Growth of (K0.5Na0.5)NbO3–SrTiO3 lead-free piezoelectric single crystals by the solid state crystal growth method and their characterization
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]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 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.
References (47)
- et al.
Grain boundary faceting and abnormal grain growth in BaTiO3
Acta Materialia
(2000) - et al.
Growth of (Na, K, Li)(Nb, Ta)O3 single crystals by solid state crystal growth
Journal of the European Ceramic Society
(2007) - et al.
The effects of sintering temperature on the properties of lead-free (Na0.5K0.5)NbO3–SrTiO3 ceramics
Journal of Alloys and Compounds
(2008) - et al.
Growth of potassium sodium niobate single crystals by solid state crystal growth
Journal of Crystal Growth
(2007) - et al.
Microstructural changes in (K0.5Na0.5)NbO3 ceramics sintered in various atmospheres
Journal of the European Ceramic Society
(2009) - et al.
Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3–SrTiO3 ceramics
Solid State Communications
(2004) - et al.
Piezoelectric properties and temperature stabilities of Mn- and Cu-modified BiFeO3–BaTiO3 high temperature ceramics
Journal of the European Ceramic Society
(2013) - et al.
Dielectric and piezoelectric properties of alkaline-earth titanate doped (K0.5Na0.5)NbO3 ceramics
Materials Letters
(2007) - et al.
Properties of (Na0.5K0.5)NbO3–SrTiO3 based lead-free ceramics and surface acoustic wave devices
Sensors and Actuators A: Physical
(2007) - et al.
Solid state growth of Na1/2Bi1/2TiO3–BaTiO3 single crystals and their enhanced piezoelectric properties
Journal of Crystal Growth
(2011)
On the study of zinc doping in congruent LiTaO3 crystals
Materials Chemistry and Physics
Growth and electrical properties of 0.95Na0.5Bi0.5TiO3–0.05K0.5Bi0.5TiO3 lead-free piezoelectric crystal by the TSSG method
Journal of Crystal Growth
Growth behaviour of potassium sodium niobate single crystals grown by solid-state crystal growth using K4CuNb8O23 as a sintering aid
Journal of the European Ceramic Society
The kinetics of precipitation from supersaturated solid solutions
Journal of Physics and Chemistry of Solids
Effects of donor concentration and oxygen partial pressure on interface morphology and grain growth behavior in SrTiO3
Acta Materialia
Effect of Ta-doping on the ionic conductivity of lithium titanate
Fusion Engineering and Design
The dependence of Nb and Ta rutile–melt partitioning on melt composition and Nb/Ta fractionation during subduction processes
Earth and Planetary Science Letters
High-temperature X-ray diffraction and Raman spectroscopy study of (K0.5Na0.5)NbO3 ceramics sintered in oxidizing and reducing atmospheres
Materials Chemistry and Physics
Piezoelectric Ceramics
Handbook of Advanced Dielectric, Piezoelectric and Ferroelectric Materials: Synthesis, Properties and Applications
Relaxor-like dielectric properties and history dependent effects in a new lead-free K0.5Na0.5NbO3–SrTiO3 ceramic system
Applied Physics Letters
High spatial resolution structure of (K,Na)NbO3 lead-free ferroelectric domains
Journal of Materials Chemistry
Toxicological profile for lead
US Department of Health and Human Services
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