Beta-alumina conduction

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. 

Beta-alumina, mentioned in Section 1.2.2.2, is just the best known and most exploited of this family. They have been developed by intensive research over more than three decades since Yao and Kummer (1967) first reported the remarkably high ionic conductivity of sodium beta-alumina. Many other elements have been used in place of sodium, as well as different crystallographic variants, and various processing procedures developed, until this material is now poised at last to enter battery service in earnest (Sudworth et al. 2000). 

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. 

Another example of this type of intercalation compound is sodium beta alumina where the sodium ions are free to move between the spinel layers. The sodium ions can be replaced by almost any +1 cation such as Li. K, Rb+, Cs. NHJ, H 0 Tl+, Ga+, NO+, etc. The conductivity of these materials varies with the size of the ions moving between the fixed-distance (A)—0—Ai) layers. 

What makes the sodium-sulfur cell possible is a remarkable property of a compound called beta-alumina, which has the composition NaAlnOiy. Beta-alumina allows sodium ions to migrate through its structure very easily, but it blocks the passage of polysulfide ions. Therefore, it can function as a semipermeable medium like the membranes used in osmosis (see Section 11.5). Such an ion-conducting solid electrolyte is essential to prevent direct chemical reaction between sulfur and sodium. The lithium-sulfur battery operates on similar principles, and other solid electrolytes such as calcium fluoride, which permits ionic transport of fluoride ion, may find use in cells based on those elements. 

Advances in electrochemical systems rest in large measure with the evolution of new materials that exhibit chemical stability in severe environments, high electrocatalytic activity, rapid ion conductivity, etc. Examples include RuOx-TiOy-Ti electrocatalysts, the polymer Nafion, yttrium-stabilized zirconate and beta-alumina electrolytes, and metastable alloys produced by rapid solidification processing. 

Optimum composition of lithia-stabilized beta"-alumina for maximum sodium ion conductivity in polycrystalUne ceramics 18,... 

The research conducted on the development of an electrochemical sensor made of ionic materials led to many investigations concerning beta-alumina associated with metallic elements, such as gold or platinum and used in electrodes. 

In order to locate and count the different oxygen species found on the surface of the materials in question, calorimetric experiments were conducted on different samples. These samples are made with beta-alumina ordy, beta-alumina associated with gold or platinum, with gold and with platinum. 

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. 

In the case of zirconia, however, the impedance spectmm is more complex than beta-alumina s. There is, at high measnrement freqnencies, an additional semicircle that might be dne to intragranular condnction, which is an intrinsic conduction mechanism. 

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. 

Beta-alumina, rich in sodium, possesses a good adhesion on the support and a good ionic conduction. Its weak point is linked to its reactions with some gaseous compounds. 

The deposits containing 60% and 40% of beta-alumina, though, seem to possess a superior conductivity. For the film containing 40% of beta-alumina, this value is independent of the thermal treatment. However, at 60% it decreases when the thermal treatment temperature decreases. 

Ingram, M. D. (1980) Conduction and Dielectric Loss Mechanisms in beta-Alumina and Glass A Discussion Based on the Paired Interstitialcy Model, J. Am. Ceram. Soc., 63, 248-263. 

P. G. Bruce, A. R. West, and D. P. Almond [1982] A New Analysis of ac Conductivity Data in Single Crystal Beta-Alumina, Solid State Ionics 7,... 

L. C. De Jonghe [1979] Grain Boundaries and Ionic Conduction in Sodium Beta-Alumina. J. Mat. Sci. 14, 33-48. 

Electrolytes are distinguished from pure electronic conductors by the fact that the passage of an electric current is only insured by displacement of charged species called ions and hence accompanied by a transfer of matter. Therefore, electrolytes are entirely ionic electrical conductors without exhibiting any electronic conductivity (i.e., no free electrons). They can be found in the solid state (e.g., fluorite, beta-aluminas, yttria-stabilized zirconia, and silver iodide), liquid state (e.g., aqueous solutions, organic solvents, molten salts and ionic liquids), and gaseous state (e.g., ionized gases and plasmas). The ions (i.e., anions or cations)... 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

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