2TiO3‒KTaO3 lead‒free piezoelectric ceramics under low electric field

Sensors and Actuators A 293 (2019) 1–6

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

High electromechanical strain properties in SrTiO3–modified Bi1/2Na1/2TiO3–KTaO3 lead–free piezoelectric ceramics under low electric field Guo Wang a , Young-Hwan Hong a , Hoang Thien Khoi Nguyen a , Byeong Woo Kim b , Chang Won Ahn c , Hyoung-Su Han a,∗ , Jae-Shin Lee a a

School of Materials Science and Engineering, University of Ulsan, 14, Techno saneop-ro 55 beon-gil, Nam-gu, Ulsan, Republic of Korea Department of Electrical Engineering, University of Ulsan, 93, Daehak-ro, Nam-gu, Ulsan, Republic of Korea c Department of Physics and EHSRC, University of Ulsan, 93, Daehak-ro, Nam-gu, Ulsan, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 16 November 2018 Received in revised form 8 April 2019 Accepted 13 April 2019 Available online 15 April 2019 Keywords: Lead-free Relaxor Piezoelectric ceramics Ternary system Phase transition

a b s t r a c t This study investigated the structures, dielectric, ferroelectric and piezoelectric properties of ˜ (0.99–x)Bi1/2 Na1/2 TiO3 –0.01KTaO3 –xSrTiO3 (BNT–KT–100xST, x = 0.20 0.235) lead–free piezoelectric ceramics. These piezoceramics were synthesized by conventional solid–state reaction method. As a consequence, a large electrical strain and normalized strain (d33 * ≈ 793 pm/V) can be obtained even under 3 kV/mm as low electric field for BNT–KT−22.5ST ceramics. The phase transition between nonergodic relaxor (NER) and ergodic relaxor (ER) under electric field might be responsible for its large strain. It means that BNT–KT–100xST lead–free ceramics can be a promising candidate for actuator applications. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Piezoelectric ceramics play important role in micro–controllable sensors and actuators nowadays because they can convert mechanical energy to electrical energy interchangeably. Up to now, the practically available materials for piezoelectric applications are lead–based piezoelectric ceramics, such as lead zirconate titanate (PZT) because of their excellent piezoelectric properties. However, it is well known that PZT contains high content of toxic element Pb, which does harm to the environment as well as human health [1–4]. Moreover, the Restriction of Hazardous Substances Directive (RoHS) has been widely applied in many countries to restrict the use of hazardous substances including lead in electrical and electronic equipment [1–4]. Consequently, it is a great importance to develop environmentally friendly lead–free piezoelectric materials. Lots of efforts have been made to find out lead–free alternatives to lead–based piezoelectric ceramics over the decades [5–14]. (Bi, Na)TiO3 (BNT) is one of the most important and superior lead–free materials that was discovered by Smolenskii and Agranovskaya [15] in 1960. BNT–based piezoelectric ceramics can be a promising candidate

∗ Corresponding author. E-mail address: [email protected] (H.-S. Han). https://doi.org/10.1016/j.sna.2019.04.016 0924-4247/© 2019 Elsevier B.V. All rights reserved.

for actuator applications to replace lead–based piezoceramics because of their excellent electromechanical strain properties with an incipient piezoelectricity based on the relaxor [8,16–22]. However, it is a critical problem that high electric field (6 kV/mm≤) is required for such large strain properties in BNT–based incipient piezoceramics. In fact, most of applications relating to piezo actuator are based on PZT–based ceramics with 2–3 kV/mm operating field [1]. Therefore, the realization of large strain under low operating electric field is an urgent issue. One promising material to overcome this problem is BNT–SrTiO3 (BNT–ST). SrTiO3 (ST) doping was beneficial for improving the piezoelectric properties of BNT ceramics [23–25], and was able to reduce their remanent polarization, yielding relaxor behavior in all compositions [26]. It has been reported that a large normalized strain (d33 * ) of about 600 pm/V under low electric field for BNT–25ST ceramics was ascribed to the core–shell structure in recent reports [27,28]. Besides, we have recently clarified that nonergodic relaxor (ferroelectrics) to ergodic relaxor (relaxor) phase transition in BNT ceramics is induced by ST modification [29]. Considering the favorable properties of BNT-ST binary solid solution, some efforts had been made to further improve the actuating performance by forming a ternary solid solution, such as BNT − BaTiO3 −K0.5 Na0.5 NaO3 (BNT–BT–KNN) [30], BNT–(Bi1/2 K1/2 )TiO3 –ST (BNT–BKT–ST) [24], BNT–BT–ST [31], BNT–KNbO3 –ST (BNT–KN–ST) [32], BNT–ST–LiNbO3 (BNT–ST–LN) [33], BNT–ST–AgNbO3 (BNT–ST–AN) [34] and so on.

