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Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition
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Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition Mouteng Yao, Yongping Pu n, Hanyu Zheng, Lei Zhang, Min Chen, Yongfei Cui School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xuefu-Zhonglu 3, Xi’an 710021, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2016 Received in revised form 24 February 2016 Accepted 25 February 2016
0.475BNT–0.525BCTZ–x wt% MgO ceramics were prepared through a solid–solid state route and their phase structure, microstructure, dielectric and energy storage properties were investigated with the focus on optimizing properties for capacitor applications. The secondary phases of MgO and Mg2TiO4 were observed when the content of MgO is higher than a critical amount (5–7 wt%). Diffuse phase transition was observed due to the large differences of ion valences and sizes in B-site ions and the presence of secondary phases. The dielectric breakdown strength of these ceramics were significantly enhanced along with the addition of MgO, leading to a significant improvement of discharge energy density. The sample with x ¼5 shows a high dielectric breakdown strength of 15.67 kV/mm and the highest maximum discharge energy density of 1.04 J/cm3. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Solid-state reaction Diffuse phase transition Energy storage density Dielectric breakdown strength
1. Introduction Capacitors are a key component in pulsed power technologies due to their fast charge and discharge rate. Interest in high energy storage density dielectric materials has surged recently, mainly driven by the increasing demands for compact electronics [1,2]. At present, polymer or polymer-based composites [3–5], glass–ceramics [6], antiferroelectrics [7,8], and relaxor ferroelectrics [9] are four mainly dielectrics for energy storage under extensive study. Among these materials for energy storage, relaxor ferroelectrics draw more attention recently due to its low remanent polarization which can provide high electric displacement, low loss and nonlinearity. For example, PZN–PMN–PT relaxor thin films prepared by Yao et al. [10] showed a high energy storage density of 15.8 J/cm3. However, this material contains a large number of lead, which has serious impact on the environment and human health. The lead-based materials must be limited to use, with the highlights of the global environmental problems [11–13]. Hence, leadfree relaxor must be developed for energy storage. In the midst of various relaxor ferroelectrics, barium strontium titanate (BST)based ceramics, which possess high dielectric constant and relatively low dielectric loss, are receiving increasing interest for storing energy [1,9,14–16]. However, pure BST ceramics exhibits an energy storage density of 0.3 J/cm3 and a low dielectric breakdown strength of 9 kV/mm [17]. Studies have shown that the dielectric breakdown strength values of BST ceramic can be n
Corresponding author. E-mail address: [email protected] (M. Yao).
enhanced by adopting glasses as additives, and the energy storage density can be increased to 0.89 J/cm3 [16]. Unfortunately, the energy storage density of BST modified with glass is still too low to meet the requirements of practical applications. Huang et al. [9] reported Ba0.4Sr0.6TiO3 ceramic prepared by spark plasma sintering (SPS) possesses a high dielectric breakdown strength of 210 kV/cm and a highest maximum energy storage density of 1.2 J/cm3. However, SPS is expensive and difficult for large-scale production. Thus, it is necessary to explore a new relaxor ferroelectrics possess high energy storage density to replace BST-based ceramics. In our previous work, it was found that 0.475Bi0.5Na0.5TiO3– 0.525Ba0.85Ca0.15Ti0.9Zr0.1O3 (0.475BNT–0.525BCTZ) ceramic showed a large maximum polarization of 25 μC/cm2 and a high energy storage density of 0.86 J/cm3 despite of a low dielectric breakdown strength ( 9 kV/mm). To further improve the energy density of 0.475BNT–0.525BCTZ, the most effective way is to enhance its dielectric breakdown strength. It has been demonstrated that the addition of MgO can significantly improve the dielectric breakdown strength of BST [9,15] due to the extra high dielectric breakdown strength of MgO ( 100 kV/mm). Hence, in this work 0.475BNT–0.525BCTZ–x wt% MgO (x¼ 0, 3, 5, 7) were prepared with the focus on enhancing the dielectric breakdown strength of 0.475BNT–0.525BCTZ and improving its energy storage density.
