Solid Explosives Research Articles

Identification of initial decomposition reactions in liquid-phase HMX using quantum mechanics calculations

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, (HMX), also known as octogen, is a commonly used ingredient in solid propellants and explosives. As a result, many experimental and theoretical studies have investigated HMX to elucidate its liquid-phase and gas-phase decomposition behavior. In various experimental efforts, including both fast and slow thermolysis, the results indicate that ring opening occurs very early due to the presence of carbon-containing species, such as HCN, H2CO, HNCO, CO and CO2. After a lengthy search using quantum mechanics, ring-opening and subsequent reactions have been identified which may occur in both liquid and gas phases. Two existing pathways – 1) HONO elimination and 2) NNO2 homolysis – are re-examined for liquid-phase decomposition, and early ring-opening reactions have been identified. Three additional pathways – 1) reaction with NO and formation of ONTNTA, 2) prompt oxidation via HONO and ONNO2 addition, and 3) hydrogen abstraction via NO2– are also identified along with early ring-opening reactions in each pathway. The proposed pathways for decomposition of HMX are similar to that of RDX presented earlier [37]. The quantum mechanics calculations are performed using density functional theory (DFT) at the B3LYP/6-311 + + G(d,p) level. Conductor-like Polarizable Continuum Model (CPCM) is used to account for solvation effects with water as solvent within the Gaussian program package. Intrinsic reaction coordinate calculations have also been performed to verify that the reactants are indeed connected to the expected products. Similar to RDX, HONO elimination appears to be the initiation reaction followed by early ring-opening reactions. Results also indicate simultaneous formation of formaldehyde and N2O from early ring-opening reactions. Competing bimolecular reactions with NO2, HONO and ONNO2 are also investigated in the liquid phase, which explain the experimentally observed autocatalytic behavior. These reactions can assist in the development of a detailed liquid-phase chemical kinetics mechanism of nitramine propellants containing RDX and HMX.

Internal sub-sonic burning during an explosion viewed via dynamic X-ray radiography

We observe internal convective and conductive burn front propagation and solid consumption subsequent to thermal ignition for plastic bonded formulations of the solid organic secondary explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-triamino-2,4,6-trinitrobenzene. This work describes x-ray radiographic diagnostics enabling the study of solid density in a fully encased explosive during internal burning subsequent to ignition. The result of this study is the ability to directly observe and measure rates of energy release during a thermal explosion.

Snowpack response to directed gas explosions on level ground

The artificial release of avalanches is an important mitigation measure in avalanche control. The explosion to trigger an avalanche is either produced by igniting solid (or liquid) explosives or a gas mixture. Whereas there have been several studies on the impact of explosives, there is little research on the effect of directed gas explosions on a snowpack. We performed experiments with a prototype gas exploder above snow and measured air pressure at different distances from the point of explosion and accelerations within the snowpack. By measuring along different directions from the point of explosion we assessed the lateral propagation of the pressure wave caused by the directed explosion. Air pressure decreased distinctly with distance from the point of explosion. For example, air pressure was about (6.0±0.2) kPa at 20m and (0.59±0.02) kPa at 80m with 1.8kg of propane‑oxygen gas mixture. Within a forward cone of half angle of about 37°, the impact was independent of the direction from the exploder axis. Within the snowpack, accelerations decreased distinctly with depth and distance from the point of explosion as it is observed with explosives. The frequency content of the air pressure signal of the directed gas explosion was similar compared to experimental results previously obtained with solid explosives. We conclude that in the gas exploder axis, the impact of a directed gas explosion is comparable to an explosion with solid explosives with similar energy density. Hence, gas explosions are well suited to artificially trigger snow avalanches. In the future, side-by-side experiments will be needed to further analyze differences and similarities between the effect of gas and solid explosives. Moreover, additional measurements at operational gas exploders will allow further validation of the experimental results.

Energy estimation and modeling solid thermal explosion containment on reactor for three organic peroxides by calorimetric technique

In this paper, we describe thermokinetic properties and decomposition characteristics of benzoyl peroxide, dicumyl peroxide, and lauroyl peroxide, which are widely used in the polymerization process as energy boosters. In the past, many accidents occurred that involved overpressure and runaway excursion of the process and thermal explosion. One reason for accidents is because of the peroxy group (–O–O–) of organic peroxides (OPs) due to its thermal instability and high sensitivity when exposing to heat. Apparent activation energy and pre-exponential factor were obtained during decomposition via non-isothermal well-recognized kinetic equation, fitting curve tests, and approximate solution to design safer reaction conditions when OPs are used as fuel. Moreover, the storage conditions were investigated for the simulation of thermal explosion in a 24-kg cubic box package and a 400-kg barrel reactor for commercial application. Experimental results established the novel features of solid explosion hazard of OPs.

A parametric study of the dynamic failure of energetic composites

Heating by frictional sliding of cracks is often considered to be one of the most important causes of localized melting and ignition in solid explosives. Furthermore, recent high speed X-ray phase contrast experiments on energetic composites under dynamic compression [Parab et al., Appl. Phys. Lett. 109(13) (2016)] show that most fracture events appear inside the particles. Initial cracks develop in regions where particles are close, and widespread fragmentation is observed in the interior of the particles as the stress waves propagate through the sample. However, most simulations have focused on interface debonding of energetic composites and, in general, do not include fracture of the particles explicitly. A phase field damage approach is used to model the dynamic response of a system of cyclotetramethylene-tetranitramine particles embedded in a Sylgard matrix. The simulations show several damage mechanisms observed in the experiments. The effects of the energy release rate and the initial crack distribution on the energy dissipation due to fracture are studied. The numerical results confirm that initial cracks play an important role in the evolution of damage, energy dissipation and consequently, the formation of hot-spots.

Performance and Kinetics Study of Self-Repairing Hydroxyl-Terminated Polybutadiene Binders Based on the Diels⁻Alder Reaction.

Based on the Diels–Alder reaction and hydroxyl-terminated polybutadiene (HTPB) binders of solid propellants, two novel compounds—furfuryl-terminated polybutadiene (FTPB) and trifurfuryl propane (TFP)—were designed and synthesized, and their structures were characterized. A new kind of reversible Diels–Alder reaction system was formed by FTPB as main resin, N,N′-1,3-phenylenedimaleimide (PDMI) as curing agent and TFP as chain extender. The results showed that this system had good mechanical properties with a tensile strength of 1.76 MPa and a tensile strain of 284% after curing, and the repair efficiency of the crack was 88%. Therefore, it could be used as a novel binder of energetic materials such as solid propellant and PBX explosives to provide them with self-repairing characteristics and improve the reliability for application.

High-resolution simulations of cylindrical void collapse in energetic materials: Effect of primary and secondary collapse on initiation thresholds

Void collapse in solid explosives leads to hot spot formation and enhanced sensitivity. Here a grid resolution study is presented for a two-dimensional circular void for various shock strengths. We find that mesoscale simulations of voids may contain significant predictive uncertainty.

Micro-fibre based Porous Composite Propellants with High Regression Rates

Harnessing energy at micro-scale from high energy sources has gained significance in recent times for space propulsion and other applications. Conventional solid rocket propellants have advantages in terms of being efficient, compact and safe to handle, though with much lower regression rates as compared to solid explosives. An approach to high regression rates in composite propellants is demonstrated in the present work by the enhancement of fuel-oxidiser interaction, and by the incorporation of micro-scale porosity into the propellant grain. The porous polystyrene-ammonium perchlorate grain designed in this work, based on electrospun micro-fibres and aqueous impregnation, exhibits burning rates more than 25 times as compared to the non-porous grain. Such high regression rates using insensitive propellant compositions have practical implications in the development of micro-thrusters, and in gas generating devices such as MAV launch systems and turbine starters. Detailed preparatory procedure, characterisation techniques, and flame regression studies are included in this paper.

Long-Wave Infrared (LWIR) Molecular Laser-Induced Breakdown Spectroscopy (LIBS) Emissions of Thin Solid Explosive Powder Films Deposited on Aluminum Substrates.

