Elsevier

Ceramics International

Volume 40, Issue 2, March 2014, Pages 3199-3207
Ceramics International

Growth of (K0.5Na0.5)NbO3–SrTiO3 lead-free piezoelectric single crystals by the solid state crystal growth method and their characterization

https://doi.org/10.1016/j.ceramint.2013.09.122Get rights and content

Abstract

Lead zirconate titanate (PZT) piezoelectric ceramics are commonly used in various applications, e.g. gas igniters, high-voltage generators and microbalances. However, due to increasing health and environmental concerns over their high lead content, lead-free piezoelectric ceramics are being developed. Lead-free piezoelectric single crystals offer superior performance over their polycrystalline counterparts but are difficult to grow by conventional methods. In this paper, (K0.5Na0.5)NbO3–SrTiO3 (KNN–ST) single crystals are grown for the first time by the solid state crystal growth (SSCG) method. 〈100〉 KTaO3 single crystal seeds are buried in the center of pellets of pressed KNN-ST powder. The single crystal grows from the seed crystal during sintering at 1100 °C for 20 h. The grown single crystals contain porosity, which is incorporated from the matrix during growth. The effect of SrTiO3 addition on single crystal growth behavior, chemical composition and structure is evaluated.

Introduction

Piezoelectric materials have the ability to convert electrical energy into mechanical energy and vice versa [1]. They have found a broad spectrum of applications, e.g. microelectromechanical systems (MEMS), ultrasonic baths, medical ultrasound imaging and scanning probe microscopes [2], [3], [4]. Lead zirconate titanate (PZT) based piezoelectric ceramics are commonly used for these applications because of their superior piezoelectric and dielectric performance [5]. However, PZT based ceramics contain a substantial amount of Pb. The high vapor pressure of PbO during production of PZT and Pb release from waste material contaminates the environment[6], [7], [8] and the toxicity of Pb is well documented [9]. Based on these environmental and health concerns, the European Union (EU) has limited or banned the use of Pb in many products [10]. If suitable alternatives to PZT based ceramics become available, the EU is expected to ban the use of PZT as well.

In the quest to find suitable replacements for PZT, many ceramic systems e.g. (K0.5Na0.5)NbO3 (KNN) [5], [11], [12], KNN with additions of SrTiO3 (ST) [13] or LiTaO3 [6], (Na0.5Bi0.5)TiO3–BaTiO3 [14] and BiFeO3–BaTiO3 [15] have been explored. Among these systems, KNN-based materials are the most promising candidates. The KNbO3-NaNbO3 system is a pseudo-binary system with a morphotropic phase boundary between two orthorhombic phases close to the (K0.5Na0.5NbO3) composition [1]. KNN ceramics show reasonable piezoelectric and dielectric constants [16]. However, the performance of KNN ceramics is lower than that of PZT ceramics [5], [17].

Various additions to KNN ceramics have been made to improve the dielectric and piezoelectric properties [6], [7], [16]. Previous reports on ST addition show improved dielectric constants and lower dielectric loss [7], [13]. Addition of ST broadens the dielectric constant versus temperature peak, eventually changing the behavior from a normal ferroelectric ceramic to relaxor-like [4], [18]. ST additions have also been found to improve the resistance to polarization switching fatigue and dielectric aging [4].

Polycrystalline piezoelectric ceramics have randomly oriented grains, meaning that the ferroelectric domains cannot be aligned perfectly in one direction during poling. This considerably deteriorates the properties of the material. In single crystals, a much improved alignment of the domains with the poling field can be achieved, leading to improved properties. Single crystals generally have better sensitivity and acoustic power, lower strain hysteresis, lower acoustic impedance and better efficiency when compared with polycrystalline piezoelectric ceramics [6], [17], [19]. Hence, the use of single crystals can boost the performance of lead-free systems and make them comparable to PZT [20].

Single crystals can be produced from a melt or flux [21], [22] or by the solid state crystal growth (SSCG) method [8], [19], [23]. The SSCG method has many advantages over growth from a liquid. This method does not involve the melting of the starting materials, reduces contamination from the crucible walls and is particularly suitable for materials which melt incongruently [6]. Lower operating temperatures and the use of relatively inexpensive equipment make the SSCG method cost effective as well [2], [19]. This method has been employed to grow single crystals of BaTiO3 [23], BaZrO3–BaTiO3 [24] and (Na0.5Bi0.5)TiO3–BaTiO3 [19].

Although single crystals of KNN [8] and KNN with LiTaO3 [6] additions have been previously grown by SSCG, there are few reports on the single crystal growth of KNN-based ceramics by this method. In this paper, we present our work on the single crystal growth behavior of KNN-ST ceramics by the SSCG method for the first time. The effect of ST content on the single crystal and matrix grain growth is evaluated by microscopy. The effect of ST content on chemical composition and structure is evaluated by electron probe microanalysis and micro-Raman scattering.

Section snippets

Experimental

The (1−x)[K0.5Na0.5NbO3]−xSrTiO3 (x=0,1,2,3,4 mol%) powders, hereafter termed KNN(0–4)%ST, are produced by the solid state synthesis method. K2CO3 (Alfa Aesar, 99%), Na2CO3 (ACROS Organics, 99.5%), Nb2O5 (CEPA, 99.9%), SrCO3 (Aldrich, 99.9%) and TiO2 (Alfa Aesar, 99.8%) powders are used as starting materials. These powders are dried at 250 °C for 5 h to remove any adsorbed water. Then, weighed amounts of these powders are ball milled for 24 h in high-purity ethanol in polypropylene jars with ZrO2

Results

Fig. 1 shows the grown single crystals in the KNN(0–4)%ST samples. A single crystal layer has grown on the seed crystal in the samples with 0–3 mol% SrTiO3, but not in the sample with 4 mol% SrTiO3. The single crystal growth distances are given in Fig. 2. Each data point is the mean value of 50 measurements and the error bars represent the standard deviation. The single crystal thickness initially increases with ST addition. The maximum thickness of the single crystal is ~97 µm with 2% ST solid

Discussion

The growth of single crystals by SSCG is dependent upon the driving force for single crystal growth and upon the grain boundary structure of the ceramic, which determines whether normal or abnormal grain growth takes place. The driving force for growth of a grain in a polycrystalline matrix is given by [31], [32]Δμ=σΩ(1r¯1r)where σ is the average interfacial energy (grain boundary energy or solid / liquid interfacial energy), Ω the molar volume, r is the radius of the growing grain and r¯ is

Conclusions

Single crystals of (1−x)[K0.5Na0.5NbO3]-xSrTiO3 (x=0,1,2,3 mol%) are grown for the first time by the solid state crystal growth (SSCG) method. A 〈001〉 oriented KTaO3 single crystal is used as a seed to grow KNN-ST single crystals. SrTiO3 solid solution addition reduces the grain size and increases the driving force for single crystal growth. The grown single crystal thickness increases with the SrTiO3 content, reaches a maximum of 97 µm with 2% SrTiO3 addition, and then declines. The falloff in

Acknowledgments

This work was funded by the National Research Foundation of Korea (Ministry of Education, Science and Technology, project no. 2012R1A1A2000925). The authors would like to thank Dr. Sang-Hun Jeong (Korea Basic Science Institute, Gwangju center) for carrying out the micro-Raman scattering experiments, and Cheol Kim, Chan Yoon and Hye-Jeong Kim for operating the XRD, EPMA and SEM respectively.

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