Preparation And Characterizations Of Monodisperse α-Al₂O₃ Nanoparticles

Two strategies forα-Al₂O₃ nanopowder preparation are presented in this dissertation. One is to develop a new method with low cost and high yield to prepare nanosizedα-Al₂O₃ powders. And the other one is to realize artificial control ofα-Al₂O₃ nanoparticle size and shape during the preparation process. A simple chemical precipitation method was explored to prepare monodiperseα-Al₂O₃ nanoparticles. The cheap aluminum nitrate and ammonia were used as raw materials to prepared alumina precursors, aluminum hydroxides, by chemical precipitation. Thermal behavior, grain growth, andα-Al₂O₃ productivity of aluminum hydroxides with different crystal structures during phase transformation process were analyzed. During calcinations, the phase transformation of precursors withα-Al₂O₃ seeding and grinding treatment and the nucleation and grain growth ofα-Al₂O₃ were investigated. The particle size and agglomeration degree of calcinatedα-Al₂O₃ nanopowder were analyzed. Then, a novel calcinations method, namely the isolating phase assistant calcination, was developed to prepare monodisperseα-Al₂O₃ nanoparticles. That is, by introducing a salt as isolating phase to isolate alumina precursor particles to avoid the contacts between the alumina precursor particles, between the transition alumina particles as well as between theα-Al₂O₃ particles to eliminate the particle growth and agglomeration of theα-Al₂O₃ particles during calcinations.It was found that addition ofα-Al₂O₃ seeds in alumina precursors could supply the heterogeneous nucleation sites, reduce the activation energy of heterogeneous nucleation ofα-Al₂O₃ grains in transition Al₂O₃ matrix, and accelerate the formation of theα-Al₂O₃ crystal phase at relative low temperatures to obtainα-Al₂O₃ nanopowder with low-degree agglomeration.Soft grinding treatment of precursors refines the precursor particles, results in the lattice distortion of the initial particles, and leads to a strained framework of disordered Al-atom sublattice to easily rearrange into the stable framework ofα-Al₂O₃ phase. Meanwhile, the energy introduced by soft grinding might lead to the removal of a part of active hydroxyl but is not enough to bond the hydroxyl adsorbed on the surface of particles; thus no serious agglomeration due to the formation of hydrogen bond between alumina precursor particles was formed. Due toα-Al₂O₃ seeding and soft-grinding treatment, bayerite undergoes a series of phase transformations via theα-Al(OH)₃→γ-AlOOH→γ-Al₂O₃→α-Al₂O₃ path without the formation of theθ-Al₂O₃ transition phase and transforms intoα-Al₂O₃ crystal phase. The onset and completion temperatures of the transformation toα-Al₂O₃ in the ground bayerite are about 800℃and 150℃lower than that in the unground bayerite, respectively. The obtainedα-Al₂O₃ nanoparticles with an average diameter of 40 nm are relatively disperse.Due toα-Al₂O₃ seed addition and soft-grinding treatment, during calcinations of fine gibbsiteγ-Al(OH)₃, a small part ofγ-Al(OH)₃ transformed intoα-Al₂O₃ at 300℃viaγ-Al(OH)₃→χ-Al₂O₃→α-Al₂O₃ phase transition sequence, and mostγ-Al(OH)₃ transformed intoα-Al₂O₃ viaγ-Al(OH)₃→χ-Al₂O₃→κ-Al₂O₃→α-Al₂O₃ phase transition path. The similar structures ofα-Al₂O₃,κ-Al₂O₃, andχ-Al₂O₃ (hexagonal closed-packing of oxygen atoms) lead to more easy heterogeneous nucleation; disperseα-Al₂O₃ nanoparticles with an average diameter of 20 nm were achieved by calcining ground gibbsite at 750℃for 10 days.The isolating phase assistant calcination method was employed to preapre monodieperseα-Al₂O₃ nanoparticles. In this novel method, the inorganic salt (NaCl) serving as isolating phase was introduced in preparation process of precursor,α-Al(OH)₃. The content of isolating phase influences directly the particle size, shape, and agglomeration of theα-Al₂O₃ nanoparticles. When a small quantity of isolating phase was used, the Al₂O₃ nanoparticles could not be isolated completely; and the sintering, agglomeration, and grain growth ofα-Al₂O₃ nanoparticles occurred. With increasing content of the isolating phase, the sintering, agglomeration, and grain growth ofα-Al₂O₃ nanoparticles obviously reduce, but the completation temperature of phase transformation ofα-Al₂O₃ increases. Meanwhile, during calcinations at high temperatures, the solid phase-liquid phase segregation occurred in the system composed of a liquid isolating phase (NaCl) and a solid phase of the Al₂O₃ nanoparticles. The solid phase-liquid phase segregation occurring during high temperature calcinations results in two sorts ofα-Al₂O₃ nanoparticles. One sort ofα-Al₂O₃ nanoparticles formed in the region of relatively higher isolating phase content and were small single-crystal nanoparticles of about 10 nm in size. The other sort ofα-Al₂O₃ nanoparticles formed in the region of relatively lower isolating phase content and were large polycrystal nanoparticles of 20-30 nm in size. For preparation of monodisperseα-Al₂O₃ nanoparticles, the isolating phase content has a critial value. When the molar ratio of salt to aluminum nitrate is about 20:1, the isolating phase can isolate the Al₂O₃ nanoparticles well to avoid the sintering, agglomeration, and grain growth of Al₂O₃ nanoparticles during calcinations. Monodisperse and sphericalα-Al₂O₃ nanoparticles with a narrow size distribution and an average diameter of 10 nm were obtained by a 1000℃calcination.