Solid–solid interactions in Fe 2O 3/MgO system doped with aluminium and zinc oxides

Abstract

The solid–solid interactions between MgO and Fe 2O 3 in the presence and absence of small amounts of Al 2O 3 and ZnO (2 and 6 mol%) have been investigated using DTA and XRD techniques. Equimolar proportions of magnesium basic carbonate and α-Fe 2O 3 were employed and aluminium and zinc were added as aluminium and zinc nitrates. Pure and doped mixed solids were subjected to heat treatment at 500–1100 °C. The results obtained revealed that the addition of small amounts of dopant nitrates to the reacting mixed solids much enhanced the thermal decomposition of magnesium carbonate to an extent proportional to their amounts added. The decomposition temperature of magnesium carbonate decreased from 520.6 to 382.8 and 375.7 °C in the presence of 12 mol% aluminium nitrate and 6 mol% zinc nitrate, respectively. MgO underwent solid–solid interaction with Fe 2O 3 at temperatures starting from 700 °C producing MgFe 2O 4. The degree of propagation of this reaction increases progressively by increasing the precalcination temperature and the amounts of dopant added. The heating of pure and doped mixed solids at 1100 °C for 6 h was not sufficient to effect the complete conversion of the reacting solids into MgFe 2O 4. However, the amounts of unreacted oxides much decreased as a function of the amount of ZnO or Al 2O 3 added. The stimulation effect of ZnO, and Al 2O 3 towards the thermal decomposition of magnesium carbonate and the ferrite formation was attributed to an effective increase in the mobility of the various reacting cations. The activation energy of formation (Δ E a) of magnesium ferrite was determined for pure and different doped solids and the values obtained were 114, 78, 62, 91 and 76 kJ mol −1 for pure mixed solids and those doped with 2 and 6 mol% ZnO and 2 and 6 mol% Al 2O 3, respectively.