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2) TiO3–0.065BaTiO3 lead-free piezoelectric ceramics
Journal of Alloys and Compounds 632 (2015) 580–584
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure, electrical properties of Bi2NiMnO6-doped 0.935(Bi1/2Na1/2) TiO3–0.065BaTiO3 lead-free piezoelectric ceramics Renfei Cheng a, Zhijun Xu a,⇑, Ruiqing Chu a, Jigong Hao a, Juan Du a, Guorong Li b a b
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 November 2014 Received in revised form 14 January 2015 Accepted 15 January 2015 Available online 2 February 2015 Keywords: Lead-free piezoelectric ceramics Bi2NiMnO6-doped Electrical properties
a b s t r a c t The 0.935Bi1/2Na1/2TiO3–0.065BaTiO3–xmolBi2NiMnO6(x = 0–0.008) (abbreviated as BNT–BT6.5– xBNMO) lead-free piezoelectric ceramics were fabricated by conventional solid-state reaction method and the effects of BNMO addition on microstructure and electrical properties of the ceramics were investigated. Results show that all samples have formed dense structures with a large relative density > 95%. X-ray diffraction (XRD) patterns show all compositions had a pure perovskite structure, suggesting BNMO effectively diffused into the BNT–BT6.5 lattice to form a solid solution. SEM images indicate that BNMO modified ceramics have a clear grain boundary and a uniformly distributed grain size. The measurements of electrical properties reveal that the electrical properties of BNT–BT–xBNMO ceramics have been greatly improved by certain amount of BNMO substitutions. At room temperature, the BNT–BT–xBNMO ceramics with appropriated BNMO exhibited optimum ferroelectric and piezoelectric properties with a relatively high remnant polarization Pr of 31 lC/cm2, high planar electromechanical coefficient kp of 31% and large piezoelectric constant d33 of 229 pC/N, respectively. These results indicate that the modified BNT–BT6.5 ceramics are promising lead-free piezoelectric candidates for practical applications. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Lead-based ferroelectric materials with excellent electromechanical properties have been widely utilized for piezoelectric or dielectric materials in applications and other electromechanical devices [1–3]. However, the high volatility and strong toxicity of lead oxide during sintering have caused severe ecological and environmental problems [4,5]. Therefore, extensive efforts have been done to find a promising way to solve this problem and develop new lead-free piezoelectric ceramics with excellent properties to replace lead based ceramics [6]. Compared with lead-based piezoelectric ceramics, Bi1/2Na1/2 TiO3–BaTiO3 or BNT–BT is currently of interest as a candidate for lead-free piezoelectric applications. It has received considerable attention due to its excellent ferroelectric and piezoelectric properties [7,8]. Similar to the (1 x) PbZrO3–xPbTiO3 ceramics, (1 x) (Bi0.5Na0.5)TiO3–xBaTiO3 system has a morphotropic phase boundary (MPB) between rhombohedral phase and tetragonal phase [7] near x = 0.06–0.07. When rhombohedral and tetragonal systems exist simultaneously, electric domain walls turn easily, increasing spontaneous polarization intensity and remnant polarization ⇑ Corresponding author. Tel./fax: +86 635 8230923. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.01.090 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
greatly, which can provide substantially improved piezoelectric properties [9,10]. This directive gave a boost to the development of BNT–BT system piezoelectric ceramics, and a variety of BNT–BT systems have developed in recent decades. However, the MPB compositions of (1 x) (Bi0.5Na0.5)TiO3–xBaTiO3 exhibits a low depolarization temperature due to the occurrence of a metastable antiferroelectric state [11]. Therefore, the electrical properties of 0.935(Bi0.5Na0.5)TiO3–0.065BaTiO3 (abbreviated as BNT–BT6.5) ceramics need to be further enhanced for practical applications [12]. In order to further enhance the properties of BNT–BT6.5 ceramics and meet the requirements for practical uses, it is necessary to develop new multicomponent piezoelectric ceramics. Nickel and manganese are often used as dopant to improve the electrical properties for electrical ceramics [13–15]. Besides, the compound of Bi2NiMnO6 (abbreviated as BNMO) has been of interest due to its double-perovskite structure with monoclinic symmetry and multiferroic materials [16,17], and it was assumed that the piezoelectric and ferroelectric properties of BNT–BT6.5 ceramics could be improved by BNMO doping. In this study, BNMO doped BNT–BT6.5 ceramics were firstly prepared by conventional solid-state reaction method, and their microstructure and electrical properties were studied systematically. As we expected, the electromechanical and piezoelectric performances of BNT–BT–xBNMO ceramics were well improved.
