The microstructure of the damaged area created during a flash sintering event in BaTiO 3 was examined by high resolution transmission electron microscopy (HRTEM) and electron energy loss spectrometry (EELS) measurement. A DC electric field of 133 V/cm was applied to a fully-densified BaTiO 3 body prepared by conventional sintering, and the specimen temperature was elevated at a constant heating rate. The flash sintering event, at which the electric current in the specimen abruptly increases at the threshold field and temperature, took place at 890°C. After the flash event, tunnellike physical damage was observed in the direction of the field through the specimen. Formation of grain boundary second phase layers around the damaged area was confirmed by HRTEM observations. The grain boundary second layer was a crystalline phase with a low Ba/Ti atomic ratio of less than one. The temperature in some of the grain boundaries must increase by Joule heating during the flash event, thus the chemical composition in the vicinity of the grain boundaries changed due to vaporization of the Ba cations, resulting in the formation of the grain boundary second phase layers.

DC electric fields ranged from 25 to 350 V/cm enhanced the sintering rates of BaTiO 3 . At the field of 25 V/cm, the sintering densification occurred by field-assisted sintering (FAST), and flash-sintering preceded by FAST was observed at the field strength of more than 75 V/cm. Most of the densification, however, was produced by the FAST process. At high field strength of more than 250 V/cm, the sintering densification was not finished owing to discharging during the flash-sintering. By means of FAST process at 100 V/cm and the current limit of 100 mA, in contrast, relative density of 97% and a fine grain size about 500 nm were achieved at 1020°C in the present material. The FAST process, where the electric current was set to the transition current between FAST and flash-sintering, is highly effective to fabricate uniform and fine grained BaTiO 3 polycrystal.

In BaTiO3, flash‐sintering associated with a surge of the specimen electric current sometimes results in an inhomogeneous microstructure including Ti‐excess secondary phases because of discharging. We applied field‐assisted sintering technique (FAST) under precisely controlled specimen current that was set just below the threshold value for the occurrence of flash event for BaTiO3, to avoid the occurrence of the discharging. As a result, uniform and fine‐grained compacts were obtained without any secondary phases. A relative density of approximately 92% was achieved under FAST condition of 100 V/cm with a limiting current of 72 mA and soaking time of 3 hours at 1070°C. The voltages during sintering under a constant current of 72 mA were found to decrease during the soaking process. Electron energy loss spectroscopy revealed the generation of excess oxygen vacancies at/near grain boundaries. The excess oxygen vacancies induced by application of DC electric fields were confirmed to reduce the voltages and to retard the shrinkage rate in a final sintering stage.