Mechanochemical Mechanism for Fast Reaction of Metastable Intermolecular Composites Based on Dispersion of Liquid Metal

A new mechanism for fast reaction of Al nanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of small liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). A number of theoretical predictions are confirmed experimentally. Main controlling physical parameters are determined. Some methods to expand the melt-dispersion mechanism for micron particles are formulated. This mechanism resolves some basic puzzles in combustion of Al particles. Some basic parameters of the melt-dispersion mechanism (strength of the oxide shell, heating rate necessary to activate the mechanism and size of the dispersed clusters) are estimated. It is found that the melt-dispersion mechanism may induce a new mode of energy transfer and heating ahead of flame front. Molten and reacting Al clusters are dispersed at speeds that exceed the macroscale flame velocities of the mixture. In this way, the molten Al clusters can heat the reactant mixture prior to flame propagation. An equation for the flame velocity versus Al nanoparticle geometrical parameters, thermomechanical properties, and synthesis parameters is formulated. The melt-dispersion mechanism was also found to be in agreement with experiments for 1–3 micron diameter Al particles and fluorination. Our results completely change the direction in which the Al nanoparticle synthesis progresses.