Theoretical study of breaking and slipping processes for HMX/graphite interface

Interface dynamics is an important issue to understand the hot spot formation mechanism in high energy explosives. We have studied the interface between octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and graphite. The former is a good performance explosive but has high sensitivity. The latter is the desensitizer for the former. Two kinds of dynamic processes have been investigated: breaking and slipping. The structure evolution, energy variation, and breaking/slipping stresses were calculated. We found that different interface processes lead to different energy dissipation ways. For breaking, it is by surface relaxation. For slipping, it is by interface friction. Both the two ways contribute to the hot spot formation and shock sensitivity of explosives.     

Comparison of simplified models for nitramine propellant combustion

Although 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) and Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are very similar molecularly and their burning rates as a function of pressure are nearly identical, it is well known that they differ significantly in temperature sensitivity, especially at low pressures. To understand these differences better, three simple models were applied to HMX and RDX combustion. Both the Denison–Baum–Williams and Li–Williams–Margolis models have previously been calibrated for use with RDX. However, the RDX calibration of the Ward–Son–Brewster model was developed in the present work. All three models were compared with relevant measured data including: burning rate, flame stand-off/thickness, combustion stability, and temperature sensitivity. It was shown that all models are capable of accurately determining the burning rate of HMX and RDX as a function of pressure at the baseline initial temperature, but only two of the models are capable of capturing the variation in temperature sensitivity for both HMX and RDX, and only one model can replicate all the other measured characteristics within experimental uncertainty. Analysis using this model suggests that the surface reaction of RDX is much less exothermic than HMX and that there is a shifting between the gas phase and surface reaction dominance with pressure for HMX. This explains why the temperature sensitivity for RDX is nearly flat for low pressures while the temperature sensitivity for HMX increases significantly as the pressure decreases. Importantly, these trends are achieved without adding significant model complexity or having parameters change with pressure or initial temperature.     

A method to calculate the thermal conductivity of HMX under high pressure

A method based on Debye theory is developed to calculate the thermal conductivity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The phonon–phonon interaction model is built up for solid HMX. The phonon lifetime formula is derived by the phonon–phonon scattering mechanism, and the thermal conductivity tensor is derived by the phonon dispersion model. The thermal conductivities of α/β/δ-HMX are calculated in the temperature range 0–700?K and pressure range of 0–10?GPa. The phonon softening process of HMX is investigated. We have proven that the Debye frequency and thermal conductivity tend to 0 at the phonon softening point. A physical picture of the phonon–phonon interaction, phonon lifetime and phonon softening is built up.     

Effects of Explosive Particle Microstructure Parameters on NQR Parameters in RDX

The effects of the particle size, density and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) content on 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) nuclear quadrupole resonance (NQR) line have been studied at 5, 10, 15 and 20?°C. The RDX line at 3.41?MHz has been measured in 17 different quality lots. The RDX line was not modified in this temperature range but was strongly altered in some lots. No significant correlation was found between line characteristics and particle sizes or particle bulk densities. Correlation coefficients were computed between the HMX content measured using high-performance liquid chromatography and the NQR line intensity and NQR line width. Significant correlations were found. They were based on the study of 11 RDX lots which exhibited 4 different HMX contents from 0 to about 9 percent in weight. Further studies are needed to precise the HMX effect in relation with the HMX location. HMX can be located inside or outside the RDX crystal. Further studies are also needed to determine the line broadening mechanism.     

Raman spectroscopy as a tool for long‐term energetic material stability studies

Raman spectroscopy is shown to be useful as a tool for long‐term stability studies. The stability of a novel submicron‐size γ‐polymorph form of the explosive HMX (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) was monitored up to 1 year using Raman spectroscopy. The preparation and characterization of this submicron HMX has been previously discussed. It is important to know the long‐term stability of this submicron γ‐polymorph material under ambient conditions for its eventual utilization. This submicron HMX material has been found to be stable both chemically and polymorphically (to the order of 1% impurity level), even though HMX normally converts to the β‐polymorph at ambient conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

β-1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷热膨胀各向异性的量子化学计算与分子动力学模拟

在303～383 K和NPT系综和COMPASS力场下对β-1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷(HMX)超晶胞初始结构的分子动力学模拟,得到常压下各温度的晶体平衡构型并发现分子的堆积方式不变;通过线性拟合求算出线膨胀系数与实验值相近,体现出明显的各向异性. 采用密度泛函理论方法对沿各晶轴方向膨胀率变化(100%～105%)的HMX单胞模型进行了总能计算,得到的能量变化率体现各向异性并与热膨胀系数值关联,建立了关联方程. 由此阐 释了HMX晶体热膨胀各向异性的本质即特定的分子堆积模式.     

