Journal of the Japan Society of Powder and Powder Metallurgy

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 CO_0.75-Fe_2.25O₄. Hence, the main reaction during firing-reaction of magnet formation-was considered to be
6 CoFe₂O₄+3 Fe₂O₃→8 Co_0.75Fe_2.25O₄+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.

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For the purpose of pre-paring Co-Ni alloy powder, the solid solution of hydroxides of Co and Ni, Co_1-xNi_x (OH)₂, were prepared by simultaneous-precipitation from mixture of aqueous solution of Co(NO₃)₂⋅6H₂O and Ni(NO₃)₂⋅6H₂O with 0.1N-NH4OH at 85°C and then these precipitates were reduced in hydrogen at various temperatures ranging from 300°C to 700°C. Analysis of crystal structure by X-ray and electron diffraction and chemical analysis of polarograph showed that the precipitates were solid solution of Co(OH)2 and Ni(OH)₂ and their composition also corresponded to the mixed ratio of the nitrates of Co and Ni. From thermomagnetic analysis, the alloy powders reduced were homogeneous solid solution of Co and Ni with composition corresponded to the raw materials respectively. Observation of morphological properties of these powders by electron microscope showed that alloy powders consisted of the smallest unit particles which were linked up with each other, resulting formation of aggregates resembled in shape with original crystals of their mother salts. The size of the unit particles increased with increasing temperature of reduction. Metal-lographical phases of alloy powders prepared were obeerved by X-ray diffraction.

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