Raj’s research group found a very unique sintering technique called flash sintering in 2010. Since then, the flash sintering technique was applied for various oxide ceramics to give valuable knowledge in a research field of ceramic sintering. The characteristic point of the flash sintering is the occurrence of a specimen electric current spike called flash event. A sintering process completes immediately with the flash event and densified ceramics can be obtained at lower temperature than that by a conventional sintering technique, e.g., zirconia ceramics can be obtained around 850℃ for a few seconds. The rapid shrinkage during flash sintering can be considered to result from Joule heat given by the input electric power mostly. However, other interesting phenomenon is confirmed to occur under electric fields in addition with the acceleration of shrinkage given by Joule heat. One of them is the occurrence of severe reduction. The reducing effect becomes remarkable to form the area with dark bluish color from an electrode inside a compact by keeping a flash state. Nitrized zirconia can be obtained using the reducing effect even in air. Grain boundaries after flashed even by AC electric fields are reduced slightly, which was confirmed by STEM-EEL analysis. The inclusion of the excess oxygen vacancies retards a shrinkage rate at a final sintering process. The obtained density was revealed to reach higher density by switching off electric fields at a final sintering process in BaTiO3. In addition, the heterogeneity of electric current flow appears by increasing electric conductivity at local areas given by the excess free electrons due to reduction. A shrinkage rate is limited by discharges due to the heterogeneity of local current flow in BaTiO3. The reducing effect, ionic current flow, and so on result in the heterogeneity of microstructure of flashed sintered compacts. To obtain flashed compact with uniform microstructure, a control of electric fields DC/AC/Pulse must be necessary. Further, the furnace temperature have to be controlled carefully by considering a balance between flash event temperature and electric fields.
In this presentation, we would like to summarize our recent study performed for some techniques to obtain uniform microstructure in flash sintering.