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Coexistence of excellent piezoelectric performance and thermal stability in KNN-based lead-free piezoelectric ceramics
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Coexistence of excellent piezoelectric performance and thermal stability in KNN-based lead-free piezoelectric ceramics Weiwei Yang, Peng Li, Shuanghao Wu, Feng Li, Bo Shen∗, Jiwei Zhai∗∗ Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji University, Shanghai, 201804, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lead-free Multiphase coexistence Phase diagram Temperature stability
With close attention being paid to environmental issues and more legislation coming into force to limit the application of Pb-based materials, accelerating research on lead-free piezoelectric ceramics has become increasingly requisite and urgent. Herein, we have devised and synthesized (1-x)(K0.5Na0.5)0.98Ag0.02(Nb0.96Sb0.04) O3-x(Bi0.5Na0.5)ZrO3 [abbreviated as (1-x)KNANS-xBNZ, x = 0.01, 0.02, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06] Pb-free ceramics. Phase transition, microstructure, electrical properties, and temperature stability of the ceramics have been comprehensively investigated. The findings illustrate that optimizing BNZ content can give rise to a rhombohedral-tetragonal (R-T) phase boundary when x = 0.04, 0.045, 0.05. The specimens with x = 0.04 show improved piezoelectric properties (d33 ~ 440 pC/N, kp ~ 53%, TC ~ 250 °C, d33* ~ 553 pm/V) and good temperature stability. The overall performance is excellent and indicates that (1-x)KNANS-xBNZ ceramics have great potential for replacing their lead-based counterparts.
1. Introduction Piezoelectric materials deal with the interconversion of mechanical and electrical energy. This feature makes them suitable for a wide range of applications in various fields [1]. Pb-rich Pb(Zr,Ti)O3 (PZT) ceramics have proven to be the most representative of piezoelectric materials. They have seen wide use in electromechanical devices because of their distinguished electromechanical properties and relatively high tunability [2,3]. Unfortunately, PZT ceramics will be eventually removed from the market eventually because their high lead content has an adverse effect on the environment and poses a threat to human health [4]. With more regulations against harmful substances in electronic devices taking effect, the development of lead-free piezoceramics with excellent performance becomes more urgent. During the past few years, the research focus has been mainly on ferroelectrics of perovskite structure, including BaTiO3 (BT), (Bi,Na)TiO3 (BNT), (K,Na)NbO3 (KNN), and BiFeO3 (BF) [5,6]. Owing to their low driving voltage, high Curie temperature (TC), and fine piezoelectric property, KNN-based ceramics have caught researchers’ attention, especially after 2004 when Saito et al. made an immense achievement in textured KNN-based ceramics [7,8]. Since then, research on KNN-based ceramics entered a golden age, with numerous achievements. To name a few, in 2016, Wu et al. reported high-
∗
performance KNN-based ceramics (d33 ~ 570 pC/N) through component optimization, approaching the piezoelectricity of PZT-based ceramics [9]. The year 2017 witnessed a breakthrough in KNN-based ceramics (d33 ~ 700 pC/N), which were synthetized by template grain growth (TGG) technique [10]. Although notable research findings have been made, there still exists a gap between scientific research and practical application of KNN-based ceramics. The main reason for this is that the KNN-based ceramics usually have a narrow sintering temperature range, poor density, and inferior temperature stability [11]. Therefore, persistent efforts should be made to overcome these barriers preventing KNN-based ceramics from being widely used. It has been accepted that KNN-based ceramics can obtain excellent piezoelectricity by constructing polymorphic phase boundaries (PPB), specially the rhombohedral-tetragonal (R-T) phase boundary [12]. According to previous research, Sb5+ and (Bi0.5Na0.5)ZrO3 are proved to be effective to build the R-T phase boundary because they can simultaneously make the rhombohedral-orthorhombic phase transformation temperature (TR-O) increase and the orthorhombic-tetragonal phase transformation temperature (TO-T) decrease [13]. However, the enhancement of piezoelectricity always causes a sharp decline of TC in KNN-based ceramics, which inevitably weakens their temperature stability and narrows the applicable temperature range. Furthermore, it has been confirmed that Li+ and Ag+ can decrease TO-T but not at the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Shen), [email protected] (J. Zhai).
∗∗
https://doi.org/10.1016/j.ceramint.2019.09.102 Received 22 August 2019; Received in revised form 4 September 2019; Accepted 11 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Weiwei Yang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.102
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Fig. 1. (a) XRD patterns of (1-x)KNANS-xBNZ (x = 0.01, 0.02, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06) ceramics; (b) enlarged XRD patterns in the 2θ range of 44°–47°.
