Containerless solidification of oxide material using an electrostatic levitation furnace in microgravity

Containerless solidification of BiFeO3 has been carried out in microgravity with an electrostatic levitation furnace (ELF) on board a sounding rocket (TR-IA). This was the first time the ELF was used in microgravity to study the solidification behavior of oxide insulator material. A spherical BiFeO3 specimen with a diameter of 5 mm was laser heated and solidified in an oxygen and nitrogen mixture atmosphere. The microstructure resulting from solidification in the ELF was compared with that obtained from solidification in a 10 m drop tube and in crucibles. In the crucible experiments, the segregation of the primary Fe2O3 phase could not be suppressed, even if the cooling speed increased to 5000 K/s. However it did suppress in a 0.3 mm diameter droplet solidified in the drop tube experiment. This suggests that containerless processing effectively promoted the undercooling of the BiFeO3 phase. In the microgravity experiment, although a homogeneous BiFeO3 phase was not observed in the 5 mm spherical specimen, an anomalous fine cellular microstructure appeared due to high undercooling. In addition, the phase transitions of BiFeO3 were measured by DTA from room temperature to 1523 K and its liquidus temperature was estimated to be 1423 K. (C) 2001 Elsevier Science B.V. All rights reserved.