Oxidation semiconduction pressures

The electronic part of conduction of ThOa electrolytes could be observed increasing with the oxygen partial pressure (oxidation semiconduction) even with pure white mixed oxides (purified from polyvalent cations). During the establishment of the electrode potentials there were signs of solubility of oxygen in the lattice. These facts led to the conclusion that the electronic conduction arose in the anion sublattice and that generally, in mixed oxides with oxide ion vacancies, holes can exist in the form of monovalent negative oxide ions. 

At high oxygen pressures, oxide phases show defect electron (hole) conduction (oxidation semiconduction) and at low oxygen pressures excess electron conduction (reduction semiconduction). The transport number of excess electrons in Zro.ssCao.isOi.ss as a function of the oxygen partial pressure could be determined by measurements with a Ca,CaO/air cell [79]. The hole conduction of zirconia-based solid electrolytes was noticed for the first time when cells with Ni,NiO reference electrodes for gas potentiometry [44,91] were tested in air. The harmful oxygen permeability was measured potentiometrically in 1965 [92]. 

A comparative study of oxides which were closely related, but had different electrical properties, showed that both n- and p-type semiconduction promoted the oxidation reaction, forming CO as the major carbon-containing product. In a gas mixture which was 30% methane, 5% oxygen, and 65% helium, reacted at 1168 K the coupling reactions were best achieved with the electrolyte Lao.9Sro.1YO 1.5 and the /i-lype semiconductor Lao.sSro MntL A and the lily pe semiconductor LaFeo.sNbo.2O1 a produced CO as the major oxidation product (Alcock et al., 1993). The two semiconductors are non-stoichiometric, and the subscript 3 — x varies in value with the oxygen pressure and temperature. Again, it is quite probable that the surface reactions involve the formation of methyl radicals and O- ions. 

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 m value depends on the semiconductive nature of metal oxides in the surrounding oxygen partial pressure. For example, m is 4 or 6 for p-type semiconductors and - 4 or - 6 for n-type semiconductors (11). 

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

The fact that the two n-type oxides Fe203 and ZnO and the two p-type oxides NiO and Cr203 fall into separate classes when the speed and pressure-dependence of their interaction with oxygen is concerned, adsorption of the gas being more difficult and much less in extent on the n-type oxides, is in accord with the view, supported by semiconductivity evidence, that the chemisorption step involves the transfer of electrons from solid to adsorbate. 

In semiconducting oxides such as iron-doped SrTi03 (e.g., SrTio.8Feo.2O3) the conductivity depends thermodynamically on the oxygen partial pressure. In the thermodynamic equilibrium a metal oxide exchanges lattice oxygen Oq with the ambient gas phase. 

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