Structure and detonation performance of a novel HMX/LLM‐105 cocrystal explosive

Intermolecular interactions and properties of octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐ tetrazocine (HMX) / 2,6‐diamino‐3,5‐dinitropyrazine‐1‐oxide (LLM‐105) cocrystal were studied by using the dispersion‐corrected density functionals (ωB97XD, B97D) and meta‐hybrid functional (M062x) methods. Binding energies, heats of formation, thermodynamic properties, atoms in molecules, and natural bond orbital analysis were performed to investigate HMX/LLM‐105 complexes. Results show that the main intermolecular interactions between HMX and LLM‐105 are CH…O, NH…O, N…O, and O…O interactions. In addition, Monte Carlo simulation was employed to predict the crystal structure of HMX/LLM‐105 cocrystal. The HMX/LLM‐105 cocrystal is most likely to crystallize in C2/c space group, and its corresponding cell parameters are Z = 8, a = 41.63 Å, b = 6.77 Å, c = 45.63 Å, ß = 164.55°, and ρ = 1.99 g/cm³. Detonation velocity and pressure of HMX/LLM‐105 cocrystal are 8.95 km/s, 37.69GPa, a little lower than those of HMX (9.10 km/s, 37.76GPa). However, according to the net charges of nitro group, HMX/LLM‐105 cocrystal exhibits less sensitive than HMX. Finally, bond dissociation energy calculation shows that HMX/LLM‐105 complexes are thermally stable. Considering thermal stability, sensitivity, and detonation performance, HMX/LLM‐105 cocrystal meets the requirements of insensitive high energy density materials. Copyright © 2013 John Wiley & Sons, Ltd.     

The pressure-induced phase transition of the solid β–HMX

The structural, vibrational and thermodynamic properties of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β–HMX) crystal have been studied using the isothermal-isobaric molecular dynamics (NPT-MD) simulations. The variations of cell volume, lattice constants and molecular geometry of solid β–HMX are presented and discussed at different pressure and temperature. It was found that the N–N bond is significantly lengthened with increasing temperature, which suggests that it is relevant to the initial decomposition. An abrupt change at 27 Gpa for the volume and internal geometrical parameters was observed. This is in good accord with the experimental observation that there is a phase transition at 27 GPa, which is clearly due to conformational change, not chemical reaction. The vibrational frequencies at ambient conditions agree well with experimental results, and the pressure/temperature-induced frequency shifts of these modes are discussed. Frequency discontinuity was also observed at pressure when the phase transition occurred. The Grüneisen parameter was obtained using the vibrational frequency.     

原位红外光谱法研究HMX炸药的热分解过程

原位红外光谱法是一种新兴的动态研究方法.该方法具有原位实时监控和红外光谱精确分析物质化学结构的优点,能够实时跟踪材料在不同温度下的化学变化,测定材料的微观结构与温度的关系.文章采用原位红外光谱研究了炸药1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷(HMX)在5℃·min-1升温条件下的热分解过程.研究结果表明:HMX在205℃发生C-N键和N-N键的断裂,随着温度的升高,C-N键的断裂速率远高于N-N键的断裂速率,表明C-N键的断裂是HMX的主要断键方式,在C-N键的断裂中伴随着N-N键的断裂.同时环的张力增大,表明断键的HMX产生分子内重新结合.检测到HMX的分解所释放出的CO2,N2O,CO,NO,HCHO,HONO,NO2和HCN等八种气体.根据HMX分解中凝聚相结构的变化和气相产物,推出HMX的分解机理:HMX产生C-N键的断裂,会释放出HCHO和N2O以及HONO和HCN;N-N键的断裂会释放出NO2.     

Theoretical insight into the influence of molecular ratio on the binding energy and mechanical property of HMX/2-picoline-N-oxide cocrystal,cooperativity effect and surface electrostatic potential

Molecular dynamics method was employed to study the binding energies and mechanical properties of the selected crystal planes of the energetic/nonenergetic 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX)/2-picoline-N-oxide cocrystal in different molecular ratios. The solvent effect in HMX:2-picoline-N-oxide (1:1) was calculated, and the cooperativity effect was discussed in HMX···HF/CH₄···2-picoline-N-oxide by using the M06-2X and MP2(full) methods at the 6-311++G^** basis set. The density, oxygen balance and detonation velocity were calculated. The results indicate that HMX/2-picoline-N-oxide prefers cocrystalising in the 1:1, 2:1 and 3:1 molar ratios, in which the cocrystals own the highest binding energy and best ductility. The cooperativity effect is present in the cyclic complex with CH₄, while the anti-cooperativity effect is found in the HF complex. Thus, in order to obtain stable HMX/2-picoline-N-oxide cocrystal, the solvent with low dielectric constant should be chosen, as is in accordance with the result from solvent effect. The reduced density gradient (RDG) and surface electrostatic potential analysis confirms the cooperativity effect and reveals the nature of decreased sensitivity in complex (or cocrystal). The cocrystals in the molar ratios of 5:1–10:1 could be satisfactory in view of explosive properties.     

