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Ultra-high piezoelectric and dielectric properties of low-temperature-sintered lead hafnium titanate-lead niobium nickelate ceramics
Journal Pre-proof Ultra-high piezoelectric and dielectric properties of low-temperature-sintered lead hafnium titanate-lead niobium nickelate ceramics Yangxi Yan, Zhimin Li, Yushun Xia, Mo Zhao, Maolin Zhang, Dongyan Zhang PII:
S0272-8842(19)33029-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.166
Reference:
CERI 23234
To appear in:
Ceramics International
Received Date: 29 September 2019 Revised Date:
16 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Y. Yan, Z. Li, Y. Xia, M. Zhao, M. Zhang, D. Zhang, Ultra-high piezoelectric and dielectric properties of low-temperature-sintered lead hafnium titanate-lead niobium nickelate ceramics, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.166. 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. © 2019 Published by Elsevier Ltd.
Ultra-high piezoelectric and dielectric properties of low-temperature-sintered lead hafnium titanate-lead niobium nickelate ceramics Yangxi Yan1, Zhimin Li1*, Yushun Xia1, Mo Zhao2*, Maolin Zhang1, Dongyan Zhang1 1
2
School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071, China
State Key Laboratory of Intense Pulsed Radiation Simulation and Effect (Northwest Institute of Nuclear Technology), Xi’an 710024, China
Abstract Equimolar Li+ and Bi3+ co-doped 0.51Pb(Hf0.3Ti0.7)O3-0.49Pb(Nb2/3Ni1/3)O3 (PHT-PNN) piezoelectric ceramics were synthesized by conditional solid state method. The effects of Li+-Bi3+ co-doping on crystalline structure, morphological structure, piezoelectric and dielectric properties of PHT-PNN ceramics were systematically analyzed. Results revealed that Pb sites of PHT-PNN were replaced with Li+-Bi3+ after co-doping. Li+-Bi3+ co-doping also improved electrical performance of ceramics, formed liquid phase, and decreased sintering temperature during sintering process. Li+-Bi3+ co-doping resulted in ultra-high piezoelectric properties (e.g., d33=1025 pC/N and εr=7974) for PHT-PNN/1.0 mol% Li+-Bi3+ ceramic sintered at 1100
. Furthermore,
detailed mechanism responsible for improved piezoelectric response of Li+-Bi3+ co-doped ceramics was proposed. This study reports the synthesis of ultra-high performance piezoelectric ceramics with low-temperature co-firing properties, which is highly desirable for wide range of applications. Keywords: ultra-high piezoelectric performance; Li+-Bi3+co-doping; low sintering temperature
*
Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (M. Zhao)
1. Introduction Nowadays, lead-free piezoelectric ceramics such as K0.5Na0.5NbO3 (KNN) and BaTiO3 ceramics are commonly used in replacement of lead-based piezoelectric ceramics. However, these lead-free ceramics are costly and their dielectric and piezoelectric properties are not better than those of lead-based ceramics at present. In this sense, lead-based piezoelectric ceramics such as PbZr0.5Ti0.5O3 (PZT) [1], 0.55Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3 (PNN-PZT) [2] or other related ceramics have been utilized as electronic materials in various devices including precision actuators, piezoelectric transformers, and smart sensors. Recently, the multilayer piezoelectric devices have been increasingly developed, which require the employed materials to have excellent properties, such as high piezoelectric constant, high
dielectric
constant
and
low
dielectric
dissipation
factor.
Pb(Hf,Ti)O3-Pb(Ni,Nb)O3 ceramics have attracted wide attention due to their ultra-high piezoelectric properties. These materials showed good piezoelectric and dielectric constants of 970 pC/N and 6000, respectively [3]. However, the high sintering temperature (~1250 oC) restrains them to use as the multilayer piezoelectric materials. In addition, lead volatilization issues have been increasingly considered in view of the ever-increasing environmental protection awareness. Thus, there is a pressing need to develop ceramics with high piezoelectricity and low sintering temperature such that lead volatilization can be mitigated. This can be achieved by ion doping since the sintering temperature and electrical properties of the ceramics are highly
sensitive to ion substitution. For instance, piezoelectric ceramics are usually doped with donor or acceptor ions to tailor their piezoelectric and dielectric properties. Dopants such as Li+, Bi3+, Li+-Al3+, Li+-Bi3+, and Al3+-N3- (A-sites) can significantly change various properties of the ceramics [4–8]. Lei et al. revealed that Li+ cations occupied A sites of BaTiO3-Bi0.5(Na,K)0.5TiO3 ceramics upon addition of Li2CO3 below 0.75 mol%, while B sites are occupied at higher concentrations [9]. Saito et al. found that the piezoelectric performance of KNN ceramics could be enhanced by doping with LiTaO3 and LiSbO3 [10]. Additionally, Zeng et al. found that Bi3+-Fe3+ co-doping can remarkably improve the piezoelectric and ferroelectric properties of PZT ceramics, resulting in materials with large strains [11]. Limpichaipanit et al. prepared Li+ and Bi3+ co-doped Pb0.91La0.09Zr0.65Ti0.35O3 (PLZT) ceramics with a high dielectric constant of 7819 [12]. Despite this progress, to the best of our knowledge, Li+-Bi3+ co-doped PHT-PNN piezoelectric ceramics have not been prepared yet. Therefore, in this work, we synthesized high-performance piezoelectric ceramics sintered at low temperature by co-doping PHT-PNN with Li+-Bi3+cations. Owing to similar ionic radii of the host and replacing ions, Li+-Bi3+cations tend to occupy the A sites of the perovskite structure [13]. The effects of Li+-Bi3+ co-doping on the microstructure, sintering temperature, piezoelectric and dielectric properties of the ceramics were analyzed. Finally, we believe that our research can be very important for developing PHT-PNN ceramics with improved piezoelectricity.
2. Experimental procedures The materials studied herein were 0.51Pb(Hf0.3Ti0.7)O3-0.49Pb(Nb2/3Ni1/3)O3/x
mol% (Li+-Bi3+) ceramics with x ranging from 0 to 2. Firstly, high-purity nickel oxide and niobium pentoxide were used as raw materials to synthesize the nickel niobate (NiNb2O6) precursor at 1100 oC for 6 h. Secondly, stoichiometric amounts of Pb3O4 (99.9%), HfO2 (99.99%), TiO2 (99.9%), Bi2O3 (99.9%), Li2CO3 (99.0%) and the as-prepared NiNb2O6 were weighed and milled for 15 h. The ball milled mixture was calcined for 2 h at 850 oC before preparing disks of 12 mm in diameter by uniaxial pressing (200 MPa). The compacted sample was immersed in lead tetroxide powder and then calcined for 3 h at 900–1300 oC. Next, the sintered samples were ground to 1 mm thick and coated at room temperature with silver slurry on the upper and lower surfaces. Finally, the sample was polarized for 30 min under DC electric field (30 kV/cm) before testing its performance. The crystal structure of the specimens was assessed by XRD (Bruker D8 Advance) in a diffractometer equipped with Cu Kα radiation. The surface morphology was observed by scanning electron microscopy (SEM, JEOL JSM 6700F). The piezoelectric constant (d33) was measured on a piezoelectric-d33 device (ZJ-3A, Chinese Academy of Science). The dielectric-temperature curves were obtained on a high-temperature dielectric spectrometer (HDMS-1000) manufactured by Partulab Co., Ltd.
3. Results and discussion
Fig. 1. XRD patterns of the PHT-PNN /x mol% (Li+-Bi3+) ceramics.
Fig. 1 presents the XRD patters (2θ: 15–78º) of the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100 oC. As can be seen, the peaks of all the prepared samples indicate that the crystalline phase is the rhombohedral perovskite with space group symmetries of R3m. XRD also revealed that Li+ and Bi3+ ions were completely incorporated into the PHT-PNN lattice during the sintering process since no secondary phases were detected. In order to further study the effect of Li+ and Bi3+ ions co-doping on the crystal structure of the PHT-PNN ceramics, we analyzed the 2θ=37–41° region in detail (Fig. 1(b)) for the PHT-PNN/Li+-Bi3+ ceramics. The (111) peak clearly shifted towards higher diffraction angles as the Li+ and Bi3+ contents increased, revealing a slight decrease of the lattice constant calculated by the Bragg formula. This decrease resulted from the lower ionic radii of the Li+ and Bi3+ substituents compared to Pb2+.
Fig. 2. SEM images of the PHT-PNN /x mol% (Li+-Bi3+) ceramics sintered at 1100 oC:(a) x=0, (b) x= 0.5, (c) x=1.0, (d) x=1.5, (e) x=2.0.
Fig. 2 shows SEM micrographs of the PHT-PNN/x mol% (Li+-Bi3+) ceramics. As shown in Fig. 2(a), the un-doped sample contained grains with different sizes and many pores distributed at the grain boundaries, revealing insufficient sintering. In contrast, for doping concentrations up to 1.0 mol%, the samples showed a denser structure and a more uniform grain size distribution centered at 2～2.5 µm (Fig. 3(c)). However, the average grain size of pure PHT-PNN was only 0.9 µm (Fig. 3(a)). Thus, the addition of Li2CO3 and Bi2O3 favored grain growth to a large extent. Li2CO3 and Bi2O3 were added to PHT-PNN ceramics to obtain dense samples with low sintering temperatures. Transition liquid phase sintering was the dominant low-temperature sintering mechanism [14, 15]. Bi2O3 and Li2CO3 are known to form a liquid phase at ca. 690 oC [16]. This liquid phase wetted and covered the surface of the grains, thereby facilitating dissolution and migration of substances. Accordingly, the formation of this liquid phase would favor densification while lowering the sintering temperature of the ceramics. Moreover, the sintering of the liquid phase
promoted grain growth, as shown in Fig. 3.
Fig. 3. Grain size distributions of the PHT-PNN /x mol% (Li+-Bi3+) ceramics: (a) x=0, (b) x=0.5, (c) x=1.0, (d) x=1.5, (e) x=2.0.
Fig. 4. d33 for the 1 mol% (Li+-Bi3+) doped and un-doped PHT-PNN ceramics as a function of the sintering temperature.
Fig. 4 shows the piezoelectric constant d33 for the 1.0 mol% (Li+-Bi3+) doped PHT-PNN ceramics as a function of the sintering temperature. The doped samples showed remarkably higher d33 values compared to the un-doped sample. The d33 of the doped specimens increased fast with the sintering temperature and reached a
maximum. This maximum value is higher than that the optimum values reported for PHT-PNN ceramics [3]. Additionally, the sintering temperature was lowered by ca. 150 oC, compared with the sintering temperature of un-doped PHT-PNN ceramics (1250 oC). The observed variations in the piezoelectric constant of our samples might be caused by the grain size and the defect dipole (Li+-Bi3+ ionic pairs). Okazaki et al. found that the d33 increased with the increase of grain size [17]. Owing to the grain size increases, the grain boundary phase content associated with the space-charge volume will decrease, and the “pinning effect” on the domain motion also decrease, which makes the domain easily switch and results in high piezoelectric performance. Feng et al. reported that doped smaller acceptor and donor ions substituting bivalent A-sites are utilized to bring local lattice distortion and lower symmetry and greatly improve the piezoelectric properties of PbTiO3-based ceramics [6]. Thus, the smaller radii of Li+ and Bi3+ could occupy the A sites of the perovskite structure by substituting Pb2+ in the PHT-PNN ceramics, and formed the Li+-Bi3+ ionic pairs which could improve the piezoelectric constant of the material.
Fig.5. Piezoelectric performance of conventional piezoceramics [3,10,18–29].
Fig. 5 compares the d33 of the PHT-PNN/ 1.0 mol% (Li+-Bi3+) ceramic and those of conventional lead-free and Pb-based ceramics sintered at optimum temperatures. In this study, the d33 (1025 pC/N) of PHT-PNN /1.0 mol% (Li+-Bi3+) was nearly twice that of conventional BZT-BCT [29] ceramics (currently the best lead-free material) and several times higher compared to other lead-free piezoelectrics. When compared with the conventional lead-based family, PHT-PNN/1.0 mol% (Li+-Bi3+) showed similar piezoelectricity to PNN-PZT [20] (d33=1070 pC/N) and PHT-PNN [3] (d33=970 pC/N). However, these systems suffer from very high sintering temperature (1250
). These results indicated that introducing Li+ and Bi3+ into PHT-PNN
ceramics can significantly enhance the piezoelectric properties of the ceramics while reducing their sintering temperature.
Fig. 6. Permittivity (εr) for the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100 ºC measured at 1 kHz as a function of temperature.
Fig. 6 shows the temperature dependence of the dielectric constant (εr) for the PHT-PNN/x mol% (Li+-Bi3+) ceramics (x=0.0, 0.5, 1.0, 1.5, and 2.0 mol%) at 1 kHz sintered at 1100
. The peak of the relative permittivity became more intense and
narrower as x increased. When x=1.0, the dielectric constant peak reached the maximum. According to the dielectric theory, the energy of the ceramic system would decrease by the presence of pores in a non-dense structure, deteriorating the dielectric properties of the ceramic. As revealed by SEM, the content of Li2CO3 and Bi2O3 can increase the density of the ceramics and the grain size significantly, thereby improving the dielectric properties of the PHT-PNN ceramics.
Fig. 7. Permittivity (εr) of the as-prepared PHT-PNN/x mol% (Li+-Bi3+) ceramics as a function of temperature and frequency.
Fig. 7 shows the temperature dependence of the dielectric constant (εr) measured at varying frequencies (100 Hz, 1 kHz, 10 kHz, and 100 kHz) with x=0.0, 0.5, 1.0, 1.5, and 2.0 mol%. Obviously, the temperature of the maximum dielectric constant corresponds to the Curie temperature (TC). Remarkably, the dielectric constant broadened and the temperature of the capacitance peak (Tmax) shifted to higher temperatures with the test frequency, revealing a typical relaxor ferroelectric behavior [30–32].
Fig. 8. Log(1/εr -1/εm) and log(T-Tm) curves of the PHT-PNN/x mol% (Li+-Bi3+) ceramics.
With the aim to further identify the effect of the doping loading on the diffuse phase transformation behavior of the PHT-PNN ceramics, the dielectric constant temperature plots were fitted and analyzed by using the Uchino and Nomura functions [33]. Fig. 8 shows the log(1/εr -1/εm) and log(T-Tm) curves for the different samples. All samples showed a linear relationship, and the slope of the fitting line provides the diffusion factor (γ). When γ approaches 1, the system behave like a conventional ferroelectric system with a sharp phase transformation, while γ close to 2 was characteristic of a relaxation system with a diffuse phase transformation. As can be seen in Fig. 8, both the pure and doped ceramics were relaxation ferroelectrics. Relaxation ferroelectric systems are well-known for having superior piezoelectric properties, which result from the polar nano-regions formed by the disordered arrangement of B-site ions on the octahedral sublattices of the ABO3 perovskite
structure. These polar nano-regions have small domains that facilitate domain wall motion and polarization rotation, thereby improving the piezoelectric properties. Additionally, γ showed a maximum of 1.84 for 0.5 mol% Li2CO3 and Bi2O3 contents and decreased thereafter. This increase of γ with the Li2CO3 and Bi2O3 contents can be attributed to the positional disorder of A-site ions which favor the diffuse phase transformation [34]. The sample with 1.0 mol% Li2CO3 and Bi2O3 showed a γ of 1.82, which is slightly lower than that of the 0.5 mol% sample. However, the sample with 1.0 mol% Li2CO3 and Bi2O3 have better grain homogeneity than the 0.5 mol% sample. Thus, considering the grain homogeneity and relaxation, the sample with 1.0 mol% Li2CO3 and Bi2O3 showed the best performance among the samples tested herein.
4. Conclusions In summary, the effects of Li2CO3 and Bi2O3 addition on the microstructure and electrical properties of PHT-PNN ternary ceramics were studied by preparing samples with conventional processing routes at a low sintering temperature of 1100
. We
found that Li+ and Bi3+ diffused into the lattice (despite the low solid solubility) and generated lattice distortion on the PHT-PNN ceramics. Li2CO3 and Bi2O3 doping also promoted densification and ensured homogeneous growth of grains by inducing the formation of a liquid phase. The 1.0 mol% Li2CO3 and 1.0 mol% Bi2O3 doped PHT-PNN ceramics showed optimum piezoelectric and dielectric properties. We suggest that the ceramics herein prepared are promising candidates for base-metal electrode multilayer device applications.
Acknowledgments
The authors greatly acknowledge financial support from the National Natural Science Foundation of China (Projects 51602240, 61701369, and 51701147), the National Natural Science Foundation of the Shaanxi Province (Projects 2018JM5060, 2017JQ5112, and 2018JM6070), and the Fundamental Research Funds for the Central Universities of China (Project JB181406).
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Figure captions Fig. 1. XRD patterns of the PHT-PNN/x mol% (Li+-Bi3+) ceramics. Fig. 2. SEM images of the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100
: (a) x=0, (b)
x=0.5, (c) x=1.0, (d) x=1.5, and (e) x =2.0. Fig. 3. Grain size distributions of the PHT-PNN/x mol% (Li+-Bi3+) ceramics: (a) x=0, (b) x=0.5, (c) x=1.0, (d) x=1.5, and (e) x=2.0. Fig. 4. d33 of the 1.0 mol% (Li+-Bi3+) doped and un-doped PHT-PNN ceramics as a function of the sintering temperature. Fig. 5. Piezoelectric performance of conventional piezoceramics [3,10,18–29]. Fig. 6. Permittivity (εr) at 1 kHz of the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100 as a function of temperature. Fig. 7. Permittivity (εr) of the as-prepared PHT-PNN/x mol% (Li+-Bi3+) ceramics as a function of the temperature and the frequency. Fig. 8. Log(1/εr -1/εm) and log(T-Tm) curves of the PHT-PNN/x mol% (Li+-Bi3+) ceramics.
Declaration of interests 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)33029-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.166
Reference:
CERI 23234
To appear in:
Ceramics International
Received Date: 29 September 2019 Revised Date:
16 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Y. Yan, Z. Li, Y. Xia, M. Zhao, M. Zhang, D. Zhang, Ultra-high piezoelectric and dielectric properties of low-temperature-sintered lead hafnium titanate-lead niobium nickelate ceramics, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.166. 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. © 2019 Published by Elsevier Ltd.
Ultra-high piezoelectric and dielectric properties of low-temperature-sintered lead hafnium titanate-lead niobium nickelate ceramics Yangxi Yan1, Zhimin Li1*, Yushun Xia1, Mo Zhao2*, Maolin Zhang1, Dongyan Zhang1 1
2
School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071, China
State Key Laboratory of Intense Pulsed Radiation Simulation and Effect (Northwest Institute of Nuclear Technology), Xi’an 710024, China
Abstract Equimolar Li+ and Bi3+ co-doped 0.51Pb(Hf0.3Ti0.7)O3-0.49Pb(Nb2/3Ni1/3)O3 (PHT-PNN) piezoelectric ceramics were synthesized by conditional solid state method. The effects of Li+-Bi3+ co-doping on crystalline structure, morphological structure, piezoelectric and dielectric properties of PHT-PNN ceramics were systematically analyzed. Results revealed that Pb sites of PHT-PNN were replaced with Li+-Bi3+ after co-doping. Li+-Bi3+ co-doping also improved electrical performance of ceramics, formed liquid phase, and decreased sintering temperature during sintering process. Li+-Bi3+ co-doping resulted in ultra-high piezoelectric properties (e.g., d33=1025 pC/N and εr=7974) for PHT-PNN/1.0 mol% Li+-Bi3+ ceramic sintered at 1100
. Furthermore,
detailed mechanism responsible for improved piezoelectric response of Li+-Bi3+ co-doped ceramics was proposed. This study reports the synthesis of ultra-high performance piezoelectric ceramics with low-temperature co-firing properties, which is highly desirable for wide range of applications. Keywords: ultra-high piezoelectric performance; Li+-Bi3+co-doping; low sintering temperature
*
Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (M. Zhao)
1. Introduction Nowadays, lead-free piezoelectric ceramics such as K0.5Na0.5NbO3 (KNN) and BaTiO3 ceramics are commonly used in replacement of lead-based piezoelectric ceramics. However, these lead-free ceramics are costly and their dielectric and piezoelectric properties are not better than those of lead-based ceramics at present. In this sense, lead-based piezoelectric ceramics such as PbZr0.5Ti0.5O3 (PZT) [1], 0.55Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3 (PNN-PZT) [2] or other related ceramics have been utilized as electronic materials in various devices including precision actuators, piezoelectric transformers, and smart sensors. Recently, the multilayer piezoelectric devices have been increasingly developed, which require the employed materials to have excellent properties, such as high piezoelectric constant, high
dielectric
constant
and
low
dielectric
dissipation
factor.
Pb(Hf,Ti)O3-Pb(Ni,Nb)O3 ceramics have attracted wide attention due to their ultra-high piezoelectric properties. These materials showed good piezoelectric and dielectric constants of 970 pC/N and 6000, respectively [3]. However, the high sintering temperature (~1250 oC) restrains them to use as the multilayer piezoelectric materials. In addition, lead volatilization issues have been increasingly considered in view of the ever-increasing environmental protection awareness. Thus, there is a pressing need to develop ceramics with high piezoelectricity and low sintering temperature such that lead volatilization can be mitigated. This can be achieved by ion doping since the sintering temperature and electrical properties of the ceramics are highly
sensitive to ion substitution. For instance, piezoelectric ceramics are usually doped with donor or acceptor ions to tailor their piezoelectric and dielectric properties. Dopants such as Li+, Bi3+, Li+-Al3+, Li+-Bi3+, and Al3+-N3- (A-sites) can significantly change various properties of the ceramics [4–8]. Lei et al. revealed that Li+ cations occupied A sites of BaTiO3-Bi0.5(Na,K)0.5TiO3 ceramics upon addition of Li2CO3 below 0.75 mol%, while B sites are occupied at higher concentrations [9]. Saito et al. found that the piezoelectric performance of KNN ceramics could be enhanced by doping with LiTaO3 and LiSbO3 [10]. Additionally, Zeng et al. found that Bi3+-Fe3+ co-doping can remarkably improve the piezoelectric and ferroelectric properties of PZT ceramics, resulting in materials with large strains [11]. Limpichaipanit et al. prepared Li+ and Bi3+ co-doped Pb0.91La0.09Zr0.65Ti0.35O3 (PLZT) ceramics with a high dielectric constant of 7819 [12]. Despite this progress, to the best of our knowledge, Li+-Bi3+ co-doped PHT-PNN piezoelectric ceramics have not been prepared yet. Therefore, in this work, we synthesized high-performance piezoelectric ceramics sintered at low temperature by co-doping PHT-PNN with Li+-Bi3+cations. Owing to similar ionic radii of the host and replacing ions, Li+-Bi3+cations tend to occupy the A sites of the perovskite structure [13]. The effects of Li+-Bi3+ co-doping on the microstructure, sintering temperature, piezoelectric and dielectric properties of the ceramics were analyzed. Finally, we believe that our research can be very important for developing PHT-PNN ceramics with improved piezoelectricity.
2. Experimental procedures The materials studied herein were 0.51Pb(Hf0.3Ti0.7)O3-0.49Pb(Nb2/3Ni1/3)O3/x
mol% (Li+-Bi3+) ceramics with x ranging from 0 to 2. Firstly, high-purity nickel oxide and niobium pentoxide were used as raw materials to synthesize the nickel niobate (NiNb2O6) precursor at 1100 oC for 6 h. Secondly, stoichiometric amounts of Pb3O4 (99.9%), HfO2 (99.99%), TiO2 (99.9%), Bi2O3 (99.9%), Li2CO3 (99.0%) and the as-prepared NiNb2O6 were weighed and milled for 15 h. The ball milled mixture was calcined for 2 h at 850 oC before preparing disks of 12 mm in diameter by uniaxial pressing (200 MPa). The compacted sample was immersed in lead tetroxide powder and then calcined for 3 h at 900–1300 oC. Next, the sintered samples were ground to 1 mm thick and coated at room temperature with silver slurry on the upper and lower surfaces. Finally, the sample was polarized for 30 min under DC electric field (30 kV/cm) before testing its performance. The crystal structure of the specimens was assessed by XRD (Bruker D8 Advance) in a diffractometer equipped with Cu Kα radiation. The surface morphology was observed by scanning electron microscopy (SEM, JEOL JSM 6700F). The piezoelectric constant (d33) was measured on a piezoelectric-d33 device (ZJ-3A, Chinese Academy of Science). The dielectric-temperature curves were obtained on a high-temperature dielectric spectrometer (HDMS-1000) manufactured by Partulab Co., Ltd.
3. Results and discussion
Fig. 1. XRD patterns of the PHT-PNN /x mol% (Li+-Bi3+) ceramics.
Fig. 1 presents the XRD patters (2θ: 15–78º) of the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100 oC. As can be seen, the peaks of all the prepared samples indicate that the crystalline phase is the rhombohedral perovskite with space group symmetries of R3m. XRD also revealed that Li+ and Bi3+ ions were completely incorporated into the PHT-PNN lattice during the sintering process since no secondary phases were detected. In order to further study the effect of Li+ and Bi3+ ions co-doping on the crystal structure of the PHT-PNN ceramics, we analyzed the 2θ=37–41° region in detail (Fig. 1(b)) for the PHT-PNN/Li+-Bi3+ ceramics. The (111) peak clearly shifted towards higher diffraction angles as the Li+ and Bi3+ contents increased, revealing a slight decrease of the lattice constant calculated by the Bragg formula. This decrease resulted from the lower ionic radii of the Li+ and Bi3+ substituents compared to Pb2+.
Fig. 2. SEM images of the PHT-PNN /x mol% (Li+-Bi3+) ceramics sintered at 1100 oC:(a) x=0, (b) x= 0.5, (c) x=1.0, (d) x=1.5, (e) x=2.0.
Fig. 2 shows SEM micrographs of the PHT-PNN/x mol% (Li+-Bi3+) ceramics. As shown in Fig. 2(a), the un-doped sample contained grains with different sizes and many pores distributed at the grain boundaries, revealing insufficient sintering. In contrast, for doping concentrations up to 1.0 mol%, the samples showed a denser structure and a more uniform grain size distribution centered at 2～2.5 µm (Fig. 3(c)). However, the average grain size of pure PHT-PNN was only 0.9 µm (Fig. 3(a)). Thus, the addition of Li2CO3 and Bi2O3 favored grain growth to a large extent. Li2CO3 and Bi2O3 were added to PHT-PNN ceramics to obtain dense samples with low sintering temperatures. Transition liquid phase sintering was the dominant low-temperature sintering mechanism [14, 15]. Bi2O3 and Li2CO3 are known to form a liquid phase at ca. 690 oC [16]. This liquid phase wetted and covered the surface of the grains, thereby facilitating dissolution and migration of substances. Accordingly, the formation of this liquid phase would favor densification while lowering the sintering temperature of the ceramics. Moreover, the sintering of the liquid phase
promoted grain growth, as shown in Fig. 3.
Fig. 3. Grain size distributions of the PHT-PNN /x mol% (Li+-Bi3+) ceramics: (a) x=0, (b) x=0.5, (c) x=1.0, (d) x=1.5, (e) x=2.0.
Fig. 4. d33 for the 1 mol% (Li+-Bi3+) doped and un-doped PHT-PNN ceramics as a function of the sintering temperature.
Fig. 4 shows the piezoelectric constant d33 for the 1.0 mol% (Li+-Bi3+) doped PHT-PNN ceramics as a function of the sintering temperature. The doped samples showed remarkably higher d33 values compared to the un-doped sample. The d33 of the doped specimens increased fast with the sintering temperature and reached a
maximum. This maximum value is higher than that the optimum values reported for PHT-PNN ceramics [3]. Additionally, the sintering temperature was lowered by ca. 150 oC, compared with the sintering temperature of un-doped PHT-PNN ceramics (1250 oC). The observed variations in the piezoelectric constant of our samples might be caused by the grain size and the defect dipole (Li+-Bi3+ ionic pairs). Okazaki et al. found that the d33 increased with the increase of grain size [17]. Owing to the grain size increases, the grain boundary phase content associated with the space-charge volume will decrease, and the “pinning effect” on the domain motion also decrease, which makes the domain easily switch and results in high piezoelectric performance. Feng et al. reported that doped smaller acceptor and donor ions substituting bivalent A-sites are utilized to bring local lattice distortion and lower symmetry and greatly improve the piezoelectric properties of PbTiO3-based ceramics [6]. Thus, the smaller radii of Li+ and Bi3+ could occupy the A sites of the perovskite structure by substituting Pb2+ in the PHT-PNN ceramics, and formed the Li+-Bi3+ ionic pairs which could improve the piezoelectric constant of the material.
Fig.5. Piezoelectric performance of conventional piezoceramics [3,10,18–29].
Fig. 5 compares the d33 of the PHT-PNN/ 1.0 mol% (Li+-Bi3+) ceramic and those of conventional lead-free and Pb-based ceramics sintered at optimum temperatures. In this study, the d33 (1025 pC/N) of PHT-PNN /1.0 mol% (Li+-Bi3+) was nearly twice that of conventional BZT-BCT [29] ceramics (currently the best lead-free material) and several times higher compared to other lead-free piezoelectrics. When compared with the conventional lead-based family, PHT-PNN/1.0 mol% (Li+-Bi3+) showed similar piezoelectricity to PNN-PZT [20] (d33=1070 pC/N) and PHT-PNN [3] (d33=970 pC/N). However, these systems suffer from very high sintering temperature (1250
). These results indicated that introducing Li+ and Bi3+ into PHT-PNN
ceramics can significantly enhance the piezoelectric properties of the ceramics while reducing their sintering temperature.
Fig. 6. Permittivity (εr) for the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100 ºC measured at 1 kHz as a function of temperature.
Fig. 6 shows the temperature dependence of the dielectric constant (εr) for the PHT-PNN/x mol% (Li+-Bi3+) ceramics (x=0.0, 0.5, 1.0, 1.5, and 2.0 mol%) at 1 kHz sintered at 1100
. The peak of the relative permittivity became more intense and
narrower as x increased. When x=1.0, the dielectric constant peak reached the maximum. According to the dielectric theory, the energy of the ceramic system would decrease by the presence of pores in a non-dense structure, deteriorating the dielectric properties of the ceramic. As revealed by SEM, the content of Li2CO3 and Bi2O3 can increase the density of the ceramics and the grain size significantly, thereby improving the dielectric properties of the PHT-PNN ceramics.
Fig. 7. Permittivity (εr) of the as-prepared PHT-PNN/x mol% (Li+-Bi3+) ceramics as a function of temperature and frequency.
Fig. 7 shows the temperature dependence of the dielectric constant (εr) measured at varying frequencies (100 Hz, 1 kHz, 10 kHz, and 100 kHz) with x=0.0, 0.5, 1.0, 1.5, and 2.0 mol%. Obviously, the temperature of the maximum dielectric constant corresponds to the Curie temperature (TC). Remarkably, the dielectric constant broadened and the temperature of the capacitance peak (Tmax) shifted to higher temperatures with the test frequency, revealing a typical relaxor ferroelectric behavior [30–32].
Fig. 8. Log(1/εr -1/εm) and log(T-Tm) curves of the PHT-PNN/x mol% (Li+-Bi3+) ceramics.
With the aim to further identify the effect of the doping loading on the diffuse phase transformation behavior of the PHT-PNN ceramics, the dielectric constant temperature plots were fitted and analyzed by using the Uchino and Nomura functions [33]. Fig. 8 shows the log(1/εr -1/εm) and log(T-Tm) curves for the different samples. All samples showed a linear relationship, and the slope of the fitting line provides the diffusion factor (γ). When γ approaches 1, the system behave like a conventional ferroelectric system with a sharp phase transformation, while γ close to 2 was characteristic of a relaxation system with a diffuse phase transformation. As can be seen in Fig. 8, both the pure and doped ceramics were relaxation ferroelectrics. Relaxation ferroelectric systems are well-known for having superior piezoelectric properties, which result from the polar nano-regions formed by the disordered arrangement of B-site ions on the octahedral sublattices of the ABO3 perovskite
structure. These polar nano-regions have small domains that facilitate domain wall motion and polarization rotation, thereby improving the piezoelectric properties. Additionally, γ showed a maximum of 1.84 for 0.5 mol% Li2CO3 and Bi2O3 contents and decreased thereafter. This increase of γ with the Li2CO3 and Bi2O3 contents can be attributed to the positional disorder of A-site ions which favor the diffuse phase transformation [34]. The sample with 1.0 mol% Li2CO3 and Bi2O3 showed a γ of 1.82, which is slightly lower than that of the 0.5 mol% sample. However, the sample with 1.0 mol% Li2CO3 and Bi2O3 have better grain homogeneity than the 0.5 mol% sample. Thus, considering the grain homogeneity and relaxation, the sample with 1.0 mol% Li2CO3 and Bi2O3 showed the best performance among the samples tested herein.
4. Conclusions In summary, the effects of Li2CO3 and Bi2O3 addition on the microstructure and electrical properties of PHT-PNN ternary ceramics were studied by preparing samples with conventional processing routes at a low sintering temperature of 1100
. We
found that Li+ and Bi3+ diffused into the lattice (despite the low solid solubility) and generated lattice distortion on the PHT-PNN ceramics. Li2CO3 and Bi2O3 doping also promoted densification and ensured homogeneous growth of grains by inducing the formation of a liquid phase. The 1.0 mol% Li2CO3 and 1.0 mol% Bi2O3 doped PHT-PNN ceramics showed optimum piezoelectric and dielectric properties. We suggest that the ceramics herein prepared are promising candidates for base-metal electrode multilayer device applications.
Acknowledgments
The authors greatly acknowledge financial support from the National Natural Science Foundation of China (Projects 51602240, 61701369, and 51701147), the National Natural Science Foundation of the Shaanxi Province (Projects 2018JM5060, 2017JQ5112, and 2018JM6070), and the Fundamental Research Funds for the Central Universities of China (Project JB181406).
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Figure captions Fig. 1. XRD patterns of the PHT-PNN/x mol% (Li+-Bi3+) ceramics. Fig. 2. SEM images of the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100
: (a) x=0, (b)
x=0.5, (c) x=1.0, (d) x=1.5, and (e) x =2.0. Fig. 3. Grain size distributions of the PHT-PNN/x mol% (Li+-Bi3+) ceramics: (a) x=0, (b) x=0.5, (c) x=1.0, (d) x=1.5, and (e) x=2.0. Fig. 4. d33 of the 1.0 mol% (Li+-Bi3+) doped and un-doped PHT-PNN ceramics as a function of the sintering temperature. Fig. 5. Piezoelectric performance of conventional piezoceramics [3,10,18–29]. Fig. 6. Permittivity (εr) at 1 kHz of the PHT-PNN/x mol% (Li+-Bi3+) ceramics sintered at 1100 as a function of temperature. Fig. 7. Permittivity (εr) of the as-prepared PHT-PNN/x mol% (Li+-Bi3+) ceramics as a function of the temperature and the frequency. Fig. 8. Log(1/εr -1/εm) and log(T-Tm) curves of the PHT-PNN/x mol% (Li+-Bi3+) ceramics.
Declaration of interests 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: