The preparation of magnetic iron oxide

Two methods for the preparation of magnetic iron oxide starting from FeSO4 · 7H2O are described. The traditional method involves preparation of Fe(OH)2 seed material, conversion of the seed to acicular γ-FeOOH crystals, dehydration of the crystals to α-Fe2O3, hydrogen reduction of Fe2O3 to magnetite, and finally reoxidation of magnetite particles to γ-Fe2O3. Magnetic iron oxide particles thus obtained, after a total processing time of about 25 h, have a coercivity between 330 and 380 Oe. The other method, known as the direct method, involves fewer steps, a shorter overall processing time, and yields magnetic iron oxide having an equivalent or higher coercivity value. A sample from the seed preparation solution is recrystallized at 250 °C, yielding a product, γ-Fe2O3, which has a coercivity value of 320 Oe. In another procedure the seed is prepared in the presence of H2O2 as the oxidizing agent and recrystallized at 300 °C for 2 h to yield γ-Fe2O3 having 390 Oe coercivity.