Ultrasonic motors provide high torques and quick responses compared to their magnetic counterparts; therefore, they are widely used in small-scale applications such as mobile phones, microrobots, and auto-focusing modules in digital cameras. To determine the feasibility of lead-free piezoceramics for ultrasonic motor applications, we fabricated a ring-type piezoceramic with a KNN-based lead-free piezoceramic (referred to as CZ5), intended for use in an auto-focusing module of a digital camera. The vibration of the lead-free stator was observed at 45.1kHz. It is noteworthy that the fully assembled lead-free ultrasonic motor exhibited a revolution speed of 5-7 rpm, even though impedance matching with neighboring components was not considered. This result suggests that the tested KNN-based piezoceramic has great potential for use in ultrasonic motor applications, requiring minimal modifications to existing lead-based systems.ope

Gwon, Jung-Ho

Han, Hyoung-Su

Hong, Chang-Hyo

Jo, Wook

Jung, Jin-Kyung

Lee, Sae-Shin

Li, Jing-Feng

Quyet, Nguyen Van

Wang, Ke

Yao, Hang-Zhou

Journal of Sensor Science and Technology

ScholarWorks@UNIST

228 J. Sensor Sci. & Tech. Vol. 24, No. 4, 2015Journal of Sensor Science and TechnologyVol. 24, No. 4 (2015) pp. 228-231http://dx.doi.org/10.5369/JSST.2015.24.4.228pISSN 1225-5475/eISSN 2093-7563Ring-Type Rotary Ultrasonic Motor Using Lead-free CeramicsChang-Hyo Hong1, Hyoung-Su Han1, Jae-Shin Lee2, Ke Wang3, Fang-Zhou Yao3,Jing-Feng Li3, Jung-Ho Gwon4, Nguyen Van Quyet4, Jin-Kyung Jung2, and Wook Jo1,+AbstractUltrasonic motors provide high torques and quick responses compared to their magnetic counterparts; therefore, they arewidely used in small-scale applications such as mobile phones, microrobots, and auto-focusing modules in digital cameras. Todetermine the feasibility of lead-free piezoceramics for ultrasonic motor applications, we fabricated a ring-type piezoceramicwith a KNN-based lead-free piezoceramic (referred to as CZ5), intended for use in an auto-focusing module of a digital camera.The vibration of the lead-free stator was observed at 45.1 kHz. It is noteworthy that the fully assembled lead-free ultrasonicmotor exhibited a revolution speed of 5–7 rpm, even though impedance matching with neighboring components was not con-sidered. This result suggests that the tested KNN-based piezoceramic has great potential for use in ultrasonic motor applications,requiring minimal modifications to existing lead-based systems.Keywords: Ultrasonic motors, Lead-free, (K0.5Na0.5)NbO3, Piezoelectrics, Auto-focusing module1. INTRODUCTIONBecause the RoHS (Restriction of Hazardous Substances)directive has been effective in the European Union; thedevelopment of environmentally friendly materials with salientproperties for electronic applications has been of great interestto the electronic materials community. Successive release oftougher regulations similar to RoHS worldwide has made thelead-free community one of the biggest within the electronicmaterials community. In spite of great effort over more thantwo decades, the market has seen only a few cases in whichlead-free piezoceramics have been utilized in consumerproducts. This is because the piezoelectric properties ofmarket-dominating PZT-based piezoceramics are very goodand easy tunable; no known lead-free piezoceramics cancompete with their versatility. This means that technologytransfer to industry has seldom been attempted; a material thatcompetes with PZT-based materials in terms of both propertiesand versatility is eagerly anticipated [1-5]. Although not