced by GAP. CL-20 could accelerate the high-temperature decomposition of AP, whereas the decomposition of HMX was greatly enhanced by AP. Mole ratio of [NO2]/[N2O] in decomposition gas products of CL-20 was 3.07, whereas the result for HMX was 0.43. More oxidative gases were generated for CL-20 or CL-20-based propellants. Molar reaction heat in luminescent flame zone and dark zone for CL-20-based propellants was much higher than that for HMX-based propellants, resulting in a higher burning rate for CL-20-based propellants. Value of [NO2]/[N2O] for gas-phase products of thermal decomposition of CL-20/ammonium perchlorate (AP) mixed system was also much higher than of HMX/AP mixed system, resulting in more oxidative gases, such as NO2, involved in the thermal decomposition and combustion of CL-20-based propellants with ammonium perchlorate, further leading to higher burning rates of CL-20-based propellants.

Reaction Mechanism of Embedding Oxidizing Small Molecules in Energetic Materials to Improve the Energy by Reactive Molecular Dynamics Simulations

Novel host–guest/multicomponent energetic materials can be obtained by embedding hydrogen- or nitrogen-containing oxidizing small molecules between the molecules of high-energy explosives, which can improve their explosive energy. To better understand the mechanism of oxidizing small molecules in the reaction and improve the energy, ReaxFF-lg reactive molecular dynamics simulations were performed to investigate the thermal decomposition reaction at different temperatures of the CL-20/H2O2 solvate formed by embedding H2O2 in the cavity of CL-20. We propose an analytical method to investigate the mechanism of H2O2 in the CL-20 reaction by tracing the interactions between the H and O atoms of H2O2 and the C, H, N, and O atoms of CL-20. During thermal decomposition of CL-20/H2O2, CL-20 and H2O2 first separately decompose, and then, the decomposition products react. The H atoms, O atoms, and hydroxyl (HO) groups generated by H2O2 decomposition connect with the O atoms of nitro groups, leading to N–O bond cleav...

Thermal properties of decomposition and explosion for CL-20 and CL-20/n-Al

ABSTRACTNano-sized aluminum powders (n-Al) are usually added to high energy explosives and propellants o promote their detonation and combustion performance. From the perspective of thermal safety, this work focused on the thermal decomposition and explosion properties of CL-20 in the absence and presence of 30 wt% n-Al (CL-20/n-Al). The results of DSC experiments showed that the activation energy () of thermal decomposition of CL-20 slightly decreased upon addition of n-Al, implying that n-Al exhibited a certain degree of catalytic effect on the thermal decomposition process of CL-20, but the catalytic activity is quite low. By nonlinear multivariate regression method, the kinetic model for the thermal decomposition of CL-20 and CL-20/n-Al was determined as n-th order reaction with autocatalysis, and both the reaction order (n) and autocatalytic kinetic rate constant () of CL-20 were found to be lower than those of CL-20/n-Al, indicating that the complexity of the decomposition reaction system and autocatalytic ability of CL-20 during thermal decomposition increased upon addition of n-Al. By the deflagration point tests, the Five-Second explosion temperature () of CL-20 and CL-20/n-Al was determined to be 277.7°C and 282.3°C, respectively. Moreover, the activation energy () of thermal explosion of CL-20 and CL-20/n-Al were calculated as 159.98 kJ mol−1 and 100.7 kJ mol−1, respectively. From the more than one-third reduction of value, it can be concluded that the thermal sensitivity of CL-20 explosive could be remarkably decreased by adding n-Al, especially under the low-temperature condition.

