Preparation of µ-HMX/C-Based Composite Energy Composite Microspheres by Microdroplet Technology.

Taking µ-HMX particles as the main research subject, a set of microdroplet sphericalization coating technology platforms was designed and constructed to realize the preparation of composite microspheres by sphericalization coating of µ-HMX. The suspension stability of µ-HMX particles and the mechanism of droplet formation were investigated, and the application effect of nanocarbon materials was also analyzed. The results showed that the prepared sample microspheres all showed a better spherical morphology, as well as good dispersibility; the samples with micron-sized particles for spherical coating had a lower thermal decomposition temperature, a higher energy release efficiency, lower mechanical sensibility, and better combustion performance; the incorporation of CNFs changed the combustion mode of the system, which resulted in the microsphere system of µ-HMX having a good safety performance. The stability and feasibility of uniform spheronization when the dispersed phase is a low-concentration particle suspension system in the spheronization encapsulation process by microdroplet technology were verified.

Efficient Construction of MXene/V2O5 Nanocomposites for Optimizing the Combustion of Ammonium Perchlorate-Based Composite Solid Propellants.

The combustion performance of composite solid propellants (CSPs) significantly affects their application in the field of military and civil aircraft. Ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) composite propellants are one of the common CSPs, and their combustion performance is mainly affected by AP thermal decomposition. In this work, a simple strategy was put forward to effectively construct MXene-supported vanadium pentoxide nanocomposites (MXene/V2O5, MXV). MXene provided a good loading interface for V2O5 nanoparticles, which made MXV obtain a large specific surface area and simultaneously improved the catalytic performance of MXV for AP thermal decomposition. The catalytic experiment results showed that the decomposition temperature of AP mixed with MXV (MXV-4, 2.0 wt %) was 83.4 °C lower than that of pure AP. Moreover, the ignition delay of the AP/HTPB propellant was significantly reduced by 80.4% after adding MXV-4. The burning rate of the propellant was also increased by 202% under the catalytic action of MXV-4. Based on the above results, MXV-4 was expected to be an additive for optimizing the burning process of AP-based composite solid propellants.

Large-Area Nanosphere Self-Assembly Monolayers for Periodic Surface Nanostructures with Ultrasensitive and Spatially Uniform SERS Sensing.

Colloidal lithography provides a rapid and low-cost approach to construct 2D periodic surface nanostructures. However, an impressive demonstration to prepare large-area colloidal template is still missing. Here, a high-efficient and flexible technique is proposed to fabricate self-assembly monolayers consisting of orderly-packed polystyrene spheres at air/water interface via ultrasonic spray. This "non-contact" technique exhibits great advantages in terms of scalability and adaptability due to its renitent interface dynamic balance. More importantly, this technique is not only competent for self-assembly of single-sized polystyrene spheres, but also for binary polystyrene spheres, completely reversing the current hard situation of preparing large-area self-assembly monolayers. As a representative application, hexagonal-packed silver-coated silicon nanorods array (Si-NRs@Ag) is developed as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate with very low limit-of-detection for selective detection of explosive 2,4,6-trinitrotoluene down to femtomolar (10-14 m) range. The periodicity and orderliness of the array allow hot spots to be designed and constructed in a homogeneous fashion, resulting in an incomparable uniformity and reproducibility of Raman signals. All these excellent properties come from the Si-NRs@Ag substrate based on the ordered structure, open surface, and wide-range electric field, providing a robust, consistent, and tunable platform for molecule trapping and SERS sensing for a wide range of organic molecules.

Noncovalent Interactions and Crystal Structure Prediction of Energetic Materials.

The crystal and molecular structures, intermolecular interactions, and energy of CL-20, HATO, and FOX-7 were comparatively predicted based on molecular dynamic (MD) simulations. By comparison, the 2D fingerprint plot, Hirshfeld surface, reduced density gradient isosurface, and electrostatic potential surface were studied to detect the intermolecular interactions. Meanwhile, the effects of vacuum and different solvents on the crystal habit of CL-20, HATO, and FOX-7 were studied by AE and MAE model, respectively. The energy calculation was also analysed based on the equilibrium structures of these crystal models by MD simulations. Our results would provide fundamental insights for the crystal engineering of energetic materials.

