Structural and piezoelectric properties of MnO2 ...

Sensors and Actuators A 200 (2013) 47–50

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Structural and piezoelectric properties of MnO2 -added 0.95(Na0.5 K0.5 )NbO3 –0.05SrTiO3 ceramics In-Tae Seo a , Chang-Hoi Choi a , Min-Soo Jang a , Bo-Yun Kim a , Guifang Han a , Sahn Nahm a,∗ , Kyung-Hoon Cho b , Jong-Hoo Paik c a

Department of Materials Science and Engineering, Korea University, 1-5 Ka, Anam-Dong, Sungbuk-Ku, Seoul 136-701, Republic of Korea Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, VA, USA c Korea Institute of Ceramic Engineering and Technology, Seoul, 153-801, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 30 May 2012 Received in revised form 14 September 2012 Accepted 31 October 2012 Available online 9 November 2012 Keywords: Piezoelectric Lead free NKN-ST MnO2 Microstructure

a b s t r a c t The structural and piezoelectric properties of the x mol% MnO2 -added 0.95(Na0.5−x/200 K0.5−x/200 )NbO3 –0.05SrTiO3 [(NK)xMN-ST] ceramics were investigated. A dense microstructure with enlarged grains was formed for the specimens sintered at 1040 ◦ C through liquid-phase sintering. However, the grain size decreased for the specimen sintered at 1060 ◦ C because of the formation of a large amount of the liquid phase. Therefore, the microstructural variations in the (NK)xMN-ST ceramics can be explained by liquid-phase-assisted abnormal grain growth. The specimens sintered at 1020 and 1060 ◦ C showed low piezoelectric properties because of their low relative density and small grain size, respectively. On the other hand, the (NK)xMN-ST ceramic with x = 0.1 sintered at 1040 ◦ C showed promising piezoelectric properties: d33 = 260 pC/N, kp = 0.42, Qm = 202 and εT33 /ε0 = 1291.

1. Introduction Pb(Zr1−x Tix )O3 (PZT)-based ceramics have been widely used in piezoelectric devices because of their outstanding piezoelectric properties [1]. However, since PZT-based ceramics contain more than 60 wt% Pb, which induces environmental problems, a large amount of research has been conducted with the aim of replacing these PZT-based ceramics. (Na0.5 K0.5 )NbO3 (NKN) ceramics have been investigated extensively as candidate piezoelectric ceramics because they have promising piezoelectric properties and high Curie temperatures [2–4]. However, NKN ceramics decompose easily in water, and Na2 O (or K2 O) evaporates during sintering at high temperatures. Therefore, it is very difficult to obtain specimens with dense microstructures and reliable piezoelectric properties. These problems can be solved by making NKN-based solid solutions. In particular, a lot of research interest has been concentrated on (1 − x)NKN-xLi(Nb, Ta, Sb)O3 solid solutions, because they exhibit outstanding piezoelectric properties [5–8]. In general, a Na-deficient liquid phase is formed in NKN-based ceramics because of the evaporation of Na2 O during sintering, and the densification of these ceramics occurs through liquid-phase

∗ Corresponding author. E-mail address: [email protected] (S. Nahm). 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.10.040

© 2012 Elsevier B.V. All rights reserved.

sintering [9,10]. For 0.95NKN–0.05SrTiO3 (NKN-ST) ceramics, however, a Na-deficient liquid phase is not formed. Therefore, it is very difficult to sinter NKN-ST ceramics using the conventional solid-state sintering method. Since the evaporation of Na2 O is not observed in NKN-ST ceramics, Na2 O was intentionally removed from the NKN-ST ceramics to produce a Na-deficient liquid phase, which assisted the densification of the NKN-ST ceramics [11]. The Na-subtracted NKN-ST ceramics sintered at 1080 ◦ C exhibited promising piezoelectric properties of d33 = 220 pC/N, kp = 0.4, Qm = 72 and εT33 /ε0 = 1447 [11]. In addition, NKN-ST ceramics, which were produced by spark-plasma-sintering method and hot press method, also exhibited the enhanced piezoelectric properties [12,13]. However, their sintering temperature was relatively high and the Qm value was low [11]. Therefore, a small amount of CuO was added to the NKN and NKN-ST ceramics to reduce the sintering temperature as well as to improve their piezoelectric properties [14,15]. The addition of CuO reduced the sintering temperature of the NKN-ST ceramics considerably to 960 ◦ C, but their d33 and kp values also decreased to 200 pC/N and 0.35, respectively, although Qm increased to 300. Therefore, it is necessary to find a new additive that can decrease the sintering temperature without reducing the piezoelectric properties such as the d33 and kp values. The MnO2 additive was generally used to reduce the sintering temperature as well as to increase the Qm value of the PZT- and NKN-based ceramics [16–19]. Therefore, in this work, MnO2 was added to the