2. Experiment procedure A conventional solid-state reaction method was used to prepare 0.475BNT–0.525BCTZ–x wt% MgO ceramics. The oxides or carbonates Bi2O3 (99.975% purity), BaCO3 (99.8% purity), CaCO3
http://dx.doi.org/10.1016/j.ceramint.2016.02.155 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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(99.8% purity), Na2CO3 (99.5% purity), TiO2 (99.6% purity), ZrO2 (99.6% purity) and MgO (98.5% purity) (Sinopharm Chemical Reagent Co., Shanghai) were used as starting raw materials. According to their stoichiometric formula, raw materials for BNT and BCTZ were mixed in planetary ball mill using Y2O3-stabilized ZrO2 grinding media for 24 h. After being milled, the mixed powder for BNT and BCTZ synthesis were calcined at 840 °C and 1270 °C for 4 h, respectively. After calcinations, the powder were ball-milled again separately for 24 h. According to the chemical formula 0.475BNT–0.525BCTZ–x wt% MgO (x ¼0, 3, 5 and 7), BNT, BCTZ and MgO powders were weighed and mixed in planetary ball mill for 24 h. The mixture were dried in oven at 80 °C for 24 h and granulated with 5 wt% PVA as a binder, and then pressed into green disks under a pressure of 100 MPa, followed by sintering in air at 1170 °C for 2 h. The phase structures of these ceramics were identified by powder X-ray diffraction (CD-MAX 2200 pc, Rigaku Co., Tokyo, Japan) at a working voltage and current of 40 kV and 30 mA. XRD data was collected in the range of 20-70° with a 0.02° step and scanning speed of 5°/min. The sintered samples were polished and thermally etched at 1020 °C for 10 min to investigate the microstructure using a scanning electron microscope (SEM, S4800, Rigaku Co., Japan). For the electrical measurements, the sintered pellets were ground, polished, and painted with silver paste. Temperature and frequency-dependent permittivity and dielectric loss tangent were measured with an impedance analyzer (E4980A, Agilent, U.S.A.). The data were collected at four frequencies (1 kHz,10 kHz, 100 kHz and 1000 kHz) from room temperature to 400 °C with a heating rate of 2 °C/min. P–E hysteresis loops were obtained using a commercial aix PES setup (Aix ACCT Systems GmbH, Aachen, Germany). The discharge energy storage density J was calculated from the P–E loops and it is equal the area between the polarization axis and the discharge curve, which is given by the following equation:
J=
∫0
Eb
EdP
(1)
where P and E are the polarization and the electric field, Eb is the dielectric breakdown electric field [18,19].
3. Results and discussion Fig. 1(a) shows the XRD patterns of 0.475BNT–0.525BCTZ– x wt% MgO ceramics with various x values. As for the samples, x¼ 0
and x ¼3, only a pure perovskite phase can be identified and no secondary phases were observed, indicating that MgO diffuse into 0.475BNT–0.525BCTZ lattices when its content is low. However, when the MgO content is higher than a critical amount (5–7 wt%), some secondary phases of MgO and Mg2TiO4 will emerge. Li et al. observed the additional phase of MgTiO3 in BST/MgO composite and related this phenomenon to the reaction between BST and MgO [14]. Thus, the secondary phase of Mg2TiO4 in this work could be attributed to the reaction between MgO and the matrix as well. As for the appearance of MgO, it has been reported to be related to its low diffusion rate and tendency of aggregation at grain boundaries [9,15]. For better illustration of (111) and (200) peaks, the enlarged XRD patterns of the two peaks are given in Fig. 1 (b) and (c), respectively. The formation of single (111) and (200) peaks suggests a cubic symmetry as described in the literature [20,21]. With the increasing content of MgO, the (111) and (200) reflection peaks monotonously shift to a lower degree, indicating the expansion of cell volume because of partial substitution of Ti4 þ (r ¼0.061 nm) by Mg2 þ (r ¼0.072 nm) [9,14]. Fig. 2 illustrates the SEM-micrographs of the polished and thermal-etched surfaces of 0.475BNT–0.525BCTZ–x wt% MgO ceramics. A dense microstructure is developed for all these ceramics. Some large-size pores can be found at grain boundaries or inside the grains for the sample with x ¼0, while the pores gradually disappear with the increasing content of MgO. The grain size are calculated by using a linear intercept method, the grain size of pure 0.475BNT–0.525BCTZ sample is 4-10 μm. With the increasing content of MgO, the grain size of the samples are gradually reduced and distribute uniformly. For the specimen with x ¼7, the grain size decreases to 2–5 μm. It has been reported that MgO can inhibit the grain growth due to its low diffusion rate [9,15]. The secondary phases (MgO and Mg2TiO4) can not be seen in the SEM-micrographs, this may be related to its low content. The temperature dependence of permittivity (ε) and dielectric loss tangent (tanδ) for 0.475BNT–0.525BCTZ–x wt% MgO ceramics from room temperature to 400 °C are shown in Fig. 3. A single maxima is clearly discerned in the frequency dependent permittivity curves for all samples. The dielectric anomaly is shifted to higher temperature and the magnitude of permittivity decreases with increasing frequency. The same behavior is observed in dielectric loss tangent curves, but the magnitude of maxima increases with increasing frequency. The features of frequency dependent dielectric properties are normally considered as a fingerprint for relaxor materials [22,23], consistent with other
Fig. 1. (a) XRD patterns of 0.475BNT–0.525BCTZ–x wt% MgO ceramics, the enlarged XRD patterns of (b) (111) and (c) (200) peaks.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Fig. 2. SEM micrographs of the polished and thermal-etched surfaces of 0.475BNT–0.525BCTZ–x wt% MgO ceramics.
Fig. 3. Temperature dependence of permittivity and dielectric loss tangent for 0.475BNT–0.525BCTZ–x wt%MgO ceramics.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Fig. 4. (a) Temperature dependence of permittivity and dielectric loss tangent measured at 1 kHz, (b1)–(b3) Tm, permittivity and dielectric loss tangent (at room temperature) as a function of MgO content.
reports on the relaxor ferroelectrics [24,25]. Moreover, the frequency dependent permittivity curves gradually become a broad plateau-like maxima over a broad temperature range with the increasing content of MgO. The diffuse behavior caused by the addition of MgO in the broad peak can be explained by large differences of ion valences and sizes in B-site ions, disturbing the long range ferroelectric order of ceramics [2]. The temperature dependent permittivity and dielectric loss tangent measured at 1 kHz are summarized in Fig. 4(a). The dielectric peak becomes more flat and the magnitude of permittivity decreases with the increasing content of MgO. The suppression of dielectric anomaly is attributed to the existence of secondary phases [26] as shown in Fig. 1(a). Furthermore, the matrix of 0.475BNT–0.525BCTZ and the secondary phases of MgO and
Mg2TiO4 possess various dielectric responses and Curie temperatures (Tc), leading to the diffuse phase transition and the reduction of permittivity. The temperature at the dielectric maxima (Tm) increases slightly with the increasing content of MgO as presented in Fig. 4(b1). The similar phenomenon was observed in Bi(Mg0.5Ti0.5)O3-modified BNT ceramics [27]. The permittivity and dielectric loss tangent (at room temperature) as a function of the content of MgO are summarised in Figs. 4(b2) and (b3), respectively. The permittivity decreases from 2300 (for x ¼0) to about 1000 (for x Z3) is due to partial substitution of Ti4 þ by Mg2 þ and the presence of secondary phases. The dielectric loss tangent decreases monotonously with the increasing content of MgO, and this is consistent with some other works where the addition of MgO has been verified to reduce the dielectric loss of BST [15,28].
Fig. 5. P–E hysteresis loops measured at room temperature and 1 Hz for 0.475BNT–0.525BCTZ–x wt% MgO ceramics.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Fig. 7. Discharge energy storage density as a function of electric field for 0.475BNT–0.525BCTZ–x wt% MgO ceramics. Fig. 6. (a) Dielectric breakdown field (Eb), (b) discharge energy density (Jd), (c) charge energy density (Jc) and (d) energy efficiency (η) as a function of MgO content.