Fig. 2. Temperature dependence of dielectric constant and tanδ measured from −60 °C to 130 °C at 10 kHz (a) and room temperature to 450 °C at 10 kHz (b); (c) phase diagram of the (1-x)KNANS-xBNZ ceramics; (d) d33 and kp values as a function of x.
expense of sacrificing TC. More precisely, doping Li+ or Ag+ can increase TC [13,14]. In consideration of the volatilization of alkali metal, Ag+, Sb5+, and (Bi0.5Na0.5)ZrO3 were adopted in this work to yield KNN-based ceramics with outstanding piezoelectricity and relatively high TC. The results reveal that a high piezoelectric constant (~440 pC/ N) and relatively high TC (~250 °C) were accomplished. Moreover, a high d33* value (553 pm/V) was achieved at room temperature and the d33* value can maintain at 435 pm/V until 180 °C, showing excellent temperature stability. Our results confirm that KNN-based ceramics have great research significance in the field of lead-free piezoelectric materials.
2. Experimental procedure (1-x)KNANS-xBNZ ceramics were synthesized through a traditional solid-state reaction route. Analytical reagents: K2CO3 (99%), Na2CO3 (99.95%), Ag2O (99.98%), Nb2O5 (99.98%), Sb2O3 (99.9%), Bi2O3 (99.975%), and ZrO2 (99.99%) were used as raw materials. The reagents were accurately weighed according to the nominal composition (1-x)(K0.5Na0.5)0.98Ag0.02(Nb0.96Sb0.04)O3-x(Bi0.5Na0.5)ZrO3. Since appropriate amount of zirconia balls and alcohol were added, the powders were mixed by planetary ball mill for 24 h. After drying, the mixtures were calcined at 850 °C for 6 h and then were ball-milled again for 24 h. The dried powders were pressed into pellets after the addition of 5 wt% polyvinyl alcohol (PVA) as a binder. Before sintering, the samples were heated to 600 °C for 6 h to remove the PVA. Aiming at restraining the 2
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Fig. 3. SEM images of (1-x)KNANS-xBNZ ceramics with different BNZ content.
Fig. 4. (a) P-E hysteresis loops of the ceramics measured at 10 Hz; (b) remnant polarization (Pr) and coercive field (EC) derived from the P-E loops.
Fig. 5. Bipolar strain-electric field (S–E) curves (a) and unipolar strain-electric field (S–E) curves (b) of the ceramics measured at 10 Hz; (c) values of d33, d33*, and εrPr as a function of x.
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the ground specimens. The dielectric constant (εr) and dissipation factor (tanδ) were collected from −60 °C to 130 °C and 25 °C to 450 °C by an E4980A LCR meter (Agilent, USA) and a HP 4284A LCR meter, respectively. The piezoelectric coefficient was measured by a quasi-static d33 m (ZJ-6A, Institute of Acoustics, China) after the samples were poled under a DC electric field of 2 kV/mm for 20 min at room temperature. The electromechanical coupling factor (kp) was determined by resonance/anti-resonance frequencies obtained by an impedance analyzer (HP4294A, Agilent, USA). A field emission scanning electron microscope (FE-SEM, HITACHI S-4700) was employed to characterize the surface morphology. Polarization hysteresis loops (P-E) and strain curves (S-E) of the ceramics were tested at 10 Hz by a ferroelectric test system (aixACCT TF-analyzer 1000). The d33* was calculated from the unipolar strain curves using the formula: d33* = Smax/Emax. To characterize the temperature stability of the ceramics, the in situ d33 values were collected by a quasi-static d33 m (YE2730A, Sinocera, China) that was connected to a small heating furnace. Fig. 6. Temperature-dependent (x = 0.01–0.06).
in
situ
d33
for
(1-x)KNANS-xBNZ
3. Results and discussion The room temperature XRD patterns and temperature dependence of dielectric constant curves were utilized to figure out the phase structure of the samples. Fig. 1(a) shows the XRD patterns of the unpoled (1-x)KNANS-xBNZ (x = 0.01–0.06) specimens in the 2θ scope of 20°–60°. These patterns demonstrate that the whole specimens exhibit a typical perovskite structure, and the absence of impurities. The enlarged XRD patterns from 44° to 47° are displayed in Fig. 1(b). We can see that the BNZ content has a distinctive effect on the peak intensities of (002) and (200). By assessing the relative intensities of the peaks of (002) and (200), the phase structure can be determined. A pure tetragonal (T) phase features a (002)/(200) ratio of 1:2, while an orthorhombic (O) phase has a (002)/(200) ratio of 2:1. A ratio approaching
volatilization of alkali metals, the pellets were covered with the calcined powders and placed in a sealed alumina crucible during sintering. A two-step sintering method was adopted. Firstly, the pellets were heated up to 1190 °C at a heating rate of 3 °C/min and then cooled to 1060–1110 °C at a high cooling rate of 10 °C/min. The holding time was 3 h. For electrical tests, the as-sintered specimens were polished to get flat surfaces. Following this, the specimens were covered with silver paste on both the sides and heated to 560 °C for 10 min to form the electrodes. An X-ray diffraction meter (XRD, D/MAX 2550V, Rigaku, Japan) with Cu Kα radiation was employed to analyze the phase structure of
Fig. 7. (a) P-E hysteresis loops and (b) unipolar strain curves of the 0.96KNANS-0.04BNZ ceramics at 10 Hz and different temperatures; (c) temperature-dependence of Pr, EC, and d33* values of the 0.96KNANS-0.04BNZ ceramics; (d) normalized d33* of the 0.96KNANS-0.04BNZ ceramics in this work and other representative piezoceramic systems for comparison. Note that the normalized d33* values of other works are obtained from previous reports. 4
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field (S-E) loops of the ceramics are shown in Fig. 5(a). It can be observed in Fig. 5(a) that all the specimens exhibit butterfly-shape curves while the value of Sneg is strongly affected by the compositions. The values of Sneg rise at first and then drop as x increases. The large Sneg value around x =0.03 is mainly due to the O-T phase boundary, while the relatively small Sneg in the R-T phase boundary indicates weaker ferroelectricity. This phenomenon is in agreement with P-E hysteresis loops [24]. Fig. 5(b) displays the unipolar strain-electric field (S-E) curves and the d33* (Smax/Emax) calculated from the unipolar strain curves, which are displayed in Fig. 5(c). The maximum d33* value is reached in the specimens with x = 0.03, which feature the O-T phase coexistence. The peak of d33 and d33* values are obtained in the samples with different type of phase boundary. The R-T phase boundary boosts the d33, while the O-T phase boundary improves the d33*. Concerning the practical application, the temperature stability of the ceramics must be taken into account. The temperature-dependent in situ d33 values of all the samples are depicted in Fig. 6. Note that the d33 values of the ceramics with 0.01 ≤ x ≤ 0.35 increase at first and then gradually decrease, reaching a peak near TO-T. This characteristic is in line with εr-T curves with a diffused peak corresponding to the O-T phase boundary [25]. As temperature increases, the in situ d33 values of the samples with 0.04 ≤ x ≤ 0.6 monotonously decrease. This is mainly because TR-T of the ceramics has already been decreased around or below room temperature. The ceramics with x = 0.04, characterized by the optimal d33 at room temperature, can maintain a high d33 value (above 250 pC/N), even though the temperature reaches 100 °C. Temperature-dependent P-E and S-E curves of the samples with x = 0.04 are shown in Fig. 7(a) and (b). The corresponding Pr, EC and d33* values at different temperatures are summarized in Fig. 7(c). The reduction of Pr suggests the gradually weakened ferroelectricity of the ceramics when the temperature increases. The EC decreases monotonously as temperature increases. The d33* shows excellent temperature stability and its values slightly increase and then decrease slowly. When the temperature reaches 180 °C, the d33* value can still maintain at 435 pm/V. The normalized d33* values in this work, together with other representative systems, are displayed in Fig. 7(d) [8,26,27]. Note that the temperature stability of the 0.96KNANS-0.04BNZ ceramics in this work is superior.
