Increase in Coercivity of Iron Oxides by Coating with Sodium Metaphosphate or Diphosphorus Pentaoxid e

Abstract

The coercivity of nonstoichiometric iron oxide parti cles has been increased remarkably by coating with chain like sodium metaphosphate (NaPO₃), t and by subsequent heating under a reduced pressure. In this paper, the chemical properties of coated (NaPO₃), , onto an acicular iron oxides and a bond-state between phosphate and iron ion were studied to explain the observed increase in coercivity.
No fundamental ch anges were observed by means of X-ray analysis or infrared absorption spectroscopy of (NaPO₃), coated onto the iron oxide particles compared with nonheated (NaO₃). The coercivity increased in the same manner with addition of Ca ion, which is likely to be linked to the site of the terminal position in the side chain of (NaPO₃). This suggests that Fe ion is linked to the site of oxygen at the terminal position of main chain in (NaPO₈) n. The coercivity tends to approach a maxmium when the acicular iron oxide pa rticle are coated in (NaPO₃)7, solution of pH range from 7 to 9. However, the effect of pH was not so great. The increase in the coercivity was more significant when the iron oxides were coated with preheated hydrous (NaPO₃)_n, at 150°C under a reduced pressure than with unpreheated one. The coerciv i ty also increased by coating with P₂O₅. Therefore, it is considered that chain like phosphate (HPO₃)_n, formed by hydrolysis and condensation of P₂O₅ improves the coercivity.