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Enhanced energy storage and dielectric properties of Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 anti-ferroelectric ceramics
Journal of Alloys and Compounds 664 (2016) 632e638
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Enhanced energy storage and dielectric properties of Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 anti-ferroelectric ceramics Guocai Liu, Huiqing Fan*, Guangzhi Dong, Jing Shi, Qi Chang State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 December 2015 Received in revised form 30 December 2015 Accepted 31 December 2015 Available online 4 January 2016
In order to hunt for the promising materials in energy-storage field, we prepared a series of compositions Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 (BNTC1000x, x ¼ 1.8, 2.2, 2.6, 3.0 wt%) by the solidestate reaction method. CeO2 addition degraded the ferroelectricity of initial components and all these samples exhibited antiferroelectricity at room temperature. The optimum energy-storage density of 0.94 J/cm3 was obtained at BNTC22 with the external electric field of 75 kV/cm. In addition, the energy-storage property not only possessed good temperature stability but also maintained high values over 0.81 J/ cm3 around 20e120 C. These characteristics indicated that Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 system could be the promising lead-free materials for energy-storage capacitor application. The investigation on dielectric properties suggested that the dielectric relaxation behavior was involved in BNTC1000x system and concerned with oxygen vacancy migration. © 2016 Elsevier B.V. All rights reserved.
Keywords: Energy storage materials Temperature stability Dielectric relaxation Ceramics
1. Introduction Now piezoelectric ceramics are widely used in electronic components for its excellent piezoelectric and dielectric properties. Much attention is being attracted to high-energy storage materials for their promising applications and the demands of miniaturization scale in the electronic devices [1e3]. Among various dielectric materials, antiferroelectric (AFE) lead-based materials have larger energy storage density compared to the ferroelectric (FE) materials and the linear dielectric materials [4,5]. That's due to the small remnant polarization (Pr) and large chargeedischarge displacements. However, the promising AFE materials such as Pb(Zr,Ti)O3, Pb(Zr,Sn,Ti)O3 and (Pb,La)ZrO3 [6,7] that have been extensively studied are mostly lead-based. The toxicity of the lead is causing serious environmental problems. Therefore, the environmentfriendly materials with high energy storage densities must be investigated for future development. The lead-free piezoelectric material, (Bi0.5Na0.5)TiO3 (BNT) had drawn much attention in recent years for its strong ferroelectric properties. BNT is also a promising candidate for energy-storage application near depolarization temperature (Td) due to the large polarization and low Pr [8]. BNT has a ferroelectric (FE)
rhombohedral phase at room temperature and shows a AFE-like behavior at depolarization temperature Td [9,10]. This behavior was clarified with a “non-polar” phase, which was confirmed an ergodic relaxor state [11e13]. Dorcet V. et al. has justified Rhombohedral R3c and tetragonal P4bm nanoregions merged in cubic matrix in very wide temperature range [14]. Tetragonal P4bm is weakly polar phase or non-polar phase, which can transform to polar ferroelectric state under the application of electric field and turn back to the initial state once the external field is removed. The pinched P-E loops with high maximum polarization (Pmax) and low remnant polarization (Pr) would be induced by this process, which is beneficial for application in energy storage field. Many researchers have done a lot of jobs to search a high energy density material. Zhang et al. [15] lowered Td by introducing (K0.5Na0.5) NbO3, BaTiO3 into BNT system and obtained AFE featured materials with “slanted” P-E hysteresis loops at room temperature (RT). Ni et al. reported that a simple compound KNbO3 can induce a decreased depolarization temperature Td for BNBT ceramics, leading to a small remnant polarization in a wide temperature range [16]. The energy storage capacity of ceramics can be calculated by the Eq. (1), Pmax Z
W1 ¼ * Corresponding author. E-mail address: [email protected] (H. Fan). http://dx.doi.org/10.1016/j.jallcom.2015.12.260 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Edp Pr
(1)
G. Liu et al. / Journal of Alloys and Compounds 664 (2016) 632e638
The energy-storage density W1 is obtained by integrating the area between the polarization axis and the discharge curve of the unipolar P-E hysteresis loops. E is the electric field that is applied in the materials. The maximum of polarization Pmax and remnant polarization Pr are obtained from the discharge curve of the unipolar P-E hysteresis loops. On the contrary, the energy loss density W2 is calculated by integrating the area between the charge and discharge curve of the unipolar P-E hysteresis loops. The system Bi0.487Na0.427K0.06Ba0.026TiO3 studied by Shieh et al. [17] suggested that this composition situated at the morphotropic phase boundary (MPB), possessing relative high dielectric constant (ε0 ). The large ε0 was beneficial to improve energy storage density. In addition, ternary system Bi0.487Na0.427K0.06Ba0.026TiO3 typically exhibited higher Curie temperature than BNT and BNBT. According to our previous work [18], introducing CeO2 into BNT system would decrease Td. The AFE featured materials with “slanted” P-E hysteresis loops was appeared at room temperature. But the high energy-storage density wasn't obtained in that work. In this paper, the appropriate amount of CeO2 different from prior work was doped into Bi0.487Na0.427K0.06Ba0.026TiO3 system to search the promising material for energy storage. The effects of CeO2 on the microstructure, dielectric properties and energy density of Bi0.487Na0.427K0.06Ba0.026TiO3 system were systematically investigated. 2. Experimental The solidestate reaction method was used to prepare the Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 (x ¼ 1.8, 2.2, 2.6, 3.0 wt%, abbreviated as BNTC1000x) ceramics using BaCO3 (99.0%), Bi2O3 (99.9%), TiO2 (99.0%), K2CO3 (99.0%), Na2CO3 (99.8%) and CeO2 (99.99%) as starting materials. The starting materials were mixed in alcohol by ball-milling for 24 h. Then the dried slurries were calcined at 830 C for 1.5 h in a covered alumina crucible and ballmilled again for 24 h. The dried slurries were cold isostatically pressed into pellets of 12 mm in diameter at a pressure of 250 MPa. Finally the pellets were sintered at 1150 C for 2 h. Crystalline structure of the crushing sintered ceramics was examined by powder X-ray diffraction (XRD) and data were collected by using an automated diffractometer (X'Pert PRO MPD, Philips, Eindhoven, The Netherlands) with a nickel monochromator (Cu Ka radiation) in 2q range of 20e70 at ambient temperature. Morphological characterizations were analyzed by scanning electron microscope (SEM; S3500N, Hitachi, Tokyo, Japan). The resulting pellets were polished to a final thickness around 0.6 mm for property investigations. Electric measurements were carried out on polished ceramics whose both surfaces were coated with sliver electrodes and fired at 550 C for 30 min. The P-E hysteresis loops measurements were carried out at 10 Hz using a standard ferroelectric analyzer (TF-2000, aixACCT, Aachen, Germany) in the temperature range from 20 C to 120 C. The dielectric properties were measured using a precision impedance analyzer (4294A, Agilent, Santa Clara, CA, USA) associated with a temperature controller (TP94, Linkam, Surrey, UK) from 20 C to 430 C at a heating rate of 3 C/min. 3. Results and discussion 3.1. Phase and microstructure analysis XRD results of the BNTC1000x sintered ceramics were presented in Fig. 1. As could be seen from these patterns, all samples showed a single perovskite structure without apparent trace of secondary phases. Fig. 1(b) showed the diffraction lines of all samples arround 2q of 40 in detail. It could be observed that there was no peak
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Fig. 1. XRD patterns of BNTC1000x ceramics with different CeO2 contents.
splitting in (111) which was charactered by tetragonal symmetry. In addition, the X-ray diffraction patterns moved towards the higher diffraction angle with increasing CeO2 contents indicating the CeO2 decreased the lattice parameters. In order to further confirm the chemical composition and oxidationstate, the XPS data of BNTC0 and BNTC22 samples were measured and shown in Fig. 2. The full scanned spectra (Fig. 2(a)) indicated the existent elements K, Ba, Bi, Ti, O, C in BNTC0 with additional Ce in BNTC22. The C 1s peak located at 284.6 eV was used as the reference to calibrate the binding energy of XPS spectra [19]. The Ce 3d XPS spectra of BNTC22 were presented in Fig. 2(b). Two banding energy peaks which located at 899.28 eV, 885.8 eV suggested the element Ce existing in BNTC22 by the state of Ce3þ. The radius of Ce3þ (0.134 nm) was smaller than irons occupied A site (Bi3þ 0.138 nm, Ba2þ 0.161 nm, K1þ 0.164 nm) and larger than irons occupied B site (Ti4þ 0.0605 nm). If Ce3þ occupied A site this would make the X-ray diffraction patterns move towards the high diffraction angle. On the contrary, if Ce3þ occupied B site, the XRD patterns would move towards the low diffraction angle. This could be explained by Bragg Eq. (2),
2d sinq ¼ l
(2)
where d, q, l, were diffraction separation, diffraction angle, X-ray wavelength, respectively. According to our analysis, X-ray diffraction patterns moved towards the high diffraction angle. This was in accordance with Fig. 1(b). Therefore, Ce3þ occupied A site in BNTC22 ceramic. Fig. 3 showed the SEM images of BNTC1000x sintered ceramics. It could be found that the grain size decreased gradually with increasing CeO2 addition. The microstructure appeared homogeneous and nearly no pores without any apparent second phase. It suggested that CeO2 additive was also effective on decreasing grain size and improving the microstructure homogeneity. 3.2. Energy-storage property of BNTC1000x system P-E loops of BNTC1000x ceramics with different CeO2 contents were shown in Fig. 4. It could be seen from Fig. 4(a) that hysteresis loops of BNTC1000x ceramics which were measured under 75 kV/ cm exhibited constricted and almost no ferroelectricity. The hysteresis loops of BNTC26, BNTC30 were more flattened and slanted than that of BNTC18, BNTC22 suggesting the weaker ferroelectric performance of them. Besides, no obvious the remnant polarization Pr could be observed. That indicated all samples exhibited antiferroelectricity at room temperature which was consistent with our
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Fig. 2. XPS spectra of (a) wide-scan for BNTC0, BNTC22 (b) Ce 3d for BNTC22.
Fig. 3. SEM micrographs of the Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 ceramics with x ¼ (a) 1.8, (b) 2.2, (c) 2.6 and (d) 3.0.
Fig. 4. (a) P-E hysteresis loops, (b) the remnant polarization Pr, the maximum polarization Pmax, and (c) the energy storage density W1, the energy loss density W2 of BNTC1000x ceramics measured at 30 C.
G. Liu et al. / Journal of Alloys and Compounds 664 (2016) 632e638
previous results [18]. The value of Pr for sample BNTC22 was 10.2 mC/cm2, much smaller than ~35 mC/cm2 [20]. Fig. 4(c) illustrated energy storage density (W1) and energy loss density (W2) of BNTC1000x system. The energy storage density decreased after first increasing with the increment of CeO2 contents and reached the maximum value of 0.94 J/cm3 at BNTC22. These were also shown in Table 1. The large energy-storage density could be attributed to the degenerated ferroelectricity [8,21] caused by CeO2 addition. Furthermore, the energy loss density W2 decreased continuously with increasing CeO2 addition. It could be noted from Fig. 4(b) that Pr decreased gradually and the maximum polarization Pmax had the maximum value at BNTC22. Therefore, the low Pr together with a large Pmax under electric field of 75 kV/cm were the key factors to improve the energy storage density of the BNTC1000x system. Energy storage efficiency (h) was also an important factor in energy storage field. The h could be obtained from the specific value of W1/ (W1 þ W2) and the values of h for BNTC1000x were shown in Table 1. 3.3. Energy-storage property of BNTC22 in detail To evaluate the potential application of BNTC1000x ceramics in energy storage field, the sample BNTC22 was selected to study the dependence of energy-storage property on applied field, temperature and the results were shown in Fig. 5. The polarization hysteresis loops and the calculated energy density W1 of BNTC22 under different electric fields at 30 C were presented in Fig. 5(a). As the electric field increased, the nonlinearity of P-E loops increased gradually and the maximum polarization was improved from 5 mC/ cm2 to 39.5 mC/cm2 with the applied electric field increasing from 20 kV/cm to 75 kV/cm. It was reasonable to infer that higher electric fields could enhance polarization state. As seen from the inset of Fig. 5(a), the slope of W1-E curves underwent an obviously gradual increment as the electric field increased up to 40 kV/cm and then maintained linearity to the maximum applied electric field. The energy storage density W1 reached 0.94 J/cm3 under at 75 kV/cm which was much higher than the reported values 0.89 J/cm3 (BNBTKN at 100 kV/cm) [8], 0.59 J/cm3 (BNBT-KNN at 56 kV/cm) [22]. That indicated that the higher electric field was favorable for energystorage property and it could be further improved if the breakdown strength was enhanced. The polarization hysteresis loops and the calculated energy density W1 of BNTC22 measured at different temperatures under 70 kV/cm were presented in Fig. 5(b). As the temperature rose, the P-E loops changed more slim and Pr decreased. The energy storage density W1 increased from 0.81 J/cm3 to 0.978 J/cm3 as the temperature rose from 20 C to 100 C and then decreased to 0.963 J/ cm3 at 120 C. Although the W1 varied with the temperature range of 20e120 C, this material possessed the excellent temperature stability of energy-storage density and maintained an acceptable level over 0.8 J/cm3. This feature was attributed to the stable AFE
635
phase over a wide temperature range [10,22]. 3.4. Dielectric constant and dielectric loss Fig. 6 showed the dielectric constant and dielectric loss curves. It could be noted from Fig. 6(a) that every ε0 -T curve had one dielectric anomaly peak corresponding to the maximum ε0 value and this temperature was called Tm. The peak was the AFE to paraelectric phase transition [23]. The dielectric constant and Tm decreased gradually as the CeO2 contents increased with a sharp decreasing at BNTC22. However, there was no alike change in dielectric loss. There was the minimum value of dielectric loss for BNTC22 and this was favorable to the enhancement of energy storage. According to our previous work [18], Td decreased with increasing CeO2 contents and decreased to below room temperature (RT) when x reached up to 1.0%. So our materials in this paper all were antiferroelectrics at RT. As was shown in Fig. 6(b), the broad and frequency dependent peaks in dielectric constant and loss indicated that there was a dielectric relaxation character in BNTC22 ceramic. This feature could also be seen from Fig. 6(c) ε0 dropped with the frequency rising which due to the decrease in net polarization [24,25]. Besides, the dielectric loss varied also with changing the frequency. 3.5. Dielectric relaxation analysis The electric modulus (M0 , M00 )vs. frequency (f) curves and the M00 vs. f normalized plots of BNTC22 ceramic were plotted in Fig. 7. These data could be used to elucidate the relaxation behavior existing in BNTC1000x ceramics which was induced by oxygen vacancy migration [26]. The dielectric relaxation on high temperature could reflect the carriers' movement and help us to explain the defect compensation mechanism [27]. The complex electric modulus (M*) was defined in terms of complex dielectric constant (ε*):
1 M * ¼ ε*
(3) ε
0
M þ iM00 ¼
0
2
0
00
00
ðε Þ þ （ε Þ
2
þi
ε 2
(4)
ðε Þ þ （ε Þ2 0
00
where M0 , M00 and ε0 , ε00 were the real and imaginary parts of the electric modulus and dielectric constants, respectively. M0 , M00 in the temperature range of 510 Ce630 C at different frequency were calculated using Eq. (4) and shown in Fig. 7(a), (b). As frequency rose, M0 increased gradually and reached the maximum at different temperatures, which was corresponding to M∞ ¼ (ε∞)1. M0 value became larger at high temperatures and these characteristics were due to the relaxation process. Besides the M00 peaks shifted towards high frequency and the M00 value increased with increasing temperatures. All these suggested that the temperature-
Table 1 Comparison in energy-storage and ferroelectric properties of BNTC1000x ceramics with others. Composition
W1 (J/cm3)
W2 (J/cm3)
h
E (kV/cm)
Pr (mC/cm2)
Pmax (mC/cm2)
Ref.
