Solid-solid interactions between ferric and cobalt oxides as influenced byAl2O3-doping

The solid-solid interactions between pure and alumina-doped cobalt and ferr ic oxides have been investigated using DTA, IR and XRD techniques. Equimola r proportions of basic cobalt carbonate and ferric oxide and different amou nts of aluminum nitrate were added as dopant substrate. The amounts of dopa nt were 0.75, 1.5, 3.0 and 4.5 mol% Al2O3. The results obtained revealed th at solid-solid interaction between Fe2O3 and Co3O4 takes place at temperatu res starting from 700 degrees C to produce cobalt ferrite. The degree of pr opagation of this reaction increases progressively as a function of precalc ination temperature and Al2O3-doping of the reacting solids. However, the h eating of pure mixed solids at 1000 degrees C for 6 h. was not sufficient t o effect the complete conversion of the reacting solids into CoFe2O4, while the addition of a small amount of Al2O3 (1.5 mol%) to ferric/cobalt mixed solids followed by precalcination at 1000 degrees C for 6h conducted the co mplete conversion of the reacting solids into cobalt ferrite. The heat trea tment of pure and the 0.75 mol%-doped solids at 900 and 1000 degrees C effe cted the disappearance of most of IR transmission bands of the free oxides with subsequent appearance of new bands characteristic for the CoFe2O4 stru cture. An increase in the amount of Al2O3 added from 1.5-4.5 mol% to the mi xed solids precalcined at 1000 degrees C led to the disappearance of all ba nds of free oxides and appearance of all bands of cobalt ferrite. The promo tion effect of Al2O3 in cobalt ferrite formation was attributed to an effec tive increase in the mobility of the various reacting cations. The activati on energy of formation (Delta E) of CoFe2O4 phase was determined for pure a nd doped solids. The computed values of Delta E were, respectively, 99.6, 8 7.8, 71.9, 64.7 and 48.7 kJ mol(-1) for the pure solid and those treated wi th 0.75, 1.5, 3 and 4.5 mol% Al2O3. (C) 2000 Elsevier Science B.V. All righ ts reserved.