- Email: [email protected]
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics
Journal Pre-proof Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu PII:
S2352-8478(19)30289-8
DOI:
https://doi.org/10.1016/j.jmat.2020.03.005
Reference:
JMAT 290
To appear in:
Journal of Materiomics
Received Date: 26 December 2019 Revised Date:
10 February 2020
Accepted Date: 9 March 2020
Please cite this article as: Deng A, Wu J, Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics, Journal of Materiomics (2020), doi: https://doi.org/10.1016/j.jmat.2020.03.005. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
ABSTRACT: In this work, the effects of rare earth (RE) dopants on the phase structure, microstructure, and electrical properties of lead-free bismuth sodium titanate (BNT)-based ceramics were studied. Rare earth elements (i.e., Sm, Gd, Ho, Pr, Y, and Yb) were selected as the dopants to replace the Bi site of Bi0.5(Na0.8K0.2)0.5TiO3 ceramics. It was found that the addition of moderate RE ions reduced the temperature of ferroelectric-relaxor transition (Tf-r) and the dielectric maximum at lower temperature, while the slight influence on the dielectric maximum at higher temperature and microstructure were observed. Hereby, the three-stage evolution of phase structure after introducing RE ions was confirmed. A high piezoelectric coefficient d33 of ~190 pC/N was achieved due to the improved dielectric constant in the state with stable electric field-induced ferroelectric phase. The high strain property and converse piezoelectric coefficient d33* of ~340 pm/V were obtained in the state with the coexistence of nonergodic and ergodic relaxor phases when Tf-r approached room temperature. The degenerated overall electromechanical properties exhibited in the state with complete ergodic relaxor phase. This study affords a significant guidance for optimizing the piezoelectricity of BNT-based ceramics utilizing rare earth dopants.
Keywords: BNT-based ceramics, rare earth doping, phase structure, electrical properties
1
1. Introduction Piezoelectric ceramics have been used in many electronic components including actuators, transducers, and sensors because of their excellent electromechanical properties [1-5]. Nowadays, bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT)-based lead-free ceramics are one of the most promising counterparts to replace the toxic lead-based piezoelectric ones because of their good ferroelectric performance and strain property [4-16]. Although a large remnant polarization Pr of ~38 µC/cm2 exhibits in pure BNT ceramic, a high coercive field EC of >50 kV/cm results in the difficulty in fully poling process, finally inducing a low piezoelectric coefficient d33 of ~70-90 pC/N [17-19]. The general strategy of optimizing the electromechanical properties of BNT-based ceramics is to manipulate the phase structure through chemical modifications.
Ions substitution or introducing ABO3 compounds can decrease the coercive field of BNT and improve the density by modifying microstructure, and thus its piezoelectricity can be optimized. For example, d33 of Li+, K+, Sr2+, Ba2+ or Mn2+ -doped BNT ceramics would increase up to ~100-150 pC/N by improving its density or constructing a morphotropic phase boundary (MPB) [4-7, 17]. Sn4+, Hf4+, Zr4+, Nb5+ or Ta5+ substitution for Ti sites can promote the strain property and the converse piezoelectric coefficient d33* utilizing the electric field-induced phase transition by reducing the transition temperature between ferroelectric and relaxor phases (Tf-r) [4-6, 14-17]. The addition of other ABO3 compounds, such as BaTiO3 (BT), SrTiO3 (ST), Bi0.5K0.5TiO3 (BKT), (K, Na)NbO3 (KNN), also optimizes the piezoelectric and/or strain properties through constructing the phase boundaries or the transition between ferroelectric and relaxor phases [8-11, 17, 19]. Among them, BNKT (or BNT-BKT) 2
ceramics present a high piezoelectric property of d33~150 pC/N due to its alleged MPB, together with a relatively high depolarization temperature (Td or Tf-r), where ferroelectric polarization and piezoelectric coefficient begin to rapidly decrease to near zero [7, 9]. Rare earth (RE) elements possess peculiar external electron shells and afford abundant orbital bonding routes with other ions. Therefore, rare earth dopants usually give rise to effective random fields and random bonds, thus resulting in local structural heterogeneity and reducing the interfacial energy barriers among different phases/polarization states [20]. This finally facilitates the polarization rotation and optimizes piezoelectric response, or introduce unstable phase to promote the electric filed-induced strain. For example, the addition of Sm enhances the piezoelectric and strain properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics/single crystals [21, 22], and the addition of La or Nd further improve the piezoelectric coefficient of BNT-based ceramics [23, 24]. However, there are no systematical investigations on the effects of RE dopants on the phase structure, microstructure, and electrical properties of BNT-based ceramics.
In this work, the Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) ceramics with relatively high piezoelectric properties were selected as the matrix to study the effects of rare earth substitutions on the structure and property of BNT-based ceramics. Six RE elements (i.e., Sm, Gd, Ho, Pr, Y, and Yb) were used to replace the Bi3+ site. The composition dependence
of
the
phase/microstructure
and
ferroelectric/piezoelectric/
dielectric/strain properties was studied. It was found that the three-stage evolution of phase structure and electromechanical properties was observed after introducing whatever RE ions. The improved piezoelectric and strain behavior was achieved in different stages charactering by different phase structures. 3
2. Experimental procedure (Bi1-xSmx)0.5(Na0.8K0.2)0.5TiO3 (abbreviate as BNKT-xSm, x=0, 0.015, 0.030, 0.045, 0.060, 0.080, and 0.100) and (Bi1-yREy)0.5(Na0.8K0.2)0.5TiO3 (abbreviate as BNKT-yRE, y=0.015, 0.030, 0.045, 0.060, RE=Sm, Gd, Ho, Pr, Y, Yb) ceramics were fabricated by the conventional solid-state reaction route. The raw materials include K2CO3 (99%), Na2CO3 (99.8%), Bi2O3 (99%), TiO2 (98%), Sm2O3 (99.9%), Gd2O3 (99.99%), Ho2O3 (99.99%), Pr6O11 (99.9%), Y2O3 (99.99%), and Yb2O3 (99.99%). All the corresponding raw materials were weighed stoichiometrically and ball milled using ZrO2 ball in the alcohol for 24 h. The mixed powders were calcined at 850 oC (6 h) in air and then directly grinded into the fine powders and pressed into disks under a pressure of 10 MPa with a binder of 7 wt% polyvinyl alcohol. All the pellets were sintered at 1130 °C (2 h) in air after burning out the binder at 850 oC. Finally, silver paste was coated on the surfaces of the sintered samples and then kept at 600 °C (10 min) in air. A direct current electric field of 4-5 kV/mm was used to pole the pellets (20 min) in a silicone oil bath at room temperature.
X-ray diffractometer (Bruker D8 Advanced XRD, Bruker AXS Inc., Madison, WI) with Cu Kα radiation (λ=1.5406 Å) was used to analyze the phase structure. Surface microstructure, energy-dispersive X ray spectroscopy (EDS) and elemental mapping analysis of sintered samples were examined using a field-emission scanning electron microscope
(FE-SEM)
(JSM-7500,
Musashino
Akishima,
Tokyo,
Japan).
Room-temperature dielectric constant (εr) and dielectric loss (tan δ) as well as the temperature-dependent dielectric constant (εr-T) and dielectric loss (tan δ-T) curves were all obtained by an LCR analyzer (Tonghui 2816A, Changzhou, China). 4
Room-temperature direct piezoelectric coefficient (d33) was measured by a quasi-static d33 meter (ZJ-3A, Institute of Acoustics, Beijing, China). Planar electromechanical coupling coefficient (kp) was determined by an impedance analyzer (HP 4294A, Agilent, Santa Clara, USA). The electric field-induced bipolar/unipolar strain (S-E) curves, ferroelectric hysteresis (P-E) loops and current density (J-E) loops were acquired utilizing a ferroelectric analyzer (aixACCT TF2000, Aachen, Germany).
3. Results and discussion
Fig. 1. Composition dependence of XRD patterns of undoped BNKT-xSm ceramics with different Sm contents: (a) 2θ=20-70o, (b) 2θ=39.5-40.5o, and (c) 2θ=45.5-47.5o.
Fig. 1(a) exhibits the room-temperature XRD patterns of unpoled BNKT-xSm ceramics, measured at 2θ=20-70o. It was observed that all the samples presented typical ABO3 perovskite structure. There were no other secondary phases, suggesting that Sm3+ totally entered into the BNT-based matrix to form a single solid solution. To further identify the evolution of phase structure, the XRD patterns at 2θ=41-42o and 5
2θ=45.5-47.5o were expanded [Figs. 1(b) and (c)], as indicated by the crystal indices of (111) and (200), respectively. According to the expanded diffraction peaks, both (111) and (200) peaks tended to shift to a higher angle when the composition increases from x=0 to 0.100. It is well known that the ionic radius of Sm3+ is smaller than that of Bi3+ under the same coordination [25]. By replacing Bi3+ by Sm3+, the reduced lattice parameters and shrinkage of unit cell were presented, and therefore, the characteristic diffraction peaks moved to a higher angle according to the Bragg formula. Besides, the single peaks without any obvious splitting peaks exhibited in all the samples, implying a pseudocubic phase structure for all the unpoled ceramics [9, 26, 27].
Fig. 2. Surface FE-SEM images of BNKT-xSm ceramics with different Sm contents: (a) x=0, (b) x=0.030, (c) x=0.060, and (d) x=0.100.
6
Fig. 3. Element mappings of BNKT-xSm ceramics with x=0.100.
Figs. 2(a)-(d) show the surface morphologies of BNKT-xSm ceramics with x=0, 0.030, 0.060, 0.100, respectively. All the ceramics exhibited a dense microstructure. The cuboid-like grains exhibited in pure BNKT ceramic (x=0), and their average grain size was ~1 µm. After introducing Sm3+, the grain shape and the average grain sizes were basically unchanged, indicating that the addition of Sm3+ slightly affected the grain growth of BNKT ceramics. Fig. 3 displays the elements mapping of BNKT-0.100Sm ceramics. It was observed that there were no elements enrichment between grains and grain boundaries. Because there were also no secondary phases in XRD results (Fig. 1), the dopant (Sm) was considered to homogeneously distribute into the ceramic matrix at the grain-size scale approximatively [28]. This result affords another piece of evidence for the doped Sm3+ can be well merged into the matrix of BNKT ceramics.
7
Fig. 4. (a) Schematic of identifying the ferroelectric-relaxor transition temperature Tf-r according to εr-T and tan δ-T curves of poled sample [10, 29]. (b)-(h) Composition dependence of εr-T and tan δ-T curves measured at frequency = 100 Hz, 1 kHz, 10 kHz, 100 kHz for BNKT-xSm ceramics with different Sm contents.
In order to confirm the influence of Sm3+ substitution on the phase structure of BNKT ceramics, the temperature/frequency dependence of dielectric properties for the poled ceramics were performed, as shown in Fig. 4. The ferroelectric-relaxor transition temperature (Tf-r) was confirmed through combining the εr-T and tan δ-T curves of poled samples, and the values of Tf-r acquired from the anomalous points of εr-T and tan δ-T curves, as shown in Fig. 4(a) [10, 29]. For pure BNKT (x=0) [Fig. 4(b)], there are two dielectric peaks in the εr-T curves, where the low-temperature one is defined as the first dielectric maximum (Tfm) and the high-temperature one is called as the second dielectric maximum (Tsm). Tsm is maximal permittivity over the whole εr-T curves. Tfm is a dielectric shoulder, in which the dielectric constant gradually decreases and shifts to a higher temperature when the measured frequency increases from 100 Hz to 100 kHz. In the vicinity of Tfm, a phase transition from ferroelectric R3c phase to relaxor phases with P4bm + Pm3m would happen, and the temperature is 8
denoted as Tf-r [29], which can be observed in the εr-T and tan δ-T curves of poled samples, as dotted box shown in Figure 4A. The Tf-r, Tfm, and Tsm of pure BNKT ceramics are about 76 oC, 140 oC, 327 oC, respectively. After introducing Sm3+, maximum dielectric anomalies decrease, and the first and second dielectric anomalies became flattened when the composition reached x=0.080 and 0.100 [Figs. 4(g) and (h)]. In addition, Tf-r moved to a lower temperature and gradually disappears in εr-T curves, and it can only accurately obtain Tf-r in the expanded tan δ-T curves [Figs. 4(c)-(h)]. This result indicates that the Sm substitution for Bi has some influence on maximum permittivity and decreases the ferroelectric-relaxor transition temperature Tf-r.
Fig. 5. (a) Plots of ln(1/εr−1/εm) as a function of ln(T–Tm) according to corresponding εr-T curves at 1 kHz. (b) and (c) Composition dependence of Tf-r, Tfm, Tsm, and diffuseness exponent γ from x=0 to x=0.100.
In order to figure out the influence of rare earth Sm3+ on the structure, the evolution of εr-T curves and the corresponding analysis as a function of Sm contents was carried out. For analyzing the diffuseness degree, a modified Curie-Weiss law is use to describe the diffuseness of phase transition [29], 9
1/εr–1/εsm = ( T–Tsm)γ/C (1≤γ≤2),
(1)
where εsm is dielectric constant at Tsm, γ=1 represents a normal ferroelectrics which fully complied with the Curie-Weiss law, while γ=2 describes an ideal relaxor ferroelectrics with complete diffuse phase transition. The fitting results are displayed in Fig. 5(a). Composition-dependent Tf-r, Tfm, Tsm and diffuseness exponent γ are displayed in Figs. 5(b) and (c). According to the evolution of dielectric anomalies in εr-T curves [Fig. 5(b)], it can deduce that there were two stages of phase-structure evolution for the Sm3+-doped BNKT ceramics. Tfm basically maintained unchanged when the composition increased from x=0 to x=0.100, whereas both Tf-r and Tsm firstly decreased at x=0-0.030, then almost remained unchanged from x>0.030 to x=0.100. Therefore, these two stages were divided in the vicinity of x=0.030. From the evolution of diffuseness exponent γ [Fig. 5(c)], there were also two stage of structure state for the Sm3+-doped BNKT ceramics, but was divided in the vicinity of x=0.060. Combining all of above results, it can deduce that there are actually three stages of structure evolution by introducing Sm3+. It can define them as stageⅠ, stage Ⅱ, and stage Ⅲ, divided by near x=0.030 and x=0.060, as shown in Fig. 5(b) and (c). This result will be further verified through the evolution of electric properties of BNKT-xSm ceramics.
10
Fig. 6. Composition dependence of (a) ferroelectric hysteresis P-E loops and (b) electric field-induced current density (J-E) loops for the BNKT-xSm ceramics. (c) Evolution of maximum polarization (Pmax), remnant polarization (Pr), and coercive field (Ec) for corresponding P-E loops. Composition dependence of (d) maximum current density (Jmax) and (E) its corresponding electric field (Es).
Fig. 6(a) presents P-E loops of BNKT-xSm ceramics. It was observed that typically square ferroelectric loops exhibited in the samples (x=0-0.030) (stageⅠ) with high maximum polarization Pmax of ~35 µC/cm2, remnant polarization Pr of ~27 µC/cm2, and large coercive field Ec of ~26-28 kV/cm. This result indicates that the samples in this stage exhibit nonergodic relaxor state, in which the electric field-induced phase transition from nonergodic relaxor state to ferroelectric state takes place and then presents a strong electric-induced ferroelectric polarization under a high applied electric field [29]. At x=0.045-0.060 (stageⅡ), the ferroelectric loops became slim with the pinched shape and exhibited the rapidly decreased remnant polarization and coercive field, but was accompanied by a still large maximum polarization. This result 11
indicates the coexistence of nonergodic and ergodic relaxor states [31]. When the composition approached x=0.080 and 0.100 (stage Ⅲ), the P-E loops became very slim and exhibited the greatly degenerated maximum polarization, implying the involvement of an ergodic relaxor state in which the field-induced ferroelectric phase transformed to the initial relaxor state again when the applied electric field was removed [29, 31]. Similar phenomenon was observed in the evolution of J-E curves [Fig. 6(b)]. There was only one high current peak of domain switching for the samples (x=0-0.030) (stageⅠ). For x=0.045 and 0.060, another current peak appeared at a low electric field except for the current peak of domain switching. It was demonstrated that this current peak derives from the decomposition of oriented ferroelectric domain under applying a backward electric field [31]. Under applying an external electric field, the coexisted nonergodicity and ergodicity would transform to ferroelectric state, and incipient short-range polar orders grow up to large ferroelectric domains and arrange along with applied electric field. After applying an external electric field along the opposite direction, partial ferroelectric domains derived from ergodic relaxor state do not switch but firstly decompose into short-range polar orders again and release the domain-decomposed current. Then the decomposed short-range polar orders grow up to new long-range ferroelectric orders and switch along with inverse electric field, showing a domain switching current [31-33]. As a result, two current peaks for x=0.045 and 0.060 (stageⅡ) were found with the coexistence of nonergodic and ergodic relaxor states. When the composition reached x=0.080 and 0.100 (stage Ⅲ), the complete ergodic relaxor state made the ceramics small electric field-induced polarization and thus presented a square J-E loops [31]. Consequently, corresponding to the evolution of three stages in εr-T curves, the evolutions of Pmax Pr, and Ec [Fig. 6(c)], maximum current density Jmax [Fig. 6(d)], and its corresponding electric field Es 12
[Fig. 6(e)] also presented three stages of the variation. The addition of Sm3+ resulted in phase transition of BNKT ceramics, and the different phase structure in the three stages is responsible for the performance change of electric field-induced ferroelectricity.
Fig. 7. Composition dependence of (a) bipolar S-E curves and (b) unipolar S-E curves for the BNKT-xSm ceramics. (c) Evolution of poling strain (Spol), negative strain (Sneg), and the corresponding electric field of maximum negative strain (Eneg) for bipolar S-E loops. (d) Evolution of the maximum strain and corresponding converse piezoelectric coefficient (d33*) as a function of x for bipolar/unipolar strain curves.
The same phenomenon was also observed in the evolutions of strain, piezoelectric, and dielectric properties as a function of compositions. Fig. 7(a) displays the bipolar strain S-E curves as a function of x, measured at 50 kV/cm. It was seen that the typical S-E curves with butterfly shape, large poling strain Spos, and negative strain Sneg appeared in the samples with low Sm3+ contents. After increasing the Sm3+ contents up to x=0.080, the S-E curves became slim. With increasing the compositions from x=0 to 0.100, the poling strain Spos firstly remained unchanged at x=0-0.030 in stageⅠ 13
and then increased to a high strain value at 0.030≤x≤0.060 in stage Ⅱ. Finally, it rapidly decreased to a small value at 0.060
14
Fig. 8. Composition dependence of (a) dielectric constant (εr) and dielectric loss (tan δ) and (b) direct piezoelectric coefficient (d33) and planar electromechanical coupling coefficient (kp) measured at room temperature. (c) Comparison of d33 for BNKT-yRE ceramics (y=0 and 0.015, RE=Sm, Gd, Ho, Pr, Y, Yb). (d) Composition dependence of d33 for BNKT-yRE ceramics (RE=Sm, Gd, Ho, Pr, Y, Yb).
Fig. 8(a) displays the room-temperature dielectric properties (εr and tan δ) of the ceramics as a function of x. In stageⅠ(x=0-0.030), dielectric constant εr gradually increased from ~800 to ~1000 and then rapidly increased to ~1350 at x=0.060 in stage Ⅱ.When x got into stageⅡ, εr begun to degenerate to ~1200. Different from εr, dielectric loss always remained a low value of ~0.05 (x=0~0.100). The addition of optimum Sm3+ improved the piezoelectric constant of BNKT ceramics. As shown the stageⅠ [Fig. 8(b)], d33 increased from 150 pC/N to 190 pC/N with the increase of x from 0 to 0.015, and maintained a high value of ~165-180 pC/N at 0.015
15
near zero when the composition reaches stage Ⅲ . Meanwhile, the planar electromechanical coupling coefficient kp of the samples exhibited similar variation, as shown in Fig. 8(b). Usually, piezoelectric response of a ferroelectric material is proportion to the product values between remnant polarization Pr and dielectric constant εr, that is, d33 ~ Pr·εr [34]. Therefore, the improved dielectric constant εr [Fig. 8(a)] and high remnant polarization Pr [Fig. 6(c)] should attribute to the enhanced d33 for the BNKT-xSm ceramics with stable electric field-induced ferroelectric phase in stageⅠ. Introducing optimum Sm enhanced the piezoelectric property of BNT-based ceramics, and the addition of other RE elements (i.e., Pr, Y, Gd, Yb, Ho) also optimized their piezoelectric properties. As shown in Fig. 8(c), a high d33 of ~165-180 pC/N was obtained in the 0.15%RE -doped BNKT ceramics. According to the evolution of d33, the phase-transition region after doping RE and the corresponding compositional range were roughly confirmed, as shown in Fig. 8(d). It was seen that both Gd and Pr had a similar effect on phase transition of BNKT ceramics with respect to Sm, while Y, Yb, and Ho exhibited the accelerated effect on phase transition with respect to Gd, Pr, and Sm.
4. Conclusions Rare earth-doped BNKT ceramics were prepared by the conventional solid-state reaction method, and then the relationship between structure & properties and doped content & type of rare earth was studied. Introducing rare earth exhibited significant effects on the phase structure through shifting the ferroelectric-relaxor transition temperature of BNKT ceramics and thus manipulated the piezoelectric and strain properties in different phase-structure regions. Consequently, high piezoelectric and strain property was achieved. This work offers a new paradigm for further optimizing 16
piezoelectricity of BNT-based ceramics utilizing rare earth dopants.
Conflicts of interest Authors declare that there are no conflicts of interest.
Acknowledgments Authors gratefully acknowledge the support of the National Natural Science Foundation of China (51972215). Thank Hui Wang (Analytical & Testing Center, Sichuan University) for measuring the FE-SEM images.
References 1.
Jaffe, B., Piezoelectric ceramics. Elsevier: 2012; Vol. 3.
2.
Uchino, K., Piezoelectric actuators and ultrasonic motors. Springer Science &
Business Media: 1996; Vol. 1. 3.
Thong, H.-C.; Zhao, C.; Zhou, Z.; Wu, C.-F.; Liu, Y.-X.; Du, Z.-Z.; Li, J.-F.;
Gong, W.; Wang, K., Technology transfer of lead-free (K, Na)NbO3-based piezoelectric ceramics. Mater. Today 2019; 29: 39-48 . 4.
Wu, J., Advances in Lead-free Piezoelectric Materials. Springer: 2018.
5.
Zheng, T.; Wu, J.; Xiao, D.; Zhu, J., Recent development in lead-free perovskite
piezoelectric bulk materials. Prog. Mater Sci. 2018, 98, 552-624. 6.
Hao, J.; Li, W.; Zhai, J.; Chen, H., Progress in high-strain perovskite piezoelectric
ceramics. Materials Science and Engineering: R: Reports 2019, 135, 1-57. 7.
Lin, D.; Xiao, D.; Zhu, J.; Yu, P., Piezoelectric and ferroelectric properties of
[Bi0.5(Na1−x−yKxLiy)0.5]TiO3 lead-free piezoelectric ceramics. Appl. Phys. Lett. 2006, 88(6), 062901. 8.
Zhang, S.-T.; Kounga, A. B.; Aulbach, E.; Ehrenberg, H.; Rödel, J., Giant strain
in lead-free piezoceramics Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3 system. Appl. Phys. Lett. 2007, 91 (11), 112906. 17
9.
Hiruma, Y.; Nagata, H.; Takenaka, T., Phase diagrams and electrical properties of
(Bi1/2Na1/2)TiO3-based solid solutions. J. Appl. Phys. 2008, 104 (12), 124106. 10. Jo, W.; Schaab, S.; Sapper, E.; Schmitt, L. A.; Kleebe, H.-J.; Bell, A. J.; Rödel, J., On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol%BaTiO3. J. Appl. Phys. 2011, 110 (7), 074106. 11. Jo, W.; Daniels, J. E.; Jones, J. L.; Tan, X.; Thomas, P. A.; Damjanovic, D.; Rödel, J., Evolving morphotropic phase boundary in lead-free (Bi1/2Na1/2)TiO3-BaTiO3 piezoceramics. J. Appl. Phys. 2011, 109 (1), 014110. 12. Yin, J.; Liu, G.; Lv, X.; Zhang, Y.; Zhao, C.; Wu, B.; Zhang, X.; Wu, J., Superior and anti-fatigue electro-strain in Bi0.5Na0.5TiO3-based polycrystalline relaxor ferroelectrics. J. Mater. Chem. A 2019, 7 (10), 5391-5401. 13. Zhang, J.; Pan, Z.; Guo, F.-F.; Liu, W.-C.; Ning, H.; Chen, Y.; Lu, M.-H.; Yang, B.; Chen, J.; Zhang, S.-T., Semiconductor/relaxor 0-3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics. Nat. Commun. 2015, 6, 6615. 14. Schütz, D.; Deluca, M.; Krauss, W.; Feteira, A.; Jackson, T.; Reichmann, K., Lone-pair-induced covalency as the cause of temperature- and field-induced instabilities in bismuth sodium titanate. Adv. Funct. Mater. 2012, 22 (11), 2285-2294. 15. Liu, X.; Tan, X., Giant strains in non-textured (Bi1/2Na1/2)TiO3-based lead-free ceramics. Adv. Mater. 2016, 28 (3), 574-578. 16. Yin,
J.;
Zhao,
C.;
Zhang,
Y.;
Wu,
J.,
Ultrahigh
strain
in
site
engineering-independent Bi0.5Na0.5TiO3-based relaxor-ferroelectrics. Acta Mater. 2018, 147, 70-77. 17. Jo, W.; Dittmer, R.; Acosta, M.; Zang, J.; Groh, C.; Sapper, E.; Wang, K.; Rödel, J., Giant electric-field-induced strains in lead-free ceramics for actuator applications– status and perspective. J. Electroceram. 2012, 29 (1), 71-93. 18. Sung, Y.; Kim, J.; Cho, J.; Song, T.; Kim, M.; Park, T., Effects of Bi nonstoichiometry in (Bi0.5+xNa)TiO3 ceramics. Appl. Phys. Lett. 2011, 98 (1), 012902. 19. Hiruma, Y.; Nagata, H.; Takenaka, T., Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics. J. Appl. Phys. 2009, 105 (8), 084112. 18
20. Zhang, Y.; Li, J.-F., Review of chemical modification on potassium sodium niobate lead-free piezoelectrics. J. Mater. Chem. C 2019, 7 (15), 4284-4303. 21. Li, F.; Lin, D.; Chen, Z.; Cheng, Z.; Wang, J.; Li, C.; Xu, Z.; Huang, Q.; Liao, X.; Chen, L.-Q., Ultrahigh piezoelectricity in ferroelectric ceramics by design. Nat. Mater. 2018, 17 (4), 349. 22. Li, F.; Cabral, M. J.; Xu, B.; Cheng, Z.; Dickey, E. C.; LeBeau, J. M.; Wang, J.; Luo, J.; Taylor, S.; Hackenberger, W., Giant piezoelectricity of Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals. Science 2019, 364 (6437), 264-268. 23. Zheng, Q.; Xu, C.; Lin, D.; Gao, D.; Kwok, K., Piezoelectric and ferroelectric properties of (Bi0.
94−xLaxNa0.94)0.5Ba0.06TiO3
lead-free ceramics. J. Phys. D: Appl.
Phys. 2008, 41 (12), 125411. 24. Lin,
D.;
Kwok,
K.,
Dielectric
and
piezoelectric
properties
of
(Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics. Curr. Appl Phys. 2010, 10 (2), 422-427. 25. Chon, U.; Kim, K.-B.; Jang, H. M.; Yi, G.-C., Fatigue-free samarium-modified bismuth titanate (Bi4−xSmxTi3O12) film capacitors having large spontaneous polarizations. Appl. Phys. Lett. 2001, 79 (19), 3137-3139. 26. Dinh, T. H.; Kang, J.-K.; Lee, J.-S.; Khansur, N. H.; Daniels, J.; Lee, H.-Y.; Yao, F.-Z.; Wang, K.; Li, J.-F.; Han, H.-S., Nanoscale ferroelectric/relaxor composites: origin of large strain in lead-free Bi-based incipient piezoelectric ceramics. J. Eur. Ceram. Soc. 2016, 36 (14), 3401-3407. 27. Daniels, J. E.; Jo, W.; Rödel, J.; Jones, J. L., Electric-field-induced phase transformation at a lead-free morphotropic phase boundary: Case study in a 93%Bi0.5Na0.5TiO3-7%BaTiO3 piezoelectric ceramic. Appl. Phys. Lett. 2009, 95 (3), 032904. 28. Zhao, C.; Wu, H.; Li, F.; Cai, Y.; Zhang, Y.; Song, D.; Wu, J.; Lyu, X.; Yin, J.; Xiao, D.; Zhu, J.; Pennycook, S. J., Practical high piezoelectricity in barium titanate ceramics utilizing multiphase convergence with broad structural flexibility. J. Am. Chem. Soc. 2018, 140(45): 15252-15260. 29. Vögler, M.; Novak, N.; Schader, F.; Rödel, J., Temperature-dependent volume 19
fraction of polar nanoregions in lead-free (1−x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics. Phys. Rev. B 2017, 95 (2), 024104. 30. Tang, X.; Chew, K.-H.; Chan, H., Diffuse phase transition and dielectric tunability of Ba(ZryTi1−y)O3 relaxor ferroelectric ceramics. Acta Mater. 2004, 52 (17), 5177-5183. 31. Yin, J.; Zhao, C.; Zhang, Y.; Wu, J., Composition-induced phase transitions and enhanced electrical properties in bismuth sodium titanate ceramics. J. Am. Ceram. Soc. 2017, 100 (12), 5601-5609. 32. Ehara, Y.; Novak, N.; Ayrikyan, A.; Geiger, P. T.; Webber, K. G., Phase transformation induced by electric field and mechanical stress in Mn-doped (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 ceramics. J. Appl. Phys. 2016, 120 (17), 174103. 33. Ehara,
Y.;
Novak,
N.;
Yasui,
S.;
Itoh,
M.;
Webber,
K.
G.,
Electric-field-temperature phase diagram of Mn-doped Bi0.5(Na0.9K0.1)0.5TiO3 ceramics. Appl. Phys. Lett. 2015, 107 (26), 262903. 34. Zhao, C.; Yin, J.; Huang, Y.; Wu, J., Polymorphic characteristics challenging electrical properties in lead-free piezoceramics. Dalton Trans. 2019, 48, 11250-11258.
20
Author Photo for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Anping Deng:
Jiagang Wu:
Highlights for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Highlights: 1. Effects of rare earth on the structure and properties of BNT-based ceramics. 2. Three-stage evolution of phase structure after introducing rare earth ions. 3. The improved d33 of ~190 pC/N and high d33* of ~340 pm/V after doping rare earth. 4. Relationships of composition-structure-property in rare-earth engineering ceramics.
Author Biography for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Anping Deng received his B.S. degree in materials physics from Sichuan University in 2002 and Master degree in management science and engineering from Chongqing University in 2008. He is currently working in Chongqing University of Posts and Telecommunications, and studying in School of Materials Science and Engineering as a Ph.D. candidate at Sichuan University. His main research interest is the composition design and property modification of lead-free bismuth sodium titanate (BNT)-based piezoelectric ceramics.
Jiagang Wu received his B.S. degree from Sichuan University in 2003 and Ph.D. degree from Sichuan University in 2008, and worked as a Singapore Millennium Postdoctoral Fellow (SMF-PDF) at the National University of Singapore from 2008 to 2010 (two years). He has been an Associate Professor at the Department of Materials Science, Sichuan University, since 2011. In 2015, he has been promoted to professor in Sichuan University, and is vice dean of the College of Materials Science and Engineering in Sichuan University. His main research interest is the composition design and property modification of ferroelectric/piezoelectric/multiferroic materials. He has published >170 papers in prestigious, top international refereed journals together with the SCI cited times of >4500.
Conflicts of interest for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Conflicts of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S2352-8478(19)30289-8
DOI:
https://doi.org/10.1016/j.jmat.2020.03.005
Reference:
JMAT 290
To appear in:
Journal of Materiomics
Received Date: 26 December 2019 Revised Date:
10 February 2020
Accepted Date: 9 March 2020
Please cite this article as: Deng A, Wu J, Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics, Journal of Materiomics (2020), doi: https://doi.org/10.1016/j.jmat.2020.03.005. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
ABSTRACT: In this work, the effects of rare earth (RE) dopants on the phase structure, microstructure, and electrical properties of lead-free bismuth sodium titanate (BNT)-based ceramics were studied. Rare earth elements (i.e., Sm, Gd, Ho, Pr, Y, and Yb) were selected as the dopants to replace the Bi site of Bi0.5(Na0.8K0.2)0.5TiO3 ceramics. It was found that the addition of moderate RE ions reduced the temperature of ferroelectric-relaxor transition (Tf-r) and the dielectric maximum at lower temperature, while the slight influence on the dielectric maximum at higher temperature and microstructure were observed. Hereby, the three-stage evolution of phase structure after introducing RE ions was confirmed. A high piezoelectric coefficient d33 of ~190 pC/N was achieved due to the improved dielectric constant in the state with stable electric field-induced ferroelectric phase. The high strain property and converse piezoelectric coefficient d33* of ~340 pm/V were obtained in the state with the coexistence of nonergodic and ergodic relaxor phases when Tf-r approached room temperature. The degenerated overall electromechanical properties exhibited in the state with complete ergodic relaxor phase. This study affords a significant guidance for optimizing the piezoelectricity of BNT-based ceramics utilizing rare earth dopants.
Keywords: BNT-based ceramics, rare earth doping, phase structure, electrical properties
1
1. Introduction Piezoelectric ceramics have been used in many electronic components including actuators, transducers, and sensors because of their excellent electromechanical properties [1-5]. Nowadays, bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT)-based lead-free ceramics are one of the most promising counterparts to replace the toxic lead-based piezoelectric ones because of their good ferroelectric performance and strain property [4-16]. Although a large remnant polarization Pr of ~38 µC/cm2 exhibits in pure BNT ceramic, a high coercive field EC of >50 kV/cm results in the difficulty in fully poling process, finally inducing a low piezoelectric coefficient d33 of ~70-90 pC/N [17-19]. The general strategy of optimizing the electromechanical properties of BNT-based ceramics is to manipulate the phase structure through chemical modifications.
Ions substitution or introducing ABO3 compounds can decrease the coercive field of BNT and improve the density by modifying microstructure, and thus its piezoelectricity can be optimized. For example, d33 of Li+, K+, Sr2+, Ba2+ or Mn2+ -doped BNT ceramics would increase up to ~100-150 pC/N by improving its density or constructing a morphotropic phase boundary (MPB) [4-7, 17]. Sn4+, Hf4+, Zr4+, Nb5+ or Ta5+ substitution for Ti sites can promote the strain property and the converse piezoelectric coefficient d33* utilizing the electric field-induced phase transition by reducing the transition temperature between ferroelectric and relaxor phases (Tf-r) [4-6, 14-17]. The addition of other ABO3 compounds, such as BaTiO3 (BT), SrTiO3 (ST), Bi0.5K0.5TiO3 (BKT), (K, Na)NbO3 (KNN), also optimizes the piezoelectric and/or strain properties through constructing the phase boundaries or the transition between ferroelectric and relaxor phases [8-11, 17, 19]. Among them, BNKT (or BNT-BKT) 2
ceramics present a high piezoelectric property of d33~150 pC/N due to its alleged MPB, together with a relatively high depolarization temperature (Td or Tf-r), where ferroelectric polarization and piezoelectric coefficient begin to rapidly decrease to near zero [7, 9]. Rare earth (RE) elements possess peculiar external electron shells and afford abundant orbital bonding routes with other ions. Therefore, rare earth dopants usually give rise to effective random fields and random bonds, thus resulting in local structural heterogeneity and reducing the interfacial energy barriers among different phases/polarization states [20]. This finally facilitates the polarization rotation and optimizes piezoelectric response, or introduce unstable phase to promote the electric filed-induced strain. For example, the addition of Sm enhances the piezoelectric and strain properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics/single crystals [21, 22], and the addition of La or Nd further improve the piezoelectric coefficient of BNT-based ceramics [23, 24]. However, there are no systematical investigations on the effects of RE dopants on the phase structure, microstructure, and electrical properties of BNT-based ceramics.
In this work, the Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) ceramics with relatively high piezoelectric properties were selected as the matrix to study the effects of rare earth substitutions on the structure and property of BNT-based ceramics. Six RE elements (i.e., Sm, Gd, Ho, Pr, Y, and Yb) were used to replace the Bi3+ site. The composition dependence
of
the
phase/microstructure
and
ferroelectric/piezoelectric/
dielectric/strain properties was studied. It was found that the three-stage evolution of phase structure and electromechanical properties was observed after introducing whatever RE ions. The improved piezoelectric and strain behavior was achieved in different stages charactering by different phase structures. 3
2. Experimental procedure (Bi1-xSmx)0.5(Na0.8K0.2)0.5TiO3 (abbreviate as BNKT-xSm, x=0, 0.015, 0.030, 0.045, 0.060, 0.080, and 0.100) and (Bi1-yREy)0.5(Na0.8K0.2)0.5TiO3 (abbreviate as BNKT-yRE, y=0.015, 0.030, 0.045, 0.060, RE=Sm, Gd, Ho, Pr, Y, Yb) ceramics were fabricated by the conventional solid-state reaction route. The raw materials include K2CO3 (99%), Na2CO3 (99.8%), Bi2O3 (99%), TiO2 (98%), Sm2O3 (99.9%), Gd2O3 (99.99%), Ho2O3 (99.99%), Pr6O11 (99.9%), Y2O3 (99.99%), and Yb2O3 (99.99%). All the corresponding raw materials were weighed stoichiometrically and ball milled using ZrO2 ball in the alcohol for 24 h. The mixed powders were calcined at 850 oC (6 h) in air and then directly grinded into the fine powders and pressed into disks under a pressure of 10 MPa with a binder of 7 wt% polyvinyl alcohol. All the pellets were sintered at 1130 °C (2 h) in air after burning out the binder at 850 oC. Finally, silver paste was coated on the surfaces of the sintered samples and then kept at 600 °C (10 min) in air. A direct current electric field of 4-5 kV/mm was used to pole the pellets (20 min) in a silicone oil bath at room temperature.
X-ray diffractometer (Bruker D8 Advanced XRD, Bruker AXS Inc., Madison, WI) with Cu Kα radiation (λ=1.5406 Å) was used to analyze the phase structure. Surface microstructure, energy-dispersive X ray spectroscopy (EDS) and elemental mapping analysis of sintered samples were examined using a field-emission scanning electron microscope
(FE-SEM)
(JSM-7500,
Musashino
Akishima,
Tokyo,
Japan).
Room-temperature dielectric constant (εr) and dielectric loss (tan δ) as well as the temperature-dependent dielectric constant (εr-T) and dielectric loss (tan δ-T) curves were all obtained by an LCR analyzer (Tonghui 2816A, Changzhou, China). 4
Room-temperature direct piezoelectric coefficient (d33) was measured by a quasi-static d33 meter (ZJ-3A, Institute of Acoustics, Beijing, China). Planar electromechanical coupling coefficient (kp) was determined by an impedance analyzer (HP 4294A, Agilent, Santa Clara, USA). The electric field-induced bipolar/unipolar strain (S-E) curves, ferroelectric hysteresis (P-E) loops and current density (J-E) loops were acquired utilizing a ferroelectric analyzer (aixACCT TF2000, Aachen, Germany).
3. Results and discussion
Fig. 1. Composition dependence of XRD patterns of undoped BNKT-xSm ceramics with different Sm contents: (a) 2θ=20-70o, (b) 2θ=39.5-40.5o, and (c) 2θ=45.5-47.5o.
Fig. 1(a) exhibits the room-temperature XRD patterns of unpoled BNKT-xSm ceramics, measured at 2θ=20-70o. It was observed that all the samples presented typical ABO3 perovskite structure. There were no other secondary phases, suggesting that Sm3+ totally entered into the BNT-based matrix to form a single solid solution. To further identify the evolution of phase structure, the XRD patterns at 2θ=41-42o and 5
2θ=45.5-47.5o were expanded [Figs. 1(b) and (c)], as indicated by the crystal indices of (111) and (200), respectively. According to the expanded diffraction peaks, both (111) and (200) peaks tended to shift to a higher angle when the composition increases from x=0 to 0.100. It is well known that the ionic radius of Sm3+ is smaller than that of Bi3+ under the same coordination [25]. By replacing Bi3+ by Sm3+, the reduced lattice parameters and shrinkage of unit cell were presented, and therefore, the characteristic diffraction peaks moved to a higher angle according to the Bragg formula. Besides, the single peaks without any obvious splitting peaks exhibited in all the samples, implying a pseudocubic phase structure for all the unpoled ceramics [9, 26, 27].
Fig. 2. Surface FE-SEM images of BNKT-xSm ceramics with different Sm contents: (a) x=0, (b) x=0.030, (c) x=0.060, and (d) x=0.100.
6
Fig. 3. Element mappings of BNKT-xSm ceramics with x=0.100.
Figs. 2(a)-(d) show the surface morphologies of BNKT-xSm ceramics with x=0, 0.030, 0.060, 0.100, respectively. All the ceramics exhibited a dense microstructure. The cuboid-like grains exhibited in pure BNKT ceramic (x=0), and their average grain size was ~1 µm. After introducing Sm3+, the grain shape and the average grain sizes were basically unchanged, indicating that the addition of Sm3+ slightly affected the grain growth of BNKT ceramics. Fig. 3 displays the elements mapping of BNKT-0.100Sm ceramics. It was observed that there were no elements enrichment between grains and grain boundaries. Because there were also no secondary phases in XRD results (Fig. 1), the dopant (Sm) was considered to homogeneously distribute into the ceramic matrix at the grain-size scale approximatively [28]. This result affords another piece of evidence for the doped Sm3+ can be well merged into the matrix of BNKT ceramics.
7
Fig. 4. (a) Schematic of identifying the ferroelectric-relaxor transition temperature Tf-r according to εr-T and tan δ-T curves of poled sample [10, 29]. (b)-(h) Composition dependence of εr-T and tan δ-T curves measured at frequency = 100 Hz, 1 kHz, 10 kHz, 100 kHz for BNKT-xSm ceramics with different Sm contents.
In order to confirm the influence of Sm3+ substitution on the phase structure of BNKT ceramics, the temperature/frequency dependence of dielectric properties for the poled ceramics were performed, as shown in Fig. 4. The ferroelectric-relaxor transition temperature (Tf-r) was confirmed through combining the εr-T and tan δ-T curves of poled samples, and the values of Tf-r acquired from the anomalous points of εr-T and tan δ-T curves, as shown in Fig. 4(a) [10, 29]. For pure BNKT (x=0) [Fig. 4(b)], there are two dielectric peaks in the εr-T curves, where the low-temperature one is defined as the first dielectric maximum (Tfm) and the high-temperature one is called as the second dielectric maximum (Tsm). Tsm is maximal permittivity over the whole εr-T curves. Tfm is a dielectric shoulder, in which the dielectric constant gradually decreases and shifts to a higher temperature when the measured frequency increases from 100 Hz to 100 kHz. In the vicinity of Tfm, a phase transition from ferroelectric R3c phase to relaxor phases with P4bm + Pm3m would happen, and the temperature is 8
denoted as Tf-r [29], which can be observed in the εr-T and tan δ-T curves of poled samples, as dotted box shown in Figure 4A. The Tf-r, Tfm, and Tsm of pure BNKT ceramics are about 76 oC, 140 oC, 327 oC, respectively. After introducing Sm3+, maximum dielectric anomalies decrease, and the first and second dielectric anomalies became flattened when the composition reached x=0.080 and 0.100 [Figs. 4(g) and (h)]. In addition, Tf-r moved to a lower temperature and gradually disappears in εr-T curves, and it can only accurately obtain Tf-r in the expanded tan δ-T curves [Figs. 4(c)-(h)]. This result indicates that the Sm substitution for Bi has some influence on maximum permittivity and decreases the ferroelectric-relaxor transition temperature Tf-r.
Fig. 5. (a) Plots of ln(1/εr−1/εm) as a function of ln(T–Tm) according to corresponding εr-T curves at 1 kHz. (b) and (c) Composition dependence of Tf-r, Tfm, Tsm, and diffuseness exponent γ from x=0 to x=0.100.
In order to figure out the influence of rare earth Sm3+ on the structure, the evolution of εr-T curves and the corresponding analysis as a function of Sm contents was carried out. For analyzing the diffuseness degree, a modified Curie-Weiss law is use to describe the diffuseness of phase transition [29], 9
1/εr–1/εsm = ( T–Tsm)γ/C (1≤γ≤2),
(1)
where εsm is dielectric constant at Tsm, γ=1 represents a normal ferroelectrics which fully complied with the Curie-Weiss law, while γ=2 describes an ideal relaxor ferroelectrics with complete diffuse phase transition. The fitting results are displayed in Fig. 5(a). Composition-dependent Tf-r, Tfm, Tsm and diffuseness exponent γ are displayed in Figs. 5(b) and (c). According to the evolution of dielectric anomalies in εr-T curves [Fig. 5(b)], it can deduce that there were two stages of phase-structure evolution for the Sm3+-doped BNKT ceramics. Tfm basically maintained unchanged when the composition increased from x=0 to x=0.100, whereas both Tf-r and Tsm firstly decreased at x=0-0.030, then almost remained unchanged from x>0.030 to x=0.100. Therefore, these two stages were divided in the vicinity of x=0.030. From the evolution of diffuseness exponent γ [Fig. 5(c)], there were also two stage of structure state for the Sm3+-doped BNKT ceramics, but was divided in the vicinity of x=0.060. Combining all of above results, it can deduce that there are actually three stages of structure evolution by introducing Sm3+. It can define them as stageⅠ, stage Ⅱ, and stage Ⅲ, divided by near x=0.030 and x=0.060, as shown in Fig. 5(b) and (c). This result will be further verified through the evolution of electric properties of BNKT-xSm ceramics.
10
Fig. 6. Composition dependence of (a) ferroelectric hysteresis P-E loops and (b) electric field-induced current density (J-E) loops for the BNKT-xSm ceramics. (c) Evolution of maximum polarization (Pmax), remnant polarization (Pr), and coercive field (Ec) for corresponding P-E loops. Composition dependence of (d) maximum current density (Jmax) and (E) its corresponding electric field (Es).
Fig. 6(a) presents P-E loops of BNKT-xSm ceramics. It was observed that typically square ferroelectric loops exhibited in the samples (x=0-0.030) (stageⅠ) with high maximum polarization Pmax of ~35 µC/cm2, remnant polarization Pr of ~27 µC/cm2, and large coercive field Ec of ~26-28 kV/cm. This result indicates that the samples in this stage exhibit nonergodic relaxor state, in which the electric field-induced phase transition from nonergodic relaxor state to ferroelectric state takes place and then presents a strong electric-induced ferroelectric polarization under a high applied electric field [29]. At x=0.045-0.060 (stageⅡ), the ferroelectric loops became slim with the pinched shape and exhibited the rapidly decreased remnant polarization and coercive field, but was accompanied by a still large maximum polarization. This result 11
indicates the coexistence of nonergodic and ergodic relaxor states [31]. When the composition approached x=0.080 and 0.100 (stage Ⅲ), the P-E loops became very slim and exhibited the greatly degenerated maximum polarization, implying the involvement of an ergodic relaxor state in which the field-induced ferroelectric phase transformed to the initial relaxor state again when the applied electric field was removed [29, 31]. Similar phenomenon was observed in the evolution of J-E curves [Fig. 6(b)]. There was only one high current peak of domain switching for the samples (x=0-0.030) (stageⅠ). For x=0.045 and 0.060, another current peak appeared at a low electric field except for the current peak of domain switching. It was demonstrated that this current peak derives from the decomposition of oriented ferroelectric domain under applying a backward electric field [31]. Under applying an external electric field, the coexisted nonergodicity and ergodicity would transform to ferroelectric state, and incipient short-range polar orders grow up to large ferroelectric domains and arrange along with applied electric field. After applying an external electric field along the opposite direction, partial ferroelectric domains derived from ergodic relaxor state do not switch but firstly decompose into short-range polar orders again and release the domain-decomposed current. Then the decomposed short-range polar orders grow up to new long-range ferroelectric orders and switch along with inverse electric field, showing a domain switching current [31-33]. As a result, two current peaks for x=0.045 and 0.060 (stageⅡ) were found with the coexistence of nonergodic and ergodic relaxor states. When the composition reached x=0.080 and 0.100 (stage Ⅲ), the complete ergodic relaxor state made the ceramics small electric field-induced polarization and thus presented a square J-E loops [31]. Consequently, corresponding to the evolution of three stages in εr-T curves, the evolutions of Pmax Pr, and Ec [Fig. 6(c)], maximum current density Jmax [Fig. 6(d)], and its corresponding electric field Es 12
[Fig. 6(e)] also presented three stages of the variation. The addition of Sm3+ resulted in phase transition of BNKT ceramics, and the different phase structure in the three stages is responsible for the performance change of electric field-induced ferroelectricity.
Fig. 7. Composition dependence of (a) bipolar S-E curves and (b) unipolar S-E curves for the BNKT-xSm ceramics. (c) Evolution of poling strain (Spol), negative strain (Sneg), and the corresponding electric field of maximum negative strain (Eneg) for bipolar S-E loops. (d) Evolution of the maximum strain and corresponding converse piezoelectric coefficient (d33*) as a function of x for bipolar/unipolar strain curves.
The same phenomenon was also observed in the evolutions of strain, piezoelectric, and dielectric properties as a function of compositions. Fig. 7(a) displays the bipolar strain S-E curves as a function of x, measured at 50 kV/cm. It was seen that the typical S-E curves with butterfly shape, large poling strain Spos, and negative strain Sneg appeared in the samples with low Sm3+ contents. After increasing the Sm3+ contents up to x=0.080, the S-E curves became slim. With increasing the compositions from x=0 to 0.100, the poling strain Spos firstly remained unchanged at x=0-0.030 in stageⅠ 13
and then increased to a high strain value at 0.030≤x≤0.060 in stage Ⅱ. Finally, it rapidly decreased to a small value at 0.060
14
Fig. 8. Composition dependence of (a) dielectric constant (εr) and dielectric loss (tan δ) and (b) direct piezoelectric coefficient (d33) and planar electromechanical coupling coefficient (kp) measured at room temperature. (c) Comparison of d33 for BNKT-yRE ceramics (y=0 and 0.015, RE=Sm, Gd, Ho, Pr, Y, Yb). (d) Composition dependence of d33 for BNKT-yRE ceramics (RE=Sm, Gd, Ho, Pr, Y, Yb).
Fig. 8(a) displays the room-temperature dielectric properties (εr and tan δ) of the ceramics as a function of x. In stageⅠ(x=0-0.030), dielectric constant εr gradually increased from ~800 to ~1000 and then rapidly increased to ~1350 at x=0.060 in stage Ⅱ.When x got into stageⅡ, εr begun to degenerate to ~1200. Different from εr, dielectric loss always remained a low value of ~0.05 (x=0~0.100). The addition of optimum Sm3+ improved the piezoelectric constant of BNKT ceramics. As shown the stageⅠ [Fig. 8(b)], d33 increased from 150 pC/N to 190 pC/N with the increase of x from 0 to 0.015, and maintained a high value of ~165-180 pC/N at 0.015
15
near zero when the composition reaches stage Ⅲ . Meanwhile, the planar electromechanical coupling coefficient kp of the samples exhibited similar variation, as shown in Fig. 8(b). Usually, piezoelectric response of a ferroelectric material is proportion to the product values between remnant polarization Pr and dielectric constant εr, that is, d33 ~ Pr·εr [34]. Therefore, the improved dielectric constant εr [Fig. 8(a)] and high remnant polarization Pr [Fig. 6(c)] should attribute to the enhanced d33 for the BNKT-xSm ceramics with stable electric field-induced ferroelectric phase in stageⅠ. Introducing optimum Sm enhanced the piezoelectric property of BNT-based ceramics, and the addition of other RE elements (i.e., Pr, Y, Gd, Yb, Ho) also optimized their piezoelectric properties. As shown in Fig. 8(c), a high d33 of ~165-180 pC/N was obtained in the 0.15%RE -doped BNKT ceramics. According to the evolution of d33, the phase-transition region after doping RE and the corresponding compositional range were roughly confirmed, as shown in Fig. 8(d). It was seen that both Gd and Pr had a similar effect on phase transition of BNKT ceramics with respect to Sm, while Y, Yb, and Ho exhibited the accelerated effect on phase transition with respect to Gd, Pr, and Sm.
4. Conclusions Rare earth-doped BNKT ceramics were prepared by the conventional solid-state reaction method, and then the relationship between structure & properties and doped content & type of rare earth was studied. Introducing rare earth exhibited significant effects on the phase structure through shifting the ferroelectric-relaxor transition temperature of BNKT ceramics and thus manipulated the piezoelectric and strain properties in different phase-structure regions. Consequently, high piezoelectric and strain property was achieved. This work offers a new paradigm for further optimizing 16
piezoelectricity of BNT-based ceramics utilizing rare earth dopants.
Conflicts of interest Authors declare that there are no conflicts of interest.
Acknowledgments Authors gratefully acknowledge the support of the National Natural Science Foundation of China (51972215). Thank Hui Wang (Analytical & Testing Center, Sichuan University) for measuring the FE-SEM images.
References 1.
Jaffe, B., Piezoelectric ceramics. Elsevier: 2012; Vol. 3.
2.
Uchino, K., Piezoelectric actuators and ultrasonic motors. Springer Science &
Business Media: 1996; Vol. 1. 3.
Thong, H.-C.; Zhao, C.; Zhou, Z.; Wu, C.-F.; Liu, Y.-X.; Du, Z.-Z.; Li, J.-F.;
Gong, W.; Wang, K., Technology transfer of lead-free (K, Na)NbO3-based piezoelectric ceramics. Mater. Today 2019; 29: 39-48 . 4.
Wu, J., Advances in Lead-free Piezoelectric Materials. Springer: 2018.
5.
Zheng, T.; Wu, J.; Xiao, D.; Zhu, J., Recent development in lead-free perovskite
piezoelectric bulk materials. Prog. Mater Sci. 2018, 98, 552-624. 6.
Hao, J.; Li, W.; Zhai, J.; Chen, H., Progress in high-strain perovskite piezoelectric
ceramics. Materials Science and Engineering: R: Reports 2019, 135, 1-57. 7.
Lin, D.; Xiao, D.; Zhu, J.; Yu, P., Piezoelectric and ferroelectric properties of
[Bi0.5(Na1−x−yKxLiy)0.5]TiO3 lead-free piezoelectric ceramics. Appl. Phys. Lett. 2006, 88(6), 062901. 8.
Zhang, S.-T.; Kounga, A. B.; Aulbach, E.; Ehrenberg, H.; Rödel, J., Giant strain
in lead-free piezoceramics Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3 system. Appl. Phys. Lett. 2007, 91 (11), 112906. 17
9.
Hiruma, Y.; Nagata, H.; Takenaka, T., Phase diagrams and electrical properties of
(Bi1/2Na1/2)TiO3-based solid solutions. J. Appl. Phys. 2008, 104 (12), 124106. 10. Jo, W.; Schaab, S.; Sapper, E.; Schmitt, L. A.; Kleebe, H.-J.; Bell, A. J.; Rödel, J., On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol%BaTiO3. J. Appl. Phys. 2011, 110 (7), 074106. 11. Jo, W.; Daniels, J. E.; Jones, J. L.; Tan, X.; Thomas, P. A.; Damjanovic, D.; Rödel, J., Evolving morphotropic phase boundary in lead-free (Bi1/2Na1/2)TiO3-BaTiO3 piezoceramics. J. Appl. Phys. 2011, 109 (1), 014110. 12. Yin, J.; Liu, G.; Lv, X.; Zhang, Y.; Zhao, C.; Wu, B.; Zhang, X.; Wu, J., Superior and anti-fatigue electro-strain in Bi0.5Na0.5TiO3-based polycrystalline relaxor ferroelectrics. J. Mater. Chem. A 2019, 7 (10), 5391-5401. 13. Zhang, J.; Pan, Z.; Guo, F.-F.; Liu, W.-C.; Ning, H.; Chen, Y.; Lu, M.-H.; Yang, B.; Chen, J.; Zhang, S.-T., Semiconductor/relaxor 0-3 type composites without thermal depolarization in Bi0.5Na0.5TiO3-based lead-free piezoceramics. Nat. Commun. 2015, 6, 6615. 14. Schütz, D.; Deluca, M.; Krauss, W.; Feteira, A.; Jackson, T.; Reichmann, K., Lone-pair-induced covalency as the cause of temperature- and field-induced instabilities in bismuth sodium titanate. Adv. Funct. Mater. 2012, 22 (11), 2285-2294. 15. Liu, X.; Tan, X., Giant strains in non-textured (Bi1/2Na1/2)TiO3-based lead-free ceramics. Adv. Mater. 2016, 28 (3), 574-578. 16. Yin,
J.;
Zhao,
C.;
Zhang,
Y.;
Wu,
J.,
Ultrahigh
strain
in
site
engineering-independent Bi0.5Na0.5TiO3-based relaxor-ferroelectrics. Acta Mater. 2018, 147, 70-77. 17. Jo, W.; Dittmer, R.; Acosta, M.; Zang, J.; Groh, C.; Sapper, E.; Wang, K.; Rödel, J., Giant electric-field-induced strains in lead-free ceramics for actuator applications– status and perspective. J. Electroceram. 2012, 29 (1), 71-93. 18. Sung, Y.; Kim, J.; Cho, J.; Song, T.; Kim, M.; Park, T., Effects of Bi nonstoichiometry in (Bi0.5+xNa)TiO3 ceramics. Appl. Phys. Lett. 2011, 98 (1), 012902. 19. Hiruma, Y.; Nagata, H.; Takenaka, T., Thermal depoling process and piezoelectric properties of bismuth sodium titanate ceramics. J. Appl. Phys. 2009, 105 (8), 084112. 18
20. Zhang, Y.; Li, J.-F., Review of chemical modification on potassium sodium niobate lead-free piezoelectrics. J. Mater. Chem. C 2019, 7 (15), 4284-4303. 21. Li, F.; Lin, D.; Chen, Z.; Cheng, Z.; Wang, J.; Li, C.; Xu, Z.; Huang, Q.; Liao, X.; Chen, L.-Q., Ultrahigh piezoelectricity in ferroelectric ceramics by design. Nat. Mater. 2018, 17 (4), 349. 22. Li, F.; Cabral, M. J.; Xu, B.; Cheng, Z.; Dickey, E. C.; LeBeau, J. M.; Wang, J.; Luo, J.; Taylor, S.; Hackenberger, W., Giant piezoelectricity of Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals. Science 2019, 364 (6437), 264-268. 23. Zheng, Q.; Xu, C.; Lin, D.; Gao, D.; Kwok, K., Piezoelectric and ferroelectric properties of (Bi0.
94−xLaxNa0.94)0.5Ba0.06TiO3
lead-free ceramics. J. Phys. D: Appl.
Phys. 2008, 41 (12), 125411. 24. Lin,
D.;
Kwok,
K.,
Dielectric
and
piezoelectric
properties
of
(Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics. Curr. Appl Phys. 2010, 10 (2), 422-427. 25. Chon, U.; Kim, K.-B.; Jang, H. M.; Yi, G.-C., Fatigue-free samarium-modified bismuth titanate (Bi4−xSmxTi3O12) film capacitors having large spontaneous polarizations. Appl. Phys. Lett. 2001, 79 (19), 3137-3139. 26. Dinh, T. H.; Kang, J.-K.; Lee, J.-S.; Khansur, N. H.; Daniels, J.; Lee, H.-Y.; Yao, F.-Z.; Wang, K.; Li, J.-F.; Han, H.-S., Nanoscale ferroelectric/relaxor composites: origin of large strain in lead-free Bi-based incipient piezoelectric ceramics. J. Eur. Ceram. Soc. 2016, 36 (14), 3401-3407. 27. Daniels, J. E.; Jo, W.; Rödel, J.; Jones, J. L., Electric-field-induced phase transformation at a lead-free morphotropic phase boundary: Case study in a 93%Bi0.5Na0.5TiO3-7%BaTiO3 piezoelectric ceramic. Appl. Phys. Lett. 2009, 95 (3), 032904. 28. Zhao, C.; Wu, H.; Li, F.; Cai, Y.; Zhang, Y.; Song, D.; Wu, J.; Lyu, X.; Yin, J.; Xiao, D.; Zhu, J.; Pennycook, S. J., Practical high piezoelectricity in barium titanate ceramics utilizing multiphase convergence with broad structural flexibility. J. Am. Chem. Soc. 2018, 140(45): 15252-15260. 29. Vögler, M.; Novak, N.; Schader, F.; Rödel, J., Temperature-dependent volume 19
fraction of polar nanoregions in lead-free (1−x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics. Phys. Rev. B 2017, 95 (2), 024104. 30. Tang, X.; Chew, K.-H.; Chan, H., Diffuse phase transition and dielectric tunability of Ba(ZryTi1−y)O3 relaxor ferroelectric ceramics. Acta Mater. 2004, 52 (17), 5177-5183. 31. Yin, J.; Zhao, C.; Zhang, Y.; Wu, J., Composition-induced phase transitions and enhanced electrical properties in bismuth sodium titanate ceramics. J. Am. Ceram. Soc. 2017, 100 (12), 5601-5609. 32. Ehara, Y.; Novak, N.; Ayrikyan, A.; Geiger, P. T.; Webber, K. G., Phase transformation induced by electric field and mechanical stress in Mn-doped (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 ceramics. J. Appl. Phys. 2016, 120 (17), 174103. 33. Ehara,
Y.;
Novak,
N.;
Yasui,
S.;
Itoh,
M.;
Webber,
K.
G.,
Electric-field-temperature phase diagram of Mn-doped Bi0.5(Na0.9K0.1)0.5TiO3 ceramics. Appl. Phys. Lett. 2015, 107 (26), 262903. 34. Zhao, C.; Yin, J.; Huang, Y.; Wu, J., Polymorphic characteristics challenging electrical properties in lead-free piezoceramics. Dalton Trans. 2019, 48, 11250-11258.
20
Author Photo for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Anping Deng:
Jiagang Wu:
Highlights for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Highlights: 1. Effects of rare earth on the structure and properties of BNT-based ceramics. 2. Three-stage evolution of phase structure after introducing rare earth ions. 3. The improved d33 of ~190 pC/N and high d33* of ~340 pm/V after doping rare earth. 4. Relationships of composition-structure-property in rare-earth engineering ceramics.
Author Biography for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Anping Deng received his B.S. degree in materials physics from Sichuan University in 2002 and Master degree in management science and engineering from Chongqing University in 2008. He is currently working in Chongqing University of Posts and Telecommunications, and studying in School of Materials Science and Engineering as a Ph.D. candidate at Sichuan University. His main research interest is the composition design and property modification of lead-free bismuth sodium titanate (BNT)-based piezoelectric ceramics.
Jiagang Wu received his B.S. degree from Sichuan University in 2003 and Ph.D. degree from Sichuan University in 2008, and worked as a Singapore Millennium Postdoctoral Fellow (SMF-PDF) at the National University of Singapore from 2008 to 2010 (two years). He has been an Associate Professor at the Department of Materials Science, Sichuan University, since 2011. In 2015, he has been promoted to professor in Sichuan University, and is vice dean of the College of Materials Science and Engineering in Sichuan University. His main research interest is the composition design and property modification of ferroelectric/piezoelectric/multiferroic materials. He has published >170 papers in prestigious, top international refereed journals together with the SCI cited times of >4500.
Conflicts of interest for
Effects of rare-earth dopants on phase structure and electrical properties of lead-free bismuth sodium titanate-based ceramics Anping Deng, Jiagang Wu* Department of Materials Science, Sichuan University, Chengdu, 610064, China *Corresponding author. Email: [email protected] and [email protected]
Conflicts of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.