Solid RDX Research Articles

Pressure and Polymorph Dependent Thermal Decomposition Mechanism of Molecular Materials: A Case of 1,3,5,-Trinitro-1,3,5,-triazine.

1,3,5,-Trinitro-1,3,5,-triazine (RDX) serves as an important energetic material and is widely used as various solid propellants and explosives. Understanding the thermal decomposition behaviors of various polymorphs of RDX at high pressure and high temperature is significantly important for safe storage and handling. The present work reveals the early thermal decay mechanisms of two polymorphs (α- and ε-forms) of RDX at high pressure and high temperature by ReaxFF reactive molecular dynamic simulations and climbing image nudged elastic band (CI-NEB) static calculations. It is found that the thermal decomposition rate has positive and negative effects on the pressure for α- and ε-RDX, respectively. This difference originates from the difference of pressure effect on the intermolecular H transfer of the two polymorphs, as we confirm that the bimolecular H transfer rather than the NO2 partition initiates the decay with a significantly lower energy barrier therein. This finding may be beneficial to understand the pressure and polymorph dependent effect on the decay of RDX and to develop a kinetic model for the combustion of solid RDX.

Enhanced shockwave-absorption ability of the molecular disorder rooting for the reactivity elevation of energetic materials

It is universally believed that the molecular disorder possesses a high reactivity compared to the molecular order and can start the ignition of shocked energetic materials (EMs) from defects, interfaces, and surfaces. However, the origin of this high reactivity is still not fully understood. To this end, this study performs a series of simulations of reactive molecular dynamics on a solid model and a liquid model of 1,3,5-trinitro-1,3,5-triazinane (RDX), which are the representatives of ordered and disordered energetic molecules with ReaxFF potentials, respectively. A total of five heating conditions and three shock conditions are considered. The high reactivity of the molecular disorder in liquid RDX is remarkably discriminated under shock, with the discrimination degree depending on shock strength, followed by adiabatic heating. This high reactivity is attributed to the increase in internal energy induced by stronger shock, which is originated from a high shockwave-absorption ability of the liquid RDX with amorphous structures compared to the solid RDX with ordered structures. Therefore, the high shockwave-absorption ability is just the root of the high reactivity of molecular disorder in defected EMs and usually starts the ignition. Thereby, for the first time, the high reactivity of molecular disorder in shocked EMs can be well understood through its high shockwave-absorption ability.

Laser ionization of solid RDX: a density functional theory study

Structural changes induced via ionization in an RDX lattice have been studied by using optimized [(RDX)2]0 conformers comprising eight combinations of four RDX isomers using DFT. Structures, dissociation enthalpy, and free energies of the resulting [(RDX)2]·+ clusters are calculated. The gas-phase and medium polarizable continuum model (PCM) calculations of the most stable [(RDX)2]·+ cluster produced similar structural results. Electron removal caused 75% of the [(RDX)2]·+ conformers to be unstable in their neutral isomeric composition and orientation. Charge distributions and structural factors indicate that [(RDX)2]·+ are in the general form RDX·+·RDX. Ionization causes charge polarization, hydrogen transfer, N-N dissociation, and assisted HONO formation in solid RDX. The assisted HONO formation occurs via and suggests hydrogen mobility within [(RDX)2]·+, causing a stabilization by a minimum of 114 kJ/mol more than the other conformers. The RDX conformational identity is a determining factor in the emerging dissociation pathways. The energy costs of ion-neutral dissociation are comparable to the hydrogen transfer and NO2 loss processes. Ionization of the RDX surface is expected to produce NO2 and HONO precursors of the NO+ ion observed previously.

Integrated rheology model: Explosive Composition B-3

ABSTRACTComposition B-3 (Comp B-3) is a high explosive formulation composed of 60/40wt% RDX (1,3,5-trinitroperhydro-1,3,5-triazine) /TNT (2,4,6 trinitrotoluene). Above approximately 78°C this formulation partially melts to form a multiphase system with solid RDX particles in a molten TNT matrix. This multiphase system presents a number of phenomena that influence its apparent viscosity. In an earlier study explosive Composition B-3 (Comp B-3, 60/40wt% RDX/TNT) was examined for evidence of yield stress using a non-isothermal falling ball viscometer and a yield stress model was proposed. An integrated viscosity model suitable for use in computational fluid dynamics (CFD) simulations is developed to capture the transition from a heterogeneous solid to a Bingham viscoplastic fluid. This viscosity model is used to simulate the motion of imbedded spheres falling through molten Comp B-3. Comparison of the simulations to physical tests show agreement between the positions predicted by the model and the measured locations of the spheres as a function of temperature between 90C and 165C.

LAMMPS integrated materials engine (LIME) for efficient automation of particle-based simulations: application to equation of state generation

We describe the development, accuracy, and efficiency of an automation package for molecular simulation, the large-scale atomic/molecular massively parallel simulator (LAMMPS) integrated materials engine (LIME). Heuristics and algorithms employed for equation of state (EOS) calculation using a particle-based model of a molecular crystal, hexahydro-1,3,5-trinitro-s-triazine (RDX), are described in detail. The simulation method for the particle-based model is energy-conserving dissipative particle dynamics, but the techniques used in LIME are generally applicable to molecular dynamics simulations with a variety of particle-based models. The newly created tool set is tested through use of its EOS data in plate impact and Taylor anvil impact continuum simulations of solid RDX. The coarse-grain model results from LIME provide an approach to bridge the scales from atomistic simulations to continuum simulations.

Response surface optimization of the purification process of cyclotrimethylenetrinitramine explosive via digestion in binary solvent mixtures of acetone/water

ABSTRACTThe present study introduces a new route for the purification of cyclotrimethylenetrinitramine (RDX) explosive containing cyclotetramethylenetetranitramine (HMX) impurity, based on the digestion of crude materials in the binary solvent of acetone/water. Here, first, the solubility data, enthalpy of dissolution and mixing of RDX and HMX were determined experimentally in binary solvent. Then, the effects of various factors on the purification of RDX were studied statistically via response surface methodology (RSM) experimental design. In this technique, unlike the solubility data, unexpectedly HMX was dissolved with higher ratio and solid RDX was obtained with purity >99.5 wt.% and improved crystal morphology.

Modeling and Simulation of Laser‐Induced Ignition of RDX Using Detailed Chemical Kinetics

AbstractA laser‐induced ignition model of RDX is developed, in which a detailed chemical kinetics scheme, containing 45 species and 231 reactions, is employed to describe the reaction in the gas phase. The model is spatially one‐dimensional and includes the transient development of two regions: the condensed phase and the gas phase. The condensed phase is composed of solid RDX, liquid RDX, and some decomposition products. The main physicochemical processes include melting, decomposation, vaporization, and radiation absorption. The gas phase is composed of RDX vapor and reaction products and the main processes include convection, diffusion, heat conduction, chemical reaction, and radiation absorption. With interfacial boundary conditions, the governing parameters for the condensed phase are conservation equations of energy and species concentration, whereas those for the gas phase are the conservation equations of mass, momentum, energy, and species concentration. A finite difference program using FORTRAN is compiled and numerical simulation is carried out. The ignition process of RDX is discussed from the distribution and evolution of temperature and species concentration. The model can provide a reasonable prediction of the phenomenon that the flame moves towards the surface immediately after ignition, and then departed from the surface.

Effects of hydrogen bonds on solid state TATB, RDX, and DATB under high pressures

To probe the behavior of hydrogen bonds in solid energetic materials, we conduct ReaxFF and SCC–DFTB molecular dynamics simulations of crystalline TATB, RDX, and DATB. By comparing the intra- and inter-molecular hydrogen bonding rates, we find that the crystal structures are stabilized by inter-molecular hydrogen bond networks. Under high-pressure, the inter- and intra-molecular hydrogen bonds in solid TATB and DATB are nearly equivalent. The hydrogen bonds in solid TATB and DATB are much shorter than in solid RDX, which suggests strong hydrogen bond interactions existing in these energetic materials. Stretching of the C–H bond is observed in solid RDX, which may lead to further decomposition and even detonation.