In addition, a denser microstructure after the addition of MgO also contribute to the decrease of the dielectric loss as illustrated in Fig. 2. Fig. 5 displays the P–E hysteresis loops measured at room temperature and 1 Hz for 0.475BNT–0.525BCTZ-x wt% MgO ceramics just under the electrical field of dielectric breakdown strength, and the green stripe area represents the maximum energy storage density. The dielectric breakdown strength of these ceramics slightly increases as the content of MgO increases as presented in Fig. 6(a). This can be attributed to the extra high dielectric breakdown strength of MgO ( 100 kV/mm) [15], the uniform grain size and the dense microstructure after the addition of MgO. The maximum polarization of 0.475BNT–0.525BCTZ– x wt% MgO ceramics slightly decreases with increasing amount of MgO due to the presence of secondary phases of MgO and Mg2TiO4. The substitution of Ti4 þ (r ¼0.061 nm) by Mg2 þ (r ¼0.072 nm) in the B-site may shorten the Ti–O bond dipole moment and thus lead to the reduction in polarizability of the unit cell. The area enclosed by discharge curve and polarization axis represents the charge energy density and the area of the loop represents the energy loss as shown in Fig. 5. The ratio of discharge energy density to charge energy density is the energy efficiency. The discharge energy density (Jd), charge energy density (Jc), and energy efficiency (η) are summarized in Fig. 6(b), (c) and (d), respectively. The discharge energy density of 0.475BNT– 0.525BCTZ–x wt% MgO ceramics was significantly improved from 0.86 J/cm3 to 1.04 J/cm3 by the addition of MgO due to the great improvement of dielectric breakdown strength. However, the discharge energy density of sample with x ¼7 is lower than that of sample with x ¼5, although it possesses the highest dielectric breakdown strength. When MgO is added to 0.475BNT–0.525BCTZ, the significantly enhancement in dielectric breakdown strength is usually companied by a notable decrease of maximum polarization, hence, the sample with the highest dielectric breakdown strength not always exhibit the maximum discharge energy density. Furthermore, some other lead-free materials with high energy storage density was also reported. For examples, BNT–BT–CZ ceramics prepared by Li et al. [29] showed a high energy storage density of 0.7 J/cm3, a high energy storage density of 0.598 J/cm3 was obtained in BNT–BT–KNN [30] ceramics and BNT–KNN [31] ceramics showed a high energy storage density of 1.2 J/cm3.
The discharge energy density as a function of electric field for 0.475BNT–0.525BCTZ–x wt%MgO ceramics are shown in Fig. 7. When compared at a given electric field, 0.475BNT–0.525BCTZ exhibits the highest discharge energy density due to its largest maximum polarization. However, 0.475BNT–0.525BCTZ undergoes a dielectric breakdown at the electric field of 9 kV/mm and thus the maximum discharge energy density that 0.475BNT–0.525BCTZ can achieve is only 0.86 J/cm3. Therefore, it can be assumed that much higher discharge energy density may be obtained by the improvement of dielectric breakdown strength. In this work, the sample with x ¼5 possesses the largest maximum discharge energy of 1.04 J/cm3 at a field of 15.67 kV/mm. This credit is given to the enhanced dielectric breakdown strength due to the addition of MgO.
4. Conclusions Solid-state route was used to prepare 0.475BNT–0.525BCTZ– x wt% MgO ceramics. MgO can diffuse into the matrix lattices and no secondary phases of MgO and Mg2TiO4 were observed until xZ 5. The dielectric breakdown strength of these ceramics were greatly enhanced with increasing the content of MgO. The highest maximum discharge energy density of 1.04 J/cm3 was obtained with a dielectric breakdown strength of 15.67 kV/mm in 0.475BNT–0.525BCTZ–5 wt% MgO ceramic.
Acknowledgment This research was supported by the National Natural Science Foundation of China (51372144), the Key Program of Innovative Research Team of Shaanxi Province (2014KCT-06).
