Ultrasonically-assisted Hydrothermal Method for Ferroelectric Material Synthesis

Transcription

1 Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp Ultrasonically-assisted Hydrothermal Method for Ferroelectric Material Synthesis Ryo Ageba, Yoichi Kadota, Takafumi Maeda, Norihito Takiguchi and Takeshi Morita Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba , Japan Mutsuso Ishikawa Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama , Japan Peter Bornmann and Tobias Hemsel Mechatronics and Dynamics, University of Paderborn, Fuerstenallee 11, Paderborn, Germany (Received 8 January 2010, in final form 8 July 2010) The hydrothermal method enables the production of high-quality piezoelectric materials. In this study, we propose to irradiate the reaction solutions with ultrasonic power during the hydrothermal method to obtain a shorter reaction time and a smooth film surface. A high-pressure reaction container for the ultrasonic transducer was newly developed, and the ultrasonically-assisted hydrothermal method was examined by using this container. The effect of ultrasonic assist on the synthesis of lead-zirconate-titanate (PZT) thin films and (K,Na)NbO 3 powders was verified. Thicker PZT film, thickness around 10 µm, could be obtained in one process, and (K,Na)NbO 3 powder was synthesized in half the previous reaction time. PACS numbers: n Keywords: Hydrothermal method, High-power ultrasonic, PZT thin film, Lead-free piezoelectric materials DOI: /jkps I. INTRODUCTION The hydrothermal method utilizes a chemical reaction in solution to obtain piezoelectric thin films or powders [1 10]. The obtained materials have interesting features due to the low reaction temperature because the small residual stress inside results in high-quality crystals. In addition, the three-dimensional structure is acceptable as a substrate for the thin film deposition because the chemical reaction is carried out in solution [1 6]. The above-mentioned properties are unique ones that other methods, such as the sol-gel, CVD and sputtering methods, don t have. However, the hydrothermal method has some problems: for example, the slow reaction rate and the rough surface of the thin film in the case of polycrystalline lead-zirconate-titanate (PZT) deposition [1 6]. The rough surface of the PZT thin film causes electrical breakdown. Therefore, a thicker film is needed and can be obtained by repeating the deposition process. However, the slow reaction rate becomes an obstacle to efficient fabrication. To solve these problems, we have proposed a strong ul- trasonic irradiation system for use during the hydrothermal reaction [8,10]. High-power ultrasonic irradiation of a liquid-phase reaction is well known to be able to accelerate the reaction by the force of ultrasonic cavitations and the acoustic flow. Such trials are difficult and have never been done before because the hydrothermal method is carried out in a sealed stainless-steal container under high pressure and high temperature. Therefore, we have made a reaction container with an ultrasonic transducer [8,10]. Until now, we were able to confirm some effectiveness, increasing the reaction rate of the KNbO 3 powder [8] and smoothing the surface of the PZT thin film [10], by using the previous ultrasonic transducer. However, in these cases, the performance degradation due to temperature rise was crucial, and the reaction temperature had to be lower than the optimum temperature. In this study, a new ultrasonic transducer was developed to improve the vibration speed even under high-temperature conditions and its effect was examined for PZT thin film deposition and (K, Na)NbO 3 powder fabrication. ageba@ems.k.u-tokyo.ac.jp; Fax:

2 Ultrasonically-assisted Hydrothermal Method for Ferroelectric Material Synthesis Ryo Ageba et al Fig. 1. (Color online) Ultrasonic transducer and reaction container. Fig. 2. (Color online) Schematic of the ultrasonic assist system. II. ULTRASONICALLY-ASSISTED HYDROTHERMAL METHOD 1. Ultrasonic Transducer Design As mentioned above, the hydrothermal method is carried out in a sealed container at high temperature and high pressure. In addition, the high alkaline condition is severe for the ultrasonic transducer [1 9]. To overcome these problems, a high-power ultrasonic transducer for the hydrothermal method was designed. First of all, the ultrasonic transducer must withstand a hightemperature condition, for example, 210 C, as an optimal temperature for the KNbO3 powder [7]. In the previous transducer [8,10], the ultrasonic vibration amplitude was degraded at high temperatures, which reason for adopting the 190 C reaction temperature used in a previous study [7]. In order to withstand the strong alkaline condition, the transducer was made of Hastelloy, which is a highly-corrosion-resistant alloy. A photo of the transducer and the schematic vibration mode are shown in Figs. 1 and 2 respectively. The piezoelectric parts are outside the reaction vessel, and the irradiation surface is inside. Such a design enables the solution to be directly exposed to the high-power ultrasonic irradiation. The performance degradation of the previous ultrasonic transducer under a high-temperature condition was thought to be reduced because of the lower pre-stress to the piezoelectric devices caused by the ther- Fig. 3. (Color online) Frequency characteristics of the transducer. The upper graph shows the admittance curve, and the bottom graph shows the relationship between the vibration speed and the applied frequency. Both graphs show that the resonant frequency of the transducer is around 31 khz. mal expansion of the bolt-tightening metal parts and piezoelectric parts. Therefore, to improve the performance under a high temperature condition, we clamped the piezoelectric device between duralumin washers with a high thermal expansion coefficient. Such a structure can prevent a degradation of the pre-stress to the piezoelectric device due to the temperature rise. In addition, to magnify the vibration speed at the tip, we designed a horn structure. The transducer was held tightly at the nodal position, as shown in Fig. 2. This metal part for holding is also utilized as one part of the container lid. 2. Ultrasonic Transducer Characteristics Different from the previous transducer [8,10], the present transducer was driven with the fundamental mode. Figures 3(a) and (b) show the admittance property measured with an impedance analyzer (Agilent 4294A) and the vibration velocity measured with a LDV (laser Doppler vibrometer) (Polytec NLV-2500). The resonant frequency of the transducer was 31.4 khz. However, this frequency was slightly different for each mea-

3 -920- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010 Table 1. Conditions for the synthesis of PZT thin films. Fig. 4. Relationship between the vibration speed and the applied voltage. The velocity of the irradiation surface increases with increasing applied voltage. The magnification by the horn-shaped tip is about 5 times. Hydrothermal Method Pb(NO 3) g ZrCl 2O H 2O g TiO g KH (8N) ml H 2O 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 transducer was put into a constant-temperature oven while a resonant frequency AC voltage was being applied. The driving frequency was automatically controlled to follow the resonant frequency because it changed slightly due to temperature rise. Previously, the current probe (model , Yokogawa, Tokyo, Japan), the lock-in-amplifier (5610B, NF Corporation), the function generator (KEN- WOOD FG-281) and a PC were connected to a GPIB interface, and the PC controlled the applyed frequency [8 10]. In this study, in order to simplify the system, the outsourced PLL circuit controlled the driving frequency. Both systems utilize the same principle that the phase difference between the current and the driving voltage is kept constant to maintain the resonance frequency. Fig. 5. This figure shows the temperature characteristic of the vibration speed. The new transducer can endure a high temperature of 250 C. surement setup. The frequency range around 30 khz is more adequate for the ultrasonic cavitations. The relationship between the applied voltage and the vibration speed at room temperature is shown in Fig. 4. The magnification factor of the horn structure was 5.2. In addition, the temperature dependency on the vibration speed was measured using the LDV. The ultrasonic transducer was put into the oven (Yamato, DKN 302), and the laser was irradiated from the outside through the window of the oven. From Fig. 5, no vibration performance degradation occurred, not even at temperature higher than 250 C. Compared to the previous transducer [8,10], this performance was improved enough. This maximum temperature of 250 C is sufficient because the optimum reaction temperature for a lead-free piezoelectric powder is 210 C [7] and that for PZT is 140 C. The ultrasonic transducer was introduced as one part of the lid of the conventional pressure resistant con- III. THE EFFECT OF ULTRASONIC ASSIST 1. Synthesis of PZT Thin Film Using Hydrothermal Method Until now, the ultrasonic motor [4], the tactile sensor [5] and the ultrasonic transducer [6] have been investigated as applications of PZT thin-films applications fabricated by using the hydrothermal method. For thick PZT films and three-dimensional structures, acceptability are great advantages. However, the hydrothermal method has some problems such as the rough surface and slow reaction rate, as mentioned before. Therefore, the ultrasonically-assisted hydrothermal method using an improved ultrasonic transducer was tried. 2. Smoothing the Surfaces of the PZT Thin Films and Increasing the Deposition Rate The experimental procedure was the following: First, the starting materials were thrown into a reaction vessel with a titanium substrate. After the reaction vessel had

4 Ultrasonically-assisted Hydrothermal Method for Ferroelectric Material Synthesis Ryo Ageba et al Table 2. Conditions of the ultrasonic assist. Adnittance at resonant freq. (room temp.) Applied voltage Driving frequency Current Ultrasonic assist 1.30 ms 350 V pp khz (automatically controlled) 250 ma pp 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 thickness around 10 µm could be obtained with one process by using ultrasonic radiation. Fig. 6. SEM images of the surfaces of the PZT thin films: The surface of the ultrasonically-assisted sample becomes smoother than that without ultrasonic assist. left: (a) ultrasonically assisted sample and right: (b) not assisted sample. been closed, it was put into the pre-heated oven. The reaction temperature was 140 C, and the reaction time was 12 hours. The details of the synthesis conditions are shown in Table 1. The titanium substrate was fixed on a Teflon stage at a 5 mm distance from the irradiation surface of the transducer. Table 2 shows the conditions of the ultrasonic irradiation. After a 12-hour synthesis, the surfaces and the cross sections of the obtained PZT thin films were observed by using a scanning electron microscope (JEOL JSM- 5310LV), as shown in Figs. 6 and 7. The crystal structure was measured by using X-ray diffraction (Rigaku Mini- FlexII), as shown in Fig. 8. By comparing Fig. 6(a) with Fig. 6(b), we find that the surface of the ultrasonicallyassisted sample becomes smoother than that without ultrasonic assist. The same tendency was observed in our previous study [10]. By comparing Fig. 7(a) with Fig. 7(b), the deposition rate was increased by the ultrasonic assist. This is the first observation that such a thick film with a thickness around 10 µm can be obtained with one Fig. 8. X-ray diffraction results for PZT thin films. The upper one is the ultrasonically-assisted sample, and lower one is not assisted. The ultrasonically-assisted sample shows a clearer PZT pattern. process by using ultrasonic radiation. Figure 8 shows that neither film contained an impurity. 3. Synthesis of The Lead-free Materials Recently, a need for environmentally-friendly piezoelectric materials to replace the PZT in piezoelectric devices has been established. We have proposed to utilize

