Structure and electrical properties of (0.965 − x)(K_0.48Na_0.52)NbO₃–xBiGaO₃–0.035(Bi_0.5Na_0.5)ZrO₃ piezoelectric ceramics

Abstract

(0.965 − x)(K_0.48Na_0.52)NbO₃–xBiGaO₃–0.035(Bi_0.5Na_0.5)ZrO₃ [(0.965 − x)KNN–xBG–BNZ] lead-free piezoelectric ceramics were prepared using normal sintering for improving the piezoelectric properties and temperature stability of KNN-based ceramics. The effects of the BiGaO₃ (BG) content on their phase structure, microstructure, and piezoelectric properties were investigated. Orthorhombic–tetragonal phase coexistence has been found in the ceramics with the composition range of 0 ≤ x ≤ 0.001, and the coexistence of rhombohedral, orthorhombic and tetragonal phases at room temperature was obtained with 0.002 ≤ x ≤ 0.006. Then enhanced piezoelectric properties and a high Curie temperature (e.g., d₃₃ = 312 pC N⁻¹, k_p ∼ 44%, and T_C ∼ 341 °C) were observed. All the results show this system of potassium–sodium niobate-based ceramics demonstrates promising potential in practical applications.

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1. Introduction

Piezoelectric ceramics have been widely used in actuators, sensors, transducers and other electronic devices.^1–3 Due to the good piezoelectric and electromechanical coupling properties, lead zirconate titanate (PZT)-based ceramics have received lots of attention.^4–6 However, lead toxicity causes a serious threat to human health and the environment, which raises increasing environmental concerns and leads to the search to find other lead-free alternatives.⁷ The perovskites barium titanate BaTiO₃ (BT), bismuth sodium titanate Bi_0.5Na_0.5TiO₃ (BNT) and potassium sodium niobate K_0.5Na_0.5NbO₃ (KNN) based ceramics have been regarded as a focus of research.⁸ Among them, potassium–sodium niobate (KNN) have attracted much attention owing to their excellent ferroelectric properties, high Curie temperature and better environmental compatibility, which has been widely recognized as one of the most promising candidates for lead-free piezoelectric ceramics.^9–12 But pure KNN ceramics suffers from a number of technological problems and drawbacks, such as volatility of alkali elements at high temperature, narrow sintering temperature range, degraded properties.^13–15

The utilization of various preparation methods have been considered as one of useful approaches to improve properties of KNN based ceramics, such as spray pyrolysis,¹⁶ spark-plasma sintering (SPS),¹⁷ hydrothermal synthesis,^18,19 tape casting,¹³ ultrasonic irradiation.²⁰ However, all of these fabrication methods are costly for production and practical application in industry.²¹ Conventional solid state reaction method suits to scale manufacture because of simple technology, low cost, precise control and high reliability.

KNN ceramic undergoes successive phase transitions from low temperature to high temperature: rhombohedral to orthorhombic (R–O) transition at ∼−123 °C, orthorhombic to tetragonal (O–T) transition at ∼200 °C and tetragonal to cubic (T–C) transition at ∼410 °C.^12,22,23 Another research strategy has been adopted to improve the electric properties is shifting polymorphic phase transition (PPT) temperature to room temperature by adding some dopants.^24,25 Therefore, enhanced piezoelectric properties have been achieved in KNN due to the involvement of new phase boundary near room temperature.

The dopant of (Bi_0.5Na_0.5)²⁺ can be used to decrease orthorhombic–tetragonal transition temperature (T_O–T) and Zr⁴⁺ can increase rhombohedral–orthorhombic transition temperature (T_R–O). Since bismuth substitution could improve the piezoelectric properties of KNN-based ceramics. Ga-doping²⁶ can be used to improve dielectric and piezoelectric properties in KNN-based ceramics. In addition, theoretical studies on the compound BiGaO₃ predicted it should have high piezoelectric and ferroelectric performances with a very large spontaneous polarization,^27,28 which is consistent with the result of an enhanced piezoelectric properties in other ceramic system.²⁹ BiGaO₃ ceramics were also researched as high Curie temperature (T_C) material.³⁰ In this work, Bi_0.5Na_0.5ZrO₃ and BiGaO₃ were chosen to develop a new ternary material system (0.965 − x)(K_0.48Na_0.52)NbO₃–xBiGaO₃–0.035(Bi_0.5Na_0.5)ZrO₃ [(0.965 − x)KNN–xBG–BNZ] by the conventional solid state method to obtain a system with large piezoelectric properties and high Curie temperature simultaneously. Effects of BiGaO₃ content on their phase structure, microstructure and electric properties were investigated. A phase boundary consisting of rhombohedral, orthorhombic and tetragonal phases (R–O–T) could be found in the composition of x ≥ 0.002. An enhanced d₃₃ (312 pC N⁻¹) and a high T_C (341 °C) were simultaneously observed in the ceramic with x = 0.004, together with a good thermal stability.