Thin solid films made of high nitro (NO2)/nitrate (NO3) content explosives were deposited on sand-blasted aluminum substrates and then studied using a mercury-cadmium-telluride (MCT) linear array detection system that is capable of rapidly capturing a broad spectrum of atomic and molecular laser-induced breakdown spectroscopy (LIBS) emissions in the long-wave infrared region (LWIR; ∼5.6-10 µm). Despite the similarities of their chemical compositions and structures, thin films of three commonly used explosives (RDX, HMX, and PETN) studied in this work can be rapidly identified in the ambient air by their molecular LIBS emission signatures in the LWIR region. A preliminary assessment of the detection limit for a thin film of RDX on aluminum appears to be much lower than 60 µg/cm2. This LWIR LIBS setup is capable of rapidly probing and charactering samples without the need for elaborate sample preparation and also offers the possibility of a simultaneous ultraviolet visible and LWIR LIBS measurement.

Quantum mechanics investigation of initial reaction pathways and early ring-opening reactions in thermal decomposition of liquid-phase RDX

Cyclotrimethylene trinitramine (RDX) is a commonly used ingredient in solid propellants and explosives. As a result, RDX has been the subject of many experimental and theoretical investigations to elucidate its liquid-phase and gas-phase decomposition. In our experimental effort involving Fourier Transform Infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToFMS) of fast thermolysis of RDX, the results indicate that ring-opening occurs very early due to the presence of carbon-containing species among the detected gas-phase species, such as HCN, H2CO, CO and CO2. Two existing pathways – 1) HONO elimination and 2) NNO2 homolysis – are re-examined for liquid-phase decomposition and early ring-opening reactions have been identified after a lengthy search using quantum mechanics. Three additional pathways – 1) reaction with NO and formation of ONDNTA, 2) prompt oxidation via HONO and ONNO2 addition, and 3) hydrogen abstraction via NO2– are also identified along with early ring-opening reactions in each pathway. The quantum mechanics investigation is based on using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. The liquid-phase studies use the Conductor-like Polarizable Continuum Model (CPCM) for solvation with water as solvent within the Gaussian program package. Intrinsic reaction coordinate calculations have also been performed to verify that the reactants indeed are connected to the expected products. Results explain the mechanistic details of the formation of early carbon containing species, simultaneous formation of formaldehyde and N2O, as well as the formation of minor species such as HNCO and HOCN. Proposed reactions can account for the experimentally observed autocatalytic behavior and can assist in the development of a detailed chemical kinetics mechanism of nitramine propellants containing RDX and possibly HMX.

Interaction of the conical impactor with barriers containing an explosive

This paper presents the actual problem of the dynamic interaction of a high-speed impactor with a screened explosive. The interaction mathematical model is based within the framework of continuum mechanics considering the mechanism of the shock-wave initiation of detonation in solid explosives. Testing of the method was conducted using experimental data in one dimension. Some problems of interaction of the conical impactor with the explosive protected one-layered and spaced metal barriers are numerically considered.

Analytical method for the estimation of transfer and detection efficiencies of solid state explosives using ion mobility spectrometry and smear matrix

Smearing method can be used as a simple and convenient sampling method for detection of trace explosives using a portable ion mobility spectrometer (IMS) in the field such as for security checks.

INVESTIGATING EXPLOSIVE SENSITIVITY TO ELECTROSTATIC DISCHARGE ACCORDING TO NATO STANDARDIZATION DOCUMENTS

Some small scale methods for testing the sensitivity against ESD (Electro Static Discharge) of solid explosives like primary explosives, booster explosives, high explosives, gun and rocket propellants (smokeless powders and solid rocket propellants) and pyrotechnic compositions are overviewed in the paper from the point of NATO standardisation documentation that includes Standard Agreement (STANAG) 4490 [1], Allied Ordnance Publication AOP-7 [2] and STANAG 4170 [3]. The overview is supplemented with analyses and assessments of methods and received results. ESD sensitivity tests of explosives in small scale have not been unified yet in NATO standardization documentation in contradiction to NATO large scale tests. Such unification would be beneficial as it could lead to a greater reliability of results of explosive ESD susceptibility tests performed e.g. in the frame of interlaboratory tests.

Phase Transition in Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under Static Compression: An Application of the First-Principles Method Specialized for CHNO Solid Explosives.

