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piezoelectric behavior in BiScO3–(Na0.5Bi0.5)TiO3–PbTiO3 piezoelectric ceramics
Journal of Alloys and Compounds 577S (2013) S488–S492
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Relaxor and ferro/piezoelectric behavior in BiScO3 –(Na0.5 Bi0.5 )TiO3 –PbTiO3 piezoelectric ceramics Zhonghua Yao ∗ , Zhe Song, Hanxing Liu ∗∗ , Hua Hao, Minghe Cao, Zhiyong Yu Key Laboratory of Advanced Technology for Specially Functional Materials, Ministry of Education and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 16 November 2011 Received in revised form 6 March 2012 Accepted 7 March 2012 Available online 3 April 2012 Keywords: Ceramics X-ray diffraction Dielectric properties Piezoelectric properties Ferroelectrics
a b s t r a c t (1 − x)[0.5BiScO3 –0.5(Na0.5 Bi0.5 )TiO3 ]–xPbTiO3 ((BS–NBT)–PTx, 0.30 ≤ x ≤ 0.70) piezoelectric ceramics have been fabricated by conventional solid-state reaction method. A morphotropic phase boundary (MPB) was confirmed in the range of PT content x between 0.35 and 0.43 by phase structure analysis. As PT content decreases, the relaxor behavior can be induced due to the appearance of polar nanodomains. The optimal ferroelectric and piezoelectric property was obtained at the composition of 0.31BS–0.31NBT–0.38PT with the following optimum properties: d33 = 141 pC/N, kp = 32.9%, Tm =350 ◦ C, Pr = 20.1 C/cm2 and Ec = 2.06 kV/mm, indicating a promising candidate for high-temperature piezoelectric application. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, both the industrial and scientific communities have expressed a strong need for device application at higher temperatures compared to commercial Pb(Zr,Ti)O3 (PZT) ceramics. So, much attention has been paid to the innovation of new piezoelectric ceramic system. Fortunately, a new Bi-based perovskite-type piezoelectric ceramic system, BiScO3 –PbTiO3 has been found and attracted great interests due to its high transition temperature of 450 ◦ C and excellent piezoelectric properties at ∼64% PbTiO3 near morphotropic phase boundary (MPB), which is much higher than those of PZT ceramics [1]. However, scandium oxide is expensive and lead oxide is toxic. Considering that the increasing demand for environmentally benign materials points in the direction of lead-free or low-lead ceramic materials with comparable properties for different electronic applications. Many methods have been carried out on to decrease the cost of Bi-based system. One of these methods is to introduce the low-cost third component into BiScO3 –PbTiO3 or relative system [2–8]. It is well known that (Na1/2 Bi1/2 )TiO3 (NBT) is a lead-free ferroelectric at room temperature with a diffuse phase transition (DPT) [9]. NBT is considered to be an excellent candidate of
∗ Corresponding author. Tel.: +86 2787864681; fax: +86 2787879468. ∗∗ Corresponding author. E-mail addresses: [email protected] (Z. Yao), [email protected] (H. Liu). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.038
lead-free piezoelectric ceramics because it is rhombohedral symmetry at room temperature with a relatively large remnant polarization Pr = 38 C/cm2 and relatively high transition temperature, which undergoes ferroelectric (rhombohedral)–antiferroelectric (tetragonal) phase transition at ∼230 ◦ C and the symmetry changes to cubic symmetry at ∼520 ◦ C [10]. Moreover, NBT can form solid solutions with many perovskite-type compositions, such as PbTiO3 [11], BiScO3 [12], (K0.5 Bi0.5 )TiO3 [13–15], BaTiO3 [14–20], SrTiO3 [21], PbZrO3 [22], NaNbO3 [23,24], etc. In this case, the lead-free compound, (Na0.5 Bi0.5 )TiO3 (NBT) was chosen as the third end member of BiScO3 –PbTiO3 compositions to develop a new cheap piezoelectric system with low-lead content. We will investigate ferroelectric, piezoelectric properties, and relaxor behavior in (1 − x)[0.5BiScO3 –0.5(Na0.5 Bi0.5 )TiO3 ]–xPbTiO3 ((BS–NBT)–xPT, 0.30 ≤ x ≤ 0.70) system. 2. Experimental (BS–NBT)–PTx ceramics were synthesized by conventional solid-state reaction method. Powders of TiO2 , Sc2 O3 , Bi2 O3 , PbO and Na2 CO3 (purity > 99%) were weighed according to the stoichiometry. Starting materials weighed and mixed by ball milling for 24 h with stabilized zirconia media and ethanol. After drying, the mixed powder was calcined at 750 ◦ C for 2 h, then ground. The as-calcined powders were mixed, ball-milled, then dried and pressed into disks of 12 mm in diameter and 1.5 mm in thickness by uniaxial pressing at 200 MPa using 2.5 wt% polyvinyl alcohol binders. These pellets were burned out the binder at 650 ◦ C for 2 h and sintered at 1020–1080 ◦ C depending on composition for 2 h in air. The bulk density was measured by Archimedes method. Phase structures were measured by a Philips vertical X-ray diffractometer (PW3050/60, MPSS) using Cu K␣1 radiation. An electrode of fired-on silver paste was used for the measurement of electrical properties, such as dielectric, ferroelectric, and piezoelectric
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Fig. 1. The graph of bulk density and relative density of the sintered ceramics with the compositions in the range of x = 0.30–0.70.
