Energy-temperature diagram, oxygen pressur

Figure 3.6 Free energy-temperature diagram with scales showing oxygen pressures, C0/C02 and H2/H20 pressure gas ratios.

A standard free energy versus temperature diagram for oxides is often presented with a scale for oxygen pressures. Such a diagram is shown in Figure 3.6. This scale is designed to... 

Figure 8.19 F.llingham diagram for the free energy of formation of metallic oxides. (After F. D. Richardson and J. H. F. Jeffes, J. Iron Steel Inst. 160, 261 (1948).) The oxygen dissociation pressure of a given M - MO system at a given temperature is obtained by joining on the lop left hand to the appropriate point on the M-MO frec-energy line, and extrapolating to the scale on the right hand ordinate for POi (atm).

To relate the concentrations of point and electronic defects to temperature and externally imposed thermodynamic conditions such as oxygen partial pressures, the defects are treated as chemical species and their equilibrium concentrations are calculated from mass action expressions. If the free-energy changes associated with all defect reactions were known, then in principle diagrams, known as Kroger-Vink diagrams, relating the defect concentrations to the externally imposed thermodynamic parameters, impurity levels, etc., can be constructed. 

We know this to be the case within a certain temperature, oxygen partial pressure domain (21). Consequently oxygen partial pressure measurement can be done using the above formula in the same domain. This is illustrated in fig. 7 where standard free energies of formation of some oxides are plotted in a familiar way as a function of temperature. There is also on this diagram a logarithmic scale allowing to plot RT log Pq and thus to determine equilibrium partial pressure for oxides at various temperatures from the relation ... 

Figure 2-1. Richardson-Ellingham diagram for several oxidation reactions of technical importance. In the diagram the standard free energies of formation of oxides per mole oxygen is plotted against temperature. The auxiliary scales serve for the determination of the oxygen partial pressure in Hj/HjO and CO/CO2 gas mixtures, respectively. The outermost auxiliary scale stands for the oxygen partial pressure. Lines of constant oxygen partial pressure poj are constructed by connecting the 0-point in the upper left corner with the appropriate mark on the P02 scale (Kofstad, 1988). (1 atm = 1.013 x lO N m". )...

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