Crystallisation of spray-dried amorphous precursors in the SrO–Al2O3 system: a DSC study

The crystallisation of amorphous precursors in the SrO–Al2O3 system has been studied by DSC and XRD. Amorphous precursors were obtained for compositions ranging from 12SrO.7Al2O3 to Al2O3 by spray-drying aqueous solutions of strontium and aluminium nitrates followed by heating the powders to decompose the nitrates. Of the stable compounds SrA, SrA2 and SrA6, only SrA crystallises directly at a usual heating rate of 5 °C/min (in this notation Sr=SrO and A=Al2O3). All the alumina-richer compositions are characterised by the first and strong crystallisation of metastable solid solutions of SrA in its hexagonal form, stabilised by excess aluminium cations, or γ-Al2O3 stabilised by strontium cations, or even the two solid solutions, depending on the composition. The hexaaluminate SrA6 is formed as single phase by an exothermic transformation of the γ-Al2O3 solid solution at 1183 °C. The formation of SrA2 is complicated by the easy and competitive formation of Sr4A7, of close composition. The latter compound is stable only up to 1500 °C and is decomposed at higher temperatures. However SrA2, which is characterised by a slow kinetics of formation from the amorphous state as well as from the high temperature liquid state, may be obtained pure by slow heating or annealing at 900–1000 °C.