| 摘要 |
PURPOSE:To obtain ferromagnetic fine powder which has good spicularity, accordingly a large coercive force and assures high density recording by applying an AC magnetic field which is 0.8-1.2 times the coercive force of ferromagnetic powder obtained to the powder under the heat processing. CONSTITUTION:The spicular goethite (alpha-FeOOH) powder of 5g in average grain size of 0.4mum and axial ratio of 10:1-20:1 is put into an almost cylindrical vessel 1, the powder is desiccated by heating the air in the vessel 1 up to 600 deg.C with a heater 2 and is then reduced at a temperature of 380 deg.C in the mixed gas of H2 and CO2, oxidized at 250 deg.C in the mixed gas of air and N2. Thereby, the powder of gamma-Fe2O3 can be obtained. During the heat processing for executing any of the desiccassion, reduction and oxidation, an AC external magnetic field is applied to the container 1 from two pairs of electromagnets 5a, 5b and 6a, 6b which are arranged almost orthogonally around the container 1. In case an AC external magnetic field applied has an intensity HAC of 500Oe, movement of particles becomes active while frequency of magnetic field is 20-100Hz, particularly when it is about 50Hz, and the sintering is most suppressed and accoringly the best spicularity of gamma-Fe2O3 can be obtained. |
