Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 918∼923

Ultrasonically-assisted Hydrothermal Method for Ferroelectric MaterialSynthesis

Ryo Ageba,∗ Yoichi Kadota, Takafumi Maeda, Norihito Takiguchi and Takeshi Morita

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

Mutsuso Ishikawa

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

Peter Bornmann and Tobias Hemsel

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

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

The hydrothermal method enables the production of high-quality piezoelectric materials. In thisstudy, we propose to irradiate the reaction solutions with ultrasonic power during the hydrother-mal method to obtain a shorter reaction time and a smooth film surface. A high-pressure reactioncontainer for the ultrasonic transducer was newly developed, and the ultrasonically-assisted hy-drothermal method was examined by using this container. The effect of ultrasonic assist on thesynthesis of lead-zirconate-titanate (PZT) thin films and (K,Na)NbO3 powders was verified. ThickerPZT film, thickness around 10 µm, could be obtained in one process, and (K,Na)NbO3 powder wassynthesized in half the previous reaction time.

PACS numbers: 85.50.-nKeywords: Hydrothermal method, High-power ultrasonic, PZT thin film, Lead-free piezoelectric materialsDOI: 10.3938/jkps.57.918

I. INTRODUCTION

The hydrothermal method utilizes a chemical reactionin solution to obtain piezoelectric thin films or powders[1–10]. The obtained materials have interesting featuresdue to the low reaction temperature because the smallresidual stress inside results in high-quality crystals. Inaddition, the three-dimensional structure is acceptableas a substrate for the thin film deposition because thechemical reaction is carried out in solution [1–6].

The above-mentioned properties are unique ones thatother methods, such as the sol-gel, CVD and sputter-ing methods, don’t have. However, the hydrothermalmethod has some problems: for example, the slow reac-tion rate and the rough surface of the thin film in thecase of polycrystalline lead-zirconate-titanate (PZT) de-position [1–6]. The rough surface of the PZT thin filmcauses electrical breakdown. Therefore, a thicker filmis needed and can be obtained by repeating the deposi-tion process. However, the slow reaction rate becomesan obstacle to efficient fabrication.

To solve these problems, we have proposed a strong ul-

∗E-mail: ageba@ems.k.u-tokyo.ac.jp; Fax: +82-4-7136-4615

trasonic irradiation system for use during the hydrother-mal reaction [8,10]. High-power ultrasonic irradiationof a liquid-phase reaction is well known to be able toaccelerate the reaction by the force of ultrasonic cavita-tions and the acoustic flow. Such trials are difficult andhave never been done before because the hydrothermalmethod is carried out in a sealed stainless-steal containerunder high pressure and high temperature. Therefore,we have made a reaction container with an ultrasonictransducer [8,10]. Until now, we were able to confirmsome effectiveness, increasing the reaction rate of theKNbO3 powder [8] and smoothing the surface of the PZTthin film [10], by using the previous ultrasonic trans-ducer. However, in these cases, the performance degra-dation due to temperature rise was crucial, and the re-action temperature had to be lower than the optimumtemperature. In this study, a new ultrasonic transducerwas developed to improve the vibration speed even underhigh-temperature conditions and its effect was examinedfor PZT thin film deposition and (K, Na)NbO3 powderfabrication.

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Fig. 1. (Color online) Ultrasonic transducer and reactioncontainer.

Fig. 2. (Color online) Schematic of the ultrasonic assistsystem.

II. ULTRASONICALLY-ASSISTEDHYDROTHERMAL METHOD

1. Ultrasonic Transducer Design

As mentioned above, the hydrothermal method is car-ried out in a sealed container at high temperature andhigh pressure. In addition, the high alkaline conditionis severe for the ultrasonic transducer [1–9]. To over-come these problems, a high-power ultrasonic transducerfor the hydrothermal method was designed. First ofall, the ultrasonic transducer must withstand a high-temperature condition, for example, 210 ◦C, as an op-timal temperature for the KNbO3 powder [7]. In theprevious transducer [8,10], the ultrasonic vibration am-plitude was degraded at high temperatures, which reasonfor adopting the 190 ◦C reaction temperature used in aprevious study [7]. In order to withstand the strong al-kaline condition, the transducer was made of Hastelloy,which is a highly-corrosion-resistant alloy.

A photo of the transducer and the schematic vibra-tion mode are shown in Figs. 1 and 2 respectively. Thepiezoelectric parts are outside the reaction vessel, andthe irradiation surface is inside. Such a design enablesthe solution to be directly exposed to the high-power ul-trasonic irradiation. The performance degradation of theprevious ultrasonic transducer under a high-temperaturecondition was thought to be reduced because of the lowerpre-stress to the piezoelectric devices caused by the ther-

Fig. 3. (Color online) Frequency characteristics of thetransducer. The upper graph shows the admittance curve,and the bottom graph shows the relationship between the vi-bration speed and the applied frequency. Both graphs showthat the resonant frequency of the transducer is around 31kHz.

mal expansion of the bolt-tightening metal parts andpiezoelectric parts. Therefore, to improve the perfor-mance under a high temperature condition, we clampedthe piezoelectric device between duralumin washers witha high thermal expansion coefficient. Such a structurecan prevent a degradation of the pre-stress to the piezo-electric device due to the temperature rise. In addition,to magnify the vibration speed at the tip, we designed ahorn structure. The transducer was held tightly at thenodal position, as shown in Fig. 2. This metal part forholding is also utilized as one part of the container lid.

2. Ultrasonic Transducer Characteristics

Different from the previous transducer [8,10], thepresent transducer was driven with the fundamentalmode. Figures 3(a) and (b) show the admittance prop-erty measured with an impedance analyzer (Agilent4294A) and the vibration velocity measured with a LDV(laser Doppler vibrometer) (Polytec NLV-2500). Theresonant frequency of the transducer was 31.4 kHz. How-ever, this frequency was slightly different for each mea-

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Fig. 4. Relationship between the vibration speed and theapplied voltage. The velocity of the irradiation surface in-creases with increasing applied voltage. The magnificationby the horn-shaped tip is about 5 times.

Fig. 5. This figure shows the temperature characteristic ofthe vibration speed. The new transducer can endure a hightemperature of 250 ◦C.

surement setup. The frequency range around 30 kHz ismore adequate for the ultrasonic cavitations. The rela-tionship between the applied voltage and the vibrationspeed at room temperature is shown in Fig. 4. The mag-nification factor of the horn structure was 5.2. In addi-tion, the temperature dependency on the vibration speedwas measured using the LDV. The ultrasonic transducerwas put into the oven (Yamato, DKN 302), and the laserwas irradiated from the outside through the window ofthe oven. From Fig. 5, no vibration performance degra-dation occurred, not even at temperature higher than250 ◦C. Compared to the previous transducer [8,10], thisperformance was improved enough. This maximum tem-perature of 250 ◦C is sufficient because the optimum re-action temperature for a lead-free piezoelectric powderis 210 ◦C [7] and that for PZT is 140 ◦C.

The ultrasonic transducer was introduced as one partof the lid of the conventional pressure resistant con-

Table 1. Conditions for the synthesis of PZT thin films.

Hydrothermal Method

Pb(NO3)2 2.7 g

ZrCl2O·H2O 0.604 g

TiO2 0.100 g

KH (8N) 12.50 ml

H2O 37.50 ml

Solution volume 50 ml

Titanium substrate 17 mm × 25 mm × 50 µm

Vessel 125 ml

Temperrature 140 ◦C

Reaction Time 12 hours

tainer. The whole container with the ultrasonic trans-ducer was put into a constant-temperature oven while aresonant frequency AC voltage was being applied. Thedriving frequency was automatically controlled to followthe resonant frequency because it changed slightly due totemperature rise. Previously, the current probe (model700938, Yokogawa, Tokyo, Japan), the lock-in-amplifier(5610B, NF Corporation), the function generator (KEN-WOOD FG-281) and a PC were connected to a GPIBinterface, and the PC controlled the applyed frequency[8–10]. In this study, in order to simplify the system, theoutsourced PLL circuit controlled the driving frequency.Both systems utilize the same principle that the phasedifference between the current and the driving voltage iskept constant to maintain the resonance frequency.

