Journal of Alloys and Compounds 613 (2014) 219–225 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom Structure and electrical properties of Li-doped BaTiO3–CaTiO3–BaZrO3 lead-free ceramics prepared by citrate method Xing Liu a, Min Zhu a, Zhihui Chen a, Bijun Fang a,⇑, Jianning Ding a,b,⇑, Xiangyong Zhao c, Haiqing Xu c, Haosu Luo c a School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China b School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China c Key Laboratory of Inorganic Function Material and Device, Chinese Academy of Sciences, Shanghai 201800, China a r t i c l e i n f o Article history: Received 14 May 2014 Received in revised form 6 June 2014 Accepted 6 June 2014 Available online 16 June 2014 Keywords: BaTiO3-based ceramics Citrate method Doping Perovskite structure Electrical properties a b s t r a c t 1 wt% Li-doped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics were prepared by the citrate method. XRD measurement showed that pure perovskite BCZT precursor powders are obtained calcined at 600–700 °C, which exhibit rather homogeneous microstructure morphology with nm-scale grain size. The sintered BCZT ceramics exhibit pure perovskite structure with composition locating at rhombohedral side around the morphotropic phase boundary (MPB). Oxide doping and citrate method exert great inﬂuences on microstructure and electrical properties of the Li-doped BCZT ceramics. All the synthesized ceramics exhibit excellent dielectric and ferroelectric properties. The Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C exhibit the best dielectric, ferroelectric and piezoelectric properties. The high piezoelectric response of the synthesized Li-doped BCZT ceramics is considered as relating to the MPB effect, Li-doping and wet chemical synthesis method. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Lead-based ferroelectric perovskites, represented by (1 x)PbZrO3–xPbTiO3 (PZT), have dominated the ﬁeld of industrial piezoelectric applications, including sonars, buzzers, motors and ultrasonic cleaners, owing to their superior piezoelectric, dielectric, and electromechanical properties especially with the compositions around the morphotropic phase boundary (MPB) [1,2]. The MPB of PZT is known to locate at the composition of x = 0.48, which is considered as a region connecting different ferroelectric phases, allowing facile domain orientation reversal during the poling process. Further modiﬁcations using acceptor and donor dopants provide us the wide range of piezoelectric compositions we have today . However, such lead-based materials contain almost 60 wt% lead, which tends to release into the environment. This occurs during calcination and sintering processes, and after usage accompanied recycling and waste disposal, causing the ⇑ Corresponding authors. Address: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China (B. Fang and J. Ding). Tel.: +86 519 86330100; fax: +86 519 86330095. E-mail addresses: firstname.lastname@example.org, email@example.com (B. Fang), dingjn@cczu. edu.cn (J. Ding). http://dx.doi.org/10.1016/j.jallcom.2014.06.046 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. environmental pollution and harming to human health. Hence, the high toxicity of PZT and the growing demand of replacing these lead containing piezoceramics have driven considerable research of environment-friendly materials . Over the past years, much efforts have been done to search for lead-free piezoceramics to substitute PZT. Recent reports on lead-free piezoelectrics are focus on three perovskite families: K0.5Na0.5NbO3 (KNN) [4,5], Na0.5Bi0.5TiO3 (NBT) [6,7], and BaTiO3 (BT) [8,9]. In the KNN and NBT systems, the alkali components become highly unstable and easily to evaporate during high temperature sintering, which makes it difﬁcult to maintain chemical stoichiometry and leads to the deterioration of electrical properties . As a classical perovskite ferroelectric, BT is another lead-free ceramic system and possesses good composition controllability. It