R. Cheng et al. / Journal of Alloys and Compounds 632 (2015) 580–584 2. Experimental procedures The produced materials 0.935Bi1/2Na1/2TiO3–0.065BaTiO3–xmolBi2NiMnO6 (abbreviated as BNT–BT6.5–xBNMO) were prepared by the conventional solid-state reaction method using reagent-grade metal oxides or carbonate powders of Bi2O3 (99.64%), TiO2 (99.5%), Na2CO3 (99.8%), BaCO3 (99%), MnO2 (97.5%) and NiO (99.98%) as starting materials. All raw materials made by Sinopharm Chemical Reagent Co., Ltd., were weighed at stoichiometric proportion and then mixed homogenized by planetary ball milling in a polyethylene with stabilized zirconia balls for 12 h, using anhydrous ethanol as the liquid medium. After drying, the mixed powders were calcined at 850 °C for 2 h. After calcination, the mixture was milled again for 6 h. The powders were mixed with an appropriate amount of polyvinyl butyral (PVB) binder, and pressed into pallets with a diameter of 12 mm and a thickness of 1.0 mm. After burning off PVB, the ceramics were sintered in an alumina crucible at 1100–1180 °C for 2 h in air. To reduce the loss of bismuth and sodium oxides during sintering, the pellets were embedded into preprepared powder with the same composition. The crystal structure of the ceramics was determined by X-ray diffraction (XRD) using a Cu Ka radiation (k = 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The surface morphology of the ceramics was observed by scanning electron microscope (SEM) (JSM-6380, Japan). Silver electrodes were coated on the top and bottom surfaces of the ceramics for the subsequent electrical measurements. The ferroelectric hysteresis loops were measured through standardized ferroelectric test system (TF2000, Germany). Dielectric properties were measured using an Agilent 4294 A precision impedance analyzer (Agilent Inc., America) in the temperature range of 25–500 °C connected to a high-temperature furnace. The samples were poled in silicon oil at room temperature under 50–70 kV/cm for 20 min, and piezoelectric measurements were then carried out using a quasi-static d33-meterYE2730 (SINOCERA, China).
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the BNT–BT6.5– xBNMO samples sintered at 1150 °C for 2 h. All ceramics possess a pure perovskite structure and no second phases can be detected, implying that BNMO has diffused into BNT–BT6.5 lattices to form a new solid solution BNT–BT6.5–100xBNMO within the studied doping level. No evident diffraction peak shift can be observed, suggesting that BNMO doped into BNT–BT6.5 does not induce the change of lattice. This can be attributed to the same ionic radii of (Ni, Mn)6+ (effective ionic radii is r(Ni, Mn)6+ = 0.61 Å [18]) to Ti4+(0.61 Å). Similar results were also reported in Nb-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics and KNbO3-doped Bi0.5Na0.5TiO3– BaTiO3 ceramics [19,20]. Fig. 2 shows SEM micrographs of various BNMO doped BNT– BT6.5 ceramics. The SEM observation confirms that all the investigated samples are visibly dense with a well-developed microstructure that has a clear grain boundary and uniform grain
Fig. 1. X-ray diffraction patterns of BNT–BT–xBNMO ceramics sintered at 1150 °C for 2 h in the 2h ranges from 20° to 70°.
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sizes, which are all typical microstructures with rectangular grain shape as normally observed in BNT-based piezoceramics [12,21,22]. In addition, these micrographs also indicated that BNMO-doped causes a significant change in grain shape and size: the average grain size first increased and then decreased with increasing BNMO contents. Table 1 summarized the relative density and the average grain size of BNT–BT6.5–xBNMO ceramics with BNMO contents. The density first increases and then decreases with increasing BNMO contents, which may be related to the variation trend in grain size: with increasing grain growth, the number of pores was found to decrease. This suggested that suitable BNMO doping effectively promoted the densification and accelerated grain growth. The accelerated grain growth associated with BNMO addition may result from an emergence of oxygen vacancies, which are favorable for the transport of mass in the sintering process, and also strongly promotes grain growth [23,24]. Moreover, the emergence of oxygen vacancies is highly dependent on the site occupation of the doping cations in the perovskite structure. Thus, it is thought that the remarkable acceleration of grain growth at a high doping level may be an indicator for the B-site occupation of the Ni2+ and Mn4+ cations [25]; however, excess BNMO doping restrain grain growth, excess BNMO can concentrate near grain boundaries and decrease their mobility substantially. Thus, the mass transportation is weakened and grain growth is inhibited [26]. The ferroelectric properties of BNT–BT6.5–xBNMO ceramics are described by the P–E hysteresis loops, as shown in Fig. 3. The hysteresis curves show that the BNMO concentration considerably affects the polarization as well as the coercive field of BNT–BT6.5 ceramics. The theory of D–E hysteresis loop based on the Avrami model proposed by Orihara [27] can be used to explain the grain size dependence of P–E response in BNT–BT6.5 ceramics. For ferroelectrics, the proportion of grains contributing in polarization reverse can be expressed as, f = f0 [1 exp( Gad3/kT)], where Ga is a constant and represents the grain anisotropy energy density, d is the grain size. Based on the above findings, it can be concluded that f has relevance only with the grain size d. With increasing grain size, the number of grains contributing in polarization reverses increases, giving rise to the enhancement of the ferroelectricity. Detailed information on the response of the variation of remnant polarization (Pr) and coercive field (Ec) of BNT–BT6.5 ceramics as a function of BNMO content is provided in Fig. 4. From Fig. 3, P–E hysteresis of all samples shows typical ferroelectric features consistent with other reports on BNT-based ceramics [28,29]. The remnant polarization (Pr) and coercive field (Ec) first increase and then rapidly decrease with increasing BNMO contents, which is according with previous report [30]. The remnant polarization and coercive field reach a maximum value of 31 lC/cm2 and 4948 V/mm, when BNMO content is 0.001 and 0.003, respectively, as shown in Fig. 4. The BNMO-doped reduced the coercive field effectively when compared with the pure BNT. In comparison with other BNT-based ceramics, the remnant polarization of BNT–BT6.5–xBNMO is slightly higher than Er2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 [31] and Gd2O3 modified 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 [32]. Enhanced remnant polarization shows that ferroelectric properties of the BNT–BT6.5 ceramics have been improved with the addition of BNMO. Fig. 5 shows temperature dependence of the relative dielectric constant of BNT–BT6.5–xBNMO ceramics at a frequency of 10 kHz. For the specimen with x = 0, there are two dielectric anomalies in the measuring temperature range, a weak hump and abroad dielectric peak corresponding to the depolarization temperature (Td, the transition from a ferroelectric state to socalled ‘‘anti-ferroelectric’’ state, which is defined as one in which lines of ions in the crystal are spontaneously polarized, but with neighboring lines polarized in antiparallel directions [33]) and
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R. Cheng et al. / Journal of Alloys and Compounds 632 (2015) 580–584
Fig. 2. SEM micrographs of surfaces for BNT–BT6.5–xBNMO ceramics sintered at 1150 °C: (a) x = 0, (b) x = 0.001, (c) x = 0.003, (d) x = 0.004, (e) x = 0.005, (f) x = 0.006, (g) x = 0.008.
Table 1 Apparent density, relative density and grain size of BNT–BT6.5–xBNMO. Composition (x)
Apparent density (g/cm3)
Relative density (%)
Grain size (lm)
x=0 x = 0.001 x = 0.003 x = 0.004 x = 0.005 x = 0.006 x = 0.008
5.74 5.77 5.81 5.86 5.96 5.97 5.87
95.8 96.3 97.0 97.8 99.5 99.7 98.0
1.3 3.9 3.8 4.4 4.3 6.5 2.6
maximum temperature (Tm, a transition from an ‘‘anti-ferroelectric’’ state to a paraelectric state), respectively. This behavior is rather analogous to those previously observed in BNT–BT ceramics
[34–36]. From Fig. 5, it is also found that Tm shifts to lower temperatures slightly as the amount of BNMO increases. It is also found that incorporation of BNMO leads to a decrease in dielectric constant maximum em of BNT–BT6.5–100xBNMO ceramics. All dielectric constant except for x = 0 at room temperature is nearly 1300, which corresponded to the different grain size. The results illustrate that the permittivity of the samples at grain sizes about 4 lm are no obvious change at room temperature. The temperature-dependent dielectric curves of samples become broader with increasing BNMO contents, which implies that the samples display a diffusing phase transition character and the degree of diffusion is enhanced with the increase of BNMO. Piezoelectric constant d33 and the planar electromechanical coefficient kp in BNT–BT6.5–xBNMO system was measured and the results are shown in Fig. 6. From Fig. 6, the piezoelectric constant d33 increases with increasing x and then decreases, giving a
R. Cheng et al. / Journal of Alloys and Compounds 632 (2015) 580–584
Fig. 3. P–E hysteresis loops of BNT–BT6.5–xBNMO ceramics at room temperature.
Fig. 4. The remnant polarization Pr and coercive field Ec of the BNT–BT6.5–xBNMO ceramics as a function of x.
Fig. 5. Temperature dependence of the dielectric constant of BNT–BT6.5–xBNMO ceramics with different BNMO content (10 kHz).
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maximum value of 229 pC/N at x = 0.004. The d33 value was much higher than that observed in Ba0.77Ca0.23TiO3 doped Bi1/2Na1/2TiO3– BaTiO3 compositions by Luo et al. [37] who obtained d33 = 178 pC/ N. And then the d33 was decreasing rapidly in turn with further increasing BNMO contents. Changes of the planar electromechanical coefficient kp with x are similar to that of piezoelectric constant d33, which reaches a maximum value of 31% at x = 0.003. It is generally known that a magnitude of density is proportional to piezoelectric properties. However, the variation trend of kp and d33 obtained from this experiment did not coincide with that of density (Table 1). High values of kp and d33 in poled ceramics are believed to arise from the easy motion of domain walls under an applied field or a stress. Domain wall motion in the BNT-based system ceramics can be easily performed through methods of donor doping and selecting compositions close to the larger remnant polarization. Therefore, from the analysis of the P–E hysteresis loops (Fig. 3), it is evident that a maximum value of kp and d33 has relations with the larger remnant polarization. The results show that the addition of appropriate BNMO improves the piezoelectric properties of the BNT–BT6.5 ceramics significantly. Generally, the piezoelectric response of ceramics may be affected by many factors, including crystal structure and microstructure of ceramics, presence of impurities, dopants, defects and local variation in the composition of ceramics [38]. When the amount of BNMO is lower than 0.004, the Ni and Mn ions will enter into B-site and create oxygen vacancies, which will harden the material. The oxygen vacancies inside the material make the diffusion easier, leading to the good sinterability, increasing the density and improving the poling process. This result is in agreement with the discussions in Fig. 2 but not agrees with Table 1. Generally, the BNT-based system exists an appropriate grain size for better piezoelectric properties. It noted that 4 lm is a critical point for fabricating high-performance BNT-based ceramics. Samples with grain size around 4 lm exhibit excellent piezoelectric properties whereas in samples with grain size too large or small, the exhibition of piezoelectric properties is rather poor. Thus the piezoelectric constant d33 and the planar electromechanical coefficient kp gradually increase with the increase of BNMO addition. However, a large amount of BNMO addition would lead to drastic worsening of the sintering behavior and the formation of pores in the ceramic bulk. Besides, the excess BNMO may precipitate in the grain boundary, which may lead to the accumulation of space charges, thus limiting the movement of the domains [39,40]. Both
Fig. 6. The piezoelectric coefficient d33 and planar electromechanical coefficient kp of the BNT–BT6.5–xBNMO ceramics as a function of x.
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R. Cheng et al. / Journal of Alloys and Compounds 632 (2015) 580–584
of these effects lead to the deterioration of the piezoelectric properties [41]. 4. Conclusions Lead-free piezoelectric ceramics BNT–BT6.5–xBNMO have been successfully synthesized by the conventional solid-state reaction method. All BNT–BT6.5–xBNMO ceramics form the pure perovskite phase structure, and no obvious change in the crystal structure is observed with the addition of BNMO. The average grain size was found to increase firstly with increasing BNMO contents and then decrease, and appropriate BNMO modified ceramics have the clear grain boundary and uniformly distributed grain size. The addition of appropriated BNMO improves the piezoelectric and dielectric properties of BNT–BT6.5–xBNMO ceramics significantly. At room temperature, the ceramics doped with appropriated BNMO show quite good performance: Pr = 31 lC/cm2, d33 = 229 pC/N, kp = 31%. It is obvious that this piezoceramic is promising candidate for lead-free piezoceramic and can be used in practical applications. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51372110, 51402144, 51302124), the National High Technology Research and Development Program of China (No. 2013AA030801), the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004), the Project of Shandong Province Higher Educational Science and Technology Program (Grant Nos. J14LA11 and J14LA10), and the Research Foundation of Liaocheng University (Nos. 318011301, 318011306, 318051407). References [1] S.C. Panigrahi, P.R. Das, B.N. Parida, R. Padhee, R.N.P. Choudhary, J. Alloys Comp. 604 (2014) 73–82. [2] P. Jaita, A. Watcharapasorn, D.P. Cann, S. Jiansirisomboon, J. Alloys Comp. 596 (2014) 98–106. [3] S.-K. Acharya, B.-G. Ahn, C.-U. Jung, J.-H. Koh, I.-H. Choi, S.-K. Lee, J. Alloys Comp. 603 (2014) 248–254. [4] W. Bai, L. Li, W. Li, B. Shen, J. Zhai, H. Chen, J. Alloys Comp. 603 (2014) 149–157. [5] W. Li, H. Zeng, K. Zhao, J. Hao, J. Zhai, Ceram. Int. 40 (2014) 7947–7951.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructure, electrical properties of Bi2NiMnO6-doped 0.935(Bi1/2Na1/2) TiO3–0.065BaTiO3 lead-free piezoelectric ceramics Renfei Cheng a, Zhijun Xu a,⇑, Ruiqing Chu a, Jigong Hao a, Juan Du a, Guorong Li b a b
