Diffusion and thermite reaction process of film-honeycomb Al/NiO nanothermite:Molecular dynamics simulations using ReaxFF reactive force field

The diffusion and thermite reaction process of Al/NiO nanothermite composed of Al nanofilm and NiO nano honeycomb are investigated by molecular dynamics simulations in combination with the Reax FF. The diffusion and thermite reaction are characterized by measuring energy release, adiabatic reaction temperature, and activation energy. Based on time evolution of atomic configuration and mean square displacement, the initialization of the thermite reaction process of Al/NiO nanothermite results from the diffusion of Al atoms. Under the microcanonical ensemble, it is found that the adiabatic reaction temperature of the thermite reaction process of Al/NiO nanothermite reaches over 5500 K, and activation energy is 8.43 k J/mol. The release energy of the thermite reaction process of Al/NiO nanothermite is 2.2 k J/g, which is in accordance with the available experimental value. With the same initial temperature, the adiabatic reaction temperature of the thermite reaction process of Al/NiO nanothermite has a tendency to decrease dramatically as the equivalence ratio increases. On the basis of chemical bond analysis, the initial temperature and equivalence ratio have great effects on the thermite reaction process, but do not significantly affect the average length of Al–Ni nor Al–O bond. Overall, the thermite reaction of film-honeycomb Al/NiO nanothermite is a complicated process instead of a theoretical equation.     

Burning structures and propagation mechanisms of nanothermites

Nanothermites demonstrate attractive combustion characteristics such as tunable reactivity and high energy density. There is however a lack of fundamental understanding on their burning structures and reaction mechanisms due to the multi-scale complexity associated with the material and reaction heterogeneities. This gap in turn hinders the optimization of nanothermite design with desirable microstructures and controllable burning properties. In this work, a high-speed microscopy imaging system was used to reveal the burning structure of Al/CuO nanothermites and to investigate the propagation mechanism of its flame front at micron and sub-millimeter scales which have not been studied. An Al/CuO nanothermite film was fabricated as a model structure. First, the previously proposed reactive sintering was confirmed as a micron-scale burning characteristic. Then, at the sub-millimeter scale, it was demonstrated that the non-uniform burning propagation of nanothermite films is featured with distinguishable roles of the active burning sites and the pre-ignition sites. The active burning sites are clusters of reactive sintering particles and the pre-ignition sites appear in the preheating regions where Al and CuO particles have not yet participated in the reaction due to insufficient ignition energy. These pre-ignition sites form randomly and are subsequently ignited by heat transferred from the adjacent active burning sites, resulting in an active burning propagation tangentially along the propagation front. At the same time, as the thermite reaction of nanoparticles in the unburnt region is initiated, the propagation front advances in the normal direction. This experimental work reveals that the burning propagation mechanism of nanothermite films is governed by active burning propagation in both tangential and normal directions of the propagation front. Although the rates of these two modes are on the same order of magnitude, the tangential propagation of active burning is slightly faster, implying that pre-ignition sites are readily ignited with lower ignition energy.     

Thermodynamics and molecular dynamics investigation of a possible new critical size for surface and inner cohesive energy of Al nanoparticles

In this study, the authors first review the previously developed, thermodynamics-based theory for size dependency of the cohesion energy of free-standing spherically shaped Al nanoparticles. Then, this model is extrapolated to the cubic and truncated octahedron Al nanoparticle shapes. A series of computations for Al nanoparticles with these two new shapes are presented for particles in the range of 1–100 nm. The thermodynamics computational results reveal that there is a second critical size around 1.62 and 1 nm for cubes and truncated octahedrons, respectively. Below this critical size, particles behave as if they consisted only of surface-energy-state atoms. A molecular dynamics simulation is used to verify this second critical size for Al nanoparticles in the range of 1–5 nm. MD simulation for cube and truncated octahedron shapes shows the second critical point to be around 1.63 and 1.14 nm, respectively. According to the modeling and simulation results, this second critical size seems to be a material property characteristic rather than a shape-dependent feature.     

