Lead-Free Piezoelectrics Shashank Priya l Sahn Nahm Editors Lead-Free Piezoelectrics Editors Shashank Priya Department of Mechanical Engineering Virginia Tech Blackburg, VA 24061, USA spriya@vt.edu Sahn Nahm Korea University Anam-dong 5-1 136-701 Seoul Korea, Republic of (South Korea) snahm@korea.ac.kr ISBN 978-1-4419-9597-1 e-ISBN 978-1-4419-9598-8 DOI 10.1007/978-1-4419-9598-8 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011940129 # Springer Science+Business Media, LLC 2012 All rights reserved This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preface Piezoelectric materials form the backbone of several components utilized in communication systems, defense systems, industrial automation, medical diagnostics, energy storage and harvesting, and information technology In high performance piezoelectric applications the material of choice is based upon lead-based composition with Pb(Zr,Ti)O3 (PZT) being the base Recently worldwide environmental considerations are demanding the elimination of lead-based materials from all the consumer items This has created urgency for finding alternative to PZT Elimination of lead in applications such as actuators still remains a challenge but in some other applications such as high frequency electronic components it has been possible to utilize lead-free materials At a recent Electronic Materials and Applications 2011 meeting, a group discussion was held on the topic of lead-free materials under the umbrella of National Science Foundation Few important points agreed upon at this discussion were: (1) development of lead-free materials should be application specific, (2) emphasis should also be on design-based research in addition to discovery-based research, and (3) a compilation of all the results on important family of lead-free materials is needed Further, the group identified the need for lead-free materials from Wikipedia These recommendations were guiding factors in the arrangement of chapters in this book All the important families of lead-free materials were addressed and each part/chapter provides relevant data for a given family The book is addressed to students, researchers, application engineers, educators, developers, and producers of piezoelectric materials and applications The chapters mainly consist of technical reviews, discussions, and basic knowledge in the design, synthesis, and microstructure characterization of lead-free piezoelectric materials The book brings the leading researchers from academia and industry in the world in the field of piezoelectric materials and applications on to one platform to provide a comprehensive overview of the fundamentals and developments All the important classes of lead-free piezoelectric materials were addressed by the leading authors Furthermore, the book covers the principles and design rules of the lead-free materials in depth The chapters on applications of the lead-free materials will allow readers to conceptualize the promise of the field v vi Preface The first part in the book provides discussions on domain engineering and phase transformations covering the role of morphotropic phase boundary, electric-field induced phase transition, intermediate bridging phases, polarization rotation and adaptive phase theory, polymorphic phase boundary, and grain texturing The second part covers the history, progress and current status of alkali niobate ceramics covering random polycrystalline ceramics, textured ceramics, and single crystals The third part covers the progress made in synthesis and characterization of sodium bismuth titanate-based ceramics The fourth part covers the fundamentals and properties of bismuth-layered structures Thus, Parts II–IV provide in-depth coverage of the important lead-free materials Last part provides an overview on the application of lead-free materials and their role in the emerging topic of magnetoelectrics The chapters published here are mostly the invited technical submissions from the authors The editors did not make any judgment on the quality and organization of the text in the chapters and it was mostly left to the decision of the authors In this regard, the editors not accept the responsibility for any technical errors present in the chapters and those should be directly discussed with the authors of the relevant chapter It was an honor editing this book consisting of contributions