Electrical conductivity oxygen pressure

One feature of oxides is drat, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than dre bulk of the cations, refened to as positive holes because their effect of oxygen partial pressure on dre electrical conductivity is dre opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

For all /7-type oxides, the defect concentration, and hence the electrical conductivity, increases with the oxygen pressure. 

Fig. 1.20. The dependence of equilibrium electric conductivity of ZnO sample on oxygen pressure...

As it has been mention in preceding section, the vast effect of the mechanism of adsorption-caused change in electrophysical characteristics of adsorbent is provided by availability of defects [32]. However, various admixtures play similarly important role on effects of properties of oxides including the sensitivity of their electrophysical properties to adsorption [4, 5]. Small amounts of admixtures (of the order of 0.5 -1 mol.-%) can both increase the sensitivity of oxide for instance to oxygen (addition of Y2O3 to calcium oxide over pressure interval lO -10 Torr [189]) and decrease it (addition of Ga203 to ZnO [190]), or can result in insensitivity of electric conductivity on the pressure of the gas in question (as it is the case with respect to O2 while adding 0.5 -1 mol.-% of lithium to NiO [190]). 

One of basic experimental facts is provided by existence of the power law between the value of equilibrium electric conductivity of adsorbent and pressure of oxygen in ambient atmosphere ... 

It is evident from this expression that the higher the pressure of oxygen in ambient volume and higher the temperature the faster is the rate of the change in adsorbent electric conductivity. 

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. 

Figure 6.34 Electrical conductivity of two oxide semiconductors as a function of oxygen partial pressure. From W. D. Kingery H. K. Bowen, and D. R. Uhhnann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

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... 

Noteworthy in this connection are Muscheid s (39) results on the dependence of the electrical conductivity of single crystals of CdS upon their treatment in vacuo or in oxygen at various pressures. This author observed a strong and sudden decrease of the conductivity when oxygen at... 

The value of non-stoichiometry d in Nii O is 1 x 10 at most, that is, there is only one vacancy in 1000 lattice points of Ni. Osburn and Vest measured the electrical conductivity, a, of high purity NiO (single crystal) as a function of temperature (1000-1400 °C) and oxygen partial pressure (1—10 atm), to elucidate the conduction mechanism. Figure 1,38 shows a versus temperature curves at fixed Po. values. The following relation between measured temperature, T, and oxygen... 

Figure 1.43 shows the electrical conductivity o versus oxygen pressure Po. curves at fixed temperatures for Zro gjCao ijOj 5 or (Zr02)o.85(CaO)o,i5 (called stabilized zirconia), as an example. In the oxygen pressure range... 

It has been shown that the accurate measurement of the diffusion constant ), combined with such measurements as electrical conductivity and quantity of non-stoichiometry as functions of temperature and oxygen pressure, afford us significant knowledge on lattice defects. 

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