BNTC18 BNTC22 BNTC26 BNTC30 0.7BT-0.3BS PNZST 0.94BNBT-0.06 KN BNKT-0.16KNN 0.9BNT-0.1 KN 0.89BNT-0.06BT-0.05KNN
0.81 0.94 0.76 0.75 6.1 0.6 0.89 1.20 1.17 0.59
0.88 0.79 0.58 0.54 e e ~0.34 e ~0.25 e
0.49 0.54 0.54 0.58 e e ~0.72 e ~0.82 e
75 75 75 75 730 56 100 100 104 56
12.2 10.2 7.2 6.9 ~1 ~1 e ~5 e ~3
38.8 39.5 30.1 29.8 ~25 ~27 28 ~34 30 ~27
Current Current Current Current [1] [6] [8] [10] [21] [22]
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Fig. 5. The P-E loops and the calculated energy storage density W1 of BNTC22 as the function of (a) electric fields and (b) temperatures.
Fig. 6. Dielectric constant and dielectric loss as a function of (a) temperature measured at 10 kHz for BNTC1000x ceramics, (b) temperature measured at different frequencies for BNTC22 and (c) frequency measured at room temperature for BNTC22.
dependent relaxation behavior was involved in BNTC1000x ceramics. Long et al. [28] hold the belief that the long-distance migration of oxygen vacancies existed in the frequency region below the M00 peak. Ions were spatially confined to short-distance hopping and oxygen vacancies only migrated in short range above the peak. The normalized curves of M00 vs. f were shown in Fig. 7(c). It could be noted that all curves at different temperatures well overlapped on a single curve. This suggested that the relaxation mechanism kept unchanged at different temperatures in the measured frequency range. Furthermore, the conductivity (s) could be determined from the fitting impedance semicircle, and the lns vs.1000/T plots at temperatures were shown in Fig. 7(d). The activation energy (Ea) for electrical conduction was detected by using the Arrhenius equation:
s ¼ sc exp
Ea kT
(5)
Where s was the dc conductivity, sc the pre-exponent constant, Ea the activation energy, k the Boltzmann's constant, T the absolute temperature. The activation energy values of BNTC1000x could be estimated from the slope of the theoretical fitting line which were shown in inset of Fig. 7(d). All values were in the range of 0.5e2 eV which was closely related to oxygen vacancies. In addition, the BNTC22 sample possessed the minimum value of Ea. This indicated that the relaxation behavior induced by oxygen vacancy migration was more predominant in BNTC22 than other samples for it's the smallest Ea value of 1.42 eV. In comparison with reported papers, this value was larger than 1.41 eV [29], and smaller than 1.49 eV
G. Liu et al. / Journal of Alloys and Compounds 664 (2016) 632e638
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Fig. 7. (a) M0 vs. f plots, (b) M00 vs. f plots, (c) the normalized plots of M00 vs. f for the BNTC22 ceramic (fmax is the M00 peak frequency) and (d) Arrhenius plots of the dc conductivity and the activation energy of BNTC1000x ceramics (the symbols: experimental data, the solid lines: fitting lines).
[30]. 3.6. Conductivity behaviors Variable frequency impedance spectra at 490 C were shown in Fig. 8. The complex impedance spectra of BNTC1000x ceramics with different CeO2 contents were presented in Fig. 8(a). It could be observed that the resistivity decreased after first increased with the doping contents and reached the maximum at BNTC22. There was only a single, substantially undistorted Debye-like semicircle in BNTC22, indicating that this sample contained essentially one electrical component around this temperature. But for BNTC30 sample, two semicircular plots with different radii were observed in the complex impedance spectrum. This phenomenon could be further confirmed by the combined Z00 /M00 spectroscopic plots showed in Fig. 8(b)-(c). The Z00 /M00 spectroscopic curves of BNTC22 showed a single peak in both cases and appeared maxima at similar frequencies, indicating that this sample was electrically homogeneous [31]. However, another shoulder Z00 peak appeared in the Z00 plot of BNTC30 corresponding to the two semicircles in the complex impedance spectrum. The M00 plot showed one single peak
correlated to the higher frequency Z00 peak, which was due to the bulk response [32]. The magnitude of mismatch between the peaks of both parameters represented a change in the apparent polarization. The overlapping of peaks was an evidence of long-range conductivity whereas the difference was an indication of shortrange conductivity [33]. It could be concluded that the Debye-like semicircle at high frequency represented the contributions from grain effect and the semicircle at low frequency was attributed to the grain boundary effect. Based on discussion above, CeO2 not only could enhance energy storage property but also had significant effects on dielectric performance for BNT based ceramics. 4. Conclusions A series of compositions Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 (BNTC1000x, x ¼ 1.8, 2.2, 2.6, 3.0 wt%) were synthesized by the solidestate reaction method. XRD patterns showed that BNTC1000x ceramics all possessed the pure single perovskite phase. According to analysis on XPS and XRD data, the element Ce existed in BNTC22 by the state of Ce3þ and occupied A site. All these samples exhibited antiferroelectricity at room temperature because
Fig. 8. (a) The complex impedance spectra of BNTC1000x ceramics, and the combined Z00 /M00 spectroscopic plots of (b) BNTC22, (c) BNTC30.
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of the CeO2 addition. The calculated results indicated that BNTC22 had a high energy-storage density of 0.94 J/cm3 at 75 kV/cm. In addition, the energy-storage property of this material not only possessed good temperature stability but also maintained the high value over 0.81 J/cm3 in the temperature range of 20e120 C. These results supported that Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 system might be promising lead-free materials for energy-storage capacitor application. It could be noted from impedance spectra that the dielectric relaxation behavior was involved in the BNTC1000x system. This characteristic was associated with oxygen vacancy migration and more predominant in BNTC22 than other samples. Acknowledgments This work was supported by the National Natural Science Foundation (51172187), the SPDRF (20116102130002, 20116102120016) and 111 Program (B08040) of MOE, the Xi'an Science & Technology Foundation (CXY1510-2), the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.
