Study On The Microstructure Evolution Of α-Al₂O₃ In The Formation Process And It's Controlling

a-Al2O3 is an important inorganic non-metallic material. The energy consumption is huge in the preparation process of a-Al2O by sintering. Reduction of the calcination temperature and energy cost is the main target inα-Al2O3 industry. As the properties ofα-Al2O3 are mainly depending on its microstructure characteristics, it is of great theoretical and practical significance to study on the microstructure evolution during the phase transformation ofα-Al2O3.The phase transformation of aluminum hydroxide and the microstructure evolution of aluminum oxide have been studied using XRD and SEM measurements. The influence of additives on the phase transformation has been studied using XRD quantitative analysis. The crystal growth kinetics of a-Al2O3 has been studied using laser particle size analysis and SEM technique. The method of microstructure control of a-Al2O3 has been proposed based on above studies. As a result of that, plate and near-spherical a-Al2O3 have been successfully obtained. Sinterable a-Al2O3 has been made by adjusting the microstructure ofα-Al2O3. The research results are mainly as follows:1. The dehydration process from aluminum hydroxide to aluminum oxide proceeds in steps. The smaller the particle size of the sample, the faster the dehydration process. The phase transformation between different transition alumina belongs to displacement, while the formation of a-Al2O3 from transition alumina belongs to reconstruction. There is no obvious change of microscope between transition alumina. But apparent shrinkage appears whenα-Al2O3 was formed. In the calcination process, the real density of alumina rises and the bulk density of alumina decreases with the increase of temperature. The specific surface area of alumina reaches its maximum at about 400℃, the aperture of alumina reaches the maximum at about 900℃.2. The phase transformation from transition alumina toα-Al2O3 completes above 1300℃. Na2O and MgO can depress the phase transformation, while the addition of additives containing F-, and Cl-grinding and the addition of crystal seeds can promote the phase transformation process. Due to heat conduction, there is a phase interface between transition alumina andα-Al2O3.The percent of a-Al2O3 rises with the increase of the calcination temperature. There is a maximum rate of phase transformation at Tm when the percent of a-Al2O3 is about 20-45%.3. Grinding combined with laser particle analysis is a suitable way to determine the crystal size ofα-Al2O3. a-Al2O3with worm-like structure was formed by solid-phase mass transfer. a-Al2O3 with a specific microstructure was formed by gas-phase mass transfer. Additives could influence the crystal growth of a-Al2O3 in the following order:NH4F> AIF3> H3BO3> MgF2> None> MgOAnd additives could change the nucleation temperature ofα-Al2O3 in the order as follows:NH4F> AIF3> None>H3BO3>MgONanocrystallineα-Al2O3 of about 20nm has been successfully made by grinding and the addition of such additives as NH4Cl and Mg(HCO3)2 because NH4Cl can restrict the process of phase transformation and Mg(HCO3)2 can depress the crystal growth ofα-Al2O34.The morphology of a-Al2O3 particles is similar to that of aluminum hydroxide. The crystallization habits of a-Al2O3 do not change with the increase of calcination temperature. Na2O and MgO can inhibit the crystal growth of a-Al2O3. H3BO3, and additives containning F-, Cl- and NO3" can accelerate the crystal growth ofα-Al2O3. H3BO3 can promote the formation of worm-like a-Al2O3 crystal, while additives containing F-can promote the formation of flake a-Al2O3 crystal. The influence of additives to the microstructure ofα-Al2O3 is in the order as follows:AlF3>H3BO3>NH4Cl>MgO5. a-Al2O3 with different microstructure can be made by the addition of different additives.α-Al2O3 with specific microstructure such as spherical, columnar, large flake crystal have been successfully obtained by the addition of seed crystal or additives. By selecting the appropriate additives and sintering temperature, the original crystal less than 0.5 microns has been acquired with the addition of additive 0.4%MgO+0.3%NH4Cl at 1350℃. The sintering technology for the preparation of lower temperature alumina ceramic with the bulk density of 3.92g/cm3 has been developed at the optimum sintering temperature of 1620℃.