Transition from Impact‐induced Thermal Runaway to Prompt Mechanochemical Explosion in Nanoscaled Ni/Al Reactive Systems

Abstract

The effect of microstructure on ignition sensitivity and reaction behavior is investigated for nanoscaled Ni/Al gasless reactive systems. Nanometric homogeneity of the reactive media was achieved through (a) conventional mixing of nanometric powders; (b) short‐term high‐energy ball milling (HEBM) of micrometer‐sized powders. Sensitivity to thermal inputs is investigated by differential thermal analysis and mechanical sensitivity is studied by high‐rate shear impacts. The composite Ni/Al particles prepared by HEBM were extremely thermally sensitive, with reaction initiating at 220 °C, compared to 559 °C for nanometric powder samples and 640 °C for un‐milled, micrometer‐sized Ni+Al powder mixture. In contrast, nanometric powder mixtures were more susceptible to ignition through mechanical means, exhibiting a high‐speed reaction mode that is not observed in HEBM samples. The high‐speed mode preferentially appears in high‐shear regions and is interpreted as a mechanically‐induced thermal explosion. Its progression is tied to the passage of a stress wave in the heterogeneous media that heats and mixes the materials, rather than being propagated due to chemical energy release. The microstructures unique to each material are considered responsible for their individually ignition sensitivities. Specifically, the finely interspersed porosity in nanometric powder mixtures allows direct heating of the reactive interface between Ni and Al particles during compression through pore collapse and plastic deformation, which leads to exceptionally high mechanical sensitivity. The HEBM materials have high specific reactant interface area in the bulk of each composite particle that enhances thermal sensitivity, but the relatively low specific interface area between particles is unfavorable to mechanical ignition.

Experimental Study on Behind‐Plate Overpressure Effect by Reactive Material Projectile

“…Significantly different from traditional energeticm aterials, such as explosives and propellants,t he traditional initiationm anners (e.g. shock or flame) are not sufficient to initiate the reactivematerial projectile (hereafter called reactive projectile) as elf-sustaining reaction.H owever,w hen impacting with ah igh-strain-rate plastic deformation process, the reactive projectilei sp rovided the required energy to drive the reaction, releasing ag reat chemical energy [1][2][3][4][5].…”

“…Significantly different from traditional energeticm aterials, such as explosives and propellants,t he traditional initiationm anners (e.g. shock or flame) are not sufficient to initiate the reactivematerial projectile (hereafter called reactive projectile) as elf-sustaining reaction.H owever,w hen impacting with ah igh-strain-rate plastic deformation process, the reactive projectilei sp rovided the required energy to drive the reaction, releasing ag reat chemical energy [1][2][3][4][5].Particularly,w hen perforating the skin of the target, the reactive projectile deflagrates in the interioro ft he target and releases ag reat of chemical energy.D amage enhancement is achievedn ot only by the ignition capacity but also the behind-plate overpressure inside the target [6][7][8]. Therefore, it is crucial to investigate the behind-plate overpressure effect by reactivep rojectile for its damagea ssessment and poweri mprovement.M ock et al have conducted Ta ylor impact experiments to explore the influences of particle size and impact velocity on the time after impact for initiation,o btaining an empirical relationship of impact pressure and the time after impact for initiation [4].A mes et al have used av ented chamber to investigate the energy release characteristic of reactive projectile by ballistic impact experiments, giving the relationship between the pressure in the chamber and the chemical energy released [9, 10].M oreover,d ynamicc ompressiont ests were performed on PTFE/Al/W reactive materials to investigate the influence of tungsten content on thee nergy release and impact sensitivity [ 11].However,i nfluences of the plate thickness on the behind-plate overpressure effect haven ot been well known.…”

Shock-induced reaction synthesis of cubic boron nitride

“…SRS has been applied to many reactive systems, resulting in the synthesis of a significant number of solid compounds including carbides, 3,4 borides, 5 silicides, 6 and aluminides. 7 However, fundamentally, there are two distinguishing cases for SRS. 8 In the first case, the shock wave heats the material enough that, after pressure release, the material reacts through a deflagration combustion wave with a characteristic reaction time on the order of several milliseconds.…”

“…[9][10][11][12][13] In the second case, a gasless reaction takes place directly in the shockwave within several microseconds. 7,14 In general, two methods have been suggested to prove the existence of such ultra-fast gasless reaction. 15 The more common approach involves an in-situ measurement and comparison of the shock Hugoniot for reactive and inert mixtures which allows an inference of reaction occurring on the timescale of the shockwave.…”