Piezoelectric Properties of Li-Doped (K0:48Na0:52)NbO3 Ceramics Synthesized

Using Hydrothermally-Derived KNbO3 and NaNbO3 Fine Powders

Takafumi Maeda1�, Tobias Hemsel2, and Takeshi Morita1

1Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Chiba 277-8563, Japan2Mechatronics and Dynamics, University of Paderborn, D-33098 Paderborn, Germany

Received November 4, 2011; revised May 7, 2012; accepted May 12, 2012; published online September 20, 2012

[Lix (Na0:52K0:48)1�x ]NbO3 (0 � x � 0:091) ceramics were synthesized using hydrothermal powders and the lithium doping content was controlled

to optimize their piezoelectric properties. The raw KNbO3 and NaNbO3 powders were obtained separately by a hydrothermal method and LiNbO3

powders were prepared by milling a commercial LiNbO3 single crystal. These powders were mixed with ethanol at a molar ratio

LiNbO3 : ðNa0:52K0:48ÞNbO3 ¼ x : 1� x . The synthesized powders were sintered at 1060–1120 �C for 2 h. We succeeded in obtaining highly

dense [Lix (Na0:52K0:48)1�x ]NbO3 ceramics using hydrothermal powder. The X-ray diffraction patterns revealed that the crystal phase changed

from orthorhombic to tetragonal at around x ¼ 0:06. At this morphotropic phase boundary (MPB), the c=a ratio changed from 1.016 to 1.024 and

the highest piezoelectric constant was obtained with the chemical component of [Li0:065(K0:48Na0:52)0:935]NbO3. The obtained piezoelectric

properties were as follows: k33 ¼ 0:51, "T33="0 ¼ 836, cE33 ¼ 46GPa, d33 ¼ 203 pC/N, and Tc ¼ 482 �C.

# 2012 The Japan Society of Applied Physics

1. Introduction

Alkaline niobate-based ceramics are promising candidatesfor lead-free piezoelectric ceramics and have been investi-gated intensively.1–18) It is well known that Li, Ta, andSb dopings are effective for improving the piezoelectricproperties of KNbO3–NaNbO3 (KNN) systems.7–12) Saitoet al. succeeded in realizing high-piezoelectric-constant ce-ramics by the texturing technique,7) and Guo et al. discover-ed the morphotropic phase boundary of (Na0:5K0:5)NbO3–LiNbO3 ceramics whose piezoelectric constant d33 reached235 pC/N.8) As a practical application, Li-doped KNNceramics (KNLN) were used for a share mode ultrasonicmotor12) by Enzhu et al. In these studies, the high-piezo-electric properties were obtained near the morphotropicphase boundary (MPB).

On the other hand, many problems exist in alkalineniobate ceramics powder fabrication with the conventionalsolid-phase process. For example, during a lengthy calcina-tion process, the Kþ ions evaporate, which results in a largeleakage current. To overcome this evaporation, a potassiumsource should be used in order to compensate for thisevaporation; however, the instability of K2CO3 becomes anobstacle, especially in the weighing process. In our previousstudy, the hydrothermal method could overcome theseproblems6) because this method utilizes a solution reactionat high temperatures and pressures, and because Nb2O5 andKOH are used to synthesize KN powder rather than K2CO3.The hydrothermal method utilizes the crystallization processfrom the solution; thus, the pure crystal powders areobtained without secondary phases. Therefore, the potas-sium-to-niobium ratio is automatically controlled to be one.A simple process and a low reaction temperature of around200 �C are other advantages of the hydrothermal method.These features enabled the fabrication of a nondopedpotassium niobate ceramic, which has been considered tobe difficult to obtain as a piezoelectric material. Further-more, the optimum chemical composition between potas-sium and sodium was verified to be (K0:48Na0:52)NbO3

by using the hydrothermal powder.13–17) This (K0:48Na0:52)-NbO3 ceramic has difficulty in obtaining a high density by a

conventional method.1–3) In our present study, highly dense(K0:48Na0:52)NbO3 ceramics were prepared by a hydrother-mal method.17)

In this study, Li was doped to this chemical compositionto improve piezoelectric properties. By X-ray diffractionanalysis, crystal structures versus lithium doping concentra-tion were investigated, and their properties are referred to astheir piezoelectric and ferroelectric properties.

