Oxygen Partial Pressure pO

First, we discuss the phase relation from the viewpoint of the phase rule. Because Mn- Zn ferrite is composed of four elements (Mn, Zn, Fe, O), the freedom F in the phase rule is expressed as f = 6 — p. Considering one solid phase with a spinel structure (see Section 2.5 and Ref. 3), equilibrated with the oxygen (partial) pressure Po., we get the relation F = 4. The starting compounds for the preparation of Mn-Zn ferrite are ZnO, MnO, and Fe2O3, in the molar ratio a / 3 — (a -I- j6). This mixture is heated at fixed temperature and partial oxygen pressure, and the product obtained has the composition Zn Mn Fcj Under conditions of fixed temperature, fixed oxygen... 

The model equation which shows a better fitting to the experimental data presents two parameters ki (the constant rate of toluene oxidation) and k2 (the constant rate of catalyst oxidation). The reaction rate is second order dependent on toluene partial pressure (Pt) and first order dependent on oxygen partial pressure (Po), leading to the following equation... 

At a given temperature and standard pressure (ambient 1 atm), the oxidation of a metal to its oxide or the reduction of an oxide to its suboxide or the corresponding metal occurs at a well-defined oxygen partial pressure (Po)- At a given temperature, on either side of this unique one of the two coexisting phases must disappear. This is illustrated in the following by considering a hypothetical metal oxidation reaction ... 

The chemical potential /u-Oj is connected with the oxygen partial pressure po by the following equation ... 

A and B. Conductivity relaxations, related to the microstructure of the composite. Their activation energies are about 1 eV, similar to those of the grain or grain boundary conductivities and are independent of the oxygen partial pressure (Po ). One or both of these arcs may be absent in some composite cathodes, but are absent with point contact electrodes. 

SOFCs, SOECs, and SOERs all consist of three parts an electrolyte, an anode, and a cathode. The electrolyte is an oxygen anion conducting but electrically insulating solid oxide. This dense ceramic layer also acts as a barrier to prevent gas mixing between the two gas chambers. If we take the SOFC as an example. Fig. 1, the cathode is in contact with air, while the anode is in contact with fuel. The difference in oxygen partial pressure (pO-2)... 

If a defined equilibrium oxygen partial pressure (pO 2) in the chamber at temperature T and... 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

Figure 9.28 shows the dependence of the catalytic rate of oxygen consumption, r0, on the oxygen partial pressure P02 at fixed pH2 under open-circuit conditions and for a potentiostatically fixed catalyst potential Urhe (=UWr). As also shown in Fig. 9.28, the open-circuit potential UrhE increases from 0.33 to 0.8 V as the Po Ph2 ratio increases from 0.2 to 3.6. 

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... 

NiOi + y could be measured precisely by use of the solid electrolyte, without chemical analysis. This result is shown in Fig. 1.51, as log Po. versus T curves, in which the dotted lines indicate the iso-y values. This figure shows that in order to get stoichiometric NiO, the sample has to be prepared at low temperature and low oxygen partial pressure in the NiO phase region. 

Fig. 3.10 Non-stoichiometry, 6, versus Pqj/Poj for Mn- and Ni-ferrites. is the oxygen partial pressure where the lattice parameter versus Po. exhibits a maximum. ...

The result of a particular demixing experiment on (Mg,Co)0 is shown in Figure 8-3a [H. Schmalzried, W. Laqua (1981)]. We see that an oxygen partial pressure ratio of only three (Po/P o1 = 3) for the two opposite surfaces of the crystal results in a 15-20% demixing of (Mg.Co)O. The mobility ratio bCo/bMg is ca. 10. Also, the corresponding (schematic) reaction path in a /zG vs. composition diagram of the A-B-O system is shown in Figure 8-3 b. 

Provided that the electronic transference number is known, the ionic (and electronic) conductivity may be obtained by differentiation of experimental data. Assume that we have produced a data set for different Po -gradients, keeping the oxygen partial pressure Pq" at the permeate side fixed. Differentiating Eq. (10.10) with respect to the lower integration limit yields. 

In the above equation r is the radial position within the perovskite layer, tq and Tb the outer and inner radii and Po r and Po2,rt/ the corresponding oxygen partial pressures at these positions. The preexponential factor A and the activation energy E were fitted to the experimental data of Teraoka et al. [101]. 

Henry s law (i.e., c = po/H) relates the equilibrium oxygen solubility in the liquid (c ) to oxygen partial pressure in the gas (po) by the corresponding Henry s law constant (H). Because the liquid film mass transfer coefficient, l, is difficult to measure independently, kj a (i.e., times a) is used. It is a lumped parameter known as the volumetric mass transfer coefficient to characterize the overall mass transfer rate. 

Figure 7. Schematic of transport processes through an oxide layer growing on a metal. Two limiting cases may be distinguished. First, metal ions and electrons may migrate from the metal toward the oxide gas interface and second, oxygen ions may migrate toward the metal-oxide interface with electrons migrating in the opposite direction. In any volume element of the oxide, electrical neutrality is required. The chemical potential of oxygen is fixed at both the metalr-oxide and-the oxide-gas interface. The former is fixed by the dissociation pressure of the oxide, po/, and the latter by the ambient oxygen partial presure, po"-...

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