The structure, dielectric and piezoelectric properties of 0.96(Bi0.5Na0.5)TiO3-(0.04-x)BaTiO3-xLiNbO3 (x = 0, 0.01, 0.02, 0.03, 0.04) ceramics were investigated by a conventional solid-state reaction method. All compositions show a single perovskite structure without any second phases. It was ascertained that the morphotropic phase boundary (MPB) of ferroelectric rhombohedral and relaxor tetragonal lies in the range of 0.02 ≤ x ≤ 0.04 at room temperature. As a result, the MPB compositions show improved properties; the saturated polarization, remnant polarization and coercive field are 43.7 μC/cm2, 38.7 μC/cm2 and 48.7 kV/cm for x = 0.02 and 43 μC/cm2, 37.9 μC/cm2 and 47 kV/cm for x = 0.03, respectively. The maximum of piezoelectric constant d33 (124pC/N) was obtained for the ceramic with x = 0.03, which was located at the MPB region. Our results do not only supplement for BNT-based lead-free piezoelectric ceramics but also provide a way to develop new high-performance Pb-free piezoceramics.
At present, lead zirconate titanate (PZT)-based ceramics are the most widely applied piezoelectric materials due to their excellent piezoelectric properties. However, the evaporation of toxic lead during the fabrication of the ceramics would cause environmental problem. Therefore, there is an increasing interest to develop lead-free piezoelectric materials to replace PZT-based ceramics.
(Bi0.5Na0.5)TiO3 (abbreviated as BNT) is considered to be a promising candidate of lead-free piezoelectric ceramics because of its large remnant polarization (Pr = 38μC/cm2) at room temperature and relatively high Curie temperature (Tc = 320°) (Hagiyev, Ismailzade, and Abiyev 1984; Suchanicz et al. 1988). However, it is difficult to pole for its large coercive field (Ec = 73 kV/cm), which makes its piezoelectric properties much lower than PZT based ceramics. Recently, a number of studies have been carried out to improve the properties of BNT ceramics by introduction other perovskite oxides, such as BaTiO3 (Chu et al. 2002; Qu, Shan, and Song 2005), (Bi1/2K1/2)TiO3 (KBT) (Sasaki et al. 1999; Jiang et al. 2006; Yang et al. 2008), (K0.5Na0.5)NbO3 (KNN) (Kounga et al. 2008; Zhang et al. 2008), NaNbO3 (Li et al. 2004), SrTiO3 (Zhou, Liu, and Meng 2007) and so on. It has been proved that these modifications on BNT show much better piezoelectric properties and it is easier to handle their poling process compared with pure BNT ceramics.
It is well known that the MPB plays a very important role in PZT-based ceramics because the piezoelectric and dielectric properties show a maximum over a specific compositional range around the MPB. Among BNT-based solid states, the BNT-BT system has rhombohedral–tetragonal MPB in the range of 0.06–0.07 mol BT and reveals relatively high piezoelectric and ferroelectric properties at the composition near the MPB (Takenaka, Maruyama, and Sakata 1991). However, Ba2+ entering into A-site of BNT lattice will induce the Tc shifting to lower temperature, which suggested that the materials were not suitable for high-temperature applications. Nevertheless, by doping lithium niobate (LiNbO3) with super high Tc at about 1,210°C(Jaffe, Cook, and Jaffe 1971), the Tc of BNT-BT ceramics will be increased correspondingly. Furthermore, LiNbO3 as one of the ABO3-type ferroelectrics has been widely used to modify the BNT-based materials (Kim et al. 2011; Kim et al. 2012; Hao et al. 2013). Jin et al. (Kim et al. 2011) reported that the BNT-LN ceramics at the BNT-rich have rhombohedral–tetragonal MPB in the range of 0.03–0.05 mol LN. This work suggests that the MPB will be existed in BNT-BT-LN ceramics at the BNT rich. Usually, the multicomponent system of piezoelectric materials will be effective to improve the piezoelectric properties. Until now, there has been no report about the work on BNT-BT-LN system. In this study, the crystal structures and electric properties of ternary ceramics 0.96(Bi0.5Na0.5)TiO3-(0.04-x)BaTiO3-xLiNbO3 system is to be investigated.
