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Novel lead-free ceramic capacitors with high energy density and fast discharge performance
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Novel lead-free ceramic capacitors with high energy density and fast discharge performance Xu Lia, Xiuli Chena,∗∗, Jie Suna, Mingxing Zhoub, Huanfu Zhoua,∗ a Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, School of Materials Science and Engineering, Guilin University of Technology, Guilin, 541004, China b Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Jiading District, Shanghai, 201800, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase structure Dielectric properties Energy storage Pulsed charge-discharge
Dielectric capacitors with high energy storage density, good frequency/temperature stability, and fast chargedischarge capability are highly demanded in pulsed power systems. In this work, we design and prepare a novel lead-free 0.88BaTiO3-0.12Bi(Li1/3Zr2/3)O3 (0.12BLZ) relaxor ferroelectric ceramic for dielectric capacitor application. The microstructure, conduction mechanism, dielectric properties, and energy storage behavior of the 0.12BLZ ceramic were systematically studied. The impedance analysis demonstrates that the introduction of BLZ enhances the insulation ability and breakdown strength of the 0.12BLZ ceramic. Meanwhile, the introduction of BLZ can also reduce the polarization nonlinearity of the BaTiO3 matrix due to the weakly coupled relaxor behavior. As a result, ultrahigh energy storage density (Urec) of 3 J/cm3 and efficiency (η) of 93.8% were simultaneously achieved in 0.12BLZ ceramic. The significantly improved Urec was far superior to most of the recently reported lead-free bulk ceramics. Additionally, excellent frequency and temperature stability (variations of Urec less than 15% in different frequencies (1–100 Hz) and temperatures (25–140 °C)) can be also observed. In addition, the 0.12BLZ ceramic presents an ultrahigh current density (Ddis) of 759 A/cm2, a giant power density (Pm) of 37.9 MW/cm3, and a fast discharge time of 80 ns. The pulsed charge-discharge performances of the 0.12BLZ ceramic are obviously better than those of other lead-free ceramics. These results indicate that the 0.12BLZ relaxor ferroelectric may be an excellent candidate material applied for pulsed power systems.
1. Introduction The progressively salient energy and environmental problems have encouraged to developing and utilizing renewable and environmentally friendly energy materials for the past few years [1–3]. Currently, energy storage devices in service or on the way of exploitation cover a wide range, including batteries, electrochemical capacitors and dielectric capacitors, among which dielectric capacitors have been widely concerned due to large power density, fast charge or discharge rates, and long service life [4–6]. However, the inferior Urec of dielectric capacitors, which is mainly restricted via the dielectric materials, is still the bottleneck of its application. Normally, there are four representative dielectric materials for energy storage application: linear dielectrics (LDs), ferroelectrics (FEs), relaxor ferroelectrics (RFEs) and antiferroelectrics (AFEs) [7,8]. The Urec and η of the above materials could be calculated based on the
∗
polarization-electric field (P-E) curves by the following formula: P P U Urec = ∫P max EdP , Utotal = ∫0 max EdP , and η = U rec × 100% [9,10], where r total E, Pmax, Pr and Utotal are the electric field, maximum polarization, remnant polarization, and total energy storage density, respectively. One can be seen that high Pmax, low Pr and large dielectric breakdown strength (Eb) are critical to achieve a large Urec. LDs materials usually have large Eb and small Pr, but low Pmax (or small permittivity) restricts their further applications in the field of energy storage [11,12]. In contrast, AFEs materials with moderate Eb, high Pmax, negligible Pr always achieve a high Urec [13]. Most materials, however, are based on lead, which is harmful to the human's health and environment. Recently, a series of new lead-free AgNbO3-based AFEs ceramics with a large Urec (> 2 J/cm3) have been successfully prepared [2,14,15]. However, Ag oxide is expensive and difficult to prepare due to the easily produced defects over the sintering process [16]. Meanwhile, double hysteresis loops induced via AFE-FE transformation result in
Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected] (H. Zhou).
∗∗
https://doi.org/10.1016/j.ceramint.2019.10.055 Received 9 September 2019; Received in revised form 4 October 2019; Accepted 6 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Xu Li, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.055
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size of the 0.12BLZ ceramic, the grain size distributions are displayed in the inset of Fig. 1(c). One can be observed that the grain size of this system is only 1.81 μm on average, which is far lower than the average grain size of the unmodified BT. This dense microstructure, low porosity, and small grain size are favorable to acquire large Eb [30]. The estimated Eb is exhibited in Fig. S2. As expected, large Eb of 313 kV/cm is obtained in 0.12BLZ ceramic, which was larger than most lead-free dielectric ceramics [7,8,20,21]. To investigate the conduction mechanism of 0.12BLZ ceramic, impedance spectroscopy is employed, as exhibited in Fig. 2. It can be observed that the two superimposed semicircles are presented in Fig. 2(a), indicating that there have two dielectric relaxations. One is related to the grain response in the high frequency region, another is linked to the grain boundary response, which can also be verified by the frequency dependence of impedance imaginary part (-Z″) and modulus imaginary part (M″), as exhibited in Fig. 2(b) [31,32]. Meanwhile, two sets of equivalent RQC (Q is constant phase element, CPE) circuit elements in series are employed to fit impedance data, where the 2RQC components represent grain and grain boundary, respectively, as depicted in the inset of Fig. 2(a). Good correlations are attained in Fig. 2(a). Generally, the resistance of the grain and grain boundary can be considered as a thermal activation process. Hence, the resistance data for the grain and grain boundary conform to the Arrhenius relationship, respectively, which can be expressed as follows:
