Lead-free piezoelectric ceramics

• Main Author: Zhu, Zangyuan
• Published: University of Leeds 2012
• Subjects: 620.14
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.581971

Legislation arising from health and environmental concerns has intensified research into finding suitable alternatives to lead-based electroceramics. Lead zirconate titanate (PZT) has been developed over several decades to become the market-leading piezoelectric ceramic. Lead-free solid solutions based on sodium potassium niobate, Na0.5K0.5NbO3 (NKN), show promising dielectric and piezoelectric properties. 1-2 The related (l-x)( Na0.5K0.5NbO3)-xBiScO3 binary system (NKN-BS) has been reported to exhibit maximum d33 values of 200 pCIN at 2 mol% BS.3 Similarly, an optimal d33 value has been reported for the binary NKN-LT system at 5-6 mol% LiTa03.4 In this work, a series of compositions along the compositional join in the ternary NKN-LT-BS system, extending from 0. Na0.5K0.5NbO3 -0.02BiScO3 toward LiTa03 have been prepared and characterized. A 0.98[0.98NKN - 0.02(LiTaO3)] - 0.02[BiScO3] (NKN- 2L T -2BS) composition showed enhanced piezoelectric properties, relative to similar compositions, with d33 values of 215 pCIN. This can be attributed to a phase content of mixed orthorhombic (or monoclinic) and tetragonal phases at ambient temperatures. Variable temperature X-ray diffraction (XRD), and dielectric measurements as a function of temperature, indicated phase transitions (on heating) from an orthorhombic (or monoclinic) crystal system to tetragonal and then cubic crystal systems at ~25°C and ~370°C respectively. Different types of dielectric behaviour were observed on increasing the LT content. A NKN-5%LT-2%BS composition exhibited twin dielectric peaks at high temperatures (~370°C and ~470°C), along with broad X-ray diffraction peaks and a fine grain size, < 0.5 μm. The twin dielectric peaks suggest that chemical inhomogeneities may have been present; this was examined using transmission electron microscopy (TEM) with energy dispersive X-ray analysis (EDX). Elemental segregation was observed within individual grains, such that a core-shell grain structure was evident. The twin high temperature dielectric peaks are attributed to the separate response from the core and shell regions, each of which have a characteristic Curie temperature range. Subsequently, a series of other compositions were prepared in the wider Na0.5K0.5NbO3 - LiTaO3-BiScO3 ternary system. Considering the combined data from XRD, dielectric measurements, SEM, TEM and piezoelectric properties for a wide range of compositions within the NKN-rich region of the NKN-LT-BS system, materials may be grouped into three categories, exhibiting the following defining characteristics. Type I: single, sharp dielectric Curie peak (~ 370°C); single phase by XRD; large grain size (5-10μm); chemically uniform by TEM-EDX. Type II: broad, single dielectric peak (~ 350°C); single phase by XRD; large grain size; no obvious chemical segregation. Type Ila: twin, broad dielectric peak(s) (~ 370°C and ~ 470°C); broad XRD peaks; small grain size (~ O.5μm); chemical segregation (core-shell structure) identified by TEM-EDX. Reasons for the properties of these three classes of material are discussed; comparisons are drawn with other lead-free dielectrics and piezoelectrics; finally, the potential of the materials in future device applications are considered.