Oxygen conductivity relaxation

R = 2[V""]/[Vo] conductivity relaxation kinetics should appear to be onefold or with a single relaxation time, namely, Xxi and Xq, respectively. In the intermediate oxygen activity region, then, the overall kinetics may appear twofold. Thus, the kinetics will have to shift from onefold (Xxi) to twofold (TTi. Co) to onefold (xo) as the oxygen activity decreases from uq = 1. 

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. 

Evidence of Interaction between Oxygen Ions from Conductivity Relaxation and Quasielastic Light Scattering Data of Yttria-Stahilized Zirconia, Phil. Mag. B 77, 187-195. 

Measuring oxygen diffusion and oxygen surface exchange by conductivity relaxation. Solid State Ionics, 136-137, 997-1001. 

Mauvy, F. Bassat, J.M. Boehm, E. Dordor, P. Grenier, J.C. Loup, J.P. (2004). Chemical oxygen diffusion coefficient measurement by conductivity relaxation - Correlation between tracer diffusion coefficient and chemical diffusion coefficient, /. Ettr. Ceram. Soc., Vol. 24, pp. 1265-1269... 

Electrical conductivity relaxation (ECR) method. ECR method is rather simple, it does not require expensive equipment and provides high precision in determination of the oxygen chemical diffusion and surface exchange coefficients in mixed ionic-electronic conductors [64-66],... 

For comparison with results of conductivity relaxation studies, values of the oxygen chemical diffusion coefficient, Dchem, were estimated by following the pellet weight relaxation using a STA 409 PC "LUXX" NETZSCH machine after a step-wise change of O2 content in the N2 stream from 14 to 1.4%. 

Conductivity relaxation studies. In agreement with weight relaxation studies, electrical conductivity relaxation method revealed that oxygen chemical diffusion coefficients in composites are approximately an order of magnitude higher than those in pure perovskite... 

Let us discuss some experimental results before we analyse these cases and let us turn to our optical experiment, which we used to measiure the effective rate constant of oxygen incorporation / excorporation, i.e. k. (Conductivity relaxation measurements yield analogous results, see Fig. 6.27.)... 

We used polycrystalline films of ZnO and Sn02 as adsorbents. The films were deposited from the water suspension of respective oxides on quartz substrates. These substrates contained initially sintered contacts made of platinum paste. The gap between contacts was of about lO" cm. All samples were initially heated in air during one hour at T 500 C. We used purified molecular oxygen an acceptor particle gas. H and Zn atoms as well as molecules of CO were used as donor particles. We monitored both the kinetics of the change of ohmic electric conductivity and the tangent of inclination angle of pre-relaxation VAC caused by adsorption of above gases and the dependence of stationary values of characteristics in question as functions of concentrations of active particles. 

If the tip is contaminated, its apex is most likely attached to a hydrogen molecule or H atoms. As a consequence, the conductance of this tip should be much lower than that of a clean tungsten tip. Since this conductance change has not been reported, it can be concluded that the reduced 0—0 distance is not the effect of a contaminated tip. Surface-tip interactions are evaluated by calculating the interaction between the reacted surface and a tungsten cluster at low distance. Here, the calculations indicate that there is no substantial relaxation due to interactions between the two leads. Consequently, the only possibility left is that the electronic surface structure somehow changes the appearance of the oxygen positions. 

Asymmetry of the response curve to the point of the exposition end reflects the different nature of the exposition and relaxation output signals. A transition from an exposition into relaxation phase corresponds to a return of gas-sensitive matter contact with the initial atmosphere. A variety of processes take place simultaneously in that phase. They may include oxidation of adsorbed molecules by the air oxygen, desorption of the previously adsorbed molecules, competitive adsorption of the ambient atmosphere components. These circumstances cause a complicated shape of the relaxation curve. In general, its course reflects the dynamics of the surface concentration of conductivity clusters. Almost all relaxation curves are characterized by presence of a maximum. It is often more prominent that the corresponding exposition maximum. The origin of this phenomenon is determined by higher conductivity of clusters formed by the oxidized molecules of compounds adsorbed during the exposition phase. 

Finally, the dynamics of the back front of the relaxation maximum formation is also informative. This part of the curve characterizes the gross process of the conductivity clusters decomposition. The decomposition process comprises (1) desorption of the exhaled air components from the gas sensitive matter and (2) oxidation of these components by the atmospheric oxygen at the begitming of relaxation. 

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