Oxygen Ion Migration

The main experimental techniques used to study oxygen migration in doped eerias are based on the AC impedance analysis of the measured eleetrical conductivity. It is found that the oxygen ion conductivity of ceria-based oxides depends strongly upon the dopant size and concentration. Both these faetors are related to defect association between oxygen vacancies (the charge carriers) and other defects (mainly dopant substitutionals) which were discussed in section 8.3.2.. 

It is known that as the dopant content increases, the conductivity rises, but reaches a maximum followed by a steep decrease. The exact dopant concentration for maximum conductivity depends to some extent upon the dopant type, but is generally around 10 — 15% mol for trivalent dopants. 

At very low dopant concentration (less than 1%) oxygen migration is adequately modeled by taking into account the formation equilibrium of simple pair clusters (equations (8.8) or (8.9) with n = 1), so that the concentration of charge carriers (oxygen vacancies) is regulated by the following equations (in the case of a trivalent dopant)  

In the Monte Carlo method of Murray etal. the long-range Coulombic interaction between charged defects was neglected. By using the same Monte Carlo techniques, Adler et al. tested a model for oxygen transport in yttria-doped ceria that includes 

However, the variation in conductivity with increasing dopant concentration based on attractive interactions between vacancies and dopant substitutionals has been recently questioned by Meyer et al. These authors performed Monte Carlo sim- 

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Figure 8 Schematic illustration of ihe energy profile for an oxygen ion migrating to an adjacent vacant site...

At the same time as aluminum is reduced at the cathode, oxygen ions migrate to the suspended anode elements and are oxidized to atomic and possibly some molecular oxygen (Eq. 12.13). 

M. Cherry, M.S. Islam and C.R.A. Catlow, Oxygen ion migration in perovkite-type oxides, J. Solid State Chem., 118 (1995) 125-132. 

These simulation studies, based on either static lattice, MD or ab initio methods, have been able to provide deeper insight as to the fundamental solid state properties at the atomic level, particularly in the following key areas (a) defect chemistry and dopant-vacancy association, (b) mechanisms of oxygen ion migration, (c) structures and stability of surfaces (mainly (110) and (111)), (d) energetics of redox reactions... 

The mechanism of scale formation also determines whether a crack in the film surface layer can mend. If metal ions and electrons migrate outward, the oxide film propagates at the oxide atmosphere interface and the crack quickly closes. But when oxygen ions migrate inward, the oxide film propagates at the metal oxide interface and the crack cannot close. 

Figure 1.19 Maximum conductivity in the binary Zr02-Ln203 systems at 1000°C, and the oxygen-ion migration and association enthalpies versus radius of Ln cations [243],...

Tolchard J R, Islam M S and Slater P R (2003), Defect chemistry and oxygen ion migration in the apatite-type materials 1 33814026 and LagSr2Si6026 , J Mater Chem, 13,1956-1961. 

Thus, when a sample gas is introduced on one side, oxygen ions migrate within the crystal lattice to form a concentration gradient from the higher partial pressure side to the lower pressure side. The gradient is determined by the ratio of the partial pressures between a sample gas on one side... 

The electromotive force (emf) of this cell in purified nitrogen gas has been measured. The observed emf was 0.205 0.005 V at 250°C, whieh is close to the theoretical value ealculated from Nernst s equation given in Eq. (3.1), where Po2(l) and Po2(2) were thermodynamic oxygen partial pressures in each gas chamber. This result means that conduction was purely ionie, as described in the previous section. The authors reported that there was no eonclusive evidence as to whether the ionic conduction is caused by a eation or an anion migration. However, from the observed cell behavior, the authors considered that at least some of the conduction is contributed by oxygen ion migration. 

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