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Effects of Ca substitution on structure, piezoelectric properties, and relaxor behavior of lead-free Ba(Ti0.9Zr0.1)O3 piezoelectric ceramics
Journal of Alloys and Compounds 541 (2012) 396–402
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Effects of Ca substitution on structure, piezoelectric properties, and relaxor behavior of lead-free Ba(Ti0.9Zr0.1)O3 piezoelectric ceramics Shukai Ye, Jerry Fuh, Li Lu ⇑ Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
a r t i c l e
i n f o
Article history: Received 14 August 2011 Received in revised form 15 June 2012 Accepted 16 June 2012 Available online 4 July 2012 Keywords: Ceramics Sintering Microstructure Piezoelectricity SEM X-ray diffraction
a b s t r a c t Lead-free piezoelectric (Ba1xCax)(Ti0.9Zr0.1)O3 (BCTZ-x, x = 0, 0.05, 0.10, 0.15, 0.20) ceramics are synthesized via a conventional solid state reaction method. The effects of Ca substitution on the microstructure, dielectric properties, piezoelectric properties, and relaxor behavior of BCTZ ceramics were systematically investigated. Structural characterization of the synthesized BCTZ shows that a structure with morphortropic phase boundary (MPB) between rhombohedral phase and tetragonal phase at about x = 0.15 has been obtained. The (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics exhibit the most homogeneous microstructure with the largest grains. The dielectric and piezoelectric properties are found to be enhanced near the phase transition composition due to the lattice distortion caused by Ca2+ substitution. The dielectric relaxor behavior of the BCTZ ceramics was enhanced with increasing Ca content. The relaxor nature of the BCTZ ceramics may be attributed to the formation of polar clusters induced by local distortion. The increased relaxor behavior in the BCTZ ceramics also contributes to the improvement of the piezoelectric properties. The (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramic exhibits optimum properties with a dielectric constant, eT3 =e0 , of 5800, piezoelectric constant, d33, of 350 pC/N, d31 of 122 pC/N, electromechanical coupling factor, kp, of 33%, and k31, of 16%, indicating that the (Ba,Ca)(Ti,Zr)O3 ceramic is a promising candidate for lead-free piezoelectric materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Piezoelectric ceramics are the most widely used materials for actuator, transducer, and sensor applications [1]. For the past few decades, lead zirconate titanate (PZT) based materials have dominated the piezoelectric ceramic market because of their superior dielectric and piezoelectric properties [2]. Among the lead based materials, ferroelectric realxors, such as Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) [3], Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) [4], and their derived compounds, have received extensive studies and have been widely used in multilayer capacitor, high performance sensor, and hysteresis-free actuator [1] due to their giant electromechanical response. However, because of the malignant effects of lead on human health and the high vapor pressure during processing, the search for lead-free relaxor with high piezoelectric response has attracted much attention in recent years [5–8]. Barium titanate, BaTiO3 (BT), is known as the first polycrystalline piezoelectric ceramic with perovskite structure which shows a d33 of 191 pC/N [2]. In the previous research, BT is often used to form solid solution with other lead-free compounds to create
⇑ Corresponding author. Fax: +65 6779 1459. E-mail address: [email protected] (L. Lu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.084
an MPB which can improve the piezoelectric and dielectric properties [9,10]. However, the interest in BaTiO3-based lead-free piezoelectric ceramics has been renewed since Rehrig et al. [11] reported a d33 higher than 400 pC/N in the Ba(Zr0.085Ti0.915)O3 single crystal. Subsequent studies in Ba(Zr,Ti)O3 ceramics [12,13] also achieved promising results. Meanwhile, BaTiO3-based lead-free relaxor ferroelectrics such as BaTiO3–BaZrO3 [14], BaTiO3–BiAlO3 [15], and BaTiO3–CaTiO3–SrTiO3 [16], have been widely studied due to their relatively high dielectric and piezoelectric properties compared with other lead-free compounds. The ultrahigh piezoelectric effect in relaxors is probably due to the decreased energy cost and electric field necessary to induce ferroelectric domain switching [17]. More recently, a new (Ba,Ca)(Ti,Zr)O3 (BCTZ) system has attracted increasing interest due to the development of a pseudobinary ferroelectric system Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 [18] which possesses a d33 as high as 600 pC/N. It is believed that the morphotropic phase boundary (MPB) plays an important role in improving the electrical properties of piezoelectric ceramics [19,20]. Several possible MPB have been reported in BCTZ ceramics with different compositions. Li et al. [21] reported a d33 of 365 pC/ N in (Ba0.92Ca0.08)(Ti0.95Zr0.05)O3 ceramics with an MPB between orthorhombic and tetragonal phases. While Zhang et al. [22] obtained a d33 of 338 pC/N in (Ba0.95Ca0.05)(Ti0.96Zr0.04)O3 ceramics with a coexistence of orthorhombic and pseudocubic phases.
S. Ye et al. / Journal of Alloys and Compounds 541 (2012) 396–402
Additionally, the electrical properties of BCTZ ceramics can be further improved to above 400 pC/N by optimizing sintering temperature [23] and poling condition [24], and doping with CuO [25] and BiFeO3 [26]. Moreover, the temperature stability of BCTZ ceramics can be enhanced by Ho doping [27] and Y doping [28]. However, the systematic study of effects of Ca substitution on the structure, piezoelectric properties and relaxor behavior of Ba(Ti0.9Zr0.1)O3 ceramics is still rare. In this paper, the synthesis of (Ba1xCax)(Ti0.9Zr0.1)O3 (0 6 x 6 0.2) piezoelectric ceramics as a lead-free material candidate and the effects of Ca substitution on the phase structure, microstructure, piezoelectric properties, and relaxor behavior of BCTZ ceramics are reported.