2. Experimental

The raw powders used for this work include K₂CO₃, Na₂CO₃, Nb₂O₅, Sb₂O₃, Bi₂O₃, Ga₂O₃ and ZrO₂ more than 99% purity. The designed composition selected for this study is (0.965 − x)(K_0.48Na_0.52)NbO₃–xBiGaO₃–0.035(Bi_0.5Na_0.5)ZrO₃ where the amount of BiGaO₃ was varied from 0 to 0.006. The powders were weighed and milled in a conventional ball mill with zirconium medium and alcohol as solvent for 12 h. The mixture was dried and calcined at 850 °C for 6 h then was pressed into disks with diameter of 10 mm and thickness of 1.0 mm at 10 MPa using 8 wt% polyvinyl alcohol (PVA) as a binder. After burning off PVA, those samples with x = 0.000, 0.001, 0.002, 0.003, 0.004, 0.005 and 0.006 were sintered in air at the temperature of 1080 °C for 3 h. Silver electrodes were painted on the sample's top and bottom sides and sintered at 700 °C for 10 min. The samples were poled in a room temperature oil bath by applying a direct current electric field of ∼4 kV mm⁻¹ for 15 min. All the electrical measurements were conducted on aged specimens (24 h after poling).

The crystal structures of the sintered samples were examined by the X-ray diffraction (XRD) with Cu Kα radiation (DX 2700, Dandong, China) at room temperature. In addition, after grinding the solid material into powder, the crystal structure analyzed by Rietveld refinement were collected from a X′Pert Pro MPD (DY120 PANalytical, Netherlands) with Cu radiation (λKα₁/λKα₂ = 1.540598 Å/1.544426 Å) and 2θ in the range from 10° to 60°, working at 40 kV and 40 mA, a step scan of 0.0065651° per step and a counting time of 13.77 s per step. The crystal structure was analyzed by Rietveld refinement using the software package Materials Analysis using Diffraction (MAUD). The surface microstructures were measured by the scanning electron microscopy (SEM, S-3400N, HIYACHI, Japan). The temperature dependence of the dielectric constant (ε_r) of the sintered samples was measured using an LCR analyzer (HP 4980 and TH2816A, Agilent, U.S.A.). Their piezoelectric constant (d₃₃) was characterized using a Belincourt meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, China) and the planar electromechanical coupling factor (k_p) was calculated using an impedance analyzer (HP 4294A, Agilent, U.S.A.) according to the IEEE standards. The polarization versus electric field (P–E) hysteresis loops of the ceramics were measured using a Radiant Precision Work-station (USA) at room temperature.

3. Results and discussion

In the present work, the phase structure of the KNN-based ceramics is analyzed by considering XRD patterns and ε_r–T curves. Fig. 1(a) shows the XRD patterns of (0.965 − x)KNN–xBG–0.035BNZ ceramics as a function of BG content, measured at room temperature. All the ceramics show a pure perovskite structure, and no secondary phases were observed in the detected range. The splitting of the reflections at 2θ of ∼45.5° suggests that the crystal structure of the ceramic exists phase coexistence near room temperature.

Fig. 1 XRD patterns of the ceramics in the 2θ range of (a) 20–60°, (b) 44–47°.

In order to further characterize the structures of the KNN–BG–BNZ ceramics, powder diffraction refinements with Rietveld method for (0.965 − x)KNN–xBG–BNZ with x = 0.000, 0.001, 0.002, 0.004, 0.006 were carried out by using the MAUD^31,32 program. Since the KNN–BG–BNZ ceramics with x = 0.000 and 0.001 have orthorhombic (O) and tetragonal (T) phases, the coordinates of KNbO₃ orthorhombic Amm2 and tetragonal P4mm were used as the initial models.^4,33 The KNN–BG–BNZ ceramics with x ≥ 0.002 present the coexistence of rhombohedral (R), orthorhombic and tetragonal phases at room temperature, then the initial models were KNbO₃ rhombohedral R3m: R, orthorhombic Amm2 and tetragonal P4mm. Fig. 2 shows the measured and calculated diffraction profiles and the difference curves taken at room temperature for KNN–BG–BNZ ceramics. The solid line is the calculated intensity and plus sign is the observed intensity. The short vertical lines show the possible Bragg reflections. The bottom corresponds to difference between observed and calculated intensities. The observed and calculated intensity profiles are well matched, which is obvious by their difference as shown in the bottom of the figure. The smoother solid line on the bottom is, the more precise refinement presents.