Photolysis of RDX and nitroglycerin in the context of military training ranges

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroglycerin (NG) are two energetic materials commonly found in the environment on military training ranges. They are deposited on the ground in the form of solid particles, which can then dissolve in infiltration water or in surface water bodies. The objective of this study was to evaluate whether photolysis by sunlight can significantly contribute to the natural attenuation of RDX and NG (as solid particles or dissolved in surface water) at mid-northern latitudes, where training ranges of Canada and many European countries are located. Experiments conducted at 46.9°N show that both compounds are degraded by sunlight when dissolved in water, with half-lives between 1 and 120d, depending on the compound and time of year. Numerical models may be useful in predicting such photolysis rates, but the models should take into account current ozone levels, as older radiation datasets, collected before the ozone depletion observed since the late 1970s, underestimate the RDX/NG photolysis rate. For solid RDX or NG-bearing particles, photolysis is slower (half-lives of 2–4months), but the degradation rate is still rapid enough to make this process significant in a natural attenuation context. However, photolysis of NG embedded within solid propellant particles cannot proceed to completion, due to the stable nitrocellulose matrix of the propellant. Nonetheless, photolysis clearly constitutes an important attenuation mechanism that should be considered in conceptual models and included in numerical modeling efforts.

Low-limit photo-acoustic detection of solid RDX and TNT explosives with carbon dioxide laser

We report the absorption spectra of RDX and TNT explosive samples in solid form at room temperature using a carbon dioxide laser-based photo-acoustic technique. A continuous wave 12C16O2 laser, tunable between the 9.25–10.74 μm wavelength region, was used as an energy source. Interference-free limits of detection of solid RDX and TNT in simulated samples are estimated to be 10.0 and 16.5 ppbw, respectively.

Microcalorimetry and DSC study of the compatibility of energetic materials

This study included accelerated aging of cyclotetramethylene tetranitramine (HMX) and CH-6 (>97% RDX), coating of HTPB propellant with two sealants (A, B) and reaction kinetics between cyclotrimethylenetrinitramine (RDX) and organic salt (ammonium benzoate). Isothermal microcalorimetry (IMC) analyses at 55–75°C and DSC analyses at decomposition temperatures (180–300°C) were in agreement regarding compatibility. IMC heat flow measurements indicated that aging conditions did not affect HMX and CH-6. DSC rates of reaction at 200°C were similar and thus indicated no reaction. DSC onset of reaction and IMC heat flow measurements showed that one sealant (A) had an interaction with the HTPB propellant while the other did not. When the sealants were combined their reaction with HTPB decreased as the amount of sealant B increased. Reruns of HTPB with sealants A and sealants A+B after 3 weeks at room temperature showed that there was still an additional reaction of ∼9%. IMC and DSC kinetic analyses were carried out on HTPB propellant+sealant A admixture. IMC gave a value of 5 kcal/mol for the reaction in the 55–75°C region while DSC measured the decomposition reaction where the activation energy was lowered from 48 kcal/mol for the propellant to 43 kcal/mol for the admixture. DSC kinetic study of RDX+organic salt showed a shift of the RDX peak temperature from 217°C (neat RDX) to 180°C (admixture). The activation energy for decomposition of the admixture was 25 kcal/mol. Literature values for neat RDX is 47 kcal/mol. Thus DSC kinetic data indicated that the admixture was incompatible at elevated temperature. An IMC kinetic study at temperatures from 65 to 77°C for this admixture gave a value of 33 kcal/mol. The reaction mechanism in the DSC and IMC experiments may be different since in the former case it involves a reaction between a gas (NH 3) and a liquid (RDX) while in the latter case it involves a gas (NH 3) and solid RDX which is a much slower reaction as also reflected in the higher activation energy.

