Two-Step Flash Sintering of ZnO Ceramics at Room Temperature

The properties of ceramics are closely related to its grain size. For example, the smaller the grain size, the better the hardness and strength of ceramics. Inspired by the conventional two-step sintering, Luo et al. proposed two-step flash sintering. By reducing the current in the “second step” of flash sintering to inhibit the grain growth, ZnO ceramics with a grain size of about 370 nm were obtained, whose grains are far smaller than those obtained by conventional flash sintering. And we propose a different two-step flash sintering method: using high voltage AC power supply to apply a higher AC electric field to the specimens, we increased the voltage until breakdown occurred. Due to the local arc, a densified conductive channel was formed on the surface of the sample. When applying the electric field for the second time, there will be a larger current through the green, resulting in a large amount of Joule heat to make the whole green body densified rapidly. We used this method to sinter zinc oxide ceramics with a grain size of about 330 nm and a relative density of 93.3%. In addition, the ceramic grain size can be controlled by adjusting the output current during the second step. The larger output current will result in bigger grain size. More importantly, the whole two-step flash sintering process is carried out at room temperature without additional heating device, which can greatly simplify the sintering equipment and operation.