Effect of pre-heat treatment and cobalt doping of hydrargillite on the kinetics of the hydrargillite-corundum transformation in supercritical water fluid

We have studied the kinetics of corundum formation in supercritical water fluid from aluminum hydroxide (hydrargillite) and from the products of its thermal decomposition at 300°C over a period of 5 h. In all cases, the reaction involves the formation of boehmite, AlOOH, as an intermediate. The use of the product of the thermal decomposition of hydrargillite as a precursor considerably increases the reaction rate and reduces the induction period in comparison with the parent hydrargillite. Cobalt doping further increases the reaction rate and reduces the induction period. When the product of the thermal decomposition of hydrargillite is used as a precursor, the temperature dependence of the reaction rate constant shows Arrhenius behavior, whereas that for synthesis from uncalcined aluminum hydroxide has a more complex character.