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100 ◦ C for 24 h. After proper mixing and milling, the dried milled powders were calcined at 850 ◦ C for 2 h with a heating rate of 5 ◦ C/min. The resulting powders were ball milled again with the same parameters. After drying, polyvinyl alcohol solution (PVA) was added as a binder. After that disk-shaped green bodies of 12 mm in diameter and about 1.2 mm in thickness were prepared. These green bodies were later sintered in the covered alumina crucibles at 1175 ◦ C for 2 h with a heating rate of 5 ◦ C/min. 2.2. Characterization

Fig. 1. Linear shrinkage and density values of BNT–KT–100xST ceramics as function of ST content.

this study tried to synthesize Accordingly, (0.99–x)Bi1/2 Na1/2 TiO3 –0.01KTaO3 –xSrTiO3 (BNT–KT–100xST, ˜ x = 0.20 0.235) ceramics as BNT–based a new ternary system. To clarify performances of this study, we systemically investigated the structures, phase transition behaviors, dielectric, ferroelectric, and piezoelectric properties of BNT–KT–100xST ceramics. 2. Experimental procedure 2.1. Preparation of materials (0.99-x)Bi1/2 Na1/2 TiO3 –0.01KTaO3 –xSrTiO3 (BNT–KT–100xST, x = 0.20, 0.21, 0.215, 0.225, 0.235) were synthesized by conventional solid–state reaction method. The raw materials Bi2 O3 (99.99%), Na2 CO3 (99.0%), TiO2 (99.99%), SrCO3 (99.9%), K2 CO3 (99.0%), Ta2 O5 (99.9%) were weighted according to the nominal compositions after drying at 100 ◦ C for 24 h. After weighing the starting raw powders according to their stoichiometric formula, they were mixed and milled thoroughly inside a polyethylene jar with ZrO2 balls (5–10 mm in diameter) as a mixing media and ethanol as solvent for 24 h, at the rate of 400 rpm. During milling process, when ball and ball or ball and jar wall collide, some amount of powder between them is repeatedly fractured, cold welded, flattened and re–welded, which reduce the particle size, and form uniform particles slurry. The wet slurry is then dried at

The density of the sintered ceramics was measured by using Archimedes’ immersion principle method using an electronic densimeter (SD-120 L, A&D, Japan). The X–ray diffraction (XRD) were characterized by a diffractometer (XRD RAD III, Rigaku, Japan) by using monochromatic CuK˛ radiation with the wavelength ␭K␣ = 1.54178 Å. The detection range was 20–70 degrees with a step size of 0.02◦ and a speed of 2◦ /min. the microstructure was studied by SEM and field–emission electron microscope (FE–SEM, JEOL, JSM–65OFF, Japan). For measuring the electrical properties, the silver paste was coated on both sides of the pellets and sintered at 700 ◦ C for 30 min to form electrodes. Temperature and frequency dependent permittivity and dielectric loss for unpoled and poled samples were measured by using a high temperature electric prober system (KEYSIGHT–E4980AL Precision LCR Meter, USA). The electric–field–induced polarization and strain curves for all samples were carried out at 1 Hz by the commercial aixPES setup (aixACCT aixPES, Aachen, Germany). 3. Results and discussions Fig. 1 shows the linear shrinkage and relative density values of (0.99–x)Bi1/2 Na1/2 T–0.01KTaO3 –xSrTiO3 (BNT–KT–100xST, x = 0.20, 0.21, 0.215, 0.225, and 0.235) ceramics as a function of ST content after sintering at 1175 ◦ C for 2 h. Linear shrinkage and relative density values for all samples reached about 16% and over 97% respectively. These results indicate that 1175 ◦ C as the sintering temperature is suitable for BNT–KT–ST ceramics regardless of ST modification. The polished and thermally–etched surface images of BNT–KT–100xST ceramics are displayed in Fig. 2. All samples revealed dense microstructures. The calculated average grain sizes of BNT–KT–100xST ceramics were increased from 3.9 ␮m for

Fig. 2. The polished and etched surface images of BNT–KT–100xST ceramics, (a) x = 0.20, (b) x = 0.215, (c) x = 0.215, (d) x = 0.225, and (e) x = 0.235, respectively.