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, People’s Republic of China Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 November 2014 Received in revised form 14 January 2015 Accepted 15 January 2015 Available online 2 February 2015 Keywords: Lead-free piezoelectric ceramics Bi2NiMnO6-doped Electrical properties
a b s t r a c t The 0.935Bi1/2Na1/2TiO3–0.065BaTiO3–xmolBi2NiMnO6(x = 0–0.008) (abbreviated as BNT–BT6.5– xBNMO) lead-free piezoelectric ceramics were fabricated by conventional solid-state reaction method and the effects of BNMO addition on microstructure and electrical properties of the ceramics were investigated. Results show that all samples have formed dense structures with a large relative density > 95%. X-ray diffraction (XRD) patterns show all compositions had a pure perovskite structure, suggesting BNMO effectively diffused into the BNT–BT6.5 lattice to form a solid solution. SEM images indicate that BNMO modified ceramics have a clear grain boundary and a uniformly distributed grain size. The measurements of electrical properties reveal that the electrical properties of BNT–BT–xBNMO ceramics have been greatly improved by certain amount of BNMO substitutions. At room temperature, the BNT–BT–xBNMO ceramics with appropriated BNMO exhibited optimum ferroelectric and piezoelectric properties with a relatively high remnant polarization Pr of 31 lC/cm2, high planar electromechanical coefficient kp of 31% and large piezoelectric constant d33 of 229 pC/N, respectively. These results indicate that the modified BNT–BT6.5 ceramics are promising lead-free piezoelectric candidates for practical applications. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Lead-based ferroelectric materials with excellent electromechanical properties have been widely utilized for piezoelectric or dielectric materials in applications and other electromechanical devices [1–3]. However, the high volatility and strong toxicity of lead oxide during sintering have caused severe ecological and environmental problems [4,5]. Therefore, extensive efforts have been done to find a promising way to solve this problem and develop new lead-free piezoelectric ceramics with excellent properties to replace lead based ceramics [6]. Compared with lead-based piezoelectric ceramics, Bi1/2Na1/2 TiO3–BaTiO3 or BNT–BT is currently of interest as a candidate for lead-free piezoelectric applications. It has received considerable attention due to its excellent ferroelectric and piezoelectric properties [7,8]. Similar to the (1 x) PbZrO3–xPbTiO3 ceramics, (1 x) (Bi0.5Na0.5)TiO3–xBaTiO3 system has a morphotropic phase boundary (MPB) between rhombohedral phase and tetragonal phase [7] near x = 0.06–0.07. When rhombohedral and tetragonal systems exist simultaneously, electric domain walls turn easily, increasing spontaneous polarization intensity and remnant polarization ⇑ Corresponding author. Tel./fax: +86 635 8230923. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.01.090 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
greatly, which can provide substantially improved piezoelectric properties [9,10]. This directive gave a boost to the development of BNT–BT system piezoelectric ceramics, and a variety of BNT–BT systems have developed in recent decades. However, the MPB compositions of (1 x) (Bi0.5Na0.5)TiO3–xBaTiO3 exhibits a low depolarization temperature due to the occurrence of a metastable antiferroelectric state [11]. Therefore, the electrical properties of 0.935(Bi0.5Na0.5)TiO3–0.065BaTiO3 (abbreviated as BNT–BT6.5) ceramics need to be further enhanced for practical applications [12]. In order to further enhance the properties of BNT–BT6.5 ceramics and meet the requirements for practical uses, it is necessary to develop new multicomponent piezoelectric ceramics. Nickel and manganese are often used as dopant to improve the electrical properties for electrical ceramics [13–15]. Besides, the compound of Bi2NiMnO6 (abbreviated as BNMO) has been of interest due to its double-perovskite structure with monoclinic symmetry and multiferroic materials [16,17], and it was assumed that the piezoelectric and ferroelectric properties of BNT–BT6.5 ceramics could be improved by BNMO doping. In this study, BNMO doped BNT–BT6.5 ceramics were firstly prepared by conventional solid-state reaction method, and their microstructure and electrical properties were studied systematically. As we expected, the electromechanical and piezoelectric performances of BNT–BT–xBNMO ceramics were well improved.
