Relationship Between Microstructure And Laser Ignition Property Of RDX Explosive Crystals

As one of the important energy sources of modern weapons,energetic materials are crucial for the entire armament systems.The previous studies indicate that the ignition properties of energetic materials initiated by various methods(such as shock waves,mechanical initiation,etc.)are significantly dependent on their microstructures.Compared to traditional initiation techniques,laser ignition has some important applications in fields such as aerospace industries and national defense due to its high security and strong anti-interference ability.However,there are few papers focusing on the relationship between microstructure of energetic materials and laser ignition property up to now.In this dissertation,the relationships between microstructure and laser ignition properties of RDX energetic crystals have been studied.Meanwhile,interaction mechanisms between laser and energetic materials have been discussed.Besides,ColdField Emission Gun Cs-Corr Transmission Electron Microscopy,combined with low temperature sample frozen techniques,has been successfully applied to characterize the high-resolution fine structures and defects of RDX crystals.The main results are as follows:1.The Relationships between micro-defects of RDX explosive crystals and laser ignition properties have been studied.It indicates that micro-defects on the RDX facets have significant influence on the surface roughness root mean square(RMS)of RDX crystals.Moreover,laser ignition energy threshold of RDX explosives decreases with the increasing surface roughness RMS.Meanwhile,both parameters,B(fit coefficient)and Hmin(minimum laser fluence that the explosives can be ignited),which are related with laser ignition probability,are found to have an exponential relationship with the crystal surface roughness RMS.2.Laser ignition properties of RDX single crystal slices have been studied under ultraviolet(UV)laser(355 nm)and near-infrared(near-IR)laser(1064 nm)irradiations,respectively.It has been observed that the RDX crystal can be ignited more easily under UV laser due to its different absorption coefficients at varied light wavelengths.The results of laser-induced damages of energetic crystals show that:(1)UV laser irradiation: Due to strong absorption of RDX crystals at UV wavelength,min*()1B H Hp e--(28)-laser-induced damages appear on the incident surface of RDX crystal slice.The damage morphologies can be classified as three types: heat-induced fine cracks,fine cracks as well as some pits,and large cavities.The cavity damage morphology includes core region and peripheral region of stripped material surrounding the core.As the laser fluence increases,the area of peripheral crack region always increases,while the area of core region and its damage depth firstly increase and then remain constant when the laser fluence is over 12 J/cm2.(2)Near-IR laser irradiation: Owing to the weak absorption of the RDX slice at nearIR wavelength and restriction effects of plasma,laser-induced damages occur on the exit surface of the RDX crystal slices.Laser damage can be also divided into three types: melt effects induced irregular long fine-cracks,heat-mechanical effects induced orientated fine cracks,and large cavities.(3)Comprehensive analysis of experimental results shows that the dynamic process of laser igniting RDX crystals can be described as follows: firstly,plasma fireball appears.Then expansion of the plasma fireball generates shock wave which also propagates through the air and RDX crystal respectively,and finally results in crystal materials ejection and bulk damages near the surface.3.The scratches,micro-voids,and inclusions are not only the common structure defects of RDX explosives,but also the primary area for hot spots generations.In this work,based on the three-dimensional finite difference time domain(3D-FDTD)methods,effects of various microstructure defects of RDX crystals on modulations of the incident laser and their laser ignition sensitivities have been systemically studied.The simulated results are as follows:(1)Scratch defects: For single scratch,the width of scratch affects more on the light intensification than its depth.Meanwhile,triangular scratch defects exhibit stronger modulation than the parabolic ones.The coupling effects of multiple scratches make it more easy to generate hot spots under laser irradiation than single one.RDX crystals are more easily initiated when the intervals between two parallel scratches are ?-4? or the intersecting scratches are perpendicular with each other.(2)Void defects: For single void,the wider or deeper void has higher laser sensitivity.RDX materials can be more easily ignited when laser incidents from the crystal surface with voids.For two voids,modulation effects are stronger when the intervals are 0.75?-3?.With the increasing number of voids,the modulation increases and RDX crystals can be initiated more easily.(3)Inclusion defects: For ellipsoid air bubble,it presents largest enhancement on light intensification when the laser incidents through the long axis of the defects.While for acetone inclusion,RDX materials can be more easily ignited when the laser incidents through the short axis.Besides,when the relative dielectric constant ?r of the inclusion increases,modulation increases firstly and then decreases.RDX materials have a lowest laser ignition threshold when ?r=7.4.Advanced analytical methods of TEM have been utilized to obtain electron irradiation parameters of RDX crystals for structural stabilities.Local multiple twinned crystal phenomena have been found.It indicates that the electron injection rate can better reflect the anti-radiation resistance of RDX crystal on the electron beams compared to the total dose and the damage threshold is in the level of 103 e/nm2?s-1.Besides,significant polycrystallines and high-density atomic heap stacking faults have been observed in the edge of RDX crystals.This study is expected to provide reference for explaining influences of crystal structure type,defect distribution and density of energetic materials on their laser initiation sensitivities in the atomic-scale level.