2. Experimental procedures (Ba1xCax)(Ti0.9Zr0.1)O3 (x = 0, 0.05, 0.1, 0.15, 0.2) (abbreviated as BCTZ-x) ceramics were prepared via a conventional solid state reaction technique. Commercial powders of BaCO3 (99%, Sinopharm Chemical Reagent), CaCO3 (99%, Sigma–Aldrich), TiO2 (99.9%, Sigma–Aldrich), and ZrO2 (99%, Sigma–Aldrich) were selected as starting materials. The raw materials were finely weighted according to the stoichiometric ratio, high-energy ball-milled (SPEX SamplePrep 8000 M Mixer/Mills, Metuchen, New Jersey, United States) for 2 h and calcined at 1150 °C for 6 h and followed by ball-milling for another 2 h again. Five weight percent of polyethylene glycol (PEG) was added into the milled powder, which was uniaxially cold-compacted to pellets of 10 mm in diameter and 1.5 mm in thickness. The cold-compacted pellets were first heated to 600 °C for 1 h to burn out the binder and then sintered at 1500 °C for 6 h. The densities of the sintered specimens were measured by the Archimedes method. The crystal structures of the ceramics were examined by X-ray diffractometer with CuKa radiation (Shimadzu XRD-600, Kyoto, Japan) and the grain microstructures were observed by a field emission scanning microscope (Hitachi S-4100, Osaka, Japan). To characterize the electrical properties, the specimens were ground and polished to obtain parallel surfaces, and gold electrodes were sputtered on both upper and bottom surfaces. The poling process was conducted by using a dc power supply (Matsusada Precision, Inc., Japan) at 3 kV/mm at room temperature in silicon oil for 30 min. The temperature dependence of the dielectric constant of the ceramics was studied at a temperature range of 30–150 °C in an environmental test chamber (Delta 9023, Delta Design, California, United States) under different frequencies using a precision impedance analyzer. The piezoelectric constant, d33, was measured using a Berlincourt-type d33 meter (YE2730A, Sinocera Piezotronics, Jiangsu, China). The piezoelectric properties kp, d31, k31, and Qm were determined by a resonance and anti-resonance method based on IEEE standards [29] using a precision impedance analyzer. The polarization–electric field (P–E) hysteresis loop was measured by a Radiant Precision workstation based on a standard Sawyer–Tower circuit at room temperature.
Fig. 1. X-ray diffraction patterns of the BCTZ-x (0 6 x 6 0.20) ceramics in the 2h range: (a) from 20° to 60°, (b) from 43° to 47°.
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3. Results and discussion Fig. 1 shows the XRD patterns of the as-sintered BCTZ-x (x = 0– 0.20) ceramics. As revealed from Fig. 1(a), all the samples are of the single pervoskite structure indicating that Ca2+ may have entirely diffused into the Ba(Ti0.9Zr0.1)O3 (BTZ) lattice to form a solid solution, in which Ca2+ substituted Ba2+. Furthermore, all the diffraction peaks shift monotonically to higher 2h angles with the increase of Ca content, suggesting a gradual decrease of the lattice parameter of the samples, which may be ascribed to the smaller ionic radius of Ca2+ (1.34 Å, coordinate number/CN: 12) than that of Ba2+ (1.61 Å, CN: 12) [30]. Fig. 1(b) shows XRD peak profile of (2 0 0) of all the samples. The crystal structure of pure BTZ at room temperature is rhombohedral [31], which is indicated by the single (2 0 0) peak. With the increase of Ca content, pronounced splitting of the (0 0 2) peak from the (200) peak is observed, demonstrating a phase transition from rhombohedral phase to tetragonal phase, which is caused by the lattice distortion induced by the substitution of Ba2+ by Ca2+. Additionally, the increased separation between (0 0 2) and (2 0 0) peaks indicates an enhancement of tetragonality in the BCTZ solid solutions [32]. Therefore, it can be inferred that the rhombohedral and tetragonal phase may coexist in the composition range x = 0.10–0.20. The variation of the relative densities of the BCTZ-x (x = 0–0.20) ceramics sintered at optimum sintering temperature is shown in Fig. 2(a). The FESEM images of the ceramics are shown in Fig. 2(b–f). As shown in Fig. 2(a), the relative density increases significantly from 90% to 95% as the Ca content increases from 0 to 0.05, and then remains almost unchanged. It is evident that Ca substitution can improve the sinterability of the BCTZ ceramics. This may be attributed to the fluxing effect of Ca2+ ions during the period of sintering facilitating the process of densification [33]. Fig. 2(b) shows the FESEM image of the BTZ ceramics with an unconsolidated microstructure and inhomogeneous grains ranging from 2 to 8 lm. With increasing Ca content, the grain size increased and the grain-size distribution became more homogeneous, led to a significant decrease of porosity. When Ca content reached 15 mol%, the most homogeneous grain-size distribution with the largest average grain size around 11 lm was obtained, as shown in Fig. 2(e). With further increasing Ca content to 20 mol%, the average grain size decreased to 5 lm. Such evolution in grain size may be explained by the change of interface atomic structure or grain boundary structure caused by Ca substitution, which significantly affects the microstructure evolution during ceramics sintering [34–36]. This result indicates that appropriate amount of Ca substitution can increase the grain size and improve the homogeneity. It is important to get the optimum microstructure of BCTZ ceramics, since it influences various properties in piezoelectric materials [2,33]. The composition dependence of dielectric properties for the BCTZ-x (x = 0–0.20) ceramics under 1 kHz at room temperature is shown in Fig. 3. The dielectric loss tan d of all the samples are below 1.2% with slight variation. Whereas the relative dielectric constant eT3 =e0 increased dramatically from 2000 at x = 0 to a maximum of 5800 at x = 0.15, and then decreased to 4300 at x = 0.2. The maximum eT3 =e0 is often obtained around the morphortropic phase boundary (MPB) composition in both lead-based systems and lead-free systems, due to the enhanced mobility of the domain and domain walls [9,33]. Therefore, the anomaly of the relative dielectric constant in BCTZ-x system may suggest the existence of another possible MPB between rhombohedral phase and tetragonal phase at about x = 0.15. Fig. 4 shows the temperature dependences of the dielectric constant and loss of BCTZ-x (x = 0–0.2) ceramics at different frequencies (100–100 kHz). Only one dielectric constant peak was observed in all the samples for each frequency. All the samples show weak frequency dispersion. However, the temperature
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Fig. 2. (a) Bulk and relative densities of the BCTZ-x ceramics as a function of Ca content; FESEM images of the BCTZ-x ceramics sintered at 1500 °C for 6 h: (b) x = 0; (c) x = 0.05; (d) x = 0.10; (e) x = 0.15; (f) x = 0.20.
