2019 Volume 127 Issue 3 Pages 150-157
We successfully prepared the c-axis-oriented polycrystalline sodium titanoaluminate (NTAO) by the reactive diffusion between solid Al2TiO5 and liquid [25 mol % Na2O and 75 mol % TiO2]. When the Na2Ti3O7/Al2TiO5 diffusion couple was isothermally heated at 1673 K for 3 h, the Na0.78Ti2.22Al4.78O12 polycrystal was readily formed in the presence of a liquid phase. The resulting polycrystalline material was characterized by X-ray diffractometry, electron microscopy, and impedance spectroscopy. We mechanically processed the annealed diffusion couple, and obtained the thin-plate electrolyte consisting mostly of the grain-aligned NTAO polycrystal. The ionic conductivity of the electrolyte along the common c-axis direction steadily increased from 1.29 × 10−2 to 1.31 × 10−2 S cm−1 as the temperature increases from 923 to 1073 K, with the activation energy being 0.095 eV. The crystal structure of NTAO (space group Pbam) was isomorphous with that of NaTi2Al5O12. The Na+ ions occupied ∼78% of the Wyckoff position 2d site, the deficiency of which would contribute to the relatively high ionic conductivity along the c-axis. The present solid–liquid reactive diffusion technique could be widely applicable to the preparation of the other grain-aligned ceramics of multi-component systems.