New Phase Boundary Originating from Coexistence of Ferroelectrics and Relaxor in Lead-free Piezoelectric KNN-based Composites

Abstract

Lead‒based piezoelectric materials have been used in various applications for sensors and actuators, based on their mutual conversion between mechanical energy and electrical energy. However, it seems that the high toxicity of lead (Pb) in wastes of those devices causes serious problems to environment and human body. Some laws and regulations were legislated to prohibit the use of lead, including European Union passed the Restriction of Hazardous Substances (RoHS) law in 2003, Household Electronic Products Recycling Law in Japan, and Electronic Information Product Pollution Control Management Regulation in China in 2006. Recently, in order to replace lead‒based piezoelectric materials, lead‒free ones were strongly developed. Several kinds of lead‒free piezoceramics have been extensively investigated, such as BaTiO3‒based, (Bi1/2Na1/2)TiO3‒based and (K0.5Na0.5)NbO3‒based (KNN) materials. Among them, KNN and its compounds have been considered as one of promising candidates because of their excellent dielectric and piezoelectric properties. The unique advantages of KNN ceramics such as a high Curie temperature (TC ≈ 420oC) and a high piezoelectricity, represents a promising candidate for replacement of lead‒based materials. From that, many studies focused on addition, substitution, modification chemical elements to improve the properties for KNN‒based materials defined the concept of polymorphic phase transition (PPT) at room temperature, and morphotropic phase boundary (MPB). The PPT has improved the piezoelectric properties in KNN‒based materials; however, it remains the temperature sensitive problem since the phase boundary corresponding to R‒O or O‒T possesses the PPT characteristic depending on not only the compositions but also the temperatures. To prevent this problem, many studies paid their attentions on MPB. The MPB is temperature independent and shows a nearly vertical phase boundary of ferroelectric phases, such as rhombohedral (R), tetragonal (T), monoclinic (M). There are many studies for MPB, which mentioned about the improvement in piezoelectric properties and the temperature stability of d33. However, the explanation for these behaviors still remains a challenge. Recently, some studies suggested ferroelectric‒relaxor phase boundary with the crystal structure transit from non cubic phase (orthorhombic or tetragonal) to pseudocubic (PC) phase. Though the enhancement of piezoelectric constant and/or electromechanical strain can be observed, the temperature stability of piezoelectric properties was not clearly considered. In order to saperate and maintain the phase in materials, the composite method was mentioned in this work, by mixing the calcined powders of two phase components. For tetragonal phase stabilization, SrTiO3 is a promising candidate which can improve not only the sinterability but also piezoelectric properties of KNN ceramics. Tetragonal phase was stabilized and maintained good properties for the modification content of 0.06 mol. Such high amount of SrTiO3 causes the higher sintering temperature and significantly reduces TC. Then Li+ was doped into KNN‒4ST to solve problems. On the other hand, we found the stabilized PC phase with relaxor features in BaZrO3‒modified KNN ceramics. Finally, the ferroelectric/relaxor piezocomposites were compounded by T‒phase KNN‒4ST‒4Li and PC‒phase KNN‒8BZ in different T‒phase fraction f. The phase boundary was found at the composites of 70 mol% of T phase and 30 mol% of PC phase with the enhanced piezoelectric constant d33 and coupling factor kp values of 210 pC/N and 34%, respectively. Besides, the enhanced strain properties at f = 0.7 composites are related with the coexistence of ferroelectric domain, tetragonal micro domains and pola nanoregions, which originates from the relatively contribution of the ferroelectric and relaxor components in composites. Moreover, the low variation (less than 10%) of d33 in the temperature range of 25oC‒225oC performs good temperature stalility of the piezoelectric constant, corresponding with phase transition in temperature dependent XRD patterns.