Transmission Electron Microscopy Studies of Mechanical Alloying in the Immiscible Fe2O3-SnO2 System

Microstructural development and nanoscale compositional variations in mechanically alloyed Fe2O3SnO2 powders have been examined by TEM and energy dispersive X-ray spectrometry. The mean grain size was found to stabilize around 10 nm after 19 h milling time, in close agreement with that estimated from XRD line broadening measurements, whereas dissolution of SnO2 grains was incomplete even after 110 h. Isolated grains with the SnO2 cassiterite structure, of diameter greater than 10 nm, persisted up to the maximum milling time. These observations are discussed in relation to previous measurements in the same system by XRD and Moessbauer spectroscopy, which suggested that alloying on the atomic scale occurred after 110-h milling. The present studies confirm that the amount of Sn dissolved in the Fe2O3 hematite lattice increases with longer milling times, indicating that a supersaturated solid solution is formed, but that mixing may be locally inhomogeneous at the atomic level. The tendency for SnO2 grains above a certain critical size to remain undissolved, while smaller grains can more easily enter into solid solution with Fe2O3, is consistent with the expected behavior due to the increased chemical contribution to the interfacial energy with decreasing grain size. (Author)