5 -922- Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010 Table 3. Conditions for the synthesis of (K,Na)NbO 3 powders. Hydrothermal Method NaOH (9N) KOH (9N) Nb 2O 5 Temperrature (Mantol Heater) Reaction Time 59.5 ml 10.5 ml 3.72 g 210 C hours the hydrothermal method for potassium-niobate-based ceramics that have superior piezoelectric properties [7 9]. Through the use of the hydrothermal method, highquality KNbO 3 and NaNbO 3 powders can be obtained as source materials and the simplicity of the process and the large productivity contribute to the low cost of the fabrication process [7 9]. In previous research, it was confirmed that the ultrasonic assist could shorten the reaction time for the hydrothermal synthesis of the KNbO 3 powder [8]. In this case, however, even though the suitable reaction temperature for the synthesis of KNbO 3 was 210 C [7], the reaction temperature was selected as 190 C because the performance of the previous transducer was degraded by increasing the temperature. In this experiment, the new transducer could endure high temperatures, so the reaction temperature was 210 C. In addition, (K,Na)NbO 3 was obtained through one process thanks to the addition of NaOH as one of the source materials. In a previous study [9], KNbO 3 and NaNbO 3 were synthesized separately, and they were sintered together to become a solid state ceramics, (K,Na)NbO Increasing the Reaction Rate The ultrasonic-assist effect for the synthesis of (K,Na)NbO 3 powders was investigated. The starting materials, as shown in Table 3, were thrown into the reaction vessel. The mixing ratio of NaOH and KOH was determined from the experiment to produce a (K 0.5,Na 0.5 )NbO 3 chemical component as the obtained material. Then, the whole vessel with the ultrasonic transducer attached was put into the pre-heated oven. The reaction temperature was 210 C. The condition for the ultrasonic irradiation was the same to that for the PZT deposition, as shown in Table 2. After hydrothermal synthesis, the obtained powder was filtered and dried at 100 C, and its weight was measured. If all the Nb ion sources reacted to form (K,Na)NbO 3 crystal powders, 4.81 g of (K,Na)NbO 3 crystal powders were expected. By comparing with this value, the yield constant was calculated from the obtained powders weight. Figure 9 shows the relationship between the reaction time and the yield constant with Fig. 9. Changes of the reaction rate by the ultrasonic assist. The ultrasonic assist doubles the reaction rate for NKN powders. and without the ultrasonic assist. The X-ray diffraction measurements are also shown in Fig. 9, providing information as to whether the obtained materials were pure (K,Na)NbO3 powders or not. This result indicates that the ultrasonic assist realizes a higher reaction speed and the required time to complete the reaction becomes less than half that without ultrasonic assist. IV. CONCLUSIONS AND FUTURE WORKS We have improved an ultrasonic transducer, and highpower ultrasound irradiation, even under high temperature conditions, became possible during the hydrothermal reaction. In addition, the effect of the ultrasonic assist was verified using this transducer. We found that the surface of the obtained PZT thin film became smoother due to the transducer. Furthermore, the ultrasonic assist with the transducer could accelerate the reaction rate in the hydrothermal synthesis of the PZT thin films and produced thicker films with thickness of about 10 µm. The proposed method could also shorten the reaction time for synthesizing (K,Na)NbO 3 crystal powders as source materials for lead-free piezoelectric ceramics. Now, we are trying to clarify the effect of ultrasonic assist from the view of fundamental physics. By improving the ultrasonic power, its effect should be increased. In addition, piezoelectric characteristics are being measured to compare them with those obtained without the ultrasonic assist. ACKNOWLEDGMENTS This research was supported by the New Energy and Industrial Technology Organization (NEDO) and by a

6 Ultrasonically-assisted Hydrothermal Method for Ferroelectric Material Synthesis Ryo Ageba et al Grant-in-Aid for Scientific Research (Research Project No.: ). The kind support from Furuuchi Chemical Co., Ltd. and that from Taiatsu Techno Co., Ltd. are highly appreciated. REFERENCES [1] K. Shimomura, T. Tsurumi, Y. Ohba and M. Daimon, Jpn. J. Appl. Phys. 30, 2174 (1991). [2] T. Morita and Y. Cho, Appl. Phys. Lett. 58, 2331 (2004). [3] T. Morita, Y. Wagatsuma, H. Morioka, H. Funakubo, N. Setter and Y. Cho, J. Mater. Res. 19, 1862 (2004). [4] T. Morita, Y. Wagatusma and T. Higuchi, Sens. Actuators. A. 83, 225 (2000). [5] T. Kanda, T. Morita and M. K. Kurosawa, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 46, 875 (1999). [6] M. Ishikawa, M. K. Kurosawa, A. Endoh and S. Takeuchi, Jpn. J. Appl. Phys., Part 1 44, 4342 (2005). [7] M. Ishikawa, N. Takiguchi, H. Hosaka and T. Morita, Jpn. J. Appl. Phys. 47, 3824 (2008). [8] M. Ishikawa, Y. Kadota, N. Takiguchi, H. Hosaka and T. Morita, Jpn. J. Appl. Phys. 47, 7673 (2008). [9] T. Maeda, N. Takiguchi, M. Ishikawa, T. Hemsel and T. Morita, Mater. lett. 64, 125 (2010). [10] Y. Kadota, M. Ishikawa, H. Hosaka and T. Morita, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 56, 9 (2009).