Perovskite structured lead-free materials for dielectric and piezoelectric applications

Abstract

Piezoelectricity is a phenomenon based on the electromechanical interaction signifying the charge generation on the application of force/pressure (direct piezoelectric effect) or induced strain in the system on the application of electric field/voltage (converse piezoelectric effect). Accordingly, piezoelectrics materials have been used in various electronic devices including, sensors, actuators, hydrophones, microphones, sonar applications, and vibration control [1]. The majority of the piezoelectric market is dominated by high performance PbZrxTi(1-x)O3 (PZT) and other lead-based materials such as Pb(Mg1/3Nb2/3)O3 (PMN), Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) and Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) owing to their excellent piezoelectric & ferroelectric properties [2]. However, lead-based materials are restricted by global regulations, such as RoHS (Restriction of certain Hazardous Substances) and WEEE (Waste of Electrical and Electronic Equipment) due to the volatilization of high levels of hazardous lead during mining, processing, and disposal. Extensive research activities have been undertaken to develop lead-free alternatives in response to the call for reducing the health and environmental risk factors, in the last 2 decades. Among various lead-free alternatives, perovskite structured K0.5Na0.5NbO3 (KNN) based materials are the most promising replacement due to the excellent piezoelectric performance and large operating temperature window and high Curie temperature. KNN exhibits orthorhombic phase with Amm2 space group at room temperature and undergoes three polymorphic phase transitions: rhombohedral to orthorhombic (TR–O) around -110 °C, orthorhombic to tetragonal (TO–T) around 180 °C, and tetragonal to cubic (TT–C) around 400 °C. Several promising investigations have recently been done to improve the piezoelectric performance and thermal stability of the KNN based materials via various approaches such as chemical modifications, advanced fabrication techniques, and post-fabrication processing.