On gas release by thermally-initiated fully-dense 2Al·3CuO nanocomposite powder

A recently developed model for low-temperature exothermic reactions in nanocomposite Al-CuO thermites described the evolution of an alumina layer growing between Al and CuO and changes in its diffusion resistance as critically affecting ignition of the composite reactive material. The model was successful in describing ignition of individual composite particles in a CO₂ laser beam. However, it was unable to conclusively predict ignition of the same powder particles coated onto an electrically heated filament. In this work, ignition of fully-dense 2Al·3CuO nanocomposite powder prepared by arrested reactive milling was studied using a modified electrically heated filament experiment, located in a miniature vacuum chamber. Thin layers of the powders coated on a nickel-chromium filament were ignited at heating rates between 200 and 16,000 K/s. The ignition was accompanied by both optical emission and pressure signals. The pressure signals occurred before emission, with increasing delay at higher heating rates. Ignition temperatures were only slightly affected by the heating rate. The results are interpreted proposing that the low-temperature redox reaction produces a metastable CuO_1-x phase with 0 < x « 1 which releases oxygen upon heating. It is shown that despite a relatively small heat release, the low-temperature reactions in nanocomposite thermites are important as producing destabilized, partially reduced oxides that decompose with gas release upon heating. In the present experiments, the gas release changed thermal properties of the powder coating, reducing the efficiency of heat exchange with the supporting filament and thus enabling its thermal runaway and ignition.