from knowledgeable and generous colleagues Thanks to all the authors for their timely assistance and cooperation during the course of this book Without their continual support, this work would not have been possible We hope that readers will find the book informative and instructive and provide suggestions and comments to further improve the text in eventual second edition Blackburg, VA, USA Shashank Priya Contents Part I Domain Engineering and Phase Transformations Domain Engineering and Phase Transformations Wenwei Ge, Jiefang Li, and D Viehland Ferroelectric Domains and Grain Engineering in SrBi2Ta2O9 H Amorin, I Coondoo, M.E.V Costa, and A.L Kholkin Part II 53 Alkali: Niobate-Based Ceramics Development of KNN-Based Piezoelectric Materials Shashaank Gupta, Deepam Maurya, Yongke Yan, and Shashank Priya 89 Low Temperature Sintering of the Alkali-Niobate Ceramics Hwi-Yeol Park and Sahn Nahm 121 Lead-Free KNN-Based Piezoelectric Materials Ahmad Safari and Mehdi Hejazi 139 Alkali Niobate Piezoelectric Ceramics Akira Ando 177 Influence of the A/B Stoichiometry on Defect Structure, Sintering, and Microstructure in Undoped and Cu-Doped KNN Michael J Hoffmann, Hans Kungl, Jerome Acker, ă ¨ Christian Elsasser, Sabine Korbel, Pavel Marton, ¨ ¨ Rudiger-A Eichel, Ebru Erunal, and Peter Jakes 209 vii viii Contents Part III Sodium Bismuth Titanate-Based Ceramics Sodium Bismuth Titanate-Based Ceramics Tadashi Takenaka and Hajime Nagata 255 Perovskite Lead-Free Piezoelectric Ceramics Hyeong Jae Lee and Shujun Zhang 291 10 Processing and Properties of Textured BNT-Based Piezoelectrics Toshihiko Tani and Toshio Kimura 311 Crystal Growth and Electric Properties of Na0.5Bi0.5TiO3-BaTiO3 Single Crystals Qinhui Zhang, Xiangyong Zhao, and Haosu Luo 337 11 12 Nonstoichiometry in (Bi0.5Na0.5)TiO3 Ceramics Yeon Soo Sung and Myong Ho Kim Part IV 13 14 15 353 Bismuth Layer Structured Ferroelectric Resonator Characteristics of Bismuth Layer Structured Ferroelectric Materials Akira Ando and Masahiko Kimura 373 Defect Control and Properties in Bismuth Layer Structured Ferroelectric Single Crystals Yuji Noguchi and Masaru Miyayama 405 Processing and Properties of Textured Bismuth Layer-Structured Ferroelectrics Toshio Kimura and Toshihiko Tani 461 Part V Applications 16 Self-Biased Lead-Free Magnetoelectric Laminates Su-Chul Yang and Shashank Priya 487 17 Applications of Lead-Free Piezoelectrics Kenji Uchino 511 Part I Domain Engineering and Phase Transformations 514 K Uchino in the device) and in heat generation in color TVs, leading to the termination of production for decades However, recent laptop computers with a liquid crystal display requiring a very thin, and electromagnetic-noise free transformer to start the glow of a fluorescent back-lamp have accelerated the development after 1990s This is a good example how the social demand creates the technology development (needs-pull ) The four-Chinese-character slogans discussed in Table 17.1 are typical social motivations for new product developments Remember that “美, 遊, 潤, 創 (beautiful, amusing, tasteful, and creative)” is the key in the first 20 years of twenty first century, and the societal sustainability, including Pb-free piezoceramic development, is really in this direction Discovery of a new principle or material, and invention of a new device sometimes change the world drastically Polyvinylidene difluoride, PVDF, a piezoelectric polymer discovered accidentally in early 1970s by Dr Kawai., and a hightemperature superconducting ceramic discovered by Drs Bednortz and Muller in 1980s are good examples of serendipity The author’s photostrictive materials were discovered along a systematic consideration of a composite effect; functionality matrixes of photovoltatic and piezoelectric effects A new idea arising from “research” will create a seed-push market, while development is initiated by a need-pull force The multilayer piezoelectric actuators have been widely adopted in diesel injection valve mechanisms However, their expensive manufacturing cost limits the spread-out speed in the world Some researchers are working to reduce the drive voltage by reducing the layer thickness of piezoelectric multilayer actuators, in particular for mobile phone applications However, it is not recommended for the large ML actuators for diesel injection valve control applications in diesel automobiles Figure 17.1 