The beta-aluminas

Sudworth, J.L. et al. (2000) Toward commercialization of the beta-alumina family of ionic conductors, MRS Bull. 25(3), 22. 

Sodium ion conduction appears to be common because of the well-known properties of the beta-aluminas and, to a lesser extent, the NASICONs (see Section 2.12.1), Table 2.1. There are, however, relatively few other examples of high Na ion conductivity, especially at room temperature. In contrast to Ag, the usual coordination number of Na is high, often 7-9, and the sites may be distorted. The bonding of Na in such structures is much more ionic than that of Ag, therefore. 

The beta-alumina structures show a strong resemblance to the spinel structure. They are layered structures in which densely packed blocks with spinel-like structure alternate with open conduction planes containing the mobile Na ions. The and /S" structures differ in the detailed stacking arrangement of the spinel blocks and conduction planes. Fig. 2.9. 

The beta-alumina structures (j8,j8"-alumina and the analogous gallates) can be prepared as H30 or NH4 derivatives by ion exchange and some of these are good proton conductors at temperatures up to 200-400 °C, until they decompose by loss of H2O/NH3. 

The Prospects For Solid Electrolyte SBs. For reasons discussed above, the Agl-based cells, being useful for some special types of primary batteries, are not very promising for secondary ones. The beta-alumina cells, on the contrary, have already been developed to the pilot-plant stage and their prospects are fairly good to become commercialized. They are the most advanced among the candidate batteries for traction. The high operating temperature could be lowered if a solid... 

Moreover, it confirms the fact that the effects we observed on the beta-alumina only as the temperature varied are suggestive of an endothermic process, which is characterized by an increase in the amount of formed species, and as a result, by an increase of the matching heat quantity as temperature rises. 

While conducting the required measurements, we did not note any surface potential change on the beta-alumina, whatever the pressure and temperature conditions. This total absence of information leads us to the idea that the endothermic oxygen species we have observed in calorimetiy, if it is under the same experimental conditions, stays electrically neutral upon contact with the material. 

For research into the manufacture of gas sensors of a potentiometric type, we have tested different solid electrolytes. The unique property of these sensors is that the two different metalhc electrodes ate located in the same gaseous phase. This property has prompted us to study particularly the beta-alumina and calcium sulfate. 

In order to crystallize the beta-alumina or the beta -alumina, the amorphous powder, located in an alumina container resistant to high temperatures, is heated at 5°C/nm until the temperature reaches 1,200°C in ambient atmosphere. 

Whether the powder is obtained using sodium oxalate or acetate, the X-rays diffraction spectra shows the presence of the beta-alumina and beta -alumina stages. 

In the case of sodium oxalate, the ratio of the beta-alumina/beta -alumina stages is calculated using d = 1.976 A (beta -alumina stage) and d = 2.690 A (beta-alumina stage). The value of this ratio is about 0.75. 

The beta-alumina powder has been conditioned by two different methods mono-axial compression and serigraphic depositing. 

The glass and the beta-alumina are mixed by grinding in a planarian grinder in a liquid, until the granulometry is less than about 10 pm (whieh is neeessaiy for... 

The first is the diffractogram of a sample of beta-alumina not treated with SO2. The characteristic peaks of the beta-alumina can be recognized. If we compare this diffractogram with that of the S02-treated sample, we can distinguish three peaks, located at 22.6°, 23.6° and 25.5° characteristic of the sodium sulfate (formula Na2S04). 

A scanning electron microscope allows us to observe the evolution of the beta-alumina before and after sulfatatioa The photograph (Figure 7.41) shows at great magnification (x 3,500), the sample surface before the SO2 treatment. We note the presence of needle-shaped peaks or inflorescence whose stmcture is similar to the stmcture of hydrated sodium carbonate. This compound confirms the reactivity of the beta-alumina even at ambient temperature. 

The film made with 40% of beta-alumina possesses the properties of wettability which allow the beta-alumina to form an amorphous matrix. Nevertheless, the structure shows cracks and cavities. 

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. 

Figure 10.14. Evolution of the surface potential as a function of oxygen pressure for the beta-alumina/platinum structure (1 mbar = 100 Pa)...

The three boundary point is a specific zone that has its own adsorption sites different from the sites of the sohd electrolyte and of the metal. If we note by p the adsorption sites at the surface of the beta-alumina, Si those present at the level of the three boundary point and S2 those of the metal, the different steps of the formation of the oxygen species present in the system can be expressed as follows ... 

FARRINGTON Both structural and NMR results indicate that sodium ions occupy non-equivalent sites within the beta alumina conduction-p1ane. It is unclear exactly how this non-equivalency is manifest on the microscopic level whether at unit cells or in larger domains. Similar non-equivalency does not seem to be the case in alumina. I submit that detailed models of interfacial behaviour in g-alumina must take into consideration these structural data presently available despite their ambiguities. Complete site equivalency and ion mobility should not be assumed for g-alumina. 

Solid state ionics, that is solids which possess unusually high diffusion coefficients and conductances for specific ions, have assumed considerable importance in recent years for battery research. This stems from the di og ry in 1966/7 of the alkali silver halides KAg I and RbAg I- and the beta alumina family of oxides. The nalides, which are pure Ag ion conductors, have resistivities as low as 5-10 ohm-cm at room temperature. 

A further useful classification of solid electrolytes is based upon whether the fast ion-conduction process is three-dimensional throughout the crystal lattice, as in the high temperature electrolytes and the silver salts, or is confined to two-dimensional layers, as in the beta alumina family of compounds, or to onedimensional tunnels, as in the hollandite materials such as... 

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. 

The other ternary oxides also occur as spinel superstructures such as CaFe204 (ferrite) or potassium iron oxides which contain the spinel structure as a building block. These complex structures of the beta alumina type consist of the formal composition (K2O x 11 Fc203) and can be described as a layer structure with unit cells of spinel linked by layers of potassium ions octahedrally coordinated to the (4 -h 4) oxygen ions of two adjacent spinel units. 

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