The mechanochemical self-propagating reaction between hexachlorobenzene and calcium hydride

We report on studies of the solid state reaction between hexachlorobenzene and calcium hydride carried out by high-energy ball milling. The transformation behavior depends on the intensity of the mechanical energy transferred to the reactants at the impact. At lower energy regimes, chemical conversions increase gradually as a function of the milling time and a large excess of calcium hydride was found to favor the reaction rate. Calcium hydride-chloride and benzene are formed as end products. Beyond an impact energy threshold, self-sustaining transformations were observed leading to an instantaneous transformation to hydrogen, graphite, and, depending upon the reactant molar ratio, to calcium hydride-chloride or calcium chloride. The sudden increase of the reactor-vial temperature was proportional to the hexachlorobenzene content in the reacting mixture and the total heat evolved was found to be in good agreement with the forecasted reaction enthalpies. The ignition time, i.e., the milling time at which the combustion-like event occurs, was followed as a function of the reactant composition. The incubation period rapidly decreases by increasing the calcium hydride to hexachlorobenzene molar ratio, that is, moving away from the stoichiometric composition at which calcium chloride forms predominantly. Some suggestions concerning the activation energy of the two competing end products were inferred from the mechanochemical yield which has been calculated as the ratio between the moles of reacted hexachorobenzene divided by the total injected energy dose. Keeping the molar composition constant and modulating the shock power intensity, the self-sustaining reaction takes place only when the same dose of mechanical energy has been supplied to the reacting system, irrespective of the single impact energy. (C) 1997 Academic Press.