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High breakdown strength and energy density in antiferroelectric PLZST ceramics with Al2O3 buffer
Ceramics International 46 (2020) 722–730
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High breakdown strength and energy density in antiferroelectric PLZST ceramics with Al2O3 buffer
T
Chunyu Lia, Manwen Yaoa,∗, Wenbin Gaob, Xi Yaoa a
Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, No. 4800, Cao'an Road, Shanghai, 201804, China Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an, 710049, China
b
ARTICLE INFO
ABSTRACT
Keywords: Al2O3 buffer Antiferroelectric PLZST High energy storage density
In this work, a new core-shell structure of antiferroelectric ceramic powder (Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3PLZST) coated with linear dielectric (Al2O3) has been successfully prepared to realize high energy density through tape-casting process. According to the experimental results of electron microscope, the sol-gel derived Al2O3 layer was uniformly coated on the PLZST particles and the Al2O3 layer can be taken as the buffer layer to effectively refine the grain growth as well. Therefore, the modified PLZST particles were fine and uniform compared with the pure PLZST. It was found that the buffer layer could undertake higher electric field and the electric field applied to PLZST particles was weakened based on finite element analysis, which can avoid the premature breakdown of PLZST. And the actual measured breakdown strength was significantly enhanced from 22.2 kV/mm to 35.5 kV/mm. Correspondingly, an extremely high recoverable energy storage density of 5.3 J/ cm3 was obtained for PLZST with 0.5%wt Al2O3, an 204% enhancement over the pure PLZST ceramics (2.6 J/ cm3), and the corresponding efficiency was up to 88.3%. In addition, impedance spectroscopy measurement was carried out to further confirm the better insulation of the ceramic with Al2O3 buffer.
1. Introduction Due to the shortage of natural resources such as oil, coal and natural gas, the generation of electricity from renewable and non-conventional resources is being investigated on a large scale and much attention is being paid to the topic of pulse power system [1–5]. Among the art-ofthe-state of energy storage devices, dielectric capacitors have the advantages of ultrahigh power density coupled with extremely short discharge time duration [6,7]. However, their energy density is approximately 3–5 orders of magnitude lower than those of electrochemical counterparts, such as batteries and electrochemical capacitors, which has restricted their further applications [8,9]. Hence, improving the energy density of dielectric capacitors is highly desirable to meet the ever-increasing demands for reliable and efficient energy storage devices. Among popular dielectric materials, antiferroelectric (AFE) ceramics are one of the strongest contenders for high energy storage ceramic capacitors. AFE materials possess low dielectric loss, low coercive field, low remnant polarization and fast discharge rate, which make AFE materials a lucrative research direction [10,11]. According to previous studies, the energy density stored in the
∗
polarized dielectric (stored energy density-Wst), energy density recovered from the depolarized dielectric (recoverable energy storage density-Wre) and energy efficiency(ŋ) of antiferroelectric can be obtained by numerical integration of the area between the polarization axis and the curves of P-E loops and can be defined as follows [12,13] (1–3): Pmax
Wst =
EdP 0
(1)
Pmax
Wre =
EdP Pre
=
Wre × 100% Wst
(2) (3)
where E is the applied electric field, Pmax is the maximum polarization, and Pre is the remnant polarization. On the basis of equations (1) and (2), the energy density is strongly determined by E, which is constrained by the breakdown strength (BDS). In this light, increasing the breakdown strength is of great beneficial to enhance the energy storage. Currently, lead zirconate (PbZrO3 or PZ) based AFE family
Corresponding author. E-mail address: [email protected] (M. Yao).
https://doi.org/10.1016/j.ceramint.2019.09.025 Received 22 June 2019; Received in revised form 23 August 2019; Accepted 3 September 2019 Available online 03 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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materials, as a group of important electronic materials, especially PLZST [14–16], have attracted increasing attention for their potential applications in high energy storage capacitors because of their external electric field-induced phase switching behavior between antiferroelectric (AFE) state and ferroelectric (FE) state and high recoverable energy density [17]. Nevertheless, the breakdown strength of PLZST ceramics is relatively low which restrains their application to a great extent. Consequently, enhancing the breakdown strength of the PLZST ceramic is a big challenge. With the purpose of solving this problem, aluminum oxide (Al2O3) derived from sol-gel wet chemistry was introduced to coat the PLZST ceramic powder to form PLZST@ Al2O3 core-shell structured composite powder. Aluminum oxide, as a superior dielectric material, possesses a high breakdown strength (300–700 kV/ mm) and high band gap (~9 eV) in conjunction with its excellent chemical and thermal properties. Moreover, aluminum is abundant in the Earth and inexpensive [18]. The outstanding performances of Al2O3 indicate that combining it with the PLZST might be highly beneficial to improve the dielectric properties of the ceramics. Wang et al. have successfully prepared Al2O3/Cu composites by solid-state addition of Al2O3 and achieved a high breakdown field strength of up to 56 kV/mm with 1.2%wt content of Al2O3 [19]. This result demonstrates that Al2O3 is a promising additive to achieve the intention of breakdown strength enhancement. At present, the main way to introduce additives into a ceramic system is by solid-state mixing. This method would result in ununiform distribution and aggregation, which will tend to generate various structure defects and thus damage the electrical properties [20]. In contrast, the method of sol-gel coating is more inclined to obtain a highly homogeneous dispersion by coating a layer outside of the particles to form the core-shell structure. The sol-coating layer can then act as an insulating layer to further improve the electrical performance. Wang et al. have successfully reduced the dielectric loss of Ba0.7Sr0.3Ti0.9925Tm0.01O3 ceramic by coating SiO2 to form the coreshell structure [21]. Additionally, Tong et al. have obtained an enhanced dielectric constant and breakdown strength via coating SiO2 layer on the H2 treated BaTiO3 nanoparticles [22]. However, little research has been done to coat the lead-based AFE particles with a layer of Al2O3 to change the electrical properties. In this study, we successfully fabricated an Al2O3 layer onto the surface of Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3 particles by sol-gel process, which was confirmed by TEM images. SEM images exhibited a few hexagonal small particles dispersed along the grain boundary, which were identified as α-Al2O3 by EDS mapping and X-ray diffraction. Al2O3, as an electrical buffer, not only restricted the abnormal grain growth to increase grain boundaries, but also withstood the majority of field strength and improved the electrical properties significantly. The breakdown strength was greatly enhanced from 22.2 kV/mm to 35.5 kV/mm with 1%wt Al2O3-coating buffer. Furthermore, the calculated energy density from P-E loops was increased by 204% and the highest value was up to 5.3 J/cm3. It is worthwhile to note that in this work the thickness of the ceramic is about 0.1 mm and the electrode pad is 6 mm in diameter, which are well compatible with the dimension requirements of the multilayer ceramic capacitor (MLCC) for high energy storage applications.
