Abstract
Thermal dilatometric curves of OP magnets, Co-Fe ferrite, were measured in the temperature range from room temperature to 800°C. Measurement was made in air and also in low pressure on OP magnets which were sintered in air and in low pressure.
In this measurement an anomalous curve was obtained. When OP magnet which was field-cooled in low pressure of about 0.5mm Hg was heated in air, it showed a remarkable shrinkage in the temperatures from 300° to 400°C. This shrinkage is explained as follows ; In the temperatures between 300° and 400°C, Fe304 contained in OP magnet becomes γ-Fe2O3 by oxidation. In this case, increase in volume of the specimen seems to happen by increase of the number of mols due to combination of oxygen with Fe2O4, but actually only the volume of pores in sintered body decreases and the length of specimen is almost the same. This is because oxygen is thought to combine with Fe2O4 only on the surface of particles. When Fe2O4 become γ-Fe2O3 by oxidation, its lattice constant becomes from 8.37 to 8.30 Å, i. e. the specimen shrinks in length to 99.2%. When Fe2O4 which is about 1/4 of OP magnet shrinks to 99.2% in its length, OP magnet shrinks to 99.8% (Δl/l≅2×10-3).
Such assumption has been supported by dilatometric measurement and at the same time by thermal analysis.