The 0.96(Bi0.5Na0.5)TiO3-(0.04-x)BaTiO3-xLiNbO3 (abbreviated as 0.96BNT-(0.04-x)BT-xLN, with x varying from 0 to 0.04) ceramics were prepared by solid state reaction using Bi2O3 (99.99%, Alfa Aesar), Na2CO3 (99.5%, Alfa Aesar), Li2CO3 (99.0, Alfa Aesar), Nb2O5 (99.5%, Alfa Aesar), BaCO3 (99.95%, Alfa Aesar) and TiO2 (99.9%, Alfa Aesar) as starting raw materials. For each composition, the starting materials were first put in the oven at 120°C for 2 h to dry, then weighed according to the stoichiometric formula and ball-milled for 20 h in ethanol with zirconia balls. The dried slurries were calcined at 800°C for 2 h, and then ball-milled again for 20 h. The powders were subsequently pulverized, mixed with polyvinyl alcohol (PVA) as a binder for granulation, and pressed into green disks with diameters of 13 mm under a pressure of 150 MPa. Sintering was performed at 1,000–1,060°C in covered alumina crucibles for 2 h. To minimize the evaporation of Bi, Na and Li, the samples were embedded in atmospheric powder of the same composition.
Phase structures of the sintered ceramics were characterized by using X-ray diffraction (D8 Advance, Bruker, Germany). Electrical property measurements were done on ground disk-shaped specimens with a thin layer of silver paste as an electrode, fired at 700°C for 10 min. Dielectric permittivity and loss of the poled specimens were measured using an impedance analyzer (Agilent Technologies Japan, Ltd). The dependence of the polarization P on an external electric field E was measured using a ferroelectric test system (Radiant Technologies, INC, Model P-PMF). The piezoelectric constant d33 at zero field was determined by using a quasi-static d33 meter (Institute of Acoustics, Chinese Academy of Sciences, ZJ-3A) after poling the samples with an electric field of 6 kV/mm for 20 min at room temperature.
Results and Discussion
Figure 1(a) shows the XRD patterns of 0.96BNT-(0.04-x)BT-xLN (0 ≤ x ≤ 0.04) piezoceramics in the 2θ range of 20°–60°. All ceramics are crystallized into a pure perovskite phase without any second impurity phases. The X-ray diffraction patterns of these ceramics suggest a phase transition from rhombohedral to tetragonal with increasing x. The rhombohedral symmetry of 0.96BNT-0.04BT at room temperature was characterized by (003)/(021) peaks splitting at around 2θ of 40° and a single peak of (202) at around 2θ of 46.5°, as shown in Figure 1(b) and (c). However, a distinct splitting of the (002)/(200) peaks at around 2θ of 46.5° could be seen when x ≥ 0.02, corresponding to a tetragonal symmetry. The (003)/(021) peak splitting is obvious until x = 0.03, then became weak. It is demonstrated that the MPB of 0.96BNT-(0.04-x)BT-xLN system exists in the composition range of 0.02 ≤ x ≤ 0.04 at room temperature, where the rhombohedral and tetragonal phases coexist. In comparison with binary BNT-BT system (Takenaka, Maruyama, and Sakata 1991), the MPB of ternary BNT-BT-LN would move to lower BT amount when the LN content is increasing.
Figure 2 presents the ferroelectric characteristics of 0.96BNT-(0.04-x)BT-xLN ceramics. It can be seen that the polarization Ps, Pr and coercive field Ec reached the maximum values of 43.7 μC/cm2, 38.7 μC/cm2 and 48.7 kV/cm at x = 0.02, respectively. The saturated polarization Ps, remnant polarization Pr and coercive field Ec vary a lot, and the pronounced difference is near the MPB composition range. For x ≤ 0.02, the polarization and coercive field increased with the increasing concentration of x. When x ≥ 0.03, however, the polarization and coercive field decreased rapidly. It is demonstrated that the enhanced ferroelectric properties were obtained near the morphotropic phase boundary (MPB). Although we are known that the coercive field Ec will be decreased near MPB due to the increased number of degrees of freedom of domain orientation (Kounga et al. 2008), the opposite trend occurred. It is because that Bi2O3, Na2CO3 and Li2CO3, which have relatively low melting point, are volatiles at high temperature during the sintering process. Oxygen vacancies would be formed to maintain charge neutrality in the lattice, and they are known to be mobile to pin domain walls (Park and Chadi 1998; Zhang and Whatmore 2003), causing difficulties for domains to be aligned during poling. As a result, coercive field Ec increased in the vicinity of the MPB.