Ab initio simulation of defects in energetic materials. Part I. Molecular vacancy structure in RDX crystal

An attempt to simulate a point defect in energetic molecular solids and to find out its effect on the crystal electronic properties at the ab initio Hartree–Fock level using two different solid-state models was performed. As an example, we have considered a molecular vacancy in RDX crystal, in the periodic and in the molecular cluster model. Low formation energy characterizes this defect in the solid RDX indicating that a significant number of vacancies should be present in the crystal even at low temperatures. Narrowing of fundamental gap (about 1 eV) is caused by the presence of vacancies in the material. The obtained results demonstrate anisotropy of the RDX crystal with respect to the vacancy distribution. Trends received in periodic and molecular cluster models agree well.

Ab initio simulation of defects in energetic materials: Hydrostatic compression of cyclotrimethylene trinitramine

An isotropic compression of both the perfect solid cyclotrimethylene trinitramine (C3H6N6O6), also known as RDX, and of the solid containing vacancies is simulated using the ab initio Hartree–Fock method combined with two different crystal models: a periodic (band structure) and a molecular cluster. We show that an external pressure causes a significant decrease of the optical gap for both the perfect material and the crystal with vacancies. The solid RDX is found to be highly compressible; a pure crystal could be compressed to 57% of its equilibrium volume, whereas crystals containing vacancies are even more compressible. The critical pressure necessary for the insulator–metal transition is also predicted. It is shown that the voids present in a real RDX solid lower the metallization pressure by about 30%. Theoretical results are in close agreement with the experimental data on solid and porous RDX. The influence of defects present in the crystal and the relation to control of the sensitivity to detonation are discussed in detail.

An Effect of Hydrostatic Compression on Defects in Energetic Materials: AB Initio Modeling Maija

AbstractFirst-principle theoretical investigation of the basic defects such as a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack in organic explosive molecular crystals is presented. As an example we considered solid RDX (C3H6N6O6) which is well studied unstable solid. It was established that external hydrostatic pressure changes optical properties of defect-free RDX as well as of the crystal with defects narrowing the band gap. The lattice defects (especially dislocations) are identified with the so-called “hot spots.” The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbitals of N-NO2 group is analyzed. Favorable conditions for molecular dissociation due to electronic excitation are shown.

Thermal decomposition of solid RDX begins with nitrogen-nitrogen bond scission

Thermal decomposition of solid RDX begins with nitrogen-nitrogen bond scission

Deuterium Isotope Effects during RDX Combustion: Mechanistic burn rate‐controlling step determination

AbstractHigh pressure (500 psig/3.55 MPa and 1000 psigl6.99 MPa) burn rate comparisons from the combustion of solid RDX (hexahydro‐ 1.3,.5‐trinitro‐1,3,5‐triazine) and perdeuterio‐labeled RDX‐dh cylindrical pressed pellets reveals a large kinetic deuterium isotope cffect (KDIE). This experimental KDIE confirms that chemical reaction kinetics are a significant mechanistic factor in controlling the inherent RDX burn rate and further shows the six‐membered RDX hcterocycle's rate‐controlling mechanistic step during com‐ bustion is the same as that previously reported for its larger eight‐membered HMX (octahydro‐l.3,5,7‐tetranitro‐l.3.5,7 tetrazocine) homologuc. As with HMX. This experimental KDTE approach also demonstrates a direct mechanistic relationship between RDX's higher order cornbustion regime and its ambient pressure thermochemical decomposition process.

X‐ray radiation decomposition of cyclo‐1,3,5‐trimethylene‐2,4,6‐trinitramine (RDX), at low temperature: Initial reaction steps

AbstractX‐ray radiation damage has been employed as a controllable method for the generating of partial decomposition products in the energetic material RDX. X‐ray photoelectron spectra provided a real time measure of the developing chemistries during the accumulation of radition damage. Rates of NO2 loss were identical at 20 °C and −50 °C However, strong variations in the product chemistry were observed. Nitroso, triazine and for the first time, nitrite ester nitrogen chemistries were observed from the solid RDX. Initial reaction steps are suggested in light of this and previously reported work.

EPR investigations of thermal decomposition of nitramines: I. primary nitroxide radicals from RDX

Abstract We have observed that the free radical produced from solid RDX undergoing thermal decomposition can also be produced in RDX-sulfolane solutions. Under milder conditions, an earlier free radical is produced which is identified as the known nitroxide. Pronounced radical-cage effects are observed.