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Energy storage properties of BiTi0.5Zn0.5O3-Bi0.5Na0.5TiO3-BaTiO3 relaxor ferroelectrics
Ceramics International 42 (2016) 17876–17879
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Energy storage properties of BiTi0.5Zn0.5O3-Bi0.5Na0.5TiO3-BaTiO3 relaxor ferroelectrics Xiao Liu a,n, Huiling Du a, Xiangchun Liu a, Jing Shi b, Huiqing Fan c a
School of Materials Science and Engineering, Xi'an University of Science and Technology, Xi'an 710054, China School of Electro-Mechanical Engineering, Xidian University, Xi'an 710071, China c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 July 2016 Received in revised form 14 August 2016 Accepted 15 August 2016 Available online 15 August 2016
(1-x)BiTi0.5Zn0.5O3-x(0.935Bi0.5Na0.5TiO3-0.065BaTiO3) (abbreviated as BTZx) lead-free ceramics were prepared by using a conventional solid state reaction method and their composition and temperature dependent energy-storage properties were investigated. The addition of BiTi0.5Zn0.5O3 sharply decreased the content of ferroelectric phase, leading to lowered remanent polarization and coercive field. A large energy storage density (W) of 1.04 J cm 3 was achieved at 95 kV cm 1 for BTZ0.916. In the high temperature region, the remanent polarization was suppressed, while maximum polarization still maintained at a high level. Significantly enhanced W of 0.952 J cm 3 at 80 kV cm 1 was obtained with a relatively high efficiency of 0.8, making these materials attractive in energy storage applications. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Dielectric capacitors have attracted intensively attention in energy storage applications owing to their high power density, fast charge/discharge, low cost and mechanical stability, as compared with batteries [1–4]. They are of great importance to meet the sustainable energy and the growing power supply demand. The main drawback of capacitors is their limited energy density. Thus, studies have been devoted to the improvement of their energy density. There are two approaches to achieve high energy density of materials, increasing polarization or breakdown electric field. A high energy density can be expected in ferroelectric materials, since they exhibit higher polarization. They have high dielectric constants, but suffer from dielectric losses during charging and discharging cycles, as a result of the ferroelectric hysteresis [5–7]. A considerable portion of energy stored in the charging process will lose, leading to a lower efficiency. Antiferroelectric materials have lower remnant polarization (Pr) and coercive electric field (Ec), as well as faster discharge rates for dissipating stored electrical energy, because of their reversible ferroelectric to anti-ferroelectric phase transition induced by electric filed [4,8–11]. However, they exhibit either poor fatigue resistance behavior or low polarization in both lead-based, such as PbZr1 xTixO3 series, and lead-free capacitors. Environmental demands are pushing the development toward non-Pb materials. Lead-free Bi0.5Na0.5TiO3 (BNT) family is considered to be promising alternative for dielectric, ferroelectric, n
Corresponding author. E-mail address: [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.ceramint.2016.08.087 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
piezoelectric and ionic conductive applications [12–16]. Pure BNT is perovskite-structured ferroelectrics with A-site disorder at room temperature. It has high polarization, large electric field-induced strain and special dielectric relaxation. To further enhance its piezoelectric properties, extensive research are carried out to combine BNT-based solid solutions with ABO3-type ferroelectrics and non-ferroelectrics to form morphotropic phase boundary (MPB) compositions, such as tetragonal BaTiO3 (BT), orthorhombic K0.5Na0.5NbO3 (KNN), and tetragonal Bi0.5K0.5TiO3 (BKT) [17–20]. BNT experiences a complicated variation in crystal structure with composition and temperature. Foreign elements would result in nanotwins with monoclinic symmetry as an intermediate bridging phase that facilitates a pathway for polarization reorientation [21– 24]. As the content is increased, the composition displays characteristics of relaxors with tetragonal symmetry. Significantly suppressed polarization, drastic reduced remanent strain and coercive field are achieved, with the presence of slim polarization hysteresis loops [12,20,23,25]. Thus, it is necessary to find a balance between high polarization and low hysteresis to achieve high energy storage performance. Meanwhile, BNT and its derivations experience diffused phase transitions, with temperature evolution from rhombohedral ferroelectric phase to a tetragonal non-polar or weakly polar phase which is either a relaxor ferroelectric state or antiferroelectric phase [24–26]. The polarization hysteresis loops display similar change as compared with that induced by composition, where slim polarization hysteresis loop and high strain can be obtained at the temperature higher than depolarization temperature. Thus, the energy storage property will be enhanced effectively by adjusting the phase transition
