Abstract
OP magnet is industrially manufactured by nitric acid method. That is, the mixture of nitrate solutions of Fe and Co is prefired up to 850°C in air after evaporation and drying. The resulting powder is then pressed (compacted) to desired shape and fired above 1000°C in air of about 1mm Hg.
The chief constituents of the prefired powder are CoFe2O4 and a-Fe2O3, while the fired magnet consists of a solid solution CoFe2O4-Fe2O4 of spinel structure, the composition being approximately CO0.75-Fe2.25O4. Hence, the main reaction during firing-reaction of magnet formation-was considered to be
6 CoFe2O4+3 Fe2O3→8 Co0.75Fe2.25O4+1/2O2.
Authors studied about the reaction by assuming that the reaction velocity is controlled mainly by the diffusion velocity of Fe and Co ions in the solid solution, and that evolution of oxygen has no great influence upon the reaction velocity, and found that the so-called Jander's equation holds good in this case. The activation energy of the reaction was calculated to be 41Kcal/mol.