higher energy loss (lower energy efficiency) and microcracks (short lifetime) [17]. All these seriously limit the practical application of AFEs ceramics. Therefore, FEs and RFEs have been the focus of attention in the past decade. BaTiO3 (BT), as one of the representatives FEs ceramics, has attracted wonderful attention in the field of dielectric capacitors [18,19]. Due to the long range ordered spontaneous polarization, BT ceramics usually possess a large dielectric constant. Nevertheless, owing to the suppression of the domain inversion under the high E, BT ceramics usually exhibit significant dielectric nonlinearity [20]. To reduce this dielectric nonlinearity and improve the energy storage characteristic, a great deal of work has been done [21–23]. By referring to the experience of PbTiO3-BiScO3 in the field of high-temperature piezoelectric ceramics, Ogihara et al. found that BiScO3 modified BT ceramics (i.e. BT-BiScO3 ceramics) have nearly linear polarization response and revealed that the excellent energy storage characteristics are due to their weak coupling relaxation ferroelectric behaviors [24,25]. Subsequently, the researchers developed a large number of BT-BiMeO3 systems (Me symbolize trivalent or average trivalent metal ions) and found similar near linear polarization response behavior. Among these, nevertheless, the values of Urec are usually up to 1–3 J/cm3 [20,26,27]. Notably, the achieving ultrahigh Urec of ≥ 3 J/cm3 and η of ≥ 90 % simultaneously are seldom reported in both the BT-BiMeO3 and other lead-free systems. In this work, a novel RFE ceramic, 0.12BLZ system, was designed and synthesized. Excitingly, high Pmax of 26.145 μC/cm2, low Pr of 0.876 μC/cm2, and large Eb of 313 kV/cm were obtained, giving rise to the ultrahigh Urec (3 J/cm3) and η (93.8%) in the 0.12BLZ bulk ceramic. More importantly, excellent frequency and temperature stabilities of the energy storage properties, with the Urec of 2.092–2.044 J/cm3 in the frequency ranging from 1 to 100 Hz and the Urec of 2.09–1.802 J/cm3 over 25–140 °C at 220 kV/cm, are also attained, which is significantly superior to most of the recently reported lead-free ceramic capacitors. In addition, the discharge speed of the 0.12BLZ bulk ceramic was examined in this work, providing an application in excellent energy density capacitors.
E R = R 0 exp ⎛ rel ⎞, k ⎝ BT ⎠ ⎜
⎟
where R is the resistance, R0 is the constant, kB is the Boltzmann constant, and Erel is the resistance activation energy. The activation energy (Egb) of the grain boundary is higher than that of the grain (Eg) for 0.12BLZ ceramic (as exhibited in Fig. 2(c)), implying that the prime resistivity contribution at high temperatures ought to be attributed to the grain boundary. The higher Egb in grain boundary signifies the decrease of free oxygen vacancies, implying a higher barrier for the transition of oxygen vacancies (as depicted in the inset of Fig. 2(c)) [33], thus making up for the grain boundary defects. These results indicate that the introduction of BLZ enhances the insulation ability and Eb of the 0.12BLZ ceramic. The temperature dependences of the dielectric constant (ε) and dielectric loss (tanδ) of the 0.12BLZ solid solution at the various frequencies are presented in Fig. 3(a). A strong relaxor characteristic is observed and verified via the broadening dielectric peak corresponding to frequency dispersion, as displayed in Fig. 3(a) [26]. This strong relaxor behavior (as shown in Fig. S3) can be explained via the fact that the long-range dipole orientation becomes difficult due to various ionic charges and sizes, thus giving rise to local heterogeneous structures in the form of PNRs. In general, the relaxor behavior of RFE is considered to be a thermal activation process similar to that of spin or dipole glass, acting only above a finite freezing temperature (Tf) [31,34]. To further describe the relaxor behavior, Vogel-Fulcher (V–F) relation is employed as follow:
2. Experimental procedure The experiment processes for 0.12BLZ ceramic are provided in the supporting material. 3. Results and discussion Fig. 1(a) presents the bulk and relative densities of 0.12BLZ ceramic at different sintering temperatures. The bulk and relative densities exhibited an evident rise with the increase of sintering temperature from 1240 to 1300 °C, showing slow decrease as the sintering temperature is further increased, which is attributable to the abnormal growth of grains arising from over-sintering (as shown in Fig. S1) [26]. In order to determine the phase structure and the occupation of ions for 0.12BLZ ceramic, the Rietveld refinement of the XRD diffraction pattern was done and the result was shown in Fig. 1(b). Pure perovskite structure is presented in the XRD pattern, which indicates that (Bi3+, Li+, Zr4+) ions enter into the lattices of BT to form a stable solid solution [28]. The estimated lattice parameters and locally magnified (111) and (200) peaks via the Rietveld refinement are presented in the inset of Fig. 1(b), which verify a pseudo-cubic phase [29]. To further ascertain the phase structure for the test sample, the high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) pattern along the [0 1−1] and [0 1 0] zone axis are displayed in Fig. 1(d). The interplanar spacing of d011 = 2.85(3) Å, d100 = 4.02(4) Å is estimated in Fig. 1(d). These results correspond with the analysis of XRD. Fig. 1(c) exhibits the Backscattered Electron (BSE) image of the surface for 0.12BLZ solid solution. The measured sample presents homogenous and dense microstructure. And hardly any pores were detected in the surface of the test sample. To estimate the average grain
−Ea ⎤, f = f0 exp ⎡ ⎢ kB (Tm − Tf ) ⎥ ⎦ ⎣ where f is the test frequency, f0 is a pre-exponential factor, and Ea is the activation energy of the relaxation process. The fitted curves displayed good correlations, as depicted in Fig. 3(b). The Ea for the 0.12BLZ solid solution was 0.251 ± 0.012 eV, which is higher than conventional RFEs (PZN and PMN) [35,36]. This phenomenon was also found in other BT-based RFE systems, including BT-Bi(Zn0.5Sn0.5)O3 (Ea ≈ 0.246 eV) and BT-BiScO3 (Ea ≈ 0.25 eV) [25,26]. High Ea indicates that it is hard to acquire long-range dipole orientation under field cooling conditions, and the polar clusters are in an isolated and frustrated state, resulting in the weakly coupling between adjacent clusters. Macroscopic switching of the polarization of the so-called 2
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Fig. 1. (a) Bulk and relative densities of 0.12BLZ ceramic at various sintering temperature. (b) XRD Rietveld refinement results of the 0.12BLZ ceramic. (c) BSE image of the 0.12BLZ ceramic. Insert shows average grain size of the 0.12BLZ ceramic. (d) The HRTEM and SAED image along the [0 1−1] and [0 1 0] zone axis of 0.12BLZ ceramic.