properties. The dielectric properties were measured at various frequencies using an LCR meter (HP 4284A) from room temperature to 650 ◦ C. The piezoelectric constant d33 and the electromechanical coupling factor kp were measured using a quasi-static piezoelectric d33 meter (Model ZJ-3D, Institute of Acoustics Academic, China) and an impedance analyzer (HP4294A) by the resonance and anti-resonance technique on the basis of IEEE standards [25]. A Radiant RT-66A standard ferroelectric test system was used to measure the polarization–electrical field (P–E) hysteresis loop.
3. Results and discussion 3.1. Structural analysis X-ray diffraction (XRD) experiments were performed on the sintered ceramics with the compositions in the range of x = 0.30–0.70. The ratios of the observed densities to the theoretical densities on the sintered ceramics were higher than 95% (Fig. 1). As shown in Fig. 2, all samples exhibited the characteristic of single phase solid solutions without any secondary phase. For the tetragonal symmetry, the (2 0 0) pseudocubic reflection is a doublet while (1 1 1) is a singlet. However, in the rhombohedral symmetry, the (2 0 0) pseudocubic peak is a singlet while the (1 1 1) pseudocubic peak is a doublet. Fig. 3 shows the changes in the (1 0 0), (1 1 1) and (0 0 2) reflections for (BS–NBT)–PTx with respect to PbTiO3 content. It is
Fig. 3. Partial X-ray diffraction (XRD) patterns of the sintered ceramics with the compositions in the range of x = 0.30–0.70.
interesting to note that the samples show the decreasing split of the (2 0 0) peak at the compositions as PT content decreases. The splitting of the peaks can normally be ascribed to lattice distortion. Very slight lattice distortions would result in very slight peak split. It is found that the split of (1 1 1) reflection cannot be detectable in (BS–NBT)–PTx (x > 0.43) while observed in (x ≤ 0.43), indicating that the split caused by Ka2 contribution is obscure but by lattice distortion. Furthermore, compared with these reflections, a MPB region from x = 0.43 to x = 0.35 can be supposed, indicating the coexistence of the tetragonal and rhombohedral structure. 3.2. Dielectric temperature spectra Fig. 4 shows the temperature dependence of the dielectric constant and loss of (BS–NBT)–PTx ceramics measured at the frequency of 1 kHz–1 MHz, respectively. As the composition of the samples moves from x = 0.70 to x = 0.30 two main changes were observed from Fig. 4: 1) The first dielectric anomaly around dielectric maximum temperature (Tm ) can be ascribed to the transition from antiferroelectric to paraelectric, consistent with BNT, BNT–NaNbO3 , NBT–PbTiO3 lead-free ceramics system [26–29]. For the whole studied compositions, the maximum of the permittivity decreases and shifts toward lower temperatures with decreasing x, i.e., from 474 ◦ C for x = 0.70 to 333 ◦ C for x = 0.38, then gradually increases from 333 ◦ C for x = 0.38 to 354 ◦ C for x = 0.30 at a frequency of 1 MHz, as shown in Fig. 4(i). The changes of dielectric maximum temperature indicate the formation of complete solid solution. In addition, the dielectric peaks become gradually broader and more diffused with the reduction of PbTiO3 content. 2) Furthermore, another permittivity anomaly [above Tm in Fig. 4(e–h) at 1 kHz] associated with the polarization relaxations occur at low frequencies with the reduction of PT content. This type of relaxation is not associated with the phase transition but with the space-charge contribution. Therefore, dielectric measurements at 1 MHz were adopted to study the transition behavior to avoid the interference by space-charge.
Fig. 2. X-ray diffraction (XRD) patterns of the sintered ceramics with the compositions in the range of x = 0.30–0.70.
As also discussed on the first dielectric anomaly, the transition peak becomes broadened gradually and more diffused with decreasing x. This suggests that a diffuse phase transition is induced and the ferroelectric behavior tends to a relaxor ferroelectric.
