Ultrafast mechanochemical synthesis of mixed oxides

Based on experimental data on the structure of disordered milling products in MO-M'O-3 systems, a theory is proposed for ultrafast mechanochemical synthesis in oxide systems, considered as a threshold process. The key points of the theory are (1) the development of a transient dynamic state (D)* in contact regions via mass transfer by the roll mechanism in primary loading events and the formation of rotation regions (D)*2 in secondary events; (2) the formation of ordered reaction products supersaturated with vacancies as a result of the relaxation of the (D)* and (D)*2 states under unloading and quenching conditions; (3) the formation, as a result of secondary events, of a nanocomposite through mechanochemical equilibrium due to diffusion between the ordered and disordered states of different compositions; and (4) rapid charge separation and recombination, leading to reduction of oxides during milling concurrently with the disintegration of particles and mechanochemical interactions. The principal factors influencing the dynamics of mechanochemical synthesis (established by linearizing the dependence of the chemical response of the system on mechanical load) are the molecular weight of the simple oxides involved, the enthalpy of the reaction, and the difference in Mohs' hardness between the reactants. The typical structures of mechanochemical synthesis products (solid solutions) are stable to disordering and compositional changes. The theory is supported by the trimodal mechanochemical synthesis rate distribution for mixed oxides and the mechanically induced electron-hole ferromagnetism. Ways of controlling the dynamics of mechanochemical synthesis and the structure of the products are discussed.