Fig. 2. (a) Complex impedance spectra for 0.12BLZ ceramic in the temperature range of 460–560 °C (Insert shows equivalent circuit fitting impedance data.), (b) Imaginary part of impedance (-Z″) and (-M″) as a function of frequency measured in 460 °C, (c) Arrhenius fit to the resistance data of the grain and grain boundary for 0.12BLZ ceramic.
“weakly coupled relaxor” can only be carried out at very low temperatures and high electric fields [31]. This trait is extremely advantageous for energy storage because it is normally related with
depressed dielectric nonlinearity (or high polarization saturation field), slender P-E curve (or low energy loss), and eminent thermal stability (above Tf).
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Fig. 3. Dielectric properties for the 0.12BLZ ceramic.
Fig. 4. (a) Unipolar P-E loops of the 0.12BLZ ceramic at various electric fields (Insert shows the unipolar current intensity-electric field (I–E) curves at various electric fields), (b) the variations of Pm and Pr values at various electric fields, (c) Urec, Utotal, and η versus applied electric field of 0.12BLZ ceramic, (d) The comparison of Urec and η for the lead-free ceramics.
Similar phenomena have been found in other BT-BiMeO3 systems [20]. The Urec, Utotal, and η of the 0.12BLZ ceramic at the various E are calculated and displayed in Fig. 4(c). As expected, ultrahigh Urec of ≥3 J/ cm3 and η of ≥90 % simultaneously are attained in 0.12BLZ ceramic due to high Pmax of 26.145 μC/cm2, low Pr of 0.876 μC/cm2, and large E of 285 kV/cm. Fig. 4(d) presents the Urec and η of various lead-free dielectric ceramics [2,3,14,26,38–54]. For the most AFEs ceramics, they usually possess high Urec, but the high loss caused by the double hysteresis loop induced via AFE-FE transformation result in the small η. For example, AN-based AFEs can provide high Urec of ≥3 J/cm3, but low η (< 80%) restrict their further applications in the field of energy storage. On the contrary, RFEs ceramics usually present a high η due to the presence of small size PNRs. For instance, most BT-based RFEs provide a high η of ≥ 90%. However, they are constrained by low Urec of < 3 J/cm3 due to low Pmax and small Eb. These results manifest that
To better characterize the energy storage properties of 0.12BLZ ceramic, the unipolar the P-E curves were employed and shown in Fig. 4. Typical relaxor behavior is exhibited in 0.12BLZ ceramic, showing high Pmax and low Pr values, which are both conducive to obtain a high Urec. This slim P-E loop may be ascribed to the fact that small sizes of PNRs are easier to flip than those of macroscopic domains in the case of applying or removing the E. The Pmax and Pr values determined by the P-E loops were presented in Fig. 4(b). With the increase of the electric field, the value of Pmax shows a significant increase from 10.098 μC/cm2 to 26.145 μC/cm2, while the change in Pr remained very small (0.144–0.921 μC/cm2). One can be observed from the inset of Fig. 4(a) that this system has no obvious domain switching under the loading of E, indicating that the polarization is mainly determined via the coherence of the isolated PNRs in the matrix [37]. The small Pr may be ascribed to strong relaxation behavior, as exhibited in Fig. S3. 4
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Fig. 5. (a) Frequency dependent unipolar P-E loops, (b) Temperature dependent unipolar P-E loops, (c) and (d) The calculated Urec and η values from (a) and (b), respectively.
Fig. 6. (a) Pulsed overdamped discharge current curves of the 0.12BLZ ceramic at different electric fields (Insert shows the variation of current peak, Wdis and t0.9 as a function of the electric field), (b) Wdis as a function of time for the 0.12BLZ ceramic at different electric fields, (c) Underdamped discharge waveforms of the 0.12BLZ ceramic at different electric fields. (d) Variation of Imax, Ddis, and Pm as a function of the electric field.
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Fig. 6(b) presents the Wdis dependence of discharge time under different E. One can be seen that t0.9 is always a very short time (~80 ns) with increasing E, as displayed in the inset of Fig. 6(a). Meanwhile, the current peak and Wdis rise from 1.24 A and 0.011 J/cm3 to 13.2 A and 0.96 J/cm3 with the increase of E, respectively. Fig. 6(c) exhibits the influence of E on underdamped discharge properties of the 0.12BLZ ceramic at ambient temperature. One can be observed that the current peak increases gradually with increasing E, while the discharge period τ hardly changes (the definition of τ is described in the inset of Fig. 6(c)), which conforms to previous reports on RFEs ceramics [20,26]. Fig. 6(d) exhibits the variation of current (Imax), Ddis (ImaxS−1), and Pm (EImax/2S, S is the effective area of the measured sample) of the 0.12BLZ ceramics at various E. The value of Imax for the 0.12BLZ ceramic increases from 5.44 (E = 1 kV mm−1) to 53.6 A (E = 10 kV mm−1). Moreover, the values of Ddis and Pm are 759 A/cm2 and 37.9 MW/cm3 at 10 kV mm−1, respectively. Fig. 7 presents the dielectric ceramic system with good charge-discharge property. Compared with most traditional antiferroelectric, BNT-based and BT-based ceramics, the 0.12BLZ ceramic exhibits a higher Ddis, larger Pm value and faster discharge time [26,55–59]. These results indicated that 0.12BLZ ceramic is a promising candidate for advanced pulsed power capacitor.
