Structure and electrical properties of Bi_1/2Na_1/2TiO₃-based lead-free piezoelectric ceramics

Abstract

Lead-free piezoelectric ceramics (1 − x)(0.935Bi_1/2Na_1/2TiO₃–0.065BaTiO₃)–xAl₆Bi₂O₁₂ (BNT–BT6.5–xAB, with x = 0–0.020) were prepared using a conventional solid-state reaction method and the crystal structure and electrical properties were systematically investigated. For all BNT–BT6.5–xAB ceramics form the pure perovskite phase structure, SEM-analysis revealed an increase first and then a decrease in the grain size with increasing AB content with no obvious change in grain morphology. Appropriate AB doping into BNT–BT6.5 ceramics induces the enhancement of piezoelectric and ferroelectric properties. Improved P_r = 32.8 μC cm⁻², low E_c of 18.2 kV cm⁻¹, and high d₃₃ = 234 pC N⁻¹ were observed at x = 0.005. Furthermore, electric field-induced strain was enhanced to its maximum value (S_max = 0.33%) with normalized strain (d₃₃^* = S_max/E_max = 413 pm V⁻¹) at an applied electric field of 80 kV cm⁻¹ for x = 0.010. The enhanced strain can be attributed to the coexistence of ferroelectric and relaxor ferroelectric phases. It is obvious that this piezoceramic is promising candidate for lead-free piezoceramics and can be used in practical applications.

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1. Introduction

Traditional lead-based piezoelectric ceramics with perovskite structures, such as Pb(Co_1/3Nb_2/3)O₃–PbTiO₃–PbZrO₃ (ref. 1) and Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃–PbZrO₃,² have been widely applied in modern piezoelectric devices owing to their excellent electrical properties.^3,4 However, the high volatility and strong toxicity of lead oxide during sintering have caused severe ecological and environmental problems.^5,6 Therefore, extensive efforts have been done to find a promising way to solve this problem and develop new lead-free piezoelectric ceramics with excellent properties to replace lead based ceramics.^7,8 Recently, experimental studies on (Bi_1/2Na_1/2)TiO₃ (BNT) have been performed extensively. BNT is considered to be an excellent candidate of lead-free piezoelectric ceramics because of its strong ferroelectric properties (P_r = 38 μC cm⁻²) at room temperature and high Curie temperature of 320 °C. However, the use of pure BNT materials in piezoelectric application is limited by the difficulty to pole adequately due to its large coercive fields (E_c = 73 kV cm⁻¹) and comparatively large conductivity.^9–11 A multitude of efforts have been made in order to improve the performance of BNT by forming BNT-based solid solutions.

To improve piezoelectric and ferroelectric properties of BNT ceramics, a variety of researches for a new morphotropic phase boundary (MPB) in the BNT-based solid solutions, e.g. BNT–BaTiO₃,^12,13 and BNT–Bi_1/2K_1/2TiO₃,^14,15 have been concentrated in recent years. It is well known that the MPB plays a very important role in PZT ceramics because the piezoelectric and ferroelectric properties show a maximum over a specific compositional range around the MPB. Among these solid solutions, (1 − x)(Bi_1/2Na_1/2)TiO₃–xBaTiO₃(BNT–xBT) system, similar to the (1 − x)PbZrO₃–xPbTiO₃ ceramics,¹⁶ has been attracted considerable attention owing to the existence of a rhombohedral–tetragonal MPB near x = 0.06–0.07 (ref. 17) and its enhanced ferroelectric and electromechanical properties at this boundary.¹⁸ The composition x = 0.065 for BNT–xBT ceramics belongs to the region of MPB, which has a relative high piezoelectric and ferroelectric properties.¹⁹ However, the piezoelectric properties of BNT–BT system are not good enough for practical uses. In order to further enhance the properties of BNT–BT ceramics and meet the requirements for practical uses, it is necessary to develop new BNT-based ceramics.

Theoretical studies predict that BiAlO₃ has been predicted to be promising candidate for lead-free ferroelectric materials because of their large ferroelectric polarization and piezoelectricity by theoretical calculations.²⁰ However, BiAlO₃ has to be prepared at high pressure and high temperature conditions.²¹ Consequently, a host of studies have been made in which BiAlO₃ has been used as an addition to enhance the performance of BNT-based in the form of solid solution.²² The results implied that BNT-based ceramics with BiAlO₃-doping could be excellent lead-free candidates for ferroelectric material. Yu et al. have synthesized and studied BNT–BiAlO₃ solid solution.²³ Zhen et al.²⁴ reported that Al₆Bi₂O₁₂ doped BNT enhanced ferroelectric and piezoelectric properties and increased the Curie temperature, simultaneously decreased the coercive field. Fu et al.²⁵ have observed that the addition of BiAlO₃ to BNT–BT can moderate relative dielectric constant. Based on the above results, it is expected that the Al₆Bi₂O₁₂ doped 0.935Bi_1/2Na_1/2TiO₃–0.065BaTiO₃ (BNT–BT6.5) can improve the ferroelectric and piezoelectric properties with higher Curie temperature and higher field-induced strain.

In this work, we doped BNTBT-6.5 with Al₆Bi₂O₁₂ (AB) using a conventional solid-state reaction method and we systematically studied the influence of compositional modification on the crystal structure, ferroelectric and piezoelectric properties and field-induced strain behaviors.

2. Experimental procedure

The produced materials (1 − x)(0.935Bi_1/2Na_1/2TiO₃–0.065BaTiO₃)–xAl₆Bi₂O₁₂ (BNT–BT6.5–xAB, with x = 0–0.020) were prepared by the conventional solid-state reaction method using reagent-grade metal oxides or carbonate powders of Bi₂O₃ (99%), TiO₂ (99.5%), Na₂CO₃ (99.8%), BaCO₃ (99%) and Al₂O₃(99.5%) as starting materials. All raw materials made by Sinopharm Chemical Reagent Co. Ltd. were weighed at stoichiometric proportion and then mixed homogenized by planetary ball milling in a polyethylene with stabilized zirconia balls for 12 h, using anhydrous ethanol as liquid medium. After drying, the mixed powders were calcined at 850 °C for 2 h. After calcination, the mixture was milled again for 6 h. The powders were mixed with an appropriate amount of polyvinyl butyral (PVB) binder, and pressed into pallets with a diameter of 12 mm and a thickness of 1.0 mm under the pressure of about 200 MPa. After burning off PVB, the ceramics were sintered in an alumina crucible at 1100–1180 °C for 2 h in air. To reduce the loss of bismuth and sodium oxides during sintering, the pellets were embedded into prepared powder with the same composition.

The crystal structure of the ceramics was determined by X-ray diffraction (XRD) using a Cu K_α radiation (λ = 1.54178 Å) (D8 Advance, Bruker Inc., Germany). The surface morphology of the ceramics was observed by scanning electron microscope (SEM) (JSM-6380, Japan). Silver electrodes were coated on the top and bottom surfaces of the ceramics for the subsequent electrical measurements. P–E loops and S–E curves, where P, E, and S denote the polarization, the electric field and the strain, respectively, were measured with ferroelectric analyzer (TF2000 analyzer, Aixacct, Germany) along with the laser interferometer (SIOS Meßtechnik GmbH, Germany) under an electric field of 80 kV cm⁻¹. The temperature dependence of dielectric properties was measured using a Broadband Dielectric Spectrometer (Novocontrol Germany) at temperatures ranging from room temperature to 500 °C with a heating rate of 3 °C min⁻¹. The samples were poled in silicon oil at room temperature under 50–70 kV cm⁻¹ for 20 min, and piezoelectric measurements were then carried out using a quasi-static d₃₃-meterYE2730 (SINOCERA, China).

3. Results and discussion

Fig. 1 shows X-ray diffraction patterns of BNT–BT6.5–xAB ceramics sintered at 1150 °C. It can be seen that all samples have formed the pure perovskite phase without any traces of secondary phases, as shown in Fig. 1, implying that AB has diffused into the BNT–BT6.5 ceramics lattice or the second phase cannot be detected because of the small doping amount of AB. These results indicate that the addition of AB does not lead to an obvious change in the phase structure. Fig. 2 shows the lattice constants a, c, and c/a as functions of the amount of AB in BNT–BT6.5 ceramics. Lattice constant a decreased with increasing AB amount, while lattice constant c was found to increase first and then decrease. The general trend of c/a ratio gradually increased with increasing AB content, suggesting the enhancement of tetragonality.

Fig. 1 X-ray diffraction patterns of BNT–BT6.5–xAB with x = 0–0.020.

Fig. 2 Lattice parameters a, c and c/a as a function of AB contents x for the BNT–BT6.5–xAB ceramics.

The microstructures of all the sintered samples (0 ≦ x ≦ 0.02) are depicted in Fig. 3. It can be noticed that all ceramics are dense and have uniform and compact structure. AB addition into BNT–BT6.5–xAB is found to slightly increase the grain size and then decrease it. The average grain size of these samples has been determined by line intercept method (see Fig. 3(f)). At the same time, the elemental concentrations of all samples were analyzed by EDS. Although EDS is not a very precise method for quantitative analysis at low concentration, the contents of AB increase and the peaks of Al in EDS spectrum show more and more obviously with the increase of x. Fig. 4 shows the representative results of the EDS analysis for the BNT–BT6.5–xAB piezoceramics with x = 0.005 and x = 0.010. The region enclosed by the line in the inset of the figure was analyzed, which confirmed the existence of Al element in the sintered samples.

