Preparation And Characterization Of Microcrystalline α-Al₂O₃Aggregate

In this paper, microcrystalline α-Al2O3aggregates were synthesized through combining sol-gel and two step sintering processes at atmospheric pressure. The sinter was obtained by Sol-Gel, adding sintering additives, using the as-prepared pseudoboehmite or aluminum hydroxide as raw material. The sinter was calcining at different low and high temperature, preserved to be heated at high temperature for different time. The different preparation conditions have great effect on grain size, compactness and hardness of microcrystalline α-Al2O3aggregate. The formation mechanisms of the microcrystalline α-Al2O3aggregate were also investigated. The experimental results further deepened understanding of microcrystalline α-Al2O3aggregate.1. Preparation of microcrystalline α-Al2O3aggregate with high hardness using pseudoboehmiteIn the research, using the as-prepared pseudoboehmite with different size and crystallinity as raw material, the microcrystalline α-Al2O3aggregates were prepared by Sol-Gel method combined with two step sintering processes at atmospheric pressure. The as-prepared samples were characterized in detail by XRD, SEM, EDS, TG-DSC, FTIR, N2adsorption-desorption analysis and other techniques. The experimental results show that the α-Al2O3aggregates has compact structure, high hardness of22.65GPa, and the average particle size of less than200nm after calcined at1100℃for1h. There are no impurity peaks in microcrystalline α-Al2O3aggregate except vibration peaks caused by absorbed water. The DSC curve show that phase transformation temperature of γ-Al2O3to-Al2O3is1046.6℃. The nitrogen adsorption-desorption isotherm of microcrystalline α-Al2O3aggregate show that the surface area drops to3.86m2/g and the pore volume is only0.003cm3/g. The sharp decline in surface area and pore volume reveal that the microcrystalline α-Al2O3 aggregate has bigger grains size, compact structure and basically non-existent pores. The size and crystallinity of pseudoboehmite have great effect on hardness of microcrystalline α-Al2O3aggregates.. All of sintering additives (ferric oxide and magnesia), calcination time and calcination temperature present a law of increase firstly and then decrease on hardness of microcrystalline α-Al2O3aggregates..2. Preparation of microcrystalline α-Al2O3aggregate with high hardness using aluminum hydroxideIn the research, amorphous aluminum hydroxide with small crystallite size was obtained by the precipitation of aluminum nitrate solution and ammonia. Using the as-prepared aluminum hydroxide and sintering additives as raw material, the microcrystalline α-Al2O3aggregates were prepared by Sol-Gel method combined with two step sintering processes at atmospheric pressure. The samples were characterized in detail by XRD, SEM, EDS, TG-DSC, FTIR, N2adsorption-desorption analysis and other techniques. The experimental results show that the aluminum hydroxide was amorphous and had transformed into α-Al2O3after calcined at1200℃for1h. The α-Al2O3aggregate has compact structure, high hardness of23.03GPa, and the average particle size of less than200nm after calcined at1200℃for1h. The DSC curve show that phase transformation temperature of y-Al2O3to α-Al2O3is1038.98℃. All of ball milling time, sintering additives (ferric oxide and magnesia), calcination time and calcination temperature present a law of increase firstly and then decrease on hardness of microcrystalline α-Al2O3aggregates. When prolonged the ball milling time, the grain size of microcrystalline α-Al2O3aggregates decreased.