The first-principles method is challenged by accurate prediction of van der Waals interactions, which are ubiquitous in nature and crucial for determining the structure of molecules and condensed matter. We have contributed to this by constructing a set of pseudopotentials and pseudoatomic orbital basis specialized for molecular systems consisting of C/H/N/O elements. The reliability of the present method is verified from the interaction energies of 45 kinds of complexes (comparing with CCSD(T)) and the crystalline structures of 23 kinds of typical explosive solids (comparing with experiments). Using this method, we have studied the phase transition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under static compression up to 50 GPa. Kinetically, intramolecular deformation has priority in the competition with intermolecular packing deformation by ∼87%. A possible γ → β phase transition is found at around 2.10 GPa, and the migration of H2O has an effect of kinetically pushing this process. We make it clear that no β → δ/ε → δ phase transition occurs at 27 GPa, which has long been a hot debate in experiments. In addition, the P-V relation, bulk modulus, and acoustic velocity are also predicted for α-, δ-, and γ-HMX, which are experimentally unavailable.

Effects of hypervelocity impact on ablation behavior of SiC coated C/C composites

SiC coated C/C (SiC-C/C) composites were designed to determine effects of hypervelocity impact on ablation behavior of coated carbon/carbon (C/C) composites. They were impacted at 2km/s and 3km/s by steel projectiles driven by solid explosive of pentaerythritol tetranitrate and then ablated by oxyacetylene torch for 100s at 2500°C. Results showed that ablation areas, the integrity of ablated boundary and ablation rates of all samples increased with the increase of impact velocity. Mass ablation rates of SiC-C/C composites were lower than those of C/C composites under the same test condition. On the contrary, linear ablation rates of SiC-C/C composites were higher than those of C/C composites after the same impact tests. The difference was attributed to the interfaces between SiC coatings and matrix C/C composite. These results demonstrated that morphology of interfaces between coatings and matrix C/C composite was one major factor for the usage of coated C/C composites and could affect the service time of structures with impact damages in high temperature environments.

Detonability of aqueous suspensions of explosives

The detonability of aqueous suspensions of solid explosives (pentaerythritol tetranitrate, cyclotrimethylenetrinitramine, cyclotetramethylenetetranitramine) was used as a basis for evaluating the explosion safety of these substances in production and transportation. The dependence of the critical detonation parameters of the suspensions on the content and dispersity of the explosives was determined. A new common method for calculating the critical detonation diameters of suspensions of crystalline explosives in organic media and in water was developed. A novel procedure for preparing uniform sedimentation-resistant aqueous suspensions of crystalline explosives was suggested and developed.

Many-body effects in the optical absorption of lithium azide (LiN3)

Until recently most of the understanding achieved for solid explosives has been obtained using various semi-empirical approaches due to a major role of excitonic effects in the mechanisms of decomposition. Nevertheless, during the last two decades, thanks to the ongoing progress in iterative computational methods, the inclusion of the electron-hole interaction in ab initio calculations has become a standard approach in solid-state theory. In this paper, the electronic structure and optical properties of bulk lithium azide are investigated, taking into account the electron-hole interaction via the Bethe–Salpeter equation (BSE). Here, we employ the kernel polynomial method (KPM), which significantly reduces the computational cost compared to direct diagonalization methods. The calculations of the imaginary part of the polarization dependent dielectric function including excitonic effects are reported for the first time. Then, we show a density map of the two-particle wave function and propose an alternative interpretation of the initial stages of the externally triggered chemical decomposition, based on the analysis of two-particle states near the absorption edge.

Influence of Small Change of Porosity on Shock Initiation of an HMX/TATB/Viton Explosive and Ignition and Growth Modeling

All solid explosives in practical use are more or less porous. Although it is known that the change in porosity affects the shock sensitivity of solid explosives, the effect of small changes in porosity on the sensitivity needs to be determined for safe and efficient use of explosive materials. In this study, the influence of a small change in porosity on shock initiation and the subsequent detonation growth process of a plastic-bonded explosive PBXC03, composed of 87% cyclotetramethylene-tetranitramine (HMX), 7% triaminotrinitrobenzene (TATB), and 6% Viton by weight, are investigated by shock to detonation transition experiments. Two explosive formulations of PBXC03 having the same initial grain sizes pressed to 98 and 99% of theoretical mass density (1.873 g/cm3) respectively are tested using the in situ manganin piezoresistive pressure gauge technique. Numerical modeling of the experiments is performed using an ignition and growth reactive flow model. Reasonable agreement with the experimental results is obtained by increasing the growth term coefficient in the Lee-Tarver ignition and growth model with porosity. Combining the experimental and simulation results shows that the shock sensitivity increases with porosity for PBXC03 having the same explosive initial grain sizes for the pressures (about 3.1 GPa) applied in the experiments.