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Enhanced energy storage and dielectric properties of Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 anti-ferroelectric ceramics Guocai Liu, Huiqing Fan*, Guangzhi Dong, Jing Shi, Qi Chang State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 December 2015 Received in revised form 30 December 2015 Accepted 31 December 2015 Available online 4 January 2016
In order to hunt for the promising materials in energy-storage field, we prepared a series of compositions Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 (BNTC1000x, x ¼ 1.8, 2.2, 2.6, 3.0 wt%) by the solidestate reaction method. CeO2 addition degraded the ferroelectricity of initial components and all these samples exhibited antiferroelectricity at room temperature. The optimum energy-storage density of 0.94 J/cm3 was obtained at BNTC22 with the external electric field of 75 kV/cm. In addition, the energy-storage property not only possessed good temperature stability but also maintained high values over 0.81 J/ cm3 around 20e120 C. These characteristics indicated that Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 system could be the promising lead-free materials for energy-storage capacitor application. The investigation on dielectric properties suggested that the dielectric relaxation behavior was involved in BNTC1000x system and concerned with oxygen vacancy migration. © 2016 Elsevier B.V. All rights reserved.
Keywords: Energy storage materials Temperature stability Dielectric relaxation Ceramics
1. Introduction Now piezoelectric ceramics are widely used in electronic components for its excellent piezoelectric and dielectric properties. Much attention is being attracted to high-energy storage materials for their promising applications and the demands of miniaturization scale in the electronic devices [1e3]. Among various dielectric materials, antiferroelectric (AFE) lead-based materials have larger energy storage density compared to the ferroelectric (FE) materials and the linear dielectric materials [4,5]. That's due to the small remnant polarization (Pr) and large chargeedischarge displacements. However, the promising AFE materials such as Pb(Zr,Ti)O3, Pb(Zr,Sn,Ti)O3 and (Pb,La)ZrO3 [6,7] that have been extensively studied are mostly lead-based. The toxicity of the lead is causing serious environmental problems. Therefore, the environmentfriendly materials with high energy storage densities must be investigated for future development. The lead-free piezoelectric material, (Bi0.5Na0.5)TiO3 (BNT) had drawn much attention in recent years for its strong ferroelectric properties. BNT is also a promising candidate for energy-storage application near depolarization temperature (Td) due to the large polarization and low Pr [8]. BNT has a ferroelectric (FE)
rhombohedral phase at room temperature and shows a AFE-like behavior at depolarization temperature Td [9,10]. This behavior was clarified with a “non-polar” phase, which was confirmed an ergodic relaxor state [11e13]. Dorcet V. et al. has justified Rhombohedral R3c and tetragonal P4bm nanoregions merged in cubic matrix in very wide temperature range [14]. Tetragonal P4bm is weakly polar phase or non-polar phase, which can transform to polar ferroelectric state under the application of electric field and turn back to the initial state once the external field is removed. The pinched P-E loops with high maximum polarization (Pmax) and low remnant polarization (Pr) would be induced by this process, which is beneficial for application in energy storage field. Many researchers have done a lot of jobs to search a high energy density material. Zhang et al. [15] lowered Td by introducing (K0.5Na0.5) NbO3, BaTiO3 into BNT system and obtained AFE featured materials with “slanted” P-E hysteresis loops at room temperature (RT). Ni et al. reported that a simple compound KNbO3 can induce a decreased depolarization temperature Td for BNBT ceramics, leading to a small remnant polarization in a wide temperature range [16]. The energy storage capacity of ceramics can be calculated by the Eq. (1), Pmax Z
W1 ¼ * Corresponding author. E-mail address: [email protected] (H. Fan). http://dx.doi.org/10.1016/j.jallcom.2015.12.260 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Edp Pr
(1)
G. Liu et al. / Journal of Alloys and Compounds 664 (2016) 632e638
The energy-storage density W1 is obtained by integrating the area between the polarization axis and the discharge curve of the unipolar P-E hysteresis loops. E is the electric field that is applied in the materials. The maximum of polarization Pmax and remnant polarization Pr are obtained from the discharge curve of the unipolar P-E hysteresis loops. On the contrary, the energy loss density W2 is calculated by integrating the area between the charge and discharge curve of the unipolar P-E hysteresis loops. The system Bi0.487Na0.427K0.06Ba0.026TiO3 studied by Shieh et al. [17] suggested that this composition situated at the morphotropic phase boundary (MPB), possessing relative high dielectric constant (ε0 ). The large ε0 was beneficial to improve energy storage density. In addition, ternary system Bi0.487Na0.427K0.06Ba0.026TiO3 typically exhibited higher Curie temperature than BNT and BNBT. According to our previous work [18], introducing CeO2 into BNT system would decrease Td. The AFE featured materials with “slanted” P-E hysteresis loops was appeared at room temperature. But the high energy-storage density wasn't obtained in that work. In this paper, the appropriate amount of CeO2 different from prior work was doped into Bi0.487Na0.427K0.06Ba0.026TiO3 system to search the promising material for energy storage. The effects of CeO2 on the microstructure, dielectric properties and energy density of Bi0.487Na0.427K0.06Ba0.026TiO3 system were systematically investigated. 2. Experimental The solidestate reaction method was used to prepare the Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 (x ¼ 1.8, 2.2, 2.6, 3.0 wt%, abbreviated as BNTC1000x) ceramics using BaCO3 (99.0%), Bi2O3 (99.9%), TiO2 (99.0%), K2CO3 (99.0%), Na2CO3 (99.8%) and CeO2 (99.99%) as starting materials. The starting materials were mixed in alcohol by ball-milling for 24 h. Then the dried slurries were calcined at 830 C for 1.5 h in a covered alumina crucible and ballmilled again for 24 h. The dried slurries were cold isostatically pressed into pellets of 12 mm in diameter at a pressure of 250 MPa. Finally the pellets were sintered at 1150 C for 2 h. Crystalline structure of the crushing sintered ceramics was examined by powder X-ray diffraction (XRD) and data were collected by using an automated diffractometer (X'Pert PRO MPD, Philips, Eindhoven, The Netherlands) with a nickel monochromator (Cu Ka radiation) in 2q range of 20e70 at ambient temperature. Morphological characterizations were analyzed by scanning electron microscope (SEM; S3500N, Hitachi, Tokyo, Japan). The resulting pellets were polished to a final thickness around 0.6 mm for property investigations. Electric measurements were carried out on polished ceramics whose both surfaces were coated with sliver electrodes and fired at 550 C for 30 min. The P-E hysteresis loops measurements were carried out at 10 Hz using a standard ferroelectric analyzer (TF-2000, aixACCT, Aachen, Germany) in the temperature range from 20 C to 120 C. The dielectric properties were measured using a precision impedance analyzer (4294A, Agilent, Santa Clara, CA, USA) associated with a temperature controller (TP94, Linkam, Surrey, UK) from 20 C to 430 C at a heating rate of 3 C/min. 3. Results and discussion 3.1. Phase and microstructure analysis XRD results of the BNTC1000x sintered ceramics were presented in Fig. 1. As could be seen from these patterns, all samples showed a single perovskite structure without apparent trace of secondary phases. Fig. 1(b) showed the diffraction lines of all samples arround 2q of 40 in detail. It could be observed that there was no peak
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Fig. 1. XRD patterns of BNTC1000x ceramics with different CeO2 contents.
splitting in (111) which was charactered by tetragonal symmetry. In addition, the X-ray diffraction patterns moved towards the higher diffraction angle with increasing CeO2 contents indicating the CeO2 decreased the lattice parameters. In order to further confirm the chemical composition and oxidationstate, the XPS data of BNTC0 and BNTC22 samples were measured and shown in Fig. 2. The full scanned spectra (Fig. 2(a)) indicated the existent elements K, Ba, Bi, Ti, O, C in BNTC0 with additional Ce in BNTC22. The C 1s peak located at 284.6 eV was used as the reference to calibrate the binding energy of XPS spectra [19]. The Ce 3d XPS spectra of BNTC22 were presented in Fig. 2(b). Two banding energy peaks which located at 899.28 eV, 885.8 eV suggested the element Ce existing in BNTC22 by the state of Ce3þ. The radius of Ce3þ (0.134 nm) was smaller than irons occupied A site (Bi3þ 0.138 nm, Ba2þ 0.161 nm, K1þ 0.164 nm) and larger than irons occupied B site (Ti4þ 0.0605 nm). If Ce3þ occupied A site this would make the X-ray diffraction patterns move towards the high diffraction angle. On the contrary, if Ce3þ occupied B site, the XRD patterns would move towards the low diffraction angle. This could be explained by Bragg Eq. (2),
2d sinq ¼ l
(2)
where d, q, l, were diffraction separation, diffraction angle, X-ray wavelength, respectively. According to our analysis, X-ray diffraction patterns moved towards the high diffraction angle. This was in accordance with Fig. 1(b). Therefore, Ce3þ occupied A site in BNTC22 ceramic. Fig. 3 showed the SEM images of BNTC1000x sintered ceramics. It could be found that the grain size decreased gradually with increasing CeO2 addition. The microstructure appeared homogeneous and nearly no pores without any apparent second phase. It suggested that CeO2 additive was also effective on decreasing grain size and improving the microstructure homogeneity. 3.2. Energy-storage property of BNTC1000x system P-E loops of BNTC1000x ceramics with different CeO2 contents were shown in Fig. 4. It could be seen from Fig. 4(a) that hysteresis loops of BNTC1000x ceramics which were measured under 75 kV/ cm exhibited constricted and almost no ferroelectricity. The hysteresis loops of BNTC26, BNTC30 were more flattened and slanted than that of BNTC18, BNTC22 suggesting the weaker ferroelectric performance of them. Besides, no obvious the remnant polarization Pr could be observed. That indicated all samples exhibited antiferroelectricity at room temperature which was consistent with our
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Fig. 2. XPS spectra of (a) wide-scan for BNTC0, BNTC22 (b) Ce 3d for BNTC22.
Fig. 3. SEM micrographs of the Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 ceramics with x ¼ (a) 1.8, (b) 2.2, (c) 2.6 and (d) 3.0.
Fig. 4. (a) P-E hysteresis loops, (b) the remnant polarization Pr, the maximum polarization Pmax, and (c) the energy storage density W1, the energy loss density W2 of BNTC1000x ceramics measured at 30 C.
G. Liu et al. / Journal of Alloys and Compounds 664 (2016) 632e638
previous results [18]. The value of Pr for sample BNTC22 was 10.2 mC/cm2, much smaller than ~35 mC/cm2 [20]. Fig. 4(c) illustrated energy storage density (W1) and energy loss density (W2) of BNTC1000x system. The energy storage density decreased after first increasing with the increment of CeO2 contents and reached the maximum value of 0.94 J/cm3 at BNTC22. These were also shown in Table 1. The large energy-storage density could be attributed to the degenerated ferroelectricity [8,21] caused by CeO2 addition. Furthermore, the energy loss density W2 decreased continuously with increasing CeO2 addition. It could be noted from Fig. 4(b) that Pr decreased gradually and the maximum polarization Pmax had the maximum value at BNTC22. Therefore, the low Pr together with a large Pmax under electric field of 75 kV/cm were the key factors to improve the energy storage density of the BNTC1000x system. Energy storage efficiency (h) was also an important factor in energy storage field. The h could be obtained from the specific value of W1/ (W1 þ W2) and the values of h for BNTC1000x were shown in Table 1. 3.3. Energy-storage property of BNTC22 in detail To evaluate the potential application of BNTC1000x ceramics in energy storage field, the sample BNTC22 was selected to study the dependence of energy-storage property on applied field, temperature and the results were shown in Fig. 5. The polarization hysteresis loops and the calculated energy density W1 of BNTC22 under different electric fields at 30 C were presented in Fig. 5(a). As the electric field increased, the nonlinearity of P-E loops increased gradually and the maximum polarization was improved from 5 mC/ cm2 to 39.5 mC/cm2 with the applied electric field increasing from 20 kV/cm to 75 kV/cm. It was reasonable to infer that higher electric fields could enhance polarization state. As seen from the inset of Fig. 5(a), the slope of W1-E curves underwent an obviously gradual increment as the electric field increased up to 40 kV/cm and then maintained linearity to the maximum applied electric field. The energy storage density W1 reached 0.94 J/cm3 under at 75 kV/cm which was much higher than the reported values 0.89 J/cm3 (BNBTKN at 100 kV/cm) [8], 0.59 J/cm3 (BNBT-KNN at 56 kV/cm) [22]. That indicated that the higher electric field was favorable for energystorage property and it could be further improved if the breakdown strength was enhanced. The polarization hysteresis loops and the calculated energy density W1 of BNTC22 measured at different temperatures under 70 kV/cm were presented in Fig. 5(b). As the temperature rose, the P-E loops changed more slim and Pr decreased. The energy storage density W1 increased from 0.81 J/cm3 to 0.978 J/cm3 as the temperature rose from 20 C to 100 C and then decreased to 0.963 J/ cm3 at 120 C. Although the W1 varied with the temperature range of 20e120 C, this material possessed the excellent temperature stability of energy-storage density and maintained an acceptable level over 0.8 J/cm3. This feature was attributed to the stable AFE
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phase over a wide temperature range [10,22]. 3.4. Dielectric constant and dielectric loss Fig. 6 showed the dielectric constant and dielectric loss curves. It could be noted from Fig. 6(a) that every ε0 -T curve had one dielectric anomaly peak corresponding to the maximum ε0 value and this temperature was called Tm. The peak was the AFE to paraelectric phase transition [23]. The dielectric constant and Tm decreased gradually as the CeO2 contents increased with a sharp decreasing at BNTC22. However, there was no alike change in dielectric loss. There was the minimum value of dielectric loss for BNTC22 and this was favorable to the enhancement of energy storage. According to our previous work [18], Td decreased with increasing CeO2 contents and decreased to below room temperature (RT) when x reached up to 1.0%. So our materials in this paper all were antiferroelectrics at RT. As was shown in Fig. 6(b), the broad and frequency dependent peaks in dielectric constant and loss indicated that there was a dielectric relaxation character in BNTC22 ceramic. This feature could also be seen from Fig. 6(c) ε0 dropped with the frequency rising which due to the decrease in net polarization [24,25]. Besides, the dielectric loss varied also with changing the frequency. 3.5. Dielectric relaxation analysis The electric modulus (M0 , M00 )vs. frequency (f) curves and the M00 vs. f normalized plots of BNTC22 ceramic were plotted in Fig. 7. These data could be used to elucidate the relaxation behavior existing in BNTC1000x ceramics which was induced by oxygen vacancy migration [26]. The dielectric relaxation on high temperature could reflect the carriers' movement and help us to explain the defect compensation mechanism [27]. The complex electric modulus (M*) was defined in terms of complex dielectric constant (ε*):
1 M * ¼ ε*
(3) ε
0
M þ iM00 ¼
0
2
0
00
00
ðε Þ þ （ε Þ
2
þi
ε 2
(4)
ðε Þ þ （ε Þ2 0
00
where M0 , M00 and ε0 , ε00 were the real and imaginary parts of the electric modulus and dielectric constants, respectively. M0 , M00 in the temperature range of 510 Ce630 C at different frequency were calculated using Eq. (4) and shown in Fig. 7(a), (b). As frequency rose, M0 increased gradually and reached the maximum at different temperatures, which was corresponding to M∞ ¼ (ε∞)1. M0 value became larger at high temperatures and these characteristics were due to the relaxation process. Besides the M00 peaks shifted towards high frequency and the M00 value increased with increasing temperatures. All these suggested that the temperature-
Table 1 Comparison in energy-storage and ferroelectric properties of BNTC1000x ceramics with others. Composition
W1 (J/cm3)
W2 (J/cm3)
h
E (kV/cm)
Pr (mC/cm2)
Pmax (mC/cm2)
Ref.
BNTC18 BNTC22 BNTC26 BNTC30 0.7BT-0.3BS PNZST 0.94BNBT-0.06 KN BNKT-0.16KNN 0.9BNT-0.1 KN 0.89BNT-0.06BT-0.05KNN
0.81 0.94 0.76 0.75 6.1 0.6 0.89 1.20 1.17 0.59
0.88 0.79 0.58 0.54 e e ~0.34 e ~0.25 e
0.49 0.54 0.54 0.58 e e ~0.72 e ~0.82 e
75 75 75 75 730 56 100 100 104 56
12.2 10.2 7.2 6.9 ~1 ~1 e ~5 e ~3
38.8 39.5 30.1 29.8 ~25 ~27 28 ~34 30 ~27
Current Current Current Current [1] [6] [8] [10] [21] [22]
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Fig. 5. The P-E loops and the calculated energy storage density W1 of BNTC22 as the function of (a) electric fields and (b) temperatures.
Fig. 6. Dielectric constant and dielectric loss as a function of (a) temperature measured at 10 kHz for BNTC1000x ceramics, (b) temperature measured at different frequencies for BNTC22 and (c) frequency measured at room temperature for BNTC22.
dependent relaxation behavior was involved in BNTC1000x ceramics. Long et al. [28] hold the belief that the long-distance migration of oxygen vacancies existed in the frequency region below the M00 peak. Ions were spatially confined to short-distance hopping and oxygen vacancies only migrated in short range above the peak. The normalized curves of M00 vs. f were shown in Fig. 7(c). It could be noted that all curves at different temperatures well overlapped on a single curve. This suggested that the relaxation mechanism kept unchanged at different temperatures in the measured frequency range. Furthermore, the conductivity (s) could be determined from the fitting impedance semicircle, and the lns vs.1000/T plots at temperatures were shown in Fig. 7(d). The activation energy (Ea) for electrical conduction was detected by using the Arrhenius equation:
s ¼ sc exp
Ea kT
(5)
Where s was the dc conductivity, sc the pre-exponent constant, Ea the activation energy, k the Boltzmann's constant, T the absolute temperature. The activation energy values of BNTC1000x could be estimated from the slope of the theoretical fitting line which were shown in inset of Fig. 7(d). All values were in the range of 0.5e2 eV which was closely related to oxygen vacancies. In addition, the BNTC22 sample possessed the minimum value of Ea. This indicated that the relaxation behavior induced by oxygen vacancy migration was more predominant in BNTC22 than other samples for it's the smallest Ea value of 1.42 eV. In comparison with reported papers, this value was larger than 1.41 eV [29], and smaller than 1.49 eV
G. Liu et al. / Journal of Alloys and Compounds 664 (2016) 632e638
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Fig. 7. (a) M0 vs. f plots, (b) M00 vs. f plots, (c) the normalized plots of M00 vs. f for the BNTC22 ceramic (fmax is the M00 peak frequency) and (d) Arrhenius plots of the dc conductivity and the activation energy of BNTC1000x ceramics (the symbols: experimental data, the solid lines: fitting lines).
[30]. 3.6. Conductivity behaviors Variable frequency impedance spectra at 490 C were shown in Fig. 8. The complex impedance spectra of BNTC1000x ceramics with different CeO2 contents were presented in Fig. 8(a). It could be observed that the resistivity decreased after first increased with the doping contents and reached the maximum at BNTC22. There was only a single, substantially undistorted Debye-like semicircle in BNTC22, indicating that this sample contained essentially one electrical component around this temperature. But for BNTC30 sample, two semicircular plots with different radii were observed in the complex impedance spectrum. This phenomenon could be further confirmed by the combined Z00 /M00 spectroscopic plots showed in Fig. 8(b)-(c). The Z00 /M00 spectroscopic curves of BNTC22 showed a single peak in both cases and appeared maxima at similar frequencies, indicating that this sample was electrically homogeneous [31]. However, another shoulder Z00 peak appeared in the Z00 plot of BNTC30 corresponding to the two semicircles in the complex impedance spectrum. The M00 plot showed one single peak
correlated to the higher frequency Z00 peak, which was due to the bulk response [32]. The magnitude of mismatch between the peaks of both parameters represented a change in the apparent polarization. The overlapping of peaks was an evidence of long-range conductivity whereas the difference was an indication of shortrange conductivity [33]. It could be concluded that the Debye-like semicircle at high frequency represented the contributions from grain effect and the semicircle at low frequency was attributed to the grain boundary effect. Based on discussion above, CeO2 not only could enhance energy storage property but also had significant effects on dielectric performance for BNT based ceramics. 4. Conclusions A series of compositions Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 (BNTC1000x, x ¼ 1.8, 2.2, 2.6, 3.0 wt%) were synthesized by the solidestate reaction method. XRD patterns showed that BNTC1000x ceramics all possessed the pure single perovskite phase. According to analysis on XPS and XRD data, the element Ce existed in BNTC22 by the state of Ce3þ and occupied A site. All these samples exhibited antiferroelectricity at room temperature because
Fig. 8. (a) The complex impedance spectra of BNTC1000x ceramics, and the combined Z00 /M00 spectroscopic plots of (b) BNTC22, (c) BNTC30.
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of the CeO2 addition. The calculated results indicated that BNTC22 had a high energy-storage density of 0.94 J/cm3 at 75 kV/cm. In addition, the energy-storage property of this material not only possessed good temperature stability but also maintained the high value over 0.81 J/cm3 in the temperature range of 20e120 C. These results supported that Bi0.487Na0.427K0.06Ba0.026TiO3-xCeO2 system might be promising lead-free materials for energy-storage capacitor application. It could be noted from impedance spectra that the dielectric relaxation behavior was involved in the BNTC1000x system. This characteristic was associated with oxygen vacancy migration and more predominant in BNTC22 than other samples. Acknowledgments This work was supported by the National Natural Science Foundation (51172187), the SPDRF (20116102130002, 20116102120016) and 111 Program (B08040) of MOE, the Xi'an Science & Technology Foundation (CXY1510-2), the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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