dependences of the dielectric constant of BCTZ ceramics represent a typical relaxor behavior. First, the samples showed a broader dielectric maximum with increasing Ca content which may suggest a diffuse phase transition [37,38]. Another indication of the relaxor behavior is the merging of the dielectric constant curves at different frequencies at high temperature [39]. More detailed investigation of the relaxor behavior of BCTZ ceramics will be described in the following. The BTZ ceramic exhibited a dielectric constant maximum at 70 °C corresponding to the rhombohedral to cubic phase transition. With increasing the content of Ca substitution, the Tm decreased gradually: 67 °C at 5 mol%, 61 °C at 10 mol%, 60 °C at 15 mol%, and 55 °C at 20 mol%. According to the previous discussion about the crystal structure, the peaks for the ceramics with Ca substitution should correspond to the tetragonal to cubic phase transition. The evolution of Curie point TC with increasing Ca content can be ascribed to the different crystal symmetry for different ceramics. The Tc of the BCTZ ceramics is relatively low compared with other lead-free piezoelectric ceramics. Further studies are in
Fig. 3. Variation in the relative dielectric constant eT3 =e0 and the dielectric loss tan d as a function of Ca content at 1 kHz under room temperature.
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Fig. 4. Temperature dependence of dielectric constant and loss for the BCTZ-x (x = 0–0.2) ceramics at different frequencies: (a) 0; (b) 5%; (c) 10%; (d) 15%; (e) 20%.
progress to increase the Curie point and broaden the working temperature range of this material. In most ferroelectrics, the relationship between temperature and the dielectric constant above Curie temperature can be expressed fairly accurately with simple equation called the Curie– Weiss law:
1
e
¼
T T CW ðT > T CW Þ C
ð1Þ
where e is the dielectric constant, T is the temperature, Tcw is the Curie–Weiss temperature, C is the Curie–Weiss constant. Fig. 5 exhibits the plots of inverse dielectric constant as a function of temperature at 10 kHz for BCTZ-x (x = 0–0.2) ceramics. Tm represents the temperature at which the dielectric constant reaches its maximum. TB denotes the temperature at which the dielectric constant
starts to obey the Curie–Weiss law. Therefore, the deviation from the Curie–Weiss law can be defined by the following:
DT m ¼ T B T m
ð2Þ
The fitting results for T m , T B , T CW , DT m , and C are summarized in Table 1. It is observed that DT m increased with increasing the addition of Ca, suggesting an increase of the degree of relaxor behavior. To date, various theory models have been proposed to explain the relaxor behavior [38,40,41], such as compositional fluctuation and diffuse phase transition, the superparaelectric model, the dipole glass model, the random field model, the domain wall model, and the random layer model. The common point of these theories is based on the local distortion of the crystal structure, giving rise to the polar clusters or polar nanoregions (PNRs). Considering the large difference of ionic radii between Ba2+ and Ca2+ ions, the Ca substitution can drive large distortion in crystal which will hinder
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Fig. 5. The plots of inverse dielectric constant as a function of temperature at 10 kHz for BCTZ-x (x = 0–0.2) ceramics: (a) x = 0; (b) x = 0.05; (c) x = 0.1; (d) x = 0.15; (e) x = 0.2.
the long-range dipole alignment and lead to the formation of PNRs, resulting in the improvement of relaxor behavior. However, the PNRs can hardly be detected by XRD because of their nanosize. Therefore, the large local distortion observed by XRD may suggest a large size of the PNRs. This may explain the weak relaxor behavior in this system due to the less sensitivity in larger PNRs [42].
