Investigation of solid–solid interactions between pure and Li 2O-doped magnesium and ferric oxides

Abstract

The solid–solid interactions between pure and lithium-doped magnesium and ferric oxides have been investigated using DTA and X-ray diffractograms (XRD) techniques. Equimolar proportions of basic magnesium carbonate and α-Fe 2O 3 were employed and lithium was added as lithium nitrate. The amounts of dopant were 0.75, 1.5, 3 and 6 mol% Li 2O. The results obtained showed that the addition of small amounts of lithium nitrate to the reacting mixed solids much enhanced the thermal decomposition of magnesium carbonate to an extent proportional to its amount added. The addition of 12 mol% LiNO 3 decreased the decomposition temperature of MgCO 3 from 525.5 to 362°C. MgO underwent solid–solid interaction with Fe 2O 3 at temperatures starting from 800°C yielding MgFe 2O 4. The amount of ferrite produced increased when the precalcination temperature of the mixed solids was increased. However, the completion of this reaction required prolonged heating at elevated temperature >1100°C owing to the formation of MgFe 2O 4 phase which covered the surfaces of the grains of each solid, thus, hindering the diffusion of Mg 2+ and Fe 3+ ions. Doping with Li 2O much enhanced the solid–solid interaction between the mixed oxides to an extent proportional to the amount of Li 2O added leading to the formation of MgFe 2O 4 phase at temperatures starting from 700°C. The addition of 6 mol% Li 2O to the mixed solids followed by precalcination at 1050°C for 4 h resulted in complete conversion of the reacting oxides into magnesium ferrite. The promotion effect of Li 2O towards the ferrite formation was attributed to an effective increase in the mobility of the various reacting cations. The activation energy of formation (Δ E) of magnesium ferrite was determined for pure and variously doped solids and the values obtained were 203, 126, 95 and 61 kJ mol −1 for pure mixed solids and those treated with 1.5, 3 and 6 mol% Li 2O, respectively.