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Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition Mouteng Yao, Yongping Pu n, Hanyu Zheng, Lei Zhang, Min Chen, Yongfei Cui School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xuefu-Zhonglu 3, Xi’an 710021, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2016 Received in revised form 24 February 2016 Accepted 25 February 2016
0.475BNT–0.525BCTZ–x wt% MgO ceramics were prepared through a solid–solid state route and their phase structure, microstructure, dielectric and energy storage properties were investigated with the focus on optimizing properties for capacitor applications. The secondary phases of MgO and Mg2TiO4 were observed when the content of MgO is higher than a critical amount (5–7 wt%). Diffuse phase transition was observed due to the large differences of ion valences and sizes in B-site ions and the presence of secondary phases. The dielectric breakdown strength of these ceramics were significantly enhanced along with the addition of MgO, leading to a significant improvement of discharge energy density. The sample with x ¼5 shows a high dielectric breakdown strength of 15.67 kV/mm and the highest maximum discharge energy density of 1.04 J/cm3. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Solid-state reaction Diffuse phase transition Energy storage density Dielectric breakdown strength
1. Introduction Capacitors are a key component in pulsed power technologies due to their fast charge and discharge rate. Interest in high energy storage density dielectric materials has surged recently, mainly driven by the increasing demands for compact electronics [1,2]. At present, polymer or polymer-based composites [3–5], glass–ceramics [6], antiferroelectrics [7,8], and relaxor ferroelectrics [9] are four mainly dielectrics for energy storage under extensive study. Among these materials for energy storage, relaxor ferroelectrics draw more attention recently due to its low remanent polarization which can provide high electric displacement, low loss and nonlinearity. For example, PZN–PMN–PT relaxor thin films prepared by Yao et al. [10] showed a high energy storage density of 15.8 J/cm3. However, this material contains a large number of lead, which has serious impact on the environment and human health. The lead-based materials must be limited to use, with the highlights of the global environmental problems [11–13]. Hence, leadfree relaxor must be developed for energy storage. In the midst of various relaxor ferroelectrics, barium strontium titanate (BST)based ceramics, which possess high dielectric constant and relatively low dielectric loss, are receiving increasing interest for storing energy [1,9,14–16]. However, pure BST ceramics exhibits an energy storage density of 0.3 J/cm3 and a low dielectric breakdown strength of 9 kV/mm [17]. Studies have shown that the dielectric breakdown strength values of BST ceramic can be n
Corresponding author. E-mail address: [email protected] (M. Yao).
enhanced by adopting glasses as additives, and the energy storage density can be increased to 0.89 J/cm3 [16]. Unfortunately, the energy storage density of BST modified with glass is still too low to meet the requirements of practical applications. Huang et al. [9] reported Ba0.4Sr0.6TiO3 ceramic prepared by spark plasma sintering (SPS) possesses a high dielectric breakdown strength of 210 kV/cm and a highest maximum energy storage density of 1.2 J/cm3. However, SPS is expensive and difficult for large-scale production. Thus, it is necessary to explore a new relaxor ferroelectrics possess high energy storage density to replace BST-based ceramics. In our previous work, it was found that 0.475Bi0.5Na0.5TiO3– 0.525Ba0.85Ca0.15Ti0.9Zr0.1O3 (0.475BNT–0.525BCTZ) ceramic showed a large maximum polarization of 25 μC/cm2 and a high energy storage density of 0.86 J/cm3 despite of a low dielectric breakdown strength ( 9 kV/mm). To further improve the energy density of 0.475BNT–0.525BCTZ, the most effective way is to enhance its dielectric breakdown strength. It has been demonstrated that the addition of MgO can significantly improve the dielectric breakdown strength of BST [9,15] due to the extra high dielectric breakdown strength of MgO ( 100 kV/mm). Hence, in this work 0.475BNT–0.525BCTZ–x wt% MgO (x¼ 0, 3, 5, 7) were prepared with the focus on enhancing the dielectric breakdown strength of 0.475BNT–0.525BCTZ and improving its energy storage density.
