| High-purity alumina particle,which is extra-fine?-Al2O3 powders with a purity more than 99.99%and uniform particle size,is widely used in high-tech fields such as growth of sapphire single crystals,preparation of structural ceramics and functional ceramics due to its excellent properties.At present,conventional methods for preparing high-purity alumina particles have some disadvantages such as high calcining temperature,complicated process,poor purity,and high cost.In this paper,high-purity?-Al2O3 particles were synthesized from anodic aluminum oxidation waste solution via recrystallization and microwave calcination with seeds addition.The effects of microwave and seeds,which can reduce sintering temperature and improve powder properties,contribute to preparing high-purity?-Al2O3 particles in low cost.Besides,sintering properties of?-Al2O3 powders have also been studied.Firstly,the high-purity NH4Al?SO4?2·12H2O crystals were prepared by the recrystallization method using anodic aluminum oxidation waste solution and NH3·H2O as raw materials.Effects of pH value and number of recrystallizations on the crystallinity and purity of the precursor were investigated by XRD,SEM,ICP-AES.The results show that the obtained crystals are all NH4Al?SO4?2·12H2O when pH value is between 1.5 and 4.5.When pH reaches 3.5,the yield of precursors achieves the highest,which is 96.08%.The purity of NH4Al?SO4?2·12H2O crystal,which increase with the increasing number of recrystallization,can reach 99.9991%after two recrystallizations.Then,?-Al2O3 particles can be obtained via calcining the above-mentioned high purity NH4Al?SO4?2·12H2O.The effects of different calcination temperatures and different seed contents on Al2O3 powders were studied by XRD,SEM,BET and laser particle size analysis.The results show that the products are completely transformed into?-Al2O3 after calcined at 1200? for 2 h,which have a particle size of about 200nm and a purity of 99.997%.However,the agglomeration is obvious.The nanometer?-Al2O3 seed crystals are added to precursors by high-energy ball milling.When the seed crystal content is 5wt%and calcination is performed at 1100? for 2 h,?-Al2O3powders with an average size of 150 nm,uniform particle size,and good dispersibility can be prepared.The introduction of seed crystals increases nucleation density and reduces nucleation barrier,so that the calcination temperature can be reduced by 100?,particle size is finer and dispersibility is better.The specific surface area is from the original 7.55 m2/g to 53.39 m2/g.Moreover,?-Al2O3 powders were prepared via microwave heating.When precursors are calcined at 1150? for 30 min,?-Al2O3 powders can be prepared.The crystallites are more uniform and fine,the particle size is about 140 nm,and the dispersibility is good.In the study of the synergistic effects of microwave and seeds,?-Al2O3 powders,which have a particle size of about 100 nm and a specific surface area of 108.53 m2/g,can be obtained at 1050? for 30 min by microwave calcining precursors with seed content of 5 wt%at a heating rate of 20?/min.The synergistic effects of microwave and seeds,which can lower the phase transition temperature of?-Al2O3 and reduce the particle size of products,as well as reducing agglomeration,are stronger than any single factor.Compared with the conventional calcination,the temperature is reduced by 150?,the calcination time is shortened,the energy consumption is reduced,and the product performance is excellent.Finally,sintering activity of the above high-purity?-Al2O3 powders was studied.Four different raw materials,including?-Al2O3 prepared by microwave heating precursors with 5 wt%seed content at 1050?,?-Al2O3 prepared by conventional calcination at 1200?,nano-Al2O3,and analytically pure Al2O3,were introduced for comparison.With a small amount of auxiliaries,alumina ceramics were obtained by sintering at high temperatures to investigate the sintering properties of different powders.The results show that the sintering activity of?-Al2O3 prepared by microwave calcination at 1050? with 5 wt%seed crystals is higher than others.The density of the sintered products is 3.79 g/cm3,and the bending strength is 308 MPa at1600? for 2 h,and the sintered products have a uniform microstructure. |
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