Article pubs.acs.org/crystal
Top-Seeded Solution Growth and Properties of K1−xNaxNbO3 Crystals Hao Tian,*,† Chengpeng Hu,† Xiangda Meng, Peng Tan, Zhongxiang Zhou,* Jun Li, and Bin Yang Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China ABSTRACT: A series of large-sized (maximum 16 × 16 × 20 mm3), high-quality K1−xNaxNbO3 (x = 0.118, 0.378, 0.462, 0.545, and 0.666) single crystals were successfully cultivated using the top-seeded solution growth method. The crystallization and structures of the K1−xNaxNbO3 single crystals were studied using first-principles calculations and X-ray diffraction, respectively. The segregation of the K1−xNaxNbO3 single crystals was investigated, which enabled precise control of the individual components of the crystals during growth. Excellent properties were obtained without annealing, including a low dielectric loss (minimum 0.2%), a saturated hysteresis loop with a low coercive field Ec, a large piezoelectric coefficient d33 (d33 = 161 pC/N when x = 0.378), and a low leakage current density J (10−6 A/cm2). These results indicated that the K1−xNaxNbO3 (x = 0.118, 0.378, 0.462, 0.545, and 0.666) crystals can be used as a high-quality, lead-free piezoelectric material.
1. INTRODUCTION Piezoelectric materials have been widely used in a variety of applications, including sensors, microelectromechanical systems, and ultrasonic transducers. However, these applications utilize lead-based materials, such as lead zirconate titanate (PZT) ceramics, because of their excellent piezoelectric properties.1−4 Because of the environmental and health concerns related to lead-based substances, a series of lead-free piezoelectric materials have attracted increasing attention in recent years. Potassium sodium niobate (K1−xNaxNbO3, referred to as KNN) materials are the most promising substitute for lead-based materials because of their good piezoelectric properties and high Curie temperatures compared with those of other lead-free materials such as barium titanate (BT), sodium bismuth titanate (BNT), and other compounds. In 2004, Saito et al. reported a major breakthrough in which the piezoelectric coefficient of KNN-based ceramics reached 416 pC/N.5 Since then, a number of intensive studies on ways to improve the piezoelectric properties of KNN-based materials have appeared in the literature.6−9 For the purpose of improving their dielectric and piezoelectric properties, sintering processes and element doping have been widely studied for their effects on the grain size, the temperature of the morphotropic phase boundary (MPB), and the Curie temperature of KNN materials.5 Nevertheless, the direct study of the nature of KNN is more difficult than similar studies of single crystals because of the presence of grain boundaries in ceramic materials.10 Recently, methods for growing KNN-based crystals have been greatly advanced, and each method has its own advantages. The most widely used methods for growing KNN single crystals are the flux and Bridgman methods.11−15 Although KNN crystals grown via the flux method have good quality and properties, these crystals are difficult to study © XXXX American Chemical Society
because of their small size. In 2007, Chen et al. successfully grew 0.95(K0.5Na0.5)NbO3−0.05LiNbO3 single crystals using the Bridgman method and obtained a piezoelectric coefficient of 405 pC/N; however, the oversaturated hysteresis loop at a frequency of 10 Hz indicated that the crystals contained oxygen vacancies and defects.15 The literature also contains numerous studies in which the solid-state single-crystal growth (SSCG) method was used.16−19 When KNN crystals were grown on a KTaO3 substrate, the maximum thickness was only 160 μm;16 however, the grown KNN crystals were porous, which led to difficulty in studying their properties. Davis et al. used the topseeded solution growth (TSSG) method to successfully prepare large-sized [001]-poled Lix(Na0.5K0.5)1−xNbO3 (x ≈ 0.02) single crystals with a high thickness electromechanical coupling coefficient (kt ≤ 0.7).20 Deng et al. investigated the dependence of the electrical properties of pure, large-sized KNN crystals on their orientation, which could be explained by domain engineering.10 However, few studies have discussed the segregation of crystals during growth. Elucidation of the process of segregation is important for growing crystal compositions with the best properties, especially when the potassium-tosodium is approximately 0.5:0.5. Furthermore, the growth of high-quality KNN crystals is necessary in order to study their ferroelectric properties, piezoelectric properties, and applications. In this work, a series of large-sized and high-quality pure K1−xNaxNbO3 crystals were grown via the TSSG method. The segregation, dielectric properties, piezoelectric properties, and Received: October 20, 2014 Revised: December 25, 2014
A
DOI: 10.1021/cg501554v Cryst. Growth Des. XXXX, XXX, XXX−XXX
Article
Crystal Growth & Design
which is larger than the KNN crystals reported in the literature.11−13 The crystals were transparent at temperatures above the Curie temperature (Figure 1f), and no cracks were apparent in the crystals. At room temperature, the crystals are opaque because of the presence of multidomains, which can obviously scatter the light; the crystals with a milk-white color contain few oxygen vacancies, which is the origin of the blue color.21,22 The crystals had round corners, and the surfaces of the crystals were (100) and (010), which could be calculated using first principles. We used a two-dimensional model to examine the relationship between the crystal growth morphology and the surface energy. The relationship between the crystal growth morphology and the growth rate can be simply derived from Figure 2,
ferroelectric properties of KNN crystals with different compositions were then investigated.
