Spinel interface reaction

Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

All the input data of this local model are either literature data (diffusion coefficients of vacancies and interstitials, thermodynamic constants of the formation reaction of interstitials and vacancies in the magnetite layer), or determined by experimental conditions or calculated (spinel stoichiometry, oxygen activities at the T91—spinel interface and at the spinel—magnetite interface). No adjustment is performed on the experimental points. Results obtained [65,66] show that the simulation is in relatively good agreement with the experimental points whatever the temperature, the oxygen concentration, and the fluid velocity (for temperatures between 450°C and 620°C in... 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

Figure 8.27 Markers used to determine the mechanism of spinel formation from magnsium oxide and alumina (a) inert markers at the interface between MgO and AI2O3 crystals (b) after reaction the marker will appear to be within the MgAl204 layer when a cation counter-diffusion is in operation. The ratio of the layer thickness on each side of the boundary will depend on the charges on the ions...

In the CoO/ZrOi eutectic solid, the two ceramics are both cubic, but they have very different structures. They grow in the cube-on-cube orientation, which means that all directions and planes in one material are parallel to the same directions and planes in the other material. As a possible application of such a material, oxygen diffuses rapidly in ZrOi- It can then react with the CoO to produce a layer of C03O4 at the interface between the two materials, resulting in the structure illustrated in Figure 15.18. Notice how uniform this spinel layer is the growth is controlled by the reaction, not diffusion of O through the ZrOi-... 

FIGURE 25.17 Chemical reactions by movement of steps on in interface, a) P-AI2O3growing in spinel (b,c) amphibote growing in orthodyroxene. 

By combining TEM and field emission gun (FEG)-SEM we find fhaf fhe formafion of spinel occurs more quickly along GBs in fhin-film reaction couples as illustrated in Figure 25.18. At the earliest stages of these reactions, the kinetics are controlled by the interface mobility rather than by diffusion through the reactant. 

In the second type of sample a buffer layer of the reaction product or a reaction-barrier layer is grown before growing the reactant layer. This geometry allows us to quantify the kinetics of the reaction separately from the nucleation. We can then examine the morphological development of the two moving interfaces and the effect of lattice misfit on this morphology. The expansion that occurs when the spinel forms can be readily acconuno-dated if a buffer layer is present forming a uniform layer... 

Several authors proposed slow electron transfer reactions, induced by adsorption of Fe(II) (32-36), at the interface of spinel-like iron oxides containing Fe(II) and Fe(III) and also provided spectroscopic evidence for product formation (37). 

The spinel product can only form at the iron oxide side, where it reacts with the entering cobalt ions, the electrons, and gaseous oxygen. These equations show that the marker threads end up on the left-hand boundary between the spinel and the divalent oxide (CoO) after reaction because there is no spinel formation at this interface. 

Generally, highly charged cations do not diffuse as well as cations with a smaller charge. The mechanism proposed here supposes that the trivalent iron ions are temporarily reduced to divalent iron, which is then assumed to be able to diffuse. The cobalt ions in this model are expected to remain stationary. The spinel is now formed only at the left-hand boundary between the spinel and the divalent oxide (CoO) hence the marker wires are found at the right-hand interface between the spinel and the trivalent oxide (Fe203) after the reaction. 

The bold-faced coefficients in these reactions show that after the reaction, the marker wires are in the interior of the spinel at a point that is one-third of the distance from the CoO interface. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

Spinel formation. The TGO growth leads to a decrease of A1 activity at the TGO/bond coat interface. When the A1 depletion occurs, the AI2O3 can be converted to NiAl204 (spinel) via the reaction ... 

As a result, selective oxidation of chromium is observed. The activity of chromium at the interface decreases as the reaction progresses. The equilibrium then shifts gradually towards the left, permitting thus the simultaneous existence of the two oxides. By reacting with each other they form a spinel phase ... 

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