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Lead azide (LA) is a commonly used primary explosive, the detonation growth of which is difficult to study because it is so sensitive and usually has a small charge size in applications. We used photon Doppler velocimetry (PDV) and calibrated polyvinylidene fluoride (PVDF) gauges to reveal the detonation growth in LA, which was pressed in the confinements with controlled heights. The particle-velocity profiles, output pressure, unsteady detonation velocity, reaction time, and reaction-zone width were obtained and analyzed. Three phases of detonation propagation of LA microcharges are discussed. The volume reactions occur at the beginning of detonation in LA microcharges without forming complete shock profiles. Then the shock front is fast with a slow chemistry reaction zone, which is compressed continuously between the height of 0.8 mm and 2.5 mm. Finally, the steady detonation is built at a height of 2.5 mm. The stable detonation velocity and CJ pressure are 4726 ± 8 m/s and 17.12 ± 0.22 GPa. Additionally, the stable reaction zone time and width are 44 ± 7 ns and 148 ± 11 μm. The detailed detonation process has not previously been quantified in such a small geometry.

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Shock initiation criteria are essential to the shock initiation process and applications in modern pyrotechnics. The most commonly used shock initiation criteria are Walker and James criterion, which can very well describe the threshold data of the impactor with sufficient size. However, the criteria were also found not to provide a good fit to the data of thin, curve, or small flyers. By comparing the wave structure of the shock process with the stable detonation wave structure, a shock initiation criterion is developed based on the concept of the chemical reactions during the impact, focusing more on the properties of the explosives. Furthermore, a desktop micro-flyer initiating system was designed for the initiation of the thin metal flyer. The obtained data and classical historical data were analyzed with the proposed criterion, producing an excellent fit, with R2 values greater than 0.96. Compared to the existing criteria, the proposed criterion can weaken the influence of the interfacial properties of the impact and collapse the threshold velocity data with different impactor types to a single curve. The shock sensitivities of various explosives are also discussed based on the criterion. A denser impactor or the incorporation of impurities may contribute to the generation of the hot spot during the impact, leading to an increase in the sensitivity. The proposed criterion provides insight into the development of the shock initiation criteria and may help to understand the mechanism of shock initiations.