Impact Sensitivity Of Energetic Materials Research Articles

Theoretical relationship between vibrational properties and impact sensitivity of energetic materials from the phonon upon transition theory

Based on the phonon-upon transition theory of energetic materials in molecular crystals, we analyzed the vibration properties of seven energetic materials with different chemical properties in doorway modes. The vibrational modes are divided into the phonon manifold and the intramolecular vibrational manifold. The phonon-vibron coupling is proportionally related to the energy transfer rates from the phonon manifold to the intramolecular vibration modes. Using the first-principles method of calculation, we calculated the energy transfer rates from the phonon manifold to the intramolecular vibration modes and discussed the relationship between energy transfer rates and experimental impact sensitivity. Our results show that there is a good correlation between the energy transfer rates and the impact sensitivity of energetic materials.

Predicting impact sensitivity of energetic materials: insights from energy transfer of carriers

The impact sensitivity to external stimuli is a critical parameter to evaluate and design high-performance energetic materials (EMs). Investigating factors that affect sensitivity and further revealing the sensitivity mechanism are still huge challenges. In the present study, a physical model is established to assess impact sensitivity of 16 energetic compounds based on the energy transfer of carriers using first-principles calculation. The results show that the empirical parameter ψ obtained by considering the band gap, the density of states, and the ability of electronic migration has a remarkable correlation with experimental drop energy E50. This is in favor of the basic assumption that the energetic material would be less sensitive if the electrons transfer energies quickly to other molecules, making it harder to form a hot spot.

Intermolecular Vibration Energy Transfer Process in Two CL-20-Based Cocrystals Theoretically Revealed by Two-Dimensional Infrared Spectra.

Inspired by the recent cocrystallization and theory of energetic materials, we theoretically investigated the intermolecular vibrational energy transfer process and the non-covalent intermolecular interactions between explosive compounds. The intermolecular interactions between 2,4,6-trinitrotoluene (TNT) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and between 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and CL-20 were studied using calculated two-dimensional infrared (2D IR) spectra and the independent gradient model based on the Hirshfeld partition (IGMH) method, respectively. Based on the comparison of the theoretical infrared spectra and optimized geometries with experimental results, the theoretical models can effectively reproduce the experimental geometries. By analyzing cross-peaks in the 2D IR spectra of TNT/CL-20, the intermolecular vibrational energy transfer process between TNT and CL-20 was calculated, and the conclusion was made that the vibrational energy transfer process between CL-20 and TNTII (TNTIII) is relatively slower than between CL-20 and TNTI. As the vibration energy transfer is the bridge of the intermolecular interactions, the weak intermolecular interactions were visualized using the IGMH method, and the results demonstrate that the intermolecular non-covalent interactions of TNT/CL-20 include van der Waals (vdW) interactions and hydrogen bonds, while the intermolecular non-covalent interactions of HMX/CL-20 are mainly comprised of vdW interactions. Further, we determined that the intermolecular interaction can stabilize the trigger bond in TNT/CL-20 and HMX/CL-20 based on Mayer bond order density, and stronger intermolecular interactions generally indicate lower impact sensitivity of energetic materials. We believe that the results obtained in this work are important for a better understanding of the cocrystal mechanism and its application in the field of energetic materials.

Probing impact of molecular structure on bulk modulus and impact sensitivity of energetic materials by machine learning methods

Machine learning (ML) can map the complex relationship between structures and properties, thus exhibiting great promise in various fields. The bulk modulus (mechanical property) and the impact sensitivity are crucial for energetic compounds. However, relationships have not been elucidated between the molecular structure and the bulk modulus and between the two important properties. Thus, artificial neural network (ANN) and sure independence screening and sparsifying operator (SISSO) methods coupled with selected molecular descriptors were introduced to explore their relationship models for 240 nitroaromatic compounds. In total, six ML models are constructed. The coefficient of determination, root mean squared error and the t-test from 10-fold cross-validation indicate that ANN significantly outperforms SISSO for the bulk modulus and the impact sensitivity models. However, SISSO exhibits comparable prediction accuracy with ANN for the relationship model between the impact sensitivity and the bulk modulus. Besides, the feature evaluation reveals that the average molecular weight, the number of unsaturated and oxygen-containing groups, hydrophilicity and three-dimensional structure play major roles in influencing the bulk modulus while main contributions to the impact sensitivity come from the structure features involving oxygen-containing group, distribution of atomic properties and hydrophilicity. Furthermore, the SISSO model explicitly gives an inverse-like function relationship for the two important properties. It indicates that the impact sensitivity sharply decreases with increasing bulk modulus when the bulk modulus is small, while an opposite trend occurs in the case of relatively large bulk modulus. These findings could provide useful structural information for improving comprehensive properties of energetic materials.