Factors affecting the NQR line width in nitramine explosives

A number of factors associated with crystal quality contribute to the nuclear quadrupole resonance (NQR) line width. Imperfections such as dislocations, voids, strain and impurities can be electrical sources that distort the electric field gradient at nearby quadrupolar nuclei and broaden the observed NQR line. We measured the¹⁴N NQR line widths in powdered samples of the nitramine explosives hexahydro-1,3,5-trinitro-s-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane and show correlations with sample purity, particle size distribution and density. Cast plastic-bonded explosives containing either RDX or HMX were also studied and their line widths compared with those of the powdered samples.     

Kinetics of reversible polymorphic transitions in high-energy compounds. Phase transformations in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

The kinetics of reversible phase transitions (PTs) in various polymorphs (α, β, γ, δ, and ?) of polycrystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is investigated by means of differential isothermal and scanning calorimetry. The rate of the β → δ PT is limited by the nucleation process occurring during the induction period. In a general case, the distribution density for the induction times is a superposition of continuous and discrete functions. The reverse δ → β PT obeys the first-order kinetic law. The effects of mechanical exposure on the kinetics and the PT products of the different polymorphs of HMX is investigated by FTIR spectroscopy.     

Unreacted equation of states of typical energetic materials under static compression:A review

The unreacted equation of state(EOS) of energetic materials is an important thermodynamic relationship to characterize their high pressure behaviors and has practical importance. The previous experimental and theoretical works on the equation of state of several energetic materials including nitromethane, 1,3,5-trinitrohexahydro-1,3,5-triazine(RDX),1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane(HMX), hexanitrostilbene(HNS), hexanitrohexaazaisowurtzitane(HNIW or CL-20), pentaerythritol tetranitrate(PETN), 2,6-diamino-3,5-dinitropyrazine-1-oxide(LLM-105), triamino-trinitrobenzene(TATB), 1,1-diamino-2,2-dinitroethene(DADNE or FOX-7), and trinitrotoluene(TNT) are reviewed in this paper. The EOS determined from hydrostatic and non-hydrostatic compressions are discussed and compared. The theoretical results based on ab initio calculations are summarized and compared with the experimental data.     

原位漫反射红外光谱研究升温速率对HMX炸药热分解过程的影响

原位红外光谱法是一种新兴的动态研究方法。该方法具有原位实时监控和红外光谱精确分析物质化学结构的优点,能够实时跟踪材料在不同温度下的化学变化,测定材料的微观结构与温度的关系。采用原位漫反射红外光谱研究了炸药1,3,5,7-四硝基-1,3,5,7-四氮杂环辛烷(HMX)分别在每min 5, 10, 20和40 ℃四种不同升温速率下的热分解行为。研究结果表明：在5 ℃·min-1升温速率下,断裂的HMX环发生分子内结合,在10, 20和40 ℃·min-1升温速率下,断裂的HMX发生分子间成环,形成稳定的八元环结构。随着温度的升高,C—N键的断裂数率远高于N—N键的断裂速率。随着升温速率的增加,C—N键的起始分解温度增加,表明增加升温速率会引起HMX分解的滞后。检测到HMX的分解所释放出CO₂, N₂O, CO, NO, HCHO, HONO, NO₂和HCN共八种气体,升温速率的变化未改变HMX的分解机理。     

环五甲撑五硝胺(CRX)结构和性质的DFT预示

用密度泛函理论 (DFT)B3LYP方法 ,在 6 31G 基组水平下 ,全优化计算了环五甲撑五硝胺 (CRX)的分子几何和优化构型下的电子结构 .环C -N键长为 0 .144～ 0 .148nm ,N -NO2 键长为 0 .139～ 0 .142nm ;CRX的最高占有MO(HOMO)能级和最低未占MO(LUMO)能级之间的差值ΔEg(5 .2 0 5 4eV)较大 ,预示CRX较稳定 .基于简谐振动分析求得IR谱频率和强度 .运用统计热力学方法 ,求得在 2 0 0～ 12 0 0K的热力学性质C0p ,m、S0 m 和H0 m.还运用Kamlet公式预示了它的爆速和爆压分别为 916 9m/s和 37.88GPa .     

