Exploration of the reactivities of homemade binary pyrotechnics

Understanding the properties of explosives is the basis for investigating and analyzing explosion cases. To date, due to the strict legal control of standard explosives and initiators, homemade pyrotechnics composed of oxidizers and fuels have become popular explosive sources of improvised explosive devices (IEDs) threatening greatly social stability and personal safety. The reactivity of pyrotechnics strongly depends on their intrinsic characteristics and operating conditions, which determine the efficiencies of heat and mass transfer between the reaction zone and the unreacted zone. Herein, the tests of thermodynamics, pressurization characteristics, and combustion propagation behaviors are conducted to explore the effects of oxidizer species, particle size, and loading density on the reactivity of homemade binary aluminum-based pyrotechnics. The results show that the pyrotechnics with potassium chlorate (KClO₃) have the strongest reactivity with the highest pressurization rate (dp/dt) and the shortest combustion duration. Compared with their counterparts based on aluminum microparticles(mAl), pyrotechnics consisting of Al nanoparticles (nAl) possess superior reactivity as expected, which results from the relatively short heat and mass transfer distances. The nAl-based pyrotechnics have a low reaction exothermic peak temperature, great heat release, great aluminothermic reaction completeness, and high produced peak pressure with several orders of magnitude higher pressurization rate. Increasing the loading density of the pyrotechnics over a certain value can change the dominant mode of heat transfer from convective to conduction, sharply decreasing the pressurization characteristics and combustion front propagation velocities (v_p). The results of theoretical calculations using the NASA-CEA codes show that loading density can alter the reaction process of the pyrotechnics, leading to a decrease in the predicted pressure per unit mass for Al/KNO₃ or Al/AP, and an increase for Al/KClO₃. For nAl/potassium nitrate (KNO₃), the density is between 1.0 and 1.25 g cm^-3, across which dp/dt decreases by one order of magnitude from 0.148 to 0.014 MPa ms^-1. In addition, v_p decreases by three orders of magnitude from 0.040 to 0.078 m s^-1. Distinct pressurization behaviors of nAl/AP are observed at a density of 1.5 g cm^-3, while the variation in nAl/KClO₃ reactivity fluctuates. These results are beneficial for the damage assessment of scenes caused by an explosion and for inversely calculating charge parameters.