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Adding AlH3 nanoparticles (AHNPs) into energetic materials has become a widely used strategy to enhance the efficiency of heat release. However, the underlying mechanisms in the reaction between AHNPs and energetic materials are poorly understood. The evolution of AHNPs in 2,2′,4,4′,6,6′-hexanitrodiphenyl ethylene (HNS) was simulated by reactive dynamic based on ReaxFF force field. As a whole, adding AHNPs unexceptionally increases heat release, and the particle size, passivation shell, and contents of AHNPs influence the morphological evolution and hydrogen release performance. AHNPs with smaller size can rapidly release hydrogen under the drastic micro-explosion. In addition, the aggregation mechanism of AHNPs is revealed at the earlier and later stages of the reaction. Al-O, Al-C, and Al-N bonds are generated during the reaction. A large amounts of H of AHNPs and C of HNS cause O to be desorbed from the Al-contained cluster. For the core–shell structure or/and larger size AHNPs, their oxidation process undergoes a transformation of H2–H2O in the gas cavity. Carbon clusters formed during the reaction depend on the distribution of Al. The addition of AHNPs could promote the graphitization of carbon clusters. This work is expected to deepen insight into the reaction mechanism of AHNPs-containing energetic materials with negative oxygen balance.

Metal hydride additives, aluminum hydride in particular, have extensively been applied in solid rocket motor propellants. This work employed the reactive force field molecular dynamics to elaborate the underlying mechanism for the oxidation of highly active aluminum hydride nanoparticles (AHNPs) by gaseous oxides (CO, CO2, NO, and NO2). The results showed that AHNPs first went through four stages: dehydrogenation ( 25 ps), micro-explosion (~31 ps), and oxidation (>28 ps). The dehydrogenation of AHNPs surface overlaps with the Al nucleation in the preheating stage and prevents the oxidation of Al by gaseous oxides. Only a small part of Al on the surface is oxidized to form a thin and uneven oxide film (0.18–0.54 nm). In the core, the formed H2 is hindered by the shell and gradually gathers into H2 bubbles. H2 bubbles have great kinetic energy and become a micro-explosion promoter, eventually causing nanoparticles to burst at high temperatures. The micro-explosion accelerates the dissociation of gaseous oxides. This study provides an in-depth understanding of the mechanism of dehydrogenation and oxidation of metal hydride nanoparticles.

Molecular dynamics simulations on the reactions of nano-aluminum, water with different proportions of hydrogen peroxide were performed by ReaxFF. The reaction rate of the system increases by 27% as the molar ratio of H2O2 increases from 0 to 30%. The reaction paths in each period are elucidated by the change of the numbers of reactants and products. The reaction absorbs heat and releases isolated H atoms in the initial stage through pathways Al + 3H2O → Al(OH)3 + 3H and 3Al + 2H2O → 2AlO + AlH3 + H. Subsequently, H2O2 decomposes and releases O2 through 2H2O2 → 2H2O + O2 as the temperature rises to 350 K. And these O2 are rapidly involved in the formation of Al2O3, corresponding to the process 4AlO + O2 → 2Al2O3 and 2Al(OH)3 + 2AlH3 + 3O2 → 2Al2O3 + 6H2O. In the final stage, H2 molecules generated in accordance with the pathways of Al(OH)3 + AlH3 → Al2O3 + 3H2 and 2AlH3 + 3H2O → Al2O3 + 6H2. Particularly, the production of H2 decreases by 28.2%, 49.8% and 68.6%, as the molar ratio of H2O2 increases from 0 to 10%, 20% and 30%, respectively. Meanwhile, the reaction of Al/H2O/H2O2 releases much more energy (271.40–440.53 kJ/mol) than that of Al/H2O (180.67 kJ/mol). Moreover, the ignition temperature decreases with the increase of H2O2 concentration, but is proportional to the heating rate of ignition. When the molar ratio of H2O2 increases from 0 to 30%, the adiabatic flame temperature of the mixture increases linearly from 2400 K to 3000 K according to the adiabatic simulation.

