The ball milling induced transformation of α-Fe2O3 powder in air and oxygen atmosphere

The mechanochemical treatment of α-Fe2O3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of α-Fe2O3 was investigated. Under appropriate milling conditions, α-Fe2O3 completely transforms to Fe3O4, and for prolonged milling to the Fe1−xO phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by ∼1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe1−xO→Fe3O4→α-Fe2O3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically clean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure.