Microstructure and electrical properties of Li0.5Bi0.5TiO3-modified (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics

Journal of Alloys and Compounds 493 (2010) 276–280

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Microstructure and electrical properties of Li0.5 Bi0.5 TiO3 -modified (Na0.5 K0.5 )NbO3 lead-free piezoelectric ceramics Xiang-Ping Jiang a,b,∗ , Qing Yang a , Zu-Deng Yu a , Fei Hu a,b , Chao Chen a , Na Tu a , Yue-Ming Li a a b

Department of Material Science and Engineering, Jing de zhen Ceramic Institute, Jing de zhen, 333001, Jiangxi, China Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 15 November 2009 Received in revised form 16 December 2009 Accepted 16 December 2009 Available online 4 January 2010 Keywords: Ceramics Sintering Electrical properties (K0.5 Na0.5 )NbO3

a b s t r a c t Lead-free piezoelectric ceramics (1 − x)(K0.5 Na0.5 )NbO3 –xLi0.5 Bi0.5 TiO3 (KNN–LBT) were synthesized by conventional sintering process. Characterized effects of LBT on the microstructure and electrical properties of ceramics were studied. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases lies close to 0.01–0.02. The ceramic with x = 0.02 near the MPB achieves remarkable electrical properties: d33 = 172 pC/N, kp = 0.37, εr = 1479, tan ı = 0.015 and Tc = 381 ◦ C, while the mechanical quality factor remains reasonably high (Qm = 102), revealing that the LBT-doping changes the KNN to “hard” piezoelectric ceramics. The addition of a small amount of LBT increases the relative density, which reaches an optimum value of 96.4% with x = 0.02. Moreover, impedance for high piezoelectric properties in (1 − x)KNN–xLBT (x = 0.0, 0.01, 0.02 and 0.05) ceramics were also discussed. It is found that the addition of LBT (x = 0.02) can improve poling procedures of the ceramics. These results demonstrate that the ceramic (x = 0.02) is an attractive candidate for lead-free piezoelectric materials. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Lead zirconate titanate (PZT) based ceramics are the mostly used materials for piezoelectric applications due to their excellent piezoelectric properties at the morphotropic phase boundary (MPB). However, lead has recently been expelled from many commercial applications and some materials (for example, from solder, glass and pottery glaze) owing to concerns for its toxicity. Whereupon researches on lead-free replacement have become more and more concentrated. For example, (K0.5 Na0.5 )NbO3 (KNN)-, Na0.5 Bi0.5 TiO3 (NBT)-, and BaTiO3 (BT)-based ceramics have been actively studied [1–13]. Among various lead-free piezoelectric ceramics, alkaline niobate-based perovskite-type ceramics, especially potassium sodium niobate (KNN)-based ceramics, have been considered to be a potential lead-free piezoelectric candidate because of their high Curie temperature (Tc ∼ 420 ◦ C) and adjustable piezoelectric properties. Particularly for hot pressed KNN ceramics, which exhibit excellent piezoelectric properties (d33 ∼ 160 pC/N, kp ∼ 46%) [7]. But this method is found to be unsuitable for using in industrial production. However, difficulties in ordinary sintering of the pure KNN ceramics have lead to the deviation from excellent proper-

∗ Corresponding author at: Department of Material Science and Engineering, Jing de zhen Ceramic Institute, Jing de zhen, 333001, Jiangxi, China. Tel.: +86 798 8499237. E-mail address: [email protected] (X.-P. Jiang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.079

ties, for instance, relatively low electrical properties (d33 ∼ 70pC/N, kp ∼ 25%) [5]. To improve the densification and piezoelectric properties of KNN ceramics, a number of additions were added into KNN to form new KNN-based ceramics, such as KNN–LiNbO3 [14,15], KNN–Bi2 O3 [16,17], KNN–CaTiO3 [18], KNN–NBT [19], KNN–KBT [20], etc. Besides, NBT and KBT-based MPB ferroelectric compositions with perovskite structure were also extensively investigated, owing to their high Tc (∼320 and 380 ◦ C; respectively) and strong ferroelectricity. To our knowledge, piezoelectric and dielectric properties show a maximum around the MPB bridging orthorhombic, tetragonal, and/or monoclinic ferroelectric phases in PZT ceramics. NBT and KBT are known to be orthorhombic and tetragonal structures at room temperature respectively [19–21]. But KNN is an orthorhombic ferroelectric at room temperature. Thus, it is reasonable to anticipate that both KNN–KBT and KNN–NBT systems possess MPB because of their different phase structures at room temperature, and the MPB can enhance greatly piezoelectric properties of the two systems. The supposition has been confirmed by Zuo et al. [19,20] who have reported that (1 − x)KNN–xNBT (x = 0.03) ceramics and (1 − x)KNN–xKBT (x = 0.03) ceramics show excellent piezoelectric properties (d33 ∼ 192 pC/N). It is well known that LBT has been used in the NBT-based lead-free piezoelectric ceramics, and promising results have been obtained in the latest years [22,23]. However, there are few investigations on the solid solution of KNN–LBT ceramics. So it is anticipated that the modification of LBT in the binary compound of KNN–LBT can give rise to enhanced electric properties. In this

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Table 1 The optimum sintering temperatures and the relative density of (1 − x)KNN–xLBT ceramics. LBT content

0

0.01

0.02

0.025

0.03

0.05

T (◦ C)  (%)

1120 85.5

1125 93.1

1130 96.4

1100 95.9

1085 94.9

1080 91.7

work, LBT-doped KNN ceramics had been prepared, and their phase transitions, impedance and electrical properties were investigated. 2. Experimental procedure (1 − x)(K0.5 Na0.5 )NbO3 –xLi0.5 Bi0.5 TiO3 ceramics (KNN–LBT), with x = 0.0, 0.01, 0.02, 0.025, 0.03, and 0.05 were prepared via traditional solid-state reaction method. High-purity oxides and carbonates, Nb2 O5 (99.0%), Bi2 O3 (99.0%), TiO2 (99.9%), Li2 CO3 (97%), Na2 CO3 (99.8%) and K2 CO3 (99.0%), were used as starting materials which had been treated carefully by a drying process, particularly for sodium/potassium carbonates. The weighed chemicals were wet-milled in polyethylene bottles using zirconia balls for 12 h in alcohol, and then calcined at 850 ◦ C for 2 h. After calcination, the mixture was milled again in the same conditions. At last the samples were sintered for 2 h between 1060 and 1170 ◦ C using the ordinary firing (OF) method. The crystalline phase of the crushed sample was identified by the X-ray powder diffraction (D8 Advanced, Bruker axs). The microstructure evolution was observed using a scanning electron microscope (SEM, Model JSM-6700F, Japan). The temperature dependence of the dielectric constant was made by a LCR meter in the temperature ranging from room temperature to 500 ◦ C. The bulk density was measured by the Archimedes method using distilled water as medium. The relative density was calculated by the equation, which was as follow: =

1 × 100% 0

where  is the relative density of ceramics, 0 is the theoretical density (4.51 g/cm3 ) [21] and 1 is the bulk density. To measure the electrical properties, silver paste was painted on both sides of the samples to form electrodes, subsequently fired at 800 ◦ C for 15 min. After that, sample was poled at about 80 ◦ C silicone oil bath by applying a DC field of 4.0–5.0 kV/mm for 30 min. The piezoelectric constant d33 was conducted using a piezo-d33 meter (ZJ-3A, China Academy of Acoustics). The mechanical quality factor (Qm ) and the planar mode electromechanical coupling coefficient (kp ) of the sample were determined by means of resonance (fr ) and anti-resonance frequencies (fa ) using an impedance analyzer (Agilent 4294A, Agilent Technology Inc.).

Fig. 1. X-ray diffraction patterns of the (1 − x)KNN–xLBT ceramics in the range of 2 (a) from 10◦ to 70◦ and (b) from 20◦ to 25◦ and 44◦ to 48◦ .

3. Results and discussion The XRD patterns of the ceramics with different LBT contents are shown in Fig. 1. It is evident that the ceramics (x ≤ 0.03) show pure perovskite phase and no secondary phases can be certified. The results indicate that the dopants have completely diffused into the KNN lattice to form a new solid solution when x ≤ 0.03. However, with x = 0.05, the trace amount of second phases (K4 Nb2 O7 and Bi2 O3 ) can be detected in KNN–LBT ceramics. Fig. 1(b) is the magnification of (a) in the 2 range of 20–25◦ and 44–48◦ . It can be clearly seen that the crystal structure of samples with x ≤ 0.01 exhibit an orthorhombic structure. However, with increasing LBT from 0.02 to 0.05, the samples present tetragonal structure. The variation of the lattice parameters with x for the KNN–LBT ceramics is shown in Fig. 2. Thereby it can be concluded that a MPB between orthorhombic and tetragonal phases existed approximately at 0.01–0.02. Similar phenomena have been reported in KBT-doped KNN and NBT-doped KNN [19,20]. Fig. 3 shows the SEM images of the fractured surfaces and Table 1 shows the optimum sintering temperatures and the relative density with x for the (1 − x)KNN–xLBT ceramics at the optimum sintering temperature. The pure KNN could not be sintered to obtain sufficient density, the relative density of ceramics is about 85.5% (see Table 1) and the average grain size is small compared to that of the other samples (x = 0.02, 0.03, 0.05). After the doping of LBT, the ceramics become denser, porosity degrades and the grain size increases significantly. The variation of grain size is not in good agreement with that reported by Zuo et al. [19,20]. Noticeably,

