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

?-Al₂O₃ is an important inorganic nonmetallic material,and the morphology of?-Al₂O₃ micro-powder is closely related to the application field.With the introduction of milling treatment and additives,?-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 of?-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 of?-transformation during calcination,and single-phase?-Al₂O₃ was synthesized from a 3h milled precursor at 1150?.In contrast,the precursor without mil ing was partially converted into?-Al₂O₃ at 1150?,co-existing with transitional phases such as?-Al₂O₃,?-Al₂O₃ and?-Al₂O₃.By the introduction of precursor milling treatment,the neck growth of?-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 100?m were broken into irregularly shaped particles in a?1?m size range by milling for 9 h,almost-spherical?-Al₂O₃ powders of⁰.5?m in size were synthesized at 1450?.Furthermore,thin or thick platelet-like,hexagonal,cake-like?-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 of?-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.The?-Al₂O₃ powders formed by calcination of precursor B with 1 wt.%AlF₃ additive at 910? was mainly a polyhedral?-Al₂O₃ with a grain size of about 100nm,and the morphology and grain size of?-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 of?-Al₂O₃ eventually translated into plate-like shape as the calcination temperature increased into 1300?.?3?The morphology and particle size of the?-Al₂O₃ powder prepared by the industrial alumina powder are significantly affected by the precursor pretreatment,introduction of additives,additive content and calcination schedule.Square shaped?-Al₂O₃ was synthesized at 1300? for 2 h by using 5 h milled commercially?-Al₂O₃precursor with the introduction of 1 wt.%NH₄F the mixture was calcined.As calcination time increased to 5 h,the edge contour of the?-Al₂O₃ grains was more rounded and ball-like?-Al₂O₃ appeared.By introducing higher content of NH₄F?20 wt.%?,the morphology of?-Al₂O₃ was changed from irregular to regular round cakes with the particle size of¹?m when the calcination time prolonged from 2 h to5 h at 1300?;while the morphology of?-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 formed?-Al₂O₃ presented cake-like morphology in a size range of 1²?m,accompanied by a small amount of 4?m.