New (1 − x)K_0.5Na_0.5NbO₃–x(0.15Bi_0.5Na_0.5TiO₃–0.85Bi_0.5Na_0.5ZrO₃) ternary lead-free ceramics: microstructure and electrical properties

Abstract

Lead-free piezoelectric ceramics have attracted considerable attention owing to their environmental friendliness and good electrical properties. Here the new (1 − x)K_0.5Na_0.5NbO₃–x(0.15Bi_0.5Na_0.5TiO₃–0.85Bi_0.5Na_0.5ZrO₃) [(1 − x)KNN–x(BNT–BNZ)] ternary lead-free piezoelectric ceramics synthesized by conventional solid sintering method are reported. The microstructure and electrical properties of (1 − x)KNN–x(BNT–BNZ) ternary ceramics were systematically investigated, and the ceramics with x = 0.06 possess enhanced piezoelectric properties and a high T_C (e.g., d₃₃ ∼ 318 pC N⁻¹, k_p ∼ 0.43, and T_C ∼ 326 °C), which are mainly ascribed to the R–T phase boundary involved. It is believed that, such a ceramic system is a promising candidate in the field of lead-free piezoelectric ceramics.

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1. Introduction

Lead zirconate titanate (PZT)-based piezoelectric materials are widely used in sensors, actuators and other electronic devices, owing to their excellent piezoelectric properties and high Curie temperature (T_C).^1–3 However, lead is harmful to the environment and human health. Therefore, it is a tough issue with great significance to develop lead-free piezoelectric materials for the replacement of these lead-based ceramics.^4–13

In the past decades, several kinds of lead-free piezoelectric ceramics with perovskite structure, such as BaTiO₃ (BT), (Bi_0.5Na_0.5)TiO₃ (BNT), and (K,Na)NbO₃ (KNN) ceramics, have been studied. BT is the first practically used piezoelectric ceramic. However, the poor piezoelectric properties and low Curie temperature of BT ceramics greatly limit their wider commercial application. Extensive efforts have been carried out to improve the piezoelectric properties and temperature stability of BT ceramics.^14–19 BNT is an important lead-free piezoelectric material, but BNT often exhibits poor piezoelectric properties. In order to improve the piezoelectric properties of BNT, a series of BNT-based lead-free piezoelectric ceramics with another perovskite components and the doping with other oxides were widely investigated.^20–24 Among those, the KNN-based ceramics are considered as the most promising candidates to replace lead-based ceramics because of their relatively excellent piezoelectric properties and high T_C.^{5,7–9,11–13,25–32}

Recently, it was reported that a small amount of Bi_0.5Na_0.5TiO₃ (BNT) was used to improve the sintering behavior and piezoelectric properties of KNN-based ceramics.³³ Nevertheless, these systems also show a poor piezoelectric activity. On the other hand, previous studies in the Authors' group have confirmed that the piezoelectric and ferroelectric properties of KNN-based ceramics could remarkably be improved by adding Bi_0.5Na_0.5ZrO₃ (BNZ),³⁴ and the Authors' group invented a series of KNN–(Bi, Na)(Zr,Ti)O₃ (KNN–BNZT) and KNN–[Bi,(K/Li)(Zr,Ti)]O₃ (KNN–BK/LZT)-based lead-free piezoceramics very recently.^35,36 However, there were few reports on the microstructure and electrical properties of KNN–BNT–BNZ ternary lead-free ceramics in detail.

In this work, (1 − x)K_0.5Na_0.5Nb–x(0.15Bi_0.5Na_0.5TiO₃–0.85Bi_0.5Na_0.5ZrO₃) [(1 − x)KNN–x(BNT–BNZ)] ternary ceramics were prepared by the conventional solid reaction method, the effects of BNT–BNZ content on the microstructure and electrical properties of (1 − x)KNN–x(BNT–BNZ) ternary ceramics were systematically investigated, and some related physical mechanisms were studied.

