Sodium borohydride reduction of aqueous iron–zirconium solutions: chemical routes to amorphous and nanocrystalline Fe–Zr–B alloys

Abstract

Fine particle Fe–Zr–B alloys have been prepared by sodium borohydride reduction of aqueous solutions of iron(II) and zirconium(IV) sulfate. A comprehensive survey of reaction conditions was completed and optimum parameters found for the preparation of precipitated products. These included rapid stirring, controlled pH in the range 5–6, using degassed solvents, Fe∶Zr ratios between 4∶1 and 20∶1 and sodium borohydride concentrations between 0.1 and 1.2 M. Scanning electron microscopy showed that the products comprised spherical particles of 0.1 µm diameter with metal compositions, determined by atomic absorption, electron microprobe and EDAX, which mirrored the initial starting solutions. The products fell into two categories: amorphous Fe–Zr–B alloys when the Fe∶Zr ratio was near 9∶1, and nanocrystalline alloys, comprising a small amount of nanometre scale α-Fe crystallites embedded in amorphous Fe–Zr–B alloy, when the Fe∶Zr ratio was either higher or lower than 9∶1. Minor impurities in the form of Fe(II) complex and Fe(III) oxide phases were also detected in the products. Full structural and magnetic characterisations were performed on selected samples, including X-ray diffraction, Mossbauer spectroscopy, differential scanning calorimetry, magnetometry and Zr-edge EXAFS measurements.