Grain boundary as an influential feature endues β-HMX powder with enhanced impact tolerance

The handling safety of high explosives, as routinely characterized by the drop-weight impact sensitivity (IS), has important repercussions on the development of explosives. Herein, the impressive low IS of β-HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane) spherulite and the corresponding desensitization mechanism are elaborated with the influential role of the grain boundary. The sensitivity is evaluated by a substantial number of IS tests using "X out of 25", Bruceton "up and down" and high-speed photography technology. The collected data, comprising over 12 batches of samples for a total of about 400 impact tests, are analyzed as a function of the particle structure parameters. The IS was found to positively relate to the porosity calculated from Small-Angle X-ray Scattering data which semi-quantitatively reflects the grain boundary in spherulite. The investigation into the influence of shape suggests that while particle shape did contribute to desensitization, the primary factor affecting the reduction in HMX sensitivity was the presence of grain boundaries. The grain boundary strengthening and spherical envelope are found to be influential as they may be beneficial for energy de-localization, thereby mitigating localized heating. The polycrystalline spherulite with purity (>98.5%), sphericity (>80.0%) and porosity (>0.4%) carefully controlled exhibits the lowest IS of 0% and H50 of c.a. 99.0 cm, outperforming any other monocrystalline β-HMX particles. This work introduces the grain boundary as a new consideration for desensitization in energetic crystals, which has been previously underestimated but holds promise for the design of high explosives with improved handling safety.