P"-alumina

Electrophoretic casting (38,59) is accompHshed by inducing controUed migration of charged particles under an appHed electric field to deposit on a mandrel. Thin tubular shapes and coatings of limited thickness are formed using this technique. Electrophoretic deposition (EPD) is also used to manufacture thin waU, soHd P -alumina [12005-16-2] NaAl Og, electrolytes for sodium—sulfur batteries. 

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. 

Table 5, Effect of calcination temperature of Cera hydrate on the fired properties of p" -alumina...

Table 7. Dimensions and physical properties of p" -alumina tubes (laboratory-made)...

Compared with the ionic conductivity of p" —alumina tubes, the electronic conductivity is negligible. The electronic resistivity was found to be 109Qcm at 300 °C and 7x lO3 Qcm at 1000 °C [27]. 

Figure 14. Resistivity of p" -alumina doped with zirconia.

Thermodynamic data suggest that P -alumina is stable in sulfur and that Na20-rich //"-alumina maybe unstable to some degree. In the latter event, depletion of Na20 from / " -alumina according to Eq. (5) would result in a surface layer resistant to further corrosion. A significant corrosion reaction does occur be-... 

The investigation of the stability of P -alumina in ZEBRA cells, which always contain some iron, showed an increase of resistance under certain extreme conditions of temperature (370 °C) and of voltage. This is related to the interaction of the P alumina with iron and it was shown that iron enters / -alumina in the presence of an electric field when current is passing, if the cell is deliberately overheated. However, it was found that only the P -phase but not the P"-phase was modified by the incursion of iron. The resistance of the iron-doped regions was high. It was shown that the addition of NaF inhibits access of the iron to the / " -alumina ceramic. By doping practical cells these difficulties have now been overcome and lifetime experiments show that the stability of / "-alumina electrolytes are excellent in ZEBRA cells. 

G.C. Farrington, B. Dunn, and J.C. Thomas, The multivalent P"-Aluminas in High Conductivity Solid Ionic Conductors, E.T. Takahashi, ed. World Sci. Publ., Singapore (1989) pp. 327-365. 

Figure 5.42. XPS of Pt/p -alumina acquired under fixed potential application at 227°C. A) cleaned surface, B) Na-promoted surface.58 Reprinted with permission from the Institute for Ionics.

---