Chart of PO2versus temperature and oxidation degree for Mn-Zn ferrites in the composition range: 50 < Fe2O3< 54; 20 < MnO < 35; 11 < ZnO < 30 (mole %)

We give a chart mostly useful for heating and cooling Mn-Zn-ferrites with a constant oxidation degree. This chart has five different scales : a logarithmic one for PO2(arm), temperature (I/T), scales for Fe2O3and MnO contents (mole %) and another one for the oxidation degree. In the composition range given, with fixed values of PO2and T, the accuracy is better than an absolute error of ± 0.1 weight % Fe²⁺which corresponds to an absolute error of ± 0.002 for the parameter γ. We emphasize the care that must be taken to reach a real equilibrium due to the existence of a phase boundary and of metastable region. This chart has been calculated using two equilibrium constants derived from our experimental studies. Experimentally, the curves log PO2= f(1/T) are straight lines with the same slope whatever the composition. This slope corresponds to the enthalpy of transfer of one mole oxygen from the solid phase to the gas phase (ΔH= 66.5 Kcal.)