Metal nanoparticles are easily deactivated by migration-aggregation in combustion. Encapsulated nanoparticles are one of the tools for coping with the stability challenges of metal nanoparticles. The self-assembly details of aluminum nanoparticles (ANPs) encapsulated into carbon nanotubes (CNTs) were demonstrated by molecular dynamics simulations. The simulation results show that ANPs can completely self-roll into CNTs to form a stable core-shell structure by inertial force and van der Waals force. Inside the tubes, ANPs move toward the cap at a velocity of 2.27 Å ps-1. However, it increases to 3.17 Å ps-1 when near the cap of CNTs. The initiation of the ANPs' oxidation and degradation can be effectively checked by coating CNTs. The diffusion of the Al atoms in the encapsulated ANPs occurred earlier than their oxidation in combustion, verified by using ReaxFF molecular dynamics simulations. The morphological evolutions of the nanostructures in the initial combustion of the encapsulated ANPs are predicted. The interplay between the encapsulated ANPs' responses and external stimuli is classified into core-shell separation, shell damage, and core-shell burst, which provides insights into the oxidation mechanism of encapsulated nanoparticles.

The modified attachment energy model was applied to predict the crystal morphology of hexanitrohexaazaisowurtzitane/1,3-dinitrobenzene (HNIW/DNB) cocrystal in ethanol. A double-layer interface structure was established based on experiments. Molecular dynamics simulation was employed to investigate the interaction of flat faces and ethanol solvents. We used periodic bond chains and roughness calculations to analyze the characteristics of the HNIW/DNB cocrystal. The crystal morphology of the HNIW/DNB cocrystal is mainly composed of the (001), (010), (102), and (111) faces in vacuum. The (001) face occupies the largest area (49.54%). In ethanol, the area of the (001) face increases to 68.58%. Ethanol molecules are adsorbed on the polar face through hydrogen bonding. In the slowest growth direction, two HNIW layers and one DNB layer alternately appear. The higher molecular recognition of the (001) face of HNIW/DNB resulted in this face becoming the most important growth face. Meanwhile, we also predicted the crystal morphologies of ε-HNIW and DNB in ethanol. The prediction morphologies are in excellent agreement with the experimental shapes. These simulation results can provide guidance for the recrystallization of HNIW/DNB.

Hexanitrostilbene plays an important role in aerospace and space technology because of its excellent chemical stability. However, carbon deposition during the explosion seriously affects its detonation performance. A newly parameterized reactive force field was used to simulate the decomposition behavior of nanometer aluminized hexanitrostilbene. Aluminum nanoparticles and the corresponding surface oxidized ones were mixtured with the hexanitrostilbene to build nanometer aluminized composites. The simulation results show that aluminum nanoparticles lead to an earlier decomposition of hexanitrostilbene in a new way. Aluminum nanoparticles follow diffusion oxidation theory in the heating. At high temperatures, Aluminum nanoparticle quickly splits and exposes active aluminum to participate in the reaction. The presence of oxide layer in surface oxidized aluminum nanoparticle is less attractive to oxygen, carbon and nitrogen atoms, and surface oxidized aluminum nanoparticle is difficult to split into small particles during ignition and detonation. Aluminum nanoparticles prevent the formation of larger carbon clusters and cause hydrogen and oxygen atoms escape from carbon clusters. This is the main reason for the increasing detonation pressure and energy outputs of nanometer aluminized Hexanitrostilbene. Adiabatic simulation shows that nanometer aluminized hexanitrostilbene decomposes faster and releases more energy than pure hexanitrostilbene. This work provides insights for the application of aluminized hexanitrostilbene composites in aerospace.

ReaxFF-low-gradient reactive force field with CHONAl parameters is used to simulate thermal decomposition of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and AlH3 composite. Perfect AlH3 and surface-passivated AlH3 particles were constructed to mix with HMX. The simulation results indicate HMX is adsorbed on the surface of particles to form O–Al and N–Al bonds. The decomposition of HMX and AlH3 composite is an exothermic reaction without energy barrier, but the decomposition of pure HMX needs to overcome the energy barrier of 133.57 kcal/mol. Active nano-AlH3 causes HMX to decompose rapidly at low temperature, and the primary decomposition pathway is the rupture of N–O and C–N bonds. Adiabatic simulation shows that the energy release and temperature increase of HMX/AlH3 is much larger than those of the HMX system. Surface-passivated AlH3 particles only affect the initial decomposition rate of HMX. In HMX and AlH3 composites, the strong attraction of Al in AlH3 to O and the activation of the intermediate reaction by H2 cause HMX to decompose rapidly. The final decomposition products of pure HMX are H2O, N2, and CO2, and those of HMX/AlH3 are H2O, N2, and Al-containing clusters dominated by C–Al. The final gas production shows that the specific impulse of HMX/AlH3 is larger than that of HMX.

Aluminum nanoparticles (ANPs), as an economical and effective metal fuel, are widely applied in energetic formulations. The objective of this research was to gain insights into the oxidation of ANPs in gaseous oxides (CO2, CO, NO2, and NO). The reactive molecular dynamics (RMD) simulations were performed to elucidate the detailed mechanisms of surface oxidation, chain-like products formation, and hollow formation in the evolution of ANPs. The O atoms in gaseous oxides are adsorbed on the ANPs' surface followed by the cleavage of O–C/N bond. The nucleation and growth of chain-like products occur in dense gaseous oxides. The four forms of chain products include tilted chain, twisted chain, branched chain, and cyclic chain were observed in CO atmosphere. A similar chain structure is also formed in NO atmosphere, but the chain length is significantly reduced. The oxide shell of ANPs is formed and expands rapidly in CO2 atmosphere, resulting in voids between oxide shell and Al core. Al atoms are transported from the core to the oxide shell through a bridge composed of Al atoms. The Al core gradually diffused outward and was eventually hollowed out. In addition, the final product has carbon deposits (C48 and C98) on the surface and core.

Molecular dynamics simulation on the reaction of nano-aluminum particle and water were performed by ReaxFF force field. The different reaction rates of aluminum with H2O and D2O indicate that the effect of kinetic isotope effect (KIE) is obvious. The generation rate of H2 is 20% higher than that of D2. The mechanism of formation and consumption of AlH3 as well as the generation of Al2O3 are elucidated. Specifically, the most frequent reaction pathways throughout the full period are clarified based on the changes of numbers of intermediate and final products. The reactions in the early period are endothermic and release isolated H atoms, which involves Al +3H2O → Al(OH)3 + 3H and 3Al + 2H2O → 2AlO + AlH3 + H. Afterwards, the reactions release a large amount of energy and generate H2 molecules in the later period, in accordance with the equations of 2AlO + H2O → Al2O3 + H2, Al(OH)3 + AlH3 → Al2O3 + 3H2 and 2AlH3 + 3H2O → Al2O3 + 6H2. The initial pathways are in agreement with early DFT investigations. The diffusion coefficients of the different atoms show that the replacement of H2O by D2O reduces the diffusion rate of all the atoms (5%) in the system. The KIE is confirmed, and the results agree with experiments. In addition, the simulations were performed under different maximum temperatures. Results show that the solid residues become more disperse in space as the temperature increase.

Using reactive molecular dynamics method with the reactive force field framework, the thermal decomposition of nano-AlH3/TNT and nano-AlH3/CL-20 composites was investigated to develop new high explosives. The binding energies and relaxed densities of the composites show that the compatibility of TNT/AlH3 is better than CL-20/AlH3, which is confirmed by the DFT calculations. The pyrolysis simulations showed that the potential barrier of TNT/AlH3 (1.14 kJ g−1) and CL-20/AlH3 (0.72 kJ g−1) are smaller than pure TNT (1.99 kJ g−1) and pure CL-20 (1.01 kJ g−1), respectively. This indicates the catalytic effect of AlH3 nanoparticle on the decomposition of TNT and CL-20. The catalytic effect was confirmed by the DFT calculations of the R–NO2 rupture of TNT and CL-20 molecules on AlH3 surfaces. The slower heating rate (rh) of 16 K ps−1 led to similar evolution results as rh = 32 K ps−1, which implies that the dependence of the simulation results on the heating rate is small. Moreover, TNT/AlH3 and CL-20/AlH3 release 1.07 and 1.97 kJ g−1 more energies and generate 33.8% and 14.0% more total gas products than pure TNT and CL-20, respectively. Therefore, AlH3 nanoparticles can improve the detonation performance and specific impulse for TNT and CL-20. These are in accord with the effect of AlH3 on nitromethane and ammonium perchlorate/hydroxyl-terminated polybutadiene propellants in the experiments. As a result, the nano-AlH3/TNT and nano-AlH3/CL-20 composites can be promising candidates of new high explosives.