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I.-T. Seo et al. / Sensors and Actuators A 200 (2013) 47–50

Na2 O- and K2 O-deficient NKN-ST ceramics, and their sintering behavior and piezoelectric properties were investigated. Moreover, the variation in the piezoelectric properties of these specimens is explained on the basis of the microstructural changes. 2. Experimental procedures Using conventional solid-state synthesis, x mol% MnO2 -added 0.95(N0.5−x/200 K0.5−x/200 )NbO3 -0.05SrTiO3 [(NK)xMN-ST] ceramics with 0.0 ≤ x ≤ 3.0 were prepared. The oxide compounds of K2 CO3 , Na2 CO3 , Nb2 O5 , SrO and TiO2 (all from High Purity Chemicals, >99%, Saitama, Japan) were mixed with zirconia balls in a plastic jar for 24 h, and then dried. The dried powders were calcined at 950 ◦ C for 3 h. Subsequently, the calcined (NK)xN-ST powders were re-milled with the MnO2 additive. The (NK)xMN-ST powders were dried and pressed into discs under a pressure of 100 kgf/cm2 and sintered at 1020–1060 ◦ C for various times. The thickness and diameter of a specimen are 1.0 and 14.0 mm, respectively. The structural properties of the specimens were examined by X-ray diffraction (XRD: Rigaku D/max-RC, Tokyo, Japan) and scanning electron microscopy (SEM: Hitachi S-4300, Osaka, Japan). The densities of the sintered specimens were measured using a water-immersion technique. A silver electrode was printed on the lapped surfaces, and the specimens were polled in silicone oil at 120 ◦ C by applying a DC field of 4–5 kV/mm for 60 min. The piezoelectric and dielectric properties and electromechanical coupling factor were determined using a d33 meter (Micro-Epsilon Channel Product DT-3300, Raleigh, NC, USA) and an impedance analyzer (Agilent Technologies HP 4294A, Santa Clara, CA, USA), according to IEEE standards. 3. Results and discussion Fig. 1(a)–(g) shows the XRD patterns of the (NK)xMN-ST ceramics with 0.0 ≤ x ≤ 3.0 sintered at 1040 ◦ C for 2 h. For the specimens with x < 1.5, a homogeneous (NK)xMN-ST phase was formed without a secondary phase. However, when x exceeded 1.0 mol%, peaks for the KTiNbO5 (KTN) secondary phase were observed, and their intensity increased with increasing x. The KTN phase was also observed in the Na-subtracted NKN-ST ceramics [11]. Since the Nb5+ and Ti4+ ions reacted with the K+ ions to form the KTN phase and the ionic size of the Mn4+ ion (67 pm) is very similar to that of Nb5+ (78 pm), the Mn4+ (or Mn3+ ) ions entered the Nb5+ (or Ti4+ ) sites and behaved as the hardener, leading to an increase in the Qm value of the (NK)xMN-ST ceramics. In addition, the

Fig. 1. XRD patterns of (NK)xMN-ST ceramics sintered at 1040 ◦ C for 2 h: (a) x = 0.0, (b) x = 0.5, (c) x = 1.0, (d) x = 1.5, (e) x = 2.0, (f) x = 2.5, and (g) x = 3.0.

