Degradation of Sodium β″‐Alumina: Effect of Microstructure

The effect of microstructure on failure initiation of sodium beta''‐alumina solid electrolytes have been investigated by acoustic emission detection. Mode I failure initiation follows Weibull statistics with a modulus of about 2.5. The observed average critical current densities were 145 and 640 mA/cm−2 for large and for small grained electrolytes, respectively. Nonuniform secondary current distributions near the electrode edges were considered and were found to lead to minor corrections to these values. The conditions under which nonuniformity of current would affect failure statistics have been considered in the Appendix. Considerations of failure statistics indicate conditions where proof testing may be necessary to achieve sufficient reliability. These considerations assume complete equivalence between mechanical and electrolytic failure; the indication is, however, that such equivalence is not valid.