SURFACE SEGREGATION OF CALCIUM-OXIDE IN WUSTITE AND ITS EFFECTS ON THE REDUCTION

The segregation of calcium oxide to the surface of wustite was investi gated in situ by Auger electron spectroscopy at temperatures in the ra nge of 973 to 1223 K. Calcium oxide indicated a strong tendency to seg regate, with the molar fractions at the surface being 10 to 1000 times the molar fractions in the bulk (X(CaO)bulk). The segregation isother ms showed maxima at intermediate molar fractions in the bulk (X(CaO)bu lk = 0.003 or 0.005). The enthalpy change for the segregation increase d from -43.3 to -5.20 kJ/mol with increasing X(CaO)bulk. The main driv ing force of the segregation may be the relaxation of the strain energ y generated around calcium ions. Calcium oxide also segregates at the surface of iron in contact with the wustite. By the segregation of cal cium oxide, the surface energies of wustite and iron may decrease wher eas the wustite/iron interfacial energy may increase. The enhancement of the reduction of wustite by calcium oxide is concluded to essential ly occur by the good and continuous direct contact of wustite surface with reducing gases owing to the suppression of new iron nucleus and o wing to the contraction of the wustite/iron-nuclei interfacial area co mpared with the free-surface area of wustite in order to decrease the total energy.