Review
Grain growth and piezoelectric property of KNN-based lead-free ceramics

https://doi.org/10.1016/j.cap.2011.04.014Get rights and content

Abstract

Recent advances of (K,Na)NbO3-based lead-free piezoelectric ceramics (KNN) with special emphasis on (K,Na,Li)(Nb,Sb,Ta)O3 representative system is reviewed concisely. The base KNN and its compositional derivatives are analyzed in terms of the relationship of fabrication methods and properties, in which the control of piezoelectric grain growth is extended thoroughly. The grain growth from nano- to meso- scales with self-orientation of sub-grains in large super-grains is obtained and presented by low temperature calcinations of raw powders. The experimental results of self-assembly grain growth are illustrated using a primary mechanism of their interplanar and interparticle polarizations. Besides grain size and orientation, the other object of grain engineering is microstructures consisting of domains and domain walls. Similar to that of the dielectric properties and deducing from previous research results, domainwalls should have a contribution to piezoelectric properties. Special attention is then devoted to domain wall formation and the relation to piezoelectric properties. Their formation assistance and coexistence with defects obey the law of lattice fit and misfit in crystallography. Both domains and domain walls may play remarkable roles in piezoelectric performance. Overall, the research of piezoelectric materials is a systematic engineering, and more issues need to be resolved.

Highlights

► In this review, recent progress on the piezoelectric properties and grain growth of KNN-based systems were presented. ► The process of self-ripening from sub-grain to super-grain showed a new option for grain orientation, which based on the cluster phenomena of super fine powders and intrinsic polarization of materials. ► The microstructures on domain and domain wall of KNN-based ceramics showed a substantial relations with piezoelectric constant d33. ► KNN based lead-free piezoelectric systems with improved properties will be obtained by grain engineering.

Introduction

Lead-based piezoelectric materials with high piezoelectric and electromechanical properties, such as Pb(Zr,Ti)O3 (PZT) [1], Pb(Mg,Nb)O3 (PMN) [2], have been used extensively for transducer applications. But their high concentrations of lead (Pb), e.g. for PZT up to 63 wt%, are hazardous to human health and environment. These concerns have induced considerable momentum for lead-free ceramic research in recent years. Especially, high piezoelectric properties were reported by Y. Saito et al in 2004 [3], for (K,Na,Li)(Nb,Sb,Ta)O3 (KNLNST) system which were modified simultaneously by A-site and B-site substitutions of perovskite structure ABO3.

For such a complex system, it is not simple binary even or ternary system [4], but based on (K,Na)NbO3 (KNN) which is essentially a binary solid solution of potassium niobate KNbO3 (KN) and sodium niobate NaNbO3 (NN). From the phase diagram of KNN [5], the phase transitions from rhombohedral to orthorhombic and from orthorhombic to tetragonal of KN occur near −10 °C and 225 °C, respectively. At room temperature, NaNbO3 shows antiferroelectric-liked behaviors and orthorhombic symmetry with space group of Pbcm determined by X-ray diffractionary (XRD) [6], but its competing antiferroelectric phase Pbcm and ferroelectric phase R3c interactions was obtained by the theoretical analysis of neutron diffraction data [6]. Compared with KN and NN, unclear phase transitions of alkali antimonates or tantalates still need further study. Near the morphotropic phase boundary (MPB) of Na+ and K+ molar ratio 1:1, relatively high piezoelectric properties of KNN were obtained [1]. And their higher piezoelectric properties were realized by hot-pressing or hot forging than pressureless sintering [7] without obvious grain orientation. But as the result of asymmetrical polarization P, piezoelectric constant d33 follows the formula of d33 = 2ɛ33P3Q11 [8,9], where ɛ33 is the longitudinal dielectric constant and Q11 is the horizontal electrostrictive coefficients, thereby being affected by the orientation of grains which possess polarization P. The grain growth technologies can texture ceramic grains growing along specified direction which show optimization properties, such as reactive-templated grain growth (RTGG) used by Y. Saito et al. [3] and screen-printing multi-layer grain growth techniques (MLGG) invented by our group [10]. All technologies of grain growth, orientations and their enhancement of polarization may be called by “Grain Engineering” of piezoelectric ceramics.

