Non-Conventional Synthesis And Magnetic Properties Of Plate-Like Hexagonal Iron Oxide

The synthesis of pure iron(Ill) oxide and of iron(Ill) oxide doped with cobalt or cobalt and boron is reported. Wet methods are normally used to prepare an oxyhydroxide precursor in order to yield magnetite and maghemite particles with controlled shape; the most common technique is via green rust. In this work, plate-like (hexagonal) precursors were prepared through a non-conventional way: Fe (III) - amorphous FeOOH - crystalline FeOOH and/or Fe(2)O(3). The amorphous oxyhydroxide precursor was obtained by precipitation from a solution of iron(Ill) sulfate and sodium hydroxide. The transformation to states with higher crystallinity was carried out by means of a heat treatment at 90 degrees C in alkaline solution (pH = 14) with several process times. The precursors were decomposed at several calcination temperatures followed by the reduction and oxidation of the obtained by-products. The characterization of the different phases was performed by means of solid state analytical techniques such as Scanning Electron Microscopy, X-Ray Diffraction, IR and Mossbauer spectroscopies and Magnetometry. The crystallinity of the iron(III) oxide precursor and the presence of additives are related to the physical properties of the final products (magnetite or maghemite). The coercivity of the final product increases with the time of the alkaline treatment and with the calcination temperature of the iron(Ill) oxide precursor with plate-like morphology. The results of magnetic measurements evidence an increase of the oxide coercivity in the following order: no alkaline treatment (498 Oe) - no alkaline treatment, doped with 5% w/w Co and 0.2 % w/w B (779 Oe) - 30 minutes of alkaline treatment, same concentration of Co and B (1490 Oe). Although the presence of Co on the surface of the particles yields an increasing coercivity of the product, the reduction process from hematite to maghemite is retarded.