Detonation performance of the CL-20-based explosive LX-19

The generation of constitutive detonation performance model components for high explosives (HEs) invariably involves reference to experiment, as reliable first-principles determinations of these models are beyond our current capability. Whatever its form or complexity, the detonation performance model must be able to accurately capture the detonation wave timing and the energy release that it triggers upon arrival. Specifically, the HE products equation-of-state (EOS), which largely determines the detonating HE’s ability to do useful work on its surroundings, is typically inferred from cylinder expansion tests where metal-confined HE cylinders are detonated and the ensuing outer confiner wall-expansion trajectory is recorded. Expensive, iterative comparisons to multimaterial hydrodynamic (or “hydrocode”) simulations of these experiments are then used to constrain the parameters of the chosen EOS form. Here, we report on new detonation performance experiments produced for the highly-ideal, plastic-bonded explosive and CL-20-based LX-19 which are used to produce a new sub-scale detonation performance model for the explosive. This includes new products EOS and a new Detonation Shock Dynamics front propagation law. We also confirm the capability of two new, non-hydrocode-based products EOS generation techniques to accelerate the HE model parameterization process. This latter development is particularly significant for detonation performance modeling of new HE formulations.     

爆轰产物的冷比内能与冷压的函数表达式

 猛炸药爆轰产物的状态可以用两相的强排斥-平动物态方程（简称为两相的排平物态方程）很好地描述。以爆轰产物分两段的等熵曲线为参考曲线的两相的排平（k, γ）物态方程,已经用于爆轰参数和强爆轰参数的理论估算,所得理论值与实验值符合得很好。为了更方便地估算爆温,有必要给出描述分子间相互作用的比内能项与压力项（分别简称为冷比内能与冷压）。参照描述分子间相互作用的Morse势和Mie势的排斥项,给出了带待定参数A、m、n和l的冷比能项和冷压项,这样的物态方程被称为两相的排平（A, m, n, l）物态方程。用TNT的{D, ρ₀}实验数据组,确定了两相的排平（A, m, n, l）物态方程的参数n=1和l=1/3,因此,可将其简称为两相的排平（A, m）物态方程。它适用于所有的猛炸药的爆轰产物。用硝基甲烷的强爆轰参数{p, D, T}实验数据组对其所做的检验表明,两相的排平（A, m）物态方程是恰当的爆轰产物物态方程。     

用TNT的{D,ρ0}实验数据对爆轰的ZND理论的检验

爆轰产物中或多或少含有固态碳 ,一相的排平物态方程被推广为两相的之后 ,以某种炸药的一条已知等熵线为参考曲线 ,就可以用来估算其各种初始装药密度下的爆轰参数 .用产物中含碳量较多的TNT的 {D ,ρ0 }实验数据与理论估算值相比较 ,可以对爆轰的ZND理论的假设进行检验 .检验的结果再一次表明 ,爆轰的ZND理论的假设是成立的 ,并且排平物态方程是恰当的爆轰产物的物态方程 .     

The formation of V–Al–N thin films by reactive ion beam mixing of V/Al interfaces

The reactive ion beam mixing (IBM) of V/Al interfaces by low-energy N₂⁺ ions at room temperature leads to the formation of V–Al–N ternary nitride thin films. The kinetics, growth mechanisms, composition and electronic structure of those films have been studied using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Factor Analysis and Monte Carlo TRIDYN simulations. The comparison of experimental results with those obtained from TRIDYN simulations suggests that the chemical reaction with the nitrogen partial pressure and processes driven by residual defects are the rate-controlling mechanisms during the reactive IBM of V/Al interfaces. The kinetics of mixing is characterized by two stages. During the first stage (≤4×10¹⁶ ions/cm²), the formation of vanadium nitride is observed. In the second stage, vanadium nitride is transformed into a V–Al–N ternary nitride due to Al incorporation in the near surface region. Moreover, the V/Al ratio can be varied in a broad range, whereas the nitrogen concentration slightly decreases with increasing the aluminium content of the film.     

