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High energy storage density of 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9−xZr0.1SnxO3 ceramics
Journal of Alloys and Compounds 687 (2016) 689e695
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High energy storage density of 0.55Bi0.5Na0.5TiO30.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 ceramics Yongping Pu a, Mouteng Yao a, *, Lei Zhang a, Panpan Jing b a b
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an, 710021, China Lanzhou University, Lan’zhou, 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2016 Received in revised form 1 June 2016 Accepted 20 June 2016 Available online 21 June 2016
High energy storage properties of 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 (x ¼ 0, 0.02, 0.05, 0.07) ceramics prepared by solid state route were first reported. The X-ray diffraction results reveal that all samples possess a pure perovskite structure phase. The investigation found that the moderate doping of Sn4þ in 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9Zr0.1O3 ceramic can decrease its phase transition temperature, and the slope of the P-E loops and the energy loss gradually decreased with the Sn4þ content increasing. A high breakdown strength of 13.02 kV/mm and the maximum discharge energy density of 1.21 J/cm3 were obtained for the sample with x ¼ 0.05, of which the energy efficiency was as high as 72.08%. Thus, it was believed that our work could provide a significant guidance for designing the new systems for energy storage. © 2016 Elsevier B.V. All rights reserved.
Keywords: Energy storage density Solid-state reaction Perovskite Energy loss
1. Introduction Capacitor is a key component in pulsed power technologies owing to its fast charge-discharge rate. Interest in high energy storage density dielectric materials has surged recently, which is mainly driven by the increasing demands for compact electronics [1,2]. Among various dielectric materials developed at present, barium strontium titanate (BST)-based ceramics are receiving increasing attention for energy storage because of their high dielectric constant and relatively low dielectric loss [3e5]. However, the pure BST can’t be utilized to energy storage application directly due to its such low dielectric breakdown strength (BDS) of about 9 kV/mm and energy density of about 0.37 J/cm3 [3]. Although earlier studies have reported that the BDS of BST can be respectively enhanced to as high as 23.9 kV/mm [3] and 33.1 kV/ mm [4] by introducing the glass and MgO as additives, their corresponding energy densities of 0.89 J/cm3 and 1.14 J/cm3 is low to meet the requirements of practical applications. Recently, BST/MgO composite sintered by Spark Plasma Sintering (SPS) showed a high BDS of 29 kV/mm as well as the maximum energy density of 1.50 J/ cm3, simultaneously [5]. For the large-scale production in industry, however, the SPS is too expensive.
* Corresponding author. E-mail address: [email protected] (M. Yao). http://dx.doi.org/10.1016/j.jallcom.2016.06.181 0925-8388/© 2016 Elsevier B.V. All rights reserved.
In order to get an outstanding ceramic material, sometimes, the BDS is overemphasized but the polarization, which is another key factor in achieving a high energy density, is neglected [6]. (Bi0.5Na0.5TiO3) BNT-based ceramics show a slim P-E loop at high temperature, which is favorable for energy storage property [7]. Gao et al. reported that 0.89BNT-0.06BT-0.05KNN possesses a high energy density of 0.59 J/cm3 in the temperature range of 100e150 C even though at a low electric field (5.6 kV/mm) [8]. An idea capturing our mind is to lower the phase transition temperature of BNT-based materials to obtain a new material system with a higher energy storage density. In our previous works, we have certified that 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9Zr0.1O3 ceramic showed a high energy density of 0.62 J/cm3 at 11 kV/mm. In the present work, similarly, then 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 (0.55BNT-0.45BCTZSx, x ¼ 0, 0.02, 0.05, 0.07) ceramics are prepared to further improve its energy density. 2. Experimental 0.55BNT-0.45BCTZSx ceramics were prepared by a conventional solid-state reaction method using Bi2O3, BaCO3, CaCO3, Na2CO3, TiO2, ZrO2 and SnO2 (Sinopharm Chemical Reagent Co., Shanghai) as starting raw materials. A typical preparation was as below. According to their stoichiometric formula, raw materials for BNT and BCTZSx (0.00 x 0.07) were mixed in planetary ball mill using Y2O3-stabilized ZrO2grinding meida for 24 h. After being milled, the
