Composition and stability of iron powders prepared by a borohydride process

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Abstract
Iron powders have been produced by the reduction of ferrous ion with borohydride and stabilized by a chromating procedure. The saturation moments is 140 to 160 emu/g. The coercivity can be controlled between 375 and 1150 Oe. Remanence is 0.45 to 0.5 for unoriented powders. The powders are fibers up to 3 μm long and 1000 Å wide consisting of agglomerated spherical crystallites 300 to 450 Å in diameter. Chemical analyses show that typical powders contain about 82–85% Fe, 2–3% B, 3% Cr, 5–6% O, and 0.2% H. The moment indicates that only 70% is metallic iron. Measurements up to 100 KG show no superparamagnetic fraction. Fe(OH)2 and FeOOH were identified by electron diffraction in some but not all the preparations. Chromating may result in the removal of some non‐magnetic by‐products. Following the chromating treatment, water is removed with methanol. Solvent‐wet powder containing less than 0.1% water can be stored at least 3 months without loss in moment. Cured films of dispersions in a typical epoxy‐type binder lose moment in an accelerated atmosphere test containing SO2 and NO2 at a rate indicating a lifetime of 10 years in a worst‐case indoor ambient.