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Fig. 4. (a–h) Temperature dependence of dielectric constant and loss (1 MHz) of (BS–NBT)–PTx ceramics at 1 kHz–1 MHz and (i) Dielectric maximum temperature Tm as a function of PbTiO3 content for (BS–NBT)–PTx ceramics at 1 MHz. Where, (a) x = 0.70; (b) x = 0.60; (c) x = 0.50; (d) x = 0.43; (e) x = 0.40; (f) x = 0.38; (g) x = 0.35 and (h) x = 0.30.
However, there is no obvious shift of dielectric maximum temperature Tm with measuring frequency, similar to pure NBT ceramic. Park and Hong [21] reported that in (Na1/2 Bi1/2 )1−x Srx TiO3 (x < 0.18) system the shift of Tm with frequency was not observable due to space-charge contribution in spite of diffuse phase transition
behavior. As we know, polarization relaxation near Tm originates from polar nano- or micro-domains, typically in Pb-based relaxors [30,31]. Moreover, dielectric dispersion measurements of NBT have demonstrated the occurrence of relaxational phenomena which can be connected with the existence of polar regions [32].
Fig. 5. Plots of ln(1/K − 1/Km ) versus ln(T − Tm ) of (BS–NBT)–PTx samples at 1 MHz. Where, (a) x = 0.70; (b) x = 0.60; (c) x = 0.50; (d) x = 0.43; (e) x = 0.40; (f) x = 0.38; (g) x = 0.35 and (h) x = 0.30.
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law, was determined. It is noted that with the reduction of x, the relaxation increases and the largest relaxation was observed with = 1.95 for the sample at x = 0.38, as shown in Fig. 5(i). 3.3. Ferroelectric and piezoelectric properties
Fig. 6. Piezoelectric properties of (BS–NBT)–PTx ceramics as a function of PT content.
Similiarly, with decreasing PT content, the increase of ionic displacement and space-charge contribution around Tm can explain the independence of Tm with measuring frequency. The diffuseness of a phase transition can be determined from the modified Curie–Weiss law [33], 1 (T − Tm ) = C K − 1/Km
(1)
where Km is the maximum value of the dielectric constant at the transition temperature (Tm ), C is the Curie-like constant and is the degree of diffuseness where the value of varies between 1 (normal ferroelectric) and 2 (ideal relaxor ferroelectric). Fig. 5 shows the plots of ln(1/K − 1/Km ) versus ln(T − Tm ) of (BS–NBT)–PTx samples at 1 MHz. All the samples exhibit a linear relationship. By leastsquares fitting the experimental data to the modified Curie–Weiss
For piezoelectric measurement, specimens were poled at 80 ◦ C for 15 min without electrical breakdown. As we know, the MPB is well known to play a more important role in the enhancement of piezoelectric properties. For example, in lead-based perovskite ceramics, such as PZT [34] and PMN–PT [35], their piezoelectric properties become maximal near the morphotropic phase boundary region. Piezoelectric constant d33 and electromechanical coupling factor kp for compositions with x are shown in Fig. 6. It is observed that the best piezoelectric and electrochemical properties are obtained in the composition near MPB. It is normally believed that the ceramics near the MPB may contain both the rhombohedral and tetragonal phases and thus more possible polarization states [36], which consequently lead to the significant enhancement in the piezoelectric properties. Specimens near MPB are shown to be ferroelectric. Fig. 7 shows the polarization–electric field curve of the studied compositions close to MPB. An optimal remnant polarization Pr = 20.1 C/cm2 with a coercive field Ec = 2.06 kV/mm was obtained at the composition of x = 0.38. 4. Conclusions In summary, (BS–NBT)–PTx ceramic has been fabricated by conventional sintering technique. As the PT content decreases, relaxor behavior can be induced due to polar nanodomains. Accordingly, the ceramics with 0.35 < x < 0.43 contain both rhombohedral and tetragonal phases at room temperature confirmed by XRD spectra. It is suggested that owing to the more possible polarization states resulting from the coexistence of the two phases, the
Fig. 7. Polarization–electric field curve (P–E) of the studied compositions near MPB. Where, (a) x = 0.40; (b) x = 0.38; (c) x = 0.35 and (d) x = 0.30.