graphs the piezo-stack price and its electronic driver cost as a function of drive voltage [4] The ML actuator price increases with reducing its drive voltage (i.e., reducing its each layer thickness), while the driver cost increases with increasing the voltage The minimum total cost is obtained around 160 V, leading the layer thickness of 80 mm This “a-sort-of standard” thickness can be derived from the cost minimization principle, but not from the performance Note that 20 mm layer thickness is not difficult technologically in ML actuators nowadays It is generally true that “cheaper” is more “competitive” in the product commercialization However, note that the following “Political/Legal” constraint is much stronger force in the commercialization Twenty first century is called “The Century of Environmental Management.” In 2006, European Community started RoHS (Restrictions of Hazardous Substances), which explicitly limits the usage of lead (Pb) in electronic equipments Basically, we may need to regulate the usage of PZT, most famous piezoelectric ceramics, in the future Japanese and European communities may experience governmental regulation on the PZT usage in these 10 years Pb (lead)-free piezoceramics have started to be developed after 1999 Figure 17.2 shows statistics of various lead-free piezoelectric ceramics The share of the papers and patents for 17 Applications of Lead-Free Piezoelectrics Fig 17.1 Drive voltage dependence of piezo-stack price and the electronic driver cost [4] 515 Thin Layer ML Low Energy (Low Current Capacitance) Discrete & EDU Cost Piezo Stack Price High Energy (High Current Capacitance) Thick Layer 10 V 100 V 1000 V Fig 17.2 Patent disclosure statistics for lead-free piezoelectric ceramics (total number of patents and papers is 102) bismuth compounds (bismuth layered type and (Bi,Na)TiO3 type) exceeds 61% (K,Na,Li)(Nb,Ta,Sb)O3 ceramics exhibit the best performance at present, and seem to be the most promising candidates [5] RoHS seems to be a significant threat to piezoelectric companies, who have only PZT piezoceramics However, this is an opportunity for the company which is preparing alternative piezoceramics to exchange the piezoelectric device share 516 K Uchino On the other hand, diesel engines are more recommended than normal gasoline cars, because the purification energy (extra electrical energy) of oil can be saved, which was contributing to global warming problem However, the conventional diesel engine generates toxic exhaust gases such as SOx and NOx In order to solve this problem, new diesel injection valves were developed by Siemens with piezoelectric PZT ML actuators Of course, as the reader can imagine, the final goal of the technology will be the diesel injection valves with using Pb-free piezoelectric ML actuators This paper was authored to introduce promising applications of Pb-free piezoelectrics developed recently: (1) piezoelectric transducers for ultrasonic cleaners by Honda Electronics, (2) piezoelectric transformers by the author’s group at The Penn State University, and (3) ultrasonic motors with multilayer piezoactuators by Taiyo Yuden Note that the initial motivation for this movement came from the Political Regulation (i.e., RoHS), in conjunction with Social/Cultural Pressure (i.e., sustainable society) Though this movement is not recommended from Economical Viewpoint (i.e., too expensive for the products at this moment), we need to overcome the problem from the new Technological Development in the nearest future The political force is the strongest factor for the practically successful commercialization 17.2 Lead-Free Piezoelectric Materials Patent statistics of lead-free piezoelectric ceramics in Fig 17.2 shows that the share of the patents for bismuth compounds (bismuth- layered type and (Bi,Na)TiO3 (BNT) type) exceeds 61% This is because bismuth compounds are easily fabricated in comparison with other compounds Note that the toxicity of Bi3+ is not very low compared with Pb2+, but that a regulation may not be proposed because this atomic element is not familiar with “politicians” fortunately (Sr,Ba,Ca) Bi2Nb2O9, Bi4Ti3O12 (Bi-Layer) was developed by TDK for high Qm communication application On the other hand, (Bi1/2Na1/2)TiO3–BaTiO3 (BNT) have been developed by AIST, Japan, and Honda Electronics for transducer applications (Na,K)NbO3 (NKN) systems exhibit the