The spiral growth model was applied to predict the crystal morphology of 2,2',4,4',6,6'-hexanitrostilbene (HNS). We selected solvents of N,N-dimethylformamide (DMF), N-methyl pyrrolidone (NMP), and nitric acid (NA) to control the crystal morphologies of HNS. Molecular dynamic simulations were used to relax the constructed interface model. The relative growth rate of important face is calculated by the spiral growth expression. The predicted crystal shapes are flaky in three solvents. Only (100), (001), and (011) faces are generated in DMF, NMP, and NA. The aspect ratios of the predicted HNS crystal morphologies in DMF, NMP, and NA are 23.00, 15.45, and 4.85, respectively. In addition, we analyzed the properties on each face using periodic bond chain, molecular arrangement, and roughness model. The excellent agreement between the predicted morphologies and the experimental images is clearly evident. These simulation results can provide guidance for the recrystallization of HNS. Graphical abstract.

The thermal decomposition of pure nitromethane (NM) and NM/nano-aluminum (Al) composites was simulated by reactive molecular dynamics with ReaxFF-lg corrected force field parameters. The initial decomposition pathway of NM molecules in pure NM is C–N bond rupture. However, NM is decomposed early by the initial pathway of N–O bond rupture when it mixes with nano-Al because of the strong attraction of Al to O. The decomposition process of NM/nano-Al can be divided into three stages: adsorption, slow decomposition, and rapid decomposition. The addition of nano-Al particles decreases the energy barrier in decomposition, increases the released energy, and reduces the decomposition temperature of NM. Adding 3% Al to the explosive can make the detonation pressure 3.083% higher than that of pure system. Compared with pure NM, the energy barrier of 16% Al composite is 25.63 kcal/mol lower and the energy released is 22.99 kcal/mol more. There is an optimal amount of Al contents being added to the NM composite by which the largest total numbers of gaseous products (N2, H2O, and CO2) are released. The effect of Al additives on CO2 production is the most obvious. The maximum detonation pressure can be achieved by adding an appropriate amount of nano-Al, which is similar to the experimental results.

According to the experiments, DNTF crystallizes in benzene/methylbenzene (1:1), benzene/methylbenzene/ethanol (2:3:5), and sym-dichloroethane solvents into two similar crystal shapes, namely strip and tetrahedral. There is a possibility that solvent changes the crystal morphology. In order to explain this phenomenon, the DNTF growth interface model was constructed according to the actual solution environment. The interaction energy between the solvent phase and the DNTF crystal face was studied by means of molecular dynamics simulation. The crystal morphology of DNTF was predicted using the classical modified attachment energy model (MAE) in benzene, methylbenzene, benzene/methylbenzene (1:1), benzene/methylbenzene/ethanol (2:3:5), and sym-dichloroethane. The results show that the DNTF growths are mainly dominated by the (011), (001), (101), (110), (111), and (11 $$ \overline{1} $$) faces in vacuum. However, only a few faces will remain in the solvents, of which the (011) and (101) faces are exposed in benzene, methylbenzene, and benzene/methylbenzene (1:1), and only the (111) faces constitute the crystal shape of the DNTF in benzene/methylbenzene/ethanol (2:3:5) and sym-dichloroethane. The predicted results successfully explained the observed phenomena in the experiment. The simulation results can provide some guidance for the crystallization process of DNTF.

Molecular dynamic simulation of a high explosive, RDX, mixed with AlH3 nanoparticles was performed by a newly parameterized ReaxFF force field. Testing of the ReaxFF shows that the mean absolute errors of the densities and bond lengths between calculated and experimental values are less than 7% and 3%, respectively. Using the ReaxFF, effects of AlH3 nanoparticles with different radii on the thermal decomposition of RDX were revealed. A new mechanism of the generation and the consumption of H2 was discovered in the explosion. The H2 is released by AlH3 firstly and then it reacts with NO2 and CO2 from the decomposition of RDX, leading to an increase of H2O, NO and CO. Meanwhile, the size effect of AlH3 upon the reaction was also revealed. As a result, the number of produced H2O and CO2 molecules increases by 10.38% and 56.85%, respectively, when the radius of AlH3 nanoparticles decreases from 1.10 to 0.68 nm. This showed that RDX decomposes more completely with smaller AlH3 nanoparticles, which was further demonstrated by the analysis of reaction residues and diffusion coefficients.

Aluminized explosives have important applications in civil construction and military armaments, but their thermal decomposition mechanisms are not well characterized. Here, the thermal decomposition of TNT, RDX, HMX and CL-20 on Al nanoparticles is examined by reactive dynamics simulations using a newly parameterized reactive force field with low gradient correction (ReaxFF-lg). Partially passivated Al nanoparticles were constructed and mixed with TNT, RDX, HMX and CL-20 crystals and then the mixed systems are heated to a high temperature in which the explosives are fully decomposed. The simulation results show that the aluminized explosives undergo three main steps of thermal decomposition, which were denoted "adsorption period" (0-20 ps), "diffusion period" (20-80 ps) and "formation period" (80-210 ps). These stages in sequence are the chemical adsorption between Al and surrounding explosive molecules (R-NO2-Al bonding), the decomposition of the explosives and the diffusion of O atoms into the Al nanoparticles, and the formation of final products. In the first stage, the Al nanoparticles decrease the decomposition reaction barriers of RDX (1.90 kJ g-1), HMX (1.95 kJ g-1) and CL-20 (1.18 kJ g-1), respectively, and decrease the decomposition reaction barrier of TNT from 2.99 to 0.29 kJ g-1. Comparing with the crystalline RDX, HMX and CL-20, the energy releases are increased by 4.73-4.96 kJ g-1 in the second stage. The number of produced H2O molecules increased by 25.27-27.81% and the number of CO2 molecules decreased by 47.73-68.01% in the third stage. These three stages are further confirmed by the evolutive diagram of the structure and temperature distribution for the CL-20/Al system. The onset temperatures (To) of generating H2O for all the aluminized explosives decrease, while those of generating CO2 for aluminized HMX and CL-20 increase, which are in accord with the experiment of aluminized RDX.

Metal nanoparticles tend to deactivate due to spontaneous migration-aggregation and continuous surface oxidation. Nanocapsules provide a brand-new strategy to tackle these challenges. In this work, two types of nanocapsules were designed and applied to active aluminum. The self-assembly details between carbon nanotubes (CNTs) and aluminum nanoparticles (ANPs) are elucidated through molecular dynamics methods. The stability and controllability of capsules I-VI consisting of various lengths of CNTs were revealed by means of reactive molecular dynamics simulations. The results indicate that the driving force for the self-assembly of nanocapsules is the vdW interaction. The assembled capsule has excellent stability, and the interaction energy between the tube and ANPs and between the tubes is as high as −599.55 and −1,014.78 kcal/mol, respectively. The opening of the nanocapsules during combustion is dependent on the length of the CNTs and temperature. Above 2000 K, the outer CNT can be opened when the length is greater than 31.73 A. When used as propellants, pyrotechnics and explosives, these nanocapsules can be triggered remotely by visible/infrared lasers without the need of detonating wires.

Metal hydride combustion is highly exothermic and is applied in energetic applications, such as propulsion, pyrotechnics, and fuel cell systems. Aluminum hydride (AlH3) is a fascinating material because of its high hydrogen content, combustion heat, and low toxicity. In this study, the reactive molecular dynamics simulations were preformed to investigate the fundamental oxidation mechanism of AlH3 nanoparticles (AHNPs) by H2O. Two self-dehydrogenation mechanisms of AHNPs were elucidated. One is the H2 formations on the shell, which firstly hinders the oxidation of AHNPs by H2O. Another is the emergence of H2 bubbles in the core, which induces the microexplosion of AHNPs above 1500 K. The microexplosion of AHNPs accelerates the oxidation of Al by H2O. H2O decomposes on Al surface to form Al-OH. Herein, the decomposition of OH with the assistance of three Al atoms was first reported. The oxidation of Al by H2O can directly generate H2 at 3500 K. The results shed light on the complicated interplay between morphological evolution and external conditions. It provides insights into the dehydrogenation mechanism of metal hydride nanoparticles at the atomic level.