Fig. 7. A comparison of the charge-discharge properties of the 0.12BLZ ceramic in this work with other reported ceramics.
0.12BLZ ceramic is expected to be a candidate for energy storage materials. For practical applications, the stabilities in frequency and temperature were also important parameter to evaluate the quality of materials. Here, we employed the unipolar P-E loops test project at 220 kV/cm to characterize the stabilities of this system, as displayed in Fig. 5. It can be seen from Fig. 5(c) that there is only a slight decrease in the estimated Urec and η of 0.12BLZ ceramic in the frequency range of 1–100 Hz (i.e. small changes from 2.092 J/cm3 to 2.044 J/cm3 in Urec), which may be attribution to the dense microstructure of the test sample and rapid response of the PNRs to the E [3]. In addition, with the increase of temperature from 25 to 140 °C, the values of Urec decrease from 2.09 J/cm3 to 1.802 J/cm3 (i.e. the fluctuation is below 15%), as depicted in Fig. 5(d). The decrease of Urec may be attributed to the decrease of Pmax, which can also be further verified by dielectric spectrum (as shown in Fig. 3(a)). In particular, the η for 0.12BLZ ceramic increases first in the temperature range of 25–100 °C and then decreases slightly as the temperature further increases. The increasing η may be attributed to the decrease of dielectric loss as the temperature increases. Nevertheless, with the further increase of temperature, the η of 0.12BLZ ceramic presents a decreasing trend because of the increase of leakage current at high temperature (as displayed in Fig. S4). Especially, there is a remarkable fluctuation at 140 °C, which may be on the edge of breakdown due to the accumulation of heat. Although Urec presents a decreasing trend, its change rate is controlled within 15%. The excellent temperature stability of the energy storage properties is ascribed to the adjacent PNR coupling, which can only arise in a large E and low temperature [26]. In addition, dielectric capacitors, as one of the core components of pulse power circuits, usually need to store and release an abundance of energy in an extremely short time to quickly (in nanoseconds) obtain high pulse voltage and large charging current. Therefore, for the energy storage system, the discharge time is an important parameter and ought to be as short as possible. Here, a high-speed capacitor discharge circuit (RC circuit) was employed to elucidate the actual charging-discharging speed behavior of this ceramic, as depicted in Fig. S5 and Fig. 6. The curves between the overdamped pulsed discharge electric current and time for 0.12BLZ ceramic at the various E were presented in Fig. 6(a). The current accesses a peak quickly and lasts only for a very short duration. Besides, the discharge energy density (Wdis) of this component can also be estimated based on the above curves by the following formula [26]:
∫ i2 (t )
Wdis = R
4. Conclusions Lead free 0.12BLZ ceramic has been designed and synthesized via the conventional solid-state reaction method. A pseudo cubic was manifested via XRD and TEM analysis. The material can simultaneously provide high Wrec of 3 J/cm3 and η of 93.8%, which is much higher than that of most lead-free dielectric ceramics. Meanwhile, the system displays decent stabilities (variation of Wrec > 2 J/cm3 in 1–100 Hz and Wrec < 15% over 25–140 °C). In addition, the 0.12BLZ ceramic displays excellent discharge property with an ultrahigh Ddis of 759 A/cm2, a giant Pm of 37.9 MW/cm3, and a fast discharge time (~80 ns). These results indicated that 0.12BLZ ceramic may be an expected lead-free dielectric material for future pulsed power capacitor applications. Declaration of competing interest There are no conflicts to declare. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 11664008, 61761015), Natural Science Foundation of Guangxi (Nos. 2018GXNSFFA050001, 2017GXNSFDA198027and 2017GXNSFFA198011). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.10.055. References [1] O. Ellabban, H. Abu-Rub, F. Blaabjerg, Renewable energy resources: current status, future prospects and their enabling technology, Renew. Sustain. Energy Rev. 39 (2014) 748–764. [2] N. Luo, K. Han, F. Zhuo, L. Liu, X. Chen, B. Peng, X. Wang, Q. Feng, Y. Wei, Design for high energy storage density and temperature-insensitive lead-free antiferroelectric ceramics, J. Mater. Chem. C. 7 (17) (2019) 4999–5008. [3] M.X. Zhou, R.H. Liang, Z.Y. Zhou, X.L. Dong, Superior energy storage properties and excellent stability of novel NaNbO3-based lead-free ceramics with A-site vacancy obtained via a Bi2O3 substitution strategy, J. Mater. Chem. A. 6 (2018) 17896–17904. [4] Z.H. Yao, Z. Song, H. Hao, Z.Y. Yu, M.H. Cao, S.J. Zhang, M.T. Lanagan, H.X. Liu, Homogeneous/inhomogeneous-structured dielectrics and their energy-storage performances, Adv. Mater. 29 (2017) 15. [5] N. Sun, Y. Li, Q. Zhang, X. Hao, Giant energy-storage density and high efficiency achieved in (Bi0.5Na0.5)TiO3-Bi(Ni0.5Zr0.5)O3 thick films with polar nanoregions, J.
