An investigation of the surface properties of atomized Fe_xSi powders

The results of XPS and electrokinetic measurements on spherical ferrosilicide particles (15 wt% Si) produced by atomization show that the surface consists of a mixed silica and Fe(II, III) oxide layer of at least 50 nm thickness. Although the surface is dominated by the silica phase (up to 70 at %), the electrophoretic mobility response in aqueous KCl electrolytes is complex and exhibits three points of charge reversal over the pH range 3-11. These phenomena correspond to an apparent point of zero charge of the surface at about pH 5 (which varies with surface composition), a point of charge reversal (which is independent of surface composition) at pH 7 due to the adsorption of FeOH⁺ ions originating from the substrate, and an isoelectric point at pH 7.6 due to transformation of an essentially siliceous surface to that of an iron hydroxide. The effects of pretreating the surfaces in acid and caustic solutions on the electrophoretic mobility is discussed, and the change in electrophoretic mobility with time is followed for three powders with different bulk chemical compositions between 15.4 and 19.7 wt% Si.