Faster and Cleaner Method to Mass Produce Nano HMX/TNT Energetic Particles for Significantly Reduced Mechanical Sensitivity.

Mechanochemical method is an environmentally friendly approach, which can facilely synthesize large-scale nano-scale HMX/TNT energetic particles. In the mechanochemical process, a large new structure was obtained by solid-state grinding of a starting physical mixture HMX and TNT in the presence of ethanol. Within the resulting nano HMX/TNT energetic particles, HMX and TNT molecules are crystallographically rearranged after IR and X-ray irradiation. The size and microstructure were characterized using scanning electron microscopy (SEM). The thermal decomposition of the energetic particles was analyzed using differential scanning calorimetry (DSC). Simultaneously, explosive property and impact safety performance tests and analysis were conducted. Results showed that nano HMX/TNT energetic particles offer a number of advantages in comparison with raw HMX and raw TNT including decreased size, stable thermal performance, reduced sensitivity and improved explosive property after the mechanochemical technology.

Theoretical investigations on CL-20/ANTA co-crystal explosive via molecular dynamics method.

CONTEXT: The study of CL-20 co-crystal has always been a focal point within the field of energetic material modification. In this study, we employed a combination of density functional theory and molecular dynamics simulations to investigate the properties of hexanitrohexaazaisowurtzitane (CL-20)/3-amino-5-nitro-1,2,4-triazole (ANTA) with different molar ratios ranging from 4:1 to 1:4. Additionally, EXPLO-5 software utilized to predict the detonation properties and products of pure CL-20, ANTA, and CL-20/ANTA systems. The results revealed that there was an interaction between CL-20 and ANTA molecules, which had the potential to form a co-crystal. The most likely molar ratio for co-crystal formation was 1:1, and the main driving forces for co-crystal formation were electrostatic force, dispersion force, and van der Waals force. The co-crystal explosive exhibited moderate sensitivity and excellent mechanical properties. Furthermore, the co-crystal detonation performance at a molar ratio of 1:1 was between that of CL-20 and ANTA, representing a new type of insensitive high-energy material. METHODS: The properties of CL-20/ANTA co-crystal were predicted by molecular dynamics (MD) method under Materials Studio software. For the whole MD simulations, set the temperature at 298 K, and the pressure was 0.0001 GPa. Conducted MD simulation under the NPT ensemble for a total simulation duration of 1 ns. The first 0.5 ns was used for thermodynamic equilibrium, and the last 0.5 ns was used for statistical calculation and analysis. Sampling was recorded every 10 fs during the calculation.

Rheological Impact of Particle Size Gradation on GAP Propellant Slurries.

Over the years, widespread interest has been placed on rheological properties to reflect the processability of propellant slurries. Particle gradation technology plays an essential role in the improvement of the processability of propellant slurries. In this article, rheological properties of glycidyl azide polymer (GAP) propellant slurries were measured by dynamic rheological measurements with a rheometer. Submicron-sized (d 50 = 0.221 µm) and micron-sized (d 50 = 33.02 µm) CL-20 particles and ultrafine (d 50 = 2.40 µm) and micron-sized (d 50 = 341.69 µm) AP particles were utilized to investigate the influence of the addition of CL-20 and particle size gradation on rheological properties. The test results demonstrate that the LVE region remains almost invariable while the yield transition process is delayed when the relative content of submicron-sized CL-20 increases from 10 to 20%. The values of G', G″, and |η*| increase with increasing submicron-sized CL-20. Despite this, the value of |η*| can be effectively reduced to about the same value as the slurries with bimodal AP by the size gradation of CL-20. In addition, particle porosity appears to be a suitable parameter to predict trends concerning the rheological properties of the GAP propellant slurries.

Morphology prediction of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) crystal in dimethyl sulfoxide (DMSO) solvent with different models using molecular dynamics simulation.