ZnO–CuO core–shell nanorods and CuO-nanoparticle–ZnO-nanorod integrated structures

ZnO–CuO core–shell nanorods and CuO-nanoparticle–ZnO-nanorod integrated structures were synthesized for the first time by a two-stage solution process. Scanning electron microscopy and high-resolution transmission electron microscopy show that the diameter and the length of the nanorods are around 60 and 800 nm, respectively. The morphologies of outer CuO could be varied from nanoparticles to nanoshells by adjusting the solvent and dipping processes of copper (II) nitrate solution. The CuO nanoparticles are single-crystalline or highly textured structures with size of around 30 nm. The CuO shell with thickness of around 10 nm is constructed of nanocrystals with sizes in the range of 3–10 nm embedded in an amorphous matrix. Room-temperature cathodoluminescence measurements of the CuO–ZnO nanocomposites exhibit relatively sharp ultraviolet emissions at 380 nm as well as broad green and yellow emissions at 500 and 585 nm. The p-CuO/n-ZnO one-dimensional nanocomposites are promising for optoelectronic nanodevice applications.     

A numerical study on Rayleigh-Taylor instability of aluminum plates driven by detonation

The authors,using elastic-plastic hydrodynamic code,present the Rayleigh-Taylor (RT) instability of Al plates driven by high-explosive detonation. Our numerical study assumes the material is fluid,or it is an elastic-plastic solid,and we compare the results of these simulations with the experimental data. For the numerical simulation of Rayleigh-Taylor instability of the metal driven by high-explosive detonation,the elastic-plastic effect must be assumed. The result of the simulation is different from the e...     

A modified equation of state for Xe at high pressures by molecular dynamics simulation

The exact equation of state （EOS） for the fission gas Xe is necessary for the accurate prediction of the fission gas behavior in uranium dioxide nuclear fuel, However, the comparison with the experimental data indicates that the applicable pressure ranges of existing EOS for Xe published in the literature cannot cover the overpressure of the rim fission gas bubble at the typical UO2 fuel pellet rim structure. Based on the interatomic potential of Xe, the pressure-volume-temperature data are calculated by the molecular dynamics （MD） simulation. The results indicate that the data of MD simulation with Ross and McMahan＇s potential [M. Ross and A. K. McMahan 1980 Phys. Rev. B 21 1658] are in good agreement with the experimental data. A preferable EOS for Xe is proposed based on the MD simulation. The comparison with the MD simulation data shows that the proposed EOS can be applied at pressures up to 550 MPa and 3 GPa and temperatures 900 K and 1373 K respectively. The applicable pressure range of this EOS is wider than those of the other existing EOS for Xe published in the literature.     

Electrostatic discharge sensitivity and electrical conductivity of composite energetic materials

Composite energetic material response to electrical stimuli was investigated and a correlation between electrical conductivity and ignition sensitivity was examined. The composites consisted of micrometer particle aluminum combined with another metal, metal oxide, or fluoropolymer. Of the nine tested mixtures, aluminum (Al) with copper oxide (CuO) was the only mixture to ignite by electrostatic discharge. Under the loose powder conditions of these experiments, the Al–CuO minimum ignition energy (MIE) is 25 mJ and exhibited an electrical conductivity two orders of magnitude higher than the next composite. This study showed a similar trend in MIE for ignition triggered by a discharged spark compared with a thermal hot wire source.     

并行建表化学加速算法研究及其在气相斜爆轰模拟中的应用

采用详细化学反应机理对气相斜爆轰问题开展数值计算时,由于组分之间的特征时间尺度相差很大,反应源项的直接积分(direct integration,DI)求解通常存在强烈的刚性及非线性现象,导致计算量很大.为了在不损失计算精度的基础上有效减少化学反应过程的计算时间,针对包含2H2+O2详细机理的二维斜爆轰并行计算,提出两...     