1:1 ratio indicates the coexistence of orthorhombic (O) and tetragonal (T) phase [15,16]. Therefore, it is confirmed that the O phase is formed in the specimens with x = 0.01 whereas the O-T phase boundary is constructed in the specimens with x = 0.02. Fig. 2(a) displays the cryogenic dielectric constant curves (εr-T). Note that the curves of the samples with x = 0.01–0.035 show two anomalies. The bumps at the lower temperature corresponding to TR-O go up from −48 °C to 14 °C as x increases from 0.01 to 0.035. Conversely, the bumps at higher temperatures corresponding to TO-T descend from 94 °C to 38 °C. With the converse shift of TR-O and TO-T, the O phase is reduced and, consequently, the TR-O and TO-T form a confluence near room temperature when x = 0.04–0.05. This suggests that R-T phase boundary is formed in the specimens with x = 0.04, 0.045, 0.05 around room temperature. The bump corresponding to TR-T nearly disappears when the amount of BNZ further increases. When considering the single peak of the ceramics with x = 0.06 around 2θ = 45° in the XRD patterns, it can be inferred that the specimens with x = 0.06 possess a pseudo-cubic structure [17]. Fig. 2(b) shows the εr-T and tanδ-T curves in a temperature range of 25–420 °C at 10 kHz. The peaks located at TC become broader and TC drops from 310 °C to 195 °C as the content of BNZ increases. Furthermore, the tanδ decreases at first and then surges when the temperature exceeds TC because of the conduction losses at high temperature. Based on the above analysis, the phase evolutions of the samples are confirmed, i.e., the ceramics exhibit pure O phase when x = 0.01; O-T phase coexistence emerges when 0.02 ≤ x ≤ 0.035; R-T phase boundary is constructed in the specimens with 0.04 ≤ x ≤ 0.05; and the ceramics with x = 0.06 have a pseudo-cubic structure. The phase diagram of the (1-x)KNANS-xBNZ ceramics is established in Fig. 2(c). Fig. 2(d) illustrates the d33 and kp values of the specimens as a function of x. Maximum d33 value (440 pC/N) is achieved when x = 0.04, showing the R-T phase coexistence. It has been frequently reported that R-T phase boundary could boost the piezoelectricity of KNN-based ceramics owing to its easier polarization rotation [18]. Therefore, the enhanced d33 value is primarily attributed to the establishment of the R-T phase boundary. The kp values of the specimens fluctuate around 50% and then gradually decrease when x ≥ 0.04. Surface SEM images of the samples are displayed in Fig. 3. It is noted that the BNZ content can greatly affect the microstructure. No obvious pores are observed, and all ceramics show a dense microstructure. The grain size is relatively homogeneous in ceramics with x = 0.01 and 0.02. As the BNZ content increases, abnormal grain growths can be observed, and a representative bimodal grain size distribution appears when x = 0.03, 0.035, 0.04, and 0.045. As the BNZ content further increases, the grain size is heavily refined. This difference in microstructure can undoubtedly have a substantial impact on the properties of the ceramics [19,20]. According to previous studies, the abnormal grain growth may improve the piezoelectricity of KNNbased ceramics because coarser grains can facilitate domain switching [21]. Therefore, the refined grains in the samples with x = 0.05 and 0.06 can also account for the deterioration of their electrical properties. Fig. 4(a) displays the ferroelectric hysteresis (P-E) loops of the samples measured at 10 Hz and room temperature. All the specimens exhibit a typical P-E loop, while the ferroelectricity of the ceramics is intensively influenced by the amount of BNZ. To clearly show the effect of BZT content on the ferroelectricity of the ceramics, the coercive field (EC) and remnant polarization (Pr) are depicted in Fig. 4(b). The largest Pr value is obtained when x = 0.02 and then the Pr value gradually decreases. Concerning the EC values, they are hardly affected by the phase evolution in the specimens with 0.01 ≤ x ≤ 0.04 and then they decrease. It is worthwhile to note that the ceramics with R-T phase boundary (0.04 ≤ x ≤ 0.05) show a relatively small Pr value. This reveals their weakened ferroelectricity [22]. Consequently, the enhanced piezoelectricity in the samples with R-T phase boundary is mainly due to their enhanced dielectric property. The x-dependent d33 and εrPr values are displayed in Fig. 5(c). They exhibit a similar changing trend, which is in accordance with d33 ∝ εrPr [23]. The bipolar strain-electric
4. Conclusions In this work, (1-x)KNANS-xBNZ lead-free ceramics were fabricated by a traditional solid-state reaction routine. Through BNZ content optimization, an R-T phase boundary was facilitated in specimens with 0.04 ≤ x ≤ 0.05. Resulting from the coexistence of R-T phase, a high d33 (440 pC/N) and kp (53%) were achieved in the 0.96KNANS0.04BNZ ceramics. In addition, the samples showed a high TC (250 °C). According to the tests of temperature-dependent d33* and in situ d33, the ceramics exhibit excellent temperature stability. The enhanced electromechanical properties and thermal stability prove that the 0.96KNANS-0.04BNZ ceramics exhibit tremendous potential for application in actuators and transducers, thus replacing their Pb-based counterparts. Acknowledgments The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China (Grant Nos. 51332003 and 51372171). References [1] J. Koruza, A.J. Bell, T. Frömling, K.G. Webber, K. Wang, J. Rödel, Requirements for the transfer of lead-free piezoceramics into application, J. Materiomics 4 (2018) 13–26 https://doi.org/10.1016/j.jmat.2018.02.001. [2] G.H. Haertling, Ferroelectric ceramics: history and technology, J. Am. Ceram. Soc.