The MMA and SO3Li-grafted polypropylene separator used in Li ion batteries

In the present work, the study is focused on the influence of the sulfonic (SO₃Li) and methyl methalacrylate (MMA) functional groups on the lithium metal electrode?Celectrolyte interface. We attempt to improve the cyclic performance of lithium metal anode and to suppress the dendrite formation by introducing surface functional groups into the electrode?Celectrolyte interface. Here, this introduction is realized by grafting the surface functional groups into the separator surface. The plasma technique is used to graft the surface functional groups into the surface of polypropylene (PP) separator, and the infrared experiment has also evidenced that these functional groups are successfully grafted into the surface. The symmetric cells and graphite half cells are constructed to study the effect of the functional group interaction with lithium dendrite formation on lithium?Cseparator and graphite?Cseparator interfaces. The experiment??s results have showed that the MMA and SO₃Li functional groups are effective for the suppression of the dendrite formation. The possible mechanism is proposed. The local acidic environment in the microarea near the MMA and SO₃Li functional groups can promote the dissolution reaction of the formed dendrite which competes with the formation reaction of the dendrite during Li-electrodeposition process. In addition, it is also found that these functional groups are also effective to improve the reversible capacity of the graphite negative electrode, probably due to solid electrolyte interface layer optimization.     

Molecular dynamic simulation of amorphous carbon and graphite interface

Amorphous carbon/graphite interface is modeled by molecular dynamic simulation using a Tersoff-type potential function with the Brenner parameters for in-plane interaction combined with the pair potential function for the interplanar bonding. The interface is created by compressing the amorphous carbon produced in a separate simulation with perfect crystalline graphite terminated to expose (1120) planes. The planar structure and weak interplanar bonding allow the graphitic planes to deform in order to accommodate the bonds formed at the interface, which is consistent with the HRTEM study of the interface. The simulation indicates that the generated interface mostly consists of nearly sp₂ hybridized bonding connecting the two sides. The bonds across the interface when formed are likely to maintain their equilibrium configurations. Due to the large interplanar spacing, many atoms both on the graphite and a-C sides are left unbonded leaving the interface energetically unfavorable with respect to the bulk. These unbonded radicals probably weaken the structural rigidity of the interface providing a fracture path under stress.     

Theoretical studies on the structure,sensitivity, density,thermodynamic properties and detonation performance of dinitrobitriazoles

Azodinitro- and dinitroethylene-bridged bitriazoles are of interest in the contest of high explosives, and were found to have true local energy minima at the B3LYP/aug-cc-pVDZ level of theory. The optimised structures, vibrational frequencies and thermodynamic quantities for bitriazoles were obtained in the ground state. Kamlet–Jacobs equations were used to evaluate the performance of bitriazoles based on the predicted density and the calculated heat of explosion. Detonation properties (D = 8.12 to 9.23 km s^?1 and P = 28.0 to 39.83 GPa) of bitriazoles were found to be promising compared with those of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX, D = 8.75 km s^?1 and P = 34.7 GPa) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX, D = 8.96 km s^?1 and P = 35.96 GPa). The fusion of azoles particularly appears to be a promising area for investigation, since it may lead to the desirable consequences of higher heat of explosion, higher density and thus improved detonation performance.     

Sol-gel synthesis of nanocomposite crystalline HMX/AP coated by resorcinol-formaldehyde

This work aims to improve the performance of composite explosive by using the sol-gel method to mix high explosive and oxidizer in nanoscale. Nanocomposite materials of HMX and AP were prepared by using resorcinol-formaldehyde (RF) as binder. Its structure was characterized by scanning electron microscopy (SEM), BET method, X-ray powder diffraction (XRD), and DSC. SEM images indicate that HMX/AP/RF aerogel has a laminate-like structure with uniform pores. The XRD results show that the mean crystal size of HMX is less than 100 nm; HMX and AP are mixed uniformly in nanoscale. The specific surface area of HMX/AP/RF is 27 m²/g and much less than that of RF aerogel. The mesopores and micropores of HMX/AP/RF aerogel mainly focus in the range of 2-20 and 0.6-1.6 nm, respectively. DSC analysis indicates that the thermal decomposition temperature of HMX/AP/RF is reduced compared to that of original HMX.