X. Liu et al. / Ceramics International 42 (2016) 17876–17879
temperature of ferroelectric to relaxor phase to appropriate values. Moreover, composition modification can reduce the ferroelectricrelaxor transition temperature, which facilitates room temperature application of these materials. Structural analysis in Bi0.5Na0.5TiO3-BaTiO3 (BNTBT) has revealed a cubic symmetry lying at the MPB between a rhombohedral BNT phase and a tetragonal BT phase in their virgin state [27]. Temperature dependent permittivity of BNTBT displays a less abrupt change, which is suitable for high temperature applications [28]. In addition, tetragonal Bi(Ti0.5Zn0.5)O3 (BTZ) has been reported to have large calculated ferroelectric polarization. Moreover, BTZ can be used to increase the density of ceramics, which is beneficial to increase the breakdown voltage and thus raise the energy density [29,30]. In this study, effect of BZT on ferroelectric and energy storage properties of BNTBT was studied. BZT was used to enhance the tetragonal phase to realize high polarization and low hysteresis, so as to further improve the energy storage performance. Polycrystalline (1 x)BiTi0.5Zn0.5O3-x(0.935Bi0.5Na0.5TiO3-0.065BaTiO3) with 1rxr0.7, as shown in Fig. 1, were prepared by using a conventional solid state reaction method, with mixing appropriate amount of high purity Bi2O3 (99.9%), Na2CO3 (99.8%), BaCO3 (99%), TiO2 (98%) and ZnO (99.99%) as raw powders. The powders were weighed and mixed by using ball milling in isopropyl alcohol for 12 h. The mixtures were calcined at 850 °C for 4 h. The calcined powders were remilled for 12 h and then cold isostatically pressed into pellets of 10 mm in diameter at a pressure of 250 MPa. The pressed pellets were sintered at 1070–1100 °C embedded in precursor powders to avoid Bi volatilization. Phase structures of powders and the sintered pellets were investigated by X-ray diffraction (XRD) with Cu Kα radiation (XRD7000, Shimadzu, Kyoto, Japan). The Rietveld program FullProf was used for full-pattern matching and structural refinements. Archimedes method was employed to measure density of the ceramics. The theoretical density was derived from the lattice parameters obtained from the XRD diffraction pattern. The peaks in XRD data could be indexed with a pseudocubic phase for all compositions. All samples had 494% of the theoretical density. The sintered pellets were polished for property investigations. Sliver electrodes were coated on both polished surfaces and fired at 950 °C for 3 h to ensure maximum conductivity and adherence. Dielectric properties with signal level of 50 mV were measured by using a precision impedance analyzer (4294A, Agilent, CA, USA), associated with temperature controller (TP94, Linkam, Surrey, UK), at a heating rate of 3 °C min 1. Temperature dependence of electric-
Fig. 1. Two cycles of polarization hysteresis and bipolar strain at 1 Hz at ambient temperature plotted on the BTZ-BNT-BT phase diagram. The virgin loops are shown in colors and the second in black. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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field-induced polarization and strain were measured by using a ferroelectric test unit (TF-2000, aix-ACCT, Aachen, Germany) from room temperature to 190 °C. Fig. 1 shows the virgin and the second polarization hysteresis and bipolar strain of BTZ-BNTBT. The polarization hysteresis displays a well-saturated typical ferroelectric profile. The virgin bipolar strain exhibits a poling strain of 0.470% with large remanent strain of 0.330%, with the second cycle to be a typical butterflytype strain loop. Therefore, BNTBT belongs to nonergodic relaxors, which transforms irreversibly into a ferroelectric phase upon the application of a sufficiently high electric field [12]. With the addition of BTZ, the depoling temperature is reduced, with drastic dropping of Ec and Pr. BTZ0.966 is an ergodic relaxor at room temperature, characterized by the “square” shaped pinched hysteresis loop and the absence of negative strain, so that the application of sufficiently large electric fields can reversibly induce ferroelectric long range order. For BTZ0.916, “slanted” shaped hysteresis loop is observed. It is demonstrated that the critical bias field for the ergodic relaxor to ferroelectric transition is strongly increased with increasing BTZ content and no such ferroelectric long range order can be created. According to the P–E hysteresis loops, energy storage behaviors of BTZ-BNT-BT can be evaluated by using the following equation:
W=
η=
∮ E∙dP
W × 100% W + Wloss
(1)
(2)
where W is the energy storage density, E is applied electric field and P is the polarization. W can be determined with their saturation electric field values. η and Wloss represent capacitor efficiency and energy loss, which are highly related to domain reorientation. It is represented by the area of the loop, so that the energy delivered to the capacitor is larger than the energy that can be recovered. As reflected by XRD results, BTZ-BNTBT exhibits pseudocubic phase of relaxors. The response of polar nano-regions induced micro-domains rather than macroscopic domains can be accelerated under the electric field, resulting in slim hysteresis loops. Both Pr and Ec are suppressed evidently with increasing BTZ content, which is beneficial to increase the energy storage density
Fig. 2. Polarization hysteresis of BTZ0.916 at 26 °C that indicates a high energy density of 1.04 J cm 3 at 95 kV cm 1, and its fatigue properties of Pmax and Pr. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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X. Liu et al. / Ceramics International 42 (2016) 17876–17879
Table 1 Comparison of ferroelectric and energy storage properties of several lead-free ceramics at ambient temperature. Compounds
W (J cm 3)
Pmax (μC cm 2)
Pr (μC cm 2)
E (kV cm 1)
η
Ref.
0.90BNTBT-0.10NaNbO3 0.96BNTBKT-0.04BaZrO3 BNTBT-0.06KNbO3 0.91BT-0.09BY 0.9BT-0.1Bi(Mg2/3Nb1/3)O3 0.9BNTBT-0.1Bi(Mg1/2Ti1/2)O3a BTZ0.916 BTZ0.916a
0.71 0.73 0.89 0.71 1.13 0.85 1.04 0.952
28.8 30 28 – 14 – 32.8 29.4
3.8 3.5 2.6 – 0.4 – 4.8 1.57
70 70 100 93 140 70 95 80
0.65 0.75 0.73 0.826 0.93 0.82 0.59 0.79
[19] [31] [32] [7] [33] [26] Current
a
Results from hysteresis loops at 80 °C.
Temperature dependent energy density W, capacitor efficiency
η and dielectric loss tan δ are shown in the inset (b) of Fig. 3. In the beginning, both W and η show monotonous increase with in-
Fig. 3. Polarization hysteresis of BTZ0.916 at elevated temperatures. Inset (a) of Fig. 3 shows the slim P–E loop and polarization current density curve at 70 °C, and the inset (b) is the temperature dependence of energy density W, capacitor efficiency η and dielectric loss tan δ at 1 k, 10 k and 100 kHz.
and efficiency. Fig. 2 shows energy density (marked in red area), maximum polarization (Pmax) and remanent polarization (Pr) of BTZ0.916 at 26 °C. BTZ0.916 is characterized by a high Pmax of 32.8 μC cm 2 and a low Pr of 4.8 μC cm 2 at 95 kV cm 1. Considerable large W of as high as 1.04 J cm 3 is achieved. For comparison, the energy storage properties of several lead-free ceramics are also listed in Table 1. Meanwhile, the ceramic shows excellent anti-fatigue property, so that the degradation is negligible with the variation of both Pmax and Pr to be less than 2.2% after 105th cycle. Temperature is also an important parameter to be considered. Fig. 3 displays temperature dependent ferroelectric hysteresis loops of BTZ0.916 at 80 kV cm 1 and 1 Hz. With increasing temperature, the sample experiences a transition from FE phase to relaxor phase. Traditionally, Pmax exhibits an obvious decrease in most ambient MPB as a gradual narrowing of P-E loops, induced by composition or temperature. However, in this study, Pmax maintained at a high level of 30 μC cm 2 with Pr showing a decrease within the temperature range, thus leading to high energy density, where the high Pmax originated from the relaxor FE phase and low Pr of 1.77 μC cm 2 from the coexistence of FE and RFE phase. The inset (a) of Fig. 3 shows the slim P-E loop and corresponding polarization current density curve at 70 °C. Four broad peaks are observed in the polarization current density curve, indicating the aligning of the randomly oriented polar nanoregions or nanodomains along the external field direction [34]. However, no longrange ferroelectric phase is induced, because of the loss of hybridization of the 6s2 bismuth lone pair with the oxygen p-orbitals, as BTZ content was increased, which triggers a more-disordered lattice. After removing the electric-field, this orientation is lost.