III. THE EFFECT OF ULTRASONIC ASSIST

1. Synthesis of PZT Thin Film Using Hy-drothermal Method

Until now, the ultrasonic motor [4], the tactile sensor[5] and the ultrasonic transducer [6] have been investi-gated as applications of PZT thin-films applications fab-ricated by using the hydrothermal method. For thickPZT films and three-dimensional structures, acceptabil-ity are great advantages. However, the hydrothermalmethod has some problems such as the rough surfaceand slow reaction rate, as mentioned before. Therefore,the ultrasonically-assisted hydrothermal method usingan improved ultrasonic transducer was tried.

2. Smoothing the Surfaces of the PZT ThinFilms and Increasing the Deposition Rate

The experimental procedure was the following: First,the starting materials were thrown into a reaction vesselwith a titanium substrate. After the reaction vessel had

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Table 2. Conditions of the ultrasonic assist.

Ultrasonic assist

Adnittance at resonant freq.1.30 mS

(room temp.)

Applied voltage 350 Vpp

Driving frequency30.40 kHz

(automatically controlled)

Current 250 mApp

Fig. 6. SEM images of the surfaces of the PZT thinfilms: The surface of the ultrasonically-assisted sample be-comes smoother than that without ultrasonic assist. left: (a)ultrasonically assisted sample and right: (b) not assisted sam-ple.

been closed, it was put into the pre-heated oven. Thereaction temperature was 140 ◦C, and the reaction timewas 12 hours. The details of the synthesis conditions areshown in Table 1. The titanium substrate was fixed ona Teflon stage at a 5 mm distance from the irradiationsurface of the transducer. Table 2 shows the conditionsof the ultrasonic irradiation.

After a 12-hour synthesis, the surfaces and the crosssections of the obtained PZT thin films were observedby using a scanning electron microscope (JEOL JSM-5310LV), as shown in Figs. 6 and 7. The crystal structurewas measured by using X-ray diffraction (Rigaku Mini-FlexII), as shown in Fig. 8. By comparing Fig. 6(a) withFig. 6(b), we find that the surface of the ultrasonically-assisted sample becomes smoother than that without ul-trasonic assist. The same tendency was observed in ourprevious study [10]. By comparing Fig. 7(a) with Fig.7(b), the deposition rate was increased by the ultrasonicassist. This is the first observation that such a thick filmwith a thickness around 10 µm can be obtained with one

Fig. 7. SEM images of cross sections of the PZT thin films:left: (a) ultrasonically-assisted sample, 9.89 µm, and right:(b) not assisted sample, 3.49 µm. A film with a thicknessaround 10 µm could be obtained with one process by usingultrasonic radiation.

Fig. 8. X-ray diffraction results for PZT thin films. Theupper one is the ultrasonically-assisted sample, and lower oneis not assisted. The ultrasonically-assisted sample shows aclearer PZT pattern.

process by using ultrasonic radiation. Figure 8 showsthat neither film contained an impurity.

3. Synthesis of The Lead-free Materials

Recently, a need for environmentally-friendly piezo-electric materials to replace the PZT in piezoelectric de-vices has been established. We have proposed to utilize

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Table 3. Conditions for the synthesis of (K,Na)NbO3 pow-ders.

Hydrothermal Method

NaOH (9N) 59.5 ml

KOH (9N) 10.5 ml

Nb2O5 3.72 g

Temperrature (Mantol Heater) 210 ◦C

Reaction Time 1.5 - 24 hours

the hydrothermal method for potassium-niobate-basedceramics that have superior piezoelectric properties [7–9]. Through the use of the hydrothermal method, high-quality KNbO3 and NaNbO3 powders can be obtainedas source materials and the simplicity of the process andthe large productivity contribute to the low cost of thefabrication process [7–9].

In previous research, it was confirmed that the ultra-sonic assist could shorten the reaction time for the hy-drothermal synthesis of the KNbO3 powder [8]. In thiscase, however, even though the suitable reaction tem-perature for the synthesis of KNbO3 was 210 ◦C [7], thereaction temperature was selected as 190 ◦C because theperformance of the previous transducer was degraded byincreasing the temperature. In this experiment, the newtransducer could endure high temperatures, so the reac-tion temperature was 210 ◦C. In addition, (K,Na)NbO3

was obtained through one process thanks to the additionof NaOH as one of the source materials. In a previousstudy [9], KNbO3 and NaNbO3 were synthesized sepa-rately, and they were sintered together to become a solidstate ceramics, (K,Na)NbO3.

4. Increasing the Reaction Rate

The ultrasonic-assist effect for the synthesis of(K,Na)NbO3 powders was investigated. The start-ing materials, as shown in Table 3, were thrown intothe reaction vessel. The mixing ratio of NaOH andKOH was determined from the experiment to produce a(K0.5,Na0.5)NbO3 chemical component as the obtainedmaterial. Then, the whole vessel with the ultrasonictransducer attached was put into the pre-heated oven.The reaction temperature was 210 ◦C. The condition forthe ultrasonic irradiation was the same to that for thePZT deposition, as shown in Table 2.

After hydrothermal synthesis, the obtained powderwas filtered and dried at 100 ◦C, and its weight wasmeasured. If all the Nb ion sources reacted to form(K,Na)NbO3 crystal powders, 4.81 g of (K,Na)NbO3

crystal powders were expected. By comparing with thisvalue, the yield constant was calculated from the ob-tained powders weight. Figure 9 shows the relationshipbetween the reaction time and the yield constant with

Fig. 9. Changes of the reaction rate by the ultrasonicassist. The ultrasonic assist doubles the reaction rate forNKN powders.

and without the ultrasonic assist. The X-ray diffractionmeasurements are also shown in Fig. 9, providing infor-mation as to whether the obtained materials were pure(K,Na)NbO3 powders or not. This result indicates thatthe ultrasonic assist realizes a higher reaction speed andthe required time to complete the reaction becomes lessthan half that without ultrasonic assist.

IV. CONCLUSIONS AND FUTURE WORKS

We have improved an ultrasonic transducer, and high-power ultrasound irradiation, even under high tempera-ture conditions, became possible during the hydrother-mal reaction. In addition, the effect of the ultrasonic as-sist was verified using this transducer. We found that thesurface of the obtained PZT thin film became smootherdue to the transducer. Furthermore, the ultrasonic assistwith the transducer could accelerate the reaction rate inthe hydrothermal synthesis of the PZT thin films andproduced thicker films with thickness of about 10 µm.The proposed method could also shorten the reactiontime for synthesizing (K,Na)NbO3 crystal powders assource materials for lead-free piezoelectric ceramics.

Now, we are trying to clarify the effect of ultrasonicassist from the view of fundamental physics. By improv-ing the ultrasonic power, its effect should be increased.In addition, piezoelectric characteristics are being mea-sured to compare them with those obtained without theultrasonic assist.

ACKNOWLEDGMENTS

This research was supported by the New Energy andIndustrial Technology Organization (NEDO) and by a

Ultrasonically-assisted Hydrothermal Method for Ferroelectric Material Synthesis – Ryo Ageba et al. -923-

Grant-in-Aid for Scientific Research (Research ProjectNo.: 21760250). The kind support from Furuuchi Chem-ical Co., Ltd. and that from Taiatsu Techno Co., Ltd.are highly appreciated.

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