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

Two phase samples containing similar to 30 vol.% of 3 mol% Y2O3-stabilized zirconia (TZ-3Y) and similar to 70 vol.% of of alpha-Al2O3 doped with 5 mol% Fe2O3 were fabricated by sintering powder compacts in air at 1450 degrees C. The added Fe2O3 was fully dissolved in Al2O3 as determined by X-ray diffraction. The two phase samples were packed in Na-beta-alumina powder of nominal composition 8.85 wt.% Na2O, 0.75 wt.% Li2O and balance Al2O3 and heat treated in air at 1250 degrees 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 similar to 97% of theoretical density with an approximate grain size of similar to 0.8 mu m. In the samples packed in Na-beta-alumina powder and heat treated in air, Fe-doped alpha-Al2O3 converted into Fe-doped Na-beta-alumina. A sample of 235 mm in thickness could be fully converted into iron-doped Na-beta-alumina in 17 hat 1450 degrees C. Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) over a temperature range from 197 degrees C to 485 degrees C. The bulk conductivity of the converted sample exhibited Arrhenius behavior with an activation energy of 020 eV and a pre-exponential factor of 421 Omega(-1) cm(-1) K. (C) 2016 Elsevier B.V. All rights reserved.