Abstract
The segregation of calcium oxide to the surface of wustite was investigatedin situ by Auger electron spectroscopy at temperatures in the range of 973 to 1223 K. Calcium oxide indicated a strong tendency to segregate, with the molar fractions at the surface being 10 to 1000 times the molar fractions in the bulk (X CaO bulk ). The segregation isotherms showed maxima at intermediate molar fractions in the bulk (X CaO bulk = 0.003 or 0.005). The enthalpy change for the segregation increased from −43.3 to −5.20 kJ/mol with increasing X CaO bulk . The main driving force of the segregation may be the relaxation of the strain energy generated around calcium ions. Calcium oxide also segregates at the surface of iron in contact with the wustite. By the segregation of calcium oxide, the surface energies of wustite and iron may decrease whereas the wustite/iron interfacial energy may increase. The enhancement of the reduction of wustite by calcium oxide is concluded to essentially occur by the good and continuous direct contact of wustite surface with reducing gases owing to the suppression of new iron nucleus and owing to the contraction of the wustite/iron-nuclei interfacial area compared with the free-surface area of wustite in order to decrease the total energy.
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