Fig. 2 Rietveld refinement on XRD patterns of KNN–BG–BNZ ceramics.

For each composition, the refined profile well fits to the experimental data, with the reliability factors and lattice parameters of compositions listing in Fig. 2. The KNN–BG–BNZ with x = 0.000 and 0.001 have a typical two phase coexistence composition. For x = 0.000, the proportion of orthorhombic is 50.0545% (volume fractions) and tetragonal phases 49.9455%. With the addition of BG, the orthorhombic phase was suppressed and the proportion of tetragonal phases increased. However, with further increasing content of BiGaO₃, the structures of ceramic present the coexistence of rhombohedral, orthorhombic and tetragonal phases. From Fig. 2(c)–(e), the volume fraction of T phase is reduced gradually. R phase present a tendency of increasing firstly and decreased subsequently. At the same time, the proportion of O phase shows the opposite tendency in this system with x ≥ 0.002.

The phase evolution of the ceramics will be further confirmed by the dielectric behavior at a low temperature range (Fig. 3). As shown in Fig. 3(a) and (b), T_R–O and T_O–T could be clearly observed for the ceramics with x ≤ 0.001, that is, the T_O–T is above room temperature (89–102 °C), and T_R–O is below temperature (−89 to −77 °C). As a result, the ceramics with x ≤ 0.001 possess the O–T phases coexistence. As the BG content increases up to 0.002 ≤ x ≤ 0.004 [Fig. 3(c)–(e)], the T_R–O becomes more broadened and then moves to room temperature, and the T_O–T locates at 66–86 °C, finally resulting in the formation of the coexistence of rhombohedral, orthorhombic and tetragonal phases at room temperature. Both T_O–T and T_R–O are shifted close to room temperature with increasing BG concentration. However, the T_R–O peaks evolves to be much more broadened with increasing x (0.005 ≤ x ≤ 0.006) because of the dispersion of phase boundaries, which caused by the dramatic decreased grain size [see Fig. 5(d)].

Fig. 3 Temperature dependence of the dielectric constant of (0.965 − x)KNN–xBG–BNZ ceramics (−150–200 °C).

Fig. 4 Temperature dependence of the dielectric constant of (0.965 − x)KNN–xBG–BNZ ceramics (room temperature – 500 °C).

Fig. 5 SEM micrographs of (0.965 − x)KNN–xBG–BNZ ceramics.

Fig. 4 shows the dielectric constant (ε_r) of the ceramics against temperature (from room temperature to 500 °C) for different composition of x. The ceramics demonstrate two phase transformations above room temperature, corresponding to that from a orthorhombic phase to a tetragonal phase (T_O–T) and that from a tetragonal phase to a cubic phase (T_C). The inset in Fig. 4 gives the T_C values of the ceramics as a function of BG content, derived from the ε_r and T curves in Fig. 4. It is worth noting that the T_C just drops slightly as the increasing BG content. This system of ceramics still keep high Curie temperature more than 340 °C. Both an excellent piezoelectric properties and higher T_C mean that this kind of ceramics could be more easily used in the practical applications, especially for high-temperature applications. By considering both XRD patterns and ε_r–T curves, we can understand the change of their phase structures, as shown below: these ceramics with x ≤ 0.001 are the coexistence orthorhombic and tetragonal phase, and the coexistence of rhombohedral, orthorhombic and tetragonal phases appears for the composition 0.002 ≤ x ≤ 0.006.

Fig. 5 shows the SEM micrographs of the (0.965 − x)KNN–xBG–0.035BNZ ceramics with x = 0.000, 0.002, 0.004 and 0.006. It can be seen that the grains with different shapes are not uniform, where the abnormal larger grains and very small grains are coexistent. For the matrix grains, their grain size increases with the addition of BG. A low concentration of BG has entered the lattice of KNN ceramics and promotes the grain growth. That might be a lower melting point of Bi element to form liquid phases, which promotes the grains growth.³⁴ For the matrix grains, their grain size increases with increasing BG contents, reaching a maximum value at x = 0.004. However, with x further increasing, their grain sizes reduce sharply. The excess addition of Bi-containing perovskites can retard the grain growth of the ceramics. As we observed, the grain size increases by doping optimum BiGaO₃ content, while overmuch BiGaO₃ suppresses and prohibits the grain growth.

