Abstract
Two α-alumina + YSZ samples were prepared by sintering for 3 h one at 1500 °C and the other at 1700 °C. The samples were then converted to Na-β″-alumina + YSZ by vapor phase conversion. Characterization techniques such as X-ray diffraction, scanning electron microscopy, and electrochemical impedance spectroscopy in addition to dimensional geometrical changes reveal the evolution of slight anisotropy in these samples during conversion. This results in an electrical conductivity anisotropy factor of about 5.5 and 1.8 for samples sintered at 1500 °C and 1700 °C, respectively. In all samples, the higher ionic conductivity was measured across the sample thickness as opposed to parallel to the disc faces. The ionic conductivity measurements show the conductivity of about 0.15 S/cm and 0.07 S/cm at 300 °C for samples sintered at 1500 °C and 1700 °C, respectively. The larger anisotropy in samples sintered at 1500 °C is explained by the higher aspect ratio of grains in this sample and by different Na concentrations.
Highlights
Na-based electrochemical cells are raising more interest in both research and industry due to Na abundance and availability.[1,2] To date, sodium beta alumina, Nab0-alumina, has been identified as the best sodium-ionconducting electrolyte
They demonstrated that the pellets with a grain size of 0.86 lm had 1 mm conversion thickness at 1450 °C in one hour
The results indicate that both samples are slightly textured in a similar habit, but pre-conversion grain size plays a role in the degree of texturing
Summary
Na-based electrochemical cells are raising more interest in both research and industry due to Na abundance and availability.[1,2] To date, sodium beta alumina, Nab0-alumina, has been identified as the best sodium-ionconducting electrolyte. The grain size-dependent sodium ion conductivity ranges from 0.2 to 0.35 S/cm for fine and coarse-grained samples, respectively, at 300 °C.3. The compound Na2O–6.2Al2O3 known as Na-b0-alumina has a rhombohedral structure consisting of three spinel blocks separated by a conduction plane partially filled by mobile sodium ions. The material is useful in a variety of applications. Sodium sulfur batteries marketed under the trade name of NAS batteries with a sodium anode and a sulfur cathode utilize beta alumina electrolyte
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