2. Experimental Procedure

2.1 Hydrothermal synthesis of KNbO3 and NaNbO3

powders

As the raw materials for the alkaline niobate powders, NaOHpellets (97.0%, Kanto-Kagaku), KOH pellets (85.0%,Wako), and Nb2O5 powders (99.95%, Kanto-Kagaku) wereused for the hydrothermal method. For the NaNbO3

powders, 37.2 g of Nb2O5 and 70mL of 9N NaOH weremixed in a pressure vessel (Parr 4748). The pressure vesselwas placed in a preheated oven at 210 �C and the reactiontime was 6 h. To prepare the KNbO3 powders, 140mL of8.8N KOH and 9.18 g of Nb2O5 were mixed together inthe pressure vessel (Taiatsu Techno TAF-SR Type). Thehydrothermal process was carried out for 12 h at 210 �C inthe preheated oven. After hydrothermal processing, eachpowder was filtered with Teflon filter paper (hole size:0.45 �m) and washed thoroughly with 1 L of distilled water.After drying these powders for 1 h at 130 �C, these pow-ders were weighed to be 45.0 g for NaNbO3 and 11.2 gfor KNbO3. A neutralization process is important for thehigh-resistivity performance of the sintered ceramics17) andinvolves the removal of ions, such as Kþ and Naþ, from thesurface of the obtained powders. For the neutralizationprocess, each powder was put into 300mL of distilled water,and 0.01mol/L HCl (high-grade Kanto-Kagaku) solutionwas added until the solution reached a pH of 7. The obtainedpowders were filtered again with filter paper of similar sizeand washed with 1 L of distilled water. The neutralizedpowders were then dried for 1 h at 130 �C. The crystalcharacteristics of the powders were analyzed using an X-raydiffraction (XRD) meter (Rigaku Miniflex II), and to observetheir microstructures, scanning electron microscopy (SEM;JEOL JSM 5310LV) was performed. The particle sizedistributions were measured with a diffraction particle�E-mail address: morita@k.u-tokyo.ac.jp

Japanese Journal of Applied Physics 51 (2012) 09MD08

09MD08-1 # 2012 The Japan Society of Applied Physics

REGULAR PAPERhttp://dx.doi.org/10.1143/JJAP.51.09MD08

analyzer (Shimadzu SALD-2100). To break the aggregates,the powders were treated ultrasonically for 40min beforemeasurement.

2.2 Sintering process

To synthesize [Lix(Na0:52K0:48)1�x]NbO3 ceramics, 4.000 gof KNbO3, 3.946 g of NaNbO3, and appropriate amountsof LiNbO3 powders were weighed and placed in a 500mLpolyethylene jar. The LiNbO3 powder was prepared bycrushing the commercial LiNbO3 (Yamaju-CeramicsLiNbO3 wafer) single crystal using alumina mortar andpestle. The mixed powders were ball-milled for 12 h with200mL of ethanol using one hundred zirconia balls(diameter: 10mm) and 100 g of zirconia balls (diameter:2mm). After ball milling, the mixed powder was filteredwith Teflon filter paper (hole size: 0.45 �m) and dried. Thepowders were uniaxially pressed into disks that were 10mmin diameter and 2mm in thickness, which were then pressedby cold isostatic pressing (CIP) at 200MPa. The obtaineddisks were sintered at temperatures between 1075 and1125 �C for 2 h using a tubular furnace (Yamada Denki TSR-430). Before reaching the sintering temperature, the sampleswere soaked at 600 �C for 4 h. The heating and coolingrates were 150 and 100 �C/h, respectively. The density wasmeasured by the Archimedes technique using a densitymeter (Alfamirage SD-200). The optimal sintering tempera-ture for each composition was determined in order to realizethe maximum density from various sintering temperatures.

2.3 Piezoelectric properties

In order to measure the longitudinal piezoelectric properties,the disk-shaped samples were cut using a diamond cutter(Musashino Denshi MPC-130) and their surfaces werepolished with sandpaper (#2000) into appropriate shapeswhose dimensions were 5:0� 1:3� 1:3mm3. Poling treat-ments were carried out using a high-voltage supply device(Matsusada HARb-10P10) at 3.0 kV/mm in 100 �C siliconeoil for 20min. The electromechanical coupling factor k33was calculated by the resonant–antiresonant method. Therelative free permittivity "T33="0 was determined from thecapacitance value at 1 kHz of the poled specimen. Thestiffness cE33 was calculated from the resonant frequency.With these parameters, the piezoelectric factor d33 wascalculated according to the IEEE standard.19) To measurethe dielectric properties, the sintered ceramic disks (dia-meter: around 8.5mm; thickness: 1.3mm) were polishedwith abrasive paper (#2000) and the gold electrodes weredeposited on each side using a sputtering coater (SanyuElectron Quick Coater SG-701). The dielectric permittivityand dielectric loss tan � were measured at 1 kHz fromroom temperature to 550 �C in a muffle furnace (YamadaDenki Y-1218-P) using an inductance–capacitance–resis-tance (LCR) meter (NF ZM2353) controlled by a personalcomputer.

3. Results and Discussion

Figures 1(a) and 1(b) show the SEM micrographs of theobtained powders. Figure 1(c) shows the particle sizedistributions of KNbO3 and NaNbO3. The average particlesizes were 1 �m for the KNbO3 powder and 2 �m for theNaNbO3 powder. Figure 2 shows the XRD patterns of the

powders. From the results, it was confirmed that the powdershad no impurities or secondary phases.

Figure 3(a) shows the XRD patterns of the [Lix-(Na0:52K0:48)1�x]NbO3 ceramics with various x chemicalcomponents. The strong peaks are labeled with the Millerindices of the KNN phase with the perovskite structure.Figure 3(b) shows the lattice parameters a and c, which werecalculated using the (001), (100), (002), and (200) peaks.When the chemical component x reached 0.06, the crystalphase changed from orthorhombic to tetragonal as the c=aratio increased from 1.016 to 1.024.8) From this result, weconsidered that this chemical component corresponded tothe MPB.

Figure 4 shows the comparison between the SEM imagesof the [Li0:065(Na0:52K0:48)0:935]NbO3 and (Na0:52K0:48)NbO3

0

5

10

15

0.1 1 10 100

Dis

trib

utio

n (%

)

Diameter (μm)

KNbO3

NaNbO3

(c)

Fig. 1. SEM micrographs of the (a) KNbO3 and (b) NaNbO3 powders.

(c) Grain size distributions of KNbO3 and NaNbO3 powders.

(022

)(3

11)

(131

)(1

12)

(130

)

20 30 40 50 60 70

Inte

nsity

(ar

b. u

nit)

2θ (deg.)

(110

)

(111

)

(021

)

(220

)(0

02)

(221

)

KNbO3 Powder

(400

) (222

)

(020

)(2

00)

(a)

(004

)

(202

)

(001

)

20 30 40 50 60 702θ (deg.)

(110

) (0

04)

(114

) (2

00)

(024

)

(122

)

(220

)

(121

) (0

23)

(118

)

(314

) (2

08)

(138

)

NaNbO3 Powder

(020

)

(122

)

(204

)

(008

)

(224

)

(134

)(1

33)

Inte

nsity

(ar

b. u

nit)

(b)

Fig. 2. XRD patterns of the (a) KNbO3 and (b) NaNbO3 powders.

T. Maeda et al.Jpn. J. Appl. Phys. 51 (2012) 09MD08

09MD08-2 # 2012 The Japan Society of Applied Physics

ceramic surface. The optimum sintering temperatures of[Li0:065(Na0:52K0:48)0:935]NbO3 and (Na0:52K0:48)NbO3 ce-ramics were 1085 and 1125 �C, respectively. According toFig. 4(a), large grain appeared between small grains. In thisstudy, we obtained highly dense [Lix(Na0:52K0:48)1�x]NbO3

ceramics of 97.8% at a theoretical density of 4.51 g/cm3

using hydrothermal powder.Figure 5 shows the admittance results for the longitudinal

vibration mode of the [Li0:065(K0:48Na0:52)0:935]NbO3 cera-mics. The minimum phase reached �60:5� and k33 of 0.51was obtained. Figure 6 shows the piezoelectric properties of[Lix(Na0:52K0:48)1�x]NbO3 ceramics. The piezoelectric con-stant d33 value reached a maximum at x ¼ 0:065, whosechemical component is close to the MPB found in XRDmeasurements.

The permittivity and tan � changes for [Li0:065-(Na0:52K0:48)0:935]NbO3 are shown in Fig. 7(a). Therewere two phase transitions, from orthorhombic to tetragonaland from tetragonal to cubic in the case of [Li0:065-(Na0:52K0:48)0:935]NbO3. Figure 7(b) indicates the phasediagram of [Lix(Na0:52K0:48)1�x]NbO3. The Curie tempera-ture (Tc) of [Li0:065(Na0:52K0:48)0:935]NbO3 was 480 �C,

which was 60 �C higher than that of non doped [K0:48Na0:52]-NbO3.

16)

Table I shows the properties of the obtained ceramics.By comparing the results with those of non doped(K0:48Na0:52)NbO3, the piezoelectric constant d33 and"T33="0 were found to improve. However, the dielectric losstangent tan � was worse with LiNbO3 doping. To overcomethis problem, other doping materials, such as Mn,12) arebeing examined.

4. Conclusions

In this study, [Lix(Na0:52K0:48)1�x]NbO3 ceramics weresynthesized by the hydrothermal method with NaNbO3 andKNbO3. Lithium doping was effective in obtaining excellentpiezoelectric properties. The results were as follows: (1) Thehydrothermal method enabled the synthesis of high-quality

(a)

20 30 40 50 60 70

Inte

nsity

(ar

b. u

nit)

2θ (deg.)

(001

)

(200

)

(002

)

x=0.091x=0.083x=0.074x=0.065x=0.048x=0.029x=0

(100

)

(b)

3.93.95

44.054.1

1.0121.0161.021.024

0 0.02 0.04 0.06 0.08 0.1

c a

c/a

Lat

tice

cons

tant

(Å

)

c/a

x

Fig. 3. (Color online) (a) XRD pattern of the [Lix(Na0:52K0:48)1�x]NbO3

ceramic. (b) Lattice constants of a-axis, c-axis, and a=c ratio of the

[Lix(Na0:52K0:48)1�x]NbO3 ceramic.

Fig. 4. SEM images of the (a) [Li0:065(K0:48Na0:52)0:935]NbO3 and

(b) (K0:48Na0:52)NbO3 ceramic surfaces.

0.1

1

10

100

-100-75-50-25

0255075100

350 370 390 410 430 450

Adm

ittan

ce (

μS)

Phas

e (d

eg.)

Frequency (kHz)

Fig. 5. (Color online) Admittance characteristics for longitudinal

vibration mode of [Li0:065(Na0:52K0:48)0:935]NbO3.

0

40

80

1060

1100

1140

0 0.02 0.04 0.06 0.08 0.1

Qm

Sint

erin

g Te

mp.

(°C

)x

400

600

800

0

10

20

ε 33/ε

0

tan

δ (%

)

0.3

0.4

0.5

0.6

100

150

200

k 33

d 33 (

pC/N

)

120

1000

Fig. 6. (Color online) Piezoelectric properties of

[Lix(Na0:52K0:48)1�x]NbO3.

T. Maeda et al.Jpn. J. Appl. Phys. 51 (2012) 09MD08

09MD08-3 # 2012 The Japan Society of Applied Physics

powders; (2) Highly dense [Lix(Na0:52K0:48)1�x]NbO3

ceramics were obtained using hydrothermal powder; (3) Aphase transition from orthorhombic to tetragonal at aroundx ¼ 0:06 was found and the piezoelectric properties of theceramics were improved. However, the tan � of [Lix-(Na0:52K0:48)1�x]NbO3 was worse; (4) A high Curietemperature of 482 �C was realized by Li doping.

The measured piezoelectric properties of the [Li0:065-(K0:48Na0:52)0:935]NbO3 ceramics were as follows: theelectromechanical coupling factors k31 and k33, the relativefree permittivity "T33="0, the piezoelectric factors d33, themechanical quality factor Qm (longitudinal), and the Curietemperature Tc, were 0.51, 836 and 203 pC/N, 29, and482 �C, respectively.

