PARTICLE-SIZE EFFECTS ON THE THERMAL-BEHAVIOR OF MAGHEMITE SYNTHESIZED BY FLAME PYROLYSIS

Thermal treatment of pure nanoscale maghemite (gamma-Fe2O3) powders un der argon yields different products depending on particle size. Partic les with a mean diameter above 15 nm transform to hematite (alpha-Fe2O 3). Smaller particles, which were synthesised under identical conditio ns but from solutions with lower Fe3+ concentrations, transform into m agnetite (Fe3O4) during thermal treatment. This phenomenon can be expl ained by adsorbed or incorporated reducing carboxylate groups. The ori gin of the reducing species and possible reaction mechanisms with the maghemite particles are discussed. (C) 1997 Elsevier Science B.V.