2.2. The preparation of Al2O3-coating buffer layer The calcined Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3 powders were mixed with the as-prepared Al2O3 sol in the ratio corresponding to the following content: x wt.% Al2O3 + (100-x) wt.% PLZST, where x = 0, 0.5, 1, 2, respectively. The mixture solution was ball-milled for 36 h and the resulting slurry was subsequently collected and dried at 60 °C for 12 h. After drying, the mixture was heated at 600 °C for 2 h in a muffle furnace to burn off the organic residuals. In addition, the PLZST powder doped with 0.5%wt Al2O3 powder was used as a comparative experiment. 2.3. The preparation of PLZST ceramics PLZST ceramics were fabricated through tape casting. Predetermined amounts of the composite particles, ethanol, trichloroethylene and triethyl phosphate were high-energy vibration milled for 1 h to obtain homogeneous and stable slurry. Then dibutyl phthalate (DBP), polyethylene glycol, n-butanol and polyvinyl alcohol were added to the slurry and high-energy vibration milled for another 2 h to make tape-casting slurry. The slurry was vacuumized to remove the bubbles from vibration. Green tapes were casted by a doctor blade, dried and punched into square tapes with a border length of 12 mm for subsequent electrode coating. Following binder burnout at 500 °C for 2 h, the green tapes were sintered at 1260 °C for 2.5 h in a closed Al2O3 crucible, under leadenriched environment maintained by Pb3O4 to minimize lead volatilization. The thickness of all ceramics was 0.1 mm. Finally, gold pads of 6 mm in diameter were deposited on the sample surface as top electrodes by dc sputtering. Then the gold electrodes were cured at 200 °C for 2 h. 2.4. Characterization The thermal property was determined via a thermogravimetric analysis differential scanning calorimeter (TG-DSC, SAT449C, Netzsch, Germany) from 30 °C to 1200 °C at a heating rate of 10 °C/min in ambient atmosphere. The morphologies of the Al2O3 buffer layer was observed using transmission electron microscope (TEM, JEM-2011F). The phase structure and microstructure of ceramics were examined using X-ray diffractometer (XRD, BRUKRER D8 Advance Diffractometer). The surface morphology of samples and elemental analysis (EDS) were carried out by field-emission scanning electron microscopy (FE-SEM, FEI Quanta 200 FEG). The breakdown strength measurement was performed using a high-voltage tester (TH9310 HIPOT TESTER, Tonghui) at room temperature, and the selected critical current was 2 mA. Impedance spectroscopy measurement (4 Hz ~ 3600 kHz) was carried out using an electrochemical impedance analyzer (IM 3590, Hioki, Japan). The frequency and temperature-dependent dielectric properties of the samples were measured using an Agilent 4980A LCR meter controlled by a computer. Polarization-electric field hysteresis loops were explored to calculate energy storage density by using a ferroelectric testing unit (Premier, Radiant Technologies, Inc., Albuquerque, NM). 3. Results and discussion
2. Experimental
To determine the optimum decomposition temperature of organics, TG-DSC analysis was performed for A0, A0.5, A1, A2 (A0-A2) before sintering and is showed in Fig. 1. There is a significant weight loss in the temperature range of 150 °C–450 °C, and two exothermic peaks (300 °C and 435 °C) are presented in the corresponding DSC curves, which indicate the decomposition and burning process of organic residuals in this temperature range. When the temperature continues to rise above 450 °C, the weights of ceramics do not change obviously. It can be concluded that the decomposition of organics has completed. Based on
2.1. The preparation of Al2O3 sol To prepare Al2O3 sol, (a) 0.02 mol aluminum isopropoxide was dissolved in 50 ml of glycol ether with stirring at 60 °C for 30 min on the magnetic stirrer; (b) the aluminum isopropoxide solution was mixed with 0.02 mol acetylacetone with stirring for 30 min; (c) 10 ml acetic acid as catalyst was added to the above solution at 90 °C with agitation for 30 min to get a clear and homogeneous sol. 723
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Al2O3 layer is successfully coated on the surface of the PLZST ceramic particles and the interplanar spacing estimated from the lattice fringes is about 0.443 nm, which corresponds to the d-spacing of (111) planes of PLZST. SEM images of samples are illustrated in Fig. 3 to study the effect of Al2O3 additive on the morphology of ceramics. Fig. 3 (a)–(d) exhibit the microstructures for ceramics with different Al2O3 sol contents (A0-A2). Fig. 3 (e) presents the surface morphology of PLZST ceramic doped with Al2O3 powder (D0.5). It is obvious that there are almost no pores for pure PLZST yet the grain size is not uniform, in which the largest particle diameter is up to 10 μm, and the smallest one is less than 2 μm. However, with the addition of Al2O3 sol, the surface morphologies of the Al2O3-sol coated ceramics exhibit the following differences in comparison with the pure counterpart. Firstly, several hexagonal small particles present at the grain boundary (marked in red circles), which are identified as second phase. The more Al2O3 sol is added, the more hexagonal particles. In order to analysis the composition of the new phase, EDS mapping of the surface for A0.5 was performed and showed in Fig. 4. The spot EDS analysis of Fig.4(b) indicates that grain area 1 in Fig.4(a) is pure PLZST without diffusion of Al element, while the spot EDS analysis of Fig.4(c) indicates that area 2 in Fig.4(a) is pure alumina with atomic ratio of Al to O very close to 2:3. To further study the diffusion of aluminum, a line mapping EDS from point A to point B for A0.5 was carried out. There are two distinct peaks in the element distribution map, and the peak values are located at the boundaries of hexagonal particles. However, in the surrounding PLZST phase, the Al content decreased significantly, indicating that the Al element is basically confined in the Al2O3 crystallite and hardly diffuse into the PLZST lattice. Thus, based on the EDS measurement, it can be concluded that
Fig. 1. TG-DSC analysis for A0-A2 before sintering.
above analysis, 500 °C was chosen as thermal treatment temperature to ensure the organic residuals can be fully decomposed. The particle information of PLZST coated with Al2O3 buffer layer was verified by TEM in Fig. 2. Fig. 2(a) exhibits pure PLZST particles and (b) and (c) show the ceramic particles with 2%wt Al2O3. It is clear that pure PLZST particles and Al2O3 buffer layer form a typical coreshell structure (PLZST @ Al2O3), of which the schematic structure is presented in Fig. 2(d). As shown in Fig. 2(c), an approximately 6 nm
Fig. 2. (a)TEM image for pure PLZST particles; (b) and (c) TEM images for PLZST with Al2O3-coating buffer; (d) schematic picture for the core-shell structure of PLZST@Al2O3. 724
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Fig. 3. (a)–(d) SEM images for Al2O3 sol coated PLZST ceramics (A0-A2); (e) SEM image for Al2O3 powder doped PLZST ceramic (D0.5).
