Performance analysis of Na-β″-Al2O3/YSZ solid electrolytes produced by conventional sintering and by vapor conversion of α-Al2O3/YSZ

Abstract High temperature Na2O vapor conversion of α-Al2O3 with different concentrations of yttria stabilized zirconia (YSZ) was used to produce Na-β″-Al2O3 solid electrolyte (BASE) materials. The influence of different ratios of yttria (YSZ-3 and YSZ-8) on conversion and material properties (ionic conductivity and flexural strength) was also investigated. Extent of conversion to Na-β″-Al2O3 was calculated by semi-quantitative XRD phase analysis. Al2O3 composites with 20% or less YSZ were incompletely converted to Na-β″-Al2O3 after 2 h at 1450 °C. Al2O3 composites with 30–40% YSZ-3 appear fully converted by XRD but have lower conductivity and higher strength compared to conventionally sintered Na-β″-Al2O3 composites with the same concentration of YSZ-3. SEM of the materials indicated differences in morphology and grain size. XRD of Al2O3 composites with YSZ-8 showed significantly lower conversion than those with YSZ-3. This is attributed to the larger grain size of YSZ-8, which limits the homogeneity of composites with Al2O3 and thus does not assist in oxygen diffusion as well as YSZ-3. Although the vapor converted BASE materials tended to have lower conductivities at 300 °C, they have lower activation energies (0.20 eV) compared to conventionally sintered Na-β″-Al2O3 composites (0.30 eV) and comparatively higher conductivity at ambient temperature. Vapor converted BASE materials also have better mechanical properties, which should allow fabrication of thinner electrodes.