Temperature oxygen partial pressure, electrical

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 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 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.

To assess the consistency of this KMC model, a variety of materials-independent, materials-dependent, and geometrical parameters was investigated, and the ionic current calculated from the model was used as the primary metric. The materials-independent parameters included the oxygen partial pressure, system temperature, and the external applied potential. Of these parameters, the oxygen pressure had a weak influence on the current (Figure 4), unless its value falls below a threshold of approximately 0.05 atm. As the temperature increased (from 200 to 800°C), the current showed an exponential increase, owing to the thermally activated ion transport in YSZ. As the applied electric potential of the cell increased, a similar increase was found in the calculated ionic current. The materials-dependent parameters included the dopant level (i.e., Y2O3... 

When zirconia-based electrolytes are exposed to the high temperatures (T > 1100°C) and low oxygen partial pressures P02 < 10 ° Pa), usually encountered in metal melts, they exhibit mixed ionic and n-type electronic conductivities. Under these conditions, the solid electrolyte sensor generates cm/that is influenced by the electrical properties of the solid electrolyte. Schmalzried [25] has analyzed the contribution of electronic conductivity in the zirconia electrolytes to the measured emf of an electrochemical cell in the P02 region less than 10 Pa and has shown that, in the presence of n-type electronic conductivity, the emf of the sensor can be expressed as... 

The non-stoichiometric Ca2LaFe308+x ferrite was also studied by measuring the electrical conductivity variation with oxygen partial pressure, p02, at various temperatures (Fig. 18). In each curve, three ranges of variation are observable as represented for the imaginary temperature tp... 

Fig. 18. Variation of the electrical conductivity [log a] of Ca2LaFe308+y with the oxygen partial pressure at various temperatures...

Under equilibrium conditions the electrical conductivity of many oxide phases, e.g., CugO, FeO, CoO, NiO, or ZnO at elevated temperatures is a function of the oxygen partial pressure in the ambient gas phase 12). The oxygen partial pressure determines the metal excess or deficit in the metal oxide and thereby the concentration of electrical carriers especially excess electrons and electron holes. Thus, after proper calibration, the steady-state oxygen activity ao(st) may be deduced from measurements of the conductance of a metal oxide foil used as catalyst while an oxygen transfer reaction, e.g., CO2 + H2 = CO -)- HjO or 2N2O = 2N2 + O2 proceeds at the surface of the metal oxide 13). 

The functional dependence of the electrical conductivity of an oxide on the oxygen partial pressure and temperature is shown in Fig. 1.21a. 

The measurements of ion transport numbers made by method of EMF and by sign of thermo-electric force reveal, that in all studied materials at Po2 > 10 Pa and in air the electronic conductivity of p-type (hole conductiviy) predominates, and at P02 < 10-1 Pa the electrical conductivity is predominantly ionic. The transport number of ions f alters depending on oxygen partial pressure, temperature and doping from 0,9 down to 0,1. 

The vapour pressure of V2O5 has been determined between 973 and 1473 K, and the enthalpy of evaporation calculated as ca. 143 kJ mol . The electrical conductivity of liquid V2O3 has been measured as a function of both temperature and partial pressure of oxygen. Raman and i.r. spectra or oriented single-crystals of V2O5 have been recorded and the absorption frequencies compared with those calculated using a simple transferred force-field. 2... 

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