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Coexistence of excellent piezoelectric performance and thermal stability in KNN-based lead-free piezoelectric ceramics Weiwei Yang, Peng Li, Shuanghao Wu, Feng Li, Bo Shen∗, Jiwei Zhai∗∗ Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji University, Shanghai, 201804, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lead-free Multiphase coexistence Phase diagram Temperature stability
With close attention being paid to environmental issues and more legislation coming into force to limit the application of Pb-based materials, accelerating research on lead-free piezoelectric ceramics has become increasingly requisite and urgent. Herein, we have devised and synthesized (1-x)(K0.5Na0.5)0.98Ag0.02(Nb0.96Sb0.04) O3-x(Bi0.5Na0.5)ZrO3 [abbreviated as (1-x)KNANS-xBNZ, x = 0.01, 0.02, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06] Pb-free ceramics. Phase transition, microstructure, electrical properties, and temperature stability of the ceramics have been comprehensively investigated. The findings illustrate that optimizing BNZ content can give rise to a rhombohedral-tetragonal (R-T) phase boundary when x = 0.04, 0.045, 0.05. The specimens with x = 0.04 show improved piezoelectric properties (d33 ~ 440 pC/N, kp ~ 53%, TC ~ 250 °C, d33* ~ 553 pm/V) and good temperature stability. The overall performance is excellent and indicates that (1-x)KNANS-xBNZ ceramics have great potential for replacing their lead-based counterparts.
1. Introduction Piezoelectric materials deal with the interconversion of mechanical and electrical energy. This feature makes them suitable for a wide range of applications in various fields [1]. Pb-rich Pb(Zr,Ti)O3 (PZT) ceramics have proven to be the most representative of piezoelectric materials. They have seen wide use in electromechanical devices because of their distinguished electromechanical properties and relatively high tunability [2,3]. Unfortunately, PZT ceramics will be eventually removed from the market eventually because their high lead content has an adverse effect on the environment and poses a threat to human health [4]. With more regulations against harmful substances in electronic devices taking effect, the development of lead-free piezoceramics with excellent performance becomes more urgent. During the past few years, the research focus has been mainly on ferroelectrics of perovskite structure, including BaTiO3 (BT), (Bi,Na)TiO3 (BNT), (K,Na)NbO3 (KNN), and BiFeO3 (BF) [5,6]. Owing to their low driving voltage, high Curie temperature (TC), and fine piezoelectric property, KNN-based ceramics have caught researchers’ attention, especially after 2004 when Saito et al. made an immense achievement in textured KNN-based ceramics [7,8]. Since then, research on KNN-based ceramics entered a golden age, with numerous achievements. To name a few, in 2016, Wu et al. reported high-
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performance KNN-based ceramics (d33 ~ 570 pC/N) through component optimization, approaching the piezoelectricity of PZT-based ceramics [9]. The year 2017 witnessed a breakthrough in KNN-based ceramics (d33 ~ 700 pC/N), which were synthetized by template grain growth (TGG) technique [10]. Although notable research findings have been made, there still exists a gap between scientific research and practical application of KNN-based ceramics. The main reason for this is that the KNN-based ceramics usually have a narrow sintering temperature range, poor density, and inferior temperature stability [11]. Therefore, persistent efforts should be made to overcome these barriers preventing KNN-based ceramics from being widely used. It has been accepted that KNN-based ceramics can obtain excellent piezoelectricity by constructing polymorphic phase boundaries (PPB), specially the rhombohedral-tetragonal (R-T) phase boundary [12]. According to previous research, Sb5+ and (Bi0.5Na0.5)ZrO3 are proved to be effective to build the R-T phase boundary because they can simultaneously make the rhombohedral-orthorhombic phase transformation temperature (TR-O) increase and the orthorhombic-tetragonal phase transformation temperature (TO-T) decrease [13]. However, the enhancement of piezoelectricity always causes a sharp decline of TC in KNN-based ceramics, which inevitably weakens their temperature stability and narrows the applicable temperature range. Furthermore, it has been confirmed that Li+ and Ag+ can decrease TO-T but not at the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Shen), [email protected] (J. Zhai).
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https://doi.org/10.1016/j.ceramint.2019.09.102 Received 22 August 2019; Received in revised form 4 September 2019; Accepted 11 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Weiwei Yang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.102
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Fig. 1. (a) XRD patterns of (1-x)KNANS-xBNZ (x = 0.01, 0.02, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06) ceramics; (b) enlarged XRD patterns in the 2θ range of 44°–47°.