2. EXPERIMENTAL PROCEDURE The K1−xNaxNbO3 crystals were grown via the TSSG method. Powders of K2 CO3 (99.99%), Na2CO3 (99.99%), and Nb 2O5 (99.99%) were used as the raw materials and were weighed to obtain a composition of K1−xNaxNbO3; excess Na2CO3 and K2CO3 were added as a self-flux. These raw materials were mixed by being ballmilled with ethanol for 24 h and subsequently dried in an oven at 80 °C to volatilize the ethanol. The dried mixture was placed in a platinum crucible and calcined at 850 °C for 6 h to synthesize the KNN compound while simultaneously driving carbon dioxide from the reaction. The compound was melted in a medium-frequency induction furnace at 1230 °C, which is higher than the temperature of crystal growth, to eliminate the residual carbon dioxide. The KNN crystals were grown on a seed in the [001]c direction, which was cut from a high-quality crystal when the melt reached the crystallization temperature. During the crystal growth, the velocities of rotation and pulling of the crystals were ∼20 rpm and ∼4.0 mm per day, respectively. Finally, the crystals were cooled to room temperature at a rate of approximately 0.5 °C/min. The content of the crystals was determined by electron microprobe analysis (EPMA-1720, Shimadzu, Kyoto, Japan). The crystal structures were confirmed by X-ray diffraction (XRD) (XRD-6000, Shimadzu, Kyoto, Japan), and the crystals were oriented using a Laue X-ray machine before property measurements. The (001) surface of the crystals was painted with silver electrodes, and the dielectric properties of the crystals were measured as a function of temperature using an impedance−capacitance−resistance (LCR) meter (E4980A, Agilent Technologies, Santa Rosa, CA). The samples were immersed in silicon oil at 130 °C and poled in a DC field of 25 kV/cm for 30 min. The polarization vs electric field (P−E) hysteresis loops of the crystals were recorded using a ferroelectric test system (Precision Premier II, Radiant Technology, Inc., Albuquerque, NM) at 100 Hz, and the piezoelectric constant d33 was measured using a d33 meter (Zj-3A, Institute of Acoustics, Academic Sinica, Beijing, China). None of the crystals used for the measurements were heat-treated.
Figure 2. Schematic of the crystal growth rate.
which is a schematic showing the crystal growth rate. The specific form of the relationship can be expressed as eq 1
3. RESULTS AND DISCUSSION The as-grown KNN single crystals are shown in Figure 1. The maximum size of the as-grown crystals was 16 × 16 × 20 mm3,
L100 = L110
2
( )−1 R110 R100
2 −
R110 R100
(1)
where R100 and R110 represent the growth rate of the (100) and (110) surfaces and L100 and L110 represent the respective areas of the crystal surfaces. As shown in eq 1, the growth rates of the surfaces directly affected the crystallization of the material. The surface of the crystal growth perpendicular to the [001]c direction is the (100) surface when R110/R100 ≥ √2, the surface of the crystal growth perpendicular to the [001]c direction is the (110) surface when R110/R100 ≤ √2/2, and both the (100) and (110) surfaces appear when √2/2 < R110/ R100 < √2. According to the Hartman−Perdok theory,23 which can be used to predict the shape of a crystal, the growth rate can be obtained as follows att R hkl ∝ |Ehkl |
(2)
where Rhkl is the rate of growth of the (hkl) surface in the normal direction and Eatt hkl is the energy difference between the slice and the bulk of the crystal. From eq 2, we know that faces with larger |Eatt hkl| grow faster; thus, the crystal morphology is affected by the attachment energy. To obtain a KNN single crystal with a surface perpendicular to the [001]c direction, we employed the Castep module in Materials Studio (Accelrys,
Figure 1. Photographs of K1−xNaxNbO3 (x = 0.118, 0.378, 0.462, 0.545, and 0.666) single crystals grown via TSSG. The samples in images (a−e) were photographed at room temperature. The crystal in image (f) was photographed at 850 °C. B
DOI: 10.1021/cg501554v Cryst. Growth Des. XXXX, XXX, XXX−XXX
Article
Crystal Growth & Design
single crystals grown with 10 mol % of K2CO3 + Na2CO3 added as a mixed flux.
Inc., San Diego, CA) to compute the attachment energy of the (100) and (110) surfaces using first-principles calculations. We obtained the following results
Table 2. Growth Temperature and Components of KNN Crystals Prepared with Excess K2CO3 + Na2CO3 as a Mixed Flux
surf(100) surf(110) E KNbO = 1.067 J/m 2, E KNbO = 2.728 J/m 2 3 3 surf(100) surf(110) E NaNbO = 1.437 J/m 2, E NaNbO = 1.686 J/m 2 3 3
(3)
ratio of potassium to sodium in the melt
and surf(110) R KNbO 3 surf(100) R KNbO 3
= 2.557 >
2,
= 1.173