First principles investigations of structural, electronic and elastic properties of ammonium perchlorate under high pressures

Ammonium perchlorate (NH4ClO4) is a highly energetic oxidizer widely used in solid propellants and explosives. Under extreme pressure conditions, significant changes are observed in the structures and properties of NH4ClO4. However, many studies of structural transformations of NH4ClO4 under high pressures have not formed a more consistent conclusion. In this study, the structural, electronic, and elastic properties of NH4ClO4 are investigated by first-principles calculations based on the density functional theory with dispersion correction (DFT-D) method in a range of 0-15 GPa. The unit cell volume and lattice parameters are optimized by GGA/PBE-TS, which leads to good agreement with the experimental structure parameters at 0 GPa, suggesting the reliability of the present calculation method. The calculated P-V data are fitted to the third-order Birch-Murnaghan equation of state, and the result provides better agreement with experimental result than other calculations for the unit cell with a volume V0 and bulk moduli B0 and B'. The comprehensive analyses of the lattice parameters, bond lengths, and hydrogen bonds under high pressure indicate that three structural transformations occur in NH4ClO4 at 1 GPa, 4 GPa, and 9 GPa. With increasing pressure, hydrogen bonding interaction gradually increases, and intra- and intermolecular hydrogen bonds are present in crystals. Results obtained from the band structures and state densities under high pressure indicate that NH4ClO4 exhibits good insulating properties. Valence band shifts towards low energy, conduction band shifts towards high energy, and electronic localization is enhanced. The charge density differences and Mulliken charge populations at different pressures reveal that the covalent interaction between the N-H and Cl-O bonds increases, and the ionicity of crystal decreases. The band gaps in different structural transition regions exhibit different linear increase trends with increasing pressure. The calculated elastic constants of NH4ClO4 satisfy elastic stability criteria of orthorhombic systems at pressures ranging from 0 GPa to 15 GPa, indicating that NH4ClO4 is mechanically stable. The bulk modulus, shear modulus, and Young's modulus are estimated by the Voigt-Reuss-Hill approach. The Cauchy pressures and B/G values indicate that NH4ClO4 exhibits ductility, attributed to the fact that NH4ClO4 is an ionic crystal, and ionic bonds are non-directional bonds; hence, NH4ClO4 is ductile and can be easily bended or reshaped. The results indicate that the ductility properties of NH4ClO4 increase with increasing pressure. All calculated properties are in excellent agreement with the available experimental results. These results will not only help to understand the structural transformations of NH4ClO4 under high pressures but also provide an important theoretical reference for the safe application of NH4ClO4 in solid propellants and explosives.

Modeling and simulation of preshock desensitization in heterogeneous explosives using a mesoscopic reaction rate model

To understand and predict the response of explosive materials, numerical models are utilized to simulate various scenarios. Various hazard and vulnerability scenarios for explosives involve multiple shock compression. In the present study, a kind of mesoscopic model for shock ignition of solid heterogeneous explosives is examined in order to demonstrate its availability to account for the desensitization caused by multiple shocks in explosives. Since the mesoscopic model is based on the assumption of the elastic viscoplastic pore collapse mechanism, and on the other hand the desensitization mechanism is also usually described in connection with the closure of pores, the ability of the mesoscopic model to predict the desensitization effects must be analyzed. For this purpose, the mesoscopic model has been implemented in a hydrodynamic code (LS-DYNA) as a user-defined equation of state. For verification, the double shock, reflected shock and detonation-quenching experiments have been modeled and simulated. The data show that the model can reproduce various features of some of the previously reported experiments involving the preshock desensitization of solid explosives.