Fig. 3 SEM micrographs of the BNT–BT6.5–xAB ceramics sintered at 1150 °C for 2 h: (a) x = 0, (b) x = 0.005, (c) x = 0.010, (d) x = 0.015 and (e) x = 0.020; (f) the dependence of grain size on x.

Fig. 4 Elemental concentrations of BNT–BT6.5–xAB ceramics by EDS analysis: (a) x = 0.005, (b) x = 0.010.

Hysteresis loops (P–E) of BNT–BT6.5–xAB ceramics with 0 ≤ x ≤ 0.020 measured at room temperature at 10 Hz are shown in Fig. 5(a). The P–E shape strongly depends on the composition of the ceramics. Detailed information on the response of the variation of P_r and E_c of BNT–BT6.5–xAB ceramics as a function of x is provided in Fig. 5(b). The BNT–BT6.5 ceramic exhibits typical ferroelectric behavior with P_r and E_c values of 29.3 μC cm⁻² and 49.7 kV cm⁻¹, respectively. As x increases, the P_r increases and then decreases while E_c drops dramatically. At x = 0.005, the value of P_r reaches a maximum value of 32.8 μC cm⁻² with E_c of 18.2 kV cm⁻¹. With x further increasing to 0.010, the P–E loop becomes exceedingly flattened and slanted, implying that excess AB addition greatly weakens the ferroelectricity of the samples.

Fig. 5 (a) Room-temperature P–E hysteresis loops of all BNT–BT6.5–xAB ceramics, (b) variation in remnant polarization (P_r) and coercive field (E_c) as a function of x.

Fig. 6(a) shows the bipolar field-induced strain curves of BNT–BT6.5–xAB ceramics measured at room temperature under an applied electric field of 80 kV cm⁻¹. Pure BNT–BT6.5 ceramic exhibits a typical butterfly-shaped curve with a maximum strain (S_max) of 0.17% and negative strain (S_neg) of 0.22% indicating a ferroelectric order. With x increasing (i.e., x = 0.010), the curves change shape, resulting in an increase in the maximum strain and a concurrent decrease in the negative strain. At x = 0.010, a significant enhancement in strain (S_max = 0.29%) was observed. However, above this critical composition (x = 0.010), drastic changes from the typical ferroelectric order were observed. This was evidenced by the absence of a negative strain, which is the difference between the zero-field strain and the lowest strain and is closely related to domain back-switching during bipolar cycles.²⁶ The trend for the negative strain level (S_neg) and the piezoelectric constant (d₃₃ = pC N⁻¹) of BNT–BT6.5–xAB ceramics are presented in Fig. 6(b). The d₃₃ first increased up to x = 0.005 ceramics with a maximum value of 234 pC N⁻¹, which is larger than those of BNT–BT ceramics with other additives.^27,28 Further increase in AB concentration resulted in a significant reduction in d₃₃. Generally, the piezoelectric properties of samples are determined by the microstructure and phase structure.²⁹ For small amount of AB, increase of grain size is beneficial for the enhancement of d₃₃.²⁹ However, the excess AB may precipitate in the grain boundary, which may lead to the accumulation of space charges, thus limiting the movement of the domains.^29,30 For BNT–BT6.5–0.005AB ceramics, similar to BNT–xBT binary ceramics, it may have a rhombohedral–tetragonal MPB in the range of 0.06–0.07 and reveal relatively high piezoelectric and ferroelectric properties at the composition near the MPB. The large enhancement in piezoelectric properties of the BNT–BT6.5–0.005AB ceramics can be attributed to the existence of the MPB because the number of possible spontaneous polarization directions increase at the MPB.³¹ Similar trends in S_neg were observed which increased up to 0.26% for x = 0.005 ceramic and then decreases. The observed trends of d₃₃ and S_neg are in good agreement with P–E hysteresis loops as shown in Fig. 5(a); similar behavior is also observed in other studies.^25,32 The substantial increase in d₃₃ at x = 0.005 is attributed to a large remnant polarization (P_r) and a lower coercive field (E_c). This is because a lower E_c enables the ceramics to be more easily poled, whereas a large P_r favors piezoelectricity.

Fig. 6 (a) Field induced bipolar S–E loops of BNT–BT6.5 ceramics with different AB contents, (b) negative strain (S_neg) and piezoelectric constant (d₃₃) of BNT–BT6.5 ceramics as a function of AB content.