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BS–NBT–xPT ceramic at x = 0.38 exhibits the following optimum properties: d33 = 141 pC/N, kp = 32.9%, Tm = 350 ◦ C, Pr = 20.1 C/cm2 , Ec = 2.06 kV/mm with environment-friendly character compared to high lead PZT systems, indicative of a promising candidate for hightemperature piezoelectric application. Acknowledgments The authors would like to thank the support of key programmer of Natural Science Foundation of China (no.:50932004), Natural Science Foundation of China (no.:50872102 and no.:51102189), the Key Grant Project of Chinese Ministry of Education (no.:309022), and the Fundamental Research Funds for the Central Universities (no.:2010-IV-045). References [1] R.E. Eitel, C.A. Randall, T.R. Shrout, S.E. Park, Japanese Journal of Applied Physics 41 (2002) 2099. [2] T. Song, R.E. Eitel, T.R. Shrout, C.A. Randall, Japanese Journal of Applied Physics 43 (2004) 5392. [3] Z.H. Yao, H.X. Liu, H. Hao, M. Cao, Journal of Alloys and Compounds 505 (2010) 281. [4] J. Ryu, S. Priya, C. Sakaki, K. Uchino, Japanese Journal of Applied Physics 41 (2002) 6040. [5] Z.H. Yao, H.X. Liu, Y. Liu, L. Chen, H. Hao, Materials Research Bulletin 44 (2009) 1511. [6] Z.H. Yao, H.X. Liu, H. Hao, M. Cao, Japanese Journal of Applied Physics 109 (2011) 014105. [7] Z.H. Yao, L.Y. Peng, H.X. Liu, H. Hao, M. Cao, Z.Y. Yu, Journal of Alloys and Compounds 509 (2011) 5637. [8] F. Gao, R.Z. Hong, J.J. Liu, Z. Li, L.H. Cheng, C.S. Tian, Journal of Alloys and Compounds 475 (2009) 619. [9] G.A. Smolenskii, V.A. Isupov, A.I. Agranovskaya, N.N. Kainik, Soviet Physics-Solid State 2 (1961) 2651. [10] I.P. Pronin, P.P. Syrnikov, V.A. Isupov, V.M. Egorov, N.V. Zaitseva, Ferroelectrics 25 (1980) 395.
[11] S.E. Park, K.S. Hong, Journal of Applied Physics 79 (1996) 383. [12] P. Marchet, E. Boucher, V. Dorcet, J.P. Mercurio, Journal of the European Ceramic Society 26 (2006) 3037. [13] T. OH, Japanese Journal of Applied Physics 45 (2006) 5138. [14] M. Zou, H. Fan, L. Chen, W. Yang, Journal of Alloys and Compounds 495 (2010) 280. [15] Y. Zhang, R. Chu, Z. Xu, J. Hao, Q. Chen, F. Peng, W. Li, G. Li, Q. Yin, Journal of Alloys and Compounds 502 (2010) 341. [16] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, Journal of the European Ceramic Society 22 (2002) 2115. [17] Q. Gou, J. Wu, A. Li, B. Wu, D. Xiao, J. Zhu, Journal of Alloys and Compounds 521 (2012) 4. [18] M. Cernea, L. Trupina, C. Dragoi, B.S. Vasile, R. Trusca, Journal of Alloys and Compounds 515 (2012) 166. [19] H. Ni, L. Luo, W. Li, Y. Zhu, H. Luo, Journal of Alloys and Compounds 509 (2011) 3958. [20] Z. Chen, J. Ding, M. Lin, N. Yuan, Y. Zhu, Journal of Alloys and Compounds 509 (2011) 482. [21] S.E. Park, K.S. Hong, Journal of Materials Research 12 (1997) 2152. [22] J.K. Lee, H.J. Youn, K.S. Hong, S.E. Park, Journal of Applied Physics 85 (1999) 368. [23] T. Wada, K. Toyoike, Y. Imanaka, Y. Matsuo, Japanese Journal of Applied Physics 40 (2001) 5703. [24] T. Chen, T. Zhang, G. Wang, J. Zhou, J. Zhang, Y. Liu, Journal of Alloys and Compounds 520 (2012) 7. [25] ANSI/IEEE Standard No. 176-1987, IEEE Standard on Piezoelectricity, IEEE, New York, 1987. [26] W. Sakamoto, M. Mizuno, Journal of Alloys and Compounds 538 (2006) 408. [27] S. Said, J. Mercrio, Journal of the European Ceramic Society 21 (2001) 1333. [28] T. Takenaka, T. Okuda, K. Takegahara, Ferroelectrics 196 (1997) 175. [29] K. Sakata, T. Takenaka, Y. Naitou, Ferroelectrics 131 (1992) 2. [30] A.A. Bokov, H. Luo, Z.G. Ye, Materials Science and Engineering: B 120 (2005) 206. [31] N. de Mathan, E. Husson, G. Calvarin, J.R. Gavarri, A.W. Hewat, A. Morell, Journal of Physics: Condensed Matter 3 (1991) 8159. [32] J. Suchanicz, Materials Science and Engineering: B 55 (1998) 114. [33] K. Uchino, S. Nomura, Ferroelectrics Letters Section 44 (1982) 55. [34] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic, London, 1971. [35] B. Noheda, D.E. Cox, G. Shirane, J. Gao, Z.G. Ye, Physical Review B 66 (2002) 054104. [36] C.A. Randall, N. Kim, J.P. Kucera, W. Cao, T.R. Shrout, Journal of the American Ceramic Society 81 (1998) 677.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Relaxor and ferro/piezoelectric behavior in BiScO3 –(Na0.5 Bi0.5 )TiO3 –PbTiO3 piezoelectric ceramics Zhonghua Yao ∗ , Zhe Song, Hanxing Liu ∗∗ , Hua Hao, Minghe Cao, Zhiyong Yu Key Laboratory of Advanced Technology for Specially Functional Materials, Ministry of Education and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 16 November 2011 Received in revised form 6 March 2012 Accepted 7 March 2012 Available online 3 April 2012 Keywords: Ceramics X-ray diffraction Dielectric properties Piezoelectric properties Ferroelectrics
a b s t r a c t (1 − x)[0.5BiScO3 –0.5(Na0.5 Bi0.5 )TiO3 ]–xPbTiO3 ((BS–NBT)–PTx, 0.30 ≤ x ≤ 0.70) piezoelectric ceramics have been fabricated by conventional solid-state reaction method. A morphotropic phase boundary (MPB) was confirmed in the range of PT content x between 0.35 and 0.43 by phase structure analysis. As PT content decreases, the relaxor behavior can be induced due to the appearance of polar nanodomains. The optimal ferroelectric and piezoelectric property was obtained at the composition of 0.31BS–0.31NBT–0.38PT with the following optimum properties: d33 = 141 pC/N, kp = 32.9%, Tm =350 ◦ C, Pr = 20.1 C/cm2 and Ec = 2.06 kV/mm, indicating a promising candidate for high-temperature piezoelectric application. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, both the industrial and scientific communities have expressed a strong need for device application at higher temperatures compared to commercial Pb(Zr,Ti)O3 (PZT) ceramics. So, much attention has been paid to the innovation of new piezoelectric ceramic system. Fortunately, a new Bi-based perovskite-type piezoelectric ceramic system, BiScO3 –PbTiO3 has been found and attracted great interests due to its high transition temperature of 450 ◦ C and excellent piezoelectric properties at ∼64% PbTiO3 near morphotropic phase boundary (MPB), which is much higher than those of PZT ceramics [1]. However, scandium oxide is expensive and lead oxide is toxic. Considering that the increasing demand for environmentally benign materials points in the direction of lead-free or low-lead ceramic materials with comparable properties for different electronic applications. Many methods have been carried out on to decrease the cost of Bi-based system. One of these methods is to introduce the low-cost third component into BiScO3 –PbTiO3 or relative system [2–8]. It is well known that (Na1/2 Bi1/2 )TiO3 (NBT) is a lead-free ferroelectric at room temperature with a diffuse phase transition (DPT) [9]. NBT is considered to be an excellent candidate of
∗ Corresponding author. Tel.: +86 2787864681; fax: +86 2787879468. ∗∗ Corresponding author. E-mail addresses: [email protected] (Z. Yao), [email protected] (H. Liu). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.038
lead-free piezoelectric ceramics because it is rhombohedral symmetry at room temperature with a relatively large remnant polarization Pr = 38 C/cm2 and relatively high transition temperature, which undergoes ferroelectric (rhombohedral)–antiferroelectric (tetragonal) phase transition at ∼230 ◦ C and the symmetry changes to cubic symmetry at ∼520 ◦ C [10]. Moreover, NBT can form solid solutions with many perovskite-type compositions, such as PbTiO3 [11], BiScO3 [12], (K0.5 Bi0.5 )TiO3 [13–15], BaTiO3 [14–20], SrTiO3 [21], PbZrO3 [22], NaNbO3 [23,24], etc. In this case, the lead-free compound, (Na0.5 Bi0.5 )TiO3 (NBT) was chosen as the third end member of BiScO3 –PbTiO3 compositions to develop a new cheap piezoelectric system with low-lead content. We will investigate ferroelectric, piezoelectric properties, and relaxor behavior in (1 − x)[0.5BiScO3 –0.5(Na0.5 Bi0.5 )TiO3 ]–xPbTiO3 ((BS–NBT)–xPT, 0.30 ≤ x ≤ 0.70) system. 2. Experimental (BS–NBT)–PTx ceramics were synthesized by conventional solid-state reaction method. Powders of TiO2 , Sc2 O3 , Bi2 O3 , PbO and Na2 CO3 (purity > 99%) were weighed according to the stoichiometry. Starting materials weighed and mixed by ball milling for 24 h with stabilized zirconia media and ethanol. After drying, the mixed powder was calcined at 750 ◦ C for 2 h, then ground. The as-calcined powders were mixed, ball-milled, then dried and pressed into disks of 12 mm in diameter and 1.5 mm in thickness by uniaxial pressing at 200 MPa using 2.5 wt% polyvinyl alcohol binders. These pellets were burned out the binder at 650 ◦ C for 2 h and sintered at 1020–1080 ◦ C depending on composition for 2 h in air. The bulk density was measured by Archimedes method. Phase structures were measured by a Philips vertical X-ray diffractometer (PW3050/60, MPSS) using Cu K␣1 radiation. An electrode of fired-on silver paste was used for the measurement of electrical properties, such as dielectric, ferroelectric, and piezoelectric
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Fig. 1. The graph of bulk density and relative density of the sintered ceramics with the compositions in the range of x = 0.30–0.70.