the 0.98KNN–0.02LBT ceramic develops well grain morphology with lowest porosity and the highest relative density which up to 96.4% (Table 1). The result should be attributed to the addition of a small amount of LBT which can promote the densification, as clearly seen from Fig. 3(b). As the doping level further increasing (x > 0.02), the grains are coarsened markedly and the porosity being to increase, implying that the relative densities decrease. Especially, the growths of some grains indicated by the arrows are abnormal

Fig. 2. Lattice parameters of (1 − x)KNN–xLBT ceramics as a function of x.

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Fig. 3. SEM images of fracture for the (1 − x)KNN–xLBT ceramics sintered at the optimal sintering temperature: (a) x = 0; (b) x = 0.02; (c) x = 0.03; (d) x = 0.05.

as shown in Fig. 3(c). It may be explained by the presence of the liquid phase. The formation mechanisms of the liquid phase are listed below: first, the evaporation of Na [18] and Bi in KNN–LBT [16] lead to the formation of the liquid phase. Second, large amount of Li+ which possesses the lower melting point of Li2 CO3 induce the presence of the liquid phase [10]. Furthermore, the optimum sintering temperature increases slightly with x up to 0.02. It may be illustrated by the increase of the impurity of Bi3+ [16]. And then it decreases significantly to 1080 ◦ C as x increase to 0.05, owing to the formation of the liquid phase [10]. On the whole, LBT modification can be used to improve the sintering performance of the (1 − x)KNN–xLBT ceramics. The relative permittivity at the frequency of 10 kHz as a function of temperature for the (1 − x)KNN–xLBT ceramics can be seen from Fig. 4. The two phase transition temperatures, Curie tempera-

Fig. 4. (a) Temperature dependence of εr for the ceramics with different x’s. (b) Composition dependence of the Curie temperature for the ceramics with different x’s.

ture Tc and orthorhombic-tetragonal phase transition temperature T0–t , are observed in the pure KNN, occurring at ∼420 and ∼200 ◦ C, respectively. Similar to those of pure KNN, the ceramic with x = 0.01 also have two phase transitions. Whereas the second ferroelectric phase transition temperature between room temperature and the Curie point disappeared approximately at 0.01–0.02. The samples become solely tetragonal phase transition below the Curie point. This borderline is the so-called MPB, as confirmed from the dates in Figs. 1 and 2. At x ≥ 0.025, only a solely tetragonal-cubic phase transition can be observed. In addition, the inset (b) in Fig. 4 shows the variation of the Curie temperature (Tc ) with increasing LBT content. It is shown that increasing x leads to a downward tendency for Tc , which is in agreement with the KNN–NBT system [19] and should be attributed to the co-doping of Bi3+ and Ti4+ . It had been reported that a partial substitution of A-site ions (K0.5 Na0.5 )+ by Li+ can increase Tc and by Bi3+ can decrease Tc slightly [14,16,17]. And a partial substitution of B-site ions Nb5+ by Ti4+ can decrease Tc [19]. The variation of Tc shows that the co-doping of A-site and B-site ions by Li+ , Bi3+ and Ti4+ decreases Tc of the ceramics, indicating that Bi3+ and Ti4+ additions play a more important role than Li+ substitutions on Tc of all ceramics. As a result, LBT in the ceramics degrades Tc . In addition, the similar values of Tc are found for x = 0.01 and 0.02, x = 0.025 and 0.03, as shown in Fig. 4(b). The causation for this behavior is not completely understood yet and remains under investigation and the more detailed explanation will be described in future work. The variation of dielectric loss at 10 kHz with changing temperature for all samples is shown in Fig. 5. The dielectric loss for the ceramics (0 < x < 0.05) is smaller than 0.13 from room temperature to 450 ◦ C, after which it increases rapidly owing to conductive losses [24]. And it is found that the dielectric loss at room temperature and high temperature (≥350 ◦ C) for sample of x = 0.025 is lower than the others samples. The result of the low dielectric loss at high temperature implies that the ceramic of x = 0.025 has good thermal stability, which is very important for high-temperature application of the ceramics.

