Coercivity of Iron Oxides Treated with Sodium Triphosphate

The coercivity of a nonstoichiometric iron oxide (Fe2+/Fe3+=0. 11) increased when the iron oxide was treated with sodium triphosphate, Na5P3O10, at an elevated temperature as in the treatment of sodium metaphosphate, which was already reported. The increase of the coercivity depend on the temperature of the treatment and molar ratio of sodium triphosphate to the iron oxide. The maximum of the coercivity was obtained approximately at 120°C and two certain molar ratios. The cercivity increased in this treatment changed with time on standing as follows. The coercivity obtained in the treatment at 100°C tend to converge to a constant value after irregular changes. However, when the iron oxide was treated at temperatures between 150°C and 300°C, the coercivity was also found to increase with time. The behavior of the hydrous sodium triphosphate (B), obtained by drying the aqueous solution of sodium triphosphate at room temperature, was studied to investigate the reason for the increase of the coercivity. The (B) was found to be mainly composed of Na5P3O10. 6H2O. Its composition unchanged when thermally treated at 120°C, but when it was treated at 150°C, the phase of Na5P3O10.6H2O disappeared and a small amount of sodium pyrophosphate, Na4P2O7, was found. No increase of the coercivity of the iron oxide was observed by treatment with sodium pyrophosphate. The (B) treated at 300°C transformed to anhydrous sodium triphosphate. The experiments showed that the increase of the coercivity depended on Na5P3O10 and the water of crystallization in the hydrous sodium triphosphate did not affect the increase of the coercivity.