(K,Na)NbO3 lead-free piezoelectric ceramics synthesized from ...

(K,Na)NbO3 lead-free piezoelectric ceramics synthesized from
hydrothermal powders
Takafumi Maeda a, ⁎, Norihito Takiguchi a , Mutstuo Ishikawa b , Tobias Hemsel c , Takeshi Morita a
a Frontier Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan
b Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatuda, Midori-ku, Yokohama, 226-8503, Japan
c Faculty of Mechanical Engineering Mechatronics and Dynamics, University of Paderborn, Fuerstenallee 11, 33102 Paderborn, Germany
article info
Article history:
Received 23 June 2009
Accepted 6 October 2009
Available online 12 October 2009
Keywords:
Lead-free piezoelectric material
(K,Na)NbO3 ceramics
Sintering solid solution
Piezoelectric properties
1. Introduction
abstract
To replace lead-containing piezoelectric materials such as lead
zirconate titanate (PZT), more environmentally friendly ceramics have
been studied intensively [1–8]. Especially, the alkaline niobate-based
piezoelectric ceramics has a good piezoelectric and high curie
temperature. Among them, (K,Na)NbO3 is considered as candidate for
lead-free piezoelectric ceramics. Recently, we proposed the use of the
hydrothermal method to obtain source powders for these ceramics, and
it has been verified that this method enables the production of high
quality powders [1,2]. From such a powder, non-doped potassium
niobate ceramics were successfully obtained, and their piezoelectric
performance was examined. Usually, the solid solution method is used
to obtain these powders, however potassium carbonate K2CO3, which is
a potassium source for potassium niobate, is unstable and quite difficult
to weigh due to its deliquescence. Moreover, to suppress conductivity, a
strict molar ratio between potassium and niobium in the ceramic is
required. However, during the sintering process, potassium is easily
evaporated. Therefore, an additional estimated quantity of potassium
must be added during preparation.
In contrast, the hydrothermal method overcomes the abovementioned
problems in the solid solutions of potassium niobate-based
⁎ Corresponding author. Tel.: +81 4 7136 4615.
E-mail addresses: maeda@ems.k.u-tokyo.ac.jp (T. Maeda), morita@k.u-tokyo.ac.jp
(T. Morita).
0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2009.10.012
Materials Letters 64 (2010) 125–128
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
Among various lead-free piezoelectric materials, (K,Na)NbO3 is a very promising candidate. In this study, (K,Na)
NbO3 ceramics were sintered from mixed KNbO3 and NaNbO3 powders prepared by hydrothermal reaction. These
two powders were mixed with distilled water in a KNbO3/NaNbO3 molar ratio of 1. After sintering the mixed
powder, the solid solution of (Na,K)NbO3 ceramics was obtained. The electrical properties such as the
electromechanical coupling factors k p and k 33, the mechanical quality factor, Q m, and the piezoelectric constant
d33 of the sintered (K,Na)NbO3 ceramics were 0.32, 0.48, 71 (radial mode), 118 ((33)mode), and 107 pC/N,
respectively.
© 2009 Elsevier B.V. All rights reserved.
powder, by utilizing chemical reactions in the solution[1,2]. When we
synthesized the potassium niobate-based powders, potassium hydroxide
was used. The hydrothermal method utilized crystallization from the
solution, so that pure crystal powder was obtained without secondary
phases. Therefore, the potassium to niobium ratio was automatically
controlled to be one. The simple process and low reaction temperature,
around 200°C, are other advantages of the hydrothermal method. These
features enabled the fabrication of non-doped potassium niobate
ceramic, which has been thought to be quite difficult to obtain as a
piezoelectric material.
In this study, sodium niobate materials were synthesized using the
hydrothermal method, and after mixing with potassium niobate, the
powders were sintered into potassium–sodium niobate ceramics. We
characterized their piezoelectric properties of its sintered ceramics.
2. Experimental methods
2.1. Potassium niobate and sodium niobate powders obtained by the
hydrothermal method
The (K,Na)NbO3 ceramics was synthesized with the hydrothermal
powders. As source materials for potassium niobate powder, 3.72 g of
niobate oxide was put into 70 ml 9 N KOH solution in a pressure vessel
(Parr model 4748). After the pressure vessel was sealed, it was placed
in a pre-heated oven at 210 °C. The reaction time was 24 h. For sodium
niobate powders, 9 N NaOH was used instead of KOH. Other reaction
conditions were the same as for potassium niobate.
Two kinds of powders were filtered with 0.45 μm filter paper and
dried for 10 min at 150 °C. During filtering process, these powders were

126 T. Maeda et al. / Materials Letters 64 (2010) 125–128
washed sufficiently with 1 L of distilled water. Fig. 1 shows SEM
photographs of the obtained powders. The powder sizes were
approximately 1 μm for KNbO3 and 3 μm for NaNbO3 powders,
respectively. Fig. 2 shows an XRD pattern for each powder. From these
results, it was confirmed that the powders had no impurity or secondary
phases, such as niobium oxide or pyrochlore.
Fig. 1. SEM of hydrothermally produced powder (a)KNbO 3 and (b) NaNbO 3.
2.2. Sintering process
Fig. 2. XRD patterns of hydrothermally produced powders: (a) KNbO 3 and (b) NaNbO 3.
Fig. 3. XRD patterns: (a) powder mixtures and (b) (K,Na)NbO 3 ceramic.
To synthesize (K,Na)NbO3 ceramics, KNbO3 and NaNbO3 powders
were mixed in distilled water. The amounts were 1.500 g and 1.366 g,
in order to obtain a molar ratio between potassium and sodium of 1 in
the ceramics. The mixed powder was filtered and dried for 10 min at

Fig. 4. SEM of the surface of (K,Na)NbO 3 ceramic sintered at 1100 °C.
150 °C. After drying, this powder was ground with alumina mortar
and alumina pestle, and filtered with sieve (mesh size 425 μm). From
this powder, a disk-shaped pellet was obtained using cold isostatic
pressing (CIP) at 200 MPa. The pellet was then sintered at 1100 °C for
two hours in an air environment. The heating rate was 230 °C/h and
the cooling rate was 100 °C/h.
Fig. 3 shows a comparison of XRD patterns between mixed powder
and sintered ceramics. This figure indicates that in the sintering
process, KNbO 3 and NaNbO 3 powders resulted in the solid solution of
(K,Na)NbO3. Note that the three peaks around 22° of the powder XRD
became two peaks after sintering. SEM photographs of the ceramics
verified that the grain size was 3~20 μm as shown in Fig. 4. The
density of the sintered (K,Na)NbO 3 ceramic was measured to be
4.09 ×10 3 kg/m 3 by density meter (Alfamirage SD-200 L), which
corresponds to a relative density of 90.7%.
2.3. Piezoelectric properties
The samples were cut by diamond cutter (Musashino densi MPC-
130) and polished with #2000 sandpaper into appropriate shapes for
the determination of their piezoelectric properties in the radial and
(33) modes. Gold electrodes were deposited on each side using a
sputtering coater (Sanyu Electron: Quick Coater SG-701). The
dimensions of the (K,Na)NbO 3 ceramics were ϕ6.6 mm×0.8 mm for
the radial vibration mode and 4.3 mm×1.0 mm×0.7 mm for the (33)
T. Maeda et al. / Materials Letters 64 (2010) 125–128
mode. Poling treatments were carried out using a high voltage supply
(Matsusada HARb-10P10) at 2.0 kV/mm in 100 °C silicone oil for
30 min. The admittance results for the radial vibration mode and (33)
mode are shown in Fig. 5(a) and (b). The electromechanical coupling
factors kp and k33 were calculated from the resonant–antiresonant
T
method. The relative free permittivity ε33 was determined from the
capacitance value at 1 kHz of the poled specimen. The stiffness cij
E was
calculated from the resonant frequency. With these parameters, the
piezoelectric factors dij were calculated from the equation,
d ij = k ij
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
εT 33 = cE q
ij
ð2:3:1Þ
The measured piezoelectric properties of the (K,Na)NbO3 ceramics
were as follows: the electromechanical coupling factors kp,andk33,the
T E
relative free permittivity ε33, the stiffness c33,
the mechanical quality
factor Qm, and the piezoelectric factor d33, were 0.32, 0.48, 264, 48 GPa,
71 (radial mode), 118 ((33) mode), and 107 pC/N, respectively.
3. Conclusion
In this study, potassium niobate and sodium niobate powders were
synthesized using a hydrothermal method. These two powders were
sintered into a solid solution of (K,Na)NbO 3. This solid solution process
was confirmed by XRD measurements. Compared to non-doped KNbO3,
the poling process was easier [1,2]. The sintering temperature also
became wider, from 1050 °C to 1150 °C. As a result, the piezoelectric
properties k 33=0.48, d 33=107 pC/N, and Q m=118 were obtained.
Acknowledgements
This research was supported by the New Energy and Industrial
Technology Development Organization (NEDO). We would like to
appreciate Dr. Daeyong Jeong for fruitful discussion.
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