G. Wang et al. / Sensors and Actuators A 293 (2019) 1–6

Fig. 3. X-ray diffraction patterns of BNT–KT–100xST ceramics as a function of ST content.

BNT–KT–20ST ceramics to 7.3 ␮m for BNT–KT–22.5ST ceramics, then decreased to 4.1 ␮m for BNT–KT–23.5ST ceramics. Fig. 3(a) represents XRD patterns of BNT–KT–100xST ceramics in the 2  range of 20–70◦ . All compositions revealed as single perovskite structures without secondary phase. In order to display the effects of ST modification, the diffraction patterns in the range of 46◦ –47◦ display in Fig. 3(b), which were extracted from Fig. 3(a). To clarify phase structures of BNT–KT–100xST ceramics, X–ray diffraction patterns for all samples were deconvoluted and fitted by using the Voigt profile function. Peaks for all compositions could be indexed based on a cubic perovskite structure, as is evident by the undetected splitting of any of the peaks except K␣2 that is

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consistently observed in other BNT–based lead–free relaxor materials [34–40]. It means that the crystal structure of BNT–KT–100xST ceramics was not influenced by ST modification. Fig. 4 depicts the temperature dependence of the dielectric constant (εr ) and dielectric loss (tan ı) for the unpoled and poled BNT–KT–100xST ceramics. All samples exhibited very broad peaks and obvious frequency dispersion from room temperature up to maximum dielectric constant temperature (Tm ). These features with the frequency–dependent dielectric properties are normally regarded as a fingerprint for relaxor (RE) materials, consistent with other reported literature on the BNT-based systems [41–45]. Moreover, Tm and ferroelectrics (FE or nonergodic relaxor, NER) to RE transition temperature (TF–R ) were gradually decreased with increasing ST content. The observed TF–R for BNT–KT–20ST and BNT–KT–21ST ceramics were 38 ◦ C and 32 ◦ C, respectively. However, TF–R were vanished when the modification level of ST exceeded 21.5 mol%. This implies that TF–R for these compositions exist in below room temperature. Furthermore, the disappearance of TF–R means that FE–to–RE phase transition occurred with modifying ST. This phenomenon has been commonly observed in other BNT–based ceramics [41–45]. The polarization hysteresis (P–E) curves for BNT–KT–100xST ceramics are exhibited in Fig. 5. In order to visually observe the relationship between compositions and their ferroelectric properties, the Pr as well as EC values were derived from the hysteresis curves and the results are shown in Fig. 5(f). In the case of BNT–KT–20ST ceramics, a good ferroelectricity revealed that remanent polarization (Pr ) and coercive field (Ec ) were around 22 ␮C/cm2 and 2 kV/mm, respectively. It is noted that a weakly pinched shape in BNT–KT–20ST ceramics was revealed, despite indicating a good ferroelectricity. The reason for this is that TF–R for BNT–KT–20ST

Fig. 4. Temperature dependence of dielectric constant (␧r ) and loss (tanı) for unpoled (top) and poled (bottom) BNT–KT–100xST ceramics, (a) and (f) x = 0.20, (b) and (g) x = 0.21, (c) and (h) x = 0.215, (d) and (i) x = 0.225, (e) and (j) x = 0.235.

Fig. 5. Polarization hysteresis curves for BNT–KT–100xST ceramics, (a) x = 0.20, (b) x = 0.21, (c) x = 0.215, (d) x = 0.225, (e) x = 0.235, and (f) the extracted Prmax , Pr , and Ec values.

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Fig. 6. Bipolar strain curves for BNT–KT–100xST ceramics, (a) x = 0.20, (b) x = 0.21, (c) x = 0.215, (d) x = 0.225, (e) x = 0.235, and (f) the extracted Smax and Sneg values.

