The (1 − x)BiFeO3–xBaTiO3–Bi(Zn0.5Ti0.5)O3 high-temperature lead-free piezoelectric ceramics with strong piezoelectric properties

The structure, microstructure, piezoelectric properties, ferroelectric properties and Curie temperature of (1 − x)BiFeO3–xBaTiO3–Bi(Zn0.5Ti0.5)O3 + MnO2 + Li2CO3 ceramics were investigated experimentally. The samples were prepared by the improved solid-state reaction approach. The crystalline structures were examined by X-ray diffractometry. When x = 0.3, the rhombohedral and pseudocubic phases coexist in the ceramic structure. It is considered that the morphotropic phase boundary was formed here. At this composition, the piezoelectric performance d33, Curie temperature TC, and depolarization temperature are as high as 184 pC/N, 550 °C, 530 °C at x = 0.3, respectively. It is worth noting that, when x = 0.24, the ceramics have a high TC = 580 °C and low dielectric loss tanδ = 1.9%. The results show that the BFBT-BZT ceramics with high piezoelectric properties are applicable in high-temperature fields.


Introduction
Piezoelectric ceramics are commonly used in sensors, transducers and many other fields [1][2][3]. The hightemperature piezoelectric materials may be applied in nuclear energy, oil well drilling, aerospace vehicles and geological mining [4,5]. The PZT ceramics is a kind of piezoelectric ceramic materials widely produced in industry. However, PZT piezoelectric ceramics pollute the environment and damage the body in the production process [6][7][8], and its piezoelectric performance is limited in high-temperature fields [9,10]. Therefore, the lead-free piezoelectric ceramics become a hot spot in current research. The Curie temperature of K 0.5 Na 0.5 NbO 3 -and Bi 0.5 Na 0.5 TiO 3 -based ceramics is far from satisfying the application in high-temperature fields [11][12][13]. The leadfree piezoelectric ceramics with bismuth layered are usually utilized in higher temperature fields, but the piezoelectric properties are relatively low [14,15]. BiFeO 3 is a twisted trigonal perovskite structure with high Curie temperature. The BiFeO 3 -BaTiO 3 (BFBT) binary system shows high T C when the rhombohedral and pseudocubic phases coexist [16]. Related studies have reported the influence of Mn doped on BFBT ceramics and obtained the piezoelectric ceramics with d 33 = 169 pC/N, T C = 506°C, T d = 500°C, tand & 4.4% [17]. Bi(Zn 0.5 Ti 0.5 )O 3 (BZT) is a Bi-based perovskite-type compound with large residual polarization and high tetragonality of P r-[ 150 lC/cm 2 and c/a = 1.21, respectively [18]. The BFBT-BZT ? MnO 2 ternary solid solution reduces the dielectric loss and improves the temperature stability of ceramics [19][20][21]. Li 2 CO 3 is a conventional sintering aid. The ionic radii of Bi 3? , Li ? and Ti 4? are 1.03, 0.76 and 0.61 Å , respectively. Bi 3? and Ti 4? are partially replaced by Li ? in the BF-BT ceramics [22,23]. Therefore, it can compensate for the stoichiometric deviation caused by Bi volatilization by adding Li 2 CO 3 . When Li 2 CO 3 was added into ceramics, it promotes sintering and makes BFBT-BZT ? MnO 2 ceramics dense [24,25].
However, in the sintering process, Bi 2 O 3 is volatile and the change of Fe 3? into Fe 2? in BiFeO 3 caused a number of oxygen vacancies in ceramics [26]. This phenomenon makes less compact sintering of ceramics. Therefore, the stoichiometric ratio of BF/BT in ceramics became the focus of the experiment. So, the experiment made the ceramics dense by changing the contents of BF and BT, and it was believed that the piezoelectric properties can still be improved, and the low dielectric loss can be obtained of the ceramics. In this work, (1 -x)BiFeO 3 -xBaTiO 3 -0.025Bi(Zn 0.5 Ti 0.5 )O 3 ? 0.0035MnO 2 ? 0.003Li 2 CO 3 (where x = 0.24, 0.26, 0.28, 0.30, 0.32, 0.34 mol%, abbreviated as (1 -x)BF-xBT-BZT) lead-free piezoelectric ceramics were prepared by the solid phase sintering. The phase transition, microstructure, electrical properties, Curie temperature and depolarization temperature of the ceramics were investigated experimentally. After a series of experiments, the ceramics were obtained with high piezoelectric properties and high Curie temperature in morphotropic phase boundary (MPB). For comparison, the related studies are listed in Tables 1 and 2.
The polarization hysteresis loop was tested by a ferroelectric tester (Premier II, Radiation Technology Corporation, Albuquerque, NM, USA). The dielectric temperature spectrum of samples was measured from room temperature to 700°C by a dynamic impedance analyzer (Agilent 4294A). The depolarization temperature of the samples was determined by the non-in situ method.