The interface interaction between DMSO solution and HMX crystal was simulated by molecular dynamics (MD) method, and the morphology of HMX in DMSO solvent was predicted through the occupancy model and attachment energy (AE) model. The intramolecular and intermolecular interactions in HMX crystals were dominated by O···H/H···O contacts, accounting for 63.0%. The S value and the adsorption position of DMSO on different crystal planes of HMX have been compared, which showed that (1 0 - 1) has a deep cavity for the adsorption of solvent molecules. The density of DMSO in the z direction of the different planes showed that (0 1 1) has the largest density peak and (0 2 0) has the smallest density peak. The DMSO solvent had an attraction effect with each crystal plane of HMX. The absolute value of Eint was sorted as follows: (1 1 0) > (1 0 1) > (0 2 0) > (1 0 - 1) > (0 1 1). The DMSO solvent affected the attachment energy of each crystal plane of HMX. In our simulation system, the prediction results of the occupancy model were the most consistent with the experimental values.

Inkjet Printing of GAP/NC/DNTF based Microscale Booster with High Strength for PyroMEMS.

In order to improve the mechanical strength of micro-booster based on 3,4-dinitrofurazanofuroxan (DNTF), 2,4-toluene diisocyanate (TDI) was introduced into the composite binder of nitrocotton (NC) and glycidyl azide polymer (GAP). A full-liquid explosive ink containing DNTF, binder and solvent was printed layer by layer. By the polymer cross-linking technology, the inkjet printed sample with three-dimensional network structure was obtained. The morphology, crystal form, density, mechanical strength, thermal decomposition and micro scale detonation properties of the printed samples were tested and analyzed. The results show that the printed sample has a smooth surface and a dense internal microstructure, and the thickness of the single layer printing is less than 10 µm. Compared with the raw material DNTF, the thermal decomposition temperature and activation energy of the printed samples do not change significantly, indicating better thermal stability. The addition of curing agent TDI increases the mechanical properties and charge density of the energetic composites. The elastic modulus and hardness are increased by more than 20%. The charge density can attain 1.773 g·cm-3, which can reach 95.5% of the theoretical density. The critical detonation size of the sample can reach 1 mm × 0.01 mm or less and the detonation velocity can achieve 8686 m·s-1, which exhibits excellent micro-scale detonation ability.

Exploring the Coordination Effect of GO@MOF-5 as Catalyst on Thermal Decomposition of Ammonium Perchlorate.

Prepared composite materials based on [Zn4O(benzene-1,4-dicarboxylate)3] (MOF-5) and graphene oxide (GO) via a simple green solvothermal method, at which GO was used as platform to load MOF-5, and applied to the thermal decomposition of AP. The obtained composites were characterized by various techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption, Fourier transform infrared (FT-IR), differential scanning calorimetry and thermalgravimetric (DSC-TG). The analyses confirmed that the composite material (GO@) MOF-5 can not only improve the decomposition peak temperature of AP from the initial 409.7 °C to 321.9 °C, but also can improve the enthalpy (△H) from 576 J g-1 to 1011 J g-1 and reduce the activation energy (Ea), thereby accelerating the decomposition reaction. The high-specific surface area of the MOF material can provide a large number of active sites, so that the transition metal ions supported thereon can participate more effectively in the electron transfer process, and GO plays its role as a bridge by its efficient thermal and electrical conductivity. Together, accelerate the thermal decomposition process of AP.

Preparation of multi-scale FOX-7 particles and investigation of sensitivity and thermal stability.

Multi-scale ultrafine 1,1-diamino-2,2-dinitroethene (FOX-7) samples with different particle size were fabricated and specifically, nano-FOX-7 was successfully prepared by a green mechanophysical milling method. All samples were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Impact and friction sensitivities of the samples were tested and thermal analysis was performed by differential scanning calorimetry (DSC) and thermogravimetry (TG). Ultrafine particles with a mean size of 40 nm, 0.9 µm and 3.4 µm respectively showed less sensitivity than raw FOX-7, whose particles size was about 20 µm. The critical drop height H 50 of ultrafine FOX-7 increased from 129 cm to 172 cm, 142 cm and 136 cm, respectively and the friction sensitivity reduced from 32% to 8%, 16% and 20%, respectively. Furthermore, the apparent activation energy of ultrafine particles increased compared with raw materials, which suggested the thermal stability of the ultrafine particles was improved.

Dependence of particle morphology and size on the mechanical sensitivity and thermal stability of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine.

Three kinds of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) samples with spherical (beta-phase), needle (gamma-phase) and polyhedral (beta-phase) shapes were fabricated by wet milling, solvent/non-solvent and riddling methods, respectively. By changing the technical conditions, HMX powders with different particle sizes were obtained for each kind of sample. All as-prepared samples were characterized by laser granularity measurement, scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Taking advantage of mechanical sensitivity tests, slow cook-off tests and differential scanning calorimetry (DSC) analysis, the mechanical sensitivity and thermal stability of HMX samples were found to depend on particle size and morphology. Results indicated that particle size played a significant role in the safety of HMX, and that morphology regulated the experimental results, i.e., for each kind of HMX samples, the mechanical sensitivity and thermal stability of HMX changed if the particle size differed. However, the trends of these changes exhibit much variance if the microstructure of the HMX particles is altered. Consequently, the difference in safety for these kinds of samples has to do with their specific morphology.

High-density HNIW/TNT cocrystal synthesized using a green chemical method.

The main challenge for achieving better energetic materials is to increase their density. In this paper, cocrystals of HNIW (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, often referred to as CL-20) with TNT (2,4,6-trinitrotoluene) were synthesized using ethanol in a green chemical method. The cocrystal was formulated as C13H11N15O18 and possesses a higher density (1.934 g cm-3) than published previously (1.846 g cm-3). This high-density cocrystal possesses a new structure, which can be substantiated by the different types of hydrogen bonds. The predominant driving forces that connect HNIW with TNT in the new cocrystal were studied at ambient conditions using single-crystal X-ray diffraction, powder X-ray diffraction, Fourier transform-infrared spectroscopy and Raman spectroscopy. The results reveal that the structure of the new HNIW/TNT cocrystals consists of three one-dimensional hydrogen-bonded chains exploiting the familiar HNIW-TNT multi-component supramolecular structure, in which two hydrogen-bonded chains are between -NO2 (HNIW) and -CH (TNT), and one hydrogen-bonded chain is between -CH (HNIW) and -NO2 (TNT). The changes to the electron binding energy and type of element in the new cocrystal were traced using X-ray photoelectron spectroscopy. Meanwhile, the physicochemical characteristics alter after cocrystallization due to the hydrogen bonding. It was found that the new HNIW/TNT cocrystal is more thermodynamically stable than HNIW. Thermodynamic aspects of new cocrystal decomposition are investigated in order to explain this observation. The detonation velocity of new HNIW/TNT cocrystals is 8631 m s-1, close to that of HNIW, whereas the mechanical sensitivity is lower than HNIW.

Reduce the Sensitivity of CL-20 by Improving Thermal Conductivity Through Carbon Nanomaterials.

The graphene (rGO) and carbon nanotube (CNT) were adopted to enhance the thermal conductivity of CL-20-based composites as conductive fillers. The microstructure features were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD), and tested the properties by differential scanning calorimeter (DSC), static electricity accumulation, special height, thermal conductivity, and detonation velocity. The results showed that the mixture of rGO and CNT had better effect in thermal conductivity than rGO or CNT alone under the same loading (1 wt%) and it formed a three-dimensional heat-conducting network structure to improve the heat property of the system. Besides, the linear fit proved that the thermal conductivity of the CL-20-based composites were negatively correlated with the impact sensitivity, which also explained that the impact sensitivity was significantly reduced after the thermal conductivity increased and the explosive still maintained better energy.

One-Step Ball Milling Preparation of Nanoscale CL-20/Graphene Oxide for Significantly Reduced Particle Size and Sensitivity.

