(501f) Ignition of Reactive Material Powders in Vicinity of an Electrostatic Discharge

Electrostatic discharge (ESD) is a common ignition stimulus for powders of reactive and energetic materials. It is a common safety hazard when handling such materials; it can also be used to initiate pyrotechnic formulations. In laboratory experiments, ESD is produced by discharging a high-voltage capacitor via a gap between two electrodes. Produced electric spark typically lasts for several microseconds. In all reported studies, the powder being tested is placed in a holder, which serves as one of the ESD electrodes. It was shown that the ignition is caused by Joule heating, with the main heat release occurring at the interfaces between powder particles through which the current flows. However, ignition may occur in situations when the ESD does not pass directly through the powder. Indeed, ESD generates a shock wave, which can lift the powder from a nearby substrate, even if this substrate was not an electrode. ESD also generates an expanding kernel of plasma heated to ca. 10,000 K, which can exist much longer than the ESD current between electrodes. The effect of such a shock and plasma on a reactive material powder remains unexplored. Such effects are important for developing appropriate safety protocols for handling reactive materials. They can also be of interest for understanding the ignition mechanisms for different materials, which can be exposed to shock and plasma. In this study, a reactive material powder is placed on a substrate located in vicinity of an ESD initiated between two pin electrodes. The electric current and voltage of ESD are monitored using induction coils. The pressure generated by the ESD is recorded using a piezoelectric transducer. Experiments are performed with both inert and reactive material powders. For inert powders, the space above the substrate is illuminated using a laser sheet so that the particles lifted in air are visible using a high-speed video camera. For reactive powders, emission from the ignited particles is sufficient to observe them in similar high-speed videos. The ESD energy and distance from ESD electrode gap and powder are varied. Ignition is observed to occur for a range of distances between the ESD gap and the substrate holding the powders. Experiments are performed with powders of flammable metals, Magnesium and Zirconium, as well as with powders of nanocomposite thermites prepared by Arrested Reactive Milling. The results are interpreted considering the size and lifetime of the plasma kernel generated by ESD and also visualized by the high-speed video. A correlation between ignition of powders exposed to ESD plasma and shock and their thermally initiated ignition observed in different experiments is considered.