Directional assembly of flowerlike maghemite on graphene and its catalytic activity for the thermal decomposition of CL-20

Metal oxides/graphene composites are potential solid propellant combustion catalysts owing to their complementary property between two components. Herein, an in-situ growth method was used for preparing γ-Fe 2 O 3 /graphene composites (γ-Fe 2 O 3 /G) with intimately coupled graphene sheets and flowerlike γ-Fe 2 O 3 nanoparticles (NPs). The in-situ growth strategy can not only inhibit the aggregation of γ-Fe 2 O 3 NPs, but also reduce the particle size of γ-Fe 2 O 3. As an efficient catalyst for the thermal decomposition of hexanitrohexaazaisowurtzitane (CL-20), γ-Fe 2 O 3 /G2 (2 represents the graphene concentration is 2 mg/mL) decreases the thermal decomposition temperature of CL-20 from 252.13 °C to 245.20 °C and the apparent activation energy from 183.02 kJ/mol to 150.68 kJ/mol, respectively. The improved catalytic activity may be attributed to the synergistic effect between γ-Fe 2 O 3 and graphene, which can efficiently construct the three-dimensional framework structure for exposing more active sites.

The Effect of Microsized Aluminum Powder on Thermal Decomposition of HNIW (CL-20)

DSC experiments were conducted on the 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-tetracyclo-[5.5.0.05,9.03,11]-dodecane (also known as HNIW or CL-20) and CL-20 containing 10 wt.% microsized aluminum (Al) powders. The kinetic parameters of CL-20 and CL-20/Al were obtained by ASTM685 and Friedman methods, respectively, indicating that Al powder decreases the activation energy of CL-20 slightly and has a catalytic effect on the thermal decomposition of CL-20. By the method of nonlinear multivariate regression, kinetic models of CL-20 and CL-20/Al were derived as fα=1−αn1+kcat⋅α, where the lgkcat of CL-20 and CL-20/Al is 1.97 and 2.12, respectively, showing that the autocatalytic ability of CL-20 had been increased by adding Al powder. From the SEM images of CL-20/Al and the XRD pattern of the decomposition residues of CL-20/Al, it can be inferred that partial combustion of Al particles happened in the microscale view and led to the release of heat.

Kinetic model of thermal decomposition of CL-20/HMX co-crystal for thermal safety prediction

To promote the practical application of CL-20/HMX co-crystal, the understanding of its thermal decomposition kinetics and thermal hazard prediction are highly required. In this study, the kinetic model was evaluated based on the non-isothermal DSC data by using non-linear optimization method, which was identified as a complicated reaction comprising two parallel autocatalytic paths, and the contribution of the two reaction paths was revealed to vary depending on the heating rate. Based on the kinetic model, the thermal hazard simulation indicates that the temperature when the occurrence of thermal decomposition after 24 h (Td,24) of CL-20/HMX co-crystal is 151.64 °C, and the critical temperature of 1000th second explosion is determined as ˜196 °C. Besides, simulation results of self-accelerating decomposition temperature demonstrate that the package mass of CL-20/HMX co-crystal, rather than the package material, has a remarkable effect on the thermal safety of CL-20/HMX co-crystal.

Thermal Decomposition Mechanism of CL-20 at Different Temperatures by ReaxFF Reactive Molecular Dynamics Simulations.

Hexanitrohexaazaisowurtzitane (CL-20) has a high detonation velocity and pressure, but its sensitivity is also high, which somewhat limits its applications. Therefore, it is important to understand the mechanism and characteristics of thermal decomposition of CL-20. In this study, a ε-CL-20 supercell was constructed and ReaxFF-lg reactive molecular dynamics simulations were performed to investigate thermal decomposition of ε-CL-20 at various temperatures (2000, 2500, 2750, 3000, 3250, and 3500 K). The mechanism of thermal decomposition of CL-20 was analyzed from the aspects of potential energy evolution, the primary reactions, and the intermediate and final product species. The effect of temperature on thermal decomposition of CL-20 is also discussed. The initial reaction path of thermal decomposition of CL-20 is N-NO2 cleavage to form NO2, followed by C-N cleavage, leading to the destruction of the cage structure. A small number of clusters appear in the early reactions and disappear at the end of the reactions. The initial reaction path of CL-20 decomposition is the same at different temperatures. However, as the temperature increases, the decomposition rate of CL-20 increases and the cage structure is destroyed earlier. The temperature greatly affects the rate constants of H2O and N2, but it has little effect on the rate constants of CO2 and H2.

