Flash sintering (FS) is a novel sintering method that can sinter ceramics in seconds by directly passing a current through the sample. The resulting Joule heating densifies ceramics with an extremely high heating rate. However, a non-uniform heat distribution limits the use of FS, particularly for large components. The resistance of most ceramics decreases with increasing temperature. When the current passes through the samples, it usually forms a preferential path in the middle. At the same time, the surface is losing heat to the atmosphere. The conductivity inside becomes higher, and the heat generated is mainly in the core region. A few attempts have been made to solve this problem, such as thermal insulation, novel electrode configurations, etc. but all these methods are external solutions. We solved this problem by material system design, and the hardware needs were simplified.
The method involves adding suitable dopants to 3YSZ. The additives reacted with 3YSZ during the sintering process. The regions with higher temperatures react first. The electrical conductivities of these parts are decreased by dopants. This means that current and heating are directed away from regions that have already started to sinter. This made FS more uniform no matter the shape of the specimen or electrode configuration. The relative density of 3YSZ under specific conditions increased from 92.4% to 99.9%. The grain size difference between the corners and core parts decreased a lot as well. Besides, although the conductivity decreased a lot during FS, the heating rate was similar. The advantage of rapid heating was retained. The tetragonal structure of 3YSZ was maintained to room temperature. Comparing the conventional sintered doped 3YSZ and FSed samples, the grain size of FSed samples was much smaller. Uniform flash sintering with high relative density and small grain size was achieved by internal material system design.