creasing temperature. At 80 °C, energy density W reaches its maximum of 0.952 J cm 3, with η ¼0.79 at 80 kV cm 1, which is a remarkably high value for lead-free ceramics. The dielectric loss tanδ also keeps at a low level of r 0.08 in the measured temperature range. Additionally, the ceramics have a small variation of o3.4% in the energy density from 60 °C to 120 °C, while the capacitor efficiency is typically higher than 0.75, indicating that they are suitable for the high temperature applications. Also, it is expected that higher energy density could be achieved at high electrical fields since the sample has a high electric breakdown voltage in ceramics, making them potential alternatives especially as a form of thin film. In conclusion, remanent polarization and coercive field of BNTBT were reduced with the addition of BTZ, originated from the response of polar nano-regions induced micro-domains. A large energy density of 1.04 J cm 3 was achieved at an electric field of 95 kV cm 1 in BTZ0.916 at ambient temperature with excellent anti-fatigue property. More interestingly, Pmax maintained at a high level of 430 μC cm 2, with a decreased Pr at elevated temperature at 80 kV cm 1. As a result, the ceramics had a small variation of the high energy density from 60 °C to 120 °C, with high efficiency of 0.8. At 80 °C, W reached its maximum of 0.952 J cm 3 with η of 0.79. These results suggest that BTZ modified BNTBT ceramics are promising candidates for energy storage applications.
Acknowledgements This work was supported by the National Natural Science Foundation (51172187 and 51372197), the Key Innovation Team of Shaanxi Province (2014KCT-04), and the Major International Joint Research Program of Shaanxi Province (2012KW-10) and the Doctoral Starting-up Foundation of Xi'an University of Science and Technology (2015QDJ046).
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
Energy storage properties of BiTi0.5Zn0.5O3-Bi0.5Na0.5TiO3-BaTiO3 relaxor ferroelectrics Xiao Liu a,n, Huiling Du a, Xiangchun Liu a, Jing Shi b, Huiqing Fan c a
School of Materials Science and Engineering, Xi'an University of Science and Technology, Xi'an 710054, China School of Electro-Mechanical Engineering, Xidian University, Xi'an 710071, China c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 31 July 2016 Received in revised form 14 August 2016 Accepted 15 August 2016 Available online 15 August 2016
(1-x)BiTi0.5Zn0.5O3-x(0.935Bi0.5Na0.5TiO3-0.065BaTiO3) (abbreviated as BTZx) lead-free ceramics were prepared by using a conventional solid state reaction method and their composition and temperature dependent energy-storage properties were investigated. The addition of BiTi0.5Zn0.5O3 sharply decreased the content of ferroelectric phase, leading to lowered remanent polarization and coercive field. A large energy storage density (W) of 1.04 J cm 3 was achieved at 95 kV cm 1 for BTZ0.916. In the high temperature region, the remanent polarization was suppressed, while maximum polarization still maintained at a high level. Significantly enhanced W of 0.952 J cm 3 at 80 kV cm 1 was obtained with a relatively high efficiency of 0.8, making these materials attractive in energy storage applications. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Dielectric capacitors have attracted intensively attention in energy storage applications owing to their high power density, fast charge/discharge, low cost and mechanical stability, as compared with batteries [1–4]. They are of great importance to meet the sustainable energy and the growing power supply demand. The main drawback of capacitors is their limited energy density. Thus, studies have been devoted to the improvement of their energy density. There are two approaches to achieve high energy density of materials, increasing polarization or breakdown electric field. A high energy density can be expected in ferroelectric materials, since they exhibit higher polarization. They have high dielectric constants, but suffer from dielectric losses during charging and discharging cycles, as a result of the ferroelectric hysteresis [5–7]. A considerable portion of energy stored in the charging process will lose, leading to a lower efficiency. Antiferroelectric materials have lower remnant polarization (Pr) and coercive electric field (Ec), as well as faster discharge rates for dissipating stored electrical energy, because of their reversible ferroelectric to anti-ferroelectric phase transition induced by electric filed [4,8–11]. However, they exhibit either poor fatigue resistance behavior or low polarization in both lead-based, such as PbZr1 xTixO3 series, and lead-free capacitors. Environmental demands are pushing the development toward non-Pb materials. Lead-free Bi0.5Na0.5TiO3 (BNT) family is considered to be promising alternative for dielectric, ferroelectric, n
Corresponding author. E-mail address: [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.ceramint.2016.08.087 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
piezoelectric and ionic conductive applications [12–16]. Pure BNT is perovskite-structured ferroelectrics with A-site disorder at room temperature. It has high polarization, large electric field-induced strain and special dielectric relaxation. To further enhance its piezoelectric properties, extensive research are carried out to combine BNT-based solid solutions with ABO3-type ferroelectrics and non-ferroelectrics to form morphotropic phase boundary (MPB) compositions, such as tetragonal BaTiO3 (BT), orthorhombic K0.5Na0.5NbO3 (KNN), and tetragonal Bi0.5K0.5TiO3 (BKT) [17–20]. BNT experiences a complicated variation in crystal structure with composition and temperature. Foreign elements would result in nanotwins with monoclinic symmetry as an intermediate bridging phase that facilitates a pathway for polarization reorientation [21– 24]. As the content is increased, the composition displays characteristics of relaxors with tetragonal symmetry. Significantly suppressed polarization, drastic reduced remanent strain and coercive field are achieved, with the presence of slim polarization hysteresis loops [12,20,23,25]. Thus, it is necessary to find a balance between high polarization and low hysteresis to achieve high energy storage performance. Meanwhile, BNT and its derivations experience diffused phase transitions, with temperature evolution from rhombohedral ferroelectric phase to a tetragonal non-polar or weakly polar phase which is either a relaxor ferroelectric state or antiferroelectric phase [24–26]. The polarization hysteresis loops display similar change as compared with that induced by composition, where slim polarization hysteresis loop and high strain can be obtained at the temperature higher than depolarization temperature. Thus, the energy storage property will be enhanced effectively by adjusting the phase transition
X. Liu et al. / Ceramics International 42 (2016) 17876–17879
temperature of ferroelectric to relaxor phase to appropriate values. Moreover, composition modification can reduce the ferroelectricrelaxor transition temperature, which facilitates room temperature application of these materials. Structural analysis in Bi0.5Na0.5TiO3-BaTiO3 (BNTBT) has revealed a cubic symmetry lying at the MPB between a rhombohedral BNT phase and a tetragonal BT phase in their virgin state [27]. Temperature dependent permittivity of BNTBT displays a less abrupt change, which is suitable for high temperature applications [28]. In addition, tetragonal Bi(Ti0.5Zn0.5)O3 (BTZ) has been reported to have large calculated ferroelectric polarization. Moreover, BTZ can be used to increase the density of ceramics, which is beneficial to increase the breakdown voltage and thus raise the energy density [29,30]. In this study, effect of BZT on ferroelectric and energy storage properties of BNTBT was studied. BZT was used to enhance the tetragonal phase to realize high polarization and low hysteresis, so as to further improve the energy storage performance. Polycrystalline (1 x)BiTi0.5Zn0.5O3-x(0.935Bi0.5Na0.5TiO3-0.065BaTiO3) with 1rxr0.7, as shown in Fig. 1, were prepared by using a conventional solid state reaction method, with mixing appropriate amount of high purity Bi2O3 (99.9%), Na2CO3 (99.8%), BaCO3 (99%), TiO2 (98%) and ZnO (99.99%) as raw powders. The powders were weighed and mixed by using ball milling in isopropyl alcohol for 12 h. The mixtures were calcined at 850 °C for 4 h. The calcined powders were remilled for 12 h and then cold isostatically pressed into pellets of 10 mm in diameter at a pressure of 250 MPa. The pressed pellets were sintered at 1070–1100 °C embedded in precursor powders to avoid Bi volatilization. Phase structures of powders and the sintered