Reaction Mechanisms in the Thermal Decomposition of CL-20 Revealed by ReaxFF Molecular Dynamics Simulations

The thermal decomposition of condensed CL-20 was investigated using reactive force field molecular dynamics (ReaxFF MD) simulations of a super cell containing 128 CL-20 molecules at 800-3000 K. The VARxMD code previously developed by our group is used for detailed reaction analysis. Various intermediates and comprehensive reaction pathways in the thermal decomposition of CL-20 were obtained. Nitrogen oxides are the major initial decomposition products, generated in a sequence of NO2, NO3, NO, and N2O. NO2 is the most abundant primary product and is gradually consumed in subsequent secondary reactions to form other nitrogen oxides. NO3 is the second most abundant intermediate in the early stages of CL-20 thermolysis. However, it is unstable and quickly decomposes at high temperatures, while other nitrogen oxides remain. At all temperatures, the unimolecular pathways of N-NO2 bond cleavage and ring-opening C-N bond scission dominate the initial decomposition of condensed CL-20. The cleavage of the N-NO2 bond is greatly enhanced at high temperatures, but scission of the C-N bond is not as favorable. A bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 is observed in the initial decomposition steps of CL-20, which should be considered as one of the major pathways for CL-20 decomposition at low temperatures. After the initiation of CL-20 decomposition, fragments with different ring structures are formed from a series of bond-breaking reactions. Analysis of the ring structure evolution indicates that the pyrazine derivatives of fused tricycles and bicycles are early intermediates in the decomposition process, which further decompose to single ring pyrazine. Pyrazine is the most stable ring structure obtained in the simulations of CL-20 thermolysis, supporting the proposed existence of pyrazine in Py-GC/MS experiments. The single imidazole ring is unstable and decomposes quickly in the early stages of CL-20 thermolysis. Many C-4 and C-2 intermediates are observed after the initial fragmentation, but eventually convert into stable products. The distribution of the final products (N-2, H2O, CO2, and H-2) obtained in ReaxFF MD simulation of CL-20 thermolysis at 3000 K quantitatively agrees with the results of the CL-20 detonation experiment. The comprehensive understanding of CL-20 thermolysis obtained through this study suggests that ReaxFF MD simulation, combined with the reaction analysis capability of VARxMD, would be a promising method for obtaining deeper insight into the complex chemistry of energetic materials exposed to thermal stimuli.

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.

Enhanced Thermal Decomposition Properties of CL-20 through Space-Confining in Three-Dimensional Hierarchically Ordered Porous Carbon.

High energy and low signature properties are the future trend of solid propellant development. As a new and promising oxidizer, hexanitrohexaazaisowurtzitane (CL-20) is expected to replace the conventional oxidizer ammonium perchlorate to reach above goals. However, the high pressure exponent of CL-20 hinders its application in solid propellants so that the development of effective catalysts to improve the thermal decomposition properties of CL-20 still remains challenging. Here, 3D hierarchically ordered porous carbon (3D HOPC) is presented as a catalyst for the thermal decomposition of CL-20 via synthesizing a series of nanostructured CL-20/HOPC composites. In these nanocomposites, CL-20 is homogeneously space-confined into the 3D HOPC scaffold as nanocrystals 9.2-26.5 nm in diameter. The effect of the pore textural parameters and surface modification of 3D HOPC as well as CL-20 loading amount on the thermal decomposition of CL-20 is discussed. A significant improvement of the thermal decomposition properties of CL-20 is achieved with remarkable decrease in decomposition peak temperature (from 247.0 to 174.8 °C) and activation energy (from 165.5 to 115.3 kJ/mol). The exceptional performance of 3D HOPC could be attributed to its well-connected 3D hierarchically ordered porous structure, high surface area, and the confined CL-20 nanocrystals. This work clearly demonstrates that 3D HOPC is a superior catalyst for CL-20 thermal decomposition and opens new potential for further applications of CL-20 in solid propellants.