A computational strategy in consideration of attachment energy, temperature, solubility and supersaturation unravels details of the solvent effect on the crystal morphology. The crystal morphologies were predicted by the advanced screw dislocation growth model. This research sheds much light on the crystal growth mechanisms with the example of 3,4-bis(3-nitrofurazan-4-yl)furoxan (DNTF) in ethanol. The solvation model based on the experiment situation was established into periodic supercell. Molecular dynamics simulation was performed for obtaining the adsorption energy at the equilibrium state of the interface layer. The growth characteristics of relevant growth faces are introduced. At the same time, a periodic bond chains analysis can be applied to the existence and evolution of crystal growth units. The prediction results are in remarkable agreement with the experimental results. We found that crystal morphology of DNTF is composed of (002), (111), (11 1 ¯ ) and (101) faces in ethanol. As the saturation temperature rises, the (101) face becomes smaller and eventually disappears.

Metal hydride nanoparticles exhibit different oxidation dynamic characteristics from bulk ones due to size effects as well as the existence of M-H bonds. We used ReaxFF-lg molecular dynamics to elucidate the dehydrogenation and oxidation mechanisms of aluminum hydride nanoparticle (AHNP). Results show that the morphological evolution of AHNP is induced by temperature and structure, and classified into four shapes: sphere, notched sphere, large branch, and small string. The surface dehydrogenation and oxidation are almost co-occurring. The initial dehydrogenation inhibits the rapid oxidation of the surface layer of AHNP. The increase in temperature induces the instability of the oxide shell and provides sufficient kinetic energy for the hydrogen bubbles. As a result, the oxidation shell ruptures, and the internal hydrogen bubbles physically escape from the nanoparticles. Some simultaneous notches appear in the shell, making its appearance to be branchy at 3000 K. AHNP explodes and ejects small strings of atoms at 3500 K. We also investigated the effect of different temperatures and initial oxygen densities on the morphologies of AHNP. Our results emphasize the complicated interplay between the structural evolution of nanoparticles and environmental conditions. It provides insights into the atomic-level dehydrogenation and oxidation mechanism of metal hydride nanoparticles.

The solubility parameters of TNAD, HMX, RDX, DINA, DNP propellants were predicted by molecular dynamics (MD) simulation in order to evaluate the miscibility of TNAD and the other four propellants. The results show that the order of miscibility is TNAD/DINA > TNAD/DNP > TNAD/RDX > TNAD/HMX from the analysis of miscibility. The densities and binding energies of TNAD/propellants blends were further investigated. The results indicate that the better the miscibility between TNAD and the propellants, the smaller the variation of the density rate. The larger the intermolecular interaction, the better the miscibility between components. The analysis of radial distribution function shows that the main interaction ways between TNAD and other energetic components are short-range interactions.

Polyhedral oligomeric silsesquioxanes (POSS) are nanometer-size molecules suitable for the production of organic-inorganic nanocomposite materials. These organic-inorganic nano-building blocks show promise for enabling the production of polymeric materials of exceptional mechanical properties as well as novel composite materials. While the experimental studies of these materials have rapidly evolved in the past decade, their theoretical investigation is still in its infancy. Toward the validation of force fields for the molecular simulation of POSS-containing systems, we present the charge-transfer reactive (CTR) force field for the molecular simulation of polyhedral oligomeric silsesquioxane (POSS) molecules and compare the ability of this, and several force fields taken from the literature, to predict the thermophysical properties of POSS-containing systems. The literature force fields compared include the universal force field (UFF) and the COMPASS and Hybrid-COMPASS force fields. Predictions from molecular dynamics simulations of the structural parameters (unit cell vectors), melting temperature, and FT-IR spectra of crystals of POSS monomers are presented. The POSS monomers investigated are octahydride, octamethyl, and octapropyl POSS. Predicted quantities are compared to experimental results where available and provide molecular-level physical insight into several aspects of the behavior of POSS molecules. While all the force fields tested perform reasonably well, our results indicate that the Hybrid-COMPASS and CTR force fields predict structural properties that are in good agreement with experimental data.