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mixed powders for BNT synthesis was calcined at 850 C for 4 h and the dried slurries for BCTZSx synthesis was calcined at 1260 C for 2 h. After calcinations, the powders were again ball milled separately for 24 h with the above mentioned ball-milling method. According to the chemical formula 0.55BNT-0.45BCTZSx, the BNT and BCTZSx powders were weighed and mixed in planetary ball mill for 24 h. At last, the uniform powder mixtures were granulated with 5 wt% PVA and pressed into green disks under a pressure of 100 MPa. They were then sintered in air at 1180 C for 2 h in covered alumina crucibles. To minimize the loss of volatile elements, samples were covered with a powder of the same composition to create an enriched atmosphere of the respective components. The phase structures of these ceramics were identified by powder X-ray diffraction (CD-MAX 2200pc, Rigaku Co., Tokyo, Japan) at a working voltage and current of 40 kV and 30 mA. XRD data was collected in the range of 20e80 with a 0.02 step and scanning speed of 5 /min. The sintered amples were polished and thermally etched at 1030 C for10 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 which sintered at 500 C for 30 min. Temperature and frequencydependent permittivity and dielectric loss 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 1 MHz) for every 2 C from RT to 400 C with a heating rate of 2 C/min. P-E hysteresis loops were obtained using ferroelectric test system (Premier II, Radiant, USA). 3. Results and discussion Fig. 1 (a) shows the XRD patterns of 0.55BNT-0.45BCTZSx ceramics with various x values. All samples have a pure perovskite phase without secondary phases within the resolution of the XRD instrument, indicating that a stable solid solution was formed between BNT and BCTZSx. The enlarged views of the peaks of (111) and (200) crystallographic plane are given in Fig. 1 (b) for better illustration. As Sn4þ content increases, the (111) and (200) reflection peaks shift gradually to a lower degree indicating the expansion of cell volume according to Bragg’s law 2dsinq ¼ nl. The result can be attributed to the substitutions of smaller ions Ti4þ (r ¼ 0.061 nm) by bigger ions Sn4þ (r ¼ 0.069 nm) [9]. Furthermore, the formation of single (111) and (200) peaks suggests a pseudocubic symmetry as described in the literature [10e12]. Fig. 1(c) displays the SEM images of the polished and thermally etched
surfaces of the ceramics. It is proved that all ceramics exhibit high density and low porosity, which is independent of the Sn4þ content. Using a linear intercept method, the grain size is about 2e10 mm for all samples. No clear relationship between microstructure and BCTZ content was observed. The temperature dependence of permittivity and dielectric loss tangent for 0.55BNT-0.45BCTZSx ceramics from room temperature to 400 C are shown in Fig. 2 (a)e(d). A single dielectric maxima is clearly discerned in the frequency dependent permittivity curves for all samples. The position of dielectric maxima shifts to higher temperature and the magnitude of permittivity decreases with increasing frequency. The position of peak of dielectric loss tangent curves also shifts to higher temperature with increasing frequency, whereas the magnitude of maxima increases. The features of frequency dependent dielectric properties are normally considered as a fingerprint for relaxor ferroelectric materials [13,14], consistent with other reports on the relaxor ferroelectric materials [15,16]. Moreover, the frequency dependent permittivity curves gradually become a broad plateau-like maxima over a broad temperature range with the increasing content of Sn4þ. The diffuse behavior caused by the addition of Sn4þ 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 [17]. Fig. 2 (e) summarizes the temperature dependent permittivity and dielectric loss tangent measured at 1 kHz for 0.55BNT-0.45BCTZSx ceramics with various x values. The temperature at the dielectric maxima slightly decreases with increasing the Sn4þ content, which conforms our expectation. The decrease of the phase transition temperature implies the increasing content of weakly polar phase in ceramics, which is duo to the destruction of the long-range Coulomb potential in the crystal [9]. The presence of weakly polar phase will cause a slim P-E loop, which is favorable for energy storage and will be discussed in the following. Fig. 3 (a), (b), (c) and (d)shows the P-E loops and I-E curves measured at 10 Hz of 0.55BNT-0.45BCTZSx ceramics with the maximum applied electric field of 11 kV/mm. For x ¼ 0 and x ¼ 0.02, as shown in Fig. 3 (a) and (b), their P-E loops are pinched, which is attributed to the presence of weakly polar phase [18,19]. Recent studies suggested that this “non-polar” phase can be converted to a polar phase on the application of high electric field [20]. Furthermore, four obvious current peaks are also observed in their I-E curves, indicating that there is a reversible structure change between weakly polar phase and ferroelectric phase [19]. During a reversible structural change, this transient current induced by the electric field may also weaken the BDS of the samples. With
Fig. 1. (a) XRD patterns, the enlarged XRD patterns of (b) (111) peak and (200) peak and (c) SEM images of the polished and thermally etched surfaces of 0.55BNT-0.45BCTZSx ceramics.
