Kinetics of the reduction of hematite to magnetite near its Curie transition

I report the reduction of Fe2O3 to Fe3O4 in approximately 5% H-2 around the Curie transition temperature of 575 degrees C of Fe3O4. Powder samples of hematite from J.T. Baker Chemical Co. were sintered in air at approximately 1000 degrees C for 8 h to coarsen the material in order to ensure a reaction rate around 575 degrees C. SEM and TEM measurements were performed to verify the particle size of these materials. Image analyses were performed using a desktop computer to calculate the particle size. X-ray data revealed that the starting samples were single-phase hematite with an impurity phase of less than 5% Fe3O4. The purpose is to investigate whether there is any anomalous behavior during the reduction of Fe2O3 to Fe3O4 in the vicinity of the magnetic transition temperature of either magnetite or gamma Fe2O3. Hydrogen reductions were performed in the temperature range of 500-630 degrees C using a Perkin-Elmer TG-7. I will discuss the data in light of the regular Hedvall effect which occurs upon the oxidation of magnetite. (C) 1999 Elsevier Science B.V. All rights reserved.