The corresponding piezoelectric constant d33 of 0.96BNT-(0.04-x)BT-xLN ceramics was described in Figure 3. The piezoelectric properties of the specimens show strong compositional dependence close to MPB. It is observed that the optimal piezoelectric properties can be obtained at the composition of x = 0.03, which exhibits a d33 of 124 pC/N, indicating the existence of MPB. Away from this boundary, the properties decrease rapidly as a function of LN content. Although the composition of x = 0.02 and 0.04 are located in the area of MPB, the piezoelectric d33 of x = 0.03 is higher than x = 0.02 and 0.04. It is proposed that high remnant polarization and low coercive field are responsible for high piezoelectric properties (Wang, Tang, and Chan 2004), and decreasing coercive field was usually regarded as an important method to modify poling process and improve piezoelectric properties. For x = 0.03, the relatively high remnant polarization and lower coercive field were identified as the critical reasons for higher piezoelectric properties than x = 0.02 and 0.04. It is noted that the piezoelectric constant d33 increased near MPB. However, we have discussed that oxygen vacancies would be formed, and it would cause d33 decrease due to the difficulties for domains to be aligned (Sung et al. 2010). This suggests that although Ec could be influenced by oxygen vacancies in the vicinity of MPB, the domain would reorient when the electric field exceeded Ec. In other words, oxygen vacancies induced by chemical inhomogeneity could be overcome as the polarization field is 6 kV/mm, which is larger than Ec (about 4 kV/mm).
Figure 4 shows the relative dielectric constant εr and dielectric loss tanδ of the 0.96BNT-(0.04-x)BT-xLN ceramics with x = 0–0.04 at room temperature. It can be clearly seen that εr increases with increasing LN (x) and reaches the maximum of 674 at x = 0.03. Actually, the corresponding increase in εr values from x = 0 (rhombohedral structure) to x = 0.03 (rhombohedral + tetragonal structures) may be mainly due to the presence of phase coexistence in the MPB compositions. The similar tendency of the dielectric loss tanδ was also found to increase up to x = 0.03, above which it increases slightly with increasing LN content.
Figure 5 provides the temperature dependence of dielectric constant and loss tangent measured for poled 0.96BNT-(0.04-x)BT-xLN samples with x from 0 to 0.04 under 100 Hz to 100 kHz. Two dielectric anomalies are observed for all of the compositions. One is permittivity maximum temperature Tm. The other is located at low temperature defined as ferroelectric to relaxor temperature (TF-R). It is evident that the phase structure has been transferred from ferroelectric rhombohedral to relaxor tetragonal. Although TF-R had been referred to as depolarization temperature Td (Hiruma, Nagata, and Takenaka 2009), recent studies by Anton et al. (2011) stated that Td is determined by thermally stimulated depolarization current (TSDC). When the temperature is beyond TF-R, obvious frequency dispersion remained, which exhibits that external poling field could not set up the long-range-order rhombohedral, and the macrodomains of tetragonal were formed. Apart from Tm and Td, another characteristic temperature where the frequency dispersion vanished was also found, which is defined as TDV (Wang et al. 2012). However, the permittivity kept rising beyond the Curie temperature for the samples with x = 0.01, 0.03 and 0.04 at low frequencies. As known, such permittivity anomaly is usually attributed to space charges. This kind of space charges existed in many BNT-based ceramics (Zuo et al. 2008; Cernea et al. 2010; Lin and Kwok 2010), which is also severed as charged vacancies. It has been proved that the space charges are produced by A-site deficiency (Zuo et al. 2008). And it could also be existed in the stoichiometric composition as A-site cations volatilize at high temperature. Although the specimens were embedded in atmospheric powder of the same composition, and 0.5% mol Bi was excessed, the evaporated A-site elements happened inevitably.
Based on the above electric data and structure analysis, a schematic phase diagram was provided for poled 0.96BNT-(0.04-x)BT-xLN system shown in Figure 6. It is noted that the transition temperature from ferroelectric rhombohedral to relaxor tetragonal phase (TF-R) decreased with increasing LN content. However, the inverse regulation was observed for frequency dispersion vanished temperature TDV. It is obvious that an MPB was existed near the composition of x = 0.02–0.04. Furthermore, the Curie temperature rising up from 228°C to 266°C when the LN content increased from 0 to 0.04. This suggests that this system would be of great interest for high-temperature applications.
In conclusion, the crystal structure, ferroelectric, piezoelectric and dielectric properties of the ternary 0.96(Bi0.5Na0.5)TiO3-(0.04-x)BaTiO3-xLiNbO3 were investigated and a schematic diagram was constructed. Phase transition from ferroelectric rhombohedral to relaxor tetragonal was observed with increasing LN fraction in the range from 0 to 0.04 mol%, and the MPB lies where the LN amount is near 0.02–0.04. The maximum value of piezoelectric constant d33 (124 pC/N) was obtained for the composition of x = 0.03 near the MPB region. The saturated polarization, remnant polarization and coercive field are 43 μC/cm2, 37.9 μC/cm2 and 47 kV/cm for x = 0.03. Dielectric measurement indicates typical relaxation process due to A- and B-site cation mixture.
This work was supported by the National Key Project for Basic Research of China (2012CB619406), the 111 project (Grant no. B12021) and NUAA research funding (no. NJ20120012).
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