To improve the piezoelectric properties, it is very effective to find structure regions of morphotropic phase boundary (MPB) by changing various compositions, and is the aim to composite new systems. For the inside physical mechanism, the polymorphic phase transition (PPT) between orthorhombic and tetragonal phases near room temperature is proposed which plays a very important role for higher piezoelectric properties [11], [12], [13]. For details, orthorhombic phase mm2 should result in 12-fold degenerate domain variants, and 6-fold degenerate domain variants should been resulted in from tetragonal phase mm4. So that their phase transition should at least induce 18-fold degenerate domain variants in total, which suggests the extrinsic contribution to d33 should be substantial [14], [15]. The total contribution of each phase to Ps (and d33) would be governed by the lever rule on the coexistence of different domains with various phases in grains [9]. For the crystal structures of grains is equal to those of domains, therefore, the objects of the research of phases and phase transitions are various domains contained in grains. Except single crystal grain with single domain, lots of domains are exist in one grain for piezoelectric ceramics, as piezoelectric grains also contain domain walls and other defects like faults in addition to domains. Evidently, domains and domain walls in grains have direct relationships with piezoelectric properties [16], [17], [18], and further research indicated that the grain size [19], [20], and defects [21], [22], have effective actions on domain density. So far, the properties of alkali niobate based piezoelectric should be very processing sensitive, especially in regard to building-in process of domains.

Since the asymmetrical character of polarization, macroscopical piezoelectric properties are not only based on domain density and the amplitudes of their inherent polarization, but also based on the uniform orientation of grains and arrangement of domains inside grains, which are achieved usually through polarizing procedure. For a remarkable increase of d33, it should be attribute to the MPB in the system and to grain texturing, which essentially are polarizations of domains with various phases and their performance, respectively.

In this review article, a concise overview of the recent progress in KNN-based piezoelectric ceramics will be provided. The grain growth and formation of domains and domain walls will be elaborated with special emphasis on KNLNST system which is the most available lead-free system up to date. This review article draws heavily on our research results, while also includes major data published by other researchers. The pointed message in this article is that the lead-free piezoelectric study will begin a new research stage based on the integration of grain growth and properties improvement after the last four decades since the basic research of KNbO3 and NaNbO3 in the 1960’s.

Section snippets

Experimental

As follows, the detailed stages of grain growth crystallization and the formation in crystallography of domains and domain walls which are related with piezoelectric properties are described from a phenomenological perspective. The experimental methodologies herein can be traced by comparing different works cited as references in this review.

KNLNST ceramics with chemical formula of (K0.44Na0.56-xLix)(Nb0.95-xSbxTa0.05)O3 (KNLNST1-4 for x = 0.035, 0.040, 0.045, 0.050) were synthesized by

Overview of KNN-based piezoelectric ceramics

Most piezoelectric ceramics principally contain lead which based on PZT solid solutions. After new systems with the compositions of (K0.5Na0.5)(1-x)(Nb1-yTay)O3 and (K0.44Na0.52Li0.04)(Nb0.86Sb0.04Ta0.10)O3 were reported [3], a worldwide effort is underway to remove lead from some commercial products by adopting alkali niobate based materials. In those systems, Li+, Sb5+, Ta5+ ions were tried to replace partial alkali metal elements and/or niobium to form MPB solid solutions. Only with the

Conclusions

In this review, recent progress on the piezoelectric properties and grain growth of KNN-based systems has been presented. It was shown that the endeavor for seeking advanced lead-free materials attained the impressive new results of KNLNST system which showed excellent piezoelectric properties. It was also shown that grain texturing can be realized by reactive-templated grain growth method, template grain growth method, even Cu-doping method. Eventually, the entire stages of self-ripening from

Acknowledgements

We thank Dr. Adrian Trinchi of CSIRO Materials Science & Engineering, Clayton VIC, Australia, for his kind help. This work was supported by the Ministry of Sciences and Technology of China through 973-Project (2009CB613305), The Major Program of the National Natural Science Foundation of China (50932007), and The Science & Technology Commission of Shanghai Municipality (08JC1420500, 10XD1404700).

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