Results and discussion
The XRD patterns of (1 -x)BF-xBT-BZT ceramics are shown in Fig. 1. The ABO 3 -type perovskite structure is dominant in all samples. This phenomenon shows that the ceramics have formed a homogeneous ternary solid solution [38,39]. In order to descript the phase structure transition of the ceramics more carefully, the characteristic peak near 39°is enlarged as displayed in Fig. 1b. When the content of BaTiO 3 increases from 0.24 to 0.34, the characteristic peak evolved from the double peak (003 and 021) to a single peak (111). The shift is mainly near x = 0.3. When the range of x is 0.24-0.28, the phase structure of ceramics is rhombohedral. When x C 0.32, the phase structure converted to pseudocubic phase. The rhombohedral and pseudocube phase coexist when x is around 0.3, and it indicates the existence of MPB in 0.7BF-0.3BT-BZT sample [28,30,40]. Table 3 shows the lattice parameters of (1 -x)BF-xBT-BZT ceramics. The transformation of phase structure also can be seen from the table that when x = 0.3, a = b = c = 90°, a = b = c = 3.99132, the ceramic is cubic phase structure. We know that BFBT-based ceramics cannot be complete cubic phases, pseudocubic phase is not reflected in the analysis of lattice structure [28,29,35]. For all ceramics, with the increase of x, the lattice parameters increased, which is ascribed to that the ionic radii of Ba 2? (1.35 Å ) are much larger than that of Bi 3? (1.03 Å ). So the rhombohedral phases disappear with x increased, ceramics change from rhomboid phase to pseudocubic phase [41][42][43][44]. The surface microstructures of (1 -x)BF-xBT-BZT ceramics sintered at 980°C are displayed in Fig. 2. The average grain size distributions of the ceramics are given in Fig. 3. All samples have no obvious holes, which could be reflected in the SEM micrographs of different BT contents [45]. At x = 0.24, the average grain size of ceramics is relatively large. When the content of BT increases, the grain size of the ceramics decreases significantly. At x C 0.28, the  [34] 324 466 -0.696BiFeO 3 -0.014Bi(Zn 0.5 Ti 0.5 )O 3 -0.28BaTiO 3 ? 0.26wt%MnO 2 [35] 147 514 -0.99[0.715BiFeO 3 -0.285BaTiO 3 ]-0.01Bi(Zn 0.5 Ti 0.5 )O 3 [36] 195 505 400 0.67BiFeO 3 -0.33BaTiO 3 -0.03Bi(Zn 0.5 Ti 0.5 )O 3 ? 0.0035MnO 2 ? 0.004CuO [21] 188 420 426 0.7BiFe 0.95 -0.05(Zn 0.5 Ti 0.5 ) 0.03 O 3 -0.3BaTiO 3 [37] 135 482 -0.98[0. grain size becomes stable. When the content of BT is low, the low melting point material BF promotes sintering of ceramics, the low melt liquid phase causes the ceramic grains grow abnormally [46][47][48].
When the content of BT increased to a certain range, the content of BF decreased; the ceramic grain size becomes smaller and tends to be stable. Furthermore, with the increase in BT, the oxygen vacancies decrease during BF sintering process (the change of Fe 3? to Fe 2? and the volatility of Bi 2 O 3 ) [16,35,48], which is not conducive to grain transfer, and ultimately inhibits grain growth [49]. It is worth noted that the liquid phase appears clearly at x = 0.3, which indicates that the BF and BT content reaches the optimal ratio and promoted sintering. In Fig. 4, the variation trend is presented for piezoelectric constant d 33 , electromechanical coupling coefficient k p , mechanical mass factor Q m and dielectric loss tand with BT content variation. d 33 and k p increase first and reach the maximum of d 33 = 184 pC/N, k p = 0.335 at x = 0.3 and then decrease with the change of BT content. The tand value increases and the Q m shows a downward trend with the rise of x. One can see tand = 1.9% and Q m = 72.71 at x = 0.24. According to the XRD results, the MPB range is formed near x = 0.3. In the ceramic with the MPB structure, the drive energy required for the domain wall movement is low, the domain activity is increased and the piezoelectric property can be improved remarkably [28,50,51]. The main reason for the decrease of Q m and the increase of tand is that, when the BT content increases, the oxygen vacancies decrease due to the change of Fe 3? to Fe 2? . Furthermore, the reduction of oxygen vacancies makes it easier to turn the domain, which makes tand increase, Q m decrease and increases the aging rate of ceramics.