Fig. 6 shows the average grain size, the piezoelectric constant d33 and d31, the electromechanical coupling factor kp and k31, and the mechanical quality factor Qm of the BCTZ-x ceramics with 0 6 x 6 0.20. The d33 value of the BCTZ ceramic increased with increasing Ca content and reached the maximum value of 350 pC/N at x = 0.15, and then dramatically degenerated to
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Table 1 The Curie–Weiss temperature (TCW), the temperature where the dielectric constant starts to follow Curie–Weiss law (TB), the temperature at the dielectric constant maximum (Tm), the deviation from Curie–Weiss law (DT m ), and the Curie–Weiss constant (C) for BCTZ-x ceramics at 10 kHz. BCTZ-x
TCW (°C)
TB (°C)
Tm (°C)
DT m (°C)
C (105 °C)
x=0 x = 0.05 x = 0.1 x = 0.15 x = 0.2
73 75 72 71 56
90 94 95 99 100
70 67 61 60 55
20 27 34 39 45
1.43 2.27 1.66 1.84 1.31
55 pC/N at x = 0.20. Because of the more possible polarization states resulting from the coexistence of two phases, the piezoelectrics exhibit improved piezoelectric response near MPB composition [22,43]. In addition, the increased grain size also contributed to the enhanced d33 value since the domain rotation becomes easier for the specimens with large grains [1]. Furthermore, the BCTZx ceramics have been suggested as relaxor ferroelectrics, and the degree of relaxor behavior increases with addition of Ca. Although the origins of the giant electromechanical response in ferroelectric realxors are still unclear [4,44], it is believed that both the energy expense and the required electric field to induce the ferroelectric polarization rotation will decrease drastically in ferroelectric relaxors [17], giving rise to the high piezoelectricity. Therefore, all these effects combine together to affect the piezoelectric properties positively and lead to a relatively high d33 value in the BCTZ-0.15 ceramic compared with other lead-free piezoelectric materials. The variation of the kp, d31, and k31 values shows similar features to that of the d33 value which suggest that the MPB, increased grain size and relaxor nature of BCTZ ceramics are also responsible for the improved kp, d31, and k31. The maximum kp, d31, and k31 values are 33%, 122 pC/N, and 16%, respectively. In contrast, Qm shows a rather low value of 80 at x = 0.15 compared to 180 at x = 0.10 and 200 at x = 0.20. The decrease of Qm generally accompanies with the increase of kp and d33 [45], and this should be ascribed to the enhanced mobility of domain walls led to the increased internal friction and energy loss in the ceramics. The hysteresis loops of polarization versus electric field of the BCTZ-x ceramics with 0 6 x 6 0.20 are shown in Fig. 7. All the
Fig. 7. Hysteresis loops of polarization versus electric field of the BCTZ-x ceramics with 0 6 x 6 0.20.
ceramics show a well-saturated hysteresis loop. The ferroelectric properties of the ceramics with x 6 0.15 increased gradually, however, deteriorated at x = 0.2. An enhanced ferroelectric property with a remnant polarization value Pr of 6.2 lC/cm2 and a coercive field Ec of 2.2 kV/cm was obtained at x = 0.15. This result indicates that appropriate amount of Ca substitution for Ba will improve the ferroelectric properties of BCTZ-x ceramics.
4. Conclusions Lead-free (Ba1xCax)(Ti0.9Zr0.1)O3 (0 6 x 6 0.20) piezoelectric ceramics were successfully synthesized by conventional solid state reaction. The phase structure, microstructure, electrical properties, and the relaxor behavior of BCTZ ceramics were systematically investigated. The XRD analysis and the electrical properties results indicate another possible MPB between rhombohedral phase and
Fig. 6. Composition dependence of the average grain size, d33, kp, d31, k31, and Qm of the BCTZ-x ceramics with 0 6 x 6 0.20 at room temperature.
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tetragonal phase existing at about x = 0.15. The (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 ceramic exhibits homogenous microstructure with large grains. The degree of relaxor behavior increased with increasing Ca content. Due to the existence of MPB, the increased grain size, and the relaxor behavior, the optimum electrical properties of eT3 =e0 = 5800, d33 = 350 pC/N, kp = 33%, d31 = 122 pC/N, and k31 = 16% were obtained in (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics. These results indicate that appropriate Ca substitution can significantly improve the piezoelectric properties of the (Ba1xCax)(Ti0.9Zr0.1)O3 ceramics, thereby demonstrating that (Ba1xCax)(Ti0.9Zr0.1) O3 ceramics have a promising potential to be a suitable candidate for lead-free piezoelectric ceramics. Acknowledgements This research is supported by the National University of Singapore. The author (Shukai Ye) is also grateful to Mr. Shijie Wang for assistance and discussion. References [1] U. Kenji, Ferroelectric Devices, Marcel Dekker, New York, 2000. [2] B. Jaffe, W.R. Cook, H.L. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971. [3] E.V. Colla, N.K. Yushin, D. Viehland, J. Appl. Phys. 83 (1998) 3298–3304. [4] S.-E. Park, T.R. Shrout, J. Appl. Phys. 82 (1997) 1804–1811. [5] R.A. Eichel, H. Kungl, Funct. Mater. Lett. 3 (2010) 1–4. [6] J. Rodel, W. Jo, K.T.P. Seifert, E.-M. Anton, T. Granzow, D. Damjanovic, J. Am. Ceram. Soc. 92 (2009) 1153–1177. [7] D. Damjanovic, N. Klein, J. Li, V. Porokhonskyy, Funct. Mater. Lett. 3 (2010) 5–13. [8] M. Kosec, B. Malic, A. Bencan, T. Rojac, J. Tellier, Funct. Mater. Lett. 3 (2010) 15–18. [9] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, J. Eur. Ceram. Soc. 22 (2002) 2115–2121. [10] Y.J. Dai, S.J. Zhang, T.R. Shrout, X.W. Zhang, J. Am. Ceram. Soc. 93 (2010) 1108– 1113. [11] P.W. Rehrig, S.E. Park, S. Trolier-McKinstry, G.L. Messing, B. Jones, T.R. Shrout, J. Appl. Phys. 86 (1999) 1657–1661. [12] Z. Yu, C. Ang, R.Y. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 1489–1493.
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Effects of Ca substitution on structure, piezoelectric properties, and relaxor behavior of lead-free Ba(Ti0.9Zr0.1)O3 piezoelectric ceramics Shukai Ye, Jerry Fuh, Li Lu ⇑ Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
a r t i c l e
i n f o
Article history: Received 14 August 2011 Received in revised form 15 June 2012 Accepted 16 June 2012 Available online 4 July 2012 Keywords: Ceramics Sintering Microstructure Piezoelectricity SEM X-ray diffraction
a b s t r a c t Lead-free piezoelectric (Ba1xCax)(Ti0.9Zr0.1)O3 (BCTZ-x, x = 0, 0.05, 0.10, 0.15, 0.20) ceramics are synthesized via a conventional solid state reaction method. The effects of Ca substitution on the microstructure, dielectric properties, piezoelectric properties, and relaxor behavior of BCTZ ceramics were systematically investigated. Structural characterization of the synthesized BCTZ shows that a structure with morphortropic phase boundary (MPB) between rhombohedral phase and tetragonal phase at about x = 0.15 has been obtained. The (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics exhibit the most homogeneous microstructure with the largest grains. The dielectric and piezoelectric properties are found to be enhanced near the phase transition composition due to the lattice distortion caused by Ca2+ substitution. The dielectric relaxor behavior of the BCTZ ceramics was enhanced with increasing Ca content. The relaxor nature of the BCTZ ceramics may be attributed to the formation of polar clusters induced by local distortion. The increased relaxor behavior in the BCTZ ceramics also contributes to the improvement of the piezoelectric properties. The (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramic exhibits optimum properties with a dielectric constant, eT3 =e0 , of 5800, piezoelectric constant, d33, of 350 pC/N, d31 of 122 pC/N, electromechanical coupling factor, kp, of 33%, and k31, of 16%, indicating that the (Ba,Ca)(Ti,Zr)O3 ceramic is a promising candidate for lead-free piezoelectric materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Piezoelectric ceramics are the most widely used materials for actuator, transducer, and sensor applications [1]. For the past few decades, lead zirconate titanate (PZT) based materials have dominated the piezoelectric ceramic market because of their superior dielectric and piezoelectric properties [2]. Among the lead based materials, ferroelectric realxors, such as Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) [3], Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) [4], and their derived compounds, have received extensive studies and have been widely used in multilayer capacitor, high performance sensor, and hysteresis-free actuator [1] due to their giant electromechanical response. However, because of the malignant effects of lead on human health and the high vapor pressure during processing, the search for lead-free relaxor with high piezoelectric response has attracted much attention in recent years [5–8]. Barium titanate, BaTiO3 (BT), is known as the first polycrystalline piezoelectric ceramic with perovskite structure which shows a d33 of 191 pC/N [2]. In the previous research, BT is often used to form solid solution with other lead-free compounds to create
⇑ Corresponding author. Fax: +65 6779 1459. E-mail address: [email protected] (L. Lu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.084
an MPB which can improve the piezoelectric and dielectric properties [9,10]. However, the interest in BaTiO3-based lead-free piezoelectric ceramics has been renewed since Rehrig et al. [11] reported a d33 higher than 400 pC/N in the Ba(Zr0.085Ti0.915)O3 single crystal. Subsequent studies in Ba(Zr,Ti)O3 ceramics [12,13] also achieved promising results. Meanwhile, BaTiO3-based lead-free relaxor ferroelectrics such as BaTiO3–BaZrO3 [14], BaTiO3–BiAlO3 [15], and BaTiO3–CaTiO3–SrTiO3 [16], have been widely studied due to their relatively high dielectric and piezoelectric properties compared with other lead-free compounds. The ultrahigh piezoelectric effect in relaxors is probably due to the decreased energy cost and electric field necessary to induce ferroelectric domain switching [17]. More recently, a new (Ba,Ca)(Ti,Zr)O3 (BCTZ) system has attracted increasing interest due to the development of a pseudobinary ferroelectric system Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 [18] which possesses a d33 as high as 600 pC/N. It is believed that the morphotropic phase boundary (MPB) plays an important role in improving the electrical properties of piezoelectric ceramics [19,20]. Several possible MPB have been reported in BCTZ ceramics with different compositions. Li et al. [21] reported a d33 of 365 pC/ N in (Ba0.92Ca0.08)(Ti0.95Zr0.05)O3 ceramics with an MPB between orthorhombic and tetragonal phases. While Zhang et al. [22] obtained a d33 of 338 pC/N in (Ba0.95Ca0.05)(Ti0.96Zr0.04)O3 ceramics with a coexistence of orthorhombic and pseudocubic phases.
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Additionally, the electrical properties of BCTZ ceramics can be further improved to above 400 pC/N by optimizing sintering temperature [23] and poling condition [24], and doping with CuO [25] and BiFeO3 [26]. Moreover, the temperature stability of BCTZ ceramics can be enhanced by Ho doping [27] and Y doping [28]. However, the systematic study of effects of Ca substitution on the structure, piezoelectric properties and relaxor behavior of Ba(Ti0.9Zr0.1)O3 ceramics is still rare. In this paper, the synthesis of (Ba1xCax)(Ti0.9Zr0.1)O3 (0 6 x 6 0.2) piezoelectric ceramics as a lead-free material candidate and the effects of Ca substitution on the phase structure, microstructure, piezoelectric properties, and relaxor behavior of BCTZ ceramics are reported.