dt / V ,
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Novel lead-free ceramic capacitors with high energy density and fast discharge performance Xu Lia, Xiuli Chena,∗∗, Jie Suna, Mingxing Zhoub, Huanfu Zhoua,∗ a Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi, Key Laboratory of Nonferrous Materials and New Processing Technology, Ministry of Education, School of Materials Science and Engineering, Guilin University of Technology, Guilin, 541004, China b Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Jiading District, Shanghai, 201800, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase structure Dielectric properties Energy storage Pulsed charge-discharge
Dielectric capacitors with high energy storage density, good frequency/temperature stability, and fast chargedischarge capability are highly demanded in pulsed power systems. In this work, we design and prepare a novel lead-free 0.88BaTiO3-0.12Bi(Li1/3Zr2/3)O3 (0.12BLZ) relaxor ferroelectric ceramic for dielectric capacitor application. The microstructure, conduction mechanism, dielectric properties, and energy storage behavior of the 0.12BLZ ceramic were systematically studied. The impedance analysis demonstrates that the introduction of BLZ enhances the insulation ability and breakdown strength of the 0.12BLZ ceramic. Meanwhile, the introduction of BLZ can also reduce the polarization nonlinearity of the BaTiO3 matrix due to the weakly coupled relaxor behavior. As a result, ultrahigh energy storage density (Urec) of 3 J/cm3 and efficiency (η) of 93.8% were simultaneously achieved in 0.12BLZ ceramic. The significantly improved Urec was far superior to most of the recently reported lead-free bulk ceramics. Additionally, excellent frequency and temperature stability (variations of Urec less than 15% in different frequencies (1–100 Hz) and temperatures (25–140 °C)) can be also observed. In addition, the 0.12BLZ ceramic presents an ultrahigh current density (Ddis) of 759 A/cm2, a giant power density (Pm) of 37.9 MW/cm3, and a fast discharge time of 80 ns. The pulsed charge-discharge performances of the 0.12BLZ ceramic are obviously better than those of other lead-free ceramics. These results indicate that the 0.12BLZ relaxor ferroelectric may be an excellent candidate material applied for pulsed power systems.
1. Introduction The progressively salient energy and environmental problems have encouraged to developing and utilizing renewable and environmentally friendly energy materials for the past few years [1–3]. Currently, energy storage devices in service or on the way of exploitation cover a wide range, including batteries, electrochemical capacitors and dielectric capacitors, among which dielectric capacitors have been widely concerned due to large power density, fast charge or discharge rates, and long service life [4–6]. However, the inferior Urec of dielectric capacitors, which is mainly restricted via the dielectric materials, is still the bottleneck of its application. Normally, there are four representative dielectric materials for energy storage application: linear dielectrics (LDs), ferroelectrics (FEs), relaxor ferroelectrics (RFEs) and antiferroelectrics (AFEs) [7,8]. The Urec and η of the above materials could be calculated based on the
∗
polarization-electric field (P-E) curves by the following formula: P P U Urec = ∫P max EdP , Utotal = ∫0 max EdP , and η = U rec × 100% [9,10], where r total E, Pmax, Pr and Utotal are the electric field, maximum polarization, remnant polarization, and total energy storage density, respectively. One can be seen that high Pmax, low Pr and large dielectric breakdown strength (Eb) are critical to achieve a large Urec. LDs materials usually have large Eb and small Pr, but low Pmax (or small permittivity) restricts their further applications in the field of energy storage [11,12]. In contrast, AFEs materials with moderate Eb, high Pmax, negligible Pr always achieve a high Urec [13]. Most materials, however, are based on lead, which is harmful to the human's health and environment. Recently, a series of new lead-free AgNbO3-based AFEs ceramics with a large Urec (> 2 J/cm3) have been successfully prepared [2,14,15]. However, Ag oxide is expensive and difficult to prepare due to the easily produced defects over the sintering process [16]. Meanwhile, double hysteresis loops induced via AFE-FE transformation result in
Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected] (H. Zhou).
∗∗
https://doi.org/10.1016/j.ceramint.2019.10.055 Received 9 September 2019; Received in revised form 4 October 2019; Accepted 6 October 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Xu Li, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.055
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size of the 0.12BLZ ceramic, the grain size distributions are displayed in the inset of Fig. 1(c). One can be observed that the grain size of this system is only 1.81 μm on average, which is far lower than the average grain size of the unmodified BT. This dense microstructure, low porosity, and small grain size are favorable to acquire large Eb [30]. The estimated Eb is exhibited in Fig. S2. As expected, large Eb of 313 kV/cm is obtained in 0.12BLZ ceramic, which was larger than most lead-free dielectric ceramics [7,8,20,21]. To investigate the conduction mechanism of 0.12BLZ ceramic, impedance spectroscopy is employed, as exhibited in Fig. 2. It can be observed that the two superimposed semicircles are presented in Fig. 2(a), indicating that there have two dielectric relaxations. One is related to the grain response in the high frequency region, another is linked to the grain boundary response, which can also be verified by the frequency dependence of impedance imaginary part (-Z″) and modulus imaginary part (M″), as exhibited in Fig. 2(b) [31,32]. Meanwhile, two sets of equivalent RQC (Q is constant phase element, CPE) circuit elements in series are employed to fit impedance data, where the 2RQC components represent grain and grain boundary, respectively, as depicted in the inset of Fig. 2(a). Good correlations are attained in Fig. 2(a). Generally, the resistance of the grain and grain boundary can be considered as a thermal activation process. Hence, the resistance data for the grain and grain boundary conform to the Arrhenius relationship, respectively, which can be expressed as follows:
higher energy loss (lower energy efficiency) and microcracks (short lifetime) [17]. All these seriously limit the practical application of AFEs ceramics. Therefore, FEs and RFEs have been the focus of attention in the past decade. BaTiO3 (BT), as one of the representatives FEs ceramics, has attracted wonderful attention in the field of dielectric capacitors [18,19]. Due to the long range ordered spontaneous polarization, BT ceramics usually possess a large dielectric constant. Nevertheless, owing to the suppression of the domain inversion under the high E, BT ceramics usually exhibit significant dielectric nonlinearity [20]. To reduce this dielectric nonlinearity and improve the energy storage characteristic, a great deal of work has been done [21–23]. By referring to the experience of PbTiO3-BiScO3 in the field of high-temperature piezoelectric ceramics, Ogihara et al. found that BiScO3 modified BT ceramics (i.e. BT-BiScO3 ceramics) have nearly linear polarization response and revealed that the excellent energy storage characteristics are due to their weak coupling relaxation ferroelectric behaviors [24,25]. Subsequently, the researchers developed a large number of BT-BiMeO3 systems (Me symbolize trivalent or average trivalent metal ions) and found similar near linear polarization response behavior. Among these, nevertheless, the values of Urec are usually up to 1–3 J/cm3 [20,26,27]. Notably, the achieving ultrahigh Urec of ≥ 3 J/cm3 and η of ≥ 90 % simultaneously are seldom reported in both the BT-BiMeO3 and other lead-free systems. In this work, a novel RFE ceramic, 0.12BLZ system, was designed and synthesized. Excitingly, high Pmax of 26.145 μC/cm2, low Pr of 0.876 μC/cm2, and large Eb of 313 kV/cm were obtained, giving rise to the ultrahigh Urec (3 J/cm3) and η (93.8%) in the 0.12BLZ bulk ceramic. More importantly, excellent frequency and temperature stabilities of the energy storage properties, with the Urec of 2.092–2.044 J/cm3 in the frequency ranging from 1 to 100 Hz and the Urec of 2.09–1.802 J/cm3 over 25–140 °C at 220 kV/cm, are also attained, which is significantly superior to most of the recently reported lead-free ceramic capacitors. In addition, the discharge speed of the 0.12BLZ bulk ceramic was examined in this work, providing an application in excellent energy density capacitors.
E R = R 0 exp ⎛ rel ⎞, k ⎝ BT ⎠ ⎜
⎟
where R is the resistance, R0 is the constant, kB is the Boltzmann constant, and Erel is the resistance activation energy. The activation energy (Egb) of the grain boundary is higher than that of the grain (Eg) for 0.12BLZ ceramic (as exhibited in Fig. 2(c)), implying that the prime resistivity contribution at high temperatures ought to be attributed to the grain boundary. The higher Egb in grain boundary signifies the decrease of free oxygen vacancies, implying a higher barrier for the transition of oxygen vacancies (as depicted in the inset of Fig. 2(c)) [33], thus making up for the grain boundary defects. These results indicate that the introduction of BLZ enhances the insulation ability and Eb of the 0.12BLZ ceramic. The temperature dependences of the dielectric constant (ε) and dielectric loss (tanδ) of the 0.12BLZ solid solution at the various frequencies are presented in Fig. 3(a). A strong relaxor characteristic is observed and verified via the broadening dielectric peak corresponding to frequency dispersion, as displayed in Fig. 3(a) [26]. This strong relaxor behavior (as shown in Fig. S3) can be explained via the fact that the long-range dipole orientation becomes difficult due to various ionic charges and sizes, thus giving rise to local heterogeneous structures in the form of PNRs. In general, the relaxor behavior of RFE is considered to be a thermal activation process similar to that of spin or dipole glass, acting only above a finite freezing temperature (Tf) [31,34]. To further describe the relaxor behavior, Vogel-Fulcher (V–F) relation is employed as follow:
2. Experimental procedure The experiment processes for 0.12BLZ ceramic are provided in the supporting material. 3. Results and discussion Fig. 1(a) presents the bulk and relative densities of 0.12BLZ ceramic at different sintering temperatures. The bulk and relative densities exhibited an evident rise with the increase of sintering temperature from 1240 to 1300 °C, showing slow decrease as the sintering temperature is further increased, which is attributable to the abnormal growth of grains arising from over-sintering (as shown in Fig. S1) [26]. In order to determine the phase structure and the occupation of ions for 0.12BLZ ceramic, the Rietveld refinement of the XRD diffraction pattern was done and the result was shown in Fig. 1(b). Pure perovskite structure is presented in the XRD pattern, which indicates that (Bi3+, Li+, Zr4+) ions enter into the lattices of BT to form a stable solid solution [28]. The estimated lattice parameters and locally magnified (111) and (200) peaks via the Rietveld refinement are presented in the inset of Fig. 1(b), which verify a pseudo-cubic phase [29]. To further ascertain the phase structure for the test sample, the high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) pattern along the [0 1−1] and [0 1 0] zone axis are displayed in Fig. 1(d). The interplanar spacing of d011 = 2.85(3) Å, d100 = 4.02(4) Å is estimated in Fig. 1(d). These results correspond with the analysis of XRD. Fig. 1(c) exhibits the Backscattered Electron (BSE) image of the surface for 0.12BLZ solid solution. The measured sample presents homogenous and dense microstructure. And hardly any pores were detected in the surface of the test sample. To estimate the average grain
−Ea ⎤, f = f0 exp ⎡ ⎢ kB (Tm − Tf ) ⎥ ⎦ ⎣ where f is the test frequency, f0 is a pre-exponential factor, and Ea is the activation energy of the relaxation process. The fitted curves displayed good correlations, as depicted in Fig. 3(b). The Ea for the 0.12BLZ solid solution was 0.251 ± 0.012 eV, which is higher than conventional RFEs (PZN and PMN) [35,36]. This phenomenon was also found in other BT-based RFE systems, including BT-Bi(Zn0.5Sn0.5)O3 (Ea ≈ 0.246 eV) and BT-BiScO3 (Ea ≈ 0.25 eV) [25,26]. High Ea indicates that it is hard to acquire long-range dipole orientation under field cooling conditions, and the polar clusters are in an isolated and frustrated state, resulting in the weakly coupling between adjacent clusters. Macroscopic switching of the polarization of the so-called 2
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Fig. 1. (a) Bulk and relative densities of 0.12BLZ ceramic at various sintering temperature. (b) XRD Rietveld refinement results of the 0.12BLZ ceramic. (c) BSE image of the 0.12BLZ ceramic. Insert shows average grain size of the 0.12BLZ ceramic. (d) The HRTEM and SAED image along the [0 1−1] and [0 1 0] zone axis of 0.12BLZ ceramic.