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Fig. 2. (a), (b), (c) and (d) temperature dependence of permittivity and dielectric loss tangent measured at 1 kHz, 10 kHz, 100 kHz and 1000 kHz, (e) temperature dependence of permittivity and dielectric loss tangent measured at 1 kHz of 0.55BNT-0.45BCTZSx ceramics.
increasing the Sn4þ content of sample with x ¼ 0.05 and x ¼ 0.07, as shown in Fig. 3 (c) and (d), the P-E loops gradually become slim, demonstrating that the area of loop decrease, which corresponds to the energy loss [5]. This is related to the existence of microdomains, which responses to the external field much faster than macroscopic domains and frequently result in a reduction of energy loss [21]. The amount of weakly polar phase gradually increases with the increasing content of Sn4þ, and then some microdomains begin to appear, leading to a slim P-E loop. It can also be observed that the slope of P-E loops progressively decreases with the increasing content of Sn4þ. With a small slope of P-E loops to avoid the polarization saturation at fields well below BDS, a higher energy density can be achieved [6]. Fig. 3 (e) shows the unipolar P-E loops measured at 10 Hz of 0.55BNT-0.45BCTZSx ceramics, it can be seen that both the maximum polarization (Pm) and the remanent polarization (Pr) decrease monotonously with the increasing content
of Sn4þ. This can be ascribed to the increasing content of weakly polar phase and the lower ionic polarizability of Sn4þ [22]. As for high power or energy storage usage, the breakdown strength is one of the key characteristics measuring the performance of the material. The breakdown strength was obtained by applying a load progressively increased during the measurement of hysteresis loops until breakdown occurred. Ten samples for each sintered ceramic were measured to obtain the average breakdown strength. Weibull distribution has been generally applied to analyze the breakdown strength both in experiment and theory [1,17]. The two parameter Weibull distribution can be described by:
Xi ¼ lnðEi Þ
(1)
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Fig. 3. (a), (b), (c) and (d) P-E loops and I-E curves, (e) unipolar P-E loops measured at 10 Hz of 0.55BNT-0.45BCTZSx ceramics.
Yi ¼ lnðlnð1=ð1 Pi ÞÞÞ
(2)
Pi ¼ i=ðn þ 1Þ
(3)
where Xi and Yi are the two parameters in Weibull distribution function, Ei is the specific breakdown voltage of each specimen in the experiments, Pi is the probability for dielectric breakdown, n is the sum of specimens of each sample, and i is serial number of specimen. The Weibull distribution of breakdown strength for 0.55BNT-0.45BCTZSx ceramics are shown in Fig. 4. There is a linear relationship between Xi and Yi as illustrated in Fig. 4, where the slop is the Weibull modules m relating to the range of breakdown strength. The value of m is observed to be higher than 14, indicating the high reliability of the materials. Fig. 5 shows the unipolar P-E loops measured at 10 Hz of the 0.55BNT-0.45BCTZSx ceramics just under the breakdown electrical field. It is found that the breakdown strength gradually increases with the Sn4þ content increasing and that it reaches the highest
value of 14.16 kV/mm for x ¼ 0.07, which is 1.28 times than that of the 0.55BNT-0.45BCTZ. The significant improvement of breakdown strength may be attributed to the disappearance of transient current with the Sn4þ doping as shown in Fig. 3. The Pm for the samples with x ¼ 0.02 and x ¼ 0.05 are comparable with that of 0.55BNT0.45BCTZ as listed in Table 1, this is because the “non-polar” phase can be converted to a polar phase and the polarization can be elongated by a higher electric field [23]. The maximum polarization of x ¼ 0.07 is lower than that of others, this may be due to the presence of large amount of “non-polar” phase. The area enclosed by discharge/charge curve and polarization axis represents the discharge/charge energy density J, which can be calculated according to the following equation:
ZE J¼
EdP
(4)
0
where P is the polarization and E is the applied electric field [5]. The
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Table 1 Average breakdown strength, discharge energy density, charge energy density, energy efficiency and maximum polarization of 0.55BNT-0.45BCTZSx ceramics.
