Diffusion and ionic conductivity in sodium beta alumina

Abstract

Measurements of sodium tracer diffusion ( D t ) and ionic conductivity (σ) have been made on the same single crystals of sodium beta-alumina of composition 1.23 Na 20.11 Al 2O 3. The σ- measurements were made over the temperature 390–660 K using reversible (liquid sodium) electrodes. A fit to the conductivity data gives σT = 2470 exp (−0.142 eV kT )Ω −1 cm −1 K . The D t , measurements employed two techniques, i.e. nondestructive profiling over the temperature range 210–750 K and cation exchange over the temperature range 505–970 K. The results of the two techniques were in close agreement and can be expressed as D = 2.12 ×10 −4 exp(−0.169 eV kT ) cm 2 sec −1 for T>520K and D = 2.45 × 10 −4 exp(−0.164 eV kT ) cm 2 sec 470 K . The “transition” region between 470 and 520 K is not observed in the conductivity measurements. Silver cation exchange was used to determine the number of mobile sodium ions. A comparison of D t and σ data yielded a Haven ratio that is temperature dependent, ranging in value from 0.45 at 870 K to 0.35 at 370 K.