Oxygen partial pressure, conductivity dependence

Figure 9.13 Examples of the oxygen partial pressure (P02) dependencies of total conductivity of various solid electrolytes and mixed conductors, including apatite-type La9 g3Si4 5Ali xfSj(O26-s [204, 205],...

J. Xue, and R. Dieckmann. Oxygen partial pressure dependence of the oxygen content of zirconia-based electrolytes in Ionic and Mixed Conducting Ceramics Second International Symposium 94-12, 191-208 (1994) ES Meeting San Francisco, California. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

The electronic conductivity is now proportional to the — power of the oxygen partial pressure. In all cases described the conductivity falls as the partial pressure of oxygen rises, but the exact power relationship, — j or - depends upon the charge state of the interstitial cation. 

The conductivity of high-purity single crystals of CoO at 1300°C was found to depend upon the oxygen partial pressure as in the following table. What conclusions can be drawn concerning the nonstoichiometry of CoO under these... 

Pure Cr203 is an intrinsic semiconductor with a band-gap of approximately 3.3 eV. Generally, Cr203 shows little stoichiometric variation. On doping with Ti02, Ti4+ ions substitute on Cr3+ sites in the structure. The conductivity of the doped solid is n-type and has a dependence upon oxygen partial pressure. The three... 

FIGURE 1.39 Oxygen partial pressure dependency of (a) total conductivity and (b) electronic conductivity of Sm0 2Ce0 8O19 [160]. 

The independence of the reaction rate on the oxygen partial pressure is confirmed by studies in which the reaction of the catalyst with propene and reoxidation of the catalyst are separately studied. Sancier et al. [275] performed conductance measurements during reduction and reoxidation using a flow system at 327 and 387° C. The rate of change of the crystal voltage, dAV/dt, is a measure of the reaction rate and appears to satisfy the dependencies... 

Figure 10. Dependence of the electrical conductivity of semiconductive metal oxides on oxygen partial pressure.

The composition of wustite is Fe < i O. The equilibrium ratio Fe/O depends on the oxygen partial pressure. The primary charge carriers are electron holes (Fe+3 ions). Explain how the conductivity of wustite changes with oxygen partial pressure. 

Figure 16. Oxygen partial pressure dependence of the electrical conductivity of doped Ce02. The steep decrease is due to excess electrons, the flat behavior to oxygen vacancies. If we refer to typical oxygen partial pressures in an SOFC, viz. to 10" bar at the cathode and 0.2 bar at the anode, we see that the conductivity changes from ionic into n-type within a high temperature Ce02 based fuel cell. Reprinted from M. Godickemeier and L.J. Gauckler, J. Electrochem. Soc. 145 (1998) 414-421. Copyright 1998 with permission from The Electrochemical Society, Inc.

Assuming that only the doubly charged zinc interstitials (or oxygen vacancies) contribute to the excess electrons leads to the following oxygen partial pressure dependence of the conductivity ... 

Wagner and also Hauffe found a significant dependence of the conductivity on the oxygen partial pressure however, the exponent in (2.4) was not... 

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