Modes of Ignition of Powder Layers of Nanocomposite Thermites by Electrostatic Discharge

ABSTRACT

Nanocomposite powders with aluminum as a fuel and oxides of molybdenum, copper, bismuth, and iron as oxidizers were prepared by arrested reactive milling. The powders were placed in 0.5-mm-thick layers and ignited by electrostatic discharge (ESD) in air. In different tests, time-resolved light emission was recorded at 568 nm or in the range of 373–641 nm. The amount of material consumed was recorded as well. Time-resolved temperatures were determined. Two distinct ignition regimes were observed. Prompt ignition occurred within 10 µs of the electric discharge, comparable to what had previously been observed for corresponding powder monolayers. This ignition mode was observed for composites with Bi₂O₃ and Fe₂O₃ ignited with a 12-kV discharge, whereas it only occurred at higher spark voltage (20 kV) and energy for CuO and MoO₃ composites. Delayed ignition, occurring after 0.1–1 ms following the discharge, was observed for all composites with consistently stronger light emission. Analysis of quenched, partially burned particles showed that the original nanostructure was preserved after prompt ignition but not after delayed ignition. It is proposed that prompt ignition represents direct ESD initiation of composite particles rapidly and adiabatically preheated to high temperatures while keeping the nanostructure intact, resulting in a heterogeneous reaction consuming most of the aluminum. Delayed ignition occurs when particles preheated to lower temperatures start oxidizing at much lower rates, leading to cloud combustion, in which thermal interaction between individual aerosolized burning particles is substantial. During this process, the nanostructure may be lost. Temperature measurements show that nanocomposites with CuO and MoO₃ burned superadiabatically with flame temperatures exceeding thermodynamic predictions.

KEYWORDS:

• Energetic materials
• heterogeneous combustion
• metal combustion
• reactive materials

Acknowledgment

The interest and encouragement of Dr. Ralph Anthenien of ARO are greatly appreciated.

Funding

This work was supported by the U.S. Army Research Office (ARO).

Additional information

Funding

This work was supported by the U.S. Army Research Office (ARO).