Beta"-alumina sintering

Sintering is a critical step with schedules extending over typically 24 h and a peak temperature of approximately 1600 °C. At this temperature there is an appreciable sodium vapour pressure over beta aluminas and special precautions are taken to maintain the required composition. The component may be enclosed in a sealed spinel (MgAl204) saggar which can be re-used. 

Figure 6.48. Nyquist representation of the impedance spectrum obtained with sintered beta-alumina associated with two gold electrodes, at various temperatures, and under air...

Some studies conducted on pulverulent zirconia post hot sintering, mention a slight decrease as temperature rises, while others give constant values in this temperature range. Concerning beta-alumina, Armstrong and Archer have established a law that slightly increases with temperature. 

As for n, which is the parameter that provides the gap between the actual capacitace and a pure capacitance, it is independent of temperature. The values are close to 0.5 (Warburg diffusion mechanism), sUghtly higher for sintered beta-alumina and sintered zirconia (0.62 and 0.59), slightly lower for glass and the... 

To overcome these different drawbacks, and to use every advantage provided by these materials, we have chosen to chemically treat sintered beta-alumina we make beta-alumina react with sulfur dioxide to form sodiirm sulfate film on the surface of the material. This process takes place at 600°C in 10,000 ppm of sulfur dioxide in... 

The chemical treatment of the sintered pellets or the thick films occurs in the presence of sulfur dioxide diluted at 1% in dry air, for 90 minutes at 600°C. Such a treatment is enough to begin the elaboration of sodium sulfate using beta-alumina. 

Figures 7.40a and 7.40b make it possible to compare the different analysis, using X-ray diffraction, of a sample of beta-alumina ciystallized at high temperature, pellet-shaped and sintered using the previously described process, before and after the SO2 treatment...

Yet, in the film containing 60% of beta-alumina, the structure shows jointed grains. Furthermore, the beta-alumina particles are not necessarily wrapped up in the glass. As in the case of sintered material, these films react in the presence of CO2 and water vapor. This causes the apparition of inflorescences we can thus assume the formation of sodium carbonate. 

At 0%, the resistance value of a thick film composed solely of glass is displayed and at 100%, those of a sintered pullet composed of beta-alumina only, of sintered pullet of sodium sulfate and a sintered pullet of nepheline, prepared for this study. 

Two general approaches have been adopted to the fabrication of electrolyte tubes (l) isostatic pressing (2) electrophoretic deposition. In either case the green tube must be sintered at high temperature in air in order to form and densify the beta alumina phase. 

Testing of coarse-grained and porous bonded refraetories, such as pressed and sintered AZS and vibro-cast AZS and alumina, should be performed on large samples and for long durations (more than a few days), in order to have a measurable and reliable result. Fusion-cast refractories, both AZS and alpha-beta alumina, will generally perform better due to their low level of open porosity and tight interlocking crystals. 

This class of materials was developed to combine the chemical stability of alumina and properties of Si3N4 with ease of sintering. A beta Sialon is formed by substitution of Al and O ions for Si and N in Si3N4 lattice, as Sie-zAlzOzNg-z- The resultant structure contains elongated beta-Sialon grains, typically with some intergranular phase, which could be amorphous or crystalline. Substitution of alumina adds to the chemical stability... 

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