(541c) Ignition Mechanisms of Metal Powders in Electrostatic Discharge

Ignition of reactive powders in electrostatic discharge (ESD) is commonly used to assess the material reactivity and safety of its handling. Despite the widespread use of this test and despite availability of several respective commercial devices, the results of testing are often inconsistent between different facilities or even between different operators using the same setup. These discrepancies are largely due to poorly established criteria of what does or does not constitute powder ignition. The problem of defining ignition criteria stems from poor understanding of the mechanisms of powder initiation by electric sparks. Our recent work suggests that Joule heating of the powder by the spark current may be the primary effect of the spark discharge. The powder heated by the passing current can ignite either inside the holding cup or outside of it, being ejected by the shock wave produced by the spark. In both cases, ignition is driven by heterogeneous exothermic reactions either on the particle surfaces or on the reactive interfaces in composite materials. Ignition occurs when the kinetics such reactions and the resulting particle self-heating becomes sufficiently rapid due to the achieved particle temperature. For pure metal particles, it is important that the oxidizer is available to maintain the reaction. This paper is aimed to validate this proposed ignition mechanism and experimentally investigate ESD ignition of different metal powders. The ESD ignition experiments were performed with Al powders with different particle sizes as well as with powders of Ti and Fe. The spark energy is measured from the recorded current and voltage traces. The emission produced by the igniting or heated particles at different wavelengths is recorded using filtered photomultipliers. The streaks produced by the igniting particles are visualized using a digital camera. The ESD ignition experiments are supplemented by the ignition experiments using the same powders coated onto an electrically heated filament. In the filament heating experiments, the heating rates are varied systematically to assess the ignition reaction kinetics. The ignition kinetics is in turn used to interpret the results of the ESD ignition tests.