Hydrothermal (K 1 x Na x )NbO 3 Lead-free Piezoelectric Ceramics

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1 Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp Hydrothermal (K 1 x Na x )NbO 3 Lead-free Piezoelectric Ceramics Takafumi Maeda, Norihito Takiguchi and Takeshi Morita Graduate School of Frontier Sciences, The University of Tokyo, Kashiwano-ha, Kashiwa, Chiba , Japan Mutsuo Ishikawa Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama , Japan Tobias Hemsel Mechatronics and Dynamics, University of Paderborn, Fuerstenallee 11, Paderborn, Germany (Received 8 January 2010, in final form 8 July 2010) As a lead-free piezoelectric ceramic, (K,Na)NbO 3 is a promising material because of its good piezoelectric properties. In this study, (K 1 xna x)nbo 3 ceramics were synthesized from a mixture of KNbO 3 and NaNbO 3 powders prepared by using the hydrothermal reaction. The hydrothermal reaction enables the production of high quality powders for the ceramic fabrication process. To obtain (K 1 xna x)nbo 3 ceramics, the KNbO 3 and the NaNbO 3 powders were mixed and sintered together. X-Ray diffraction analysis revealed that the solid solution ceramic (K 1 xna x)nbo 3 was formed by the sintering process. The K/Na ratio in the (K 1 xna x)nbo 3 ceramic was optimized for the best piezoelectric properties. The optimized form was (K 0.48Na 0.52)NbO 3, which showed the following piezoelectric properties: k 33 = 0.56 and d 33 = 114 pc/n. In addition, the ferroelectric properties P r = 7.72 µc/cm 2, E c = 857 V/mm, and T c = 420 C were measured. PACS numbers: n, j Keywords: Lead-free piezoelectric material, KNN, Hydrothermal method DOI: /jkps I. INTRODUCTION Lead-free piezoelectric ceramics have been studied [1 10] intensively in recent years. In particular, alkaline niobate-based ceramics are considered as candidate lead-free piezoelectric materials because of their good piezoelectric properties and high Curie temperature. Among these materials, (K,Na)NbO 3 is considered to be a promising candidate for a lead-free piezoelectric ceramic. To obtain the source powders for these ceramics, we proposed the hydrothermal method, and it was verified that this method enabled the formation of highquality powders [1 3]. Using such a powder, undoped potassium-niobate ceramics were successfully obtained, and their piezoelectric performance was examined in earlier studies [1 4]. Usually, the solid-solution method is used to obtain these powders. However, potassium carbonate, K 2 CO 3, which is a potassium source for potassium niobate, is unstable and is difficult to weigh due to its deliquescence. Moreover, to suppress the conductivity, the stoichiometry between the potassium and the niobium in the ceramic needs to be strictly controlled. maeda@ems.k.u-tokyo.ac.jp However, potassium atoms are easily evaporated during the calcination process. Therefore, an additional estimated quantity of potassium needs to be added during preparation using the solid-solution method. In contrast, the hydrothermal method overcomes these problems in solid solutions of potassium-niobate-based powder by utilizing chemical reactions in a solution [1 4]. To synthesize potassium niobate-based powders, a potassium-hydroxide solution is used. The hydrothermal method utilizes the crystallization reaction from the solution, so that pure crystalline powder is obtained without any secondary phases. Therefore, the potassium to niobium ratio is automatically controlled to be unity. The simple process and the low reaction temperature, around 200 C, are other advantages of the hydrothermal method. These features enable the fabrication of a non-doped potassium niobate ceramic, which has been thought to be quite difficult to obtain as a piezoelectric material [1,2]. In this study, (K 1 x Na x )NbO 3 ceramics were obtained by sintering hydrothermal KNbO 3 and NaNbO 3 powder mixtures. By optimizing the elemental ratio x, piezoelectric properties with the highest values were obtained with x = 0.52 in (K 1 x Na x )NbO 3.

2 Hydrothermal (K 1 xna x)nbo 3 Lead-free Piezoelectric Ceramics Takafumi Maeda et al II. EXPERIMENTAL PROCEDURE 1. Potassium Niobate and Sodium Niobate Powders Obtained by Using the Hydrothermal Method (K 1 x Na x )NbO 3 ceramics were synthesized with two kinds of hydrothermal powders. As source materials for the potassium niobate powder, 3.72 g of niobate oxide was put into 70 ml of a 9-N KOH solution in a pressure vessel (Parr model 4748). After had been sealed the pressure vessel, it was placed in an oven pre-heated at 210 C. The reaction time was 24 h. For the sodium-niobate powders, 9-N NaOH was used instead of KOH. The other reaction conditions were the same as there for potassium niobate. The two powders were filtered with 0.45 µm filter paper and dried for 1 h at 150 C. During the filtering process, the powders were washed sufficiently with 1 L of distilled water. Fig. 1. SEM micrographs of hydrothermal produced powders: (a) KNbO 3 and (b) NaNbO Sintering Process To obtain the (K 1 x Na x )NbO 3 ceramics, the KNbO 3 and NaNbO 3 powders were weighed and mixed in distilled water to a molar ratio of KNbO 3 :NaNbO 3 = 1-x:x (0.50 x 0.54). The mixed powders were filtered and dried for 1 h at 150 C. After drying, they were ground with an alumina mortar and pestle, and sieved using a 425 µm mesh. This powder was then pressed into disks 10 mm in diameter and 2.0 mm in thickness. The obtained disks were pressed with cold isostatic pressing (CIP) under 200 MPa, and they were sintered at 1100 C in air for 2 hours at a heating rate of 230 C/h and subsequently cooled at a rate of 100 C/h. The crystalline characteristics were examined by using X-ray diffraction (XRD: MiniFlex II Rigaku Corporation, Tokyo, Japan), and the density was measured by using Archimedes technique with a density meter (Alfamirage SD-200L). 3. Measuring the Electrical Properties The sintered ceramics were cut using a diamond cutter (Musashino Denshi MPC-130) into appropriate shapes and were polished with #2000 sandpaper for measuring their piezoelectric properties. The dimensions of the disk-shaped specimens were φ8.60 mm 1.30 mm, which were used for radial vibration modes (k p ), dielectric measurements and P E hysteresis measurements. For measuring the thickness vibration modes, stick-shaped specimens were used with dimensions of 4 mm 1 mm 1 mm. Gold electrodes were deposited on each side of the disk-shaped ceramic specimens by using a sputter coater (Sanyu Electron, Quick Coater SG-701). A conductive electrode (Dotite FA-705A Fujikura Kasei Co., Ltd.) and Fig. 2. XRD patterns of the hydrothermal produced powders: (a) KNbO 3 and (b) NaNbO 3. lead were fired on the two sides of the stick-shaped ceramic specimens. Poling treatments were carried out using a high voltage supply (Matsusada HARb-10P10) in silicone oil at 150 C. An electric field of 2 kv/mm was applied in the thickness direction for 1 h, and the samples were cooled for 30 min under the applied electrical field. III. RESULTS AND DISCUSSION Figure 1 shows SEM micrographs of the obtained powders. The powder sizes were approximately 1 µm for KNbO 3 and 3 µm for NaNbO 3. Figure 2 shows the XRD pattern for each powder. These results confirmed that the powders had no impurities or secondary phases, such as niobium oxide or pyrochlore. Figure 3 shows a comparison of the XRD patterns of the as-mixed powder and the sintered ceramic. This figure indicates that in the

