The oxygen potential of the systems Fe+FeCr₂O₄+Cr₂O₃ and Fe+FeV₂O₄+V₂O₃ in the temperature range 750-1600°C

Abstract

From electromotive force (emf) measurements using solid oxide galvanic cells incorporating ZrO₂-CaO and ThO₂-YO_1.5 electrolytes, the chemical potentials of oxygen over the systems Fe + FeCr₂O ₄ + Cr₂0 ₃ and Fe + FeV₂0₄ + V₂0₃ were calculated. The values may be represented by the equations: 2Fe(s, I) + O₂(g) + 2Cr₂O₃(s) → 2FeCr₂0₄ (s) Δμ_O2 = - 151,400 + 34.7T (±300) cal = -633,400 + 145.5T(±1250) J (750 to 1536°C) Δμ_O2 = -158,000 + 38.4T(±300) cal = -661,000 + 160.5T(±1250) J (1536 to 1700°C) 2Fe (s, I) + O₂ (g) + 2V₂0₃ (s) → 2FeV₂0₄(s) Δμ_O2 = - 138,000 + 29.8T(±300) cal = - 577,500 + 124.7T (±1250) J (750 to 1536°C) Δμ_O2 = -144,600 + 33.45T(±300) cal = -605,100 + 140.0T(±1250) J (1536 to 1700°C). At the oxygen potentials corresponding to Fe + FeCr₂O₄ + Cr₂0₃ equilibria, the electronic contribution to the conductivity of ZrO₂-CaO electrolyte was found to affect the measured emf. Application of a small 60 cycle A.C. voltage with an amplitude of 50 mv across the cell terminals reduced the time required to attain equilibrium at temperatures between 750 to 950°C by approximately a factor of two. The second law entropy of iron chromite obtained in this study is in good agreement with that calculated from thermal data. The entropies of formation of these spinel phases from the component oxides can be correlated to cation distribution and crystal field theory.