2. Experiment procedure A conventional solid-state reaction method was used to prepare 0.475BNT–0.525BCTZ–x wt% MgO ceramics. The oxides or carbonates Bi2O3 (99.975% purity), BaCO3 (99.8% purity), CaCO3
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Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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(99.8% purity), Na2CO3 (99.5% purity), TiO2 (99.6% purity), ZrO2 (99.6% purity) and MgO (98.5% purity) (Sinopharm Chemical Reagent Co., Shanghai) were used as starting raw materials. According to their stoichiometric formula, raw materials for BNT and BCTZ were mixed in planetary ball mill using Y2O3-stabilized ZrO2 grinding media for 24 h. After being milled, the mixed powder for BNT and BCTZ synthesis were calcined at 840 °C and 1270 °C for 4 h, respectively. After calcinations, the powder were ball-milled again separately for 24 h. According to the chemical formula 0.475BNT–0.525BCTZ–x wt% MgO (x ¼0, 3, 5 and 7), BNT, BCTZ and MgO powders were weighed and mixed in planetary ball mill for 24 h. The mixture were dried in oven at 80 °C for 24 h and granulated with 5 wt% PVA as a binder, and then pressed into green disks under a pressure of 100 MPa, followed by sintering in air at 1170 °C for 2 h. The phase structures of these ceramics were identified by powder X-ray diffraction (CD-MAX 2200 pc, Rigaku Co., Tokyo, Japan) at a working voltage and current of 40 kV and 30 mA. XRD data was collected in the range of 20-70° with a 0.02° step and scanning speed of 5°/min. The sintered samples were polished and thermally etched at 1020 °C for 10 min to investigate the microstructure using a scanning electron microscope (SEM, S4800, Rigaku Co., Japan). For the electrical measurements, the sintered pellets were ground, polished, and painted with silver paste. Temperature and frequency-dependent permittivity and dielectric loss tangent were measured with an impedance analyzer (E4980A, Agilent, U.S.A.). The data were collected at four frequencies (1 kHz,10 kHz, 100 kHz and 1000 kHz) from room temperature to 400 °C with a heating rate of 2 °C/min. P–E hysteresis loops were obtained using a commercial aix PES setup (Aix ACCT Systems GmbH, Aachen, Germany). The discharge energy storage density J was calculated from the P–E loops and it is equal the area between the polarization axis and the discharge curve, which is given by the following equation:
J=
∫0
Eb
EdP
(1)
where P and E are the polarization and the electric field, Eb is the dielectric breakdown electric field [18,19].
3. Results and discussion Fig. 1(a) shows the XRD patterns of 0.475BNT–0.525BCTZ– x wt% MgO ceramics with various x values. As for the samples, x¼ 0
and x ¼3, only a pure perovskite phase can be identified and no secondary phases were observed, indicating that MgO diffuse into 0.475BNT–0.525BCTZ lattices when its content is low. However, when the MgO content is higher than a critical amount (5–7 wt%), some secondary phases of MgO and Mg2TiO4 will emerge. Li et al. observed the additional phase of MgTiO3 in BST/MgO composite and related this phenomenon to the reaction between BST and MgO [14]. Thus, the secondary phase of Mg2TiO4 in this work could be attributed to the reaction between MgO and the matrix as well. As for the appearance of MgO, it has been reported to be related to its low diffusion rate and tendency of aggregation at grain boundaries [9,15]. For better illustration of (111) and (200) peaks, the enlarged XRD patterns of the two peaks are given in Fig. 1 (b) and (c), respectively. The formation of single (111) and (200) peaks suggests a cubic symmetry as described in the literature [20,21]. With the increasing content of MgO, the (111) and (200) reflection peaks monotonously shift to a lower degree, indicating the expansion of cell volume because of partial substitution of Ti4 þ (r ¼0.061 nm) by Mg2 þ (r ¼0.072 nm) [9,14]. Fig. 2 illustrates the SEM-micrographs of the polished and thermal-etched surfaces of 0.475BNT–0.525BCTZ–x wt% MgO ceramics. A dense microstructure is developed for all these ceramics. Some large-size pores can be found at grain boundaries or inside the grains for the sample with x ¼0, while the pores gradually disappear with the increasing content of MgO. The grain size are calculated by using a linear intercept method, the grain size of pure 0.475BNT–0.525BCTZ sample is 4-10 μm. With the increasing content of MgO, the grain size of the samples are gradually reduced and distribute uniformly. For the specimen with x ¼7, the grain size decreases to 2–5 μm. It has been reported that MgO can inhibit the grain growth due to its low diffusion rate [9,15]. The secondary phases (MgO and Mg2TiO4) can not be seen in the SEM-micrographs, this may be related to its low content. The temperature dependence of permittivity (ε) and dielectric loss tangent (tanδ) for 0.475BNT–0.525BCTZ–x wt% MgO ceramics from room temperature to 400 °C are shown in Fig. 3. A single maxima is clearly discerned in the frequency dependent permittivity curves for all samples. The dielectric anomaly is shifted to higher temperature and the magnitude of permittivity decreases with increasing frequency. The same behavior is observed in dielectric loss tangent curves, but the magnitude of maxima increases with increasing frequency. The features of frequency dependent dielectric properties are normally considered as a fingerprint for relaxor materials [22,23], consistent with other
Fig. 1. (a) XRD patterns of 0.475BNT–0.525BCTZ–x wt% MgO ceramics, the enlarged XRD patterns of (b) (111) and (c) (200) peaks.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Fig. 2. SEM micrographs of the polished and thermal-etched surfaces of 0.475BNT–0.525BCTZ–x wt% MgO ceramics.
Fig. 3. Temperature dependence of permittivity and dielectric loss tangent for 0.475BNT–0.525BCTZ–x wt%MgO ceramics.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Fig. 4. (a) Temperature dependence of permittivity and dielectric loss tangent measured at 1 kHz, (b1)–(b3) Tm, permittivity and dielectric loss tangent (at room temperature) as a function of MgO content.
reports on the relaxor ferroelectrics [24,25]. Moreover, the frequency dependent permittivity curves gradually become a broad plateau-like maxima over a broad temperature range with the increasing content of MgO. The diffuse behavior caused by the addition of MgO in the broad peak can be explained by large differences of ion valences and sizes in B-site ions, disturbing the long range ferroelectric order of ceramics [2]. The temperature dependent permittivity and dielectric loss tangent measured at 1 kHz are summarized in Fig. 4(a). The dielectric peak becomes more flat and the magnitude of permittivity decreases with the increasing content of MgO. The suppression of dielectric anomaly is attributed to the existence of secondary phases [26] as shown in Fig. 1(a). Furthermore, the matrix of 0.475BNT–0.525BCTZ and the secondary phases of MgO and
Mg2TiO4 possess various dielectric responses and Curie temperatures (Tc), leading to the diffuse phase transition and the reduction of permittivity. The temperature at the dielectric maxima (Tm) increases slightly with the increasing content of MgO as presented in Fig. 4(b1). The similar phenomenon was observed in Bi(Mg0.5Ti0.5)O3-modified BNT ceramics [27]. The permittivity and dielectric loss tangent (at room temperature) as a function of the content of MgO are summarised in Figs. 4(b2) and (b3), respectively. The permittivity decreases from 2300 (for x ¼0) to about 1000 (for x Z3) is due to partial substitution of Ti4 þ by Mg2 þ and the presence of secondary phases. The dielectric loss tangent decreases monotonously with the increasing content of MgO, and this is consistent with some other works where the addition of MgO has been verified to reduce the dielectric loss of BST [15,28].
Fig. 5. P–E hysteresis loops measured at room temperature and 1 Hz for 0.475BNT–0.525BCTZ–x wt% MgO ceramics.
Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i
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Fig. 7. Discharge energy storage density as a function of electric field for 0.475BNT–0.525BCTZ–x wt% MgO ceramics. Fig. 6. (a) Dielectric breakdown field (Eb), (b) discharge energy density (Jd), (c) charge energy density (Jc) and (d) energy efficiency (η) as a function of MgO content.