Fig. 7. Unipolar strain curves for BNT–KT–100xST ceramics with applying electric field as 3 kV/mm (top) and 4 kV/mm (bottom), (a) and (f) x = 0.20, (b) and (g) x = 0.21, (c) and (h) x = 0.215, (d) and (i) x = 0.225, (e) and (j) x = 0.235.

ceramics exist near room temperature as discussed in Fig. 4. A further increase in ST modification led to drastic decreases in Pr and Ec with strongly pinched P–E curves. Eventually, Pr and Ec reached minimum values as 8 ␮C/cm2 and 0.93 kV/mm when the modification level of ST was 23.5 mol%. On the other hand, the electric–field–induced maximum polarization (Pmax ) values for BNT–KT–100xST ceramics were changed slightly with increasing ST modification. These results in P–E curves imply that FE–to–RE phase transition in BNT–KT–100xST ceramics was induced by ST modification. Furthermore, this approach is responsible for highly maintaining Pmax with electric–field–induced phase transition [8]. Fig. 6 shows the bipolar strain curves for BNT–KT–100xST ceramics as a function of ST content. To clarify the changes of strain behaviors for all compositions, maximum strain (Smax ) and negative strain (Sneg ) values were compared in Fig. 6(f). The butterfly shaped strain curves with large Sneg were exhibited in BNT–KT–20ST, BNT–KT–21ST, and BNT–KT–21.5ST ceramics. These imply that BNT–KT–20ST, BNT–KT–21ST, and BNT–KT–21.5ST ceramics were dominantly stabilized with FE (more precisely nonergodic relaxor, NER) [8,24,38]. Smax value of BNT–KT–20ST ceramics was around 0.12% and increased to 0.235% as the highest value in BNT–KT–22.5ST ceramics, then decreased to 0.195% in BNT–KT–23.5ST ceramics. Besides, Sneg values were slightly increased from 0.06% for BNT–KT–20ST ceramics to 0.07% for BNT–KT–21ST. It is well known that the large strain is commonly obtained with drastically decreasing Sneg as an incipient piezoelectricity [8,24,38]. In fact, the stabilized ergodic relaxor (ER) can be reversibly transformed by an applied electric field into FE [3,8].

Therefore, the vanished Sneg in BNT–KT–22.5ST ceramics means that the long–range ordered ferroelectricity in BNT–KT–100xST ceramics was destabilized by ST modification [8,24,38]. Unipolar strain curves for BNT–KT–100xST ceramics are illustrated in Fig. 7 with applying electric field as 3 and 4 kV/mm. We obtained that the trend for changes of unipolar strain properties were similar to results for bipolar strain curves. BNT–KT–20ST ceramics as dominantly stabilized FE composition exhibited linear unipolar strain curve. On the other hand, strain properties for the stabilized ER composition as incipient piezoelectricity were monotonically improved by ST modification (0.20< x< 23.5) with getting larger hysteresis [8,24,38], and then was declined in BNT–KT–23.5ST ceramics. We obtained the largest strain properties in BNT–KT–22.5ST ceramics regardless of applied electric fields. To clarify the achievements of this study, the normalized strain (corresponding to d33 * or Smax /Emax ) values for BNT–KT–100xST ceramics as a function of ST content are depicted for two different applying electric fields in Fig. 8(a) and compared with those of other BNT–based ceramics in Fig. 8(b). d33 * values for BNT–KT–100xST ceramics increased firstly and reached maximum values of 793 and 695 pm/V for BNT–KT–22.5ST ceramics with applying electric fields of 3 and 4 kV/mm respectively, then dramatically decreased in BNT–KT–23.5ST ceramics. Based on these results, the obtained d33 * values for BNT–KT–22.5ST ceramics in this study were comparable to other BNT–based ceramics as shown in Fig. 8(b). The relatively improved strain properties under low electric field in this study as compared with other studies are obviously related to decrease electric field for electric–field–induced phase transition (or poling field) in ER. In fact, there were some efforts to decrease poling field in BNT–based relaxor ceramics [33,38,46–54]. Furthermore, we believe that the changes in nanoscale structures (more precisely the localized chemical heterogeneities [55] or presence of the unintended nanoscale composites [28,56]) are responsible for the stabilization of relaxor or ferroelectric phases relating to strain properties as well controllable poling field. However, it was difficult to clarify the exact mechanism by using general analysis (SEM or XRD). Therefore, further works are needed with nanoscale analysis such as TEM or PFM for better understanding the exact mechanism. Furthermore, 793 pm/V under 3 kV/mm as the highest d33 * value means that we succeeded in improving strain properties under low applied electric field for BNT–based lead–free ceramics. Therefore, we believe that BNT–KT–22.5ST ceramics can be a promising candidate for practical applications. 4. Conclusion This study investigated the effect of ST modification on dielectric, ferroelectric, and strain properties of lead–free BNT–KT–100xST ceramics. We obtained linear shrinkage values of all samples beyond 16% and density over 5.5 g/cm3 with

