Beta-alumina sodium

Another form of alkali metal attack on the hot faces of refractory linings involves their high temperature reaction with various components of the brick to form expansive crystalline phases which cause brick to bloat on their hot faces and, subsequently, erode or spall. An example Is the case of alumina brick exposed to sodium at temperatures from about 1700°F to 3000°F. Although sodium does not form a low temperature melt with alumina, it reacts with the alpha phase of alumina, corundum, to form beta alumina, sodium aluminate. Beta alumina has a much greater volume than the very dense corundum and, therefore, disrupts the brick bonding matrix, causing eventual bond failure. 

Variations of the sodium/sulphur battery include systems with sodium/beta-alumina/sodium tetrachloroaluminate/nickel chloride or iron chloride. The early results are encouraging and the system suffers less from the problems of the sulphur electrode, such as rechargeability, high vapour pressure, various polysulphide formations etc. 

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. 

Fig. 7.17 Relation of the spinel structure (left) io the structure of sodium beta alumina (right). The sodium ions are free to move in the open spaces between spinel blocks, held apart by Al—O—Al pillars in the "parking garage structure. [In part from Wells. A. F. Structural Inorganic Chemistry, 5th ed. Oxford University Oxford, 1984. Reproduced with permission. ...

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. 

Another solid electrolyte that may lead to important practical applications is sodium beta alumina. Its unusual name comes from a misidertificalion and an uncer-... 

Fig. 7.18 Sodium/sulfur battery with a sodium beta alumina solid electrolyte.

Consider the battery in Fig. 7.18. The sodium beta alumina barrier allows sodium ions formed at the anode to Row across to the sulfur compartment, where, together with the reduction products of the sulfur, U forms a solution of sodium trisulfide in the sulfur. The latter is held at 300 CC to keep it molten. The sodium beta alumina also acts like an electronic insulator to prevent short circuits, and it is inert toward both sodium and sulfur. The reaction is reversible. At the present state of development, when compared with lead storage cells, batteries of this sort develop twice the power on a volume basis or four times the power on a weight basis. 

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 17.10 In a sodium-sulfur battery, Na ions migrate through beta-alumina to the cathode to equalize the charge as electrons flow spontaneously from the anode to the cathode through the external circuit. 

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. 

The authors summarized the results of the soda reaction test as follows For the high-silica samples the reaction products were identified as nepheline and sodium aluminum silicate (3 2 4). The amount of nepheline decreased with increasing alumina content, whereas the amount of (3 2 4) compound increased. The lack of silica in the high alumina samples permits the free alumina not tied up as a sodium aluminum silicate to react with the alkali to form beta-alumina. Higher amounts of soda produce sodium aluminates. 

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