The sol - gel method is the effective technique to prepare high purity ultrafine alumina powder. Laser particle size analyzer and scanning electron microscopy were used to study the inflence of pH, leaching of precursor sol, surface modification of alumina powder on the secondary particle size. The impact of leaching sol on powder crystal was tested with X-ray diffraction. The results show that: aluminum nitrate solution showed different appearance with different pH values during titration aluminum nitrate solution with ammonia. When pH=3~5.5, the solution occur through no precipitation to the white precipitate, then the solution becomes relatively thin paste; when the pH was from 6.0 to 6.5, the solution becomes more thick paste. When the pH increased from 7.0 to 8.0, the solution has turned into a more dilute paste. The impact of pH on the particle size is: When the pH increased from 5.5 to 7.0, aluminum powder particle size gradually become thicker and larger. When the pH continues to rise, powder particle size is reduced, when the sol pH value is 8.0, it can get the smallest particle size for alumina powder, up to 548nm. Powder average particle size when adding TEA surfactant to sol is less than the sol without any surfactant particle size from the micrograph. Moreover, particle size is more uniform with TEA surfactant, but powder particle size which did not add surfactant to the sol is clear size differences and the dispersion is poor. When the sol containing surfactant, the effect of surface modification is not obvious. In the case of sol without surfactant powder particle size reduced after the surface modification, then powder surface modification is necessary, it can prevent the powder from congregating. The crystal type is relatively perfect and single α-Al2O3 after filtration, the crystal type without leaching is α-Al2O3 and γ-Al2O3, with a smaller particle size, crystal grain size is relatively smaller. If alumina sol isn’t be leached it contains ammonium nitrate, ammonium nitrate will decompose and release various gases during sintering, the gas emission has an crushed effect on the sintered powder.
