Effect of KTaO3 Modification on Dielectric, Ferroelectric, and Strain Properties of Lead-Free Bi1/2Na1/2TiO3-SrTiO3 Piezoelectric Ceramics

Abstract

Piezoelectric ceramics that interchangeably convert mechanical energy to electricity play a key role in micro-controllable sensors and actuators, for which lead zirconate titanate (PZT) has been primarily used during the last five decades because of their excellent electrical properties. However, European Union implements the Restriction of Hazardous Substance Directive (RoHS), which restricts electronic products or electrical appliances that use harmful substances. Therefore, it is necessary to replace lead-based materials to lead-free materials. Recently (Bi1/2Na1/2)TiO3 (BNT)-based lead-free relaxor ceramics as an incipient piezoelectrics are considered to be one of the most promising candidates for electromechanical applications because of their excellent electrical properties (large electromechanical strains of ~ 0.40% at 6 ~8 kV/mm). However there is a problem that a high electric field must be applied to obtain such high strain property. One promising material system to address this problem is BNT‒SrTiO3 (BNT‒ST) ceramics. SrTiO3 (ST) doping was beneficial for improving the piezoelectric properties of BNT ceramics, and can reduce their remanent polarization, operating electric field, and yielding relaxor behavior in all compositions. To solve this problem, lead-free (1-x-y)(Bi1/2Na1/2)TiO3-xSrTiO3-yKTaO3 (x = 0.18 ~ 0.24, y = 0 ~ 0.02) (BNT-ST-KT) piezoelectric ceramics were investigated in this study. As a consequence, a large electrical strain and normalized strain (d33* ≈ 793 pm/V) can be obtained even under 3 kV/mm as low electric field for BNT‒KT−22.5ST ceramics. The phase transition between nonergodic relaxor (NER) and ergodic relaxor (ER) under electric field might be responsible for its large strain. It means that BNT‒KT‒ST lead‒free ceramics can be a promising candidate for actuator applications.