Solid state formation of calcium ferrites from thermal decomposition of mixtures Ca3(C6H5O7)2 . 4H2O–Fe2(C2O4)3 . 6H2O. The role of mechanical activation on the mechanism of the thermal decomposition and the temperature of the ferrites formation

The present paper reports a novel synthetic procedure of both calcium ferrites (CaFe2O4 and Ca2Fe2O5) starting from mechanically activated mixtures of organic precursors (calcium citrate tetrahydrate and iron(III) oxalate hexahydrate). With the help of TG analysis the different stages of the mass loss process taking place could be individuated and their chemical nature assessed. The DSC signal, recorded on the same sample, revealed that much higher enthalpies are released from the milled mixtures than those expected on the basis of the reactions taking place. From such enthalpic excess the enthalpies of formation of Ca2Fe2O5 and CaFe2O4 have been obtained. X-ray powder diffraction (XRPD) indicated that the mechanically activated mixtures yield, after an 18 h-annealing at 800 °C (or 750 °C), Ca2Fe2O5 (or CaFe2O4). On the contrary, the same compounds could be obtained, when starting from physical mixtures, only by performing thermal treatments (for t > 18 h) at 1180 °C (Ca2Fe2O5) or 1100 °C (CaFe2O4).