properties. The dielectric properties were measured at various frequencies using an LCR meter (HP 4284A) from room temperature to 650 ◦ C. The piezoelectric constant d33 and the electromechanical coupling factor kp were measured using a quasi-static piezoelectric d33 meter (Model ZJ-3D, Institute of Acoustics Academic, China) and an impedance analyzer (HP4294A) by the resonance and anti-resonance technique on the basis of IEEE standards [25]. A Radiant RT-66A standard ferroelectric test system was used to measure the polarization–electrical field (P–E) hysteresis loop.
3. Results and discussion 3.1. Structural analysis X-ray diffraction (XRD) experiments were performed on the sintered ceramics with the compositions in the range of x = 0.30–0.70. The ratios of the observed densities to the theoretical densities on the sintered ceramics were higher than 95% (Fig. 1). As shown in Fig. 2, all samples exhibited the characteristic of single phase solid solutions without any secondary phase. For the tetragonal symmetry, the (2 0 0) pseudocubic reflection is a doublet while (1 1 1) is a singlet. However, in the rhombohedral symmetry, the (2 0 0) pseudocubic peak is a singlet while the (1 1 1) pseudocubic peak is a doublet. Fig. 3 shows the changes in the (1 0 0), (1 1 1) and (0 0 2) reflections for (BS–NBT)–PTx with respect to PbTiO3 content. It is
Fig. 3. Partial X-ray diffraction (XRD) patterns of the sintered ceramics with the compositions in the range of x = 0.30–0.70.
interesting to note that the samples show the decreasing split of the (2 0 0) peak at the compositions as PT content decreases. The splitting of the peaks can normally be ascribed to lattice distortion. Very slight lattice distortions would result in very slight peak split. It is found that the split of (1 1 1) reflection cannot be detectable in (BS–NBT)–PTx (x > 0.43) while observed in (x ≤ 0.43), indicating that the split caused by Ka2 contribution is obscure but by lattice distortion. Furthermore, compared with these reflections, a MPB region from x = 0.43 to x = 0.35 can be supposed, indicating the coexistence of the tetragonal and rhombohedral structure. 3.2. Dielectric temperature spectra Fig. 4 shows the temperature dependence of the dielectric constant and loss of (BS–NBT)–PTx ceramics measured at the frequency of 1 kHz–1 MHz, respectively. As the composition of the samples moves from x = 0.70 to x = 0.30 two main changes were observed from Fig. 4: 1) The first dielectric anomaly around dielectric maximum temperature (Tm ) can be ascribed to the transition from antiferroelectric to paraelectric, consistent with BNT, BNT–NaNbO3 , NBT–PbTiO3 lead-free ceramics system [26–29]. For the whole studied compositions, the maximum of the permittivity decreases and shifts toward lower temperatures with decreasing x, i.e., from 474 ◦ C for x = 0.70 to 333 ◦ C for x = 0.38, then gradually increases from 333 ◦ C for x = 0.38 to 354 ◦ C for x = 0.30 at a frequency of 1 MHz, as shown in Fig. 4(i). The changes of dielectric maximum temperature indicate the formation of complete solid solution. In addition, the dielectric peaks become gradually broader and more diffused with the reduction of PbTiO3 content. 2) Furthermore, another permittivity anomaly [above Tm in Fig. 4(e–h) at 1 kHz] associated with the polarization relaxations occur at low frequencies with the reduction of PT content. This type of relaxation is not associated with the phase transition but with the space-charge contribution. Therefore, dielectric measurements at 1 MHz were adopted to study the transition behavior to avoid the interference by space-charge.
Fig. 2. X-ray diffraction (XRD) patterns of the sintered ceramics with the compositions in the range of x = 0.30–0.70.
As also discussed on the first dielectric anomaly, the transition peak becomes broadened gradually and more diffused with decreasing x. This suggests that a diffuse phase transition is induced and the ferroelectric behavior tends to a relaxor ferroelectric.
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Fig. 4. (a–h) Temperature dependence of dielectric constant and loss (1 MHz) of (BS–NBT)–PTx ceramics at 1 kHz–1 MHz and (i) Dielectric maximum temperature Tm as a function of PbTiO3 content for (BS–NBT)–PTx ceramics at 1 MHz. Where, (a) x = 0.70; (b) x = 0.60; (c) x = 0.50; (d) x = 0.43; (e) x = 0.40; (f) x = 0.38; (g) x = 0.35 and (h) x = 0.30.