2. Experimental procedure

(1 − x)KNN–x(BNT–BNZ) ternary ceramics with x = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, and 0.07 were prepared by the conventional solid–state reaction route. Raw materials were K₂CO₃ (99.0%), Na₂CO₃ (99.8%), Nb₂O₅ (99.5%), Bi₂O₃ (99.0%), ZrO₂ (99%), and TiO₂ (99%) respectively. Raw powders were thoroughly mixed with ZrO₂ balls for 24 h using ethanol as the medium, and then dried and calcined at 850 °C for 6 h. Calcined powders were mixed with a poly vinyl alcohol (PVA) binder solution and compacted into disk samples with a diameter of ∼1.0 cm and a thickness of ∼1.0 mm. Those samples were sintered in air at the temperature of 1070–1130 °C for 3 h after burning out the PVA binder at 850 °C for 2 h. Silver pastes were fired at 700 °C for 10 min on both sides of these samples as electrodes for electrical measurements. All samples were poled at room temperature in a silicone oil bath under a dc field of 4.0 kV mm⁻¹ for 20 min.

The phase structure of these sintered samples was measured using X-ray diffraction (XRD) (Bruker D8 Advanced XRD, Bruker AXS Inc, Madison, WI, CuKα). The surface morphology of these sintered samples was analyzed by the field emission-scanning electron microscopy (FE-SEM) (JSP 7500, Japan). The temperature dependence dielectric behavior of these sintered samples was characterized using a programmable furnace with an LCR analyzer (HP 4980, Agilent, U.S.A.). The d₃₃ of the samples was tested using a piezo-d₃₃ meter (ZJ-3A, China), the dielectric properties of the samples were measured using an impedance analyzer (HP 4294A), and the hysteresis loops of the samples were characterized using a Radiant Precision Workstation (USA).

3. Results and discussion

Fig. 1(a) shows the XRD patterns of the ceramics as a function of BNT–BNZ content in the 2θ range of 20–60°. All ceramics are of a pure perovskite phase, and no secondary phases are observed in the composition range investigated, confirming that the stable solid solutions between KNN and BNT–BNZ are formed in this work. Their correspondingly expanded XRD patterns in the 2θ range of 44–47° are represented in Fig. 1(b). A small amount of BNT–BNZ can not change the crystal structure of KNN ceramics, that is to say, an orthorhombic phase is still observed in the ceramics with x ≤ 0.02. With the increase of BNT–BNZ content, an orthorhombic–tetragonal (O–T) phase coexistence is found in the ceramics with 0.03 ≤ x ≤ 0.04. For the ceramics with x = 0.07, the rhombohedral–tetragonal (R–T) phase is suppressed. So we can confirm that the R–T phase coexistence has been existed in the ceramics with 0.05 ≤ x < 0.07.

Fig. 1 XRD patterns of the ceramics with different BNT–BNZ content: (a) 2θ = 20–60°, (b) 2θ = 44–47°.

To further indicate the variation of phase structures, the temperature dependence of the dielectric constant (ε_r) of (1 − x)KNN–x(BNT–BNZ) ternary ceramics was measured in the temperature range of −150–200 °C, as illustrated in Fig. 2. As represented in Fig. 2(a)–(e), the T_R–O peaks can be clearly observed for the ceramics with x = 0–0.04, increasing with the increase of BNT–BNZ content, while the T_O–T peaks are gradually shifted to a lower temperature with rising BNT–BNZ content. The T_R–O and T_O–T peaks gradually develop into a single one as the BNT–BNZ further increases, as shown in Fig. 2(f) and (g). Such a result shows that T_R–O and T_O–T peaks get together, and the T_R–T peaks appear when the BNT–BNZ content raises up to 0.05. However, with continually increasing x (= 0.07), the T_R–T peaks evolve to be much more broadened and even disappear because of the dramatic decreased grain size [see Fig. 5(d)], indicating that the R–T phase boundary has been suppressed.^37,38

Fig. 2 Temperature-dependence of the dielectric constant (ε_r) of (1 − x)KNN–x(BNT–BNZ) ternary ceramics in the temperature from −150 °C to 200 °C: (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, (f) x = 0.05, (g) x = 0.06, and (h) x = 0.07.