pellets were investigated by X-ray diffraction (XRD) with Cu Kα radiation (XRD7000, Shimadzu, Kyoto, Japan). The Rietveld program FullProf was used for full-pattern matching and structural refinements. Archimedes method was employed to measure density of the ceramics. The theoretical density was derived from the lattice parameters obtained from the XRD diffraction pattern. The peaks in XRD data could be indexed with a pseudocubic phase for all compositions. All samples had 494% of the theoretical density. The sintered pellets were polished for property investigations. Sliver electrodes were coated on both polished surfaces and fired at 950 °C for 3 h to ensure maximum conductivity and adherence. Dielectric properties with signal level of 50 mV were measured by using a precision impedance analyzer (4294A, Agilent, CA, USA), associated with temperature controller (TP94, Linkam, Surrey, UK), at a heating rate of 3 °C min 1. Temperature dependence of electric-
Fig. 1. Two cycles of polarization hysteresis and bipolar strain at 1 Hz at ambient temperature plotted on the BTZ-BNT-BT phase diagram. The virgin loops are shown in colors and the second in black. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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field-induced polarization and strain were measured by using a ferroelectric test unit (TF-2000, aix-ACCT, Aachen, Germany) from room temperature to 190 °C. Fig. 1 shows the virgin and the second polarization hysteresis and bipolar strain of BTZ-BNTBT. The polarization hysteresis displays a well-saturated typical ferroelectric profile. The virgin bipolar strain exhibits a poling strain of 0.470% with large remanent strain of 0.330%, with the second cycle to be a typical butterflytype strain loop. Therefore, BNTBT belongs to nonergodic relaxors, which transforms irreversibly into a ferroelectric phase upon the application of a sufficiently high electric field [12]. With the addition of BTZ, the depoling temperature is reduced, with drastic dropping of Ec and Pr. BTZ0.966 is an ergodic relaxor at room temperature, characterized by the “square” shaped pinched hysteresis loop and the absence of negative strain, so that the application of sufficiently large electric fields can reversibly induce ferroelectric long range order. For BTZ0.916, “slanted” shaped hysteresis loop is observed. It is demonstrated that the critical bias field for the ergodic relaxor to ferroelectric transition is strongly increased with increasing BTZ content and no such ferroelectric long range order can be created. According to the P–E hysteresis loops, energy storage behaviors of BTZ-BNT-BT can be evaluated by using the following equation:
W=
η=
∮ E∙dP
W × 100% W + Wloss
(1)
(2)
where W is the energy storage density, E is applied electric field and P is the polarization. W can be determined with their saturation electric field values. η and Wloss represent capacitor efficiency and energy loss, which are highly related to domain reorientation. It is represented by the area of the loop, so that the energy delivered to the capacitor is larger than the energy that can be recovered. As reflected by XRD results, BTZ-BNTBT exhibits pseudocubic phase of relaxors. The response of polar nano-regions induced micro-domains rather than macroscopic domains can be accelerated under the electric field, resulting in slim hysteresis loops. Both Pr and Ec are suppressed evidently with increasing BTZ content, which is beneficial to increase the energy storage density
Fig. 2. Polarization hysteresis of BTZ0.916 at 26 °C that indicates a high energy density of 1.04 J cm 3 at 95 kV cm 1, and its fatigue properties of Pmax and Pr. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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Table 1 Comparison of ferroelectric and energy storage properties of several lead-free ceramics at ambient temperature. Compounds
W (J cm 3)
Pmax (μC cm 2)
Pr (μC cm 2)
E (kV cm 1)
η
Ref.
0.90BNTBT-0.10NaNbO3 0.96BNTBKT-0.04BaZrO3 BNTBT-0.06KNbO3 0.91BT-0.09BY 0.9BT-0.1Bi(Mg2/3Nb1/3)O3 0.9BNTBT-0.1Bi(Mg1/2Ti1/2)O3a BTZ0.916 BTZ0.916a
0.71 0.73 0.89 0.71 1.13 0.85 1.04 0.952
28.8 30 28 – 14 – 32.8 29.4
3.8 3.5 2.6 – 0.4 – 4.8 1.57
70 70 100 93 140 70 95 80
0.65 0.75 0.73 0.826 0.93 0.82 0.59 0.79
[19] [31] [32] [7] [33] [26] Current
a
Results from hysteresis loops at 80 °C.
