On the Gibbs energy of formation of wustite
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I.
INTRODUCTION
IN view of its significance in iron-making, the Fe-FexO equilibrium has been extensively studied in process metallurgy. With the inception of the solid-state galvanic cell technique for the measurements of oxygen potentials,l the Fe-FexO system has commonly been used as reference electrode in view of its high degree of reversibility 2 at temperatures as low as 873 K. Consequently it is necessary that the thermodynamics of this system is well understood in order to interpret the results of such measurements. In the course of solid-electrolyte galvanic cell measurements in the oxide system containing mixed oxides of alkali and iron, currently being carried out in this department, the results were not compatible with the literature data for the Fe-FexO equilibrium in the temperature range 860 to 1340 K. To the present knowledge of the authors, the functions available for the standard Gibbs energy change for the reaction 1 xFe + ~ O 2 = FexO
in the temperature range 1050 to 1550 K. It is well known that iron undergoes the o~ ---> y phase transformation at 1184 K. The AG ~ vs temperature function for reaction [1] would normally be expected to show a discontinuity in slope at this temperature due to the first order transformation. 4 In addition to this, metallic iron undergoes a second order magnetic transformation with the Curie temperature at 1043 K. A linear representation of the Gibbs energy change, AG~, would therefore be only an approximation. Recently, Jacobsson and Ros6n 5 have carried out extensive measurements with the Fe-FexO electrode. Their results indicate that AG~ is essentially linear in the temperature range 960 to 1620 K even though the authors have preferred to use a three parameter equation. The linear representation of the AG ~ function could only mean that the experimental measurements are not sensitive enough to reflect either the discontinuity due to the a ---> y transition or the nonlinearity due to the magnetic transformation.
[1] II.
in the above-mentioned temperature range do not reflect the phase transformation occurring in the system. This motivated the present redetermination of the Gibbs energy for reaction [1] with better accuracy. The Gibbs energies of formation of metal-oxides are normally measured either by direct equilibration with a gas mixture of known oxygen potential or by the solid electrolyte galvanic cell method involving ZrO2-CaO or ThO2-Y203 electrolytes. The former method is not suitable for measurements at temperatures lower than 1200 K in view of the sluggishness in attaining thermodynamic equilibrium between gas and condensed phases at these temperatures, and hence the galvanic cell method is often relied upon for such measurements. An extensive assessment of the results of the various measurements available in the literature on the Fe-FexO system has been carded out by Spencer and Kubaschewski.3 These authors recommend the equation: AG~ = -261182 + 62.93T J/tool
[2]
O. SJODI~N, formerly Research Assistant at the Department of Theoretical Metallurgy
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