| Alumina（Al2O3）ceramics are essential materials of industrial importance due to their excellent physicochemical properties and low price.Al2O3 ceramics have been widely used as structural,functional,and biological ceramics.Al2O3 ceramics are brittle at room temperature,which substantially limits their wider applications.The nanocrystalline ceramic may overcome its brittleness.To prepare fully dense Al2O3nanocrystalline ceramics with excellent properties,high-purity,disperse,fine,and equiaxed Al2O3 nanoparticles with narrow size distribution are essential.Simulation and experimental studies showed that when the specific surface area ofα-Al2O3 larger than 100 m2/g（the size of nanoparticles below 15 nm）,α-Al2O3 nanoparticles have a higer surface energy thanγ-Al2O3 and become thermodynamically unstable with respect toγ-Al2O3 at room temperature and thus are extremely difficult to fabricate.This dissertation is mainly focused on the preparation and characterization of disperse fine equiaxed Al2O3 nanoparticles with narrow size distributions and high purity.It mainly contains the following contents:Preparation ofα-Al2O3 nanoparticles by mechanochemical method combined with selective corrosion.The mixtures of Al and Fe2O3 powders were milled in argon using a high-energy ball mill andα-Al2O3 nanoparticles-embedded nanocomposite powders can be obtained after the redox reaction being finished.After removing the by-product iron and the impurities introduced by ball milling,the disperse equiaxedα-Al2O3 nanoparticles with an average particle size of 14.3 nm and a purity of 99.6%（mass percent）were prepared.Preparation ofα-Al2O3 nanoparticles by mechanical milling method combined with acid corrosion.Micron-sizedα-Al2O3 particles were directly ball-milled and subsequently corroded by HCl at room temperature,and the high-purity disperse equiaxedα-Al2O3 nanoparticles with an average particle size of 8 nm and a purity of99.96%（mass percent）were prepared.α-Al2O3 nanoparticles with smallr average particle size,high yield,and high purity can be obtained by this method.Besides,disperse fine equiaxedα-Al2O3 nanoparticles with average particle sizes（and size distributions）of 3.3（2-6）,4.8（2-10）,6.8（2-15）,and 12（3-32）nm were obtained by fractionated coagulation separation.The analyses of the crystal structure of theα-Al2O3 nanoparticles with different average particle sizes shown that,the lattice parameters of the single crystal ofα-Al2O3 nanoparticle are larger than the lattice parameters of the largeα-Al2O3 particles.For the single crystal ofα-Al2O3nanoparticle,the lattice parameters increase with the average particle size ofα-Al2O3nanoparticles decreasing.We explored that,forα-Al2O3 particles,fracture of largeα-Al2O3 particles occurs at the initial stage of ball milling.As more and more fine nanoparticles form,cold welding of fineα-Al2O3 nanoparticles occurs.These two competing processes ultimately approach a dynamic equilibrium with the minimal average particle size.The disperse fine equiaxedα-Al2O3 nanoparticles with an average particle size of 4.8 nm and a narrow size distribution of 2-10 nm were sintered in air by two-step pressureless sintering method.The green compacts were heated to 1150?C without hold,cooled to 1000?C,held at 1000?C for 40 h,and then cooled down to room temperature.We obtained Al2O3 nanocrystalline ceramic with an average grain size of the 34 nm.The relative density of the sintered bodies is99.8%.The disperse fine equiaxedα-Al2O3 nanoparticles with an average particle size of 6.2 nm and a narrow size distribution of 2-14 nm were sintered by ultra-high pressure and low sintering temperature method.The Al2O3 nanocrystalline ceramic was obtained at 20 GPa and 600?C.The obtained Al2O3 nanocrystalline ceramic has an average grain size of 9.5 nm and a size distribution of 4-30 nm.The relative density of the sintered bodies is above 95%.The value of microhardness is found to be 23.3 GPa.Commercialγ-Al2O3 particles were directly ball-milled and subsequently corroded by HCl at room temperature.γ-Al2O3 nanoparticles obtained are disperse,equiaxed,fine with an average particle size of 6.2 nm.The disperseγ-Al2O3nanoparticles are highly pure（99.994%mass percent）.After fractionated coagulation separations,disperseγ-Al2O3 nanoparticles with average particle sizes（and narrow size distributions）of 4.8（2-9）,6.6（2-12）,and 7.7（2-15）nm were obtained.Furthermore,using commercialγ-Al2O3 powders as starting material,disperse and equiaxedα-Al2O3 nanoparticles with an average particle size（and narrow size distribution）of 7.6 nm and a purity of 95.98%（mass percent）were prepared. |