Fig. 4. (a) SEM images for A0.5; (b) corresponding spot scan for selected area 1; (c) corresponding spot scan for selected area 2; (d) line mapping EDS from point A to point B for A0.5.
well-faceted Al2O3 crystallites precipitated at the grain boundary without apparent chemical reaction with the surrounding PLZST grains. Secondly, the uniformity of the grain size of the PLZST is greatly improved and the average grain size decreased significantly. Obviously, the more the sol content, the smaller the grain size. This phenomenon can be attributed to the existence of Al2O3-coating layer, as well as the Al2O3 crystallites intercalated at grain boundaries, which prevent the migration of the grain boundary during sintering. As a result, the abnormal grain growth is significantly suppressed to obtain smaller grains with a good uniformity. The more Al2O3 additive is added, the more the barrier to the grain boundary migration, the smaller the grain size. Nevertheless, there are too many Al2O3 crystals exist at the grain boundary in A2, which has a negative effect on the formation of typical sintering structure of converged triple grain boundaries among particles
(marked in yellow circles in Fig. 3(d)). Therefore, many particles cannot grow normally and keep a round particle shape (marked in green circles in Fig. 3(d)), which leaves small pores at the grain boundary and thus degrades the ceramic microstructure. Besides, comparing Fig. 3 (b) and (e), it can be seen that the introduction of Al2O3 in the form of sol instead of in the form of powder is more beneficial to eliminate pores in the sintering process to achieve dense ceramics. To determine the phase structure of PLZST main phase and Al2O3 secondary phase, XRD patterns of the sintered ceramics are exhibited in Fig. 5. For pure PLZST ceramic (A0), only peaks of perovskite phase are detected and no other secondary phase can be found. However, several diffraction peaks of α-Al2O3 appear in the composite ceramics (A0.5-A2 and D0.5), which accords with the SEM images and EDS analysis about 725
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Fig. 5. (a) XRD patterns of sintered ceramics; (b) The magnified diffraction pattern between 43° and 45°.
the secondary phase. Furthermore, there are two split peaks around 44°~44.5°(Fig. 5(b)), confirming that the PLZST main phase is typical orthorhombic antiferroelectric. The microstructure morphology of ceramics has a great impact on its electrical properties, especially the breakdown field strength. In order to investigate the effect of Al2O3 addition on breakdown strength (BDS), Weibull distribution of BDS for PLZST ceramics with different Al2O3 contents is exhibited in Fig. 6(a) [23]. The BDS could be extracted from points where the fitting lines intersect with the horizontal
line through Yi = 0 and are presented in Fig. 6(b). In the experiment, the values of the Weibull modules ( ) for the ceramics are more than 30 signifying a valid comparison of BDS is guaranteed. According to the data given in Fig. 6(b), the characteristic BDS is boosted up with the increase of Al2O3 additive and the highest value can reach 35.5 kV/mm, an 60% improvement over the pure PLZST (22.4 kV/mm). Simulations of the electric field were performed to characterize the effect of the Al2O3 buffer layer on electric breakdown strength, the electric field distribution in the ceramics was simulated by the finite
Fig. 6. (a) The Weibull distribution of BDS for A0-A2 and D0.5; (b) The BDS values obtained from Xi intercept of the fitting lines for A0-A2 and D0.5. Simulations of the electric field intensity for (c) pure PLZST (A0) and (d) PLZST@Al2O3 (A1). 726
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Fig. 7. (a)Temperature dependence of dielectric constant and dielectric loss (tanδ) of A0-A2 and D0.5 at 100 kHz; (b) Frequency dependence of dielectric constant and dielectric loss (tanδ) of A0-A2, D0.5.
element analysis. As shown in Fig. 6(c) and (d), the larger and the smaller sized grains represent the uncoated PLZST particles and the coated PLZST particles, respectively. In addition, the gap between the grains in Fig. 6(d) is the Al2O3-coating buffer layer. The A, B, C, and D are denoted as the grain A of the uncoated pure PLZST, the grain core B of the PLZST@Al2O3, the Al2O3-coating buffer layer C, and the grain D of the α-Al2O3 particles, respectively. When an external electric field is applied to the ceramics, grains of pure PLZST exhibit more intensive electric field concentration than the grain cores of the PLZST@Al2O3 (EA > EB). Consequently, ceramic of pure PLZST is more likely to breakdown in contrast to the Al2O3-coated ceramics. These results can be attributed to the existence of Al2O3 buffer layer, which has the following two effects: (1) Al2O3 addition has successfully decreased the grain size of the ceramics and is confirmed by SEM. According to previous study, ceramics have small grain size usually have higher breakdown strength and the correlation between grain size and BDS can be described by E G a , where E, G and a are the electric
breakdown strength, grain size and constant, respectively [24]. It is believed that the grain boundary is the main defender of electric field strength. Small grain size leads to an increased proportion of grain boundary. Under the condition that the breakdown strength value per unit grain boundary does not change, the total voltage value will be increased as a result. (2) Al2O3 buffer layer has the effect of attenuating the electric field applied to the PLZST particles. When there exist two phases in the ceramic in which the dielectric constants are extremely different, the electric field is mainly distributed on the phase with a low dielectric constant. The dielectric constant of Al2O3 is much lower than that of PLZST. Due to this characteristic, the Al2O3-coating layer exhibits an “electric field black hole” under an electric field. When a voltage is applied to the ceramic, the electric field is mostly “sucked” into the Al2O3-coating layer, and fewer electric fields are distributed on the grain boundaries and the PLZST particles, which corresponds to the absorption of the electric field from original crystal phase by Al2O3. For PLZST particles, Al2O3 is equivalent to an electric field buffer layer that
Fig. 8. Impedance spectra and equivalent circuits of (a) A0 and (b) A0.5. 727
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Fig. 9. (a) The polarization-electric field hysteresis loops of ceramics measured at 10 Hz with different applied fields at room temperature; inset shows the variation of Pmax for A0-A2 and D0.5; (b) The recoverable energy density versus electric field for A0-A2 and D0.5, inset shows the electric field dependence of the corresponding energy efficiency; (c) The Wst, Wre and ŋ values for A0-A2 and D0.5; (d) Energy storage performance of recently reported lead-based and lead-free AFE ceramics, in which the two PLZST are (Pb0.97La0.02)(Zr0.58Sn0.35Ti0.07)O3 [15] and (Pb0.94La0.06)(Zr0.66Sn0.23Ti0.11)O3 [16], respectively; PLZT is (Pb0.65La0.35) (Zr0.90Ti0.10)0.1625O3 [26]; PBLZST is (Pb0.90Ba0.04La0.04)[(Zr0.7Sn0.3)0.88Ti0.12]O3 [5]; ACN is Ag0.92Ca0.04NbO3 [27]; AN is AgNbO3 [28]; BNT-BKT is Bi0.5Na0.5TiO3– Bi0.5K0.5TiO3 [29]; NN-CZ is NaNbO3–CaZrO3 [30].
