Synthesis of KNN powders: Scaling effect of the milling step

The scaled-up production of lead-free potassium sodium niobate (KNN)-based piezoceramics still suffers from poor reliability and reproducibility issues that hinder their penetration into the piezoelectric market. To address this issue, pure KNN ceramics were prepared in this work by mechanochemical activation of the powders, followed by calcination at 700 °C, obtaining high density sintered pellets featuring fine and uniform-grain-size. Results show that both cumulative kinetic energy released in the system and hindrance factor, the latter being paramount, must be properly taken into account when the synthesis is scaled-up. Mechanochemical activation strongly effects both stoichiometry and morphology of the final KNN powders during the milling step, which in turn affects the piezoelectric properties of the sintered pellets. This work definitely proves that proper design and control of the powder synthesis process is a valuable tool in the scaled-up production of KNN powders for the production of reliable and performant KNN ceramics.