Fig. 4. Weibull distribution of breakdown strength of 0.55BNT-0.45BCTZSx ceramics.
ratio of discharge energy density to charge energy density is the energy efficiency. Table 1 lists the average breakdown strength, discharge energy density, charge energy density, energy efficiency and maximum polarization of 0.55BNT-0.45BCTZSx ceramics. On one hand, the energy efficiency was greatly increased to 72.29% from 58.22%, that because of the significant reduction of energy loss with the increasing Sn4þ content as shown in Fig. 5. On the other hand, the maximum charge energy density was significantly improved from 0.84 J/cm3 to 1.21 J/cm3, which is due to the enhancement of breakdown strength and the decrease of the slope of P-E loops. Furthermore, the high breakdown strength of 13.02 kV/mm, the Pm
x
0
0.02
0.05
0.07
Average breakdown strength (kV/mm) Discharge energy density (J/cm3) Charge energy density (J/cm3) Energy efficiency (%) Maximum polarization (mC/cm2)
11.05 0.84 1.45 58.22 31.07
11.96 0.95 1.67 59.48 31.21
13.02 1.21 1.69 72.08 30.58
14.16 1.17 1.62 72.29 25.38
of 30.58 mC/cm3 and a high energy efficiency of 72.08% make the sample with x ¼ 0.05 possesses a maximum discharge energy density of 1.21 J/cm3, which is almost 1.44 times than that of the 0.55BNT-0.45BCTZ. Table 2 summaries the energy storage property of 0.55BNT-0.45BCTZS5 and the other ceramics. It can be seen that the discharge energy density of 0.55BNT-0.45BCTZS5 ceramics in this work is much more higher than that in recently reported literature. For examples, BST-MgO compositions reported by Zhang et al. [4] exhibited a high energy storage density of 1.14 J/cm3, BNTBT-CZ ceramics prepared by Li et al. [24] showed a high energy storage density of 0.7 J/cm3, a high energy storage density of 0.9 J/ cm3 was obtained in 0.89BNT-0.06BT-0.05KNN ceramics [25]. The discharge energy density of 0.55BNT-0.45BCTZS5 ceramics in this work is comparable with that of BST ceramic prepared by SPS [5] and 0.84BNT-0.16KNN ceramic reported by Hao et al. [26]. Thus, the sample with x ¼ 0.05 in this work is a promising candidate materials for high energy density storage applications. The charge energy density, discharge energy density and energy efficiency as a function of electric field of 0.55BNT-0.45BCTZSx ceramics are shown in Fig. 6 (a)e(c), respectively. As observed from Fig. 6, the charge energy density and discharge energy density increase, meanwhile, the energy efficiency remains nearly constant with the ever increasing electric fields. The increase of the charge
Fig. 5. Unipolar P-E loops of 0.55BNT-0.45BCTZSx ceramics.
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Table 2 The comparison in the energy storage property of 0.55BNT-0.45BCTZS5 and the other ceramics. Composition
Average breakdown strength (kV/mm)
Discharge energy density (J/cm3)
0.55BNT-0.45BCTZS5 BST-MgO BST 0.97BNBT6-0.03CZ 0.89BNT-0.06BT-0.05KNN 0.84BNT-0.16KNN
13.02 33.1 21 7 8.6 10
1.21 1.14 1.2 0.7 0.9 1.2
In this work Ref. [4] Ref. [5] Ref. [24] Ref. [25] Ref. [26]
Fig. 6. (a) Charge energy density, (b) discharge energy density and (c) energy efficiency as a function of electric field of 0.55BNT-0.45BCTZSx ceramics.
energy density and discharge energy density is due to the enhanced hysteresis caused by the occurrence of the ferroelectric state at higher electric fields [18]. When compared at a given electric field, 0.55BNT-0.45BCTZ exhibits the highest charge energy density due to its largest maximum polarization. While the discharge energy density shows a contrary tendency, which is caused by the larger energy loss as shown in Fig. 5. 4. Conclusions In summary, 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 (x ¼ 0, 0.02, 0.05, 0.07) ceramics with a high energy storage density have been successfully prepared. Our results confirmed that the phase transition temperature slightly decreases because of the Sn4þ doping, which is favorable for energy storage. With increasing the Sn4þ content, the BDS values of the ceramics gradually increase but the energy loss is decreased, causing the energy efficiency significantly improved from 58.22% to 72.29%. When x ¼ 0.05, moreover,
the highest breakdown strength and maximum discharge energy density of the corresponding sample reach 13.02 kV/mm and 1.21 J/ cm3, respectively. Consequently, these results may lead to the development of BNT-based materials for energy storage. Acknowledgement This work was supported by the National Natural Science Foundation of China (51372144) and the Key Program of Innovative Research Team of Shaanxi Province (2014KCT-06). References [1] L.W. Wu, X.H. Wang, H.L. Gong, Y.N. Hao, Z.B. Shen, L.T. Li, Core-satellite BaTiO3@ SrTiO3 assemblies for a local compositionally graded relaxor ferroelectric capacitor with enhanced energy storage density and high energy efficiency, J. Mater. Chem. C 3 (2015) 750e758. [2] Y.F. Wang, Z.L. Lv, H. Xie, J. Cao, High energy-storage properties of [(Bi1/2Na1/ 2)0.94 Ba0.06] La(1x) ZrxTiO3 lead-free anti-ferroelectric ceramics, Ceram. Int.
