Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 924∼928

Hydrothermal (K1−xNax)NbO3 Lead-free Piezoelectric Ceramics

Takafumi Maeda,∗ Norihito Takiguchi and Takeshi Morita

Graduate School of Frontier Sciences, The University of Tokyo,5-1-5 Kashiwano-ha, Kashiwa, Chiba 277-8563, Japan

Mutsuo Ishikawa

Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan

Tobias Hemsel

Mechatronics and Dynamics, University of Paderborn, Fuerstenallee 11, 33102 Paderborn, Germany

(Received 8 January 2010, in final form 8 July 2010)

As a lead-free piezoelectric ceramic, (K,Na)NbO3 is a promising material because of its goodpiezoelectric properties. In this study, (K1−xNax)NbO3 ceramics were synthesized from a mixtureof KNbO3 and NaNbO3 powders prepared by using the hydrothermal reaction. The hydrothermalreaction enables the production of high quality powders for the ceramic fabrication process. Toobtain (K1−xNax)NbO3 ceramics, the KNbO3 and the NaNbO3 powders were mixed and sinteredtogether. X-Ray diffraction analysis revealed that the solid solution ceramic (K1−xNax)NbO3 wasformed by the sintering process. The K/Na ratio in the (K1−xNax)NbO3 ceramic was optimizedfor the best piezoelectric properties. The optimized form was (K0.48Na0.52)NbO3, which showedthe following piezoelectric properties: k33 = 0.56 and d33 = 114 pC/N. In addition, the ferroelectricproperties Pr = 7.72 µC/cm2, Ec = 857 V/mm, and Tc = 420 ◦C were measured.

PACS numbers: 85.50.-n, 77.65.-jKeywords: Lead-free piezoelectric material, KNN, Hydrothermal methodDOI: 10.3938/jkps.57.924

I. INTRODUCTION

Lead-free piezoelectric ceramics have been studied[1–10] intensively in recent years. In particular, al-kaline niobate-based ceramics are considered as candi-date lead-free piezoelectric materials because of theirgood piezoelectric properties and high Curie tempera-ture. Among these materials, (K,Na)NbO3 is consideredto be a promising candidate for a lead-free piezoelectricceramic. To obtain the source powders for these ceram-ics, we proposed the hydrothermal method, and it wasverified that this method enabled the formation of high-quality powders [1–3]. Using such a powder, undopedpotassium-niobate ceramics were successfully obtained,and their piezoelectric performance was examined in ear-lier studies [1–4]. Usually, the solid-solution method isused to obtain these powders. However, potassium car-bonate, K2CO3, which is a potassium source for potas-sium niobate, is unstable and is difficult to weigh dueto its deliquescence. Moreover, to suppress the conduc-tivity, the stoichiometry between the potassium and theniobium in the ceramic needs to be strictly controlled.

∗E-mail: maeda@ems.k.u-tokyo.ac.jp

However, potassium atoms are easily evaporated duringthe calcination process. Therefore, an additional esti-mated quantity of potassium needs to be added duringpreparation using the solid-solution method.

In contrast, the hydrothermal method overcomes theseproblems in solid solutions of potassium-niobate-basedpowder by utilizing chemical reactions in a solution [1–4]. To synthesize potassium niobate-based powders, apotassium-hydroxide solution is used. The hydrothermalmethod utilizes the crystallization reaction from the so-lution, so that pure crystalline powder is obtained with-out any secondary phases. Therefore, the potassium toniobium ratio is automatically controlled to be unity.The simple process and the low reaction temperature,around 200 ◦C, are other advantages of the hydrother-mal method. These features enable the fabrication of anon-doped potassium niobate ceramic, which has beenthought to be quite difficult to obtain as a piezoelectricmaterial [1,2].

In this study, (K1−xNax)NbO3 ceramics were obtainedby sintering hydrothermal KNbO3 and NaNbO3 powdermixtures. By optimizing the elemental ratio x, piezo-electric properties with the highest values were obtainedwith x = 0.52 in (K1−xNax)NbO3.

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Hydrothermal (K1−xNax)NbO3 Lead-free Piezoelectric Ceramics – Takafumi Maeda et al. -925-

II. EXPERIMENTAL PROCEDURE

1. Potassium Niobate and Sodium Nio-bate Powders Obtained by Using the HydrothermalMethod

(K1−xNax)NbO3 ceramics were synthesized with twokinds of hydrothermal powders. As source materials forthe potassium niobate powder, 3.72 g of niobate oxidewas put into 70 ml of a 9-N KOH solution in a pressurevessel (Parr model 4748). After had been sealed the pres-sure vessel, it was placed in an oven pre-heated at 210◦C. The reaction time was 24 h. For the sodium-niobatepowders, 9-N NaOH was used instead of KOH. The otherreaction conditions were the same as there for potassiumniobate. The two powders were filtered with 0.45 µm fil-ter paper and dried for 1 h at 150 ◦C. During the filteringprocess, the powders were washed sufficiently with 1 Lof distilled water.

2. Sintering Process

To obtain the (K1−xNax)NbO3 ceramics, the KNbO3

and NaNbO3 powders were weighed and mixed in dis-tilled water to a molar ratio of KNbO3:NaNbO3 = 1-x:x(0.50 ≤ x ≤ 0.54). The mixed powders were filteredand dried for 1 h at 150 ◦C. After drying, they wereground with an alumina mortar and pestle, and sievedusing a 425 µm mesh. This powder was then pressed intodisks 10 mm in diameter and 2.0 mm in thickness. Theobtained disks were pressed with cold isostatic pressing(CIP) under 200 MPa, and they were sintered at 1100 ◦Cin air for 2 hours at a heating rate of 230 ◦C/h and sub-sequently cooled at a rate of 100 ◦C/h. The crystallinecharacteristics were examined by using X-ray diffraction(XRD: MiniFlex II Rigaku Corporation, Tokyo, Japan),and the density was measured by using Archimedes’ tech-nique 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 shapesand were polished with #2000 sandpaper for measur-ing their piezoelectric properties. The dimensions of thedisk-shaped specimens were φ8.60 mm × 1.30 mm, whichwere used for radial vibration modes (kp), dielectric mea-surements and P−E hysteresis measurements. For mea-suring the thickness vibration modes, stick-shaped spec-imens were used with dimensions of 4 mm × 1 mm × 1mm. Gold electrodes were deposited on each side of thedisk-shaped ceramic specimens by using a sputter coater(Sanyu Electron, Quick Coater SG-701). A conductiveelectrode (Dotite FA-705A Fujikura Kasei Co., Ltd.) and

Fig. 1. SEM micrographs of hydrothermal produced pow-ders: (a) KNbO3 and (b) NaNbO3.

Fig. 2. XRD patterns of the hydrothermal produced pow-ders: (a) KNbO3 and (b) NaNbO3.

lead were fired on the two sides of the stick-shaped ce-ramic specimens. Poling treatments were carried out us-ing a high voltage supply (Matsusada HARb-10P10) insilicone oil at 150 ◦C. An electric field of 2 kV/mm wasapplied in the thickness direction for 1 h, and the sam-ples were cooled for 30 min under the applied electricalfield.

III. RESULTS AND DISCUSSION

Figure 1 shows SEM micrographs of the obtained pow-ders. The powder sizes were approximately 1 µm forKNbO3 and 3 µm for NaNbO3. Figure 2 shows the XRDpattern for each powder. These results confirmed thatthe powders had no impurities or secondary phases, suchas niobium oxide or pyrochlore. Figure 3 shows a com-parison of the XRD patterns of the as-mixed powder andthe sintered ceramic. This figure indicates that in the

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Table 1. Piezoelectric properties of the (K0.48Na0.52)NbO3 ceramic.

Material kp k33 εT33/ε0 cE

33 (GPa) Qm (radial) Qm (thickness) d33 (pC/N)

(K0.48Na0.52)NbO3 0.34 0.56 207 45 183 74 114

Fig. 3. XRD patterns: (a) powder mixture and (b)(K0.5Na0.5)NbO3 ceramic.

Fig. 4. SEM micrograph of the surface of the (K,Na)NbO3

ceramic sintered at 1100 ◦C.

sintering process, the KNbO3 and the NaNbO3 powdersformed a solid solution of (K1−xNax)NbO3. Note thatthe three peaks around 22 of the powder XRD becametwo peaks after sintering. From the XRD pattern, thecrystal structure of the sintered ceramic was confirmedto be orthorhombic. SEM micrographs of the ceramicsverified that the grain sizes were 3 ∼ 20 µm, as shownin Fig. 4. The density of the sintered (K0.5Na0.5)NbO3

ceramic was measured to be 4.09 g/cm3(90.7%).Figures 5(a) and (b) show the admittance results for

the radial vibration mode and the thickness vibrationmode. The electromechanical coupling factors kp and k33

Fig. 5. Admittance characteristics of (a) radial and (b)thickness vibration mode.

were calculated using the resonant-antiresonant methodon the basis of IEEE standards [11]. The relative freepermittivity εT

33/ε0 was determined from the capacitancevalue at 1 kHz of the poled specimen. The stiffness cE

ij

was calculated from the resonant frequency. With theseparameters, the piezoelectric factors dijwere calculatedfrom the equation

dij = kij

√εT33/cE

ij . (1)

Mason’s equivalent circuit was fitted to the admittancecurve, and the mechanical quality factor Qmwas calcu-lated.

Figure 6 indicates dependences of the the piezoelectricproperties on the x = Na/(Na+K) ratio. All parame-ters show maximum values when the Na/(Na+K) ratiox is 0.52. Therefore, the optimum chemical componentis (K0.48Na0.52)NbO3. The dielectric permittivity as afunction of the temperature was measured at 1 kHz attemperatures from room temperature to 600 ◦C by us-ing an LCR meter (NF, ZM2353). Figure 7 shows theresults of the permittivity change for (K0.48Na0.52)NbO3.The Curie temperature (Tc) of (K0.48Na0.52)NbO3 wasapproximately 420 ◦C.

Hydrothermal (K1−xNax)NbO3 Lead-free Piezoelectric Ceramics – Takafumi Maeda et al. -927-

Fig. 6. Piezoelectric properties of (K1−xNax)NbO3 ceram-ics.

Fig. 7. Temperature dependence of the dielectric constantfor the (K0.48Na0.52)NbO3 ceramic.

The D-E hysteresis curve of (K0.48Na0.52)NbO3 wasmeasured at room temperature by using a ferroelectrictester (Precision LC, Nippon FerroTechnology Corpora-tion, Japan), as shown in Fig. 8. The driving frequencywas 100 Hz. Based on the saturated hysteresis loop, theferroelectricity was, indeed, observed, and the remnantpolarization Pr and the coercive field Ec values weremeasured to be 7.72 µC/cm2 and 857 V/mm, respec-

Fig. 8. P −E hysteresis loop of (K0.48Na0.52)NbO3 at 100Hz.

tively.Table 1 summarizes the piezoelectric properties of the

(K0.48Na0.52)NbO3 ceramic. The measured piezoelectricproperties of the (K0.48Na0.52)NbO3 ceramic were as fol-lows: the electromechanical coupling factors kp, and k33

were 0.34 and 0.56, respectively, the relative free permit-tivity εT

33/ε0 was 207, the stiffness cE33 was 45 GPa, the

mechanical quality factors Qm were 183 (radial mode)and 74 (Thickness mode), and the piezoelectric factord33 was 114 pC/N.

IV. CONCLUSIONS

In this study, potassium-niobate and sodium-niobatepowders were synthesized using the hydrothermalmethod. The two powders were mixed and sinteredinto a solid solution of (K1−xNax)NbO3. This solid-solution’s properties were confirmed by using XRD mea-surements. By optimizing the chemical composition to(K0.48Na0.52)NbO3,we obtained the following piezoelec-tric properties k33 = 0.56, εT

33/ε0 = 207, cE33 = 45 GPa,

d33 = 114 pm/V and Qm = 74 (thickness). In addition,the following ferroelectric properties were measured: Pr

= 7.72 µC/cm2, Ec = 857 V/mm, and Tc = 420 ◦C.

ACKNOWLEDGMENTS

This research was supported by the New Energyand Industrial Technology Development Organization(NEDO). The kind support of Furuuchi Chemical Co.Ltd. and that of Taiatsu Techno Co. Ltd. are highlyappreciated.

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