Combustion velocities and propagation mechanisms of metastable interstitial composites

Abstract

Combustion velocities were experimentally determined for nanocomposite thermite powders composed of aluminum (Al) fuel and molybdenum trioxide (MoO3) oxidizer under well-confined conditions. Pressures were also measured to provide detailed information about the reaction mechanism. Samples of three different fuel particle sizes (44, 80, and 121nm) were analyzed to determine the influence of particle size on combustion velocity. Bulk powder density was varied from approximately 5% to 10% of the theoretical maximum density (TMD). The combustion velocities ranged from approximately 600 to 1000m∕s. Results indicate that combustion velocities increase with decreasing particle size. Pressure measurements indicate that strong convective mechanisms are integral in flame propagation.

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

“…The order of magnitude of the reaction time and the temperature rise time can be estimated based on the measured value of pressure rise time at the reaction front which is t f Ӎ 10 s. 3 Such a reaction time is consistent with the data in Ref. 6.…”

“…͑1͒ The flame propagation rate is observed to be independent of particle size for particles diameters d smaller than 80− 100 nm, 3 while for diffusion controlled oxidation it is proportional to d −2 . 9 ͑2͒ The ignition time delay is found to be independent of particle size for particle diameters d smaller than 120 nm; for diffusion controlled oxidation it is a power function of diameter.…”

“…Thus flame rates of 0.9− 1 km/ s can be reached ͑see Refs. 1-4 for low density powders of Al+ MoO 3 and Al+ Fe 2 O 3 nanocomposite powders͒, while for micron size thermites they are on the order of centimeters or meters per second. Ignition delay time also decreases by up to three orders of magnitude.…”

The effect of Si–Bi2O3 on the ignition of the Al–CuO thermite

“…Substituting micrometer-sized Al with nano-sized Al particles also changes the kinetic mechanism of the combustion reaction [18]: While flame speed increases with density for micron-scale Al particle compositions, the opposite is observed for nanoscale aluminium particle mixtures [19]. The high combustion velocity (reaching 1000 m.s -1 ), observed for nano-scale thermites [14,15], suggests that strong convective mechanisms operate in flame propagation [4,15].…”

“…Reducing the fuel particle size into the nanometer range is much more effective as the interface area is substantially increased [13,14,15]. The use of nano-aluminium as fuel enhances ignition sensitivity, i.e.…”