lattice parameters of the (NK)xMN-ST ceramics with 0.5 ≤ x ≤ 3.0 are considered to have decreased with the increase in x because the amount of subtraction of K+ and Na+ ions increased with the increase in x. Fig. 2(a)–(c) shows the SEM images of the (NK)xMN-ST ceramics with 0.0 mol% ≤ x ≤ 2.0 mol% sintered at 1040 ◦ C for 2 h. For the specimens with x ≤ 0.5, a porous microstructure with small grains was observed (see Fig. 2(a) and (b)). However, when x exceeded 0.5 mol%, grain growth occurred and a dense microstructure developed, as shown in Fig. 2(c). Moreover, the liquid phase, which is indicated by the arrow in Fig. 2(c), was observed in the specimen containing 2.0 mol% MnO2 . The Na-deficient liquid phase was observed in the Na-subtracted NKN-ST ceramics, which contained the KTN secondary phase [11]. Therefore, it was expected that this Na-deficient liquid phase would also exist in the (NK)xMNST ceramics with x ≥ 1.5 mol%. Moreover, the enlarged grains show rectangular shapes with flat surfaces, as are usually observed in specimens showing abnormal grain growth [9–11,19,20]. A similar structure was also observed for the specimens with x ≥ 2.0 mol%. Therefore, the variation in the microstructure in the MnO2 -added (NK)xMN-ST ceramics could be explained by liquid phase assisted abnormal grain growth. Fig. 3(a)–(c) shows SEM images of the 1.0 mol% MnO2 added [(NK)1.0MN-ST] ceramics sintered at various temperatures for 2 h. For the specimen sintered at 1020 ◦ C, a microstructure with small grains developed, as shown in Fig. 3(a), indicating that the amount of the liquid phase was insufficient to assist grain growth. On the other hand, a sufficient amount of the liquid phase was formed for the specimen sintered at 1040 ◦ C, and a dense microstructure with enlarged grains was formed (see Fig. 3(b)). When the sintering temperature exceeded 1040 ◦ C, however, the grain size decreased again, as shown in Fig. 3(c). A large amount of the liquid phase was considered to be formed in this specimen, and many nuclei developed and impinged each other before grain growth, resulting in a microstructure with small grains, as shown in Fig. 3(c). Similar results have been observed frequently in ceramics in which grain growth occurred abnormally because of the presence of a liquid phase [9–11,17]. These results confirm that the microstructural variation in the (NK)xMN-ST ceramics can be explained by liquidphase-assisted abnormal grain growth. Fig. 3(d)–(f) shows SEM images of the (NK)1.0MN-ST ceramics sintered at 1040 ◦ C for various times. For the specimens sintered for short times (≤1.0 h), microstructure with small grains developed, indicating that either the amount of the liquid phase or the sintering time was insufficient for grain growth. The grain size increased for the specimen sintered for 2.0 h, as shown in Fig. 3(b). However, the average grain size also decreased for the specimen sintered for 10 h, as shown in Fig. 3(f). This result also suggests that a large amount of the liquid phase was formed in the specimen sintered at 10.0 h, which inhibited grain growth. Since the grain size influences the electrical properties of the specimen, control of the sintering time is also important for good electrical properties to be obtained. Fig. 4(a) shows the relative density, εT33 /ε0 , d33 , kp and Qm values of the (NK)xMN-ST ceramics sintered at various temperatures for 2 h. All the specimens sintered at 1020 ◦ C exhibited low relative densities of less than 90% of the theoretical density. However, for the specimens sintered above 1020 ◦ C, high relative densities (above 93% of the theoretical densities) was observed, indicating that sufficient amounts of the liquid phase were formed for the densification of the specimens. For the specimens sintered at 1020 ◦ C, the εT33 /ε0 , d33 , kp and Qm values were small because of the low relative densities. The specimens sintered at 1060 ◦ C also showed small εT33 /ε0 , d33 and kp values, although they had high relative densities. The small grain size and presence of a large amount of the liquid phase could be responsible for the low dielectric and piezoelectric properties. However, for the specimen sintered at 1060 ◦ C, a high


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Fig. 2. SEM images of (NK)xMN-ST ceramics sintered at 1040 ◦ C for 2 h: (a) x = 0.0, (b) x = 0.5, and (c) x = 2.0.

Fig. 3. SEM images of (NK)1.0MN-ST ceramics sintered at various temperatures for 2 h: (a) 1020 ◦ C, (b) 1040 ◦ C, and (c) 1060 ◦ C and SEM images of (NK)1.0MN-ST ceramics sintered at 1040 ◦ C for various times: (d) 0.5 h, (e) 1.0 h, and (f) 10.0 h.

Fig. 4. Relative density, εT33 /ε0 , d33 , kp and Qm values of (a) (NK)xMN-ST ceramics sintered at various temperatures for 2 h, and (b) (NK)1.0MN-ST ceramics sintered at 1040 ◦ C for various times.