is well known that BT is widely used as a commercial dielectric material rather than piezoelectric material due to its high dielectric property and low piezoelectric coefﬁcient (d33 = 191 pC/N) . Recently, Liu and Ren  reported a Ca and Zr co-doped BT system with extraordinarily high d33 (620 pC/N), which can be comparable with PZT-5H. The high piezoactivity stems from the tricritical triple point-like MPB composition . Since then, further studies have focused on doping [12,13], sintering temperature , piezoelectric , pyroelectric , optical  properties, and manufacturing process [18,19]. X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225 The 1 wt% LiNO3-doped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics were prepared by the citrate method. Ba(NO3)2 (99.5%), Ca(NO3)24H2O (99%), Zr(NO3)45H2O (99.5%), C16H36O4Ti (98%) and LiNO3 (99.9%) were used as starting materials. In this method, the mole ratio of the citric acid to the total metal cations was 1:1. The weighed amount of citric acid was dissolved into deionized water and the pH value was adjusted to 7–9 by ammonia water. The designed amount of tetrabutyl titanate was slowly added into the citric acid solution and stirred at 70 °C to form a transparent aqueous solution. Various nitrates were then added according to the stoichiometric ratio and stirred at 80 °C to form a semitransparent precursor solution. The precursor solution was heated and dried to form a gel. The gel was pre-calcined at 300 °C for 2 h to eliminate the water, ammonia, citric acid and organics, eventually obtaining dark-brown powder. The powder was then calcined at 600– 700 °C for 2 h to form perovskite BCZT. The calcined powders were cold dry-pressed uniaxially into pellet with 13 mm in diameter and 1.5 mm in thickness with the addition of appropriate amount of 8 wt% polyvinyl alcohol (PVA) solution as binder and then sintered at 1490 °C and 1500 °C for 2 h in a covered zirconium dioxide crucibles. The sintered ceramics were polished to obtain ﬂat and parallel surfaces. Crystal structure of the calcined Li-doped BCZT powders and sintered Li-doped BCZT ceramics was investigated by a Rigaku D/max-2500/PC X-ray Diffraction meter (XRD, Rigaku Corp., Japan) using the well-polished ceramics. Simultaneous thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) analysis of the pre-calcined powder were carried out by a simultaneous thermal analyzer (TG–DSC, Setaram Instrumentation Corp., France), which was heated from room temperature to 1400 °C in air at a rate of 10 °C/min. Microstructure morphology of the calcined powders were observed by a Zeiss SupraTM 55 Sapphire Field Emission Scanning Electron Microscope (FESEM, Carl Zeiss Group, Germany). The medium diameter of the calcined powders was measured by a BT-9300S laser scattering particle analyzer (Dandong Bettersize Instruments Ltd., China). For electrical properties characterization, silver paste was coated on both surfaces of the well-polished ceramics and ﬁred at 550 °C for 30 min to provide robust electrodes. Dielectric property was measured by a computer-controlled TH2818 Automatic Component Analyzer under a weak oscillation level of 1 Vrms (Changzhou Tonghui Electronic Co. Ltd., China). For piezoelectric property measurement, the ceramics were poled at room temperature under different electric ﬁeld between 0.5 and 3 kV/mm with an interval of 0.5 kV/mm for 5 min in silicon oil to investigate the inﬂuences of polarization electric ﬁeld on piezoelectric properties. Piezoelectric properties were measured by a ZJ-6A Berlincourt-type quasi-static d33/d31 meter (Institute of Acoustics, Chinese Academy of Sciences, Beijing, China) and a TH2826 Wide Frequency LCR Meter (Changzhou Tonghui Electronic Co. Ltd., China) using the resonance-antiresonance method. Detailed electrical properties measurement procedures were described elsewhere . 0.5 100 TG 30 o T1=469.6 C 0.0 20 90 -0.5 -1.0 -1.5 10 o dTG 80 T2=528.9 C 0 o T3=599.5 C 70 -10 o T4=612.6 C 3. Results and discussion -2.0 60 3.1. Citrate method processing -2.5 50 DSC -20 Heat flow (w/g) 2. Experimental procedure pre-calcined powder. The TG–DSC curves could be helpful to determine the temperature of the solid state reaction. The strongest endothermic peak appears at 469.6 °C, accompanied by serious weight loss and extremum of derivative thermogravimetry, indicating the decomposition of nitrates. Due to the dynamic measurement characteristic of DSC and the expense time for the decomposition of nitrates, two small endothermic peaks appear subsequently at 528.9 and 599.5 °C, which also may be correlated with the combustion of residual organics. In addition, a minor exothermic peak appears at 612.6 °C, which can be attributed to the solid state reaction and the initial crystallization . The weight loss reaches nearly 40% when the temperature increases to 700 °C, indicating that most organic components and nitrates have decomposed. A wide weak exothermic peak appears around 880 °C may be also correlated with the solid state reaction since such reaction usually is a slow weak exothermic process depending on diffusion and chemical reaction. Based on the TG–DSC analysis, the calcining temperature is chosen between 600 and 700 °C considering the dynamic nature of thermal analysis. Fig. 2 shows XRD patterns of the Li-doped BCZT powders calcined between 600 and 700 °C. All the calcined powders exhibit typical perovskite structure, and almost no impurity phase can be detected. Pure perovskite structure Li-doped BCZT has already formed at calcining temperature of 600 °C, where the nitrates and organics have decomposed and the solid state reaction begins as shown by the TG–DSC analysis. No apparent difference in the phase formation is observed of the powders calcined between 625 °C and 700 °C. It is clearly evident that perovskite structure can be formed well below calcining temperature of 700 °C by the citrate method, which is much lower than that used by the conventional solid state reaction method (1300 °C) [11,14]. The decreased calcining temperature obtained in this study can be attributed to the oxide doping, and small particle size, homogenous distribution and high activity of the precursor powders prepared by the citrate synthesis process. Medium diameter and grain size of the calcined Li-doped BCZT powders are shown in Table 1, based on laser particle size analysis and XRD measurement of the (1 0 0), (1 1 0) and (1 1 1) diffraction reﬂections. The grain size was estimated by the Scherrer formula: d ¼ kk=b cos h, where d is the grain size; k is the shape factor, 0.89; k is the X-ray wavelength, 0.15406 nm; b is the full-width at halfmaximum (FWHM) of the diffraction peak; h is the Bragg’s angle. Due to the detection limitation of the laser scattering particle analyzer, the medium diameter D50 of the particles is around lm-scale, which is different from the nm-scale values of grain size calculated by the Scherrer formula. Particle size tends to increase with the increase of calcining temperature; however, no simple regularity appears in this study. In order to detect exact particle size of the calcined Li-doped BCZT powders, FESEM Weight (%) However, most Ba(ZrxTi1x)O3–(BayCa1y)TiO3 (BZT–BCT) piezoelectric ceramics reported as yet are mainly fabricated by the conventional solid-state reaction method [11–17], which requires rather high calcination temperature (1300 °C). The solid state sintering usually produces inhomogeneous microstructure and impurity phases, which may deteriorate electrical properties. By contrast, wet chemical synthesis method is beneﬁcial to obtain high chemical purity, rather dense microstructure, and better homogeneity . As a common chemical solution synthesis method, citrate method is essentially a polymeric precursor method, which uses citrate acid and water as complex agent and solvent, respectively, leading to improved homogeneity and stabilized compositions . So far as we know, there are few reports on the microstructure and piezoelectric properties of the BZT–BCT ceramics derived from the citrate method. In this work, the composition 0.75BaTiO3– 0.15CaTiO3–0.1BaZrO3 (BCZT) was fabricated by the citrate method, and LiNO3 was doped to adjust piezoelectric properties of the BCZT ceramics. Such composition locates around the reported tricritical triple point MPB composition and exhibits excellent electrical properties . The inﬂuences of ceramics processing and chemical doping on crystal structure and electrical properties of the BCZT ceramics were studied and their possible causations were interpreted. dTG (% / min) 220 Exo -30 Fig. 1 shows the simultaneous TG–DSC curves and the ﬁrst order partial differential of the TG curve of the Li-doped BCZT 0 200 400 600 800 1000 1200 1400 1600 Temperature (oC) Fig. 1. Simultaneous TG–DSC curves of the Li-doped BCZT pre-calcined powder. Intensity (a.u.) X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225 7500 700 o C 6000 675 o C 4500 650 o C 3000 625 o C 1500 600 o C 0 0 10 20 30 40 50 60 70 80 90 221 Based on the XRD measurement results, cell parameters of the sintered Li-doped BCZT ceramics are shown in Table 2. Synthesis method, chemical doping, and calcining and sintering temperature exert great effects on microstructure of the synthesized ceramics. With the increase of calcining and sintering temperature, grains grow up, porosity eliminates gradually, and microstructure becomes dense. It can be deduced from the tolerance factor that small ions (r(R3+) < 0.87 Å) will occupy the B-site, large ions (r(R3+) > 0.94 Å) will occupy the A-site, and intermediate ions will occupy both sides with different ratios of the perovskite structure . Taking radius of Li+ (r = 0.59 Å) into consideration, Li+ tends to enter interstitial sites of the perovskite structure, which exerts slight inﬂuence on crystal lattice. In general, optimum sintering conditions for ceramics can be established by analyzing cell parameters, perovskite phase content and densiﬁcation combining with electrical properties measurements. 3.3. Dielectric properties 2θ ( o ) Fig. 2. XRD patterns of the Li-doped BCZT powders calcined at different temperatures. Table 1 Laser particle size and grain size of the calcined Li-doped BCZT powders estimated by Scherrer formula. Calcining temperature (°C) 600 625 650 675 700 D50 (lm) Grain size (1 0 0) (nm) Grain size (1 1 0) (nm) Grain size (1 1 1) (nm) 2.1 21.08 21.00 20.89 1.8 22.69 21.11 20.23 2.86 18.28 19.17 17.96 1.9 19.68 18.95 18.4 1.41 22.06 18.48 17.96 analysis was undertaken and the micrographs are shown in Fig. 3. Due to the decomposition of nitrates and organics, particle size of the calcined powders is rather homogenous although different shape of particles can be observed. However, it is deﬁnitely found that grain size of all the calcined powders is in nm-scale, which is in agreement well with the values estimated by the Scherrer formula. Agglomeration exists in all the calcined Li-doped BCZT powders with different content, which is a normal phenomenon in the wet chemical synthesis method and should be improved further to eliminate its inﬂuence on sintering capability. 3.2. Structure characterization XRD patterns of the Li-doped BCZT ceramics sintered at different temperatures are shown in Fig. 4. Except slight impurity phase appearing in the Li-doped BCZT ceramics calcined at 675 °C and sintered at 1490 °C, all the other ceramics exhibit pure perovskite structure without detectable impurity within the detection limitation of the XRD measurement, indicating that Ca2+, Zr4+, and Li+ may diffuse into the BaTiO3 lattice to form complete solid solutions in the studied composition range. It is well known that lithium salts can be used as sintering acid to improve homogeneity and microstructure morphology of ceramics. As compared to the undoped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics reported before , the formation of pure perovskite Li-doped BCZT ceramics can be attributed to the Li-doping and the citrate synthesis process. Except slight broadening of the (3 1 0) and (3 1 1) diffraction reﬂections, all the other diffraction peaks exhibit rather symmetric, single and sharp diffraction peak with narrow FWHM, indicating that the Li-doped BCZT ceramics locate at the rhombohedral side around the MPB composition. Fig. 5 shows the temperature dependence of dielectric constant of the Li-doped BCZT ceramics calcined and sintered at different temperatures measured at 1 kHz upon cooling. It can be seen that the values of dielectric constant maximum (em) and Curie temperature (TC/Tm) are inﬂuenced greatly by the calcining and sintering temperatures. For the Li-doped BCZT ceramics calcined at the same calcining temperature, both the values of em and TC/Tm increase with the increase of sintering temperature since the microstructure becomes denser and the grain size grows larger . The dielectric anomalies appearing at the TC/Tm temperature can be attributed to the ferroelectric phase transition from rhombohedral ferroelectric phase to cubic paraelectric phase, which is a common critical phenomenon in a ferroelectric system near the phase transition point . The Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C exhibit the largest em, being 11,292, and the Curie temperature is 72.9 °C. Fig. 6 shows the temperature dependence of dielectric constant and loss of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C measured at several frequencies. Although the dielectric peaks are broad measured at different frequencies, dielectric frequency dispersion is not apparent, i.e., the values of em and TC/Tm maintain almost unchangeable with the variation of frequency as shown in the inset, which indicates that the Li-doped BCZT ceramics synthesized by the citrate method approach normal ferroelectrics. A dielectric shoulder appearing around 20–30 °C on the dielectric constant and loss peaks may be related to the polymorphic phase transition, where the ferroelectric phase transition from rhombohedral ferroelectric phase to tetragonal one takes place, accompanied by the improvement of electrical properties . Fig. 7 shows the linear ﬁtting of its dielectric behavior above TC/Tm at 1 kHz according to the Curie–Weiss law: e ¼ C=ðT T 0 Þ, where C is the Curie–Weiss constant, T0 is the Curie–Weiss temperature. The ﬁtting equation is determined as e ¼ 1:208 105 = ðT 94:7Þ, in which the Curie–Weiss law takes effect between 120 °C and 183.9 °C. The Curie–Weiss constant C is around 105 magnitude, indicating further the normal ferroelectric characteristic of the Li-doped BCZT ceramics prepared by the citrate method. 3.4. Ferroelectric properties The ferroelectric properties of ferroelectric ceramics are strongly inﬂuenced by calcining and sintering temperature. Figs. 8 and 9 show the room-temperature P–E hysteresis loops of the Li-doped BCZT ceramics sintered at 1490 and 1500 °C, respectively. At 2 kV/mm and 1 Hz, the hysteresis loops are rather saturated and fully developed, conﬁrming partly excellent ferroelectric property of the Li-doped BCZT ceramics synthesized by the citrate method. It 222 X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225 (110) Fig. 3. SEM micrographs of the Li-doped BCZT powders calcined at (a) 600 °C, (b) 650 °C and (c) 700 °C. 8000 Intensity (a.u.) Dielectric constant (220) (300) (310) (311) (211) (210) (111) (100) (200) 12000 o o o o o o o o o o o o 700 C-1500 C 700 C-1490 C 6000 o o 650 C-1490 C o o 650 C-1500 C o o 675 C-1490 C o o 675 C-1500 C o o 700 C-1490 C o o 700 C-1500 C 10000 8000 6000 4000 2000 675 C-1500 C 4000 675 C-1490 C 2000 0 0 0 30 60 90 120 150 180 Temperature (oC) 650 C-1500 C 650 C-1490 C 0 10 20 30 40 50 60 70 80 90 2θ ( o ) Fig. 4. XRD patterns of the Li-doped BCZT ceramics calcined and sintered at different temperatures. is found that the value of remnant polarization Pr shows great dependency on the calcining and sintering temperatures, whereas their coercive ﬁeld Ec changes little. All the ceramics exhibit low Ec value (<0.5 kV/mm), indicating that the Li-doped BCZT ceramics Fig. 5. Temperature dependence of dielectric constant of the Li-doped BCZT ceramics calcined and sintered at different temperatures measured at 1 kHz upon cooling. prepared by the citrate method are rather ‘‘soft’’ and may be feasible to tailor the poling condition. Generally, the Li-doped BCZT ceramics calcined at 650 °C and sintered at 1500 °C exhibit the optimum ferroelectric properties, whose values of Pr = 7.96 lC/cm2 and Ec = 0.20 kV/mm. The slight asymmetry of the hysteresis loops can be attributed to the existence of space-charge ﬁeld, which is constructed by point-charge defects induced by oxide doping and Table 2 Cell parameters of the Li-doped BCZT ceramics calcined and sintered at different temperatures. Composition (°C) a = b = c (Å) a (°) b (°) c (°) Cell volume (Å3) 650–1490 650–1500 675–1490 675–1500 700–1490 700–1500 3.9997(43) 4.0011(14) 4.0026(25) 4.0066(13) 3.9996(26) 3.9966(21) 90.349(578) 90.18(183) 90.375(326) 90.164(172) 90.249(340) 90.232(279) 89.873(382) 89.878(121) 89.784(215) 89.954(114) 89.831(224) 89.850(184) 90.088(95) 89.906(30) 90.007(53) 90.066(28) 89.930(55) 89.930(45) 63.985 64.052 64.123 64.317 63.979 63.835 223 80 18000 1kHz 10kHz 50kHz 100kHz 300kHz 6000 C 15000 72 o 9000 76 εm 12000 TC / Tm 68 12000 64 9000 60 6000 0 60 3000 120 180 240 300 Frequency ( kHz ) 0 0.04 0.03 0.02 0.01 0 30 60 90 120 150 Dielectric loss Dielectric constant X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225 0.00 180 Temperature (oC) Fig. 6. Temperature dependence of dielectric constant and loss tangent of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C measured at several frequencies upon cooling. 0.0008 Measured data Linear fitting 0.0007 5 ε=1.208x10 /(T-94.7) o o 120 C-183.9 C 0.0005 0.0004 0.0003 0.0002 110 120 130 140 150 160 170 180 Polarization (μC/cm2) 5 0 -5 -10 -15 -20 -10 0 -5 -10 -20 -10 0 10 20 Electric field (kV/cm) Fig. 9. Room-temperature P–E hysteresis loops of the Li-doped BCZT ceramics sintered at 1500 °C measured at 2 kV/mm and 1 Hz. 3.5. Piezoelectric properties o 650 C-1490 C o o 675 C-1490 C o o 700 C-1490 C 10 5 -15 Fig. 7. Linear ﬁtting of the dielectric behavior above TC/Tm of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C at 1 kHz according to the Curie– Weiss law. The ﬁtting parameters and the temperature range where the Curie– Weiss law takes effect are shown in the plot. o o 190 Temperature (oC) 15 o 650 C-1500 C o o 675 C-1500 C o o 700 C-1500 C 10 Polarization (μC/cm2) 0.0006 1/ ε 15 0 10 20 Electric field (kV/cm) Fig. 8. Room-temperature P–E hysteresis loops of the Li-doped BCZT ceramics sintered at 1490 °C measured at 2 kV/mm and 1 Hz. tends to exhibit pinning-down phenomenon [20,21]. The breach appearing in the hysteresis loops provides additional evidence of the existence of crystal defects. Piezoelectric constant and electromechanical coupling coefﬁcient are important parameters for piezoelectric applications. Fig. 10 shows the frequency dependence of impedance and phase degree of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C as an example. The Li-doped BCZT ceramics synthesized by the citrate method exhibit excellent resonance response peaks, which provides efﬁcient energy interconversion between electrical energy and mechanical energy. Poling conditions exert great inﬂuences on piezoelectric properties. The low poling electric ﬁeld makes the ferroelectric domains switch and reverse incompletely, whereas excessively large poling electric ﬁeld leads to high leakage current, increase of conductivity and ﬁnally dielectric breakdown of the piezoceramics . The poling electric ﬁeld dependence of piezoelectric constant d33 and electromechanical coupling factor Kp are given in Fig. 11 and Table 3, respectively. The value of Kp is calculated by the equation: pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ K p 2:51 ðfp fs Þ=fs . The gradual increase of d33 and Kp results from the easy switching of 180° domains under lower poling electric ﬁeld, whereas non-180° domains switching is responsible for the improvement of d33 and Kp during higher poling electric ﬁeld . It can be seen that most Li-doped BCZT ceramics exhibit the 224 X. Liu et al. / Journal of Alloys and Compounds 613 (2014) 219–225 Table 3 Poling electric ﬁeld dependence of electromechanical coupling factor Kp of the Li-doped BCZT ceramics calcined and sintered at different temperatures. Composition (°C) Kp (%) 650–1490 650–1500 675–1490 675–1500 700–1490 700–1500 500 V/mm 1000 V/mm 1500 V/mm 2000 V/mm 2500 V/mm 3000 V/mm 28.52 28.19 37.53 29.51 27.73 34.75 31.20 29.57 35.60 31.12 30.35 36.58 32.19 30.58 34.90 32.77 30.37 35.95 32.68 31.38 39.44 32.75 34.03 38.39 34.21 33.27 40.94 33.60 35.72 39.23 33.18 32.78 41.31 34.77 36.72 40.10 90 4.2 60 2.8 0 logZ o 30 θ logZ 3.5 -30 2.1 -60 θ 1.4 -90 240000 252000 264000 276000 4. Conclusions 288000 Frequency (Hz) Fig. 10. Frequency dependence of impedance and phase degree of the Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C in radial–extension– vibration-mode. o o 650 C-1490 C o o 650 C-1500 C o o 675 C-1490 C o o 675 C-1500 C 330 d33 (pC/N) growth of grains and further improve electrical properties. Lithium has a very small ionic radius, which tends to enter interstitial sites of the perovskite structure. Due to the low local energy barrier of the lithium ions, the rotation of ferroelectric domains becomes easier, leading to the improvement of piezoelectric properties. In addition, the wet chemical synthesis process, i.e., citrate method, is feasible to obtain homogenous, small size and high activity precursor powders, which are helpful to improve sintering capability of ceramics. Thus, enhanced ceramics with rather uniform and densiﬁed microstructure can be synthesized, leading to improved electrical properties. 300 270 240 o o 700 C-1490 C o o 700 C-1500 C 210 0.5 1.0 1.5 2.0 2.5 3.0 1 wt% Li-doped 0.75BaTiO3–0.15CaTiO3–0.1BaZrO3 (BCZT) ceramics were synthesized successfully by the citrate method. The Li-doped BCZT precursor powders calcined between 600 and 700 °C exhibit rather homogeneous microstructure with nm-scale grain size. The Li-doped BCZT ceramics sintered at 1490 and 1500 °C exhibit pure rhombohedral perovskite structure. Oxide doping and citrate method exert great inﬂuences on microstructure and electrical properties of the synthesized Li-doped BCZT ceramics. Dielectric behavior of the Li-doped BCZT ceramics prepared by the citrate method approaches normal ferroelectrics. All the sintered ceramics exhibit rather saturated and fully developed P–E hysteresis loops, whose values of Pr are inﬂuenced greatly by calcining and sintering temperatures, whereas Ec is inﬂuenced little. The Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C exhibit the best dielectric and piezoelectric properties, in which em = 11,292, TC/Tm = 72.9 °C, d33 = 336 pC/N and Kp = 40.1% poled at 3 kV/mm. The high piezoactivity of the synthesized ceramics can be attributed to the low energy barrier for the polarization rotation and the densiﬁed microstructure induced by Li-doping and the wet chemical synthesis method. Poling electric field (kV/mm ) Fig. 11. Poling electric ﬁeld dependence of piezoelectric constant d33 of the Lidoped BCZT ceramics calcined and sintered at different temperatures. maximum values of d33 and Kp under the poling electric ﬁeld of 2.5–3 kV/mm. The Li-doped BCZT ceramics calcined at 700 °C and sintered at 1500 °C exhibit the optimum piezoelectric properties, where the largest value of d33 is 336 pC/N, poled at 3 kV/mm, and its Kp reaches 40.1%. The improvement of the piezoelectric properties is related to the MPB effect, oxide doping and preparation method. The composition of the selected BCZT ceramics locates around the reported MPB composition, where polymorphic ferroelectric phases coexist at room temperature. The free energy of these ferroelectric phases around the MPB composition is nearly equivalent, which causes a low energy barrier for polarization rotation and lattice distortion, thus gives rise to high piezoelectric properties [11,26,27]. The Li-doping also takes effect. 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