Study On The Dispersion And Morphology Control Of Ultrafine Alumina Preparation

Ultrafine alumina, with extensively excellent properties such as high melting point, high heat-resistance, high corrosion-resistance, good electrical insulation and catalytic abilities, etc, is a very important branch of ultrafine powder materials and widely applied in fields of ceramics, metallurgy, electron and catalysts. Three methods were used for preparing ultrafine alumina powders in this paper, the effects of methods and the conditions on dispersion and morphology of ultrafine alumina were also discussed. Main research conclusions are as follows:(1) Ammonium aluminum carbonate hydroxide was synthesized as the precursor by the precipitation reaction of aluminum sulfate and ammonium carbonate, and the effect factors of reaction on preparation of AACH were discussed, The research demonstrated that the better conditions for synthesizing AACH were as follows: reaction temperature is 60℃, reaction time is 20min, milling speed is 1000r/min, aluminum sulfate atomizing into ammonium carbonate, adding 5% PEG2000 as dispersant, controlling the pH value about 10, and quick temperature downing. AACH with uniform particle size well-distributed and good dispersion was obtained under the better conditions.(2) The effects of drying conditions on the dispersion of precursor were studied in this paper, and according to the studies done before, the possible mechanism of the formation of agglomerates in ordinary drying was offered for the first time. Based on the studies, atomizing freeze drying, a new drying method for preventing agglomeration is put forward.(3) Under the combination of optimized conditions and atomizing freeze drying, the ultrafineα-Al₂O₃ with uniform particle size and in spherical shape and well dispersed was obtained by AACH calcined at 1200℃for 2h, which is the ideally raw material for the preparation of ceramic.(4) The ultrafine alumina powder was prepared by homogeneous precipitation method using aluminum salt and urea as raw material. The effect factors of reaction, such as the ratio of reactant, initial concentrations of Al³⁺, reaction temperature, heat-preservation time, drying condition and additives were comprehensive analyzed in this paper, and the role of anions on the preparation of ultrafine alumina were firstly studied. The results showed that univalent counterions resulted in an amorphous gel which was agglomerates having elongated fibrillar morphology, and bivalent counterion reactions resulted in the direct formation of amorphous powders which were agglomerates of spherical particles. Based on the studies, anions doped are creatively suggested to synthesize ultrafine alumina, which showed that spherical precursor with good dispersion can be synthesized by doping 30% SO42" into aluminum nitrate.(5) During the investigation process of additive, we have firstly discovered that the size distribution and the shape were evidently affected by adding alcohol; the reaction speed and the yield and the dispersion were great improved by adding glycol.(6) Low temperature heat treatment and high energy ball milling were firstly combined to prepare ultrafine alumina powder, and the effects of materials, low temperature heat treatment, and the milling parameters on the physical and chemical characteristics of alumina powder were studied in this paper. The results showed that low temperature heat treatment can effectively prevent the powders from forming aggregation. Under the low temperature heat treatment, and using ethanol as milling medium, processing with a rotation speed of 300r/min and a balls/powder weight ratio of 10/1, ultrafine flakeα-Al₂O₃ with non-aggregation characteristics and an average particles size of about 2μm and piece thickness about 0.3μm can be prepared after milling for 4 hours using aluminum sulfate as raw material, which is the ideally raw material for the preparation of Aluminates phosphor.