Importance of the surface exchange kinetics as rate limiting step in oxygen permeation through mixed-conducting oxides

Attention is drawn to the possible involvement of the surface exchange kinetics in limiting the rate of oxygen permeation through mixed-conducting oxide ceramics. A theoretical approach is provided with which it is possible to distinguish between surface exchange- and bulk diffusion controlled kinetics of oxygen permeation. New results on the oxygen permeability of perovskites La0.8Sr0.2CoO3−σ and SrCo0.8Fe0.2O3−σ are presented. The importance of the exchange reaction re to diffusion in limiting overall oxygen transport through (La,Sr)(Co,Fe)O3−σ perovskite-type oxides is emphasized.\u

Partial symmetries for nonlinear systems

We define the concept of partial symmetry for nonlinear systems, which is an intermediate notion between the concepts of symmetry and controlled invariance. It is shown how this concept can be used for a decomposition theory of nonlinear systems and is particularly suited as a framework for treating input-output decoupling problems

Low-level finite state control of knee joint in paraplegic standing

Low-level finite state (locked-unlocked) control is compared with open-loop stimulation of the knee extensor muscles in functional electrical stimulation (FES) induced paraplegic standing. The parameters were: duration of standing, relative torque loss in knee extensor muscles, knee angle stability, average stimulus output and average arm effort during standing. To investigate the impact of external mechanical conditions on controller performance, experiments were performed both under the condition of a freely moving ankle joint and of a mechanically stabilized ankle joint. Finite state control resulted in a 2.5 to 12 times increase of standing duration or in a 1.5 to 5 times decrease of relative torque loss in comparison with open-loop stimulation. Finite state control induced a limit cycle oscillation in the knee joint. Average maximum knee flexion was 6.2° without ankle bracing, and half that value with ankle bracing. Average arm support was 13.9 and 7.5% of the body weight without and with ankle bracing respectively

Optimal prediction for moment models: Crescendo diffusion and reordered equations

A direct numerical solution of the radiative transfer equation or any kinetic equation is typically expensive, since the radiative intensity depends on time, space and direction. An expansion in the direction variables yields an equivalent system of infinitely many moments. A fundamental problem is how to truncate the system. Various closures have been presented in the literature. We want to study moment closure generally within the framework of optimal prediction, a strategy to approximate the mean solution of a large system by a smaller system, for radiation moment systems. We apply this strategy to radiative transfer and show that several closures can be re-derived within this framework, e.g. $P_N$, diffusion, and diffusion correction closures. In addition, the formalism gives rise to new parabolic systems, the reordered $P_N$ equations, that are similar to the simplified $P_N$ equations. Furthermore, we propose a modification to existing closures. Although simple and with no extra cost, this newly derived crescendo diffusion yields better approximations in numerical tests.Comment: Revised version: 17 pages, 6 figures, presented at Workshop on Moment Methods in Kinetic Gas Theory, ETH Zurich, 2008 2 figures added, minor correction

Thermochemical stability and nonstoichiometry of yttria-stabilized bismuth oxide solid solutions

The thermochemical stability of fast oxygen ion conducting yttria stabilized bismuthoxide (YSB) solid solutions containing 22.0–32.5 mol% of yttria was investigated. It was shown that in the temperature range between 650–740 C the stabilized cubic δ-phase containing less than 31.8 mol% of yttria is not stable during long term annealing treatments (greater-or-equal, slanted 500 hours). During annealing at 650 C a sluggish transformation from the cubic to hexagonal phase appears, while above 740 C this hexagonal phase is converted very fast into the cubic phase again. It was shown, that the oxygen content of YSB solid solutions is a function of temperature and oxygen partial pressur

Oxygen permeation through oxygen ion oxide-noble metal dual phase composites

Oxygen permeation behaviour of three composites, yttria-stabilized zirconia-palladium, erbia-stabilized bismuth oxidenoble metal (silver, gold) was studied. Oxygen permeation measurements were performed under controlled oxygen pressure gradients at elevated temperatures. Air was supplied at one side of a dense sintered disk specimen, while helium was fed at the opposite side to sweep away the permeated oxygen. This research has demonstrated that in addition to the presence of percolative metal phase in the oxide matrix, a large ionic conductivity of the oxide phase and a high catalytic activity of the metal phase towards surface oxygen exchange are required for the dual phase composite to possess high oxygen permeability. The bismuth oxide-silver composite fulfils these requirements, hence showing the best oxygen permeability

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 mgrm and a zirconia grain size of 0.2 mgrm. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the agr alumina phase stage shifted to lower temperature. During pressureless sintering the gamma to agr alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature

Improvement of mechanical properties of zirconia-toughened alumina by sinter forging

ZTA powder with a composition of 85 wt% alumina/15 wt% zirconia was prepared by a gel precipitation method. Sinter forging was performed with this powder to enhance the mechanical properties of ZTA materials. The influence of processing flaws on mechanical properties of sinter forged materials and pressureless sintered materials was investigated. Sinter forging at 40 MPa effectively decreases process flaw sizes resulting in a homogeneous microstructure and improves the grain boundary structure because of large shear applied in this process. Sinter forging resulted in an increase in strength and toughness by a factor of 1.5–2 when compared with pressureless sintered compacts. The fracture energy is enhanced by a factor of two. The predominate mechanism for improvement of mechanical properties of these sinter-forged ZTA materials is grain boundary strengthening

Oxygen semi-permeability of erbia-stabilized bismuth oxide

The isothermal permeability of oxygen through sintered dense disks of bismuth oxide stabilized with 25 mol% erbia (BE25) has been studied at 610–810°C and oxygen pressures of 0.0001–1 atm. It is concluded that the permeating flux is rate limited both by solid state diffusion of electron holes and by the surface exchange reaction. From the results the p-type electronic conductivity of BE25 and the surface oxygen exchange rate have been evaluated. The value obtained for the latter shows excellent agreement with that obtained from 18O-16O isotope exchange reported previously. Using gold point electrodes it is demonstrated that the oxygen semipermeability flux in case of partial rate control by surface oxygen exchange leads to deviations from Nernst behaviour, though oxygen permeation measurements reveal that under the applied conditions BE25 remains a solid electrolyte with an ionic transference number close to unity.\u