Correlation between the self-sustaining ignition ability and the impact sensitivity of energetic materials

Sensitivity is one of the most important characteristics of energetic materials (EMs), and uncovering the sensitivity mechanism is still a challenge. In this work, we find that the self-sustaining ignition ability is a predominant factor governing the impact sensitivity, and propose the self-ignition propagation coefficient (SIPC) to represent the capability for molecular decomposition of energetic materials based on heat release and energy barrier. A higher correlation of the SIPC with the impact sensitivity measured experimentally is validated by applying it to more than 150 energetic compounds with a high diversity of components, structures and impact sensitivity, in contrast to those of the existing molecule-based models. The advantage of the SIPC can be attributed to the fact that it combines thermodynamics and kinetics and better represents the impact sensitivity. Moreover, the SIPC with a physical meaning can be encoded in a program for the high-throughput design of energetic compounds due to its high accessibility on the molecular level and high reliability for fast screening.

π–π Stacking Contributing to the Low or Reduced Impact Sensitivity of Energetic Materials

π–π stacking, usually together with the aid of hydrogen bonding (HB), serves as a main characteristic of low impact, sensitive, highly energetic materials (LSHEMs), which are highly desired for app...

Models for predicting impact sensitivity of energetic materials based on the trigger linkage hypothesis and Arrhenius kinetics

In order to predict the impact sensitivity of high explosives, we designed and evaluated several models based on the trigger linkage hypothesis and the Arrhenius equation. To this effect, we calculated the heat of detonation, temperature of detonation, and bond dissociation energy for 70 energetic molecules. The bond dissociation energy divided by the temperature of detonation proved to be a good predictor of the impact sensitivity of nitroaromatics, with a coefficient of determination (R2) of 0.81. A separate Bayesian analysis gave similar results, taking model complexity into account. For nitramines, there was no relationship between the impact sensitivity and the bond dissociation energy. None of the models studied gave good predictions for the impact sensitivity of liquid nitrate esters. For solid nitrate esters, the bond dissociation energy divided by the temperature of detonation showed promising results (R2 = 0.85), but since this regression was based on only a few data points, it was discredited when model complexity was accounted for by our Bayesian analysis. Since the temperature of detonation correlated with the impact sensitivity for nitroaromatics, nitramines, and nitrate esters, we consider it to be one of the leading predictive factors of impact sensitivity for energetic materials.

Reversible intramolecular hydrogen transfer: a completely new mechanism for low impact sensitivity of energetic materials.

The intramolecular H transfer of energetic NO2-compounds has been recognized as a possible primary step in triggering molecular decomposition for a long time. Nevertheless, studies on H transfer in different complex situations are limited, lacking a comprehensive understanding of its role in NO2-compounds. In this work, twenty intramolecular H transfer reactions are studied for eighteen nitro compounds and compared with the NO2 partition in thermodynamics and kinetics. Three factors, including the high planarity of molecules, the short transfer distance between the target H and O atoms and the high protonation of the H atom are identified to facilitate H transfer. If H transfer is more kinetically favorable than NO2 partition, and if a reverse H transfer occurs with a barrier less than 30 kcal mol-1, we define it as a reversible one. In our study, for those impact insensitive nitro compounds with H50 larger than that of 2,4,6-trinitrotoluene, all of them are found to be accompanied with reversible H transfer, while the impact sensitive compounds are not. Accordingly, we propose that the reversible H transfer can effectively buffer the external stimuli against the molecular decomposition through chemical energy absorption/release. Beyond the conventional understanding that H transfer triggers molecular decomposition, this work builds a new correlation between reversible H transfer and the low impact sensitivity of energetic nitro-compounds.

Predicting the reactivity of energetic materials: an ab initio multi-phonon approach

The impact sensitivity of energetic materials is successfully predicted using an ab initio model based on the concepts of phonon up-pumping.

Relationship between the crystal packing and impact sensitivity of energetic materials

The crystal packing structure–safety (usually represented by sensitivity) relationships of energetic materials (EMs) are requisite to set a basis for tailoring new ones with the desired safety by means of crystal engineering, because safety is one of the two most important properties of EMs for which there is always a high concern.