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Qm value (>200) was obtained from the specimens containing a large amount of MnO2 (>1.5 mol%) because of the hardening effect of the Mn ions. On the other hand, for the (NK)xMN-ST ceramics sintered at 1040 ◦ C, εT33 /ε0 , d33 and kp values increased considerably with the addition of MnO2 because of their increased density and enlarged grain size. Moreover, the hardening effect of the Mn ions is also responsible for the increased Qm values of these specimens. In addition, the εT33 /ε0 and d33 values decreased when x exceeded 1.0 mol%, probably because of the presence of the KTN second phase. The hardening effect of the Mn ions is also related to the decreased εT33 /ε0 and d33 values. Fig. 4(b) shows the relative density, d33 , kp , Qm and εT33 /ε0 values of the (NK)1.0MN-ST ceramics sintered at 1040 ◦ C for various times. In the case of the specimen sintered for 30 min, the relative density was approximately 93.0% of the theoretical density. For the specimens sintered for 1.0 and 2.0 h, very high relatives densities (>96.0% of the theoretical density) were obtained. However, for the specimen sintered for 10 h, the relative density decreased slightly, probably because of the presence of a large amount of the liquid phase and KTN second phase. The d33 and kp values of the specimens sintered for short times (≤1.0 h) were small, probably because of the small grain size. The maximum d33 and kp values of 260 pC/N and 0.42, respectively, were obtained for the specimens sintered for 2.0 h. However, they decreased slightly for the specimen sintered for 10.0 h because of the presence of large amounts of the liquid phase and KTN secondary phase. The Qm value exhibited a similar variation with respect to the sintering time, but the specimen containing 1.0 mol% MnO2 and sintered at 1.0 h also showed a high Qm value, indicating that 1.0 mol% MnO2 is sufficient to induce the hardening effect in (NK)xN-ST ceramics. In addition, all the specimens sintered at 1040 ◦ C showed high εT33 /ε0 values of approximately 1300, and their variation with sintering time was not significant. 4. Conclusions In (NK)xMN-ST ceramics with x < 1.5 mol%, a homogeneous (NK)xMN-ST phase was formed. However, when x exceeded 1.0 mol%, the KTN secondary phase with the liquid phase was formed. The grain size of the specimens increased with increasing x, but decreased in the specimens with a large x values. The specimens sintered at 1020 ◦ C had small grains, which increased in size with increasing sintering temperature. However, the grain size decreased for the specimens sintered at a high temperature of 1060 ◦ C. Therefore, the microstructural variations in these (NK)xMN-ST ceramics can be explained by liquid phase assisted abnormal grain growth. In addition, Mn ions entered the Nb5+ (or Ti4+ ) sites and behaved as a hardener. All the specimens sintered at 1020 ◦ C exhibited low relative densities. However, for the specimens sintered above 1020 ◦ C, high relative densities (above 93% of the theoretical density) were observed, indicating that a sufficient amount of the liquid phase was formed for the densification of the specimens. For the specimens sintered at 1020 ◦ C, the εT33 /ε0 , d33 , kp and Qm values were small because of their low relative densities. The specimens sintered at 1060 ◦ C also showed small εT33 /ε0 , d33 and kp values, which can be explained by the presence of the