highest performance because of the morphotropic phase boundary usage Figure 17.3 shows the current best data reported by Toyota Central Research Lab, where strain curves for oriented and unoriented (K,Na,Li) (Nb,Ta,Sb)O3 ceramics are shown [5] [Recited from Chap 12 of this book] Note that the maximum strain reaches up to 1,500  10À6, which is equivalent to the PZT strain Drawbacks include their sintering difficulty and the necessity of the sophisticated preparation technique (topochemical method for preparing flaky raw powder) Tungsten-bronze (TB) types are another alternative choice for resonance applications, because of high Curie temperature and low loss Taiyo Yuden demonstrated (Sr,Ca)2NaNb5O15 for ultrasonic motor applications Other materials include (Sr,Ba,Ca)Bi2Nb2O9, Bi4Ti3O12 (Bi-Layer), which TDK 17 Applications of Lead-Free Piezoelectrics 517 Fig 17.3 Strain curves for oriented and unoriented (K, Na,Li)(Nb,Ta,Sb)O3 ceramics [5] applied for high Qm and high TC communication devices Refer to various chapters in this book for the detailed performance of each material Since the piezoelectric performance of Pb-free materials is not superior to the PZTs in general in a similar randomly oriented ceramic configuration, commercialization efforts should include either (1) epitaxially oriented ceramic preparation method, or (2) sophisticated composite designing, in order to overcome the inferiority 17.3 Langevin Transducers with BNT Langevin transducers used popularly for underwater fish finders and hydrophones are also utilized for ultrasonic cleaning and hazardous material dissolving systems For these ecological applications, it is reasonable to make the systems also environment-friendly Honda Electronics, Japan developed Langevin transducers using the BNT-based ceramics for ultrasonic cleaner applications [6] The composition 0.82(Bi1/2Na1/2)TiO3–0.15BaTiO3–0.03(Bi1/2Na1/2)(Mn1/3Nb2/3) 518 K Uchino Fig 17.4 High-power performance of the Honda Electronics’ Pb-free piezoelectric, in comparison with a regular hard PZT [6] O3 exhibits d33 ¼ 110  10À12 C/N, which is only 1/3 of that of a hard PZT However, the electromechanical coupling factor kt ¼ 0.41 is larger because of much smaller permittivity (e ¼ 500) than that of the PZT More importantly, since the figure of merit of the resonant vibration level (i.e., vibration velocity) is provided by the product of the piezoelectric d constant and mechanical quality factor Qm, higher Qm is desired under a practically high excitation state The maximum vibration velocity of a rectangular plate (k31 mode) of Honda Electronics’ BNT is close to m/s (rms value), which is higher than that of typical hard PZTs Figure 17.4 demonstrates high-power piezoelectric performance of the Honda Electronics’ Pb-free piezoelectric, in comparison with a regular hard PZT The mechanical quality factor Qm was measured for a k31 type rectangular plate (43   mm3) as a function of excited vibration velocity (0–p value in m/s in this figure) Notice that their measurement has been made under a burst mode for escaping from the temperature rise of the sample This condition is different from the continuous drive measurement in the next section, which provides roughly a half of the value measured under a burst mode A constant Qm around 500 of the BNT-based ceramic is sustained up to 1.4 m/s (0–p) of the vibration velocity, while a regular hard PZT exhibits dramatic decay in Qm under 0.5 m/s (0–p), though the initial Qm under small excitation is reasonably high (~1,100) Table 17.2 summarizes properties of the bolt-clamped Langevin-type transducer (28 kHz) with Pb-free and PZT-based piezoelectric ceramics [6] The transducer design is shown in Fig 17.5a Note that the device performances are almost the same even though the material’s properties are rather different, except for capacitance and impedance The Langevin transducer shown in Fig 17.5a (28 kHz) was tested in an erosion chamber The ultrasonic power of lead-free piezoelectric transducer was equivalent to that of a hard PZT type, and the cleansing effect of both types was verified to be practically the same by comparing the erosion areas on aluminum foils (see Fig 17.5b) 17 Applications of Lead-Free Piezoelectrics 519 Table 17.2 Properties of the bolt clamped Langevin-type transducer (28 kHz) with Pb-free and PZT-based piezoelectric ceramics [6] Lead-free (Bi,Na) Hard