Flash Sintering of Alumina-based Ceramics

Flash sintering is an electrical field-assisted consolidation technology and represents a very novel technique for producing ceramic materials, which allows to decrease sensibly both processing temperature and time. Starting from 2010, when flash sintering was discovered, different ceramic materials with a wide range of electrical properties have been successfully densified. Up to date, the research on flash sintering has been mainly focused on ionic and electronic conductors and on semiconductor ceramics. In the present work, we studied the flash sintering behavior of a resistive technical ceramic like alumina also in the presence of magnesia silicate glass phase typically used for activating liquid phase sintering. The materials were studied by using different combinations of electric field and current density. Physical, structural and microstructural properties of sintered samples were extensively investigated by Archimedes’ method, SEM, XRD, XPS and pholuminescence spectroscopy. Light emission and electrical behavior during the flash process were studied,as well. The results point out the applicability of flash sintering to alumina and glass-containing alumina using electrical-field in excess of 500 V/cm, allowing an almost complete densification at temperatures lower than 1000°C. Different densification mechanisms were pointed out in the two systems, namely “solid state flash sintering” and “liquid phase flash sintering” for pure alumina and glass containing alumina, respectively. The glass addition allows a significant reduction of the current and power dissipation needed for densification, by promoting liquid phase sintering. The results suggest that unconventional mass transport phenomena are activated by the current flow in the ceramic body and they can be very likely attributed to partial reduction of the oxide induced by the electrical current. The hypothesis that the oxide gets partially reduced during DC-flash sintering experiments is supported by several experimental findings. Finally, strong affinities between flash sintering and other physical processes, like dielectric breakdown, were pointed out.