The Microstructure And Thermal-mechanical Damages Of Polymer Bonded Explosive Studied By Scattering Method

Polymer bonded explosives（PBX）is a kind of mixed explosives,which can be regarded as a composite material made of high-energy explosive crystal and polymer binder.It is widely used in weapon and industrial blasting.When PBX is used as the main charge for nuclear weapons,HMX and TATB are often used as explosive crystals,and their content is more than80%.In the process of production,processing,transportation and storage,PBX will experience complex temperature and mechanical environment,resulting in holes,microcracks and other forms of damage,affecting its detonation performance and safety performance.Therefore,studying the damage behavior of PBX from the micro-（meso-）perspective is helpful to reveal the damage evolution and mechanism,and understand the relationship between the macro properties and microstructure of energetic materials,which is of great significance to control and evaluate the quality of PBX.In this work,X-ray small angle scattering（SAXS）,ultra small angle scattering（USAXS）and wide angle scattering（WAXS）techniques were used to characterize the microstructure evolution and thermal stability of HMX crystal particles under thermal stimulation,the effect of polymer binder on HMX phase transition behavior,and the pore structure of nano-structural TATB powder during the compression molding process.The research results are as follows:（1）The defects evolution and phase transformation behavior of HMX crystal particles with average size of 5μm and 20μm under thermal stimulation were studied.WAXS results show that the starting temperature ofβ→δphase transition of 5μm-HMX is higher than that of 20μm-HMX.The fitting analysis of Guinier theorem for SAXS curve shows that microdefects with rotation radius less than 1 nm are formed in both HMX crystals when the temperature is much lower than the phase transition temperature.It is speculated that the micro defects in this study are caused by the anisotropic thermal expansion of HMX,and have nothing to do withβ→δphase transition.The defect content of 20μm-HMX was higher than that of 5μm-HMX,and both of them increased with the increase of temperature.Microdefects formed at high temperatures remain at room temperature in vacuum.Crystal cracking occurs at 20μm-HMX at lower temperature.5μm-HMX has better thermal stability.（2）In order to investigate the effect of polymer binder on the phase transition behavior of HMX,HMX-based PBX containing polyester polyurethane（HMX-Estane）,Fluororubber(HMX-F₂₃₁₄),Nitrocellulose(HMX-F₂₃₁₄-NC)and pure HMX grain were prepared by press fitting process.WAXS results show that the phase transition starting temperature（T_i）of HMX grain is 188℃,the T_i of HMX-Estane（95∶5）is 186℃,the T_i of HMX-F₂₃₁₄（95∶5）is192℃,and the T_i of HMX-F₂₃₁₄-NC（95∶3∶2）is 198℃.Small amounts（2%）of nitrocellulose added to the binder increased the T_i by 10°C compared with HMX pellet.All samples retained theδ-phase on cooling from the high temperature phase to 100°C for 12 h in vacuum,except HMX-Estane which displayed a reversible phase transition after 3.5 h.Among the three types of binders studied only Estane promoted theβ→δphase transition and its inverse transition.This result was attributed to the dissolution（on heating）and the precipitation（on cooling）ofβ-HMX at the HMX/Estane interface.（3）The defect evolution behavior of nano particle（NPD）and nano network（NND）TATB under different pressing densities was studied.Through the splicing of USAXS and SAXS data,the quantitative characterization of 1～800 nm multi-scale pore structure of energetic materials is realized for the first time in China.According to the apparent density and absolute strength calibration combined with quasi-integral invariant analysis,the total porosity of NPD and NND decreased by 30%and 40%,respectively,while the porosity of 1-800 nm decreased by 4.5%and 4.7%,respectively.The results show that the pressure mainly acts on the large size of the pores during the pressing process,and the number and size of the pores are reduced.When the pressure is less than 10 k N,the compression density of NND is larger,the total porosity is higher,the interface area is larger,and the compression performance is better.