NOVEL TRANSPORT PROPERTIES OF NANOSTRUCTURED α-Fe_2O_3

摘要

Nanophase materials with an average grain sizes ranging from 2 to 20 ran exhibit novel properties relative to conventional materials. Recently, we reported that the mechanical milling of Fe_3O_4 powders produced nanometer sized particles (～7 nm) which exhibited a phase transition from cubic (Fe_3O_4) to hexagonal (α-Fe_2O_3) phase. In this paper, we present results on the synthesis of nanosized α-Fe_2O_3 produced by mechanical milling for 1 to 10 h. Some unusual electrical conductivity characteristics were observed. The X-ray diffraction analyses show only the presence characteristic lines of hematite in the ball milled samples. Particle sizes measured from X-ray line broadening varied from 7 to 11 nm. TEM micrograph for the ball milled samples show distribution of particle size. A drastic change of electrical resistivity has been observed from the electrical measurements of different ball milled samples. The resistivity variation of the unmilled sample can be ascribed to typical band conduction. A decrease of about two orders of magnitude of resistivity was observed for the specimen ball milled for 10 h. The resistivity variation of the two activated processes was observed with activation energies of ～ 0.06 and ～ 0.7eV, respectively. Large and small particle sizes are responsible for the two activation energies.