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Fabrication of dense KNN‐based lead‐free piezoelectric ceramics at low temperatures in short time through a cost‐effective way remains a challenge. Herein, this challenge could be addressed by using reactive flash sintering. It is demonstrated that the phase transformation of KNbO3‐NaNbO3 into (K,Na)NbO3 and densification occur simultaneously during the flash event. Most importantly, ZrO2 doping can greatly decrease the onset flash temperature, which is ascribed to the increased conductivity of sample. In addition, the current limit has a significant effect on the phase transformation and densification. The flash‐sintered KNN ceramics exhibit the good ferroelectric and piezoelectric properties. Furthermore, the ZrO2 doped and undoped KNN ceramics show a comparable coercive field Ec, which may be related to the residual point defects after the flash. Besides the Joule heating, the avalanche generation of point defects is suggested to be responsible for the ultrafast solid‐state reaction and densification rates.

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