Flash sintering method is expected to be a new sintering method because it can complete sintering in shorter time and at lower temperature than time and temperature of conventional sintering method without electric field.
On the other hand, it has also been pointed out that during flash sintering, unique phenomena on sintered ceramics may appear, which are caused by applying electric field to compact.For example, it has been confirmed that flash phenomenon does not appear in pure Al₂O₃ even when high electric field is applied, whereas flash phenomenon appears when a small amount of MgO is added to Al₂O₃. It has also been reported that the grain growth rate that occurs during flash sintering increases by several orders of magnitude compared to the grain growth rate during sintering under no electric field. Such unique phenomena that appear by flash sintering suggest that changes in types of point defects are induced by electric field applying, but have not been discussed enough. Therefore, photoluminescence is focused on, which is sensitive to the type change of point defects and its concentration, and applied flash sintering to Ga₂O₃, the photoluminescence mechanism of which is already understood, and analyzed the photoluminescence property to investigate change of point defect types that may have occurred during flash sintering.
Ga₂O₃ powder was formed compact by uniaxial pressurization and hydrostatic pressing, and placed in a flash sintering furnace. The temperature of the compact was increased to 1300°C without electric field, and an electric field was applied when the temperature reached 1300°C for flash sintering. Alternating electric field of 25 to 50 V/cm and 1000 Hz was used as the applied electric field. To prevent excessive current flow during flash sintering, a current limit of 1500 mA was set on the power supply. After the flash event was confirmed, the flash sintering condition was kept for 5 minutes, then the electric field was removed and the sample was cooled in furnace. As a comparison sample, a sintered compact was also prepared by conventional sintering without an electric field. The photoluminescence properties of samples were measured using a photoluminescence spectrophotometer after cutting in the longitudinal direction and polishing the inner cross section of the specimens.
The flash event and rapid densification were observed under the electric field conditions used, and it was confirmed that flash sintering is possible for Ga₂O₃. The obtained flash sintered samples had a high relative density of over 96%. The photoluminescence property of each samples showed that fluorescence of the conventional sintered material in the ultraviolet region appeared at 375 nm, which is similar to the photoluminescence reported previously. In contrast, in the flash sintered samples, the photoluminescence intensity at 375 nm, which is peculiar to the conventional sintered sample, is reduced, while the photoluminescence intensity around 475 nm is significantly increased. This suggests that point defects responsible for certain photoluminescence were generated with the applying of an electric field during flash sintering.