PROPERTIES AND APPLICATIONS OF IRON OXIDE NANOPOWDERS PRODUCED BY ELECTROEROSION DISPERSION

Abstract

The magnetic characteristics of iron oxide nanopowder (main phase Fe₃O₄) produced by electroerosion dispersion and then consolidated at high pressure (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 that appeared at the points of contact between the granules when electric pulses of high current and voltage were passed through them. During dispersion, metal granules are in a constantly circulating liquid (water), creating a pseudo-boiling layer from these granules. The liquid (waster in this case) cools the granules and prevents them from welding, and also oxidizes the metal vapors that emerge in plasma, forming nanosized iron oxide grains that are carried by the flow of the same liquid into sedimentation tanks (powders with different grain sizes sediment in different sedimentation tanks). The room-temperature study of the magnetic characteristics of materials consolidated from iron oxide powders showed that those sintered at 1200 and 1300 °C belonged to soft magnetic materials with virtually absent 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 respectively. The material sintered at 1100 °C was 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 lower temperatures (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 showed 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, along with 32 wt.% FeO and 2 wt.% Fe, contained a significant amount of Fe₃N: 66 wt.%. The materials consolidated at 1200–1300 °C contained about 100% of the Fe₃N phase. Hence, reduction of iron oxides followed by its nitriding with nitrogen released from boron nitride, which improves the soft magnetic characteristics of sintered materials, is observed under conditions of high pressures and temperatures with increasing sintering temperature.