yetsatisfactory, Bi1/2Na1/2TiO3 (BNT), KxNa(1-x)NbO3 (KNN), andtheir derivatives in the form of solid solutions are generallyconsidered the most promising materials [4,6-8]. Recently, we reported a highly promising lead-free piezoceramicwith a nominal composition of 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3-0.05CaZrO3 with 2 wt% MnO2 addition, (referred to as CZ5),which shows highly temperature-insensitive and fatigue-resistantelectric-field-induced strains [9]. The piezoelectric coefficients d33and d33* (Smax/Emax) reach 320 pC/N and 0.13% (at 4 kV/mm),respectively, at room temperature. It is noteworthy that althoughd33 varies significantly with increasing temperature, d33* remainsalmost the same up to temperatures as high as 175 °C (variations1School of Materials Science and Engineering, Ulsan National Institute ofScience and Technology, 50 UNIST-gil, Ulsan 689-798, Republic of Korea2School of Materials Science and Engineering, University of Ulsan, 93DaeHak-Ro, Ulsan 680-749, Repulic of Korea3State Key Laboratory of New Ceramics and Fine Processing, School ofMaterials Science and Engineering, Tsinghua University, Beijing 100084,China4HanaUtech, Techno 10-Ro, DaeJeon 305-510, Repulic of Korea+Corresponding author: wookjo@unist.ac.kr(Received: July 14, 2015, Accepted: July 20, 2015)This is an Open Access article distributed under the terms of the CreativeCommons Attribution Non-Commercial License(http://creativecommons.org/licenses/bync/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.Fig. 1. Strain of CZ5 (a) as a function of cycles, (b) as a function oftemperature.Ring-Type Rotary Ultrasonic Motor Using Lead-free Ceramics229 J. Sensor Sci. & Tech. Vol. 24, No. 4, 2015of less than ±10%) [9]. This temperature-insensitive straindegrades by only as much as ±3%, even up to 107 cycles at 50 Hzat room temperature (as shown in Fig. 1) [10].To quickly assess the suitability of CZ5 for commercialapplications, we fabricated a ring-type piezoceramic ultrasonicmotor (USM) to be embedded in a commercialized auto-focusingmodule in a digital camera. Although no mechanical impedancematching was applied between CZ5 and neighboring componentssuch as the stator and rotor, the USM propelled by CZ5 exhibitsgood performance, which makes CZ5 a highly promisingcandidate for USM applications.2. EXPERIMENTALThe oxides and carbonates; (Alfa Aesar GmbH & Co. KG,Karlsruhe, Germany), Li2CO3 (99.0%), Na2CO3 (99.5%), K2CO3(99.0%), Nb2O5 (99.5%), Ta2O5 (99.0%), CaCO3 (99.5%), ZrO2(99.5%), and MnO2 (99.5%); were mixed according to the previouslyreported chemical formula of 0.95(Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O3-0.05CaZrO3 with 2 wt% MnO2, followed by ball-milling for 24h in ethanol [9]. The ball-milled powder was calcined at 900oCfor 4 h and ball-milled again for 24 h. Polyvinyl alcohol(PVA), as a binder, was added into the calcined powder beforebeing pressed into ring-type disks of 77.4 mm in externaldiameter, 68.4 mm in internal diameter, and 1.0 mm inthickness under a pressure of 100 MPa. These ring-typesamples were sintered in air at 1150oC for 2 h.Sintered samples were ground and polished to a thickness of0.4 mm, and then painted with a silver paste using a speciallydesigned electrode pattern. After burning out the silver paste at700oC, the external side of the pellets was trimmed to fit thepoling jig because of uneven shrinkage. Specimens were poledat 2.5 kV/mm and 30oC in a silicon oil bath for 30 min in sucha way that positively and negatively poled regions werealternately induced. To fabricate the assembled motor, poledsamples were glued onto a stator attached to a flexible printedcircuit board (FPCB) under a pressure of 534 kPa at roomtemperature for 24 h.The piezoelectric coefficient of the ceramic samples wasmeasured using a d33 testing meter (ZJ-3A, Institute ofAcoustics, Chinese Academy of Sciences). The resistance andcapacitance of the stator were measured using a multimeter(Fluke 177 digital multimeter, Fluke Corporation, USA). Therevolution speed and torque were measured using a customizedtester.3. RESULTS AND DISCUSSIONSFig. 