Vapor-Phase Synthesis and Characterization of Ultrafine Iron Powders

Formation of ultrafine iron powders by vapor-phase reduction of ferrous chloride with hydrogen was studied in a tubular reactor made of quartz, 3.5 cm in diameter and 1.5 m in length. Effects on the particle-size distribution of produced iron powders were investigated of various operating variables including the temperature in reaction zone, the concentration of FeCl2, and the flow rate of dilution argon gas. Ultrafine iron powders ranging from 40 to 88 nm in average particle size were produced. The geometric standard deviation was about 1.4. Particles were linked to each other to form a chain, probably due to magnetic interaction. By electron diffraction, all the particles are found to be single crystal. Over the reaction zone temperature ranging from 800 to 950°C, the average size of primary particles decreased with temperature, probably due to enhanced nucleation rate. No significant change in the particle size was observed for a reduction of reactor residence 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 the iron powder of 55 nm was 130 emu/g, about 60% of that of bulk iron