small grains and the large amount of the liquid phase. On the other hand, the 1.0 mol% MnO2 -added (NK)xN-ST ceramic sintered at 1040 ◦ C for 2.0 h showed enlarged grains and good piezoelectric and dielectric properties: d33 = 260 pC/N, kp = 0.42, Qm = 202 and εT33 /ε0 = 1291. References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, 1st ed, Academic, New York, 1971. [2] L. Egerton, D.M. Dillon, Piezoelectric and dielectric properties of ceramics in the system potassium sodium niobate, Journal of the American Ceramic Society 42 (1959) 438–442. [3] R.E. Jaeger, L. Egerton, Hot pressing of potassium-sodium niobates, Journal of the American Ceramic Society 45 (1962) 209–213. [4] G.H. Haertling, Properties of hot-pressed ferroelectric alkali niobate ceramics, Journal of the American Ceramic Society 50 (1967) 329–330. [5] Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, M. Nakamura, Lead-free piezoceramics, Nature 432 (2004) 84–86. [6] G.Z. Zang, J.F. Wang, H.C. Chen, W.B. Su, C.M. Wang, P. Qi, B.Q. Ming, J. Du, L.M. Zheng, S. Zhang, T.R. Shrout, Perovskite (Na0.5 K0.5 )1−x (LiSb)x Nb1−x O3 lead-free piezoceramics, Applied Physics Letters 88 (2006) 212908. [7] E. Hollenstein, M. Davis, D. Damjanovic, N. Setter, Piezoelectric properties of Liand Ta-modified (K0.5 Na0.5 )NbO3 ceramics, Applied Physics Letters 87 (2005), 18295-1-7. [8] D.M. Lin, K.W. Kwok, K.H. Lam, H.L.W. Chan, Structure and electrical properties of K0.5 Na0.5 NbO3 –LiSbO3 lead-free piezoelectric ceramics, Journal of Applied Physics 101 (2007), 074111-1-6. [9] H.Y. Park, C.W. Ahn, H.C. Song, J.H. Lee, S. Nahm, K. Uchino, H.G. Lee, H.J. Lee, Microstructure and piezoelectric properties of 0.95(Na0.5 K0.5 )NbO3 –0.05BaTiO3 ceramics, Applied Physics Letters 89 (2006), 062906-1-3. [10] H.Y. Park, K.-H. Cho, S. Nahm, D.-S. Paik, H.-G. Lee, D.-H. Kim, Microstructure and piezoelectric properties of the (1 − x)(Na0.5 K0.5 )NbO3 –xCaTiO3 lead-free ceramics, Journal of Applied Physics 102 (2007), 124101-1-5. [11] K.H. Cho, H.Y. Park, C.W. Ahn, S. Nahm, H.G. Lee, H.J. Lee, Microstructure and piezoelectric properties of 0.95(Na0.5 K0.5 )NbO3 –0.05SrTiO3 ceramics, Journal of the American Ceramic Society 90 (2007) 1946–1949. [12] Y. Shimojo, R. Wang, Dielectric and piezoelectric properties of MeTiO3 (Me = Ba and Sr) modified (K, Na)NbO3 , Journal of Korean Physics Society 46 (2005) 48–51. [13] R. Wang, R.J. Xie, K. hanada, K. Matsusaki, H. Bando, M. Itoh, Phase diagram and enhanced piezoelectricity in the strontium titanate doped potassium-sodium niobate solid solution, Physica Status Solidi A 202 (2005) R57–R59. [14] H.W. Park, J.Y. Choi, M.K. Choi, K.H. Cho, S. Nahm, H.G. Lee, H.W. Kang, Effect of CuO on the sintering temperature and piezoelectric properties of (Na0.5 K0.5 )NbO3 lead-free piezoelectric ceramics, Journal of the American Ceramic Society 91 (2008) 2374–2377. [15] I.T. Seo, K.H. Cho, H.Y. Park, S.J. Park, M.K. Choi, S. Nahm, H.G. Lee, H.W. Kang, H.J. Lee, Effect of CuO on the sintering and piezoelectric properties of 0.95(Na0.5 K0.5 )NbO3 –0.05SrTiO3 lead-free piezoelectric ceramics, Journal of the American Ceramic Society 91 (2008) 3955– 3960. [16] H.Y. Park, C.H. Nam, I.T. Seo, J.H. Choi, S. Nahm, H.G. Lee, K.J. Kim, S.M. Jeong, Effect of MnO2 on the piezoelectric properties of the 0.75Pb(Zr0.47 Ti0.53 )O3 0.25Pb(Zn1/3 Nb2/3 )O3 ceramics, Journal of the American Ceramic Society 93 (2010) 2537–2540. [17] C.H. Nam, H.Y. Park, I.T. Seo, J.H. Choi, S. Nahm, H.G. Lee, Effect of CuO on the sintering temperature and piezoelectric properties of MnO2 -doped 0.75Pb(Zr0.47 Ti0.53 )O3 -0.25Pb(Zn1/3 Nb2/3 )O3 ceramics, Journal of Alloys and Compounds 509 (2011) 3686–3689. [18] D.M. Lin, Q. Zheng, K.W. Kwok, C.G. Xu, C. Yang, Dielectric and piezoelectric properties of MnO2 -doped K0.5 Na0.5 Nb0.92 Sb0.08 O3 lead-free ceramics, Journal of Materials Science 21 (2010) 649–655. [19] D.M. Lin, K.W. Kwok, H.L.W. Chan, Piezoelectric and ferroelectric properties of Kx Na1−x NbO3 lead-free ceramics with MnO2 and CuO doping, Journal of Alloys and Compounds 461 (2008) 273–278. [20] S.H. Hong, D.Y. Kim, Effect of liquid content on the abnormal grain growth of alumina, Journal of the American Ceramic Society 84 (2001) 159–1600.