However, there is no obvious shift of dielectric maximum temperature Tm with measuring frequency, similar to pure NBT ceramic. Park and Hong [21] reported that in (Na1/2 Bi1/2 )1−x Srx TiO3 (x < 0.18) system the shift of Tm with frequency was not observable due to space-charge contribution in spite of diffuse phase transition
behavior. As we know, polarization relaxation near Tm originates from polar nano- or micro-domains, typically in Pb-based relaxors [30,31]. Moreover, dielectric dispersion measurements of NBT have demonstrated the occurrence of relaxational phenomena which can be connected with the existence of polar regions [32].
Fig. 5. Plots of ln(1/K − 1/Km ) versus ln(T − Tm ) of (BS–NBT)–PTx samples at 1 MHz. Where, (a) x = 0.70; (b) x = 0.60; (c) x = 0.50; (d) x = 0.43; (e) x = 0.40; (f) x = 0.38; (g) x = 0.35 and (h) x = 0.30.
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law, was determined. It is noted that with the reduction of x, the relaxation increases and the largest relaxation was observed with = 1.95 for the sample at x = 0.38, as shown in Fig. 5(i). 3.3. Ferroelectric and piezoelectric properties
Fig. 6. Piezoelectric properties of (BS–NBT)–PTx ceramics as a function of PT content.
Similiarly, with decreasing PT content, the increase of ionic displacement and space-charge contribution around Tm can explain the independence of Tm with measuring frequency. The diffuseness of a phase transition can be determined from the modified Curie–Weiss law [33], 1 (T − Tm ) = C K − 1/Km
(1)
where Km is the maximum value of the dielectric constant at the transition temperature (Tm ), C is the Curie-like constant and is the degree of diffuseness where the value of varies between 1 (normal ferroelectric) and 2 (ideal relaxor ferroelectric). Fig. 5 shows the plots of ln(1/K − 1/Km ) versus ln(T − Tm ) of (BS–NBT)–PTx samples at 1 MHz. All the samples exhibit a linear relationship. By leastsquares fitting the experimental data to the modified Curie–Weiss
For piezoelectric measurement, specimens were poled at 80 ◦ C for 15 min without electrical breakdown. As we know, the MPB is well known to play a more important role in the enhancement of piezoelectric properties. For example, in lead-based perovskite ceramics, such as PZT [34] and PMN–PT [35], their piezoelectric properties become maximal near the morphotropic phase boundary region. Piezoelectric constant d33 and electromechanical coupling factor kp for compositions with x are shown in Fig. 6. It is observed that the best piezoelectric and electrochemical properties are obtained in the composition near MPB. It is normally believed that the ceramics near the MPB may contain both the rhombohedral and tetragonal phases and thus more possible polarization states [36], which consequently lead to the significant enhancement in the piezoelectric properties. Specimens near MPB are shown to be ferroelectric. Fig. 7 shows the polarization–electric field curve of the studied compositions close to MPB. An optimal remnant polarization Pr = 20.1 C/cm2 with a coercive field Ec = 2.06 kV/mm was obtained at the composition of x = 0.38. 4. Conclusions In summary, (BS–NBT)–PTx ceramic has been fabricated by conventional sintering technique. As the PT content decreases, relaxor behavior can be induced due to polar nanodomains. Accordingly, the ceramics with 0.35 < x < 0.43 contain both rhombohedral and tetragonal phases at room temperature confirmed by XRD spectra. It is suggested that owing to the more possible polarization states resulting from the coexistence of the two phases, the
Fig. 7. Polarization–electric field curve (P–E) of the studied compositions near MPB. Where, (a) x = 0.40; (b) x = 0.38; (c) x = 0.35 and (d) x = 0.30.