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High energy storage density of 0.55Bi0.5Na0.5TiO30.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 ceramics Yongping Pu a, Mouteng Yao a, *, Lei Zhang a, Panpan Jing b a b
School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an, 710021, China Lanzhou University, Lan’zhou, 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2016 Received in revised form 1 June 2016 Accepted 20 June 2016 Available online 21 June 2016
High energy storage properties of 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 (x ¼ 0, 0.02, 0.05, 0.07) ceramics prepared by solid state route were first reported. The X-ray diffraction results reveal that all samples possess a pure perovskite structure phase. The investigation found that the moderate doping of Sn4þ in 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9Zr0.1O3 ceramic can decrease its phase transition temperature, and the slope of the P-E loops and the energy loss gradually decreased with the Sn4þ content increasing. A high breakdown strength of 13.02 kV/mm and the maximum discharge energy density of 1.21 J/cm3 were obtained for the sample with x ¼ 0.05, of which the energy efficiency was as high as 72.08%. Thus, it was believed that our work could provide a significant guidance for designing the new systems for energy storage. © 2016 Elsevier B.V. All rights reserved.
Keywords: Energy storage density Solid-state reaction Perovskite Energy loss
1. Introduction Capacitor is a key component in pulsed power technologies owing to its fast charge-discharge rate. Interest in high energy storage density dielectric materials has surged recently, which is mainly driven by the increasing demands for compact electronics [1,2]. Among various dielectric materials developed at present, barium strontium titanate (BST)-based ceramics are receiving increasing attention for energy storage because of their high dielectric constant and relatively low dielectric loss [3e5]. However, the pure BST can’t be utilized to energy storage application directly due to its such low dielectric breakdown strength (BDS) of about 9 kV/mm and energy density of about 0.37 J/cm3 [3]. Although earlier studies have reported that the BDS of BST can be respectively enhanced to as high as 23.9 kV/mm [3] and 33.1 kV/ mm [4] by introducing the glass and MgO as additives, their corresponding energy densities of 0.89 J/cm3 and 1.14 J/cm3 is low to meet the requirements of practical applications. Recently, BST/MgO composite sintered by Spark Plasma Sintering (SPS) showed a high BDS of 29 kV/mm as well as the maximum energy density of 1.50 J/ cm3, simultaneously [5]. For the large-scale production in industry, however, the SPS is too expensive.
* Corresponding author. E-mail address: [email protected] (M. Yao). http://dx.doi.org/10.1016/j.jallcom.2016.06.181 0925-8388/© 2016 Elsevier B.V. All rights reserved.
In order to get an outstanding ceramic material, sometimes, the BDS is overemphasized but the polarization, which is another key factor in achieving a high energy density, is neglected [6]. (Bi0.5Na0.5TiO3) BNT-based ceramics show a slim P-E loop at high temperature, which is favorable for energy storage property [7]. Gao et al. reported that 0.89BNT-0.06BT-0.05KNN possesses a high energy density of 0.59 J/cm3 in the temperature range of 100e150 C even though at a low electric field (5.6 kV/mm) [8]. An idea capturing our mind is to lower the phase transition temperature of BNT-based materials to obtain a new material system with a higher energy storage density. In our previous works, we have certified that 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9Zr0.1O3 ceramic showed a high energy density of 0.62 J/cm3 at 11 kV/mm. In the present work, similarly, then 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 (0.55BNT-0.45BCTZSx, x ¼ 0, 0.02, 0.05, 0.07) ceramics are prepared to further improve its energy density. 2. Experimental 0.55BNT-0.45BCTZSx ceramics were prepared by a conventional solid-state reaction method using Bi2O3, BaCO3, CaCO3, Na2CO3, TiO2, ZrO2 and SnO2 (Sinopharm Chemical Reagent Co., Shanghai) as starting raw materials. A typical preparation was as below. According to their stoichiometric formula, raw materials for BNT and BCTZSx (0.00 x 0.07) were mixed in planetary ball mill using Y2O3-stabilized ZrO2grinding meida for 24 h. After being milled, the