Temperature dependent energy density W, capacitor efficiency
η and dielectric loss tan δ are shown in the inset (b) of Fig. 3. In the beginning, both W and η show monotonous increase with in-
Fig. 3. Polarization hysteresis of BTZ0.916 at elevated temperatures. Inset (a) of Fig. 3 shows the slim P–E loop and polarization current density curve at 70 °C, and the inset (b) is the temperature dependence of energy density W, capacitor efficiency η and dielectric loss tan δ at 1 k, 10 k and 100 kHz.
and efficiency. Fig. 2 shows energy density (marked in red area), maximum polarization (Pmax) and remanent polarization (Pr) of BTZ0.916 at 26 °C. BTZ0.916 is characterized by a high Pmax of 32.8 μC cm 2 and a low Pr of 4.8 μC cm 2 at 95 kV cm 1. Considerable large W of as high as 1.04 J cm 3 is achieved. For comparison, the energy storage properties of several lead-free ceramics are also listed in Table 1. Meanwhile, the ceramic shows excellent anti-fatigue property, so that the degradation is negligible with the variation of both Pmax and Pr to be less than 2.2% after 105th cycle. Temperature is also an important parameter to be considered. Fig. 3 displays temperature dependent ferroelectric hysteresis loops of BTZ0.916 at 80 kV cm 1 and 1 Hz. With increasing temperature, the sample experiences a transition from FE phase to relaxor phase. Traditionally, Pmax exhibits an obvious decrease in most ambient MPB as a gradual narrowing of P-E loops, induced by composition or temperature. However, in this study, Pmax maintained at a high level of 30 μC cm 2 with Pr showing a decrease within the temperature range, thus leading to high energy density, where the high Pmax originated from the relaxor FE phase and low Pr of 1.77 μC cm 2 from the coexistence of FE and RFE phase. The inset (a) of Fig. 3 shows the slim P-E loop and corresponding polarization current density curve at 70 °C. Four broad peaks are observed in the polarization current density curve, indicating the aligning of the randomly oriented polar nanoregions or nanodomains along the external field direction [34]. However, no longrange ferroelectric phase is induced, because of the loss of hybridization of the 6s2 bismuth lone pair with the oxygen p-orbitals, as BTZ content was increased, which triggers a more-disordered lattice. After removing the electric-field, this orientation is lost.
creasing temperature. At 80 °C, energy density W reaches its maximum of 0.952 J cm 3, with η ¼0.79 at 80 kV cm 1, which is a remarkably high value for lead-free ceramics. The dielectric loss tanδ also keeps at a low level of r 0.08 in the measured temperature range. Additionally, the ceramics have a small variation of o3.4% in the energy density from 60 °C to 120 °C, while the capacitor efficiency is typically higher than 0.75, indicating that they are suitable for the high temperature applications. Also, it is expected that higher energy density could be achieved at high electrical fields since the sample has a high electric breakdown voltage in ceramics, making them potential alternatives especially as a form of thin film. In conclusion, remanent polarization and coercive field of BNTBT were reduced with the addition of BTZ, originated from the response of polar nano-regions induced micro-domains. A large energy density of 1.04 J cm 3 was achieved at an electric field of 95 kV cm 1 in BTZ0.916 at ambient temperature with excellent anti-fatigue property. More interestingly, Pmax maintained at a high level of 430 μC cm 2, with a decreased Pr at elevated temperature at 80 kV cm 1. As a result, the ceramics had a small variation of the high energy density from 60 °C to 120 °C, with high efficiency of 0.8. At 80 °C, W reached its maximum of 0.952 J cm 3 with η of 0.79. These results suggest that BTZ modified BNTBT ceramics are promising candidates for energy storage applications.
Acknowledgements This work was supported by the National Natural Science Foundation (51172187 and 51372197), the Key Innovation Team of Shaanxi Province (2014KCT-04), and the Major International Joint Research Program of Shaanxi Province (2012KW-10) and the Doctoral Starting-up Foundation of Xi'an University of Science and Technology (2015QDJ046).
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