| Plastic bonded explosives (PBX) composed by explosive particles and polymers are widely used in the area of explosives. PBXs are made by hot isostatic pressing during which the temperature and pressure may results in cracks and voids. The resulting damaged HMX-PBX will become more sensitive to shock or impact initiation. Therefore, the study of the microstructure evolution of HMX-based PBX during the thermal stimulus plays a key role in its performance which can help to predict the response of munitions in abnormal thermal environments and understand the physical and chemical processes of the thermal defects.Octahydro-1,3,5,7-tetranitro-1,3,5,7 tetrazocine (HMX) is a high energy materials which has a higher energy than RDX. Normally, the HMX in HMX based PBX (HMX-PBX) are the stable β phase, however, it will change to the most unstable 8 phase when subjected to high temperature, and the thermal expansion, phase transition, increase of binder viscosity may results in cracks and voids. Small-angle scattering is a powerful technique for the investigation of nano-scale defects in explosives and can provide quantitative information of void size distribution and specific surface area.In this work, the in-situ small-angle X-ray scattering (SAXS) technique was used to analyze the thermal damages of HMX-PBX during and after the thermal process:The thermal defects of HMX particles are measured by small angle X-ray scattering and atomic force microscopy. At 180℃, new populations of pores with about 10nm developed in HMX, and the pores grow bigger and decrease in number with increasing time. A large number of pores with an average size of 6 nm are found in HMX when heated at 190℃ and 200℃. The size of the pores increases by the increasing temperature.The effect of the HMX grain size on the thermal effects of HMX-PBX was studied by small angle scattering. Two kinds of HMX-PBX with different HMX particle size name by FHP (about 40um in HMX sizes) and LHP (about 100um in sizes) were analyzed, and no obvious voids change were found in LHP samples heated at 160℃, while the magnitude of pores in FHP decreased. The total pores volume of FHP and LHP was found to increase when heated at 180℃and the average pore radii decreased from 45 nm to 25 nm, the pore volume of LHP reached to a high level after heating at 180℃ for 1h, while that of FHP needs 3h. Both FHP and LHP have a high pore volume when heated at 200℃for 1 and 3 h.The thermal damages in HMX-PBX during heating and subsequent cooling were quantitatively characterized by the total interfacial surface area and void volume distribution. The interfacial surface area had an obvious decrease above 100℃, indicating a filling effect of the binder into the voids. And then the interfacial surface area increased obviously at room temperature after cooling. A new population of voids with approximately 30nm in diameter caused by δ-β phase transition was produced in the sample which implies that voids and cracks in HMX-PBX will develop not only during the elevating temperature but also after thermal treated. |
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