2. Experimental procedures (Ba1xCax)(Ti0.9Zr0.1)O3 (x = 0, 0.05, 0.1, 0.15, 0.2) (abbreviated as BCTZ-x) ceramics were prepared via a conventional solid state reaction technique. Commercial powders of BaCO3 (99%, Sinopharm Chemical Reagent), CaCO3 (99%, Sigma–Aldrich), TiO2 (99.9%, Sigma–Aldrich), and ZrO2 (99%, Sigma–Aldrich) were selected as starting materials. The raw materials were finely weighted according to the stoichiometric ratio, high-energy ball-milled (SPEX SamplePrep 8000 M Mixer/Mills, Metuchen, New Jersey, United States) for 2 h and calcined at 1150 °C for 6 h and followed by ball-milling for another 2 h again. Five weight percent of polyethylene glycol (PEG) was added into the milled powder, which was uniaxially cold-compacted to pellets of 10 mm in diameter and 1.5 mm in thickness. The cold-compacted pellets were first heated to 600 °C for 1 h to burn out the binder and then sintered at 1500 °C for 6 h. The densities of the sintered specimens were measured by the Archimedes method. The crystal structures of the ceramics were examined by X-ray diffractometer with CuKa radiation (Shimadzu XRD-600, Kyoto, Japan) and the grain microstructures were observed by a field emission scanning microscope (Hitachi S-4100, Osaka, Japan). To characterize the electrical properties, the specimens were ground and polished to obtain parallel surfaces, and gold electrodes were sputtered on both upper and bottom surfaces. The poling process was conducted by using a dc power supply (Matsusada Precision, Inc., Japan) at 3 kV/mm at room temperature in silicon oil for 30 min. The temperature dependence of the dielectric constant of the ceramics was studied at a temperature range of 30–150 °C in an environmental test chamber (Delta 9023, Delta Design, California, United States) under different frequencies using a precision impedance analyzer. The piezoelectric constant, d33, was measured using a Berlincourt-type d33 meter (YE2730A, Sinocera Piezotronics, Jiangsu, China). The piezoelectric properties kp, d31, k31, and Qm were determined by a resonance and anti-resonance method based on IEEE standards [29] using a precision impedance analyzer. The polarization–electric field (P–E) hysteresis loop was measured by a Radiant Precision workstation based on a standard Sawyer–Tower circuit at room temperature.
Fig. 1. X-ray diffraction patterns of the BCTZ-x (0 6 x 6 0.20) ceramics in the 2h range: (a) from 20° to 60°, (b) from 43° to 47°.
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3. Results and discussion Fig. 1 shows the XRD patterns of the as-sintered BCTZ-x (x = 0– 0.20) ceramics. As revealed from Fig. 1(a), all the samples are of the single pervoskite structure indicating that Ca2+ may have entirely diffused into the Ba(Ti0.9Zr0.1)O3 (BTZ) lattice to form a solid solution, in which Ca2+ substituted Ba2+. Furthermore, all the diffraction peaks shift monotonically to higher 2h angles with the increase of Ca content, suggesting a gradual decrease of the lattice parameter of the samples, which may be ascribed to the smaller ionic radius of Ca2+ (1.34 Å, coordinate number/CN: 12) than that of Ba2+ (1.61 Å, CN: 12) [30]. Fig. 1(b) shows XRD peak profile of (2 0 0) of all the samples. The crystal structure of pure BTZ at room temperature is rhombohedral [31], which is indicated by the single (2 0 0) peak. With the increase of Ca content, pronounced splitting of the (0 0 2) peak from the (200) peak is observed, demonstrating a phase transition from rhombohedral phase to tetragonal phase, which is caused by the lattice distortion induced by the substitution of Ba2+ by Ca2+. Additionally, the increased separation between (0 0 2) and (2 0 0) peaks indicates an enhancement of tetragonality in the BCTZ solid solutions [32]. Therefore, it can be inferred that the rhombohedral and tetragonal phase may coexist in the composition range x = 0.10–0.20. The variation of the relative densities of the BCTZ-x (x = 0–0.20) ceramics sintered at optimum sintering temperature is shown in Fig. 2(a). The FESEM images of the ceramics are shown in Fig. 2(b–f). As shown in Fig. 2(a), the relative density increases significantly from 90% to 95% as the Ca content increases from 0 to 0.05, and then remains almost unchanged. It is evident that Ca substitution can improve the sinterability of the BCTZ ceramics. This may be attributed to the fluxing effect of Ca2+ ions during the period of sintering facilitating the process of densification [33]. Fig. 2(b) shows the FESEM image of the BTZ ceramics with an unconsolidated microstructure and inhomogeneous grains ranging from 2 to 8 lm. With increasing Ca content, the grain size increased and the grain-size distribution became more homogeneous, led to a significant decrease of porosity. When Ca content reached 15 mol%, the most homogeneous grain-size distribution with the largest average grain size around 11 lm was obtained, as shown in Fig. 2(e). With further increasing Ca content to 20 mol%, the average grain size decreased to 5 lm. Such evolution in grain size may be explained by the change of interface atomic structure or grain boundary structure caused by Ca substitution, which significantly affects the microstructure evolution during ceramics sintering [34–36]. This result indicates that appropriate amount of Ca substitution can increase the grain size and improve the homogeneity. It is important to get the optimum microstructure of BCTZ ceramics, since it influences various properties in piezoelectric materials [2,33]. The composition dependence of dielectric properties for the BCTZ-x (x = 0–0.20) ceramics under 1 kHz at room temperature is shown in Fig. 3. The dielectric loss tan d of all the samples are below 1.2% with slight variation. Whereas the relative dielectric constant eT3 =e0 increased dramatically from 2000 at x = 0 to a maximum of 5800 at x = 0.15, and then decreased to 4300 at x = 0.2. The maximum eT3 =e0 is often obtained around the morphortropic phase boundary (MPB) composition in both lead-based systems and lead-free systems, due to the enhanced mobility of the domain and domain walls [9,33]. Therefore, the anomaly of the relative dielectric constant in BCTZ-x system may suggest the existence of another possible MPB between rhombohedral phase and tetragonal phase at about x = 0.15. Fig. 4 shows the temperature dependences of the dielectric constant and loss of BCTZ-x (x = 0–0.2) ceramics at different frequencies (100–100 kHz). Only one dielectric constant peak was observed in all the samples for each frequency. All the samples show weak frequency dispersion. However, the temperature
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Fig. 2. (a) Bulk and relative densities of the BCTZ-x ceramics as a function of Ca content; FESEM images of the BCTZ-x ceramics sintered at 1500 °C for 6 h: (b) x = 0; (c) x = 0.05; (d) x = 0.10; (e) x = 0.15; (f) x = 0.20.
dependences of the dielectric constant of BCTZ ceramics represent a typical relaxor behavior. First, the samples showed a broader dielectric maximum with increasing Ca content which may suggest a diffuse phase transition [37,38]. Another indication of the relaxor behavior is the merging of the dielectric constant curves at different frequencies at high temperature [39]. More detailed investigation of the relaxor behavior of BCTZ ceramics will be described in the following. The BTZ ceramic exhibited a dielectric constant maximum at 70 °C corresponding to the rhombohedral to cubic phase transition. With increasing the content of Ca substitution, the Tm decreased gradually: 67 °C at 5 mol%, 61 °C at 10 mol%, 60 °C at 15 mol%, and 55 °C at 20 mol%. According to the previous discussion about the crystal structure, the peaks for the ceramics with Ca substitution should correspond to the tetragonal to cubic phase transition. The evolution of Curie point TC with increasing Ca content can be ascribed to the different crystal symmetry for different ceramics. The Tc of the BCTZ ceramics is relatively low compared with other lead-free piezoelectric ceramics. Further studies are in
Fig. 3. Variation in the relative dielectric constant eT3 =e0 and the dielectric loss tan d as a function of Ca content at 1 kHz under room temperature.
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Fig. 4. Temperature dependence of dielectric constant and loss for the BCTZ-x (x = 0–0.2) ceramics at different frequencies: (a) 0; (b) 5%; (c) 10%; (d) 15%; (e) 20%.
progress to increase the Curie point and broaden the working temperature range of this material. In most ferroelectrics, the relationship between temperature and the dielectric constant above Curie temperature can be expressed fairly accurately with simple equation called the Curie– Weiss law:
1
e
¼
T T CW ðT > T CW Þ C
ð1Þ
where e is the dielectric constant, T is the temperature, Tcw is the Curie–Weiss temperature, C is the Curie–Weiss constant. Fig. 5 exhibits the plots of inverse dielectric constant as a function of temperature at 10 kHz for BCTZ-x (x = 0–0.2) ceramics. Tm represents the temperature at which the dielectric constant reaches its maximum. TB denotes the temperature at which the dielectric constant
starts to obey the Curie–Weiss law. Therefore, the deviation from the Curie–Weiss law can be defined by the following:
DT m ¼ T B T m
ð2Þ
The fitting results for T m , T B , T CW , DT m , and C are summarized in Table 1. It is observed that DT m increased with increasing the addition of Ca, suggesting an increase of the degree of relaxor behavior. To date, various theory models have been proposed to explain the relaxor behavior [38,40,41], such as compositional fluctuation and diffuse phase transition, the superparaelectric model, the dipole glass model, the random field model, the domain wall model, and the random layer model. The common point of these theories is based on the local distortion of the crystal structure, giving rise to the polar clusters or polar nanoregions (PNRs). Considering the large difference of ionic radii between Ba2+ and Ca2+ ions, the Ca substitution can drive large distortion in crystal which will hinder
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Fig. 5. The plots of inverse dielectric constant as a function of temperature at 10 kHz for BCTZ-x (x = 0–0.2) ceramics: (a) x = 0; (b) x = 0.05; (c) x = 0.1; (d) x = 0.15; (e) x = 0.2.
the long-range dipole alignment and lead to the formation of PNRs, resulting in the improvement of relaxor behavior. However, the PNRs can hardly be detected by XRD because of their nanosize. Therefore, the large local distortion observed by XRD may suggest a large size of the PNRs. This may explain the weak relaxor behavior in this system due to the less sensitivity in larger PNRs [42].
Fig. 6 shows the average grain size, the piezoelectric constant d33 and d31, the electromechanical coupling factor kp and k31, and the mechanical quality factor Qm of the BCTZ-x ceramics with 0 6 x 6 0.20. The d33 value of the BCTZ ceramic increased with increasing Ca content and reached the maximum value of 350 pC/N at x = 0.15, and then dramatically degenerated to
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Table 1 The Curie–Weiss temperature (TCW), the temperature where the dielectric constant starts to follow Curie–Weiss law (TB), the temperature at the dielectric constant maximum (Tm), the deviation from Curie–Weiss law (DT m ), and the Curie–Weiss constant (C) for BCTZ-x ceramics at 10 kHz. BCTZ-x
TCW (°C)
TB (°C)
Tm (°C)
DT m (°C)
C (105 °C)
x=0 x = 0.05 x = 0.1 x = 0.15 x = 0.2
73 75 72 71 56
90 94 95 99 100
70 67 61 60 55
20 27 34 39 45
1.43 2.27 1.66 1.84 1.31
55 pC/N at x = 0.20. Because of the more possible polarization states resulting from the coexistence of two phases, the piezoelectrics exhibit improved piezoelectric response near MPB composition [22,43]. In addition, the increased grain size also contributed to the enhanced d33 value since the domain rotation becomes easier for the specimens with large grains [1]. Furthermore, the BCTZx ceramics have been suggested as relaxor ferroelectrics, and the degree of relaxor behavior increases with addition of Ca. Although the origins of the giant electromechanical response in ferroelectric realxors are still unclear [4,44], it is believed that both the energy expense and the required electric field to induce the ferroelectric polarization rotation will decrease drastically in ferroelectric relaxors [17], giving rise to the high piezoelectricity. Therefore, all these effects combine together to affect the piezoelectric properties positively and lead to a relatively high d33 value in the BCTZ-0.15 ceramic compared with other lead-free piezoelectric materials. The variation of the kp, d31, and k31 values shows similar features to that of the d33 value which suggest that the MPB, increased grain size and relaxor nature of BCTZ ceramics are also responsible for the improved kp, d31, and k31. The maximum kp, d31, and k31 values are 33%, 122 pC/N, and 16%, respectively. In contrast, Qm shows a rather low value of 80 at x = 0.15 compared to 180 at x = 0.10 and 200 at x = 0.20. The decrease of Qm generally accompanies with the increase of kp and d33 [45], and this should be ascribed to the enhanced mobility of domain walls led to the increased internal friction and energy loss in the ceramics. The hysteresis loops of polarization versus electric field of the BCTZ-x ceramics with 0 6 x 6 0.20 are shown in Fig. 7. All the
Fig. 7. Hysteresis loops of polarization versus electric field of the BCTZ-x ceramics with 0 6 x 6 0.20.
ceramics show a well-saturated hysteresis loop. The ferroelectric properties of the ceramics with x 6 0.15 increased gradually, however, deteriorated at x = 0.2. An enhanced ferroelectric property with a remnant polarization value Pr of 6.2 lC/cm2 and a coercive field Ec of 2.2 kV/cm was obtained at x = 0.15. This result indicates that appropriate amount of Ca substitution for Ba will improve the ferroelectric properties of BCTZ-x ceramics.
4. Conclusions Lead-free (Ba1xCax)(Ti0.9Zr0.1)O3 (0 6 x 6 0.20) piezoelectric ceramics were successfully synthesized by conventional solid state reaction. The phase structure, microstructure, electrical properties, and the relaxor behavior of BCTZ ceramics were systematically investigated. The XRD analysis and the electrical properties results indicate another possible MPB between rhombohedral phase and
Fig. 6. Composition dependence of the average grain size, d33, kp, d31, k31, and Qm of the BCTZ-x ceramics with 0 6 x 6 0.20 at room temperature.
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tetragonal phase existing at about x = 0.15. The (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 ceramic exhibits homogenous microstructure with large grains. The degree of relaxor behavior increased with increasing Ca content. Due to the existence of MPB, the increased grain size, and the relaxor behavior, the optimum electrical properties of eT3 =e0 = 5800, d33 = 350 pC/N, kp = 33%, d31 = 122 pC/N, and k31 = 16% were obtained in (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics. These results indicate that appropriate Ca substitution can significantly improve the piezoelectric properties of the (Ba1xCax)(Ti0.9Zr0.1)O3 ceramics, thereby demonstrating that (Ba1xCax)(Ti0.9Zr0.1) O3 ceramics have a promising potential to be a suitable candidate for lead-free piezoelectric ceramics. Acknowledgements This research is supported by the National University of Singapore. The author (Shukai Ye) is also grateful to Mr. Shijie Wang for assistance and discussion. References [1] U. Kenji, Ferroelectric Devices, Marcel Dekker, New York, 2000. [2] B. Jaffe, W.R. Cook, H.L. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971. [3] E.V. Colla, N.K. Yushin, D. Viehland, J. Appl. Phys. 83 (1998) 3298–3304. [4] S.-E. Park, T.R. Shrout, J. Appl. Phys. 82 (1997) 1804–1811. [5] R.A. Eichel, H. Kungl, Funct. Mater. Lett. 3 (2010) 1–4. [6] J. Rodel, W. Jo, K.T.P. Seifert, E.-M. Anton, T. Granzow, D. Damjanovic, J. Am. Ceram. Soc. 92 (2009) 1153–1177. [7] D. Damjanovic, N. Klein, J. Li, V. Porokhonskyy, Funct. Mater. Lett. 3 (2010) 5–13. [8] M. Kosec, B. Malic, A. Bencan, T. Rojac, J. Tellier, Funct. Mater. Lett. 3 (2010) 15–18. [9] B.J. Chu, D.R. Chen, G.R. Li, Q.R. Yin, J. Eur. Ceram. Soc. 22 (2002) 2115–2121. [10] Y.J. Dai, S.J. Zhang, T.R. Shrout, X.W. Zhang, J. Am. Ceram. Soc. 93 (2010) 1108– 1113. [11] P.W. Rehrig, S.E. Park, S. Trolier-McKinstry, G.L. Messing, B. Jones, T.R. Shrout, J. Appl. Phys. 86 (1999) 1657–1661. [12] Z. Yu, C. Ang, R.Y. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 1489–1493.
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