weakens the field strength. Furthermore, the breakdown strength of αAl2O3 is higher than that of PLZST and thus the presence of a small amount of Al2O3 crystallites at the grain boundaries does not affect the breakdown field strength. However, an excessive addition of Al2O3 sol will lead to more Al2O3 crystallites (confirmed in Fig. 3), which prevent the formation of converged grain boundaries and leave a few pores unremoved. Thus, the breakdown strength of A2 is significantly lower than that of other components due to the failure of the microstructure to be completely grown into ceramics. Of great importance is that the characteristic BDS for D0.5 (28.2 kV/ mm) is not as high as that of A0.5 (31.5 kV/mm). The result can be ascribed to the following two factors: uniformity of the Al2O3 dispersion and the existence of pores. (1) Compared with conventional solid-state doping, sol-gel coating method can help to disperse PLZST particles so as to avoid aggregation and make the applied voltage distribute evenly. (2) There exist certain amounts of pores on the ceramic surface for D0.5, which result in a drop in BDS due to their low breakdown strength. Dielectric properties as a function of temperature for ceramics at 100 kHz are given in Fig. 7(a). All ceramics are initially in an AFE state and display only one dielectric anomaly at the temperature Tm, corresponding to the Curie temperature (Tc) for the AFE to paraelectric (PE) phase transition. The value of Tc is about 150 °C and varies slight. Additionally, the Al2O3 additive, both in the form of sol and solid-state particle, can impair the value of dielectric constant yet maintaining the dielectric loss (tanδ) at a low level. As is discussed before, the dielectric constant of Al2O3 is much lower than that of PLZST. In this light, the
introduction of Al2O3 additive (A0-A2, D0.5) can lead to the suppressed dielectric constant. The more the Al2O3, the lower the dielectric constant, which accords with the data in the figure. The frequency dependent dielectric constant and dielectric loss (tanδ) measured at room temperature from 1 kHz to 2000 kHz are presented in Fig. 7(b). The values of dielectric constant drop gradually as the frequency increases, because long-time polarization process, like space charges, stops responding to the external electric field at high frequency [25]. With the aim of further understanding the effect of Al2O3 buffer on the outstanding electrical properties, impedance spectroscopy at 4 Hz–3600 kHz is observed for A0 and A0.5 and the corresponding equivalent circuits are (RPCP)(RB1CBRB2) and (RACA)(RPCP)(RB1CBRB2), respectively. RP and CP represent the resistance and the capacitance of the PLZST grains, while RB and CB refer to the resistance and the capacitance of the grain boundary. RA describes the resistance of the Al2O3 buffer and CA represents the corresponding capacitance. The fitting curves are shown in Fig. 8 and the fitting parameters are collected in the following tables. The errors between the fitting and measured data from the impedance spectra are quite small, indicating the reasonably good compatibility of the presented model. Apparently, the equivalent circuit of A0 is formed by the PLZST grains and the grain boundary in series; for A0.5, due to the introduction of Al2O3, the equivalent circuit is formed by the PLZST grains, the grain boundary and the Al2O3 grains in series. As is showed in tables, the resistance of PLZST grains (RP) and boundary (RB1 and RB2) are increased owing to 728
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sintered ceramics, there are α-Al2O3 particles intercalated at the grain boundary. The Al2O3-coating layer and the α-Al2O3 particles inhibited grain boundary movement to obtain small particles and high proportion of grain boundary. When a voltage is applied to the ceramic, the Al2O3-coating buffer layer, coupled with high proportion of grain boundary, enabled the sample to bear more electric field and was confirmed by the simulations of the electric field. Hence, the breakdown strength was enhanced significantly from 22.2 kV/mm to 35.5 kV/mm. A better insulation of ceramics with Al2O3 buffer was exhibited by impedance spectroscopy. In addition, the maximum energy storage density was up to 5.3 J/cm3, more than twice that of pure counterpart, with 88.6% in energy efficiency. As a result, this work provides a prospective method to enhance energy storage of antiferroelectric ceramics.
Table 1 Characteristic electrical properties for ceramics in this work. Sample
BDS (kV/mm )
Pmax ( µC / cm2 )
Wst
A0 A0.5 A1 A2 D0.5
22.4 31.5 35.5 26.2 28.2
22.2 36 30.7 26.1 30.1
2.8 6 5.4 4 5
( J / cm3 )
Wre
( J / cm3 )
2.6 5.3 4.6 3.6 4.3
92.7 88.3 85.2 90 86
the buffering of Al2O3. Moreover, Al2O3 itself has an extremely high resistance, which is the highest among the resistance parameters. As a result, the total resistance of A0.5 is much higher than that of A0, which further explains the enhancement of breakdown strength for PLZST ceramics with Al2O3 additive. Obviously, the values of CP and CB drop down with Al2O3 additive, indicating a decrease in dielectric constant which is consistent with previous discussions. The polarization-electric field hysteresis loops of ceramics measured at 10 Hz with different applied fields (breakdown electric fields at alternating voltage) at room temperature are illustrated in Fig. 9(a). As shown, all ceramics display slim double loop characteristics of AFE phase, and the residual polarization is close to zero after driving electric field is removed. To investigate the effect of Al2O3 buffer on the maximum polarization, the values of Pmax are plotted in Fig. 9(a) inset. The value of Pmax shares similar variation with that of BDS and the highest value can reach 36 µ C/cm2. The recoverable energy densities (Wre) of the ceramics versus the electric field are shown in Fig. 9(b), and the inset exhibits the corresponding energy efficiencies (ŋ). The energy storage densities grow with the increase of the electric field. In particular, when the applied electric field is increased from 15 kV/mm to 20 kV/mm, the increment of energy storage density is most significant. This result is related to the forward transition electric field (EAF). When the applied electric field reaches the value of EAF, the ceramic changes from the antiferroelectric state to the ferroelectric state, and the polarization surges. Therefore, the energy density increases apparently. It can be inferred from Fig. 9(a) and (b) that the values of EAF for different ceramics vary little and are about 18 kV/mm. The energy efficiencies show an upward trend at the EAF. Due to the sudden increase of the storage energy density, the proportion of the energy loss reduces and the energy storage efficiency is increased. However, the higher the applied electric field, the greater the dielectric loss and the leakage current caused by charge migration, so the lower the energy storage efficiency. The calculated Wst, Wre and ŋ of all ceramics from polarization-electric loops are given in Fig. 9(c). The Al2O3 buffer successfully improved the energy density of PLZST ceramics with relative high efficiency. The maximum Wre is up to 5.3 J/cm3 for PLZST ceramic with 0.5%wt coating Al2O3 buffer (A0.5), an 204% enhancement over the pure counterpart (2.6 J/cm3), and the corresponding efficiency is as high as 88.3%. In addition, by comparing the data of A0.5 and D0.5, the higher recoverable energy density of A0.5 provides an evidence that the Al2O3 buffer introduced by sol-gel coating method is more effective than traditional solid-state doping in improving the energy density of PLZST ceramics. The characteristic electrical properties of ceramics in this work are listed in Table 1. Furthermore, the Wre of recently studied AFE ceramics [5,15,16,26–30] are summarized in Fig. 9(d). It proves that the recoverable energy storage in this study achieved at an electric field of 31 kV/mm is extremely high, demonstrating that the core-shell structured PLZST ceramic with Al2O3 buffer is a promising candidate for energy storage.
Acknowledgments The authors would like to thank the financial support of Chinese government by the grant number 6141A02022433 and the National Natural Science Foundation of China by grant number 51872201. References [1] Z. Liu, X. Chen, W. Peng, C. Xu, X. Dong, F. Cao, G. Wang, Temperature-dependent stability of energy storage properties of Pb0.97La0.02(Zr0.58Sn0.335Ti0.085)O3 antiferroelectric ceramics for pulse power capacitors, Appl. Phys. Lett. 106 (2015) 15–17. [2] X. Wang, Y. Zhang, X. Song, Z. Yuan, T. Ma, Q. Zhang, C. Deng, T. Liang, Glass additive in barium titanate ceramics and its influence on electrical breakdown strength in relation with energy storage properties, J. Eur. Ceram. Soc. 32 (2012) 559–567. [3] S. Jiang, L. Zhang, G. Zhang, S. Liu, J. Yi, X. Xiong, Y. Yu, J. He, Y. Zeng, Effect of Zr:Sn ratio in the lead lanthanum zirconate stannate titanate anti-ferroelectric ceramics on energy storage properties, Ceram. Int. 39 (2013) 5571–5575. [4] Q. Yuan, J. Cui, Y. Wang, R. Ma, H. Wang, Significant enhancement in breakdown strength and energy density of the BaTiO3/BaTiO3@SiO2 layered ceramics with strong interface blocking effect, J. Eur. Ceram. Soc. 37 (2017) 4645–4652. [5] R. Xu, Z. Xu, Y. Feng, H. He, J. Tian, D. Huang, Temperature dependence of energy storage in Pb0.90La0.04Ba0.04[(Zr0.7Sn0.3)0.88Ti0.12]O3 antiferroelectric ceramics, J. Am. Ceram. Soc. 99 (2016) 2984–2988. [6] H. Wang, J. Liu, J. Zhai, Z. Pan, B. Shen, Effects of Sr substitution for Ba on dielectric and energy-storage properties of SrO-BaO-K2O-Nb2O5-SiO2 glass-ceramics, J. Eur. Ceram. Soc. 37 (2017) 3917–3925. [7] B. Qu, H. Du, Z. Yang, Q. Liu, T. Liu, Enhanced dielectric breakdown strength and energy storage density in lead-free relaxor ferroelectric ceramics prepared using transition liquid phase sintering, RSC Adv. 6 (2016) 34381–34389. [8] L. Zhao, Q. Liu, J. Gao, S. Zhang, J. Li, Lead-free antiferroelectric silver niobite tantalite with high energy storage performance, Adv. Mater. 29 (2017) 1701824. [9] Z. Pan, M. Wang, J. Chen, B. Shen, J. Liu, J. Zhai, Largely enhanced energy storage capability of a polymer nanocomposite utilizing a core-satellite strategy, Nanoscale 10 (2018) 16621–16629. [10] L. Chen, N. Sun, Y. Li, Q. Zhang, X. Hao, Multifunctional antiferroelectric MLCC with high -energy storage properties and large field-induced strain, J. Am. Ceram. Soc. 101 (2018) 2313–2320. [11] H. Zhang, X. Chen, F. Cao, G. Wang, X. Dong, Z. Hu, T. Du, Discharge properties of an antiferroelectric ceramics capacitor under different electric fields, J. Am. Ceram. Soc. 93 (2010) 4015–4017. [12] R. Xu, Z. Xu, Y. Feng, X. Wei, J. Tian, D. Huang, Evaluation of discharge energy density of antiferroelectric ceramics for pulse capacitors, Appl. Phys. Lett. 109 (2016) 224103. [13] R. Xu, Z. Xu, Y. Feng, X. Wei, J. Tian, Nonlinear dielectric and discharge properties of Pb0.94La0.04[(Zr0.56Sn0.44)0.84Ti0.16]O3 antiferroelectric ceramics, J. Appl. Phys. 120 (2016) 144102. [14] X. Hao, J. Zhai, L. Kong, Z. Xu, A comprehensive review on the progress of lead zirconate-based antiferroelectric materials, Prog. Mater. Sci. 63 (2014) 1–57. [15] Z. Liu, Y. Bai, X. Chen, X. Dong, H. Nie, F. Cao, G. Wang, Linear compositiondependent phase transition behavior and energy storage performance of tetragonal PLZST antiferroelectric ceramics, J. Alloy. Comp. 691 (2017) 721–725. [16] I.V. Ciuchi, L. Mitoseriu, C. Galassi, Antiferroelectric to ferroelectric crossover and energy storage properties of (Pb1-xLax)(Zr0.9Ti0.1)1-x/4O3 (0.02≤x≤0.04) ceramics, J. Am. Ceram. Soc. 99 (2016) 2382–2387. [17] R. Xu, J. Tian, Y. Feng, X. Wei, Z. Xu, Effects of Ti content on dielectric and energy storage properties of Pb0.94La0.04[(Zr0.70Sn0.30)1-xTix]O3 ferroelectric/antiferroelectric ceramics, J. Adv. Dielectr. 06 (2016) 1650033. [18] Z. Su, M. Yao, M. Li, W. Gao, Q. Li, Q. Feng, X. Yao, A novel and simple aluminium/ sol-gel-derived amorphous aluminium oxide multilayer film with high energy density, J. Mater. Chem. C 6 (2018) 5616. [19] X. Wang, S. Liang, P. Yang, Z. Fan, Effect of Al2O3 content on electrical breakdown properties of Al2O3/Cu composite, J. Mater. Eng. Perform. 19 (2010) 1330. [20] D.F.K. Hennings, R. Janssen, P.J.L. Reynen, Control of liquid-phase-enhanced
4. Conclusions In this work, a novel type of antiferroelectric Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3 ceramics with different contents of Al2O3 coating layer derived by sol-gel method were obtained via tape casting. For 729
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High breakdown strength and energy density in antiferroelectric PLZST ceramics with Al2O3 buffer
T
Chunyu Lia, Manwen Yaoa,∗, Wenbin Gaob, Xi Yaoa a
Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, No. 4800, Cao'an Road, Shanghai, 201804, China Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an, 710049, China
b
ARTICLE INFO
ABSTRACT
Keywords: Al2O3 buffer Antiferroelectric PLZST High energy storage density
In this work, a new core-shell structure of antiferroelectric ceramic powder (Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3PLZST) coated with linear dielectric (Al2O3) has been successfully prepared to realize high energy density through tape-casting process. According to the experimental results of electron microscope, the sol-gel derived Al2O3 layer was uniformly coated on the PLZST particles and the Al2O3 layer can be taken as the buffer layer to effectively refine the grain growth as well. Therefore, the modified PLZST particles were fine and uniform compared with the pure PLZST. It was found that the buffer layer could undertake higher electric field and the electric field applied to PLZST particles was weakened based on finite element analysis, which can avoid the premature breakdown of PLZST. And the actual measured breakdown strength was significantly enhanced from 22.2 kV/mm to 35.5 kV/mm. Correspondingly, an extremely high recoverable energy storage density of 5.3 J/ cm3 was obtained for PLZST with 0.5%wt Al2O3, an 204% enhancement over the pure PLZST ceramics (2.6 J/ cm3), and the corresponding efficiency was up to 88.3%. In addition, impedance spectroscopy measurement was carried out to further confirm the better insulation of the ceramic with Al2O3 buffer.
1. Introduction Due to the shortage of natural resources such as oil, coal and natural gas, the generation of electricity from renewable and non-conventional resources is being investigated on a large scale and much attention is being paid to the topic of pulse power system [1–5]. Among the art-ofthe-state of energy storage devices, dielectric capacitors have the advantages of ultrahigh power density coupled with extremely short discharge time duration [6,7]. However, their energy density is approximately 3–5 orders of magnitude lower than those of electrochemical counterparts, such as batteries and electrochemical capacitors, which has restricted their further applications [8,9]. Hence, improving the energy density of dielectric capacitors is highly desirable to meet the ever-increasing demands for reliable and efficient energy storage devices. Among popular dielectric materials, antiferroelectric (AFE) ceramics are one of the strongest contenders for high energy storage ceramic capacitors. AFE materials possess low dielectric loss, low coercive field, low remnant polarization and fast discharge rate, which make AFE materials a lucrative research direction [10,11]. According to previous studies, the energy density stored in the
∗
polarized dielectric (stored energy density-Wst), energy density recovered from the depolarized dielectric (recoverable energy storage density-Wre) and energy efficiency(ŋ) of antiferroelectric can be obtained by numerical integration of the area between the polarization axis and the curves of P-E loops and can be defined as follows [12,13] (1–3): Pmax
Wst =
EdP 0
(1)
Pmax
Wre =
EdP Pre
=
Wre × 100% Wst
(2) (3)
where E is the applied electric field, Pmax is the maximum polarization, and Pre is the remnant polarization. On the basis of equations (1) and (2), the energy density is strongly determined by E, which is constrained by the breakdown strength (BDS). In this light, increasing the breakdown strength is of great beneficial to enhance the energy storage. Currently, lead zirconate (PbZrO3 or PZ) based AFE family
Corresponding author. E-mail address: [email protected] (M. Yao).
https://doi.org/10.1016/j.ceramint.2019.09.025 Received 22 June 2019; Received in revised form 23 August 2019; Accepted 3 September 2019 Available online 03 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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materials, as a group of important electronic materials, especially PLZST [14–16], have attracted increasing attention for their potential applications in high energy storage capacitors because of their external electric field-induced phase switching behavior between antiferroelectric (AFE) state and ferroelectric (FE) state and high recoverable energy density [17]. Nevertheless, the breakdown strength of PLZST ceramics is relatively low which restrains their application to a great extent. Consequently, enhancing the breakdown strength of the PLZST ceramic is a big challenge. With the purpose of solving this problem, aluminum oxide (Al2O3) derived from sol-gel wet chemistry was introduced to coat the PLZST ceramic powder to form PLZST@ Al2O3 core-shell structured composite powder. Aluminum oxide, as a superior dielectric material, possesses a high breakdown strength (300–700 kV/ mm) and high band gap (~9 eV) in conjunction with its excellent chemical and thermal properties. Moreover, aluminum is abundant in the Earth and inexpensive [18]. The outstanding performances of Al2O3 indicate that combining it with the PLZST might be highly beneficial to improve the dielectric properties of the ceramics. Wang et al. have successfully prepared Al2O3/Cu composites by solid-state addition of Al2O3 and achieved a high breakdown field strength of up to 56 kV/mm with 1.2%wt content of Al2O3 [19]. This result demonstrates that Al2O3 is a promising additive to achieve the intention of breakdown strength enhancement. At present, the main way to introduce additives into a ceramic system is by solid-state mixing. This method would result in ununiform distribution and aggregation, which will tend to generate various structure defects and thus damage the electrical properties [20]. In contrast, the method of sol-gel coating is more inclined to obtain a highly homogeneous dispersion by coating a layer outside of the particles to form the core-shell structure. The sol-coating layer can then act as an insulating layer to further improve the electrical performance. Wang et al. have successfully reduced the dielectric loss of Ba0.7Sr0.3Ti0.9925Tm0.01O3 ceramic by coating SiO2 to form the coreshell structure [21]. Additionally, Tong et al. have obtained an enhanced dielectric constant and breakdown strength via coating SiO2 layer on the H2 treated BaTiO3 nanoparticles [22]. However, little research has been done to coat the lead-based AFE particles with a layer of Al2O3 to change the electrical properties. In this study, we successfully fabricated an Al2O3 layer onto the surface of Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3 particles by sol-gel process, which was confirmed by TEM images. SEM images exhibited a few hexagonal small particles dispersed along the grain boundary, which were identified as α-Al2O3 by EDS mapping and X-ray diffraction. Al2O3, as an electrical buffer, not only restricted the abnormal grain growth to increase grain boundaries, but also withstood the majority of field strength and improved the electrical properties significantly. The breakdown strength was greatly enhanced from 22.2 kV/mm to 35.5 kV/mm with 1%wt Al2O3-coating buffer. Furthermore, the calculated energy density from P-E loops was increased by 204% and the highest value was up to 5.3 J/cm3. It is worthwhile to note that in this work the thickness of the ceramic is about 0.1 mm and the electrode pad is 6 mm in diameter, which are well compatible with the dimension requirements of the multilayer ceramic capacitor (MLCC) for high energy storage applications.
2.2. The preparation of Al2O3-coating buffer layer The calcined Pb0.97La0.02Zr0.85Sn0.12Ti0.03O3 powders were mixed with the as-prepared Al2O3 sol in the ratio corresponding to the following content: x wt.% Al2O3 + (100-x) wt.% PLZST, where x = 0, 0.5, 1, 2, respectively. The mixture solution was ball-milled for 36 h and the resulting slurry was subsequently collected and dried at 60 °C for 12 h. After drying, the mixture was heated at 600 °C for 2 h in a muffle furnace to burn off the organic residuals. In addition, the PLZST powder doped with 0.5%wt Al2O3 powder was used as a comparative experiment. 2.3. The preparation of PLZST ceramics PLZST ceramics were fabricated through tape casting. Predetermined amounts of the composite particles, ethanol, trichloroethylene and triethyl phosphate were high-energy vibration milled for 1 h to obtain homogeneous and stable slurry. Then dibutyl phthalate (DBP), polyethylene glycol, n-butanol and polyvinyl alcohol were added to the slurry and high-energy vibration milled for another 2 h to make tape-casting slurry. The slurry was vacuumized to remove the bubbles from vibration. Green tapes were casted by a doctor blade, dried and punched into square tapes with a border length of 12 mm for subsequent electrode coating. Following binder burnout at 500 °C for 2 h, the green tapes were sintered at 1260 °C for 2.5 h in a closed Al2O3 crucible, under leadenriched environment maintained by Pb3O4 to minimize lead volatilization. The thickness of all ceramics was 0.1 mm. Finally, gold pads of 6 mm in diameter were deposited on the sample surface as top electrodes by dc sputtering. Then the gold electrodes were cured at 200 °C for 2 h. 2.4. Characterization The thermal property was determined via a thermogravimetric analysis differential scanning calorimeter (TG-DSC, SAT449C, Netzsch, Germany) from 30 °C to 1200 °C at a heating rate of 10 °C/min in ambient atmosphere. The morphologies of the Al2O3 buffer layer was observed using transmission electron microscope (TEM, JEM-2011F). The phase structure and microstructure of ceramics were examined using X-ray diffractometer (XRD, BRUKRER D8 Advance Diffractometer). The surface morphology of samples and elemental analysis (EDS) were carried out by field-emission scanning electron microscopy (FE-SEM, FEI Quanta 200 FEG). The breakdown strength measurement was performed using a high-voltage tester (TH9310 HIPOT TESTER, Tonghui) at room temperature, and the selected critical current was 2 mA. Impedance spectroscopy measurement (4 Hz ~ 3600 kHz) was carried out using an electrochemical impedance analyzer (IM 3590, Hioki, Japan). The frequency and temperature-dependent dielectric properties of the samples were measured using an Agilent 4980A LCR meter controlled by a computer. Polarization-electric field hysteresis loops were explored to calculate energy storage density by using a ferroelectric testing unit (Premier, Radiant Technologies, Inc., Albuquerque, NM). 3. Results and discussion
2. Experimental
To determine the optimum decomposition temperature of organics, TG-DSC analysis was performed for A0, A0.5, A1, A2 (A0-A2) before sintering and is showed in Fig. 1. There is a significant weight loss in the temperature range of 150 °C–450 °C, and two exothermic peaks (300 °C and 435 °C) are presented in the corresponding DSC curves, which indicate the decomposition and burning process of organic residuals in this temperature range. When the temperature continues to rise above 450 °C, the weights of ceramics do not change obviously. It can be concluded that the decomposition of organics has completed. Based on
2.1. The preparation of Al2O3 sol To prepare Al2O3 sol, (a) 0.02 mol aluminum isopropoxide was dissolved in 50 ml of glycol ether with stirring at 60 °C for 30 min on the magnetic stirrer; (b) the aluminum isopropoxide solution was mixed with 0.02 mol acetylacetone with stirring for 30 min; (c) 10 ml acetic acid as catalyst was added to the above solution at 90 °C with agitation for 30 min to get a clear and homogeneous sol. 723
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Al2O3 layer is successfully coated on the surface of the PLZST ceramic particles and the interplanar spacing estimated from the lattice fringes is about 0.443 nm, which corresponds to the d-spacing of (111) planes of PLZST. SEM images of samples are illustrated in Fig. 3 to study the effect of Al2O3 additive on the morphology of ceramics. Fig. 3 (a)–(d) exhibit the microstructures for ceramics with different Al2O3 sol contents (A0-A2). Fig. 3 (e) presents the surface morphology of PLZST ceramic doped with Al2O3 powder (D0.5). It is obvious that there are almost no pores for pure PLZST yet the grain size is not uniform, in which the largest particle diameter is up to 10 μm, and the smallest one is less than 2 μm. However, with the addition of Al2O3 sol, the surface morphologies of the Al2O3-sol coated ceramics exhibit the following differences in comparison with the pure counterpart. Firstly, several hexagonal small particles present at the grain boundary (marked in red circles), which are identified as second phase. The more Al2O3 sol is added, the more hexagonal particles. In order to analysis the composition of the new phase, EDS mapping of the surface for A0.5 was performed and showed in Fig. 4. The spot EDS analysis of Fig.4(b) indicates that grain area 1 in Fig.4(a) is pure PLZST without diffusion of Al element, while the spot EDS analysis of Fig.4(c) indicates that area 2 in Fig.4(a) is pure alumina with atomic ratio of Al to O very close to 2:3. To further study the diffusion of aluminum, a line mapping EDS from point A to point B for A0.5 was carried out. There are two distinct peaks in the element distribution map, and the peak values are located at the boundaries of hexagonal particles. However, in the surrounding PLZST phase, the Al content decreased significantly, indicating that the Al element is basically confined in the Al2O3 crystallite and hardly diffuse into the PLZST lattice. Thus, based on the EDS measurement, it can be concluded that
Fig. 1. TG-DSC analysis for A0-A2 before sintering.