Fig. 2. (a) Complex impedance spectra for 0.12BLZ ceramic in the temperature range of 460–560 °C (Insert shows equivalent circuit fitting impedance data.), (b) Imaginary part of impedance (-Z″) and (-M″) as a function of frequency measured in 460 °C, (c) Arrhenius fit to the resistance data of the grain and grain boundary for 0.12BLZ ceramic.
“weakly coupled relaxor” can only be carried out at very low temperatures and high electric fields [31]. This trait is extremely advantageous for energy storage because it is normally related with
depressed dielectric nonlinearity (or high polarization saturation field), slender P-E curve (or low energy loss), and eminent thermal stability (above Tf).
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Fig. 3. Dielectric properties for the 0.12BLZ ceramic.
Fig. 4. (a) Unipolar P-E loops of the 0.12BLZ ceramic at various electric fields (Insert shows the unipolar current intensity-electric field (I–E) curves at various electric fields), (b) the variations of Pm and Pr values at various electric fields, (c) Urec, Utotal, and η versus applied electric field of 0.12BLZ ceramic, (d) The comparison of Urec and η for the lead-free ceramics.
Similar phenomena have been found in other BT-BiMeO3 systems [20]. The Urec, Utotal, and η of the 0.12BLZ ceramic at the various E are calculated and displayed in Fig. 4(c). As expected, ultrahigh Urec of ≥3 J/ cm3 and η of ≥90 % simultaneously are attained in 0.12BLZ ceramic due to high Pmax of 26.145 μC/cm2, low Pr of 0.876 μC/cm2, and large E of 285 kV/cm. Fig. 4(d) presents the Urec and η of various lead-free dielectric ceramics [2,3,14,26,38–54]. For the most AFEs ceramics, they usually possess high Urec, but the high loss caused by the double hysteresis loop induced via AFE-FE transformation result in the small η. For example, AN-based AFEs can provide high Urec of ≥3 J/cm3, but low η (< 80%) restrict their further applications in the field of energy storage. On the contrary, RFEs ceramics usually present a high η due to the presence of small size PNRs. For instance, most BT-based RFEs provide a high η of ≥ 90%. However, they are constrained by low Urec of < 3 J/cm3 due to low Pmax and small Eb. These results manifest that
To better characterize the energy storage properties of 0.12BLZ ceramic, the unipolar the P-E curves were employed and shown in Fig. 4. Typical relaxor behavior is exhibited in 0.12BLZ ceramic, showing high Pmax and low Pr values, which are both conducive to obtain a high Urec. This slim P-E loop may be ascribed to the fact that small sizes of PNRs are easier to flip than those of macroscopic domains in the case of applying or removing the E. The Pmax and Pr values determined by the P-E loops were presented in Fig. 4(b). With the increase of the electric field, the value of Pmax shows a significant increase from 10.098 μC/cm2 to 26.145 μC/cm2, while the change in Pr remained very small (0.144–0.921 μC/cm2). One can be observed from the inset of Fig. 4(a) that this system has no obvious domain switching under the loading of E, indicating that the polarization is mainly determined via the coherence of the isolated PNRs in the matrix [37]. The small Pr may be ascribed to strong relaxation behavior, as exhibited in Fig. S3. 4
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Fig. 5. (a) Frequency dependent unipolar P-E loops, (b) Temperature dependent unipolar P-E loops, (c) and (d) The calculated Urec and η values from (a) and (b), respectively.
Fig. 6. (a) Pulsed overdamped discharge current curves of the 0.12BLZ ceramic at different electric fields (Insert shows the variation of current peak, Wdis and t0.9 as a function of the electric field), (b) Wdis as a function of time for the 0.12BLZ ceramic at different electric fields, (c) Underdamped discharge waveforms of the 0.12BLZ ceramic at different electric fields. (d) Variation of Imax, Ddis, and Pm as a function of the electric field.
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Fig. 6(b) presents the Wdis dependence of discharge time under different E. One can be seen that t0.9 is always a very short time (~80 ns) with increasing E, as displayed in the inset of Fig. 6(a). Meanwhile, the current peak and Wdis rise from 1.24 A and 0.011 J/cm3 to 13.2 A and 0.96 J/cm3 with the increase of E, respectively. Fig. 6(c) exhibits the influence of E on underdamped discharge properties of the 0.12BLZ ceramic at ambient temperature. One can be observed that the current peak increases gradually with increasing E, while the discharge period τ hardly changes (the definition of τ is described in the inset of Fig. 6(c)), which conforms to previous reports on RFEs ceramics [20,26]. Fig. 6(d) exhibits the variation of current (Imax), Ddis (ImaxS−1), and Pm (EImax/2S, S is the effective area of the measured sample) of the 0.12BLZ ceramics at various E. The value of Imax for the 0.12BLZ ceramic increases from 5.44 (E = 1 kV mm−1) to 53.6 A (E = 10 kV mm−1). Moreover, the values of Ddis and Pm are 759 A/cm2 and 37.9 MW/cm3 at 10 kV mm−1, respectively. Fig. 7 presents the dielectric ceramic system with good charge-discharge property. Compared with most traditional antiferroelectric, BNT-based and BT-based ceramics, the 0.12BLZ ceramic exhibits a higher Ddis, larger Pm value and faster discharge time [26,55–59]. These results indicated that 0.12BLZ ceramic is a promising candidate for advanced pulsed power capacitor.