R. Cheng et al. / Journal of Alloys and Compounds 632 (2015) 580–584 2. Experimental procedures The produced materials 0.935Bi1/2Na1/2TiO3–0.065BaTiO3–xmolBi2NiMnO6 (abbreviated as BNT–BT6.5–xBNMO) were prepared by the conventional solid-state reaction method using reagent-grade metal oxides or carbonate powders of Bi2O3 (99.64%), TiO2 (99.5%), Na2CO3 (99.8%), BaCO3 (99%), MnO2 (97.5%) and NiO (99.98%) as starting materials. All raw materials made by Sinopharm Chemical Reagent Co., Ltd., were weighed at stoichiometric proportion and then mixed homogenized by planetary ball milling in a polyethylene with stabilized zirconia balls for 12 h, using anhydrous ethanol as the liquid medium. After drying, the mixed powders were calcined at 850 °C for 2 h. After calcination, the mixture was milled again for 6 h. The powders were mixed with an appropriate amount of polyvinyl butyral (PVB) binder, and pressed into pallets with a diameter of 12 mm and a thickness of 1.0 mm. After burning off PVB, the ceramics were sintered in an alumina crucible at 1100–1180 °C for 2 h in air. To reduce the loss of bismuth and sodium oxides during sintering, the pellets were embedded into preprepared powder with the same composition. The crystal structure of the ceramics was determined by X-ray diffraction (XRD) using a Cu Ka radiation (k = 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The surface morphology of the ceramics was observed by scanning electron microscope (SEM) (JSM-6380, Japan). Silver electrodes were coated on the top and bottom surfaces of the ceramics for the subsequent electrical measurements. The ferroelectric hysteresis loops were measured through standardized ferroelectric test system (TF2000, Germany). Dielectric properties were measured using an Agilent 4294 A precision impedance analyzer (Agilent Inc., America) in the temperature range of 25–500 °C connected to a high-temperature furnace. The samples were poled in silicon oil at room temperature under 50–70 kV/cm for 20 min, and piezoelectric measurements were then carried out using a quasi-static d33-meterYE2730 (SINOCERA, China).
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the BNT–BT6.5– xBNMO samples sintered at 1150 °C for 2 h. All ceramics possess a pure perovskite structure and no second phases can be detected, implying that BNMO has diffused into BNT–BT6.5 lattices to form a new solid solution BNT–BT6.5–100xBNMO within the studied doping level. No evident diffraction peak shift can be observed, suggesting that BNMO doped into BNT–BT6.5 does not induce the change of lattice. This can be attributed to the same ionic radii of (Ni, Mn)6+ (effective ionic radii is r(Ni, Mn)6+ = 0.61 Å [18]) to Ti4+(0.61 Å). Similar results were also reported in Nb-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics and KNbO3-doped Bi0.5Na0.5TiO3– BaTiO3 ceramics [19,20]. Fig. 2 shows SEM micrographs of various BNMO doped BNT– BT6.5 ceramics. The SEM observation confirms that all the investigated samples are visibly dense with a well-developed microstructure that has a clear grain boundary and uniform grain
Fig. 1. X-ray diffraction patterns of BNT–BT–xBNMO ceramics sintered at 1150 °C for 2 h in the 2h ranges from 20° to 70°.
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sizes, which are all typical microstructures with rectangular grain shape as normally observed in BNT-based piezoceramics [12,21,22]. In addition, these micrographs also indicated that BNMO-doped causes a significant change in grain shape and size: the average grain size first increased and then decreased with increasing BNMO contents. Table 1 summarized the relative density and the average grain size of BNT–BT6.5–xBNMO ceramics with BNMO contents. The density first increases and then decreases with increasing BNMO contents, which may be related to the variation trend in grain size: with increasing grain growth, the number of pores was found to decrease. This suggested that suitable BNMO doping effectively promoted the densification and accelerated grain growth. The accelerated grain growth associated with BNMO addition may result from an emergence of oxygen vacancies, which are favorable for the transport of mass in the sintering process, and also strongly promotes grain growth [23,24]. Moreover, the emergence of oxygen vacancies is highly dependent on the site occupation of the doping cations in the perovskite structure. Thus, it is thought that the remarkable acceleration of grain growth at a high doping level may be an indicator for the B-site occupation of the Ni2+ and Mn4+ cations [25]; however, excess BNMO doping restrain grain growth, excess BNMO can concentrate near grain boundaries and decrease their mobility substantially. Thus, the mass transportation is weakened and