3 -926- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010 Table 1. Piezoelectric properties of the (K 0.48Na 0.52)NbO 3 ceramic. Material k p k 33 ε T 33/ε 0 c E 33 (GPa) Q m (radial) Q m (thickness) d 33 (pc/n) (K 0.48Na 0.52)NbO Fig. 3. XRD patterns: (a) powder mixture and (b) (K 0.5Na 0.5)NbO 3 ceramic. Fig. 5. Admittance characteristics of (a) radial and (b) thickness vibration mode. were calculated using the resonant-antiresonant method on the basis of IEEE standards [11]. The relative free permittivity ɛ T 33/ɛ 0 was determined from the capacitance value at 1 khz of the poled specimen. The stiffness c E ij was calculated from the resonant frequency. With these parameters, the piezoelectric factors d ij were calculated from the equation Fig. 4. SEM micrograph of the surface of the (K,Na)NbO 3 ceramic sintered at 1100 C. sintering process, the KNbO 3 and the NaNbO 3 powders formed a solid solution of (K 1 x Na x )NbO 3. Note that the three peaks around 22 of the powder XRD became two peaks after sintering. From the XRD pattern, the crystal structure of the sintered ceramic was confirmed to be orthorhombic. SEM micrographs of the ceramics verified that the grain sizes were 3 20 µm, as shown in Fig. 4. The density of the sintered (K 0.5 Na 0.5 )NbO 3 ceramic was measured to be 4.09 g/cm 3 (90.7%). Figures 5(a) and (b) show the admittance results for the radial vibration mode and the thickness vibration mode. The electromechanical coupling factors k p and k 33 d ij = k ij ε T 33 /ce ij. (1) Mason s equivalent circuit was fitted to the admittance curve, and the mechanical quality factor Q m was calculated. Figure 6 indicates dependences of the the piezoelectric properties on the x = Na/(Na+K) ratio. All parameters show maximum values when the Na/(Na+K) ratio x is Therefore, the optimum chemical component is (K 0.48 Na 0.52 )NbO 3. The dielectric permittivity as a function of the temperature was measured at 1 khz at temperatures from room temperature to 600 C by using an LCR meter (NF, ZM2353). Figure 7 shows the results of the permittivity change for (K 0.48 Na 0.52 )NbO 3. The Curie temperature (T c ) of (K 0.48 Na 0.52 )NbO 3 was approximately 420 C.

4 Hydrothermal (K 1 xna x)nbo 3 Lead-free Piezoelectric Ceramics Takafumi Maeda et al Fig. 8. P E hysteresis loop of (K 0.48Na 0.52)NbO 3 at 100 Hz. tively. Table 1 summarizes the piezoelectric properties of the (K 0.48 Na 0.52 )NbO 3 ceramic. The measured piezoelectric properties of the (K 0.48 Na 0.52 )NbO 3 ceramic were as follows: the electromechanical coupling factors k p, and k 33 were 0.34 and 0.56, respectively, the relative free permittivity ɛ T 33/ɛ 0 was 207, the stiffness c E 33 was 45 GPa, the mechanical quality factors Q m were 183 (radial mode) and 74 (Thickness mode), and the piezoelectric factor d 33 was 114 pc/n. Fig. 6. Piezoelectric properties of (K 1 xna x)nbo 3 ceramics. IV. CONCLUSIONS In this study, potassium-niobate and sodium-niobate powders were synthesized using the hydrothermal method. The two powders were mixed and sintered into a solid solution of (K 1 x Na x )NbO 3. This solidsolution s properties were confirmed by using XRD measurements. By optimizing the chemical composition to (K 0.48 Na 0.52 )NbO 3,we obtained the following piezoelectric properties k 33 = 0.56, ɛ T 33/ɛ 0 = 207, c E 33 = 45 GPa, d 33 = 114 pm/v and Q m = 74 (thickness). In addition, the following ferroelectric properties were measured: P r = 7.72 µc/cm 2, E c = 857 V/mm, and T c = 420 C. ACKNOWLEDGMENTS Fig. 7. Temperature dependence of the dielectric constant for the (K 0.48Na 0.52)NbO 3 ceramic. The D-E hysteresis curve of (K 0.48 Na 0.52 )NbO 3 was measured at room temperature by using a ferroelectric tester (Precision LC, Nippon FerroTechnology Corporation, Japan), as shown in Fig. 8. The driving frequency was 100 Hz. Based on the saturated hysteresis loop, the ferroelectricity was, indeed, observed, and the remnant polarization P r and the coercive field E c values were measured to be 7.72 µc/cm 2 and 857 V/mm, respec- This research was supported by the New Energy and Industrial Technology Development Organization (NEDO). The kind support of Furuuchi Chemical Co. Ltd. and that of Taiatsu Techno Co. Ltd. are highly appreciated. REFERENCES [1] M. Ishikawa, N. Takiguchi, H. Hosaka and T. Morita, Jpn. J. Appl. Phys. 47, 3824 (2008).

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