In addition, a denser microstructure after the addition of MgO also contribute to the decrease of the dielectric loss as illustrated in Fig. 2. Fig. 5 displays the P–E hysteresis loops measured at room temperature and 1 Hz for 0.475BNT–0.525BCTZ-x wt% MgO ceramics just under the electrical field of dielectric breakdown strength, and the green stripe area represents the maximum energy storage density. The dielectric breakdown strength of these ceramics slightly increases as the content of MgO increases as presented in Fig. 6(a). This can be attributed to the extra high dielectric breakdown strength of MgO ( 100 kV/mm) [15], the uniform grain size and the dense microstructure after the addition of MgO. The maximum polarization of 0.475BNT–0.525BCTZ– x wt% MgO ceramics slightly decreases with increasing amount of MgO due to the presence of secondary phases of MgO and Mg2TiO4. The substitution of Ti4 þ (r ¼0.061 nm) by Mg2 þ (r ¼0.072 nm) in the B-site may shorten the Ti–O bond dipole moment and thus lead to the reduction in polarizability of the unit cell. The area enclosed by discharge curve and polarization axis represents the charge energy density and the area of the loop represents the energy loss as shown in Fig. 5. The ratio of discharge energy density to charge energy density is the energy efficiency. The discharge energy density (Jd), charge energy density (Jc), and energy efficiency (η) are summarized in Fig. 6(b), (c) and (d), respectively. The discharge energy density of 0.475BNT– 0.525BCTZ–x wt% MgO ceramics was significantly improved from 0.86 J/cm3 to 1.04 J/cm3 by the addition of MgO due to the great improvement of dielectric breakdown strength. However, the discharge energy density of sample with x ¼7 is lower than that of sample with x ¼5, although it possesses the highest dielectric breakdown strength. When MgO is added to 0.475BNT–0.525BCTZ, the significantly enhancement in dielectric breakdown strength is usually companied by a notable decrease of maximum polarization, hence, the sample with the highest dielectric breakdown strength not always exhibit the maximum discharge energy density. Furthermore, some other lead-free materials with high energy storage density was also reported. For examples, BNT–BT–CZ ceramics prepared by Li et al. [29] showed a high energy storage density of 0.7 J/cm3, a high energy storage density of 0.598 J/cm3 was obtained in BNT–BT–KNN [30] ceramics and BNT–KNN [31] ceramics showed a high energy storage density of 1.2 J/cm3.
The discharge energy density as a function of electric field for 0.475BNT–0.525BCTZ–x wt%MgO ceramics are shown in Fig. 7. When compared at a given electric field, 0.475BNT–0.525BCTZ exhibits the highest discharge energy density due to its largest maximum polarization. However, 0.475BNT–0.525BCTZ undergoes a dielectric breakdown at the electric field of 9 kV/mm and thus the maximum discharge energy density that 0.475BNT–0.525BCTZ can achieve is only 0.86 J/cm3. Therefore, it can be assumed that much higher discharge energy density may be obtained by the improvement of dielectric breakdown strength. In this work, the sample with x ¼5 possesses the largest maximum discharge energy of 1.04 J/cm3 at a field of 15.67 kV/mm. This credit is given to the enhanced dielectric breakdown strength due to the addition of MgO.
4. Conclusions Solid-state route was used to prepare 0.475BNT–0.525BCTZ– x wt% MgO ceramics. MgO can diffuse into the matrix lattices and no secondary phases of MgO and Mg2TiO4 were observed until xZ 5. The dielectric breakdown strength of these ceramics were greatly enhanced with increasing the content of MgO. The highest maximum discharge energy density of 1.04 J/cm3 was obtained with a dielectric breakdown strength of 15.67 kV/mm in 0.475BNT–0.525BCTZ–5 wt% MgO ceramic.
Acknowledgment This research was supported by the National Natural Science Foundation of China (51372144), the Key Program of Innovative Research Team of Shaanxi Province (2014KCT-06).
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Please cite this article as: M. Yao, et al., Improved energy storage density in 0.475BNT–0.525BCTZ with MgO addition, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.155i