Study On Solidification And Kinetics Of BaTiO₃and BaTi2O₅Ceramics Using Aerodynamic Levitation

Containerless method are widely used in the study of metal and metal alloysolidification mechanism and solidification dynamics. However, the studies on thesolidification mechanism of ceramic materials in the solidification conditions withoutcontainer were rarely reported. As one of the many techniques which canrealizecontainerless condition, aerodynamic levitation is now an important process forthe preparation of new ceramic materials because of its technical characteristics, but itsapplication is limited only to this aspect. As for ceramic materials solidified undercontainerless condition, aspects such as nucleation and growth of the thermodynamicand kinetic characteristics, and structure formation are rarely involved in literature.Solidification under containerless condition is a process of fast nucleation andgrowth in the condition of deep undercooling. Generally melted ceramic phase is ofquite strictly ordered lattice structure, which is very much different from metal solidsolution. Ceramic phase has relatively large entropies of melting when compared withmetal solid solution. Under equilibrium condition, the solid to liquid phase interface issmooth in atomic scale. However, under deep undercooling state which is far away fromthe equilibrium state, the role of kinetic factors in the process of rapid growth will playa more important role. Therefore, studies on the mechanism of rapid solidification indeep undercooling for ceramics can be of benefit to the preparation of ceramic materialswith unique structure and exotic properties. Taking the advantage of aerodynamiclevitation technique, kinetics of rapid solidification process for ceramic materials canbe investigated in a slow cooling process, establishingessentials for the control of rapidsolidification morphology and performance-optimized design of ceramic materials.In this paper, the investigated objects were BaTiO3, BaTi2O5and La4Ti9O24functional oxide ceramics, which are useful for ferroelectric and dielectric materials.With an aerodynamic levitation system using laser heating, by controlling the coolingrate of ceramic materials in molten state under containerless condition, the influencesof solidification cooling rate on their formation and structures were investigated, andCCT curve was built. Experimental results show that under continuous cooling, solid to liquid phase interface of BaTiO3changed into a rough interface, which moved towardsupercooled liquid in the form of dendrites, and that the solid to liquid interfaces ofBaTi2O5and La4Ti9O24were unchanged as smooth interfaces, which pushed towardsupercooled liquid in the form of facets.By controlling the degree of undercooling of the ceramic materials in a moltenstate under the containerless condition, the influences of solidification undercoolingrate on the structure formation of the ceramics were revealed, and TTT curve was built.Experimental results show that under isothermal solidification, since the undercoolingrates of BaTiO3are larger than that of BaTi2O5, the solid to liquid interface of BaTiO3changed into a rough interface, pushing into to supercooled liquid with dendrites.Meanwhile, the smooth solid to liquid interfaces of BaTi2O5and La4Ti9O24wereunchanged due to small undercooling rates, which moved toward supercooled liquid inthe form of facets.