Thermal decomposition of CL-20 via a self-modified dynamic vacuum stability test

The thermal decomposition of 2, 4, 6, 8, 10, 12-hexanitro-2, 4, 6, 8, 10, 12-hexaazaisowurtzitane (CL-20) is examined via a self-modified dynamic vacuum stability test at temperature intervals of 130–170 °C. The p versus t curves indicate the multistep thermal decomposition of CL-20 and its autocatalytic behavior. The reaction rate constant k is 3.07 × 10−8, 9.61 × 10−8, 27.25 × 10−8, 133.31 × 10−8, and 387.94 × 10−8 s−1 at 130, 140, 150, 160, and 170 °C, respectively. The polymorphic transformation of CL-20 and certain products may affect kinetic output. The activation energy is 182.5 kJ mol−1, which shows good agreement with previously published results. The storage life of CL-20 at 50, 70, 90, 110, and 130 °C are estimated using the Semenov equation. At 130 °C, the storage life of CL-20 is extrapolated to 18.14 h, 8.20, 21.22, and 27.30 days, while taking the reaction extents of 1, 2, 3, and 4%, respectively, as the end points of life. These values correspond to the values obtained in the present work.

Thermal decomposition properties and compatibility of CL-20 with binders HTPB, PBAN, GAP and polyNIMMO

The compatibility of filler 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) with rocket propellant binders: hydroxyl-terminated polybutadiene (HTPB), butadiene-acrylonitrile-acrylic acid terpolymer (PBAN), glycidyl azide polymer (GAP) and poly(3-nitratomethyl-3-methyloxetane) (polyNIMMO), has been examined. The compatibility of the compounds has been tested in accordance with the STANAG 4147 standard and its modification consisting in the change of the heating rate. As it arises from STANAG 4147 standard criterion, CL-20 is not compatible with polyNIMMO, PBAN and GAP and possibly incompatible with HTPB. Changes of relative position of peaks between measurements performed in hermetical pans and pans with a pinhole and with different heating rate were observed. In case of polyNIMMO and HTPB, changes of measurement parameters lead to estimated compatibility change. The analysis of the thermal decomposition of CL-20 revealed that it is a two-phase process. The first phase is associated with decomposition in solid phase; the second phase is associated with decomposition of volatile intermediate products of CL-20 decomposition. Due to the complex process of decomposition of tested samples, the apparent activation energy was used for the assessment of the compatibility. The apparent activation energy of the initial phase of decomposition CL-20 and its mixtures with binders are compatible with one another within the limits of measurement error. Results of measurement of apparent activation energy do not indicate a destabilizing effect of binders on the initial phase of decomposition of CL-20.

Thermal study of HNIW(CL-20) and mixtures containing aluminum powder

The thermal behavior of the energetic material 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-tetracyclo-[5.5.0.05,9.03,11]-dodecane (HNIW or CL-20) and its mixtures with aluminum under linear temperature control condition and adiabatic condition were investigated by DSC-TG-MS-FT-IR and ARC. Two different particle sizes of aluminum powder (10 μm and 50 nm) were added into CL-20. The influence of particle size on the thermal behavior of CL-20 was studied by using of these apparatuses. The enthalpies of reaction and onset temperatures were determined for various heating rates. The kinetic parameters were found according to Kissinger method, Ozawa method, and Friedman method based on DSC data. The gaseous products from the decomposition of CL-20 and its mixtures were determined by simultaneous MS-FT-IR experiments. ARC measurements were performed to investigate the thermal stability of the samples. The onset temperature, adiabatic temperature rise, self-heat rate, time to maximum rate, and pressure–temperature profile were found from the data measured by ARC. Based on these results, the catalytic effect of aluminum powder was studied.

Ab Initio Molecular Dynamics Study on the Initial Chemical Events in Nitramines: Thermal Decomposition of CL-20

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane or HNIW) is a high-energy nitramine explosive. To improve atomistic understanding of the thermal decomposition of CL-20 gas and solid phases, we performed a series of ab initio molecular dynamics simulations. We found that during unimolecular decomposition, unlike other nitramines (e.g., RDX, HMX), CL-20 has only one distinct initial reaction channelhomolysis of the N-NO2 bond. We did not observe any HONO elimination reaction during unimolecular decomposition, whereas the ring-breaking reaction was followed by NO 2 fission. Therefore, in spite of limited sampling, that provides a mostly qualitative picture, we proposed here a scheme of unimolecular decomposition of CL-20. The averaged product population over all trajectories was estimated at four HCN, two to four NO2, two to four NO, one CO, and one OH molecule per one CL-20 molecule. Our simulations provide a detailed description of the chemical processes in the initial stages of thermal decomposition of condensed CL-20, allowing elucidation of key features of such processes as composition of primary reaction products, reaction timing, and Arrhenius behavior of the system. The primary reactions leading to NO2, NO, N 2O, and N2 occur at very early stages. We also estimated potential activation barriers for the formation of NO2, which essentially determines overall decomposition kinetics and effective rate constants for NO2 and N2. The calculated solid-phase decomposition pathways correlate with available condensed-phase experimental data.

