Xiaolan Song

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.

Mechanism for thermite reactions of aluminum/iron-oxide nanocomposites based on residue analysis

Abstract Sol-gel method was employed to combine Al and iron-oxide to form nanocomposites (nano-Al/xero-Fe 2 O 3 and micro-Al/xero-Fe 2 O 3 ). SEM, EDS and XRD analyses were used to characterize the nanocomposites and the results indicated that nano-Al and micro-Al were compactly wrapped by amorphous iron-oxide nanoparticles (about 20 nm), respectively. The iron-oxide showed the mass ratio of Fe to O as similar as that in Fe 2 O 3 . Thermal analyses were performed on two nanocomposites, and four simple mixtures (nano-Al+xero-Fe 2 O 3 , nano-Al+micro-Fe 2 O 3 , micro-Al+xero-Fe 2 O 3 , and micro-Al+micro-Fe 2 O 3 ) were also analyzed. There were not apparent distinctions in the reactions of thermites fueled by nano-Al. For thermites fueled by micro-Al, the DSC peak temperatures of micro-Al/Xero-Fe 2 O 3 were advanced by 68.1 °C and 76.8 °C compared with micro-Al+xero-Fe 2 O 3 and micro-Al+micro-Fe 2 O 3 , respectively. Four thermites, namely, nano-Al/xero-Fe 2 O 3 , nano-Al+micro-Fe 2 O 3 , micro-Al/xero-Fe 2 O 3 , and micro-Al+micro-Fe 2 O 3 , were heated from ambient temperature to 1020 °C, during which the products at 660 °C and 1020 °C were collected and analyzed by XRD. Crystals of Fe, FeAl 2 O 4 , Fe 3 O 4 , α-Fe 2 O 3 , Al, γ-Fe 2 O 3 , Al 2.667 O 4 , FeO and α-Al 2 O 3 were indexed in XRD patterns. For each thermite, according to the specific products, the possible equations were given. Based on the principle of the minimum free energy, the most reasonable equations were inferred from the possible reactions.

A Versatile Methodology Using Sol-Gel, Supercritical Extraction, and Etching to Fabricate a Nitramine Explosive: Nanometer HNIW

A combinative method with three steps was developed to fabricate HNIW (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtziane) nanoexplosives with the gas anti-solvent (GAS) method improved by introducing a gel frame to limit the overgrowth of recrystallized particles and an acid-assistant to remove the used frame. Forming the mixed gel, by locking the explosive solution into a wet gel whose volume was divided by the networks, was the key for the fabrication. As demonstrated by scanning electron microscopy (SEM) analysis, a log-normal size distribution of nano-HNIW indicated that about 74.4% of the particles had sizes <120 nm and maximum particle size was ∼300 nm. Energy-dispersive X-ray spectroscopy (EDS) and infrared (IR) characterizations showed that the aerogel embedded with nanoexplosive particles was dissolved in hydrochloric acid solution, and the raw ϵ-HNIW was mostly transformed into the α phase (nano-HNIW) during recrystallization. Nano-HNIW exhibited impact and friction sensitivity almost equal to those of raw HNIW, within experimental error. Thermal analysis showed that the decomposition peak temperature decreased by more than 10°C and that the heat release increased by 42.5% when the particle size of HNIW was at the nanometer scale.

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.

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.

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.

Characterization and Thermal Decomposition of Nanometer 2,2′, 4,4′, 6,6′-Hexanitro-Stilbene and 1,3,5-Triamino-2,4,6-Trinitrobenzene Fabricated by a Mechanical Milling Method

ABSTRACT Nanometer 2,2ʹ, 4,4ʹ, 6,6ʹ-hexanitro-stilbene (HNS) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were fabricated on a high-energy ball mill. The particle sizes of nano-HNS and nano-TATB were 98.4 and 57.8 nm, respectively. An SEM analysis was employed to image the micron morphology of nano-explosives. The particle size distribution was calculated by measuring the size of 300 particles in SEM images. XRD, IR, and XPS analyses were used to confirm whether the crystal phase, molecule structure, and surface elements were changed by the milling process. Thermal decomposition of nano-HNS and nano-TATB was investigated by differential scanning calorimetry (DSC) and thermal-infrared spectrometry online (DSC-IR) analyses. Using DSC traces collected from different heating rates, the kinetic and thermodynamic parameters of thermolysis of raw and nano-explosives were calculated (activation energy (EK), pre-exponential 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). The results indicated that nano-explosives were of different kinetic and thermodynamic properties from starting explosives. In addition, the gas products for thermal decomposition of nano-HNS and nano-TATB were detected. Although HNS and TATB are both nitro explosives, the decomposition products of the two were different. A mechanism to explain the difference is proposed.

Thermochemical properties of nanometer CL-20 and PETN fabricated using a mechanical milling method

2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and pentaerythritol tetranitrate (PETN), with mean sizes of 73.8 nm and 267.7 nm, respectively, were fabricated on a high-energy ball-mill. Scanning electron microscope (SEM) analysis was used to image the micron-scale morphology of nano-explosives, and the particle size distribution was calculated using the statistics of individual particle sizes obtained from the SEM images. Analyses, such as X-ray diffractometer (XRD), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS), were also used to confirm whether the crystal phase, molecular structure, and surface elements changed after a long-term milling process. The results were as expected. Thermal analysis was performed at different heating rates. Parameters, such as the activation energy (ES), activation enthalpy (ΔH≠), activation free energy (ΔG≠), activation entropy (ΔS≠), and critical temperature of thermal explosion (Tb), were calculated to determine the decomposition courses of the explosives. Moreover, the thermal decomposition mechanisms of nano CL-20 and nano PETN were investigated using thermal-infrared spectrometry online (DSC-IR) analysis, by which their gas products were also detected. The results indicated that nano CL-20 decomposed to CO2 and N2O and that nano PETN decayed to NO2, which implied a remarkable difference between the decomposition mechanisms of the two explosives. In addition, the mechanical sensitivities of CL-20 and PETN were tested, and the results revealed that nano-explosives were more insensitive than raw ones, and the possible mechanism for this was discussed. Thermal sensitivity was also investigated with a 5 s bursting point test, from which the 5 s bursting point (T5s) and the activation of the deflagration were obtained.2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and pentaerythritol tetranitrate (PETN), with mean sizes of 73.8 nm and 267.7 nm, respectively, were fabricated on a high-energy ball-mill. Scanning electron microscope (SEM) analysis was used to image the micron-scale morphology of nano-explosives, and the particle size distribution was calculated using the statistics of individual particle sizes obtained from the SEM images. Analyses, such as X-ray diffractometer (XRD), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS), were also used to confirm whether the crystal phase, molecular structure, and surface elements changed after a long-term milling process. The results were as expected. Thermal analysis was performed at different heating rates. Parameters, such as the activation energy (ES), activation enthalpy (ΔH≠), activation free energy (ΔG≠), activation entropy (ΔS≠), and critical temperature of thermal explosion (Tb), were calculated to determine the decompositio...