Annealing effect on oxidized α-Fe particles

A pigment of acicular fine α-Fe particles for magnetic recording tape is produced by reducing goethite under hydrogen gas and oxidizing by small amount of air diluted with nitrogen and toluene. In order to improve stability, the α-Fe particles are annealed under nitrogen gas, temperature is varied between 150°C and 400°C. It is found that surface of oxidized α-Fe particles thus obtained consists of amorphous-like iron oxide and is crystallized by annealing, and its structure is assigned to Fe3O4 or γ-Fe2O3 and Fe3O4 mixture, while magnetic and morphological properties of α-Fe are essentially unchanged during annealing. As result of humid exposing test, it is confirmed that the annealed α-Fe particles are more stable than the only oxidized one.