The unipolar electric field-induced strain curves for BNT–BT6.5–xAB ceramics with different x measured at room temperature are depicted in Fig. 7(a). The unipolar field-induced strain level significantly increases with increasing x. The highest strain value (S_max = 0.33%) is obtained for composition with x = 0.010. However, a further increase in AB contents (x > 0.010) drops the strain level. The field-induced strain S_max and the normalized strain d₃₃^* of BNT–BT6.5–xAB ceramics as a function of x are presented in Fig. 7(b). The enhanced strain (S_max = 0.33%), nearly two times more than that of pure BNT–BT6.5 and the normalized strain (d₃₃^* = S_max/E_max = 413 pm V⁻¹) were obtained for x = 0.010 at an applied electric field of 80 kV cm⁻¹. Recently, similar normalized strain behaviors were also observed in other BNT-based systems, such as BNT–BA,³³ BNT–BKT,³⁴ BNT–BT³⁵ and BNT–ST.^36,37

Fig. 7 (a) Unipolar field-induced strain curves of the BNT–BT6.5 ceramics with different AB contents, (b) maximum strain and normalized strain (d₃₃^* = S_max/E_max) as a function of x in BNT–BT6.5–xAB ceramics.

To further analyze the behavior under electric field, the d₃₃ (E) and ε_r (E) of all samples were measured under an applied electric field of 60 kV cm⁻¹. As shown Fig. 8, the characteristics of both d₃₃ (E) and ε_r (E) differ significantly for 0 ≤ x ≤ 0.005 and 0.010 ≤ x ≤ 0.020. As illustrated in Fig. 8, the relative dielectric constant ε_r (E) varies under the influence of an applied electric field. Compositions with 0 ≤ x ≤ 0.005 exhibit evident hysteretic behavior, which is closely connected to switching of ferroelectric domains. Before the coercive field is reached, the domain wall density increases, and hence permittivity increases as well. Due to the stress that is associated with the strain incompatibility of switching domains, clamping occurs, which results in a decrease of ε_r (E). In addition, the number of domain walls diminishes at high field, and therefore hysteresis becomes smaller at higher fields. On the contrary, BNT–BT6.5–xAB samples (x ≥ 0.010) display indiscernible hysteretic behavior, implying little domain switching.³⁸ Judging from the evolution of ε_r (E), it is evident that compositions with x ≥ 0.010 keep permittivity high as in the undoped BNT–BT6.5 at zero electric field. In addition, it reduces the effect of the electric bias field on the permittivity, so that the relative deviation calculated through the maximum permittivity ε_max, the minimum permittivity ε_min, and the permittivity at zero field ε_{0 kV} according to eqn (1) (ref. 39) changes from 37% (x = 0) to 78% (x = 0.005), 36% (x = 0.010), 26% (x = 0.015) and eventually 25% (x = 0.020). This result is in good agreement with P–E hysteresis loops.

The d₃₃ (E) value of x = 0 changes only slightly from its value of 134 pm V⁻¹ at maximum field when the field is reduced to zero. In contrast, d₃₃ for x = 0.005 is 164 pm V⁻¹ at 6 kV mm⁻¹ and reaches a maximum at 33 kV cm⁻¹ with 221 pm V⁻¹ and then drops down to 212 pm V⁻¹ at 0 kV mm⁻¹.

Fig. 8 Electric field dependences of ε_r and d₃₃ for the BNT–BT6.5–xAB ceramics at 10 Hz.

4. Conclusions

In summary, lead-free BNT–BT6.5–xAB piezoelectric ceramics were prepared by the conventional solid-state reaction method and the crystal structure, microstructure and electrical properties were systematically investigated. All BNT–BT6.5–xAB ceramics form the pure perovskite phase structure, SEM-analysis revealed an increase first and then decrease in the grain size with increasing AB content with no obvious change in grain morphology. With increasing AB content, the piezoelectric and ferroelectric properties of BNT–BT6.5–xAB ceramics first increase and then decrease. Improved P_r = 32.8 μC cm⁻², low E_c of 18.2 kV cm⁻¹, and high d₃₃ = 234 pC N⁻¹ were observed at x = 0.005. A large field-induced strain S_max = 0.33%, corresponding to the normalized strain (d₃₃^* = S_max/E_max = 413 pm V⁻¹), was obtained for x = 0.010. These results show that BNT-based ceramics modified with AB can be considered superior candidate materials for lead-free practical applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51372110, 51402144, 51302124), the National High Technology Research and Development Program of China (no. 2013AA030801), the Natural Science Foundation of Shandong Province of China (no. ZR2012EMM004), Science and Technology Planning Project of Guangdong Province, China (no. 2013B091000001), Independent innovation and achievement transformation in Shandong Province special, China (no. 2014CGZH0904), the Project of Shandong Province Higher Educational Science and Technology Program (no. J14LA11, no. J14LA10), Research Foundation of Liaocheng University (no. 318011301, no. 318011306, no. 318051407).

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