Acknowledgements

This research was supported by the New Energy andIndustrial Technology Development Organization (NEDO)and the Japan Society for the Promotion of Science (JSPS).

1) L. Egerton and D. M. Dillon: J. Am. Ceram. Soc. 42 (1959) 438.

2) R. E. Jaeger and L. Egerton: J. Am. Ceram. Soc. 45 (1962) 209.

3) V. J. Tennery and K. W. Hang: J. Appl. Phys. 39 (1968) 4749.

4) H. Birol, D. Damajanovic, and N. Setter: J. Eur. Ceram. Soc. 26 (2006)

861.

5) B. Malic, J. Bernard, A. Bencan, and M. Kosec: J. Eur. Ceram. Soc. 28

(2008) 1191.

6) K. Kakimoto, I. Masuda, and H. Ohsato: J. Eur. Ceram. Soc. 25 (2005)

2719.

7) Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T.

Nagaya, and M. Nakamura: Nature 432 (2004) 84.

8) Y. Guo, K. Kakimoto, and H. Ohsato: Appl. Phys. Lett. 85 (2004) 4121.

9) Y. Guo, K. Kakimoto, and H. Ohsato: Mater. Lett. 59 (2005) 241.

10) E. Hollenstein, M. Davis, D. Damjanovic, and N. Setter: Appl. Phys. Lett.

87 (2005) 182905.

11) Y. Saito and H. Takao: Ferroelectrics 338 (2006) 17.

12) E. Li, H. Kakemoto, T. Hoshina, and T. Tsurumi: Jpn. J. Appl. Phys. 47

(2008) 7702.

13) M. Ishikawa, N. Takiguchi, H. Hosaka, and T. Morita: Jpn. J. Appl. Phys.

47 (2008) 3824.

14) M. Ishikawa, Y. Kadota, N. Takiguchi, H. Hosaka, and T. Morita: Jpn. J.

Appl. Phys. 47 (2008) 7673.

15) T. Maeda, N. Takiguchi, M. Ishikawa, T. Hemsel, and T. Morita: Mater.

Lett. 64 (2010) 125.

16) T. Maeda, N. Takiguchi, and T. Morita: J. Korean Phys. Soc. 57 (2010)

924.

17) T. Maeda, T. Hemsel, and T. Morita: Jpn. J. Appl. Phys. 50 (2011)

07HC01.

18) K. Matsumoto, Y. Hiruma, H. Nagata, and T. Takenaka: Jpn. J. Appl. Phys.

45 (2006) 4479.

19) ANSI/IEEE Standard 1976–1987: IEEE Trans. Ultrason. Ferroelectr. Freq.

Control 43 (1996) 717.

Table I. Piezoelectric and ferroelectric properties of the (K0:48Na0:52)NbO3 and [Li0:065(K0:48Na0:52)0:935]NbO3 ceramics.

Material k33 "T33="0tan �

(%)

cE33(GPa)

Qm

(longitudinal)

d33(pC/N)

Tc(�C)

�

(g/cm3)

(K0:48Na0:52)NbO317) 0.55 446 2.7 73 53 130 420

4.43

98.20%

[(K0:48Na0:52)0:935Li0:065]NbO3 0.51 837 7.8 46 29 203 4824.41

97.80%

0

2

4

0 100 200 300 400 500

x=0x=0.057x=0.065x=0.083

102

103

104

tan

δTemperature (°C)

ε r

Tc

Tot

Tot

Tc

(a) (b)

100

200

300

400

500

600

0 0.02 0.04 0.06 0.08 0.1

Tem

pera

ture

(°C

)

x

Orthorhombic

Tetragonal

Cubic

Fig. 7. (Color online) (a) Temperature dependence of the dielectric constant for the [Lix(Na0:52K0:48)1�x]NbO3 ceramic. (b) Curie temperature (Tc) andortho tetra transition temperature for [Lix(Na0:52K0:48)1�x]NbO3.

T. Maeda et al.Jpn. J. Appl. Phys. 51 (2012) 09MD08

09MD08-4 # 2012 The Japan Society of Applied Physics