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Microscopic Mechanisms of Femtosecond Laser Ablation of HMX from Reactive Molecular Dynamics Simulations
摘要： With ultrashort duration and ultrahigh energy, femtosecond laser (fs-laser) pulses are very promising for the precision machining of energetic materials. Compared with the mechanical machining methods of energetic materials, fs-laser machining technology has the advantages of high safety, high precision and absence of pollution. A deep understanding of the mechanisms between fs-lasers and energetic materials is the basis for the development of fs-laser machining technology. In this paper, the method of reactive molecular dynamics (ReaxFF-MD) was adopted to calculate the fs-laser ablation process of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX, a high explosive compound), and the ablation mechanisms of HMX under different fs-laser energies were studied. The results show that the fs-laser ablation mechanisms of HMX are related to the laser power density. When the laser power density is high enough (3.4×1014 W/cm2, 1.0 mJ/pulse), HMX undergoes ionization or decomposition reactions at the picosecond level (~7.65 ps) and produces a high temperature and pressure plasma. Many N, H and O single atoms and their ionic products occurs along with some small molecular fragments of NO2, H2O, CO2, N2, H2, NH, NH2, CO, OH, CNO2 and very few toxic products of NO and HNO2. In this case, the removal process of HMX occurs via a phase explosion mechanism. As the laser energy decreases, the ionization degree of ablation products decreases, in which the number of monoatomic and ionic products decreases, while the number of toxic small molecules (such as NO, HNO2, and HNO) increases. When the laser power density is relatively low (0.34×1014 W/cm2, 0.1 mJ/pulse), the removal process of HMX occurs via a photomechanical mechanism, and the compound escapes as intact initial HMX molecules. When the laser power density is close to the ablation threshold of the explosive, the HMX molecules only undergo a melting state to some extent without escaping from the surface of the crystal. Therefore, the fs-laser can be used in the precise machining of explosives and preparation of high-purity energetic nanomaterials by a reasonable selection of fs-laser energy.
关键词： energetic materials,ReaxFF-MD,femtosecond laser,HMX,ablation mechanisms
更新于2025-09-23 15:21:01
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Surface Characterization of Nanoscale Co-Crystals Enabled through Tip Enhanced Raman Spectroscopy
摘要： Atomic Force Microscopy coupled with Tip Enhanced Raman Spectroscopy (AFM-TERS) was applied to obtain information about structure and surface composition of single nano co-crystals. For this purpose, a co-crystalline system consisting of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazatetracyclo-[5.5.0.03,11.05,9]-dodecane (CL-20) and 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX) in a molar ratio of 2:1 (CL-20/HMX) were chosen. CL-20/HMX nano-plates were prepared by Spray Flash Evaporation. To ensure co-crystallinity and nanostructure, powder X-Ray Diffraction and AFM investigations were performed. Results demonstrate that coherence lengths and particle dimensions are on a similar level though coherence lengths appear shorter than measured particle dimension. According to this fact, defects inside the nano co-crystals are minimized. The co-crystallinity was additionally proven by confocal Raman spectroscopy. Here, marker bands for pristine CL-20 and HMX were chosen which appear in the CL-20/HMX spectrum in an intensity ratio of ~ 2.5:1 (CL-20:HMX). Afterwards surface investigations of single CL-20/HMX nano-plates were performed by AFM-TERS. Due to the surface sensitivity of TERS, these experiments reveal that the ratio of the Raman intensities between CL-20 and HMX inverts at CL-20/HMX nano-plate surfaces. Therefore, it is concluded that nano co-crystal surfaces consist of molecular layers of HMX. A theoretical approximation of the normal coordinates of the investigated marker vibrations supports this conclusion since it can exclude the occurrence of the intensity ratio inversion because of the given orientation between CL-20/HMX nano-plates and the Raman scattering system. Based on this finding, an impact ignition mechanism is proposed.
关键词： surface characterization,CL-20/HMX,AFM-TERS,Raman spectroscopy,nano co-crystals
更新于2025-09-23 15:19:57
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The Effect of Metal Film Thickness on Ignition of Organic Explosives with a Laser Pulse
摘要： The results of numerical ignition simulation of pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) by aluminium (Al) and molybdenum (Mo) films heated by nanosecond laser pulses in a three-layer system: glass–metal–explosive material (EM) are presented. Influence of metal film thickness on the time of EM ignition delay was considered. A non-linier dependence of time of delay of ignition of EM from thickness of a metal film is shown. The greatest critical thicknesses of Al and Mo metallic films at which ignition of EM is still possible were determined. It was established that the greater the thickness of the metal film and heat resistance of EM, the greater the heat reserve needed in EM ignition film. It was established that the ignition delay time of EM increases in the sequence of PETN, RDX, HMX and TATB.
关键词： TATB,Al film,simulation,explosive,laser ignition,HMX,Mo film,PETN,RDX
更新于2025-09-12 10:27:22
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Effect of Decomposition of CuO Film on Ignition of Organic Explosives by a Laser Pulse
摘要： This work presents experimental results of the PETN ignition in a three-layer system: glass – copper oxide (CuO) – explosive materials (EM) and results of numerical simulations of the ignition of PETN, RDX, HMX, and TATB with a CuO film heated by a YLS-150 fiber laser with a pulse duration of 20 ms. In the numerical simulations, the endothermic reaction of decomposition of the CuO film and decomposition reaction of EM were taken into account. It has been established that the endothermic effect of the CuO decomposition reaction increases the dynamic ignition delay time of EM. The expression for an estimation of width of a reaction layer in EM at heating by a constant heat stream is obtained. Calculations have shown that the dynamic threshold of ignition of organic explosives by laser pulse in a three-layer system (glass – CuO – EM) has a minimum for the thickness of the copper oxide film of 5 mm. The density of the critical ignition energy of an explosive laser pulse increases in the sequence of PETN, RDX, HMX, and TATB. The calculated dependence of the PETN dynamic ignition delay time on the thickness of the CuO film is consistent with the experimental data.
关键词： PETN,explosive,RDX,TATB,copper oxide,HMX,laser ignition,simulation
更新于2025-09-11 14:15:04