Chongwei An

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.

Preparation and Properties of Surface-Coated HMX with Viton and Graphene Oxide

To improve the safety performance of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) particles, the new carbon material graphene oxide (GO) and Viton were used to coat HMX via a solvent–slurry process. For comparison, the HMX/Viton/graphite (HMX/Viton/G) and HMX/Viton composites were also prepared by the same process. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC) were employed to characterize the morphology, composition, and thermal decomposition of samples. The impact sensitivity and shock wave sensitivity of HMX-based composites were also measured and analyzed. The results of SEM, XRD, and XPS indicate that the cladding layer of HMX-based composites is successfully constructed. HMX/Viton/GO composites exhibit better thermal stability compared to HMX and HMX/Viton. The results show that both impact and shock wave sensitivities of HMX/Viton/GO composites are much lower than that of HMX/Viton. In addition, GO sheets exhibit a better desensitizing effect than G sheets. These combined properties suggest that nano-GO has good compatibility with explosives and can be utilized as a desensitizer in HMX particles.

Nano-CL-20/HMX Cocrystal Explosive for Significantly Reduced Mechanical Sensitivity

Spray drying method was used to prepare cocrystals of hexanitrohexaazaisowurtzitane (CL-20) and cyclotetramethylene tetranitramine (HMX). Raw materials and cocrystals were characterized using scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, Raman spectroscopy, and Fourier transform infrared spectroscopy. Impact and friction sensitivity of cocrystals were tested and analyzed. Results show that, after preparation by spray drying method, microparticles were spherical in shape and 0.5ź5źµm in size. Particles formed aggregates of numerous tiny plate-like cocrystals, whereas CL-20/HMX cocrystals had thicknesses of below 100źnm. Cocrystals were formed by CźHźO bonding between źNO2 (CL-20) and źCH2ź (HMX). Nanococrystal explosives exhibited drop height of 47.3źcm, and friction demonstrated explosion probability of 64%. Compared with raw HMX, cocrystals displayed significantly reduced mechanical sensitivity.

A Fractal Approach to Assess the Risks of Nitroamine Explosives

To the best of our knowledge, this work represents the first thermal conductivity theory for fractal energetic particle groups to combine fractal and hot-spot theories. We considered the influence of the fractal dimensions of particles on their thermal conductivity and even on the sensitivity of the explosive. Based on this theory, two types of nitroamine explosives (hexahydro-1,3,5-trinitro-1,3,5-triazine [RDX] and hexanitrohexaazaisowurtzitane [HNIW]) with different sizes, size distributions, and microscale morphologies were prepared using wet milling, solvent/nonsolvent, and ridding methods. The dependence of the explosive sensitivity on the fractal characteristics of the particles was investigated. Specifically, the size distributions and scanning electron microscopy (SEM) images of the samples were used to obtain the fractal dimension (D) and surface fractal dimension (Ds), respectively, by using a least-squares method and fractal image processing software (FIPS). The mechanical sensitivity and thermal stability of the samples were characterized using mechanical sensitivity tests and differential scanning calorimetry (DSC) and were further compared with the previous results upon the investigation about HMX (octahydro-1.3.5.7-tetranitro-1,3,5,7-tetrazocine). The results indicate that the sensitivity of nitroamine explosives largely depends on the fractal dimensions of the particles. Specifically, the sample with a higher D value is more insensitive to impact action, whereas the sample with a higher Ds value is more sensitive to friction action. In addition, the sample with both higher D and Ds values has less heat release and a slower rate of thermal decomposition. All of the above observations can be attributed to the alternation of the formation of hot spots that was controlled by heat mass and thermal conductivity due to the increase of D and Ds values caused by changes in parameters such as fine particle content, specific surface area, porosity content, surface protruding points, and surface roughness. Therefore, the data in these studies were used to develop a thermal conductivity theory for fractal energetic particle groups that could be applied to the prediction of the sensitivity of energetic materials.

Formation and properties of HMX-based microspheres via spray drying

Herein, we report a facile strategy to prepare a novel HMX-based microspheres by coating a layer of energetic binders on HMX. HMX-based microspheres were synthesized and compared by different dissolution methods by spray dying. The HMX-based composites were also prepared by a water-suspension method. The formation mechanism of the hollow structure and core–shell structure is proposed. The as-prepared HMX/NC/GAP microspheres synthesized by suspension spray drying were found to possess a solid core–shell structure, display a β-form, lower impact sensitivity and higher energy performance.

