Giant Electrostrain from Inherent Strain for KNN-Based Ceramics

Lead-free potassium sodium niobate (KNN)-based piezoelectric ceramics with different sizes sintered in a reducing atmosphere are prepared. By determining the frequency dependence of the unipolar strain, it provides researchers with an effective and convenient method to distinguish between bending deformation and inherent electrostrain. A highly operable strategy to enhance inherent electrostrain by reducing the area of thin samples has been provided. There is a significant dependence between multivalent manganese ions and thickness changes in ceramics. With decreasing thickness from 400 to 200 μm, the Mn2+ ions in A-site vacancies absorb holes due to re-oxidation to become Mn4+ ions, and then, these Mn4+ ions with increased valence states substitute for Nb5+ ions in the B-site to form a defect (Mn'Nb) and increase the defect (V'Na/K) concentration. In this work, giant electrostrain in ceramics (200 μm) without the defect (V'Na/K-VO··) gradient distribution should originate from the inherent strain due to the synergistic effect of dipole defects (Mn'Nb-VO·· and V'Na/K-VO··) and stripe (100 nm) domains. The thin (200 μm) ceramics show a significant inherent electrostrain (1068 pm/V at 15 kV/cm). Our research provides an innovative way of designing high-performance lead-free piezoelectric ceramics sintered in a reducing atmosphere that are promising for low-cost multilayer piezoelectric actuator applications co-fired with base metal nickel electrodes.