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BS–NBT–xPT ceramic at x = 0.38 exhibits the following optimum properties: d33 = 141 pC/N, kp = 32.9%, Tm = 350 ◦ C, Pr = 20.1 C/cm2 , Ec = 2.06 kV/mm with environment-friendly character compared to high lead PZT systems, indicative of a promising candidate for hightemperature piezoelectric application. Acknowledgments The authors would like to thank the support of key programmer of Natural Science Foundation of China (no.:50932004), Natural Science Foundation of China (no.:50872102 and no.:51102189), the Key Grant Project of Chinese Ministry of Education (no.:309022), and the Fundamental Research Funds for the Central Universities (no.:2010-IV-045). References [1] R.E. Eitel, C.A. Randall, T.R. Shrout, S.E. Park, Japanese Journal of Applied Physics 41 (2002) 2099. [2] T. Song, R.E. Eitel, T.R. Shrout, C.A. Randall, Japanese Journal of Applied Physics 43 (2004) 5392. [3] Z.H. Yao, H.X. Liu, H. Hao, M. Cao, Journal of Alloys and Compounds 505 (2010) 281. [4] J. Ryu, S. Priya, C. Sakaki, K. Uchino, Japanese Journal of Applied Physics 41 (2002) 6040. [5] Z.H. Yao, H.X. Liu, Y. Liu, L. Chen, H. Hao, Materials Research Bulletin 44 (2009) 1511. [6] Z.H. Yao, H.X. Liu, H. Hao, M. Cao, Japanese Journal of Applied Physics 109 (2011) 014105. [7] Z.H. Yao, L.Y. Peng, H.X. Liu, H. Hao, M. Cao, Z.Y. Yu, Journal of Alloys and Compounds 509 (2011) 5637. [8] F. Gao, R.Z. Hong, J.J. Liu, Z. Li, L.H. Cheng, C.S. Tian, Journal of Alloys and Compounds 475 (2009) 619. [9] G.A. Smolenskii, V.A. Isupov, A.I. Agranovskaya, N.N. Kainik, Soviet Physics-Solid State 2 (1961) 2651. [10] I.P. Pronin, P.P. Syrnikov, V.A. Isupov, V.M. Egorov, N.V. Zaitseva, Ferroelectrics 25 (1980) 395.
[11] S.E. Park, K.S. Hong, Journal of Applied Physics 79 (1996) 383. [12] P. Marchet, E. Boucher, V. Dorcet, J.P. Mercurio, Journal of the European Ceramic Society 26 (2006) 3037. [13] T. OH, Japanese Journal of Applied Physics 45 (2006) 5138. [14] M. Zou, H. Fan, L. Chen, W. Yang, Journal of Alloys and Compounds 495 (2010) 280. [15] Y. Zhang, R. Chu, Z. Xu, J. Hao, Q. Chen, F. Peng, W. Li, G. Li, Q. Yin, Journal of Alloys and Compounds 502 (2010) 341. [16] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, Journal of the European Ceramic Society 22 (2002) 2115. [17] Q. Gou, J. Wu, A. Li, B. Wu, D. Xiao, J. Zhu, Journal of Alloys and Compounds 521 (2012) 4. [18] M. Cernea, L. Trupina, C. Dragoi, B.S. Vasile, R. Trusca, Journal of Alloys and Compounds 515 (2012) 166. [19] H. Ni, L. Luo, W. Li, Y. Zhu, H. Luo, Journal of Alloys and Compounds 509 (2011) 3958. [20] Z. Chen, J. Ding, M. Lin, N. Yuan, Y. Zhu, Journal of Alloys and Compounds 509 (2011) 482. [21] S.E. Park, K.S. Hong, Journal of Materials Research 12 (1997) 2152. [22] J.K. Lee, H.J. Youn, K.S. Hong, S.E. Park, Journal of Applied Physics 85 (1999) 368. [23] T. Wada, K. Toyoike, Y. Imanaka, Y. Matsuo, Japanese Journal of Applied Physics 40 (2001) 5703. [24] T. Chen, T. Zhang, G. Wang, J. Zhou, J. Zhang, Y. Liu, Journal of Alloys and Compounds 520 (2012) 7. [25] ANSI/IEEE Standard No. 176-1987, IEEE Standard on Piezoelectricity, IEEE, New York, 1987. [26] W. Sakamoto, M. Mizuno, Journal of Alloys and Compounds 538 (2006) 408. [27] S. Said, J. Mercrio, Journal of the European Ceramic Society 21 (2001) 1333. [28] T. Takenaka, T. Okuda, K. Takegahara, Ferroelectrics 196 (1997) 175. [29] K. Sakata, T. Takenaka, Y. Naitou, Ferroelectrics 131 (1992) 2. [30] A.A. Bokov, H. Luo, Z.G. Ye, Materials Science and Engineering: B 120 (2005) 206. [31] N. de Mathan, E. Husson, G. Calvarin, J.R. Gavarri, A.W. Hewat, A. Morell, Journal of Physics: Condensed Matter 3 (1991) 8159. [32] J. Suchanicz, Materials Science and Engineering: B 55 (1998) 114. [33] K. Uchino, S. Nomura, Ferroelectrics Letters Section 44 (1982) 55. [34] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic, London, 1971. [35] B. Noheda, D.E. Cox, G. Shirane, J. Gao, Z.G. Ye, Physical Review B 66 (2002) 054104. [36] C.A. Randall, N. Kim, J.P. Kucera, W. Cao, T.R. Shrout, Journal of the American Ceramic Society 81 (1998) 677.