Grain growth and piezoelectric property of KNN-based lead-free ceramics

Recent advances of (K,Na)NbO3-based lead-free piezoelectric ceramics (KNN) with special emphasis on (K,Na,Li)(Nb,Sb,Ta)O3 representative system is reviewed concisely. The base KNN and its compositional derivatives are analyzed in terms of the relationship of fabrication methods and properties, in which the control of piezoelectric grain growth is extended thoroughly. The grain growth from nano- to meso- scales with self-orientation of sub-grains in large super-grains is obtained and presented by low temperature calcinations of raw powders. The experimental results of self-assembly grain growth are illustrated using a primary mechanism of their interplanar and interparticle polarizations. Besides grain size and orientation, the other object of grain engineering is microstructures consisting of domains and domain walls. Similar to that of the dielectric properties and deducing from previous research results, domainwalls should have a contribution to piezoelectric properties. Special attention is then devoted to domain wall formation and the relation to piezoelectric properties. Their formation assistance and coexistence with defects obey the law of lattice fit and misfit in crystallography. Both domains and domain walls may play remarkable roles in piezoelectric performance. Overall, the research of piezoelectric materials is a systematic engineering, and more issues need to be resolved.

► In this review, recent progress on the piezoelectric properties and grain growth of KNN-based systems were presented.
► The process of self-ripening from sub-grain to super-grain showed a new option for grain orientation, which based on the cluster phenomena of super fine powders and intrinsic polarization of materials.
► The microstructures on domain and domain wall of KNN-based ceramics showed a substantial relations with piezoelectric constant d33.
► KNN based lead-free piezoelectric systems with improved properties will be obtained by grain engineering.