Synthesis and sintering behavior of a nanocrystalline α-alumina powder

Abstract Nanocrystalline α -alumina powders with a primary mean particle diameter of 10 nm were synthesized from aluminum nitrate and ammonia solution using a precipitation method. The presence of ammonium nitrate (a by-product of the precipitation reaction) in the Al(OH) 3 dry gel can reduce the formation temperatures of γ -, δ -, θ -, and α -Al 2 O 3 during heating. The combined effect of 5 wt% α -alumina seed crystals, 100 nm in diameter, and 44% ammonium nitrate can reduce the θ -Al 2 O 3 → α -Al 2 O 3 transformation temperature from 1200 to 900°C. The α -Al 2 O 3 powder milled in anhydrous alcohol has an agglomeration strength of 76 MPa (soft agglomerated), while the one milled in deionized water has an agglomeration strength of 234 MPa (hard agglomerated). For both the soft and the hard agglomerated powders initial stage sintering is controlled by grain boundary diffusion, with activation energies of 365 and 492 kJ/mol, respectively. The alumina ceramic produced by sintering the soft agglomerated powder at 1400°C for 2 h has a mean grain size of 0.93 μm, a mean flexural strength of 700 MPa, and a fracture toughness of 4.75 MPa m 1/2 .