Fabrication of highly textured fine-grained α-alumina by templated grain growth of nanoscale precursors

[0001] textured alumina ceramics with a fine grain size were fabricated between 1400°C and 1600°C via templated grain growth (TGG) using fine alumina platelets (∼0.6 and ∼3 μm diameter) aligned by tape casting in either a 50 nm α-Al₂O₃ matrix powder, or in a seeded boehmite sol. The 3 μm templates could be readily aligned by tape casting in both matrices (orientation parameters r = 0.27 and 0.18, respectively), whereas 0.6 μm diameter templates were well aligned in the seeded boehmite sol only (r = 0.29). Improved alignment in boehmite sols is attributed to inorganic gelation, resulting in a strongly pseudo-plastic rheology that preserves template alignment against the influence of Brownian motion. The in situ formation of fine α-Al₂O₃ matrix after transformation in the seeded boehmite system results in a higher driving force for TGG and improves texture development. The combination of 3 μm templates with a seeded boehmite matrix results in extremely high texture qualities (texture fraction f = 0.97-0.99, r = 0.17) while maintaining a relatively fine grain size (5-10 μm in diameter and 1.5-3 μm in thickness). Although undoped samples can be fully textured at 1600°C, adding as little as ∼0.25 wt% CaO/SiO₂ dopant improves TGG kinetics and yields full texture at 1400°C.