Effects of nano-sized BCZT on structure and electrical properties of KNN-based lead-free piezoceramics

Abstract

The (1 − x)(Li0.03Na0.5K0.47)(Nb0.92Sb0.05Ta0.03)O3–xBa0.85Ca0.15Zr0.1Ti0.9O3 (LNKNST–BCZT, x = 0, 0.5, 1.5, 2.5, and 3.5%) lead-free piezoceramics were synthesized by the conventional solid-state reaction method with adding nano-sized Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) powder to investigate their influences on crystal structure and electrical properties. The X-ray diffraction (XRD) analysis reveals that all the sintered LNKNST–BCZT ceramics exhibit pure perovskite structure with the mainly rhombohedral (R) phase or the coexistence of rhombohedral (R) phase and tetragonal (T) phase. The composition of the LNKNST–1.5%BCZT ceramics locates around the critical point, i.e., the ceramics changing from the coexistence of rhombohedral (R) phase and tetragonal (T) phase to purely rhombohedral (R) phase, and presents the most densified microstructure morphology, which can be confirmed further by the field emission scanning electron microscope (FESEM) observation. There are two dielectric anomalies in the temperature dependent dielectric properties curves of the LNKNST–BCZT ceramics, which correlate with the ferroelectric rhombohedral (R) phase changing to the ferroelectric tetragonal (T) phase, and then to the paraelectric cubic (C) phase. Both the Curie–Weiss law and the power law fittings confirm the diffuse phase transition ferroelectrics characteristic of the LNKNST–1.5%BCZT ceramics, which is considered as correlating with the polar nanoregions (PNRs). Due to the densification effect caused by the chosen amount of nano-sized BCZT doping and the construction of R–T polymorphic phase boundary, the LNKNST–1.5%BCZT ceramics exhibit the best ferroelectric and piezoelectric properties. The complex impedance spectroscopy analysis confirms that the extrinsic electrical conduction mechanism at high temperatures is dominated by the oxygen vacancies induced by the evaporation of the alkali metals during sintering.