At room temperature, the P-E hysteresis loop is displayed Fig. 5, where P, E, P r , E c represent polarization intensity, applied electric field, residual polarization intensity and coercive field, respectively. The electric field used in ferroelectric behavior test is from 30 to 60 kV/cm. In Fig. 5, the typical ferroelectric polarization hysteresis loops are shown. Except for x = 0.24, the ceramic samples are in a relatively saturated state at E = 60 kV/cm. As one can see in Fig. 5g, when x rises from 0.24 to 0.26, P r and E c increase significantly. P r slowly increases to a maximum 20.13 lC/cm 2 at x = 0.34, E c reaches 33.77 kV/ cm at x = 0.26, which is the maximum value, and  then it shows a decreasing trend. The reason why P r increases is the favorable uniformity of grain size and the improving densification of structure with the BT content increase, which induces the more completed polarization and stronger ferroelectric properties. The sudden increase of E c is caused by the decrease in grain size when x = 0.24-0.26. The increase in the thickness of grain boundaries makes it difficult to flip the domain [19][20][21], P r becomes slightly larger, but E c becomes smaller as the grain decrease continues. It is clear in Fig. 5e, f that the hysteresis loop is not smooth under E = 60 kV/cm, and this is because the high electric field caused a leak current in ceramic samples [17,19]. In Fig. 6a-f, e r and tand are shown for all samples, where e r represents dielectric constant and tand represents loss tangent. The frequencies are equal to 1, 10 and 100 kHz. All ceramic samples have good temperature dependence from room temperature to 700°C. The variation of e r is small at the beginning of temperature increase; then it rapidly increases to the peak at a certain temperature (T C ) and finally decreases. As shown in Fig. 6g, T C reaches the maximum of 580°C when x = 0.24, and Curie temperature reduces with the increase in BT. For the ceramic samples with x = 0.24-0.28, tand increases when the temperature rises to about 400°C. tand decreases at the Curie point and increases again when the temperature increases. For the ceramic samples with x C 0.3, tand increases rapidly with the disappearance of Curie peak. According to the modified Curie-Weiss law: where e m is the maximum value of e r , T m denotes the phase transition temperature, C is the Curie-like constant, and c is the degree of diffuseness. The dependence of ln(1/e r -1/e m ) on ln(T -T m ) is presented in Fig. 6h. Furthermore, the range of c is from 1 to 2, where 1 represents a normal ferroelectric, 2 represents ideal relaxor ferroelectric [32]. It can be seen from Fig. 6a-f, all (1 -x)BF-xBT-BZT samples are relaxor ferroelectrics. For the ceramic samples with x = 0.24, 0.26 and 0.28, below T C , when the frequency increase from 1 to 100 kHz, e r decreases, the dielectric loss peaks were generated, dielectric peak and the loss peak shift toward the higher temperatures. All the effects are caused by the frequency dispersion characteristics of relaxation ferroelectrics [19]. Above T C , tand of the samples increases again because the ceramics have a slight leakage current at high temperatures. With the increase of x, the Curie temperature reduces due to the increase in dielectric loss at room temperature, and it is caused by oxygen vacancy generated in the sintering process. It is worth noting that, when x \ 0.3, and below the Curie point, e r decreases with the frequency increases. However, at x C 0.3, the loss peak disappears due to weaker frequency dispersion of the relaxed ferroelectric. The trend of e r and tand for all (1 -x)BF-xBT-BZT samples is consistent with c of relaxation ferroelectrics.
The depolarization curve of the (1 -x)BF-xBT-BZT ceramics is shown in Fig. 7. The d 33 value as a function of temperature was measured by the ex situ d 33 method. With the increase in temperature, there is no obvious change of d 33 . At certain temperature (T d ), d 33 drops abruptly [29,52]. For ceramic samples with x = 0.24, when the depolarization temperature is 550°C, d 33 = 85 pC/N. At x = 0.3 and T d = 530°C, d 33 is as high as 150 pC/N. The dependence indicates that depolarization law of all ceramic samples is consistent with the variation of the Curie temperature, as shown Fig. 6g. When the BT content ranges from 0.24 to 0.34, the decrease in depolarization temperature is due to the increasing loss for the ceramics, as described in Fig. 4b. When x = 0.3, the high d 33 is caused by the ceramic at MPB. On the one hand, BZT has a twisted tetragonal structure. Its  strong tetragonality (c/a = 1.21) may help to achieve such high-temperature stability [24,53]. On the other hand, through the tolerance factor formula: it is calculated that the tolerance factor of BZT is about 0.83. When the tolerance factor is 0.88-1.09, the perovskite structure is stable, while 0.83 is far from 1 [28,54,55]. Therefore, BZT forms a solid solution with BFBT, thereby increasing the depolarization temperature of ceramics [30][31][32]. In addition, Li 2 CO 3 is a sintering aid, it promotes BFBT-BZT ceramics producing a low-melting liquid phase during the sintering process, which makes domain walls difficult to move, and the ceramic structure is more stable. It means that the appropriate Li 2 CO 3 improves the temperature stability of ceramics. Therefore, the BFBT-BZT-LC solid solution shows excellent temperature stability.

Conclusions
The (1 -x)BF-xBT-BZT ceramics were synthesized by the solid-state sintering method. The effect of BT content on the structure, micromorphology, piezoelectric, ferroelectric properties and Curie temperature of ceramics was investigated. The XRD results indicate that the increase in BT content can cause a phase transformation from rhombohedral to pseudocubic phase. The (1 -x)BF-xBT-BZT system has a MPB at x = 0.3. And the 0.7BF-0.3BT-BZT ceramics exhibit the best d 33 = 184 pC/N, and high T c-= 550°C, particularly when T d = 530°C, d 33 = 150 pC/N. It is worth pointing out that the 0.76BF-0.24BT-BZT ceramic has excellent Curie temperature T C = 580°C, and low dielectric loss tand = 1.9%. The excellent electrical properties and improved temperature stability indicate that the prepared ceramics can be effectively used at high temperature.