Fig. 6 exhibits P–E hysteresis loops of the KNN–BG–BNZ ceramics and the remnant polarization (P_r) and coercive field (E_c) as a function of x at room temperature. It is well known that the existence of P–E hysteresis loops is considered as the characteristics of ferroelectrics. As can be seen from Fig. 6(a), well saturated P–E hysteresis loops are clearly observed in ceramics with x = 0.000–0.006. To accurately evaluate the effect of BiGaO₃ content on the ferroelectric properties of the KNN–BG–BNZ ceramics, the remnant polarization (P_r) and coercive field (E_c) as a function of BiGaO₃ content x shown in Fig. 6(b). The P_r decreases with the increasing x, however, the changing tendency of the E_c is basically opposite compared with P_r with the addition of BG. The decrease of P_r may be attributed to the T_O–T and T_O–T of the ceramics near room temperature causes the instability of the polarization state, where the polarization direction can be easily rotated by external electric fields.³⁵ Then the E_c increases gradually with x increase due to the involvement of no. 90° domains in the R phases.^25,36

Fig. 6 (a) P–E hysteresis loops of the ceramics as a function of x and (b) the composition dependence of remnant polarization (P_r) and coercive field (E_c).

Fig. 7 gives the piezoelectric coefficient (d₃₃), planar electromechanical coupling factor (k_p) and dielectric constant (ε_r) of the studied ceramics. These ceramics show a strong composition dependence of the electrical properties. By adding a small amount of BG to KNN–BNZ, the piezoelectric and dielectric properties are clearly increased. The maximum values of d₃₃ as 312 pC N⁻¹ is observed for the ceramic with x = 0.004. The improvement piezoelectric at room temperature as observed in the ceramics may be attributed to the coexistence of rhombohedral, orthorhombic and tetragonal phases. Meanwhile, the k_p shows a different tendency to that of d₃₃, a maximum value is observed in the ceramic with x = 0.003. Thus tuning the phase transition close to room temperature by adding BiGaO₃ is in favor of improving piezoelectric properties. However, further addition of BG makes the piezoelectric properties deteriorate for the reduced grain sizes. Fig. 7(c) plots the dielectric constant ε_r of the ceramics as a function of x, which almost keeps rising as the x increases.

Fig. 7 (a) d₃₃ (b) k_p as well as (c) ε_r values of (0.965 − x)KNN–xBG–BNZ ceramics as a function of x.

The thermal stability and temperature stability of piezoelectric ceramics are the important factors for the practical application of ceramics. In order to further investigate the piezoelectric properties of this system, the relationship between d₃₃ and annealing temperature (T_a) in the range from room temperature to 430 °C. The d₃₃ value were measured at room temperature after annealing for 30 min at each chosen annealing temperature in air, as shown in Fig. 8. It can be observed that the d₃₃ value present a slight variation when the annealing temperature is close to or a little more than T_C, and then drops dramatically above a critical temperature due to thermal depolarization. To further indicate the varied ratio of d₃₃, Fig. 8(b) gives the variations of d₃₃/d₃₃ (25 °C) of the ceramics. The ceramics with x ≤ 0.004 have a better thermal stability of d₃₃ than those of the ones with x = 0.005 and 0.006. As a result, the KNN–BG–BNZ ceramics have good thermal stability with excellent piezoelectric properties in the range from room temperature to 370 °C, which can be used for the high-temperature applications.

Fig. 8 (a) Piezoelectric constant d₃₃ of (0.965 − x)KNN–xBG–BNZ ceramics as a function of annealing temperature. (b) Δd₃₃/d₃₃ vs. T_a of (0.965 − x)KNN–xBG–BNZ ceramics.

4. Conclusions

The (0.965 − x)(K_0.48Na_0.52)NbO₃–xBiGaO₃–0.035(Bi_0.5Na_0.5)ZrO₃ [(0.965 − x)KNN–xBG–BNZ] lead-free piezoelectric ceramics have been synthesized by the conventional solid state sintering method for improving the piezoelectric properties and temperature stability of KNN-based ceramics. Their phase transition behavior, microstructure as well as piezoelectric properties were also systematically investigated. All the samples present a pure perovskite structure, and no secondary phases were formed in the detected range. The orthorhombic–tetragonal phase coexistence has been found in the ceramics with the composition range of 0 ≤ x ≤ 0.001, and a coexistence of rhombohedral, orthorhombic and tetragonal phases at room temperature was obtained with 0.002 ≤ x ≤ 0.006. Then the enhanced piezoelectric properties and a high Curie temperature (e.g., d₃₃ = 312 pC N⁻¹, k_p ∼ 44%, and T_C ∼ 341 °C) were observed. All the results show the (0.965 − x)KNN–xBG–BNZ ceramics developed in this study are expected to be suitable substitutes for lead-based ceramics.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51332003 and 61201064).

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