Nature of high reactivity of metal/solid oxidizer nanocomposites prepared by mechanoactivation: a review

In this review, the regularities of formation, structure and high reactivity of two types of energetic metal/solid oxidizer nanocomposites (Al(Mg)/X (X = MoO₃, (–C₂F₄–) _n )) prepared by mechanoactivation are examined. One reason for the high reactivity is an increase in contact surface between the components occurring after mechanoactivation. Two methods for determination of area of contact surface S _C between the components are used, and the values of S _C for all the systems are estimated. Considerable attention is paid to the role of highly reactive defects (grain sizes, dislocations and stacking faults, paramagnetic centers, “weakly bound” oxygen in MoO₃, etc.), formed in the components under mechanical stress. For the Me/MeO₃ systems, the formation of point defects in the oxide is an important factor. It was found that, after mechanoactivation, the evolution of O₂ from MoO₃ occurs at 230–450 °C. It is argued that this process is associated with the thermal destruction of “weak” Mo–O bonds in the “bridge” oxygen. It was suggested that the formation of defect structure in MoO₃ and increasing of the oxygen mobility under heating give rise to a low-temperature peak in DSC curves and initiated self-ignition on the fuel–air mixture. For composites Mg/MoO₃, self-ignition occurs at temperature 100 °C lower than that for Al/MoO₃: The decreasing of temperature can be connected with larger S _C in the first system. In the Mg/(–C₂F₄–) _n system, the reactions of magnesium defects with (–C₂F₄–) _n are accompanied by a weak heat evolution, too low to initiate ignition. In this case, the reaction is initiated by the thermal depolymerization of (–C₂F₄–) _n , while a high values of S _C provide a complete conversion. In the case of shock-wave initiation, defects in the components play only a minor role in the conversion, whereas the value of S _C remains to be highly important.