Introduction: Mechanochemical synthesis takes place in a ball mill, thus the heavy walls of the milling vial make in situ process monitoring very difficult. The reaction mechanism is complex, as important processes take place on several length and time scales, from the macroscopic operation of the mill to the deformation of small powder batches between the colliding balls and to the deformation-induced mixing and reactions between individual grains. Although recent progress in modeling mechanochemical processes is significant , any indirect information about the mechanism and key parameters of the process is valuable. Investigating the reaction kinetics is a possible method to gain such information.
Most mechanochemical reactions take place gradually, as small amounts of powder get trapped between the colliding balls repeatedly, and the heavy plastic deformation during impacts induces local mixing and reactions . The overall kinetics reflects the combined effect of many localized changes. As the typical time scale is several hours, it is possible to investigate the phase composition and other properties of the milled powder as a function of milling time ex situ.
If the reactant powder mixture is sufficiently exothermic, ball milling can initiate a Mechanically induced Self-sustaining Reaction (MSR) after some activation time. Even though the underlying ignition criteria are not precisely understood, the ignition time is a useful marker that is well reproducible and its variation from system to system or as a function of milling conditions is a useful tool to identify the key parameters of the process.
Authors: Francesco Delogu and Laszlo Takacs
Keywords: Mechanochemical Reactions, Kinetics, Mechanism, Self-Sustaining Reactions