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mixed powders for BNT synthesis was calcined at 850 C for 4 h and the dried slurries for BCTZSx synthesis was calcined at 1260 C for 2 h. After calcinations, the powders were again ball milled separately for 24 h with the above mentioned ball-milling method. According to the chemical formula 0.55BNT-0.45BCTZSx, the BNT and BCTZSx powders were weighed and mixed in planetary ball mill for 24 h. At last, the uniform powder mixtures were granulated with 5 wt% PVA and pressed into green disks under a pressure of 100 MPa. They were then sintered in air at 1180 C for 2 h in covered alumina crucibles. To minimize the loss of volatile elements, samples were covered with a powder of the same composition to create an enriched atmosphere of the respective components. The phase structures of these ceramics were identified by powder X-ray diffraction (CD-MAX 2200pc, Rigaku Co., Tokyo, Japan) at a working voltage and current of 40 kV and 30 mA. XRD data was collected in the range of 20e80 with a 0.02 step and scanning speed of 5 /min. The sintered amples were polished and thermally etched at 1030 C for10 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 which sintered at 500 C for 30 min. Temperature and frequencydependent permittivity and dielectric loss 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 1 MHz) for every 2 C from RT to 400 C with a heating rate of 2 C/min. P-E hysteresis loops were obtained using ferroelectric test system (Premier II, Radiant, USA). 3. Results and discussion Fig. 1 (a) shows the XRD patterns of 0.55BNT-0.45BCTZSx ceramics with various x values. All samples have a pure perovskite phase without secondary phases within the resolution of the XRD instrument, indicating that a stable solid solution was formed between BNT and BCTZSx. The enlarged views of the peaks of (111) and (200) crystallographic plane are given in Fig. 1 (b) for better illustration. As Sn4þ content increases, the (111) and (200) reflection peaks shift gradually to a lower degree indicating the expansion of cell volume according to Bragg’s law 2dsinq ¼ nl. The result can be attributed to the substitutions of smaller ions Ti4þ (r ¼ 0.061 nm) by bigger ions Sn4þ (r ¼ 0.069 nm) [9]. Furthermore, the formation of single (111) and (200) peaks suggests a pseudocubic symmetry as described in the literature [10e12]. Fig. 1(c) displays the SEM images of the polished and thermally etched
surfaces of the ceramics. It is proved that all ceramics exhibit high density and low porosity, which is independent of the Sn4þ content. Using a linear intercept method, the grain size is about 2e10 mm for all samples. No clear relationship between microstructure and BCTZ content was observed. The temperature dependence of permittivity and dielectric loss tangent for 0.55BNT-0.45BCTZSx ceramics from room temperature to 400 C are shown in Fig. 2 (a)e(d). A single dielectric maxima is clearly discerned in the frequency dependent permittivity curves for all samples. The position of dielectric maxima shifts to higher temperature and the magnitude of permittivity decreases with increasing frequency. The position of peak of dielectric loss tangent curves also shifts to higher temperature with increasing frequency, whereas the magnitude of maxima increases. The features of frequency dependent dielectric properties are normally considered as a fingerprint for relaxor ferroelectric materials [13,14], consistent with other reports on the relaxor ferroelectric materials [15,16]. Moreover, the frequency dependent permittivity curves gradually become a broad plateau-like maxima over a broad temperature range with the increasing content of Sn4þ. The diffuse behavior caused by the addition of Sn4þ 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 [17]. Fig. 2 (e) summarizes the temperature dependent permittivity and dielectric loss tangent measured at 1 kHz for 0.55BNT-0.45BCTZSx ceramics with various x values. The temperature at the dielectric maxima slightly decreases with increasing the Sn4þ content, which conforms our expectation. The decrease of the phase transition temperature implies the increasing content of weakly polar phase in ceramics, which is duo to the destruction of the long-range Coulomb potential in the crystal [9]. The presence of weakly polar phase will cause a slim P-E loop, which is favorable for energy storage and will be discussed in the following. Fig. 3 (a), (b), (c) and (d)shows the P-E loops and I-E curves measured at 10 Hz of 0.55BNT-0.45BCTZSx ceramics with the maximum applied electric field of 11 kV/mm. For x ¼ 0 and x ¼ 0.02, as shown in Fig. 3 (a) and (b), their P-E loops are pinched, which is attributed to the presence of weakly polar phase [18,19]. Recent studies suggested that this “non-polar” phase can be converted to a polar phase on the application of high electric field [20]. Furthermore, four obvious current peaks are also observed in their I-E curves, indicating that there is a reversible structure change between weakly polar phase and ferroelectric phase [19]. During a reversible structural change, this transient current induced by the electric field may also weaken the BDS of the samples. With
Fig. 1. (a) XRD patterns, the enlarged XRD patterns of (b) (111) peak and (200) peak and (c) SEM images of the polished and thermally etched surfaces of 0.55BNT-0.45BCTZSx ceramics.
