Electro-static discharge ignition of monolayers of nanocomposite thermite powders prepared by Arrested Reactive Milling
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Electro-static discharge ignition of monolayers of nanocomposite thermite powders prepared by Arrested Reactive Milling Ian Monk, Rayon Williams, Xinhang Liu, Edward L. Dreizin New Jersey Institute of Technology Newark, NJ 07102 Abstract Reactive nanocomposite powders with compositions 2Al∙3CuO, 2.35Al∙Bi2O3, 2Al∙Fe2O3, and 2Al∙MoO3 were prepared by arrested reactive milling, placed in monolayers on a conductive substrate and ignited by an electro-static discharge (ESD) or spark in air, argon, and vacuum. The ESD was produced by discharging a 2000 pF capacitor charged to a voltage varied from 5 to 20 kV. Emission from ignited particles was monitored using a photomultiplier equipped with an interference filter. Experimental variables included particle sizes, milling time used to prepare composite particles, surrounding environment, and starting ESD voltage. All materials ignited in all environments, producing individual burning particles that were ejected from the substrate. The spark duration varied from 1 to 5 µs; the duration of the produced emission pulse was in the range of 80 – 250 µs for all materials studied. The longest emission duration was observed for the nanocomposite thermite using MoO3 as an oxidizer. The reaction rates of the ESD-initiated powders were defined primarily by the scale of mixing of and reactive interface area between the fuel and oxidizer in composite materials rather than by the external particle surface or particle dimensions. In vacuum, particles were heated by ESD while remaining on the substrate until they began generating gas combustion products. In air and argon, particles initially pre-heated by ESD were lifted and accelerated to ca. 100 m/s by the generated shock wave; the airborne particles continued self-heating due to heterogeneous redox reactions. Keywords: spark ignition; ignition delay, heterogeneous reactions, reactive materials Introduction Electrostatic discharge (ESD) or spark is a ubiquitous ignition stimulus for energetic materials [1-4]. However, related ignition mechanisms remain poorly understood. For composite reactive powder mixtures [5] and for pure metals [6-8], it was recently shown that the Joule heating is the primary mechanism of energy transfer from the ESD to the powder. It was also reported that different ignition regimes, involving individual burning particles or aerosolized powder clouds, were observed when the thickness of a metal powder layer struck by ESD varied [6]. Most recently, ESD ignition of reactive nanocomposite thermite powders prepared by Arrested Reactive Milling (ARM) [9] was studied [10, 11]. Such powders contain particles with dimensions in the range of 1 – 100 µm, in which metal and oxidizer are mixed on the scale of ca. 100 nm. Two distinct spark-initiated ignition regimes were observed for nanocomposite thermites using Al as a fuel and CuO and MoO3 as oxidizers: monolayers of composite particles ignite within 100-200 ns after the spark discharge onset producing multiple individually burning particles [11], whereas thicker powder la
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