Synthesis of iron-doped Na-β”-alumina + yttria-stabilized zirconia composite electrolytes by a vapor phase process

• Fe-doped α-alumina + TZ-3Y dense two phase samples sintered
• Fe-doped α-alumina + TZ-3Y converted to dense Na-β”-alumina + TZ-3Y composites
• Ionic conductivity measured by electrochemical impedance spectroscopy

Two phase samples containing ~ 30 vol.% of 3 mol% Y2O3-stabilized zirconia (TZ-3Y) and ~ 70 vol.% of α-Al2O3 doped with 5 mol% Fe2O3 were fabricated by sintering powder compacts in air at 1450 °C. The added Fe2O3 was fully dissolved in Al2O3 as determined by X-ray diffraction. The two phase samples were packed in Na-β”-alumina powder of nominal composition 8.85 wt.% Na2O, 0.75 wt.% Li2O and balance Al2O3 and heat treated in air at 1250 °C for 20 h. The samples were sectioned, polished and examined by X-ray diffraction, optical microscopy and scanning electron microscopy. The as-sintered samples were ~ 97% of theoretical density with an approximate grain size of ~ 0.8 μm. In the samples packed in Na-β”-alumina powder and heat treated in air, Fe-doped α-Al2O3 converted into Fe-doped Na-β”-alumina. A sample of 2.35 mm in thickness could be fully converted into iron-doped Na-β”-alumina in 17 h at 1450 °C. Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) over a temperature range from 197 °C to 485 °C. The bulk conductivity of the converted sample exhibited Arrhenius behavior with an activation energy of 0.20 eV and a pre-exponential factor of 421 Ω− 1cm− 1 K.