Microstructural and high-temperature dielectric, piezoelectric and complex impedance spectroscopic properties of K0.5Bi0.5TiO3 modified NBT-BT lead-free ferroelectric ceramics

Abstract

Solid solutions (1-x-y)(Na0.5Bi0.5)TiO3-xBaTiO3-y(K0.5 Bi0.5)TiO3 with (x (mol.%) = 0, 7 and 100); y(mol.%) = 0, 20 and 100) compositions have been prepared by a conventional solid-state reaction method, and their structure, dielectric properties and depolarization temperature have been examined. At room temperature, X-ray diffraction (XRD) patterns reveal that the crystalline structure of the ceramics was perovskite. The morphotropic phase boundary (MPB) of the ternary system lying between rhombohedral (R3c) and tetragonal (P4mm) phases is in the range of (x (mol.%) = 7 and y (mol.%) = 20). The Raman-active modes for 0.73NBT-0.07BT-0.20KBT were separated and identified under the framework of group theory. SEM micrographs illustrate the quasi-uniform distribution of the grains, which are compact. The dielectric properties of the ceramics were studied in the frequency range of 1 kHz–100 kHz from ambient temperature to 600 °C. Dielectric measurements indicate that all ceramics show a diffuse phase transition near the temperature (Tm) for diffusivity of the order of 1.4–1.7 and a shift of (Tm) towards high temperatures. The resistance and capacitance of the various contributors (grain and grain boundary) in our samples are also discussed using a brick-layer model. Excellent piezoelectric properties for d33 = 146 pCN−1 and electromechanical coupling factors kp = 29.4% were observed at morphotropic phase boundary (MPB), which was assumed to be associated with the coexistence of rhombohedral and tetragonal phases and accurate grain size. This work establishes a new approach for improving lead-free piezoelectric ceramics based on 0.73NBT-0.07BT-0.20KBT.