Pb(Zr,Ti) TiO3–BaTiO3–(Bi, Na)(Mn,Nb)O3 O3 (HC–50GS) Remarks Item Symbol 28.0 kHz 28.0 kHz Measurement voltage: Resonance F0 1.0 Vrms frequency Impedance Z 75 O 35 O Measurement voltage: 1.0 Vrms Capacitance Cx 1,300 pF 3,300 pF Measurement frequency: kHz ≧1  104 MO Measurement voltage Insulation IR ≧1  104 MO and time: resistance DC1000 V  Maximum V 0.5 m/s(0–p) 0.5 m/s(0–p) Vibration velocity of permissible radiation surface velocity Maximum Tmax 120 C 120 C Surface temperature of permissible ceramics temperature Storage Tst À5 C to +40 C À5 C to +40 C temperature range 60 Ỉ 20% 60 Ỉ 20% Storage humidity Hst range Fig 17.5 (a) Langevin transducer (28 kHz) for cleaner applications, and (b) cleansing effect comparison among Pb-free and PZT types on aluminum foils [6] 17.4 Piezoelectric Transformer with NKN In the trend of miniaturization of electronic devices, components used are also getting smaller Traditional electromagnetic transformers cannot catch up with this trend On the other hand, those made from piezoelectric ceramics have ease to follow this trend Piezoelectric components show much higher power density in smaller scales 520 K Uchino The first piezoelectric transformer was introduced by C A Rosen et al in 1957 as the Rosen-Type transformer [7] Piezoelectric transformers can be divided into two types One of them is the voltage transformer, exemplified by a Rosen-Type transformer as mentioned before, yields high voltage and low currents The other group is the current transformer, which produces high current and relatively low voltage (or typically stepping-down voltage) The Penn State group designed the latter purpose transformers called “ring-dot” types The common piezoelectric ceramics used so far are lead-based piezoelectric ceramics commonly known as PZT (Lead-ZirconateTitanate) By doping, PZT’s piezoelectric properties have been enhanced in terms of loss and heat generation, and by design optimization piezoelectric transformers’ power capability has also been increased Power density is typically around 10 ~ 20 W/cm3, and the top data as high as 40 W/cm3 has been reported by Laoratanakul et al [8] Though PZTs possess excellent piezoelectric properties and have been used for many applications, their continuous usage may be regulated by the RoHS (Restriction of Hazardous Substances) This raises the issue of safe disposability and recyclability of piezoelectric substances The effect of RoHS and the promising results by Saito et al of lead-free systems [5] made an impact on the number of the researches 17.4.1 Hard NKN and Its High-Power Performance In collaboration with Korea University, The Penn State University is concentrating on Sodium-Potassium-Niobate (NKN) due to its promising properties reported recently by H –Y Park et al in 2008, based on (Na0.5 K0.5) (Nb0.97Sb0.03)O3 with mol% CuO addition [9]: kp ¼ 0.41, Qm ¼ 1,333, d33 ¼ 111 pC/N, and e3 x/e0 ¼ 324 17.4.1.1 Low-Power Piezoelectric Characterization After 24 h from poling the low-power characterization was handled The d33 value was measured with the d33 meter at 100 Hz Capacitance and dielectric loss (tan d) values were measured with the HP-4272A LCR meter at kHz and V Impedance spectrum was obtained with Agilent 4294A impedance analyzer (Fig 17.6) for obtaining the mechanical quality factor Qm (or the elastic loss factor) The electromechanical parameters obtained in the newly prepared specimens (Table 17.3) seem to be a little lower than the numbers reported in Ref [9], indicating the preparation reproducibility problem 17.4.1.2 High-Power Characterization The high-power property of the disk sample and the transformer characteristic were measured by a High-Power Characterization System (HiPoCS™) developed by our group [10] An amplitude controlled sinusoidal signal was produced by a function 17 Applications of Lead-Free Piezoelectrics 521 Impedance (Ω) 100000 10000 1000 100 10 300 310 320 Frequency (kHz) 330 340 Fig 17.6 Impedance analysis of the NKN disk (10.6 diameter  1.5 mm thickness) measured under 0.5 Vrms Table 17.3 Electromechanical properties of the NKN ceramic tand (%) Qm (3dB method) d33 (pC/N) 80 1.1 639 generator (HP 33120A) and amplified by NF 4010 The sample current was detected by a clamp-on AC current sensor (Tektronix TCP 305) The voltage, current, and displacement waveforms of the piezoelectric disks and transformers were monitored