Mechanochemical activation of Al/V2O5 composites: Thermal transformations

Abstract

Thermal transformations of mechanochemically activated (MCA) thermites Al/V2O5 (mixtures of Al with bare coarse V2O5 as well as with highly defected nanosized n-V2O5) were analyzed by the methods of X-ray diffraction, calorimetry, EPR spectroscopy, adsorption and SEM. The influence of the treatment time of MCA on stages of thermal transformations during oxidation of aluminum in the reaction 10Al + 3V2O5 → 5Al2O3 + 6V was revealed and the optimal conditions of MCA were established. It was shown that the use of highly defected n-V2O5 does not lead to changing the ignition conditions of mixtures and the dynamics of thermal reactions. For the non-activated mixtures and at short-time of MCA, the intensive oxidation of Al occurred by the liquid-phase mechanism at Т > 680 °C, above melting temperatures of the both components. At the optimal conditions of MCA, transformations are started at temperatures only slightly above 150 °C by the solid-phase mechanism. Reduction of V2O5 occurs via several lower vanadium oxides: V2O5 → V4O9 → V6O13 → VO2-1 → VO2-2 → V3O5 → V2O3 → VO → V. Heating of mixtures is accompanied by the increase of concentrations of paramagnetic centers (PCs) VO2+ of two types up to 420 °C, and by their noticeable decrease at 520 °C. Assumptions concerning relations between PCs and phases V4O9, VO2-1, and on the effect of PCs on initiation of the solid-phase transitions are discussed. MCA treatment carried out upon optimal conditions promotes the explosive way of the reaction at firing the activated samples. Possibility of passing the reaction by the solid-phase mechanism as the result of MCA can be caused by the formation of large surface area of contacts ca. 7–8 m2/g, and also by the formation of vanadyl PCs clusters.