Properties and Applications of Iron Oxide Nanopowders Produced by Electroerosion Dispersion

The magnetic characteristics of iron oxide nanopowder (Fe 3 O 4 base phase) produced by electroerosion dispersion and consolidated at high pressures (2 GPa) and high temperatures (900, 1000, 1100, 1200, and 1300°C) for 0.07 h in contact with hexagonal boron nitride were studied. The nanopowder was produced by dispersing iron granules or shavings in plasma induced at contact points between the granules when electric pulses of high current and voltage were passed through them. The metal granules being dispersed are in a constantly circulating liquid (water) atmosphere, creating a pseudo-boiling layer from the granules. The liquid (water in this case) cools the granules to prevent them from being welded and oxidizes the metal vapors that emerge in plasma, forming nanosized iron oxide grains carried by the liquid flow into sedimentation tanks (powders with different grain sizes sediment in different tanks). Room-temperature studies of the magnetic characteristics of samples consolidated from iron oxide powders showed that the materials sintered at 1200 and 1300°C were soft magnetics with virtually zero hysteresis. Their specific magnetic moments at 5000 Oe were 128.4 and 126.4 emu/g and the coercive force was negligibly small: 5.1 and 4.5 Oe. The materials sintered at 1100°C were characterized by a specific magnetic moment of 90.4 emu/g and a relatively low coercive force of 9.1 Oe. The specific magnetic moments of the samples sintered at 900 and 1000°C were significantly lower and the coercive force higher: 40.2 and 42.1 emu/g and 37.9 and 32.4 Oe, respectively. X-ray diffraction with Rietveld refinement revealed that the materials consolidated at 900 and 1000°C contained 75–80 wt.% FeO and 25–20 wt.% Fe, while the materials sintered at 1100°C contained, along with 32 wt.% FeO and 2 wt.% Fe, a significant amount of Fe 3 N (66 wt.%). The materials consolidated at 1200–1300°C contained 100% Fe 3 N phase. Hence, under high pressures and increasing sintering temperatures, iron oxides are reduced and then iron is nitrided with nitrogen released from boron nitride, which improves the soft magnetic characteristics of the sintered materials.