by two digitizing oscilloscopes (Tektronix TDS 3014B) and logged by a personal computer The temperature on the surface of the sample nodal point was measured by an infrared spot thermometer (HIOKI 3445) and thermal images of the whole sample were taken with a thermal camera (ThermaCAM S40 Flir Systems outfitted with 200 mm lens) Vibration amplitudes on the edge of the piezoelectric disks/transformers were measured with laser interferometers (Polytec OFV 511) The HiPoCS™ characterized the behavior of the mechanical quality factor at various fixed vibration velocity frequency sweeps The high-power characterization was performed on 10.6 mm diameter  1.5 mm thick disks, driven under constant vibration velocity The impedance curves for the disks under various vibration velocities between 0.1 and 0.4 m/s were recorded in Fig 17.7 The “rms” value was adopted for the vibration velocity The resonance frequency appears to shift from higher to lower frequencies as the vibration velocity increased This trend is a usual fashion in high-power characteristics even in the PZT-based piezoelectric materials As the vibration velocity increases, the material tends to be softened and yields the resonance frequency to drop On the other hand, there is an unusual behavior in the resonance impedance change Even though the vibration velocity increases, the resonance impedance remained constant (Figs 17.7 and 17.8a), which is significantly different from the PZT’s case [11] This novel characteristic of the sample is a result of its mechanical quality factor (Qm) As shown in Fig 17.8b, NKN’s Qm does not drop with increased vibration velocity For high-power applications the Qm vs vibration velocity figure is 522 K Uchino Fig 17.7 Impedance curves for the NKN disk (10.6 diameter  1.5 mm thickness) at certain constant vibration velocities very important; as it represents the degradation of piezoelectric properties as power demand is increased Note that even at a temperature change of about 20 C, the NKN does not show a significant Qm drop (Refer to Fig 17.8b) The maximum vibration velocity (rms value defined at 20 C temperature rise) of 0.4 m/s for a disk shape corresponds to 1.4 m/s for a rectangular plate [11] Since the maximum vibration velocity for a PZT rectangular plate is 0.3 m/s typically and 0.6 m/s for the best, we can conclude that the NKN ceramic seems to be better than the PZT ceramics in terms of the maximum vibration velocity The unusual and promising result of an almost steady high mechanical quality factor (Qm ~ 900) at extreme vibration velocities, exceeding current PZT’s high-power performances, makes the evaluated composition a perfect candidate for high-power applications 17.4.2 Lead-Free Piezoelectric Transformers The above high-power study encouraged to examine the sample in a piezoelectric current-transformers (i.e., step-down type) Ring-dot high-power transformers with 10.6 mm diameter  0.5 mm thickness were designed, manufactured, and characterized in terms of power density, equivalent impedance, and gain [12] The picture and the design of the KNN ceramic ring-dot type transformer are shown in Fig 17.9a FEM simulation showing the vibration mode and the induced voltage is also illustrated in Fig 17.9b Using low-power impedance spectra obtained at 0.5 V with an impedance analyzer, the matching impedance was calculated by (17.1): Zm ¼ ; 2pfr Cb ðoutputÞ (17.1) 17 Applications of Lead-Free Piezoelectrics 523 a Impedance (W) 319.5 100 319 318.5 Frequency (kHz) 320 1000 Zr (Ω) 10 318 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Vibration Velocity (m/s) b 20 18 16 14 1000 12 10 100 Qm 10 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 DT(°C) Qm 10000 Vibration Velocity (m/s) Fig 17.8 (a) Impedance and resonance frequency change with vibration velocity (rms value) (b) Qm and DT as a function of vibration velocity The data were collected for the NKN disk (10.6 diameter  1.5 mm thickness) Fig 17.9 High-power NKN-based lead-free piezoelectric transformer: (a) picture of a ring-dot sample (b) FEM simulation showing the vibration mode and the induced voltage K Uchino Fig 17.10 Power density for the NKN-based ring-dot piezoelectric transformer under various external loads Note that the maximum is obtained around the impedance matching load Power Density (W/cm3) 524 30 25 20 15 10 100 1000 10000 100000 Load (Ω) where fr is