Scale formation, ceramics

In spite of having many favorable characteristics, the metallic interconnects also suffer fi-om certain drawbacks. Some of the pertinent issues are electrical contacts between metallic interconnect and ceramic electrodes (cathode and anode), matching of thermal expansion between the metallic interconnect and adjacent components, oxide scale formation on the metallic surface as well as cathode poisoning. All these issues need drastic improvement. For more than a decade, a number of alloys have been attempted, but the major interest for development of such metallic interconnect started only when SOFC developers started using metallic interconnects for SOFC operation, preferably < 750°C. 

Brady M P and TortoreUi P F (2004), Alloy Design of IntermetaUics for Protective Scale Formation and For Use as Precursors for Complex Ceramic Phase Surfaces, IntermetaUics 12, 779—789. 

Fig. 19. Simulation of soot deposition on a filter wall, (a) Evolution of soot deposits (gray) in the wall (black is solid, white is pore space) and incipient cake formation (b) pressure drop as function of challenge soot mass demonstrating the deep-bed to cake filtration transition (c) visualization of soot deposition in an extruded ceramic (granular) filter wall and (d) development of soot deposits (black) and soot mass fraction in the wall (solid material is gray) to the onset of cake formation. Soot mass fraction scale is from 0 (violet) to the inflow value (red). In (d) the velocity on a section through the filter wall is shown, with overlay of the soot deposit shapes (see Plate 9 in Color Plate Section at the end of this book).

The curves show that, before calcination of the powder, the pH rise is very steep but tapers off at pH 10. In contrast, the pH of the calcined powder increases very slowly at a constant rate. This constant rate of increase in the pH helps to produce Mg-based phosphate ceramics in large sizes and makes the process practical. Most commercially available MgO exhibits very high dissolution rates in acid solutions, and its calcination becomes a necessity for production of ceramics at a commercial scale. The titration test provides a good method for testing these powders for their suitability for CBPC formation. 

As described in Ref. [10], in spite of the partial neutralization of the acid and the precalcination of the powder, the ceramic formation is extremely rapid. Accordingly, these cements gain half of their strength in the first 10 min and 80% within an hour [24,25]. As a result, this product is most suitable for dental applications, where the dentist will produce slurry in a small scale within a few minutes, apply it to the patient, and expect its solidification within a reasonable waiting time. 

Critical for the coating system lifetime is the formation of a thermally grown oxide scale (TGO) at the interface BC/TBC during service, which mainly consists of alumina [3]. The possible oxygen diffusion in zirconia itself and the open columnar structure of the ceramic coating allows oxidation of bond coat aluminum. The scale s growth and the difference in... 

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