2 shows that the ring-type pellets for use in ultrasonicmotors were prepared with maximum tolerances in thickness anddiameter of ±10 µm and ±100 µm, respectively, to guaranteestable revolution speed, minimal noise, and maximal durability.The specimens were ground and lapped to a final thickness of 0.4mm (±9 µm). Changes in the material parameters were observed between thedisk pellets and ring-type samples, as shown in Table 1. The ring-type samples had a piezoelectric constant (d33) of 85 pC/N, anelectromechanical coupling factor (kp) of 0.18, and a mechanicalquality factor (Qm) of 103. In other words, the d33 and kp valueswere degraded by 74.3% and 61.7%, respectively; however, Qmwas enhanced by 106%.To check how well the ceramic body was glued onto the statorand electrically connected, as shown in Fig. 3, resistance andcapacitance were measured. The capacitance of PZT and CZ5were 5.6–5.8 nF and 9.0–9.2 nF respectively. The resistance of the CZ5 stator was similar to that of thecommercial PZT. Additionally, the d33 value of CZ5 was almostthe same before being glued onto the stator, as shown in Table 2.The assembled USM motor module is shown in Fig. 4. TheFig. 2. Ring-type CZ5 specimen (a) before and (b) after sintering.Table 1. Material constants of CZ5TypeDielectricpermittivity (εr)tanδ(%)d33(pC/N)kp(%)QmDisk 1735 2.10 320 47 50Ring-type 1262 3.42 85 18 103Fig. 3. A, B, and ground (G) points of stator.I 15 IChang-Hyo Hong, Hyoung-Su Han, Jae-Shin Lee, Ke Wang, Fang-Zhou Yao, Jing-Feng Li, Jung-Ho Gwon, Nguyen Van Quyet, Jin-Kyung Jung, and Wook JoJ. Sensor Sci. & Tech. Vol. 24, No. 4, 2015 230resonance of the CZ5 stator was found to be 45.1 kHz with aninput voltage of 110-120 Vp-p. The ultrasonic motor using CZ5had a revolution speed of up to 7 rpm with a torque of 50 gf·cmat 45.1 kHz. The PZT motor had a revolution speed of up to 55rpm with a torque of 450 gf·cm at 31 kHz. In spite of the overall inferior performance of CZ5 incomparison to PZT, CZ5 was found to have a lower power lossand enhanced electric power efficiency in operation because of itshigher capacitance (relative to PZT). Power storage in a capacitoris W = 1/2 × C × V2, which yields 5.39 × 10-5 J and 3.22 × 10-5J for CZ5 and PZT, respectively.In addition, the power loss in a capacitor corresponds to 2p × f× C × V2 (tangent delta), which indicates that CZ5 (30 J·s) resultedin a higher power loss than PZT (12 J·s) because of the largertangent delta and C values. This phenomenon could be one of thereasons for the lower speed and torque (in addition to theaforementioned impedance mismatch). Another reason for thelower torque in CZ5 might arise from the lower stiffness of CZ5,relative to that of PZT. The lower torque may be related to theforce generated during operation. Because the force generation isgiven by F = K (stiffness) × D (displacement) and the stiffness hasno distinct difference between CZ5 and PZT [11], the lowertorque value is considered to be caused by the lower displacementduring operation.4. CONCLUSIONSThis study reports a preliminary test