Oxygen pressure dependence, electronic conductivity

Because Dqq oc [V(5o]then if one measured the radio tracer diffusion of cobalt in CoO, the isothermal oxygen pressure dependence should exhibit a one-quarter dependence. This is exactly what Carter and Richardson ( ) did. Their results are shown in Figure 5. The electronic conductivity, o,... 

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. 

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

In this case, the number of zinc ions in interstitial positions and the number of free electrons will be decreased by an increase in the partial pressure of oxygen. These disorder reactions result in a dependence of the electrical conductivity on the oxygen pressure. This effect is a well known phenomenon in the field of semiconductors (1). Complicated relations, however, will occur at lower temperatures, at which no equilibrium can be attained between the gas phase and the lattice defects in the whole... 

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 ... 

The permeation flux of oxygen through a mixed oxide membrane described above depends on the oxygen partial pressures across the membrane, membrane thickness and temperature. The dependence, however, is embedded in a complicated implicit equation [Lin et al., 1994]. Only in special cases the permeation Oux shows a pressure dependence similar to that for palladium membranes as given in Eq. (4-10). For example, when electronic conductivity predominates, the value of the exponent, n, is equal to 0.5 for thin membranes and 0.25 [Dou et al., 1985 Itoh et al., 1993] for thick oxide films. If the oxide membrane is essentially an ionic conductor and the surface reaction is the rate-limiting step, n takes on a value of 0.5. 

Khorkunov, B.A., Nafe, H., and Aldinger, F. 2006. Relationship between the ionic and electronic partial conductivities of co-doped LSGM ceramics from oxygen partial pressure dependence of the total conductivity. Journal of Solid State Electrochemistry 10, 479-487. 

The isothermal dependencies of electronic conductivity and Seebeck coefficient on the oxygen pressure, along with the corresponding defect concentrations, are shown in Eigure 3.10. 

Figure 9.7 Temperature dependence of the electronic conductivity of La2Mo2O9-based ceramics, determined by the faradaic efficiency and total conductivity measurements at atmospheric oxygen pressure [65], The dashed line corresponds to the p-type electronic...

---