Coercive Force and Stability of Acicular Iron Oxides Treated with Sodium Metaphosphate

Abstract

An anomalous increase in coercive force and stability of the acicular iron oxide powder (Fe²⁺ /Fe³⁺: 1.1) treated with (NaP0₃)n was investigated at a room temperature∼300°C under the pressure of 0.4 mmHg in an air. The maximum coercive force was observed upon heating at 150°C. This increased coerive force decreased linearly with treating time. The coercive force of the sample treated at 150°C for 40 minutes was approximately constant. An anomalous increase in coercive force was observed when some water was present during the process of treatment. The experiments suggested that water may play an important role to form a chemical bond between iron and phosphorous ions.