(652f) Structure – Reactivity Relationship: Milling-Induced Exothermic Nano-Materials

An efficient approach that combines short-term (minutes) high-energy dry ball milling and wet grinding to tailor the nanostructure of Ni+Al, Ti+C, Si+C and Ta+C composite reactive particles is reported. Depending on the system and milling conditions two different reaction mechanisms, i.e. sudden ignition and gradual solid-state diffusion for milling-induced reactions are observed. The ignition sensitivity and reactivity of thus prepared materials are investigated as function of milling time. A direct correlation between the milling-induced nanostructures and ignition characteristics (ignition temperature, ignition delay time and kinetics of chemical reactions) are also discussed. Analysis of the microstructures and reaction kinetics suggests that different factors may be responsible for reactivity enhancement. In Ni+Al and Si+C systems such enhancement is primary related to the formation of intimate contact between the reactants. The fine mixing of reactants creates fresh oxygen-free boundaries which reduces the energy barrier for diffusion processes. In metal – carbon systems (e.g. Ti+C, Ta+C) the solid-state dissolution of carbon in the metal crystal lattice is an important factor responsible for reactivity enhancement.  Classification of the exothermic systems from the stand point of the dominant reaction mechanism under high energy ball milling conditions is suggested.