the resonance frequency and Cb is the damped capacitance of the piezoelectric transformer The matching impedance for the NKN piezoelectric transformer was about 1.6 kO As a rule of thumb, high-power characterization was performed under various impedance load (0.5, 1.6, 2, and 10 kO) until sample temperature reached 20 C above the ambient temperature (i.e., under the maximum vibration velocity drive) The output power was calculated from the output voltage and the output current drawn from the transformer Thereafter, the output power was used to calculate the power density of the transformer The power density showed dependence on the external loads, and is shown in Fig 17.10 The maximum power density was obtained as 25 W/cm3 under the matching impedance, i.e., 1.6 kO By changing the external load the power level dropped to 0.25 W/cm3, 20 W/cm3, W/cm3 for 10, 2, and 0.5 kO respectively Since the PZT-based ring-dot transformer exhibits typically 20 W/cm3, our NKN ceramic again exceeds the PZT performance in a piezo-transformer device In conclusion, (Na0.5 K0.5)(Nb0.97Sb0.03)O3, prepared with 1.5% mol CuO addition, exhibited even better high-power characteristics than PZT In this material, instead of dropping mechanical quality factor (Qm) with increasing vibration velocity, Qm remained constant up to 0.4 m/s in disk-shaped samples, which corresponds to 1.4 m/s in rectangular plates This indicates that the candidate lead-free system has better characteristics under high-power levels To explore the practical applicability, ring-dot piezo-transformers were designed and manufactured Very high-power densities (as high as 25 W/cm3) were obtained, which is comparable to that of PZT piezo-transformers 17.5 Ultrasonic Motors with TB Taking into account general consumer attitude on disposability of portable equipment (printers, cellular phones), Taiyo Yuden, Japan developed microultrasonic motors using non-Pb multilayer piezoactuators [13] Their composition is based on tungsten bronze (TB) ((Sr,Ca)2NaNb5O15) without heavy metal or even K (potassium seems to be a little more toxic in comparison with Na) 17 Applications of Lead-Free Piezoelectrics 525 Fig 17.11 Principle of ceramic powder orientation under magnetic field [13] 17.5.1 Preparation of Oriented Ceramic The basic piezoelectric parameters in TB (d33 ¼ 55 ~ 80 pC/N, TC ¼ 300 C) are not very attractive However, once the c-axis oriented ceramics are prepared, the d33 is dramatically enhanced up to 240 pC/N Further since the Young’s modulus Y33E ¼ 140 GPa is more than twice of that of PZT, the higher generative stress is expected, which is suitable to ultrasonic motor applications Taiyo Yuden developed a sophisticated preparation technology for fabricating oriented ceramics with a multilayer configuration: that is, preparation under strong magnetic field, much simpler than the flaky powder preparation introduced in Fig 17.3 Figure 17.11 illustrates the principle of ceramic powder orientation under magnetic field Since most of piezoceramics are diamagnetic, the ceramic powder suspended in slurry will be aligned along its magnetically stable axis under a strong magnetic field (such as 10 T) [13] Because the polarization axis in their particular TB composition corresponds to the magnetically unstable axis, they used the magnetic field parallel to the green sheet A cut-green-sheet was rotated practically under steady magnetic field during drying period These oriented green sheets were electroded, laminated, and sintered They reported beautiful crystal orientation degree of the sintered product with Lotgering factor 0.9, as shown in Fig 17.12 [13] 17.5.2 Compact Ultrasonic Motor with Oriented Ceramic Figure 17.13(a) shows their compact rotary ultrasonic motor with a piezoelectric multilayer actuator (MLA) [13] A canti-lever rod is wobbled by a  arrayed 526 K Uchino Fig 17.12 Columnar grains grown along c-axis are normal to the tape plane (right-hand-side pictures), shown in comparison with the randomly oriented ceramic (left-hand-side pictures) Fig 17.13 (a) Compact rotary ultrasonic motor with a piezoelectric multilayer actuator, (b)  arrayed element, and (c) cross-section view of the 18 mm-layer ML element [13] MLA element, driven by a 4-phase voltage (sine, cosine, –sine and –cosine) They fabricated monolithic  arrayed elements with the size 2.6  2.6  1.1 mm3 including buffers with