grain growth is inhibited [26]. The ferroelectric properties of BNT–BT6.5–xBNMO ceramics are described by the P–E hysteresis loops, as shown in Fig. 3. The hysteresis curves show that the BNMO concentration considerably affects the polarization as well as the coercive field of BNT–BT6.5 ceramics. The theory of D–E hysteresis loop based on the Avrami model proposed by Orihara [27] can be used to explain the grain size dependence of P–E response in BNT–BT6.5 ceramics. For ferroelectrics, the proportion of grains contributing in polarization reverse can be expressed as, f = f0 [1 exp( Gad3/kT)], where Ga is a constant and represents the grain anisotropy energy density, d is the grain size. Based on the above findings, it can be concluded that f has relevance only with the grain size d. With increasing grain size, the number of grains contributing in polarization reverses increases, giving rise to the enhancement of the ferroelectricity. Detailed information on the response of the variation of remnant polarization (Pr) and coercive field (Ec) of BNT–BT6.5 ceramics as a function of BNMO content is provided in Fig. 4. From Fig. 3, P–E hysteresis of all samples shows typical ferroelectric features consistent with other reports on BNT-based ceramics [28,29]. The remnant polarization (Pr) and coercive field (Ec) first increase and then rapidly decrease with increasing BNMO contents, which is according with previous report [30]. The remnant polarization and coercive field reach a maximum value of 31 lC/cm2 and 4948 V/mm, when BNMO content is 0.001 and 0.003, respectively, as shown in Fig. 4. The BNMO-doped reduced the coercive field effectively when compared with the pure BNT. In comparison with other BNT-based ceramics, the remnant polarization of BNT–BT6.5–xBNMO is slightly higher than Er2O3 doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 [31] and Gd2O3 modified 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 [32]. Enhanced remnant polarization shows that ferroelectric properties of the BNT–BT6.5 ceramics have been improved with the addition of BNMO. Fig. 5 shows temperature dependence of the relative dielectric constant of BNT–BT6.5–xBNMO ceramics at a frequency of 10 kHz. For the specimen with x = 0, there are two dielectric anomalies in the measuring temperature range, a weak hump and abroad dielectric peak corresponding to the depolarization temperature (Td, the transition from a ferroelectric state to socalled ‘‘anti-ferroelectric’’ state, which is defined as one in which lines of ions in the crystal are spontaneously polarized, but with neighboring lines polarized in antiparallel directions [33]) and
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Fig. 2. SEM micrographs of surfaces for BNT–BT6.5–xBNMO ceramics sintered at 1150 °C: (a) x = 0, (b) x = 0.001, (c) x = 0.003, (d) x = 0.004, (e) x = 0.005, (f) x = 0.006, (g) x = 0.008.
Table 1 Apparent density, relative density and grain size of BNT–BT6.5–xBNMO. Composition (x)
Apparent density (g/cm3)
Relative density (%)
Grain size (lm)
x=0 x = 0.001 x = 0.003 x = 0.004 x = 0.005 x = 0.006 x = 0.008
5.74 5.77 5.81 5.86 5.96 5.97 5.87
95.8 96.3 97.0 97.8 99.5 99.7 98.0
1.3 3.9 3.8 4.4 4.3 6.5 2.6
maximum temperature (Tm, a transition from an ‘‘anti-ferroelectric’’ state to a paraelectric state), respectively. This behavior is rather analogous to those previously observed in BNT–BT ceramics
[34–36]. From Fig. 5, it is also found that Tm shifts to lower temperatures slightly as the amount of BNMO increases. It is also found that incorporation of BNMO leads to a decrease in dielectric constant maximum em of BNT–BT6.5–100xBNMO ceramics. All dielectric constant except for x = 0 at room temperature is nearly 1300, which corresponded to the different grain size. The results illustrate that the permittivity of the samples at grain sizes about 4 lm are no obvious change at room temperature. The temperature-dependent dielectric curves of samples become broader with increasing BNMO contents, which implies that the samples display a diffusing phase transition character and the degree of diffusion is enhanced with the increase of BNMO. Piezoelectric constant d33 and the planar electromechanical coefficient kp in BNT–BT6.5–xBNMO system was measured and the results are shown in Fig. 6. From Fig. 6, the piezoelectric constant d33 increases with increasing x and then decreases, giving a
R. Cheng et al. / Journal of Alloys and Compounds 632 (2015) 580–584
Fig. 3. P–E hysteresis loops of BNT–BT6.5–xBNMO ceramics at room temperature.
Fig. 4. The remnant polarization Pr and coercive field Ec of the BNT–BT6.5–xBNMO ceramics as a function of x.
Fig. 5. Temperature dependence of the dielectric constant of BNT–BT6.5–xBNMO ceramics with different BNMO content (10 kHz).