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Fig. 2. (a), (b), (c) and (d) temperature dependence of permittivity and dielectric loss tangent measured at 1 kHz, 10 kHz, 100 kHz and 1000 kHz, (e) temperature dependence of permittivity and dielectric loss tangent measured at 1 kHz of 0.55BNT-0.45BCTZSx ceramics.
increasing the Sn4þ content of sample with x ¼ 0.05 and x ¼ 0.07, as shown in Fig. 3 (c) and (d), the P-E loops gradually become slim, demonstrating that the area of loop decrease, which corresponds to the energy loss [5]. This is related to the existence of microdomains, which responses to the external field much faster than macroscopic domains and frequently result in a reduction of energy loss [21]. The amount of weakly polar phase gradually increases with the increasing content of Sn4þ, and then some microdomains begin to appear, leading to a slim P-E loop. It can also be observed that the slope of P-E loops progressively decreases with the increasing content of Sn4þ. With a small slope of P-E loops to avoid the polarization saturation at fields well below BDS, a higher energy density can be achieved [6]. Fig. 3 (e) shows the unipolar P-E loops measured at 10 Hz of 0.55BNT-0.45BCTZSx ceramics, it can be seen that both the maximum polarization (Pm) and the remanent polarization (Pr) decrease monotonously with the increasing content
of Sn4þ. This can be ascribed to the increasing content of weakly polar phase and the lower ionic polarizability of Sn4þ [22]. As for high power or energy storage usage, the breakdown strength is one of the key characteristics measuring the performance of the material. The breakdown strength was obtained by applying a load progressively increased during the measurement of hysteresis loops until breakdown occurred. Ten samples for each sintered ceramic were measured to obtain the average breakdown strength. Weibull distribution has been generally applied to analyze the breakdown strength both in experiment and theory [1,17]. The two parameter Weibull distribution can be described by:
Xi ¼ lnðEi Þ
(1)
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Fig. 3. (a), (b), (c) and (d) P-E loops and I-E curves, (e) unipolar P-E loops measured at 10 Hz of 0.55BNT-0.45BCTZSx ceramics.
Yi ¼ lnðlnð1=ð1 Pi ÞÞÞ
(2)
Pi ¼ i=ðn þ 1Þ
(3)
where Xi and Yi are the two parameters in Weibull distribution function, Ei is the specific breakdown voltage of each specimen in the experiments, Pi is the probability for dielectric breakdown, n is the sum of specimens of each sample, and i is serial number of specimen. The Weibull distribution of breakdown strength for 0.55BNT-0.45BCTZSx ceramics are shown in Fig. 4. There is a linear relationship between Xi and Yi as illustrated in Fig. 4, where the slop is the Weibull modules m relating to the range of breakdown strength. The value of m is observed to be higher than 14, indicating the high reliability of the materials. Fig. 5 shows the unipolar P-E loops measured at 10 Hz of the 0.55BNT-0.45BCTZSx ceramics just under the breakdown electrical field. It is found that the breakdown strength gradually increases with the Sn4þ content increasing and that it reaches the highest
value of 14.16 kV/mm for x ¼ 0.07, which is 1.28 times than that of the 0.55BNT-0.45BCTZ. The significant improvement of breakdown strength may be attributed to the disappearance of transient current with the Sn4þ doping as shown in Fig. 3. The Pm for the samples with x ¼ 0.02 and x ¼ 0.05 are comparable with that of 0.55BNT0.45BCTZ as listed in Table 1, this is because the “non-polar” phase can be converted to a polar phase and the polarization can be elongated by a higher electric field [23]. The maximum polarization of x ¼ 0.07 is lower than that of others, this may be due to the presence of large amount of “non-polar” phase. The area enclosed by discharge/charge curve and polarization axis represents the discharge/charge energy density J, which can be calculated according to the following equation:
ZE J¼
EdP
(4)
0
where P is the polarization and E is the applied electric field [5]. The
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Table 1 Average breakdown strength, discharge energy density, charge energy density, energy efficiency and maximum polarization of 0.55BNT-0.45BCTZSx ceramics.
