Dielectric, ferroelectric and piezoelectric behaviors of thulium-doped KNN ceramics fabricated by microwave sintering.

(K,Na)NbO₃ − x at.% Tm₂O₃ (KNNTx) piezoelectric ceramics with submicron grain size were successfully prepared by solid-state method combined with microwave sintering. With the increase of content of Tm³⁺, KNNTx ceramics gradually transformed from orthorhombic (O) phase to the coexistence of O and tetragonal (T) phase at room temperature, and the phase transition temperature of O–T phase shifted to low temperature region. The leakage behaviors, dielectric, ferroelectric, and piezoelectric properties of KNNTx ceramics depend on the structure evolution, cationic vacancy and grain refinement caused by Tm³⁺ doping, and the relevant mechanisms have been illustrated. The substitution of Tm³⁺ at A sites increases the critical driving force for the rapid growth of grains and obviously refine the grain of KNNTx ceramics, resulting in its deteriorated ferroelectricity and enhanced piezoelectricity. Especially the piezoelectric constant of KNNTx ceramics increased from 109.8 to 163.6 pC/N. These findings are of great significance for developing excellent lead-free piezoelectric materials. [ABSTRACT FROM AUTHOR]