As shown in Fig. 3(a), the ceramics with x = 0–0.04 possess two dielectric peaks above room temperature, which are assigned to orthorhombic to tetragonal phase temperature (T_O–T) and tetragonal to cubic phase temperature (T_C), while only one dielectric peak (T_C) is observed in the ceramics with x = 0.05–0.07. It can be found from Fig. 3(b) that the T_C of those ceramics generally decreases with the rise of BNT–BNZ content, and the composition with x = 0.06 shows a relatively high T_C of 326 °C, which is much higher than other KNN-based ceramics.^39–42 As illustrated in Fig. 3(c), the T_O–T of the ceramics with x = 0–0.04 decreases with increasing BNT–BNZ content, which is consistent with the results of Fig. 2(a)–(e).

Fig. 3 (a) ε_r–T (30–500 °C) curves of (1 − x)KNN–x(BNT–BNZ) (x = 0–0.07) ternary ceramics, (b) the expanded ε_r–T (60–200 °C) curves of (1 − x)KNN–x(BNT–BNZ) (x = 0–0.04) ternary ceramics, and (c) the T_C of the ceramics with different BNT–BNZ content.

For further describing the phase transition of this work, the temperature-composition phase diagram of (1 − x)KNN–x(BNT–BNZ) ternary ceramics has been identified by using the temperature dependence of dielectric constant^10,43 in Fig. 2 and 3, as shown in Fig. 4. With the increase of BNT–BNZ content, the T_R–O is gradually shifted to a higher temperature, and the T_C and T_O–T have a decreasing trend. Both rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries gradually move close to room temperature, and then the rhombohedral–tetragonal phase boundary can been seen in the ceramics with 0.05 ≤ x < 0.07. Considering the combination of XRD and the temperature dependence of dielectric constant, the phase coexistence of R–T can be confirmed in the compositional range of 0.05 ≤ x < 0.07.

Fig. 4 Phase diagram of (1 − x)KNN–x(BNT–BNZ) ternary ceramics.

Fig. 5(a)–(d) plot the SEM surface morphologies of the (1 − x)KNN–x(BNT–BNZ) ternary ceramics as a function of BNT–BNZ content with x = 0, 0.03, 0.06, and 0.07, respectively. The grain size of the ceramics increases sharply with rising BNT–BNZ content, reaching a maximum value at x = 0.06, showing that a low concentration of BNT–BNZ has entered the lattice of KNN ceramics and promoted the grain growth. Nevertheless, with further increasing BNT–BNZ content, the grain size of the ceramics reduces dramatically because of the increasing of Bi³⁺, which inhibits the grain growth.^44,45 The dramatic decreased grain size results in a more broadened T_R–T peak, and T_R–T peak even disappears, as shown in Fig. 2(h).

Fig. 5 SEM patterns of (1 − x)KNN–x(BNT–BNZ) ternary ceramics as a function of BNT–BNZ content: (a) x = 0, (b) x = 0.03, (c) x = 0.06, and (d) x = 0.07.

Fig. 6 shows the composition dependence of the piezoelectric constant (d₃₃) and electromechanical coupling factor (k_p) of (1 − x)KNN–x(BNT–BNZ) ternary ceramics, where all samples were poled and measured at room temperature. The d₃₃ values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics raise with the increase of BNT–BNZ content, reaches a maximum (d₃₃ ∼ 318 pC N⁻¹) for the ceramic with x = 0.06, and drastically reduces with further rising BNT–BNZ content. The k_p values show the similar change with rising BNT–BNZ content, and obtain a maximum (k_p ∼ 0.43) for the ceramics with x = 0.06. The enhanced polarizability induced by the coupling between two equivalent energy states of the tetragonal and rhombohedral phases mainly results in large d₃₃ and k_p values. The electrical domains in the R–T region have more possible polarization states and can rotate much easier by the external stresses and electric fields.

Fig. 6 d₃₃ and k_p values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics.

In this work, the ceramics with x = 0.06 show a high d₃₃ of 318 pC N⁻¹, which is much larger than those KNN-based ceramics.^33,37,42,46 Furthermore, it is of great significance that the ceramics with x = 0.06 also possess a higher T_C (326 °C) than these KNN-based ceramics,^39–42 as shown in Table 1, and the higher T_C of the ceramics facilitates industrial applications in a wide temperature range.

Table 1 Piezoelectric properties and Curie temperature of KNN-based ceramics

Materials system	d33/(pC N^−1)	kp	TC/(°C)	Ref.
(K0.4Na0.52)(Nb0.84Sb0.08)O3–(0.08 − x)LiTaO3–xBaZrO3	365	0.45	178	Zuo^39
0.94(K0.4−xNa0.6BaxNb1−xZrx)O3–0.06LiSbO3	344	0.32	176	Liang^40
(K0.44−xNa0.52)(Nb0.95−xSb0.05)O3–xLiTaO3	321	0.52	315	Fu^41
(K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3	262	0.53	320	Zhang^42
(1 − x)(K0.48Na0.52)NbO3–xBi0.5(Na0.7K0.2Li0.1)0.5ZrO3	236	0.38	350	Cheng^37
(1 − x)K0.5Na0.5NbO3–xBi0.5Na0.5TiO3	195	0.43	375	Zuo^33
(1 − x)K0.5Na0.5NbO3–xBi0.5Li0.5TiO3	172	0.37	381	Jiang^46
(1 − x)K0.5Na0.5Nb–x(0.15Bi0.5Na0.5TiO3–0.85Bi0.5Na0.5ZrO3)	318	0.43	326	This work

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Fig. 7 displays the ε_r and tan δ values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics as a function of BNT–BNZ content, measured at room temperature. The ε_r values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics rise slightly with increasing BNT–BNZ content (x = 0–0.03), then rise sharply in the range of 0.03 < x < 0.05, and almost maintain a constant at 0.05 < x < 0.07 due to R–T phase transition near room temperature. A lower tan δ value (tan δ = 0.026) is demonstrated for the ceramics with x = 0.06.

Fig. 7 ε_r and tan δ values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics.

The P–E loops of the (1 − x)KNN–x(BNT–BNZ) ternary ceramics as a function of BNT–BNZ content, measured in the frequency of 10 Hz at room temperature, are represented in Fig. 8(a). All the samples have typical ferroelectric P–E loops, especially for x = 0.05–0.06, standing for their superior ferroelectric properties. To further display the composition dependence of their ferroelectric properties, the composition dependence of remanent polarization (P_r) and coercive field (E_c) is shown in Fig. 8(b). The P_r values significantly rise with increasing BNT–BNZ content, reach a maximum with x = 0.05–0.06, and then drop dramatically due to the change of phase structure. The relatively large P_r values obtained by the samples with x = 0.05 and 0.06 mainly originate from their unique R–T phase coexistence near room temperature, resulting in the instability of the polarization states, which can be easily rotated under the action of external electric fields. The E_c values reach maximum for the ceramics with x = 0.06, and then drop as the x increases.

Fig. 8 (a) P–E loops and (b) P_r and E_c values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics.

The generally used empirical formula of d₃₃ ∼ αε_rP_r can be introduced to analyze the new R–T structure and the effect of the dielectric and dipole properties on piezoelectric properties of the ceramics.^30,34 Fig. 9 shows the d₃₃ and ε_rP_r values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics. The d₃₃ and ε_rP_r values of the ceramics have simultaneously reached peaks for the ceramics with the R–T phase boundaries, indicating that relatively high ε_r and P_r also play a role on the large d₃₃.

Fig. 9 d₃₃ and ε_rP_r values of (1 − x)KNN–x(BNT–BNZ) ternary ceramics.

4. Conclusions

(1 − x)K_0.5Na_0.5Nb–x(0.15Bi_0.5Na_0.5TiO₃–0.85Bi_0.5Na_0.5ZrO₃) [(1 − x)KNN–x(BNT–BNZ)] ternary ceramics were prepared by the conventional solid reaction method. The suitable amount of BNT–BNZ to KNN greatly improves the piezoelectric and ferroelectric properties. The ceramics with x = 0.06 possess enhanced electrical properties and a high T_C: d₃₃ ∼ 318 pC N⁻¹, k_p ∼ 0.43, ε_r ∼ 1604, tan δ ∼ 0.026, P_r ∼ 16.8 μC cm⁻², E_c ∼ 12.3 kV cm⁻¹, and T_C ∼ 326 °C, which are mainly ascribed to the involved R–T phase boundary. As result, the material system is a promising candidate for lead-free piezoelectric applications in the near future.

Acknowledgements

This work was supported by National Science Foundation of China (NSFC nos 50772068, 50972095, 51272164, and 51332003). Thanks are also to Ms Wang Hui for her help in the SEM measurement.

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