Synthesis of Ultrafine α-Alumina Powder by Pyrolysis of Ammonium Aluminium Carbonate Hydroxide

By using ammonium aluminium sulfate and ammonium hydro-carbonate as the starting materials, the synthesis of ammonium aluminium carbonate hydroxide (NH4AlO(OH)HCO3) wasstudied. It was found that NH4AlO(OH)HCO3 can be synthesized by adding ammonium aluminium sulfate solution to ammonium hydro--carbonate solution with a adding rate less than1.2L'h--1. Under other conditions the precipitate was γ-AIOOH in the experimental range. Thesequence of phase transformation of γ-AIOOH during heating was found to be γ--AIOOH → γAl2O3 → δ -Al2O3 → θ-Al2O3 →α-Al2O3, while that of NH4AlO(OH)HCO3 was found to beNH4AlO(OH)HCO3 →amorhous A12O3 → θ→Al2O3 →αA12O3. The transformation temperatures of NH4AlO(OH)HCO3 to θ-A12O3 and α-A12O3 were both about 100℃ lower than thoseofγ-AIOOH. γ-AIOOH can transform to α--A12O3 completely by calcining at 1200℃ for 1h, andthe obtained powder, with a particle size of 150um, can be sintered to 84.46% relative density at1450℃ for 2h. NH4AlO(OH)HCO3 can transform to α-A12O3 completely by calcining at 1100℃for 1h, and the obtained powder, with a particle size of 70um, can be sintered to 97.80% relativedensity under the same sintering condition.