Fig. 4. Weibull distribution of breakdown strength of 0.55BNT-0.45BCTZSx ceramics.
ratio of discharge energy density to charge energy density is the energy efficiency. Table 1 lists the average breakdown strength, discharge energy density, charge energy density, energy efficiency and maximum polarization of 0.55BNT-0.45BCTZSx ceramics. On one hand, the energy efficiency was greatly increased to 72.29% from 58.22%, that because of the significant reduction of energy loss with the increasing Sn4þ content as shown in Fig. 5. On the other hand, the maximum charge energy density was significantly improved from 0.84 J/cm3 to 1.21 J/cm3, which is due to the enhancement of breakdown strength and the decrease of the slope of P-E loops. Furthermore, the high breakdown strength of 13.02 kV/mm, the Pm
x
0
0.02
0.05
0.07
Average breakdown strength (kV/mm) Discharge energy density (J/cm3) Charge energy density (J/cm3) Energy efficiency (%) Maximum polarization (mC/cm2)
11.05 0.84 1.45 58.22 31.07
11.96 0.95 1.67 59.48 31.21
13.02 1.21 1.69 72.08 30.58
14.16 1.17 1.62 72.29 25.38
of 30.58 mC/cm3 and a high energy efficiency of 72.08% make the sample with x ¼ 0.05 possesses a maximum discharge energy density of 1.21 J/cm3, which is almost 1.44 times than that of the 0.55BNT-0.45BCTZ. Table 2 summaries the energy storage property of 0.55BNT-0.45BCTZS5 and the other ceramics. It can be seen that the discharge energy density of 0.55BNT-0.45BCTZS5 ceramics in this work is much more higher than that in recently reported literature. For examples, BST-MgO compositions reported by Zhang et al. [4] exhibited a high energy storage density of 1.14 J/cm3, BNTBT-CZ ceramics prepared by Li et al. [24] showed a high energy storage density of 0.7 J/cm3, a high energy storage density of 0.9 J/ cm3 was obtained in 0.89BNT-0.06BT-0.05KNN ceramics [25]. The discharge energy density of 0.55BNT-0.45BCTZS5 ceramics in this work is comparable with that of BST ceramic prepared by SPS [5] and 0.84BNT-0.16KNN ceramic reported by Hao et al. [26]. Thus, the sample with x ¼ 0.05 in this work is a promising candidate materials for high energy density storage applications. The charge energy density, discharge energy density and energy efficiency as a function of electric field of 0.55BNT-0.45BCTZSx ceramics are shown in Fig. 6 (a)e(c), respectively. As observed from Fig. 6, the charge energy density and discharge energy density increase, meanwhile, the energy efficiency remains nearly constant with the ever increasing electric fields. The increase of the charge
Fig. 5. Unipolar P-E loops of 0.55BNT-0.45BCTZSx ceramics.
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Table 2 The comparison in the energy storage property of 0.55BNT-0.45BCTZS5 and the other ceramics. Composition
Average breakdown strength (kV/mm)
Discharge energy density (J/cm3)
0.55BNT-0.45BCTZS5 BST-MgO BST 0.97BNBT6-0.03CZ 0.89BNT-0.06BT-0.05KNN 0.84BNT-0.16KNN
13.02 33.1 21 7 8.6 10
1.21 1.14 1.2 0.7 0.9 1.2
In this work Ref. [4] Ref. [5] Ref. [24] Ref. [25] Ref. [26]
Fig. 6. (a) Charge energy density, (b) discharge energy density and (c) energy efficiency as a function of electric field of 0.55BNT-0.45BCTZSx ceramics.
energy density and discharge energy density is due to the enhanced hysteresis caused by the occurrence of the ferroelectric state at higher electric fields [18]. When compared at a given electric field, 0.55BNT-0.45BCTZ exhibits the highest charge energy density due to its largest maximum polarization. While the discharge energy density shows a contrary tendency, which is caused by the larger energy loss as shown in Fig. 5. 4. Conclusions In summary, 0.55Bi0.5Na0.5TiO3-0.45Ba0.85Ca0.15Ti0.9xZr0.1SnxO3 (x ¼ 0, 0.02, 0.05, 0.07) ceramics with a high energy storage density have been successfully prepared. Our results confirmed that the phase transition temperature slightly decreases because of the Sn4þ doping, which is favorable for energy storage. With increasing the Sn4þ content, the BDS values of the ceramics gradually increase but the energy loss is decreased, causing the energy efficiency significantly improved from 58.22% to 72.29%. When x ¼ 0.05, moreover,
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