Fig. 2. Temperature dependence of dielectric constant and tanδ measured from −60 °C to 130 °C at 10 kHz (a) and room temperature to 450 °C at 10 kHz (b); (c) phase diagram of the (1-x)KNANS-xBNZ ceramics; (d) d33 and kp values as a function of x.
expense of sacrificing TC. More precisely, doping Li+ or Ag+ can increase TC [13,14]. In consideration of the volatilization of alkali metal, Ag+, Sb5+, and (Bi0.5Na0.5)ZrO3 were adopted in this work to yield KNN-based ceramics with outstanding piezoelectricity and relatively high TC. The results reveal that a high piezoelectric constant (~440 pC/ N) and relatively high TC (~250 °C) were accomplished. Moreover, a high d33* value (553 pm/V) was achieved at room temperature and the d33* value can maintain at 435 pm/V until 180 °C, showing excellent temperature stability. Our results confirm that KNN-based ceramics have great research significance in the field of lead-free piezoelectric materials.
2. Experimental procedure (1-x)KNANS-xBNZ ceramics were synthesized through a traditional solid-state reaction route. Analytical reagents: K2CO3 (99%), Na2CO3 (99.95%), Ag2O (99.98%), Nb2O5 (99.98%), Sb2O3 (99.9%), Bi2O3 (99.975%), and ZrO2 (99.99%) were used as raw materials. The reagents were accurately weighed according to the nominal composition (1-x)(K0.5Na0.5)0.98Ag0.02(Nb0.96Sb0.04)O3-x(Bi0.5Na0.5)ZrO3. Since appropriate amount of zirconia balls and alcohol were added, the powders were mixed by planetary ball mill for 24 h. After drying, the mixtures were calcined at 850 °C for 6 h and then were ball-milled again for 24 h. The dried powders were pressed into pellets after the addition of 5 wt% polyvinyl alcohol (PVA) as a binder. Before sintering, the samples were heated to 600 °C for 6 h to remove the PVA. Aiming at restraining the 2
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Fig. 3. SEM images of (1-x)KNANS-xBNZ ceramics with different BNZ content.
Fig. 4. (a) P-E hysteresis loops of the ceramics measured at 10 Hz; (b) remnant polarization (Pr) and coercive field (EC) derived from the P-E loops.
Fig. 5. Bipolar strain-electric field (S–E) curves (a) and unipolar strain-electric field (S–E) curves (b) of the ceramics measured at 10 Hz; (c) values of d33, d33*, and εrPr as a function of x.
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the ground specimens. The dielectric constant (εr) and dissipation factor (tanδ) were collected from −60 °C to 130 °C and 25 °C to 450 °C by an E4980A LCR meter (Agilent, USA) and a HP 4284A LCR meter, respectively. The piezoelectric coefficient was measured by a quasi-static d33 m (ZJ-6A, Institute of Acoustics, China) after the samples were poled under a DC electric field of 2 kV/mm for 20 min at room temperature. The electromechanical coupling factor (kp) was determined by resonance/anti-resonance frequencies obtained by an impedance analyzer (HP4294A, Agilent, USA). A field emission scanning electron microscope (FE-SEM, HITACHI S-4700) was employed to characterize the surface morphology. Polarization hysteresis loops (P-E) and strain curves (S-E) of the ceramics were tested at 10 Hz by a ferroelectric test system (aixACCT TF-analyzer 1000). The d33* was calculated from the unipolar strain curves using the formula: d33* = Smax/Emax. To characterize the temperature stability of the ceramics, the in situ d33 values were collected by a quasi-static d33 m (YE2730A, Sinocera, China) that was connected to a small heating furnace. Fig. 6. Temperature-dependent (x = 0.01–0.06).