Fig. 7. A comparison of the charge-discharge properties of the 0.12BLZ ceramic in this work with other reported ceramics.
0.12BLZ ceramic is expected to be a candidate for energy storage materials. For practical applications, the stabilities in frequency and temperature were also important parameter to evaluate the quality of materials. Here, we employed the unipolar P-E loops test project at 220 kV/cm to characterize the stabilities of this system, as displayed in Fig. 5. It can be seen from Fig. 5(c) that there is only a slight decrease in the estimated Urec and η of 0.12BLZ ceramic in the frequency range of 1–100 Hz (i.e. small changes from 2.092 J/cm3 to 2.044 J/cm3 in Urec), which may be attribution to the dense microstructure of the test sample and rapid response of the PNRs to the E [3]. In addition, with the increase of temperature from 25 to 140 °C, the values of Urec decrease from 2.09 J/cm3 to 1.802 J/cm3 (i.e. the fluctuation is below 15%), as depicted in Fig. 5(d). The decrease of Urec may be attributed to the decrease of Pmax, which can also be further verified by dielectric spectrum (as shown in Fig. 3(a)). In particular, the η for 0.12BLZ ceramic increases first in the temperature range of 25–100 °C and then decreases slightly as the temperature further increases. The increasing η may be attributed to the decrease of dielectric loss as the temperature increases. Nevertheless, with the further increase of temperature, the η of 0.12BLZ ceramic presents a decreasing trend because of the increase of leakage current at high temperature (as displayed in Fig. S4). Especially, there is a remarkable fluctuation at 140 °C, which may be on the edge of breakdown due to the accumulation of heat. Although Urec presents a decreasing trend, its change rate is controlled within 15%. The excellent temperature stability of the energy storage properties is ascribed to the adjacent PNR coupling, which can only arise in a large E and low temperature [26]. In addition, dielectric capacitors, as one of the core components of pulse power circuits, usually need to store and release an abundance of energy in an extremely short time to quickly (in nanoseconds) obtain high pulse voltage and large charging current. Therefore, for the energy storage system, the discharge time is an important parameter and ought to be as short as possible. Here, a high-speed capacitor discharge circuit (RC circuit) was employed to elucidate the actual charging-discharging speed behavior of this ceramic, as depicted in Fig. S5 and Fig. 6. The curves between the overdamped pulsed discharge electric current and time for 0.12BLZ ceramic at the various E were presented in Fig. 6(a). The current accesses a peak quickly and lasts only for a very short duration. Besides, the discharge energy density (Wdis) of this component can also be estimated based on the above curves by the following formula [26]:
∫ i2 (t )
Wdis = R
4. Conclusions Lead free 0.12BLZ ceramic has been designed and synthesized via the conventional solid-state reaction method. A pseudo cubic was manifested via XRD and TEM analysis. The material can simultaneously provide high Wrec of 3 J/cm3 and η of 93.8%, which is much higher than that of most lead-free dielectric ceramics. Meanwhile, the system displays decent stabilities (variation of Wrec > 2 J/cm3 in 1–100 Hz and Wrec < 15% over 25–140 °C). In addition, the 0.12BLZ ceramic displays excellent discharge property with an ultrahigh Ddis of 759 A/cm2, a giant Pm of 37.9 MW/cm3, and a fast discharge time (~80 ns). These results indicated that 0.12BLZ ceramic may be an expected lead-free dielectric material for future pulsed power capacitor applications. Declaration of competing interest There are no conflicts to declare. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 11664008, 61761015), Natural Science Foundation of Guangxi (Nos. 2018GXNSFFA050001, 2017GXNSFDA198027and 2017GXNSFFA198011). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.10.055. References [1] O. Ellabban, H. Abu-Rub, F. Blaabjerg, Renewable energy resources: current status, future prospects and their enabling technology, Renew. Sustain. Energy Rev. 39 (2014) 748–764. [2] N. Luo, K. Han, F. Zhuo, L. Liu, X. Chen, B. Peng, X. Wang, Q. Feng, Y. Wei, Design for high energy storage density and temperature-insensitive lead-free antiferroelectric ceramics, J. Mater. Chem. C. 7 (17) (2019) 4999–5008. [3] M.X. Zhou, R.H. Liang, Z.Y. Zhou, X.L. Dong, Superior energy storage properties and excellent stability of novel NaNbO3-based lead-free ceramics with A-site vacancy obtained via a Bi2O3 substitution strategy, J. Mater. Chem. A. 6 (2018) 17896–17904. [4] Z.H. Yao, Z. Song, H. Hao, Z.Y. Yu, M.H. Cao, S.J. Zhang, M.T. Lanagan, H.X. Liu, Homogeneous/inhomogeneous-structured dielectrics and their energy-storage performances, Adv. Mater. 29 (2017) 15. [5] N. Sun, Y. Li, Q. Zhang, X. Hao, Giant energy-storage density and high efficiency achieved in (Bi0.5Na0.5)TiO3-Bi(Ni0.5Zr0.5)O3 thick films with polar nanoregions, J.
dt / V ,
where i(t), R and V are the discharge current, resistor (200 Ω), and sample volume, respectively. The discharge time (t0.9) usually refers to the time that corresponds with attain 90% saturated Wdis value. 6
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