Catalysis of a Nanometre Solid Super Acid of SO42−/TiO2 on the Thermal Decomposition of Ammonium Nitrate

Raw TiO2 nanoparticles were prepared using the hydrolysis of TiCl4. The nanoparticles were subjected to a surface treatment in diluted sulphuric acid and, subsequently, calcined at different temperatures. Then, a type of super solid acid (SO42−/TiO2) with particle sizes of 20∼30 nm was fabricated. The catalysis of SO42-/TiO2 on the thermolysis of ammonium nitrate (AN) was probed using thermal analysis. For SO42−/TiO2 (AN doped with 3%SO42−/TiO2), the onset temperature decreased by 19°C and the peak temperature decreased by 15.8°C. For TiO2 (AN doped with 3%TiO2), the peak temperature decreased by only 0.5°C. Using the DSC-IR technology, the gas products of the decomposition of 3%SO42-/TiO2-doped AN were detected. We found that the products were mainly N2O (g) and a small amount of H2O (g), and that no NH3 (g) or HNO3 (g) was detected, which ascertained the decomposition reaction of NH4NO3→N2O(g)+H2O(g). In addition, the catalysis mechanism of SO42-/TiO2 on the AN decomposition was discussed in detail.

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.

Carbon-coated copper nanoparticles prepared by detonation method and their thermocatalysis on ammonium perchlorate

Carbon-coated copper nanoparticles (CCNPs) were prepared by initiating a high-density charge pressed with a mixture of microcrystalline wax, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and copper nitrate hydrate (Cu(NO3)2·3H2O) in an explosion vessel filled with nitrogen gas. The detonation products were characterized by transmission electron microcopy (TEM), high resolution transmission electron microcopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Raman spectroscopy. The effects of CCNPs on thermal decomposition of ammonium perchlorate (AP) were also investigated by differential scanning calorimeter (DSC). Results indicated that the detonation products were spherical, 25-40 nm in size, and had an apparent core-shell structure. In this structure, the carbon shell was 3-5 nm thick and mainly composed of graphite, C8 (a kind of carbyne), and amorphous carbon. When 5 wt.% CCNPs was mixed with 95 wt.% AP, the high-temperature decomposition peak of AP decreased by 95.9...

Mechanism investigation for remarkable decreases in sensitivities from micron to nano nitroamine

Raw hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) was pulverized to nano RDX by mechanical milling, and their micron morphology and surface elements were probed by transmission electron microscope and X-ray photoelectron spectroscopy analyses. Thermal analysis was employed to take a kinetic evaluation on thermal decomposition of raw and nano RDX. The result indicated that activation energy for thermal decomposition of nano RDX is closed to the value of raw RDX, which means nano RDX had similar thermal reactivity as raw RDX. However, the sensitivity tests showed that when raw RDX was pulverized to nanoparticles, its mechanical and shock sensitivities decreased by more than 45%. Since it was impossible to use kinetic evaluation to explain the reason why the difference on sensitivities between raw and nano RDX was so distinct, we recruited classic detonation models to solve the problem. By combining the models of Khasainov’s and Merzhanov’s, we related the detonation parameters such as temperature of hot spots, critical temperature of hot spots (TC ), critical size of hot spots (δC ), and mean size of explosive particles, and concluded that: (a) under the same condition, mean size of hot spot in nano RDX charge was much smaller than that of raw RDX charge; (b) at the same δC , TC of nano RDX (776 K) was higher than that of raw RDX (459 K); and (c) particle size was not an important factor to affect sensitivities of explosives unless size of explosive particles was less than 400 nm. These results must base on a steady thermal reactivity from micron to nano RDX.

Nano-HNS Particles: Mechanochemical Preparation and Properties Investigation

Nano-2,2′,4,4′,6,6′-hexanitrostilbene (HNS) particles were successfully prepared by a mechanochemical (i.e., high energy milling) process without an organic solvent, which can be viewed as a green technology. The particle size, morphology, specific area, crystal phase, thermal decomposition properties, impact sensitivity, and short duration shock initiation sensitivity were characterized and tested. The diameter of milling HNS is about 89.2 nm with a narrow size distribution and without agglomeration of particles. The formation mechanism of nano-HNS can be viewed as the transformation from thin HNS sheets with a one-dimensional nanostructure to three-dimensional nanoparticles. The nano-HNS particles present a much higher and lower impact sensitivity than purified HNS, revealing the outstanding safety properties. From the results of the short duration shock initiation sensitivity, 50% and 100% initiation voltages are decreased compared with those of HNS-IV, indicating the higher initiation sensitivity.