A one-step method which involves exfoliating graphite materials (GIMs) off into graphene materials (GEMs) in aqueous suspension of CL-20 and forming CL-20/graphene materials (CL-20/GEMs) composites by using ball milling is presented. The conversion of mixtures to composite form was monitored by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The impact sensitivities of CL-20/GEM composites were contrastively investigated. It turned out that the energetic nanoscale composites based on CL-20 and GEMs comprising few layers were accomplished. The loading capacity of graphene (reduced graphene oxide, rGO) is significantly less than that of graphene oxide (GO) in CL-20/GEM composites. The formation mechanism was proposed. Via this approach, energetic nanoscale composites based on CL-20 and GO comprised few layers were accomplished. The resulted CL-20/GEM composites displayed spherical structure with nanoscale, ε-form, equal thermal stabilities, and lower sensitivities.

Inkjet printing of energetic composites with high density.

To explore a new manufacturing method in preparing energetic composites, an inkjet printing device possessing the ability of high precision and flexibility was utilized to deposit six 3,4-dinitrofurazanofuroxan (DNTF) and hexogen (RDX) based explosive inks. The printed quality, inner structure, printed density and crystal morphology of energetic composites were tested, as well as their thermal decomposition properties and detonation properties. The results indicate that inkjet printing provides a good formation uniformity for explosive inks. Interestingly, all energetic composites exhibit excellent printed density with all values higher than 90% theoretical maximum density (TMD). Meanwhile, the composite DNTF/RDX/EC/GAP (54/36/5/5) performs best, reaching 96.88% TMD, which has reached a new height in the three-dimensional printing of energetic composites. Further study manifests that there is no appearance of new material, and the stacking manner of rodlike structures in multilayer manufacturing is the key to achieving such an amazing result. The particles in the energetic composites are spherical with the size ranging from 500 nm to 2 µm and connect with each other closely in the matrix of binders. Moreover, the energetic composites that were directly deposited into wedge channels display a good capability in steadily detonating above the size of 1 × 0.32 mm.

Synthesis, thermolysis, and sensitivities of HMX/NC energetic nanocomposites.

1,3,5,7-Tetranittro-1,3,5,7-tetrazocane/nitrocellulose (HMX/NC) nanocomposites were successfully synthesized by an improved sol-gel-supercritical method. NC nanoparticles with a size of ∼30nm were cross-linked to form a network structure, and HMX nanoparticles were imbedded in the nano-NC matrix. The key factors, i.e., the selection of catalyst and solvent, were probed. No phase transformation of the HMX occurred before or after fabrication, and the molecular structures of the HMX and NC did not change. Thermal analyses were performed, and the kinetic and thermodynamic parameters, such as activation energy (EK), per-exponent factor (lnAK), rate constant (k), activation heat (ΔH(≠)), activation free energy (ΔG(≠)), activation entropy (ΔS(≠)), critical temperature of thermal explosion (Tb), and critical heating rate of thermal explosion (dT/dt)Tb, were calculated. The results indicate that HMX/NC presented a much lower activation energy (165.03kJ/mol) than raw HMX (282.5kJ/mol) or raw NC (175.51kJ/mol). The chemical potential (ΔG(≠)) for the thermal decomposition of HMX/NC has a positive value, which means that the activation of the molecules would not proceed spontaneously. The significantly lower ΔH(≠) value of HMX/NC, which represents the heat needed to be absorbed by an explosive molecule to change it from its initial state to an activated state, implies that the molecules of HMX/NC are much easier to be activated than those of raw HMX. Similarly, the HMX/NC presented a much lower Tb (168.2°C) than raw HMX (283.2°C). From the results of the sensitivity tests, the impact and friction sensitivities of HMX/NC were significantly decreased compared with those of raw HMX, but the thermal sensitivity was distinctly higher. The activation of the particles under external stimulation was simulated, and the mechanism was found to be crucial. Combining the thermodynamic parameters, the mechanism as determined from the results of the sensitivity tests was discussed in detail.