layer thickness as thin as 18 mm (Refer to Fig 17.13b, c) Because of this layer thinness and dramatic enhancement in the piezoelectric 17 Applications of Lead-Free Piezoelectrics 527 Fig 17.14 Ultrasonic motor characteristics (revolution speed, torque, efficiency vs input power) for SCNN ((Sr,Ca)2NaNb5O15) (a) and PZT-A (b) multilayer motors performance owing to the magnetic field alignment, the ultrasonic motor was successfully driven under only Vp–p which is low enough to be adopted in mobile phones without coupling a step-up drive circuit Figure 17.14a, b show ultrasonic motor characteristics (revolution speed, torque, efficiency vs input power) for the Pbfree SCNN ((Sr,Ca)2NaNb5O15), (a) and PZT-A (b) multilayer motors Though the actual drive voltage for the SCNN motor is Vp–p, times higher than for the PZT motor (1 Vp–p), the equivalent motor characteristics can be obtained for both motors under normal battery operation 17.6 Concluding Remarks Note that the initial motivation for the Pb-free piezoelectrics came from the Political Regulation (i.e., RoHS), in conjunction with Social/Cultural Pressure (i.e., sustainable society) Though this movement is not recommended at present from Economical Viewpoint (i.e., too expensive for the products at this moment), we need to overcome the problem from the new Technological Development in the nearest future The political force is the strongest factor for the practically successful commercialization Though the piezoelectric d constant and electromechanical coupling factor k (real parameters) of the present Pb-free materials are not superior to the values of PZTs, in general, in a similar randomly oriented ceramic configuration, commercialization efforts are overcoming this inferiority by using either (1) epitaxially oriented ceramic preparation method, or (2) sophisticated composite designing It is also intriguing to note that the maximum vibration velocity for the Pb-free piezoelectrics seem to be larger than that of the hard PZTs; that is, the vibration velocity is given by the product of piezoelectric d and mechanical quality factor Qm, and constant Qm of the Pb-free ceramic is sustained up to 1.4 m/s (rms) of the vibration velocity (in a rectangular plate specimen), while a regular hard PZT 528 K Uchino exhibits dramatic decay in Qm under 0.3 m/s (rms), even though the initial Qm under small excitation is reasonably high In conclusion, basic performances of the Pb-free piezoelectrics are almost satisfactory from the application viewpoints (transducers, transformers, and ultrasonic motors), though the off-resonance performance based on merely the piezoelectric constant d is not very excellent The remaining issues include (1) reproducibility of the product’s quality, and (2) inexpensive mass production process References Kuwata J, Uchino K, Nomura S (1982) Jpn J Appl Phys 21:1298 Hirashima Y (1981) Product planning in the feeling consumer era (in Japanese) JitsumuKyoiku Publisher, Tokyo Uchino K (2009) Entrepreneurship for engineers CRC Press, New York Fujii A (2005) Proc Smart Actuators/Sensors Study Committee JTTAS, Dec 2, Tokyo Saito Y (1996) Jpn J Appl Phys 35:5168–5173 Tou T, Hamaguchi Y, Maida Y, Yamamori H, Takahashi K, Terashima Y (2009) Jpn J Appl Phys 48:07GM03 Rosen CA (1957) Proc Electronic Compon Symp, p 205 Laoratanakul P, Carazo AV, Bouchilloux P, Uchino K (2002) Jpn J Appl Phys 41:1446–1450 Park H-Y, Seo I-T, Choi M-K, Nahm S, Lee H-G, Kang H-W, Choi B-H (2008) J Appl Phys 104(3):034103 10 Ural SO, Tuncdemir S, Zhuang Y, Uchino K (2009) Jpn J Appl Phys 48:056509 11 Ural SO, Zhuang Y, Tuncdemir S, Uchino K (2009) Jpn J Appl Phys 49:021502 12 Gurdal, EA, Ural SO, Park HY, Nahm S, Uchino K (2011) High power (Na0.5K0.5)NbO3-based lead-free piezoelectric transformer Jpn J Appl Phys 50:027101 13 Doshida Y (2009) Proc 81st Smart Actuators/Sensors Study Committee JTTAS, Dec 11, Tokyo .. .Lead-Free Piezoelectrics Shashank Priya l Sahn Nahm Editors Lead-Free Piezoelectrics Editors Shashank Priya Department of Mechanical... 461 Part V Applications 16 Self-Biased Lead-Free Magnetoelectric Laminates Su-Chul Yang and Shashank Priya 487 17 Applications of Lead-Free Piezoelectrics ... (3) a compilation of all the results on important family of lead-free materials is needed Further, the group identified the need for lead-free materials from Wikipedia These recommendations were