On the Reliability of Sensitivity Test Methods for Submicrometer-Sized RDX and HMX Particles

Submicrometer-sized RDX and HMX crystals were produced by electrospray crystallization and submicrometer-sized RDX crystals were produced by plasma-assisted crystallization. Impact and friction sensitivity tests and ballistic impact chamber tests were performed to determine the product sensitivity. Rather than reflecting the quality of the particles, we found the sensitivity tests to be unreliable for submicrometer particles. The used impact test was not accurate enough, while in the friction and ballistic impact chamber tests the submicrometer-sized crystals were distributed among the grooves of the porcelain plate or among the grains of the sandpaper used in these tests. These observations stress the need for revisiting the current standards used for determining the hazardous properties like friction and impact sensitivity of energetic materials in the case, where the sample consists of submicrometer-sized crystals. Recommendations were suggested to develop new test methods that only use the interactions between the particles and therefore allow the application of sensitivity tests for submicrometer/nano-sized energetic materials. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Modeling High Rate Impact Sensitivity of Perfect RDX and HMX Crystals by ReaxFF Reactive Dynamics

We report a methodology for rapid assessment of impact sensitivity of energetic materials which uses the ReaxFF reactive force field in reactive dynamics (RD) simulations of the high rate compression/expansion of a perfect energetic crystal. This approach is validated here to study the high rate impact sensitivity of 1,3,5-trinitrohexahydro-s-triazine (RDX) crystal and octahydro-1,3,5,7-tetrazocine (HMX) crystal at different phases (α, β, γ, and δ). These simulations found that for a compression rate of ∼8.76 km/s along the [100] direction, RDX crystals at lower volume compression ratios (x = 30, 35, 38%) led only to a few RDX molecules decomposing and only into primary products. However, at higher compression ratios (x ≥ 40%), all RDX molecules in the crystal decompose very quickly, leading to both primary and secondary decomposition reactions, including various intermediates such as NO2, NO, HONO, and OH and final products such as H2O, N2, CO, and CO2. For the various phases of HMX, these ReaxFF RD simulations found noticeably higher impact sensitivity for the δ-phase than for other three phases (α, β, and γ). At the same compression ratio x = 40%, all HMX molecules in δ-phase decompose leading to both primary and secondary reaction. However, at 40%, only few HMX molecules in α-, β-, and γ-phases decompose. For a higher compression ratio (x = 42%), increased HMX decomposition is observed for all four phases. These simulation results for both RDX and HMX crystals agree qualitatively with experiment observations. We also observe a variation of the strain energy in different HMX phases induced by the high rate compression, which could be related to the sensitivity difference of HMX phases. These simulations typically took less than 18 h to run on a single 3.0-GHz processor, demonstrating that the fast compression approach by MD simulations with the ReaxFF force field can be used for a quick evaluation of the sensitivity of energetic materials.

Interpreting Drop-Weight Impact Results in Terms of Deformation Dependent Initiation Criteria

AbstractThe impact sensitivity of energetic materials is frequently measured in small-scale drop-weight tests. Plastic deformation of the sample leads to heating, the onset of chemical reaction, and eventual ignition. An analytic model provides deformation dependent estimates of the energy delivered to the sample, its stress state, and its temperature distribution. Results for hydoxy-terminated polybutadiene (HTPB/AP) propellants are interpreted in terms of two initiation criteria: critical decomposition temperature and critical thickness for shear banding. The influence of friction, impact velocity, and sample size on initiation give possible explanations of experimental results. Sample material properties variations are studied for both initiation criteria. The model includes energy localization by varying the thickness of shear layers at the sample/machine interface.

Overview: High Speed Dynamics and Modelling as It Applies to Energetic Solids

AbstractThis paper discusses experimental techniques and modelling tools used to characterize energetic solids subjected to dynamic deformation and shock. Critical experiments have been designed to study shock response and impact sensitivity of energetic materials. For example, a simplified two dimensional experiment has been developed to study the critical phenomena involved in delayed detonation reactions (XDT). In addition, wedge tests are used to obtain equation-of-state data. Coupled with hydrocodes, these experiments give us an in-depth understanding of the response of energetic materials subjected to shock loading. A coupled methodology using both experimental and modelling tools is presented. Consisting of three parts, it addresses all possible responses to fragment impact. The three parts are: (1) Fragment impact modelling (hydrocodes and empirically based codes); (2) Experiments to obtain accurate data for predicting prompt detonation; and (3) Tests with planar rocket motor models to explore mechanisms related to bum reaction thresholds and degree of violence. This methodology is currently being used in weapon design and munitions hazard assessments.