Phase relations of iron-zinc-oxygen system at 1100 K have been determined by the X-ray microanalysis and X-ray diffraction studies of quenched samples. The partial oxygen pressure have been measured by the e.m.f. method using zirconium oxide solid electrolytes. The activities of Fe and Zn in the alloys as well as the activities of FeO and ZnO in the oxides were obtained by applying the Gibbs-Duhem relation to the alloy and oxide phases. Phase relations of Fe-Zn-ZnO-FeO system at 1100 K are composed of two-phase fields (Fe+FeO s.s., Fe s.s.+ZnO s.s., Zn(1)+ZnO) and three-phase field (Fe+ZnO s.s.+FeO s.s., Fe s.s.+Zn(1)+ZnO). The mutual solubilities between FeO and ZnO were determined. Activities in Fe-Zn alloys at 1100 K showed considerable positive deviations from Raoult's law except for the activity of zinc at Zn-rich region which exhibited a negative deviation. The activities of FeO and ZnO in solid FeO-ZnO system at 1100 K showed considerable positive deviations from Raoult's law except the activity of zinc oxide at the ZnO-rich side which exhibited no deviation the range up to the solid solubility limit. The activity coefficients at infinite dilution at 1100 K were estimated to be 4.4 and 6.6, respectively.
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