Grain-oriented polycrystalline sodium titanoaluminate formed by reactive diffusion between solid Al₂TiO₅ and liquid [25 mol % Na₂O and 75 mol % TiO₂]

抄録

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 Na_0.78Ti_2.22Al_4.78O₁₂ 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.