CO oxidation activity of Cu–CeO2 nano-composite catalysts prepared by laser vaporization and controlled condensation

Ceria supported copper catalysts were synthesized by laser vaporization and controlled condensation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and temperature programmed reduction (TPR). The catalytic activity of the nanopowders for CO oxidation reaction was tested in a fixed bed flow tube reactor in Ar–20%O₂–4%CO mixture. Irrespective of the copper content, the catalytic activity of the nanopowders is similar in the initial CO test, and the catalytic activity improves (i.e. the light-off temperature decreases) during a subsequent run. The lowest light-off temperature during the second run is recorded in the material with 20% copper. TEM studies on 20%Cu–CeO₂ sample in the as-prepared condition and after CO test exhibit two types of ceria particles namely, polygonal particles 3–5 nm in size and spherical particles of 15–20 nm in size. Rapid cooling of the nanoparticles formed during the laser ablation results in incorporation of a large amount of copper within the ceria as solid solution. Presence of solid solution of copper is confirmed by EDAX and electron diffraction analyses. In addition, copper-rich surface layer of Cu₂O is found over the spherical particles. The cerium oxide components are essentially identical before and after CO test, except that the polygonal CeO₂ particles contain newly formed fine crystals of CuO. TPR results reveal two reduction peaks, which further supports, the presence of two different copper species in the material. The shift in light-off temperature during the second run is attributed to the synergistic interaction between newly formed CuO crystals with the CeO₂ matrix.     

Acceleration ability of high explosives and their mixtures with fuel additives. 1. Assessment by the M-40 method

The relationship between the chemical composition of the detonation products and the acceleration ability of explosives is considered. Based on theoretical and experimental studies, it is concluded that, at volumetric energy content, the most important parameter is the average atomicity of the molecules constituting the detonation products. Among the CHNOF-containing explosives, groups with high acceleration ability were identified. The experimental results on the acceleration ability obtained using the M-40 method are reported for mixtures of dispersed Al with HEs with different oxygen balance and for mixtures of HMX-based HE with Al, Mg, Y, Ta, ZrH₂. An analysis of the acceleration ability on the charge density for BTNEN and mixtures thereof with Al made is possible to estimate the degree of conversion of the energy of combustion of Al under the gasdynamic conditions typical of the M-40 method, which turned out to be 20–40%.     

A particle level-set based Eulerian method for multi-material detonation simulation of high explosive and metal confinements

The multi-material numerical simulation for energetic system that consists of a high explosive charge and an inert confinement is carried out with an accurate and state-of-the-art Eulerian method. An explosively driven copper tube results in a state of extreme temperature and pressure, coupled to a high speed structural response of metal due to a detonating high explosive (HE). We use the experimentally tuned Ignition and Growth (or JWL++) rate equation for the HE while the elasto-plastic response of inert is modeled by the Mie–Gruneisen equation of state (EOS) and the Johnson–Cook strength model. A new particle level-set based reactive Ghost Fluid Method (GFM) that imposes exact boundary conditions at the material’s interface according to physical restraints is developed to simulate the multi-material detonation problem. Our calculations reproduce the experimental data of both unconfined and confined rate stick problems, suggesting that the method is suitable for detonation simulation of energetic systems.     

Perturbation approach for equation of state for hard-sphere and Lennard–Jones pure fluids

In this paper we have established the equation of state (EOS) for liquids. The EOS was established for hard-sphere (HS) fluid along with Lennard–Jones (LJ) fluid incorporating perturbation techniques. The calculations are based on suitable axiomatic functional forms for surface tension S _m (r), r ≥ d/2 with intermolecular separation r, as a variable, and m is an arbitrary real number (pole). The results for βP/ρ from the present EOS thus obtained are compared with Percus-Yevick (PY), scaled particle theory (SPT), and Carnahan–Starling (CS). In addition, we have found a simple EOS for the HS fluid in the region which represents the simulation data accurately.     

