pISSN 0374-4914 eISSN 2289-0041

## Research Paper

New Phys.: Sae Mulli 2017; 67: 1419-1425

Published online December 29, 2017 https://doi.org/10.3938/NPSM.67.1419

## High Strain of Na- and K-Deficient Bi$_{0.5}$(Na$_{0.78}$K$_{0.22}$)$_{0.5-x}$TiO$_3$ Piezoelectric Ceramics

Jin Ho CHOI, Hae Jin SEOG, Chang Won AHN, Shinuk CHO, Ill Won KIM*

Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 44610, Korea

Correspondence to:kimiw@mail.ulsan.ac.kr

Received: August 23, 2017; Revised: October 18, 2017; Accepted: October 20, 2017

This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

That the Bi$_{0.5}$(Na$_{0.78}$K$_{0.22}$)$_{0.5-x}$TiO$_3$ (BNKT) ceramics exhibit a tetragonal structure without any secondary phase was confirmed by X-ray diffraction (XRD) measurements. Scanning electron microscopy (SEM) images indicate that the shape of grains change from round to stick-like with increasing Na- and K-deficiency ratio. The dielectric constants of the BNKT ceramics are 1030~1500, and their Curie temperature ($T_c$) exists in the range 280 $^\circ$C ~ 320 $^\circ$C. The $P-E$ hysteresis loop of the BNKT ceramics changes from a normal ferroelectric to a pinched shape with increasing Na- and K-deficiency ratio. As a result, the BNKT ceramics shows a high strain ($S$ = 0.4%) state corresponding to a large normalized strain ($d_{33}^\ast$ = $S_{max}/E_{max}$ = 680 pm/V) at $x$ = 0.015. The observed large $d_{33}^\ast$ value suggests that Na- and K-deficient BNKT ceramics can be considered as potential candidates for use in lead-free actuator applications.

Keywords: Piezoelectricity, Ceramics, Ferroelectrics, Lead-free, Perovskite structure

### Figures

Fig. 1. (left) Unipolar strains of BNKT ceramics with different Na and K deficient ratio, the inset shows the dynamic d33* as a function of x.
(right) Schematic diagram of "deformation of electric dipole caused by external electric field inside BNKT ceramics with Na and K deficiency''.