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Fig. 8. (a) Normalized strain (corresponding to d33 * or Smax /Emax ) values for BNT–KT–100xST ceramics as a function of ST content, (b) comparison of d33 * values.

dense microstructure. These results indicate BNT–KT–ST ceramics were successfully synthesized. Ferroelectrics (or nonergodic relaxor)–to–relaxor (or ergodic relaxor) phase transition was induced by ST modification that was originated from the interrupted long-range ferroelectric order. As a consequence, a large electromechanical strain (d33 * ≈ 793 pm/V) was obtained even under 3 kV/mm as low electric field for BNT–KT–22.5ST ceramics. This result indicates that BNT–KT–ST lead–free piezoelectric ceramics as BNT–based a new ternary system can be a promising candidate for actuator applications. Acknowledgements This study was supported by the National Research Foundation (NRF) of Republic of Korea Grant (2016R1D1A3B01008169). Han acknowledges financial support from the National Research Foundation (NRF) of Republic of Korea Grant (2016R1C1B1014365). Ahn acknowledges financial support form Basic Science Program through the National Research Foundation (NRF) of Republic of Korea funded the Ministry of Science and ICT Grant (2018R1A2B6009210). References [1] J. Rödel, W. Jo, K.T.P. Seifert, E.-M. Anton, T. Granzow, D. Damjanovic, Perspective on the development of lead-free piezoceramics, J. Am. Ceram. Soc. 92 (2009) 1153–1177. [2] J. Rödel, K.G. Webber, R. Dittmer, W. Jo, M. Kimura, D. Damjanovic, Transferring lead-free piezoelectric ceramics into application, J. Eur. Ceram. Soc. 35 (2015) 1659–1681. [3] C.H. Hong, H.P. Kim, B.Y. Choi, H.S. Han, J.S. Son, C.W. Ahn, W. Jo, Lead–free piezoceramics – where to move on? J. Materiomics 2 (2015) 1–24. [4] J. Rödel, J.F. Li, Lead-free piezoceramics: status and perspectives, MRS Bull. 43 (2018) 576–580. [5] M. Acosta, N. Novak, V. Rojas, S. Patel, R. Vaish, J. Koruza, G.A. Rossetti Jr.,J. Rödel, BaTiO3 -based piezoelectrics: fundamentals, current status, and perspectives, Appl. Phys. Rev. 4 (2017), 041305. [6] J.F. Li, K. Wang, F.Y. Zhu, L.Q. Cheng, F.Z. Yao, D.J. Green, (K,Na)NbO3 –based lead–free piezoceramics: fundamental aspects, processing technologies, and remaining challenges, J. Am. Ceram. Soc. 96 (2013) 3677–3696. [7] J. Wu, D. Xiao, J. Zhu, Potassium–sodium niobate lead–free piezoelectric materials: past, present, and future of phase boundaries, Chem. Rev. 115 (2015) 2559–2595. [8] W. Jo, R. Dittmer, M. Acosta, J. Zang, C. Groh, E. Sapper, K. Wang, J. Rödel, Giant electric–field–induced strains in lead–free ceramics for actuator applications – status and perspective, J. Electroceram. 29 (2012) 71–93. [9] A.R. Paterson, H. Nagata, X. Tan, J.E. Daniels, Relaxor-ferroelectric transitions: Sodium bismuth titanate derivatives, MRS Bull. 43 (2018) 600–606. [10] K. Wang, B. Malic, J. Wu, Shifting the phase boundary: Potassium sodium niobate derivates, MRS Bull. 43 (2018) 607–611. [11] K. Shibata, R.P. Wang, T. Tou, J. Koruza, Applications of lead-free piezoelectric materials, MRS Bull. 43 (2018) 612–616. [12] H. Yokozawa, Y. Doshida, S. Kishimoto, T. Morita, Resonant-type smooth impact drive mechanism actuator using lead-free piezoelectric material, Sens. Actuator A Phys. 274 (2018) 179–183. [13] T. Zheng, J. Wu, D. Xiao, J. Zhu, Recent development in lead-free perovskite piezoelectric bulk materials, Prog. Mater. Sci. 98 (2018) 552–624.