Studies on thermal decomposition mechanism of CL-20 by pyrolysis gas chromatography–mass spectrometry (Py-GC/MS)

The thermal decomposition study of CL-20 (hexanitrohexaazaisowurtzitane) using pyrolysis GC/MS was carried out mainly by electron impact (EI) mode. Chemical ionization (CI) mode was used for further confirmation of identified species. Mass spectrum of CL-20 decomposition products predominantly revealed fragments with m/ z 81 and 96 corresponding to C 4H 5N 2 + and C 4H 4N 2O + ions, respectively. The total ion chromatogram (TIC) of CL-20 pyrolysis shows peak within first 2 min due to the presence of low molecular weight gases. Peaks corresponding to several other products were also observed including the atmospheric gases. Cyanogen formation (C 2N 2, m/ z 52) observed to be enriched at the scan number 300–500. The low molecular mass range decomposition products formed by cleavage of C–N ring structure were found in majority. Additional structural information was sought by employing chemical ionization mode. The data generated during this study was instrumented in determining decomposition pathways of CL-20.

Thermal study of HNIW (CL-20)

The thermal properties of the energetic material 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-tetracyclo-[5.5.0.0 5,9.0 3,11]-dodecane (HNIW), also known as CL-20, have been investigated by DSC, TG, ARC, HFC and simultaneous TG–DTA–FTIR–MS. The solid–solid phase transitions and the reversibility of these phase transitions were explored using DSC. A heating rate study was performed and the DSC traces show the possibility of a multi-step thermal decomposition of CL-20. The enthalpies of reaction and onset temperatures were determined for various heating rates. The kinetic parameters were found by means of the ASTM E698 method and were verified by ageing experiments. A model-free kinetic (MFK) analysis was performed on the DSC heating rate results. TG experiments were conducted to investigate the effects of various heating rates on the mass loss of CL-20 and its onset temperature. The TG results were analyzed using MFK and ASTM E1641 methods. ARC measurements were performed to investigate the thermal stability of CL-20. The thermal behaviour of CL-20 at various pressures of argon was studied by heat flux calorimetry (HFC). The gaseous products from the decomposition of CL-20 were determined by simultaneous TG–DTA–FTIR–MS experiments in air and in helium. The thermal stability at different isothermal temperatures (190, 195, 200 and 205 °C) was studied, using the same technique.

STUDIES ON CL-20: THE MOST POWERFUL HIGH ENERGY MATERIAL

CL-20 is an attractive HEM having density (>2 g cm-3) and velocity of detonation (9400 m s-1) superior to HMX (1.9 g cm-3 and 9100 m s-1). During this study, CL-20 was synthesized to establish viability of efficient synthesis method. The compound synthesized at HEMRL was characterized by FTIR, 1H NMR and elemental analysis. Thermal studies (dynamic DSC and isothermal TG) were undertaken to determine kinetic parameters and understand the decomposition patterns. An attempt is made to explain the mechanism of decomposition of CL-20 on the basis of the data obtained by the authors and findings of other researchers. The activation energy values obtained during this work by adopting various approaches are close to the values reported for N-NO2 bond cleavage suggesting that it is global rate determining process rather than the collapse of cage structure. Mass spectra also provides evidences in this regard. Monitoring of decomposition products at high temperature supports these findings and brings out that NO2 initiates secondary decomposition processes because of entrapment in cage structure.

Comparative Investigation of Thermal Decomposition of Various Modifications of Hexanitrohexaazaisowurtzitane (CL-20)

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