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Table 2 Various electrical properties of (1 − x)KNN–xLBT ceramics as a function of the LBT content. LBT content d33 /(pC/N) kp tan ı (10 kHz) εr (10 kHz) Qm

0 95 0.36 0.045 532 45

0.01 152 0.29 0.042 1172 46

0.02 172 0.37 0.015 1479 102

0.025 160 0.30 0.022 1499 149

0.03

0.05

148 0.25 0.039 1274 94

90 0.12 0.042 1384 61

Table 2 summarizes the various room-temperature properties of all ceramics. As shown in Table 2, by adding x, both d33 and kp increase initially, and the maximum with d33 = 172pC/N and kp = 0.37 are obtained at x = 0.02. Furthermore increasing x causes the decline in d33 and kp . It is evident that the optimized piezoelectric performances in the present study should be attributed to the improved poling state [25] (Fig. 6) and the MPB behavior [20]. It is well known that a large amount of thermodynamically equivalent states near the MPB allow a high degree of alignment of ferroelectric dipoles, which give rise to a splendid piezoelectric performance [26]. Yet the variation of tan ı is opposite to those of the d33 , kp and εr with increasing LBT amount, reaching the minimum value of 0.015 at x = 0.02. And εr reaches the optimum value of 1499 at 0.025. Especially, it is noted that the sample shows a finer value of Qm = 149 with x = 0.025, which is almost 3.5 times of the value for the pure KNN.

4. Conclusions Fig. 5. Temperature dependence of the dielectric loss of all samples at 10 kHz.

The impedance magnitude |Z| and phase angle  as functions of frequency for the (1 − x)KNN–xLBT ceramics with x = 0.01, 0.02 and 0.05 is made in Fig. 6. As far as we know, the ideal poling state is obtained when  approaches to 90◦ in the frequency range between the anti-resonance (fa ) and resonance frequencies (fr ) [25]. The phase angle increase dramatically from 10.1◦ to 61.6◦ with x increase from 0.01 to 0.02, and then decrease rapidly to 2.7◦ with x increase to 0.05, giving a maximum value of 61.6◦ at x = 0.02. The kp value (Table 2) displays a similar trend to the  value. Therefore, the doping of LBT can optimize the poling state which is important for the improvement of the kp value [25].

The effect of LBT-doped (0–0.05) on the phase, microstructure, and electrical properties of lead-free piezoelectric system of (1 − x)KNN–xLBT have been investigated. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases lies close to 0.01–0.02. The relative density is enhanced when x increase from 0 to 0.02, and dropped when x is more than 0.02, giving a first-class value of 96.4% with x = 0.02. Significantly enhanced electrical properties (d33 = 172pC/N, kp = 0.37, εr = 1479, tan ı = 0.015, Qm = 102 and Tc = 381 ◦ C) are obtained in the ceramic when x = 0.02 near the MPB. The better value of Qm reveals that the LBT-doping changes the property of KNN to “hard”. Moreover, the variation of impedance for high piezoelectric properties in the samples (x = 0.01, 0.02 and 0.05) presents that the addition of LBT (x = 0.02)

Fig. 6. Frequency dependence of impedance and phase for the (1 − x)KNN–xLBT ceramics with x = 0.01, 0.02 and 0.05 at the planar-mode resonance.

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can improve poling procedures of the ceramics. Hence, taking into accounts all data on this paper, it can be concluded that the 0.98KNN–0.02LBT ceramic is a promising lead-free piezoelectric material for high-temperature practical applications. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No: 50862005, 50562002), the New Century Excellent Talents in University (Grant No: NCET-06-0576), the Jiangxi Natural Science Foundation and Cooperative Project (Grant No: 2007GZC1258, 2008GZC0009, [2008]212), the Jiangxi Colleges and Universities “advanced ceramics” scientific and technological innovation team. Thanks are also to the Centre for Smart Materials of the Hong Kong Polytechnic University. References [1] Y.J. Dai, X.W. Zhang, G.Y. Zhou, Appl. Phys. Lett. 90 (2007) 262903–262913. [2] N.M. Hagh, K. Kerman, B. Jadidian, A. Safari, J. Eur. Ceram. Soc. 29 (2009) 2325–2332. [3] 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.J. Zhang, T.R. Shrout, Appl. Phys. Lett. 88 (2006) 212908. [4] J.G. Wu, D.Q. Xiao, Y.Y. Wang, J.G. Zhu, L. Wu, Y.H. Jiang, Appl. Phys. Lett. 91 (2007) 252907. [5] Z.P. Yang, Y.F. Chang, L.L. Wei, Appl. Phys. Lett. 90 (2007) 042911.

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