Preparation of High-Hc Ultra-fine Strontium Hexaferrite by the Amorphous Citrate Method.

Abstract

The high-Hc ultrafine hexaferrite SrO-n(Fe2O3)(n=5.4, 5.5, 5.8, 6.0)has been prepared by the amor-phous citrate process. An extra pure iron metal(>99.99%) and a pure strontium carbonate(99.7%) are dissolved into 6N HNO3 to prepare stoichiometric aqeous solution of strontium nitrate and ferric nitrate. Nitrate salts are avoided as starting materials, since they are strongly hygrosc-opic in nature. The nitrate solution is mixed with citric acid and ethyleneglycol in a uniform solution. The water and nitric oxides are evaporated at around 90 °C and ethyleneglycol is dried at around 120 °C to yield a solid precursor material. The precursor is fired to high temperature to form the desired compound. The decomposition mechanism is studied with thermogravimetric analysis, differential thermal analysisand X-ray diffraction. Also, magnetic properties and BET surface areas are measured for heat-treated powders. Coercive force He reaches 6700 Oe at 10KG for the powder obtained by firing the precursor in air to 700 °C for lhr and nearly pure-phase strontium hexaferrite powders are formed at 1000 °C for lhr.