Transmission Electron Microscopy Studies of Mechanical Alloying in the Immiscible Fe<sub>2</sub>O<sub>3</sub>-SnO<sub>2</sub> System

Abstract

Microstructural development and nanoscale compositional variations in mechanically alloyed Fe 2 O 3 -SnO 2 powders have been examined by transmission electron microscopy 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 X-ray diffraction line broadening measurements, whereas dissolution of SnO 2 grains was incomplete even after 110 h Isolated grains with the SnO 2 cassiterite structure, of diameter >10 nm, persisted up to the maximum milling time These observations are discussed in relation to previous measurements in the same system by X-ray diffraction and Mossbauer 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 Fe 2 O 3 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. Similar conclusions have been reported for studies of mechanical alloying in immiscible metallic systems The tendency for SnO 2 grains above a certain critical size to remain undissolved, while smaller grains can more easily enter into solid solution with Fe 2 O 3 , is consistent with the expected behaviour due to the increased chemical contribution to the interfacial energy with decreasing grain size Mossbauer results also showed that some of the SnO 2 had not reacted with Fe 2 O 3 after 110 h milling.