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maximum value of 229 pC/N at x = 0.004. The d33 value was much higher than that observed in Ba0.77Ca0.23TiO3 doped Bi1/2Na1/2TiO3– BaTiO3 compositions by Luo et al. [37] who obtained d33 = 178 pC/ N. And then the d33 was decreasing rapidly in turn with further increasing BNMO contents. Changes of the planar electromechanical coefficient kp with x are similar to that of piezoelectric constant d33, which reaches a maximum value of 31% at x = 0.003. It is generally known that a magnitude of density is proportional to piezoelectric properties. However, the variation trend of kp and d33 obtained from this experiment did not coincide with that of density (Table 1). High values of kp and d33 in poled ceramics are believed to arise from the easy motion of domain walls under an applied field or a stress. Domain wall motion in the BNT-based system ceramics can be easily performed through methods of donor doping and selecting compositions close to the larger remnant polarization. Therefore, from the analysis of the P–E hysteresis loops (Fig. 3), it is evident that a maximum value of kp and d33 has relations with the larger remnant polarization. The results show that the addition of appropriate BNMO improves the piezoelectric properties of the BNT–BT6.5 ceramics significantly. Generally, the piezoelectric response of ceramics may be affected by many factors, including crystal structure and microstructure of ceramics, presence of impurities, dopants, defects and local variation in the composition of ceramics [38]. When the amount of BNMO is lower than 0.004, the Ni and Mn ions will enter into B-site and create oxygen vacancies, which will harden the material. The oxygen vacancies inside the material make the diffusion easier, leading to the good sinterability, increasing the density and improving the poling process. This result is in agreement with the discussions in Fig. 2 but not agrees with Table 1. Generally, the BNT-based system exists an appropriate grain size for better piezoelectric properties. It noted that 4 lm is a critical point for fabricating high-performance BNT-based ceramics. Samples with grain size around 4 lm exhibit excellent piezoelectric properties whereas in samples with grain size too large or small, the exhibition of piezoelectric properties is rather poor. Thus the piezoelectric constant d33 and the planar electromechanical coefficient kp gradually increase with the increase of BNMO addition. However, a large amount of BNMO addition would lead to drastic worsening of the sintering behavior and the formation of pores in the ceramic bulk. Besides, the excess BNMO may precipitate in the grain boundary, which may lead to the accumulation of space charges, thus limiting the movement of the domains [39,40]. Both
Fig. 6. The piezoelectric coefficient d33 and planar electromechanical coefficient kp of the BNT–BT6.5–xBNMO ceramics as a function of x.
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of these effects lead to the deterioration of the piezoelectric properties [41]. 4. Conclusions Lead-free piezoelectric ceramics BNT–BT6.5–xBNMO have been successfully synthesized by the conventional solid-state reaction method. All BNT–BT6.5–xBNMO ceramics form the pure perovskite phase structure, and no obvious change in the crystal structure is observed with the addition of BNMO. The average grain size was found to increase firstly with increasing BNMO contents and then decrease, and appropriate BNMO modified ceramics have the clear grain boundary and uniformly distributed grain size. The addition of appropriated BNMO improves the piezoelectric and dielectric properties of BNT–BT6.5–xBNMO ceramics significantly. At room temperature, the ceramics doped with appropriated BNMO show quite good performance: Pr = 31 lC/cm2, d33 = 229 pC/N, kp = 31%. It is obvious that this piezoceramic is promising candidate for lead-free piezoceramic and can be used in practical applications. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 51372110, 51402144, 51302124), the National High Technology Research and Development Program of China (No. 2013AA030801), the Natural Science Foundation of Shandong Province of China (No. ZR2012EMM004), the Project of Shandong Province Higher Educational Science and Technology Program (Grant Nos. J14LA11 and J14LA10), and the Research Foundation of Liaocheng University (Nos. 318011301, 318011306, 318051407). References [1] S.C. Panigrahi, P.R. Das, B.N. Parida, R. Padhee, R.N.P. Choudhary, J. Alloys Comp. 604 (2014) 73–82. [2] P. Jaita, A. Watcharapasorn, D.P. Cann, S. Jiansirisomboon, J. Alloys Comp. 596 (2014) 98–106. [3] S.-K. Acharya, B.-G. Ahn, C.-U. Jung, J.-H. Koh, I.-H. Choi, S.-K. Lee, J. Alloys Comp. 603 (2014) 248–254. [4] W. Bai, L. Li, W. Li, B. Shen, J. Zhai, H. Chen, J. Alloys Comp. 603 (2014) 149–157. [5] W. Li, H. Zeng, K. Zhao, J. Hao, J. Zhai, Ceram. Int. 40 (2014) 7947–7951.
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