The Ceramic Electrolyte -Alumina

The electrolyte / -alumina has already been described in Chapter III, Sec. 9. This section relates to additional information on the manufacture of / -alumina and its application and behavior in high-tem-perature batteries (ZEBRA and Na /S). 

The P —alumina structures are remarkable not only for their ionic conductivities but also for the versatility in isomorphous replacement. There is little of the structure of (Na2S)l+Jt 11A1203 which cannot be substituted, at least in part, by an alternative ion. MgO and Li20 are preferred additives to P —alumina in order to obtain good ionic conduction with no electronic contribution. 

The difficulty of obtaining pure / -material for the electrolyte has been tackled in many production processes worked out in the past. Unless precautions are taken, sintering of a -alumina-derived / -alumina compositions invariably results in the duplex microstructure and a low-strength ceramic. Therefore a balance has to be struck between conductivity and strength. The problem arises because the conversion from —alumina to / -alumina is slow 

The results of development work on processes indicate that the two main methods of preventing the duplex microstructure from forming appear to be fast-firing, or increasing the amount of / -alumina at low temperatures. Based on these results, Duncan et al. [20] and Zyl et al. [21] have described production processes starting from aluminum oxy-hydroxides or aluminum hydroxides as precursors for the synthesis of the solid electrolyte -alumina. Duncan et al. described an alumina precursor which substitutes in part or wholly for or-alumina in an established 

A comparison of boehmite with other raw materials is included in Table 6. In this table the soda and lithia contents of compositions based on a range of raw materials and the resultant properties are detailed. The level of / -alumina was always higher with the hydrate-type raw material the hydrothermally prepared raw materials gave the highest content of / -alumina. 

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The molten salt, sodium aluminum chloride, fulfills two other tasks in the cell system. The ceramic electrolyte "-alumina is sensitive to high-current spots. The inner surface of the ceramic electrolyte tube is completely covered with molten salt, leading to uniform current distribution over the ceramic surface. This uniform current flow is one reason for the excellent cycle life of ZEBRA batteries. 

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