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Biographies Guo Wang received his early university education in polymer materials and engineering, from Shanghai University of Science and Engineering, China from 2012 and 2016. For studying graduate studies, he moved to department of materials science and engineering, University of Ulsan, South Korea in 2016. He is currently studying lead-free piezoelectric ceramics. Young-Hwan Hong received his BS in 2016 and MS in School of Materials Science and Engineering in 2018 from University of Ulsan, South Korea He is currently studying about the BNT-based lead-free piezoelectric ceramics. Hoang Thien Khoi Nguyen received his early university education in materials from Ho Chi Minh City University of Technology, Vietnam from 2010 and 2015. For studying graduate studies, he moved to department of Material Science and Engineering, University of Ulsan, South Korea in September 2015. He is currently studying leadfree ferroelectric and piezoelectric materials of perovskite structure. His narrow research focus on relaxor/ferroelectric ceramic composite materials. Byeong Woo Kim is currently a professor of the Department of Electrical Engineering at the University of Ulsan, Republic of Korea. He received his BS in 1987, MS in 1990, and Ph.D. in 2002 from Hanyang University. He worked as a Visiting Researcher in 1989 at Kosaka Laboratory in Japan, as a Junior Researcher from 1990 to 1994 at CAS Co., and as Head at Korea Automotive Technology Institute from 1995 to 2006. Since moving to the University of Ulsan in 2007, he has investigated Automotive Electric and Electronics Control field. Chang Won Ahn is a Research Professor in Department of Physics and Energy Harvest Storage Research Center (EHSRC) at University of Ulsan, Republic of Korea. He received his Ph.D. from University of Ulsan in Department of Physics in 2007. His recent research interest is focused on the synthesis of nanotubes, thin films and textured ceramics of lead-free piezoelectric materials for the applications of energy-conversion devices. Particular interests are photovoltaic and photocatalysis effects of nanoporous ferroelectric materials. He has published over 110 papers in international journals and holds 10 Korean patents. Hyoung-Su Han is currently a Research Professor the School of Materials Science and Engineering at the University of Ulsan, Republic of Korea. He received his BS in 2008, MS in 2010, and Ph.D. in School of Materials Science and Engineering in 2013 from University of Ulsan. He worked as a postdoctoral associate at Technische Universität Darmstadt and Ulsan National Institute of Science and Technology (UNIST) from 2014 to 2016. He authored and coauthored over 40 journal papers. His recent research focuses on BNT-based lead-free relaxors, KNN-based lead-free piezoelectric materials, and their applications. Jae-Shin Lee is currently a Professor of the School of Materials Science and Engineering at the University of Ulsan, Republic of Korea. He received his BS in ceramic engineering in 1982 from Seoul National University and Ph.D. in materials science and engineering in 1986 from Korea Advanced Institute of Science and Technology (KAIST). He then worked for the Electronics and Telecommunications Research Institute (ETRI), Daejeon, Republic of Korea, for six years at the telecommunication components division. Since moving to the University of Ulsan in 1993, he has investigated piezoelectric ceramic materials, ultrasonic sensors, and multilayer ceramic actuators with his graduate students who got 68 master and Ph.D. degrees. He authored and coauthored over 165 journal papers and over 75 patents. He has carried out many academic and social activities as a 2018 Vice President of KIEEME (Korean Institute of Electrical and Electronic Materials Engineers), as a member of Reviewer Board of Korean NRF (2012–2016), and as a Vice-Chair of UOU-LINC (2012–2018).