Fe-O-Zn (Iron-Oxygen-Zinc)

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hase Diagram Evaluations: Section II

Fe-O-Zn (Iron-Oxygen-Zinc) V. Raghavan

The early results on this ternary system were reviewed by [1989Rag], who presented a tentative isothermal section at 900 °C. Recently, Hayes and coworkers [2000Jak, 2001Deg, 2004Han, 2005Han] reinvestigated the system under different experimental conditions: (i) at metallic iron saturation, (ii) in air, and (iii) at oxygen pressures that lie between (i) and (ii).

Binary Systems The Fe-O phase diagram [1991Wri] exhibits the following compounds. Wustite (FeO) is a metal-deﬁcient monoxide with the B1, NaCl-type cubic structure. Magnetite (Fe3O4) has the H11, Al2MgO4-type cubic structure. Hematite (aFe2O3) is D51-type rhombohedral. Other lesscommon polymorphic forms of the above compounds are known [1991Wri]. In the Fe-Zn system [Massalski2], several intermediate phases are known in the zinc-rich region. In the Zn-O system, zincite (ZnO) has a hexagonal structure, space group P63mc. A cubic monoxide ZnO is stable at high pressures. ZnO2 has the FeS2, pyrite-type cubic structure.

Ternary Phase Equilibria Starting with high purity powders of FeO, ZnO, and Fe (99.5 mass% purity) [1993Ito] prepared briquettes, which were sealed in evacuated quartz tubes, annealed at 827 °C for 3-7 d and quenched in water. The phases were identiﬁed with x-ray powder diffraction and the phase compositions were measured with an electron probe microanalyzer (EPMA). Using a stabilized zirconia solid electrolyte, [1993Ito] measured the partial pressures of oxygen by the emf method. The composite isothermal section at 827 °C constructed by [1993Ito] in the Fe-Fe2O3-ZnO-Zn region is shown in Fig. 1. The phase distribution is very similar to the tentative isothermal section at 900 °C presented by [1989Rag]. In the three-phase equilibria of (Fe), (Zn), and ZnO, (Fe) contains 38.6 at.% Zn, liquid Zn has 11 at.% Fe and ZnO dissolves 0.46 at.% Fe. Magnetite (Fe3O4) and zinc ferrite (ZnFe2O4) form a continuous solid solution. Hematite (Fe2O3) dissolves negligible amount of Zn. At metallic iron saturation, the phase equilibria of this ternary system were investigated by [2000Jak]. With starting materials of ZnO and Fe of 99.5 mass% purity, [2000Jak] ﬁrst prepared ZnO pellets by sintering ZnO powder at 1500 °C in air. The pellets were then surrounded by Fe powder and the assembly was annealed between 900 and 1200 °C in high-purity N2. This diffusion-couple type arrangement resulted in the formation of wustite next to the

outer Fe layer and zincite (ZnO with dissolved Fe) below it. The coexisting compositions at the wustite-zincite interface were measured with EPMA and listed. The phase equilibria in air (0.21 atm of O2 and 1 atm of total pressure) were investigated by [2001Deg] and [2004Han]. Starting with oxide powders of Fe2O3, ZnO and Fe of 99.5 mass% purity, [2001Deg] pelletized and equilibrated powder mixtures between 900 and 1500 °C for annealing times of 18-240 h. The samples were quenched in a ﬂow of air or in iced water. The phase equilibria were studied with optical a

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