of the feasibility of usinga lead-free piezoceramic for commercial uses. Based on thecurrent method of using PZT ultrasonic motors in commerciallenses, we successfully demonstrated a KNN-based lead-freepiezoelectric ultrasonic motor. Although no optimization effortwas made to maximize the performance of the device, the CZ5embedded auto-focusing module exhibited reasonable functionalperformance. Two issues of importance to be further studied wereidentified: 1) the reason why the ring-type CZ5 samples showedlower piezoelectric properties but higher mechanical quality factorthan the disk samples, and 2) the influence of mechanicalimpedance matching on the final performance of the USM.ACKNOWLEDGMENTThis work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korean government (MISP)(No. 2014R1A2A1A11053597).REFERENCES[1] J. Rödel, W. Jo, K. T. Seifert, E. M. Anton, T. Granzow, andD. Damjanovic, “Perspective on the development of lead-free piezoceramics”, J. Am. Cer. Soc., Vol. 92, pp. 1153-1177, 2009.[2] T. Takenaka and H. Nagata, “Current status and prospectsof lead-free piezoelectric ceramics”, J. Eur. Cer. Soc., Vol.25, pp. 2693-2700, 2005.[3] P. Panda, “Review: Environmental friendly lead-free piezo-electric materials”, J. Mater. Sci., Vol. 44, pp. 5049-5062,2009.[4] W. Jo, R. Dittmer, M. Acosta, J. Zang, C. Groh, E. Sapper,K. Wang, and J. Rödel, “Giant electric-field-induced strainsin lead-free ceramics for actuator applications – status andperspective”, J. Electroceram., Vol. 29, pp. 71-93, 2012.[5] J. Rödel, K. G. Webber, R. Dittmer, W. Jo, M. Kimura, andD. Damjanovic, “Transferring lead-free piezoelectricceramics into application”, J. Eur. Cer. Soc., Vol. 35, pp.1659-1681, 2015.[6] J.-F. Li, K. Wang, F.-Y. Zhu, L.-Q. Cheng, F.-Z. Yao, and D.J. Green, “(K,Na)NbO3-based lead-free piezoceramics: fun-damental aspects, processing technologies, and remainingchallenges”, J. Am. Cer. Soc., Vol. 96, pp. 3677-3696, 2013.[7] J. Wu, D. Xiao, and J. Zhu, “Potassium-sodium niobatelead-free piezoelectric materials: Past, present, and future ofphase boundaries”, Chem Rev, Vol. 115, pp. 2559-2595, 2015.[8] W. Liu and X. Ren, “Large Piezoelectric effect in Pb-freeTable 2. Resistance and capacitance of ultrasonic motorsMaterialsd33 (pC/N)Resistance(Ω)Capacitance(nF)A B G A/G B/GCZ5 85 0.2 0.2 0.7 8.644 9.164PZT 320 0.3 0.4 0.3 5.283 5.364Fig. 4. Ring-type vibrator and motor: (a) stator vibrator, (b) ultra-sonic motor.Table 3. Mechanical properties of assembled statorsRevolutionspeed (rpm)Torque(gf-cm)d33 (pC/N)Operating frequency(kHz)CZ5 5–7 ~50 103 45.1PZT 45–55 450 200–320 31–32I 16 IRing-Type Rotary Ultrasonic Motor Using Lead-free Ceramics231 J. Sensor Sci. & Tech. Vol. 24, No. 4, 2015ceramics”, Phys. Rev. Lett., Vol. 103, 2009.[9] K. Wang, F.-Z. Yao, W. Jo, D. Gobeljic, V. V. Shvartsman,D. C. Lupascu, J.-F. Li, and J. Rödel, “Temperature-insen-sitive (K,Na)NbO3-based lead-free piezoactuator seramics”,Adv. Funct. Mater., Vol. 23, pp. 4079-4086, 2013.[10] F.-Z. Yao, J. Glaum, K. Wang, W. Jo, J. Rödel, and J.-F. Li,“Fatigue-free unipolar strain behavior in CaZrO3 and MnO2co-modified (K,Na)NbO3-based lead-free piezoceramics”,Appl. Phy. Lett., Vol. 103, pp. 192907, 2013.[11] F.-Z. Yao, K. Wang, and J.-F. Li, “Comprehensive inves-tigation of elastic and electrical properties of Li/Ta-modified(K,Na) NbO3 lead-freepiezoceramics”, J. App. Phys., Vol.113, pp. 174105, 2013.I 17 I

Ring-Type Rotary Ultrasonic Motor Using Lead-free Ceramics

Ring-Type Rotary Ultrasonic Motor Using Lead-free Ceramics

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