VAPOR-PHASE SYNTHESIS AND CHARACTERIZATION OF ULTRAFINE IRON POWDERS

Formation of ultrafine iron powders by vapor-phase reduction of ferrou s chloride with hydrogen was studied in a tubular reactor made of quar tz, 3.5 cm in diameter and 1.5 m in length. Effects on the particle-si ze distribution of produced iron powders were investigated of various operating variables including the temperature in reaction zone, the co ncentration of FeCl2, and the flow rate of dilution argon gas. Ultrafi ne iron powders ranging from 40 to 88 nm in average particle size were produced. The geometric standard deviation was about 1.4. Particles w ere linked to each other to form a chain, probably due to magnetic int eraction. By electron diffraction, all the particles are found to be s ingle crystal. Over the reaction zone temperature ranging from 800 to 950 degrees C, the average size of primary particles decreased with te mperature, probably due to enhanced nucleation rate. No significant ch ange in the particle size was observed for a reduction of reactor resi dence time by one-third. The coercive force was 400 Oe at the average particle size of 40 nm, increased to 900 Oe at 55 nm, and decreased to 750 Oe at the larger size of 65 nm. The saturation magnetization of t he iron powder of 55 nm was 130 emu/g, about 60% of that of bulk iron. (C) 1998 American Association for Aerosol Research.