Study On Preparation Of Ultrafine α-Al₂O₃ By Catalytic Sintering

Ultrafineα-Al₂O₃, with extensively excellent properties such as high hardness,high heat-resistance, high corrosion-resistance, high oxidation-resistance and goodelectrical insulation, etc, is a very important branch of ultrafine powder materials andwidely applied in fields of ceramics, metallurgy, electron, biomedicine and optics, etc.In this paper, the effects ofα-Al₂O₃ seeds and four kinds of catalysts on reducing thephase transformation ofα-Al₂O₃ in sintering process were investigated respectivelyon the basis of many exploratory experiments. The multiplex catalyst was synthesizedand the phase transformation temperature ofα-Al₂O₃ was reduced to 800℃firstly.Simultaneously, the mechanisms of different additives and performance ofα-Al₂O₃powder particles were also contrastively investigated. The results are as follows.The precursor of ammonium aluminum carbonate hydroxide (AACH) wassynthesized with precipitation method by using aluminum sulfate and ammoniumcarbonate as raw materials. According to Debye Scherrer Method, the grain size ofAACH particles is about 18.9nm. The phase transformation sequence of AACH insintering process is: AACH→AlOOH→amorphous Al₂O₃→γ-Al₂O₃→θ-Al₂O₃→α-Al₂O₃. Sintered at 1200℃for 2h, the phase transformation ofα-Al₂O₃ finishedcompletely but particles obtained have a broad size distribution with grain size ofabout 200-500nm. Simultaneously, high density of neckings and vermicularmorphology rigid agglomeration are formed between particles.The optimal amounts ofα-Al₂O₃ seeds and catalyst B, C, D, E added are 2%, 6%,6%, 5%, 3% (mass fraction, measured basing on the theoretical value ofα-Al₂O₃product) and the extents of transformation temperature ofα-Al₂O₃ reduced by theseadditives are 50℃, 40℃, 100℃, 100℃and 90℃respectively. Furthermore, the bestmatching ratio of multiplex catalyst is obtained by orthogonal experiments and theresults showed that the ratio ofα-Al₂O₃ seeds and catalyst B, C, E are 1.5%, 1.5%,1.0% and 0.4% respectively. Sintered at 800℃for 2h with addition of multiplexcatalyst, the phase transformation ofα-Al₂O₃ finished completely and the powderparticles obtained have homogeneous figuration and size distribution with gooddispersion of about 80-100nm.The results of phase transformation activation energy analysis indicate that theactivation energy ofα-Al₂O₃ reduced byα-Al₂O₃ seeds and multiplex catalyst are 64.9kJ/mol and 281.9kJ/mol respectively. These give the best explainationtheoretically that the multiplex catalyst has great potential synergistic effects on thephase transformation of alumina and largely reduces the phase transformationtemperature ofα-Al₂O₃.Ceramic samples were prepared after compressing moulding and sintering byusing the variousα-Al₂O₃ powder particles as raw materials. The analysis resultsindicate that the relative density, and tensile strength of ceramics gradually increase astemperature rising. Sintered at 1450℃, the relative density and tensile strength ofsamples are over 95% and 200MPa respectively. Simultaneously, the results alsoshow that the sample produced withα-Al₂O₃ powder gained at 800℃has largerrelative density and tensile strength than that of all the other samples in the sameconditions, which indicate thatα-Al₂O₃ powder particles gained at lower temperaturehave better sintering characteristics and combination properties than commonα-Al₂O₃powders. This has great potential synergistic effects on the application and promotionof ultrafineα-Al₂O₃ in fields of ceramics, metallurgy, electron, etc.