Study On Morphology Evolution And Reaction Mechanism Of α-Al₂O₃ Synthesized From Industrial Precursors

α-Al₂O₃ is an important inorganic nonmetallic material,and the morphology ofα-Al₂O₃ micro-powder is closely related to the application field.With the introduction of milling treatment and additives,α-Al₂O₃ powders with different morphology were prepared by using three different industrial precursors as raw materials.The morphology evolution and mechanism of alumina with the introduction of AlF₃additive during the calcination process were further studied.The study results are mainly as follows:（1）The phase transformation and morphology ofα-Al₂O₃ powders are closely related to milling treatment of the industrial Al（OH）₃ precursor and the introduction of additives.Mil ing activation of the industrial Al（OH）₃ precursor led to the acceleration ofα-transformation during calcination,and single-phaseα-Al₂O₃ was synthesized from a 3h milled precursor at 1150℃.In contrast,the precursor without mil ing was partially converted intoα-Al₂O₃ at 1150℃,co-existing with transitional phases such asκ-Al₂O₃,δ-Al₂O₃ andθ-Al₂O₃.By the introduction of precursor milling treatment,the neck growth ofα-Al₂O₃ nuclei formed from the milled precursor has been efficiently suppressed by the morphological changes of the Al（OH）₃ precursor.As the gibbsite agglomerates in the size of 100μm were broken into irregularly shaped particles in a≤1μm size range by milling for 9 h,almost-sphericalα-Al₂O₃ powders of⁰.5μm in size were synthesized at 1450℃.Furthermore,thin or thick platelet-like,hexagonal,cake-likeα-Al₂O₃ powders were synthesized from milled precursors with the introduction of different additives,such as AlF₃,NH₄BF₄,NH₄F,NH₄Cl and their compounds.（2）Mil ing treatment with different rotation modes has great influence on the morphology of Al（OH）₃ precursors,phase transformation temperature and morphology ofα-Al₂O₃ powders synthesized by calcination.In comparison with precursor A（bi-directional rotation）obtained by 9 h milling treatment of industrial Al（OH）₃,precursor B（unidirectional rotation）presented a honeycomb microstrucure;meanwhile,the specific surface area of the precursor B was much larger than that of the precursor A.Theα-Al₂O₃ powders formed by calcination of precursor B with 1 wt.%AlF₃ additive at 910℃ was mainly a polyhedralα-Al₂O₃ with a grain size of about 100nm,and the morphology and grain size ofα-Al₂O₃ were basically unchanged as the calcination temperature increased into 940℃.During the calcination of precursor A with the addition of 1 wt.%AlF₃,the obtained Al₂O₃ powders firstly inherited the precursor morphology at the temperature below 900℃,and then the shape of Al₂O₃ powders were obviously changed at the temperature above 900℃,resulting in an octahedral morphology with the exposure of the{101}crystal planes.At the same time,it was also found that the chemically combined water（O-H）in the lattice was reduced rapidly by infrared spectrum.With the increase of calcination time,the{100}crystal planes were exposed gradually,and the morphology ofα-Al₂O₃ eventually translated into plate-like shape as the calcination temperature increased into 1300℃.（3）The morphology and particle size of theα-Al₂O₃ powder prepared by the industrial alumina powder are significantly affected by the precursor pretreatment,introduction of additives,additive content and calcination schedule.Square shapedα-Al₂O₃ was synthesized at 1300℃ for 2 h by using 5 h milled commerciallyγ-Al₂O₃precursor with the introduction of 1 wt.%NH₄F the mixture was calcined.As calcination time increased to 5 h,the edge contour of theα-Al₂O₃ grains was more rounded and ball-likeα-Al₂O₃ appeared.By introducing higher content of NH₄F（20 wt.%）,the morphology ofα-Al₂O₃ was changed from irregular to regular round cakes with the particle size of¹μm when the calcination time prolonged from 2 h to5 h at 1300℃;while the morphology ofα-Al₂O₃ was changed from irregular block to regular square-like and round cake-like in 200⁵00 nm size range during the same calcination process with the addition of 1 wt.%NH₄F and 1 wt.%NH₄Cl composite additives.As the calcination temperature increased into 1400℃ and maintained for2 h,the formedα-Al₂O₃ presented cake-like morphology in a size range of 1²μm,accompanied by a small amount of 4μm.