Phase Transformation And Morphology Evolution Of Different Industrial Alumina Powders After Firing At Different Temperatures

Alumina has been used in a wide variety of refractory materials because of its high strength,wear resistance,and corrosion resistance.As industrial alumina powder is used to produce calcined alumina micropowder and sintered tabular corundum,it is important to investigate morphology evolution of the alumina powder in a wide range of temperature.In this paper,XRD and SEM were used to study the phase transformation and morphology evolution of different industrial alumina powder and different industrial alumina powder with presence of different additives after firing at different temperatures.The research results are mainly as follows:The phase transformation and morphological evolution of different industrial alumina powder after firing at different temperatures are basically consisitent.The phase and morphology of industrial alumina powder after firing at 700-900°C do not change noticeably.The ?-Al₂O₃ and ?-Al₂O₃ have been completely transformed to ?-Al₂O₃ and ?-Al₂O₃,and small grains began to appear on the surface of particles,and the boundaries between the grains are observable after firing at 1000 ° C.The most amounts of ?-Al₂O₃ have been transformed to ?-Al₂O₃,and small grains are separated from each other to form gaps after firing at 1100 °C.All the transitional alumina has been completely transformed to ?-Al₂O₃,and some grains are coalesced together because of the grains growing up after firing at 1200°C.The above results indicate that the morphology evolution of alumina may be associated with the phase transition after firing at 700-1200°C.Small grains of alumina gradually grow up with the temperature increasing,but some grains separated from each other and the gaps between the small grains gradually increased.The grains become spherical and elliptical,and continue to grow at 1400°C.Spherical and elliptical grains continue to grow,and they are coalesced together again at 1600°C.After firing at 1700 °C and 1800 °C,the disks of different industrial alumina powder are agglomerated by a lot of spherical,elliptical and plate-like grains,and there are many pores between the grains.Compared to the alumina fired at 1600 °C,the grain size does not change,indicating that the grains do not continue to grow when the calcination temperature increasesto 1700 °C and 1800 °C.When 2.0% content of NH₄F is present,the industrial alumina with higher ?-Al₂O₃ completely transform to ?-Al₂O₃ after firing at 900°C,but the industrial alumina with lower?-Al₂O₃ completely transform to ?-Al₂O₃ after firing at 1000°C,so the effect of NH₄F on the promotion of two different industrial alumina transformed to ?-Al₂O₃ is different.The microstructure of different industrial alumina with 2.0% NH₄F content is obviously different after firing at 1200°C,the grains of industrial alumina with higher ?-Al₂O₃ become circular plate-like,but the grains of industrial alumina with lower ?-Al₂O₃ become regular hexagonal plate-like.With the calcination temperature rising,the plate-like grains of industrial alumina gradually disappear.When 2.0% content of NH₄F is present,the industrial alumina with higher ?-Al₂O₃ is composed of a large number of granular ?-Al₂O₃ grains and a small part of the plate-like ?-Al₂O₃ grains at 1400 ° C,but the industrial alumina with lower ?-Al₂O₃ is mainly composed of granular ?-Al₂O₃ grains,and the plate-like ?-Al₂O₃ grains have completely disappeared.All the transition phases of industrial alumina with presence of H₃BO₃ have been completely transformed to ?-Al₂O₃ at 1200 ° C.The effect of H₃BO₃ on the promotion of industrial alumina transformed to ?-Al₂O₃ is weak at 900° C,but H₃BO₃ has an inhibitory effect on the phase transformation of industrial alumina at 1000°C and 1100°C.The grains of industrial alumina with 2.0% H₃BO₃ content transformed to ?-Al₂O₃ become worm-like.When 0.8% content of H₃BO₃ is present,the industrial alumina with higher ?-Al₂O₃ is composed of relatively scattered spherical ?-Al₂O₃ grains at 1200 ° C,but the industrial alumina with lower ?-Al₂O₃ is composed of relatively scattered spherical ?-Al₂O₃ grains at1300°C.The ?-Al₂O₃ grains of industrial alumina become worm-like,when the calcination temperature continues to increase.