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3. Results and discussion The room temperature XRD patterns and temperature dependence of dielectric constant curves were utilized to figure out the phase structure of the samples. Fig. 1(a) shows the XRD patterns of the unpoled (1-x)KNANS-xBNZ (x = 0.01–0.06) specimens in the 2θ scope of 20°–60°. These patterns demonstrate that the whole specimens exhibit a typical perovskite structure, and the absence of impurities. The enlarged XRD patterns from 44° to 47° are displayed in Fig. 1(b). We can see that the BNZ content has a distinctive effect on the peak intensities of (002) and (200). By assessing the relative intensities of the peaks of (002) and (200), the phase structure can be determined. A pure tetragonal (T) phase features a (002)/(200) ratio of 1:2, while an orthorhombic (O) phase has a (002)/(200) ratio of 2:1. A ratio approaching
volatilization of alkali metals, the pellets were covered with the calcined powders and placed in a sealed alumina crucible during sintering. A two-step sintering method was adopted. Firstly, the pellets were heated up to 1190 °C at a heating rate of 3 °C/min and then cooled to 1060–1110 °C at a high cooling rate of 10 °C/min. The holding time was 3 h. For electrical tests, the as-sintered specimens were polished to get flat surfaces. Following this, the specimens were covered with silver paste on both the sides and heated to 560 °C for 10 min to form the electrodes. An X-ray diffraction meter (XRD, D/MAX 2550V, Rigaku, Japan) with Cu Kα radiation was employed to analyze the phase structure of
Fig. 7. (a) P-E hysteresis loops and (b) unipolar strain curves of the 0.96KNANS-0.04BNZ ceramics at 10 Hz and different temperatures; (c) temperature-dependence of Pr, EC, and d33* values of the 0.96KNANS-0.04BNZ ceramics; (d) normalized d33* of the 0.96KNANS-0.04BNZ ceramics in this work and other representative piezoceramic systems for comparison. Note that the normalized d33* values of other works are obtained from previous reports. 4
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field (S-E) loops of the ceramics are shown in Fig. 5(a). It can be observed in Fig. 5(a) that all the specimens exhibit butterfly-shape curves while the value of Sneg is strongly affected by the compositions. The values of Sneg rise at first and then drop as x increases. The large Sneg value around x =0.03 is mainly due to the O-T phase boundary, while the relatively small Sneg in the R-T phase boundary indicates weaker ferroelectricity. This phenomenon is in agreement with P-E hysteresis loops [24]. Fig. 5(b) displays the unipolar strain-electric field (S-E) curves and the d33* (Smax/Emax) calculated from the unipolar strain curves, which are displayed in Fig. 5(c). The maximum d33* value is reached in the specimens with x = 0.03, which feature the O-T phase coexistence. The peak of d33 and d33* values are obtained in the samples with different type of phase boundary. The R-T phase boundary boosts the d33, while the O-T phase boundary improves the d33*. Concerning the practical application, the temperature stability of the ceramics must be taken into account. The temperature-dependent in situ d33 values of all the samples are depicted in Fig. 6. Note that the d33 values of the ceramics with 0.01 ≤ x ≤ 0.35 increase at first and then gradually decrease, reaching a peak near TO-T. This characteristic is in line with εr-T curves with a diffused peak corresponding to the O-T phase boundary [25]. As temperature increases, the in situ d33 values of the samples with 0.04 ≤ x ≤ 0.6 monotonously decrease. This is mainly because TR-T of the ceramics has already been decreased around or below room temperature. The ceramics with x = 0.04, characterized by the optimal d33 at room temperature, can maintain a high d33 value (above 250 pC/N), even though the temperature reaches 100 °C. Temperature-dependent P-E and S-E curves of the samples with x = 0.04 are shown in Fig. 7(a) and (b). The corresponding Pr, EC and d33* values at different temperatures are summarized in Fig. 7(c). The reduction of Pr suggests the gradually weakened ferroelectricity of the ceramics when the temperature increases. The EC decreases monotonously as temperature increases. The d33* shows excellent temperature stability and its values slightly increase and then decrease slowly. When the temperature reaches 180 °C, the d33* value can still maintain at 435 pm/V. The normalized d33* values in this work, together with other representative systems, are displayed in Fig. 7(d) [8,26,27]. Note that the temperature stability of the 0.96KNANS-0.04BNZ ceramics in this work is superior.