Effect of carbon nanotube addition on the thermite reaction in the Al/CuO energetic nanocomposite

Abstract

In this work, the Al/CNT/CuO nano-thermite samples are prepared by ultrasonic mixing with variable CNT content. The morphology of nano-thermites analysed by electron microscopy revealed that the CNTs are dispersed and there are intimate contacts between fuels (Al and CNT) and oxidiser (CuO) constituents of the nano-thermite. Raman spectroscopy technique is used to analyse the structural integrity of the CNTs in the nano-thermite. The thermite reaction characteristics are evaluated by simultaneous thermogravimetric analysis/differential scanning calorimetry technique. The exothermic enthalpy of the Al/CNT/CuO nano-thermite samples increased with increasing CNT content. The effect of Al particle size and Al/Cu molar ratio variation on the thermite reaction enthalpy is also analysed. The ignition temperature of the thermite reaction is also lowered by 71 °C than that of Al/CuO nano-thermite. The activation energy for thermite reaction of Al/CNT/CuO nano-thermite is also lowered by 23% to that of pure Al/CuO. The residues of the nano-thermites after the thermite reaction at 1010 °C are collected and analysed by the X-ray diffraction.     

拐角空间中悬浮铝粉尘爆轰后效的三维数值模拟

建立三维的铝粉-空气两相爆轰计算模型,采用时-空守恒元解元（CE/SE）方法求解,并开发了悬浮铝粉尘爆轰的三维数值模拟程序.基于消息传递接口（MPI）技术实现了程序的并行化设计.通过对激波管问题以及爆轰管中铝粉-空气两相爆轰实验的模拟验证程序的可靠性.对拐角空间中左侧浓度为368 g·m^-3的铝粉-空气混合物两相爆轰及其在拐角空间右侧和下方空气域内形成的冲击波和温压效应开展数值模拟,获得复杂空间内爆轰波或冲击波的传播、反射以及绕射过程.结果表明：两相爆轰在离铝粉尘区域2 m远的空气域内产生的后效冲击波能达到2.66 MPa的固壁反射压力,火球燃烧范围会超出初始铝粉尘区域约0.8 m,并且造成初始铝粉尘区域附近1.5 m范围内空气的温度高达1 600 K.模拟程序可用于铝粉尘爆轰的后效研究,对工业安全及其防护具有指导意义.     

Improved equation of state for ionic liquids using surface tension

The major objective of this work was the development of a reliable model to describe volumetric properties of ionic liquids (ILs). In this regard, we have applied the Ihm–Song–Mason equation of state (EOS) to some phosphonium- and imidazolium-based ILs. Three temperature-dependent parameters in the equation of state have been scaled based on the surface tension and the liquid density at room temperature. In order to improve the predictive power of the mentioned EOS for ILs, we have proposed using a simple modification. We have taken 1,228 experimental points to show the reliability of the improved EOS. The comparison of predicted densities with literature data over a broad range of temperature, 293–472 K, and pressures up to 200 MPa led to encouraging results. The average absolute deviation of calculated densities from literature values was found to be 0.75%.     

Blast waves generated in an unconfined space by a nonideal detonation of high-density aluminum-enriched formulations

Experiments on the detonation of high-density (1.8 g/cm³) aluminum-ammonium perchlorate-paraffin-RDX formulations in an unconfined space demonstrated their high efficiency at pressure amplitudes within 0.3–7.0 atm. The relative pressure amplitude and impulse of the blast waves with respect to the analogous characteristics of TNT charges of the same mass were found to be 1.3–2.4. The TNT equivalents in pressure and impulse vary with the distance nonmonotonically, ranging within 1.4–2.8. The blast wave produced by an infield explosion of a 1.42-kg composite charge demonstrated its high performance characteristics. Measurements at blast wave amplitudes of 1 to 20 atm gave a TNT equivalent in pressure of up to 3 and a TNT equivalent in impulse of 1.3 to 1.8. The high parameters of blast waves in an unconfined space originate from both the high-energy characteristics of the systems themselves and the afterburning of excess metal fuel in air. To estimate the extent of participation of the reaction of excess metal fuel with air in supporting the blast wave, numerical simulations of the generation of blast waves for various rates of mixing of detonation products with air at the contact surface were conducted. The main elements of the mechanisms of the processes that determine the efficiency of explosive systems with a heat release spread in space and time were considered. It was concluded that an optimal regime of blast wave generation, capable of ensuring a prolonged attenuation of the wave with the distance, could be realized for low-velocity detonation.