1:1 ratio indicates the coexistence of orthorhombic (O) and tetragonal (T) phase [15,16]. Therefore, it is confirmed that the O phase is formed in the specimens with x = 0.01 whereas the O-T phase boundary is constructed in the specimens with x = 0.02. Fig. 2(a) displays the cryogenic dielectric constant curves (εr-T). Note that the curves of the samples with x = 0.01–0.035 show two anomalies. The bumps at the lower temperature corresponding to TR-O go up from −48 °C to 14 °C as x increases from 0.01 to 0.035. Conversely, the bumps at higher temperatures corresponding to TO-T descend from 94 °C to 38 °C. With the converse shift of TR-O and TO-T, the O phase is reduced and, consequently, the TR-O and TO-T form a confluence near room temperature when x = 0.04–0.05. This suggests that R-T phase boundary is formed in the specimens with x = 0.04, 0.045, 0.05 around room temperature. The bump corresponding to TR-T nearly disappears when the amount of BNZ further increases. When considering the single peak of the ceramics with x = 0.06 around 2θ = 45° in the XRD patterns, it can be inferred that the specimens with x = 0.06 possess a pseudo-cubic structure [17]. Fig. 2(b) shows the εr-T and tanδ-T curves in a temperature range of 25–420 °C at 10 kHz. The peaks located at TC become broader and TC drops from 310 °C to 195 °C as the content of BNZ increases. Furthermore, the tanδ decreases at first and then surges when the temperature exceeds TC because of the conduction losses at high temperature. Based on the above analysis, the phase evolutions of the samples are confirmed, i.e., the ceramics exhibit pure O phase when x = 0.01; O-T phase coexistence emerges when 0.02 ≤ x ≤ 0.035; R-T phase boundary is constructed in the specimens with 0.04 ≤ x ≤ 0.05; and the ceramics with x = 0.06 have a pseudo-cubic structure. The phase diagram of the (1-x)KNANS-xBNZ ceramics is established in Fig. 2(c). Fig. 2(d) illustrates the d33 and kp values of the specimens as a function of x. Maximum d33 value (440 pC/N) is achieved when x = 0.04, showing the R-T phase coexistence. It has been frequently reported that R-T phase boundary could boost the piezoelectricity of KNN-based ceramics owing to its easier polarization rotation [18]. Therefore, the enhanced d33 value is primarily attributed to the establishment of the R-T phase boundary. The kp values of the specimens fluctuate around 50% and then gradually decrease when x ≥ 0.04. Surface SEM images of the samples are displayed in Fig. 3. It is noted that the BNZ content can greatly affect the microstructure. No obvious pores are observed, and all ceramics show a dense microstructure. The grain size is relatively homogeneous in ceramics with x = 0.01 and 0.02. As the BNZ content increases, abnormal grain growths can be observed, and a representative bimodal grain size distribution appears when x = 0.03, 0.035, 0.04, and 0.045. As the BNZ content further increases, the grain size is heavily refined. This difference in microstructure can undoubtedly have a substantial impact on the properties of the ceramics [19,20]. According to previous studies, the abnormal grain growth may improve the piezoelectricity of KNNbased ceramics because coarser grains can facilitate domain switching [21]. Therefore, the refined grains in the samples with x = 0.05 and 0.06 can also account for the deterioration of their electrical properties. Fig. 4(a) displays the ferroelectric hysteresis (P-E) loops of the samples measured at 10 Hz and room temperature. All the specimens exhibit a typical P-E loop, while the ferroelectricity of the ceramics is intensively influenced by the amount of BNZ. To clearly show the effect of BZT content on the ferroelectricity of the ceramics, the coercive field (EC) and remnant polarization (Pr) are depicted in Fig. 4(b). The largest Pr value is obtained when x = 0.02 and then the Pr value gradually decreases. Concerning the EC values, they are hardly affected by the phase evolution in the specimens with 0.01 ≤ x ≤ 0.04 and then they decrease. It is worthwhile to note that the ceramics with R-T phase boundary (0.04 ≤ x ≤ 0.05) show a relatively small Pr value. This reveals their weakened ferroelectricity [22]. Consequently, the enhanced piezoelectricity in the samples with R-T phase boundary is mainly due to their enhanced dielectric property. The x-dependent d33 and εrPr values are displayed in Fig. 5(c). They exhibit a similar changing trend, which is in accordance with d33 ∝ εrPr [23]. The bipolar strain-electric
4. Conclusions In this work, (1-x)KNANS-xBNZ lead-free ceramics were fabricated by a traditional solid-state reaction routine. Through BNZ content optimization, an R-T phase boundary was facilitated in specimens with 0.04 ≤ x ≤ 0.05. Resulting from the coexistence of R-T phase, a high d33 (440 pC/N) and kp (53%) were achieved in the 0.96KNANS0.04BNZ ceramics. In addition, the samples showed a high TC (250 °C). According to the tests of temperature-dependent d33* and in situ d33, the ceramics exhibit excellent temperature stability. The enhanced electromechanical properties and thermal stability prove that the 0.96KNANS-0.04BNZ ceramics exhibit tremendous potential for application in actuators and transducers, thus replacing their Pb-based counterparts. Acknowledgments The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China (Grant Nos. 51332003 and 51372171). References [1] J. Koruza, A.J. Bell, T. Frömling, K.G. Webber, K. Wang, J. Rödel, Requirements for the transfer of lead-free piezoceramics into application, J. Materiomics 4 (2018) 13–26 https://doi.org/10.1016/j.jmat.2018.02.001. [2] G.H. Haertling, Ferroelectric ceramics: history and technology, J. Am. Ceram. Soc.
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