above analysis, 500 °C was chosen as thermal treatment temperature to ensure the organic residuals can be fully decomposed. The particle information of PLZST coated with Al2O3 buffer layer was verified by TEM in Fig. 2. Fig. 2(a) exhibits pure PLZST particles and (b) and (c) show the ceramic particles with 2%wt Al2O3. It is clear that pure PLZST particles and Al2O3 buffer layer form a typical coreshell structure (PLZST @ Al2O3), of which the schematic structure is presented in Fig. 2(d). As shown in Fig. 2(c), an approximately 6 nm
Fig. 2. (a)TEM image for pure PLZST particles; (b) and (c) TEM images for PLZST with Al2O3-coating buffer; (d) schematic picture for the core-shell structure of PLZST@Al2O3. 724
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Fig. 3. (a)–(d) SEM images for Al2O3 sol coated PLZST ceramics (A0-A2); (e) SEM image for Al2O3 powder doped PLZST ceramic (D0.5).
Fig. 4. (a) SEM images for A0.5; (b) corresponding spot scan for selected area 1; (c) corresponding spot scan for selected area 2; (d) line mapping EDS from point A to point B for A0.5.
well-faceted Al2O3 crystallites precipitated at the grain boundary without apparent chemical reaction with the surrounding PLZST grains. Secondly, the uniformity of the grain size of the PLZST is greatly improved and the average grain size decreased significantly. Obviously, the more the sol content, the smaller the grain size. This phenomenon can be attributed to the existence of Al2O3-coating layer, as well as the Al2O3 crystallites intercalated at grain boundaries, which prevent the migration of the grain boundary during sintering. As a result, the abnormal grain growth is significantly suppressed to obtain smaller grains with a good uniformity. The more Al2O3 additive is added, the more the barrier to the grain boundary migration, the smaller the grain size. Nevertheless, there are too many Al2O3 crystals exist at the grain boundary in A2, which has a negative effect on the formation of typical sintering structure of converged triple grain boundaries among particles
(marked in yellow circles in Fig. 3(d)). Therefore, many particles cannot grow normally and keep a round particle shape (marked in green circles in Fig. 3(d)), which leaves small pores at the grain boundary and thus degrades the ceramic microstructure. Besides, comparing Fig. 3 (b) and (e), it can be seen that the introduction of Al2O3 in the form of sol instead of in the form of powder is more beneficial to eliminate pores in the sintering process to achieve dense ceramics. To determine the phase structure of PLZST main phase and Al2O3 secondary phase, XRD patterns of the sintered ceramics are exhibited in Fig. 5. For pure PLZST ceramic (A0), only peaks of perovskite phase are detected and no other secondary phase can be found. However, several diffraction peaks of α-Al2O3 appear in the composite ceramics (A0.5-A2 and D0.5), which accords with the SEM images and EDS analysis about 725
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Fig. 5. (a) XRD patterns of sintered ceramics; (b) The magnified diffraction pattern between 43° and 45°.
the secondary phase. Furthermore, there are two split peaks around 44°~44.5°(Fig. 5(b)), confirming that the PLZST main phase is typical orthorhombic antiferroelectric. The microstructure morphology of ceramics has a great impact on its electrical properties, especially the breakdown field strength. In order to investigate the effect of Al2O3 addition on breakdown strength (BDS), Weibull distribution of BDS for PLZST ceramics with different Al2O3 contents is exhibited in Fig. 6(a) [23]. The BDS could be extracted from points where the fitting lines intersect with the horizontal
line through Yi = 0 and are presented in Fig. 6(b). In the experiment, the values of the Weibull modules ( ) for the ceramics are more than 30 signifying a valid comparison of BDS is guaranteed. According to the data given in Fig. 6(b), the characteristic BDS is boosted up with the increase of Al2O3 additive and the highest value can reach 35.5 kV/mm, an 60% improvement over the pure PLZST (22.4 kV/mm). Simulations of the electric field were performed to characterize the effect of the Al2O3 buffer layer on electric breakdown strength, the electric field distribution in the ceramics was simulated by the finite
Fig. 6. (a) The Weibull distribution of BDS for A0-A2 and D0.5; (b) The BDS values obtained from Xi intercept of the fitting lines for A0-A2 and D0.5. Simulations of the electric field intensity for (c) pure PLZST (A0) and (d) PLZST@Al2O3 (A1). 726
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Fig. 7. (a)Temperature dependence of dielectric constant and dielectric loss (tanδ) of A0-A2 and D0.5 at 100 kHz; (b) Frequency dependence of dielectric constant and dielectric loss (tanδ) of A0-A2, D0.5.
element analysis. As shown in Fig. 6(c) and (d), the larger and the smaller sized grains represent the uncoated PLZST particles and the coated PLZST particles, respectively. In addition, the gap between the grains in Fig. 6(d) is the Al2O3-coating buffer layer. The A, B, C, and D are denoted as the grain A of the uncoated pure PLZST, the grain core B of the PLZST@Al2O3, the Al2O3-coating buffer layer C, and the grain D of the α-Al2O3 particles, respectively. When an external electric field is applied to the ceramics, grains of pure PLZST exhibit more intensive electric field concentration than the grain cores of the PLZST@Al2O3 (EA > EB). Consequently, ceramic of pure PLZST is more likely to breakdown in contrast to the Al2O3-coated ceramics. These results can be attributed to the existence of Al2O3 buffer layer, which has the following two effects: (1) Al2O3 addition has successfully decreased the grain size of the ceramics and is confirmed by SEM. According to previous study, ceramics have small grain size usually have higher breakdown strength and the correlation between grain size and BDS can be described by E G a , where E, G and a are the electric
breakdown strength, grain size and constant, respectively [24]. It is believed that the grain boundary is the main defender of electric field strength. Small grain size leads to an increased proportion of grain boundary. Under the condition that the breakdown strength value per unit grain boundary does not change, the total voltage value will be increased as a result. (2) Al2O3 buffer layer has the effect of attenuating the electric field applied to the PLZST particles. When there exist two phases in the ceramic in which the dielectric constants are extremely different, the electric field is mainly distributed on the phase with a low dielectric constant. The dielectric constant of Al2O3 is much lower than that of PLZST. Due to this characteristic, the Al2O3-coating layer exhibits an “electric field black hole” under an electric field. When a voltage is applied to the ceramic, the electric field is mostly “sucked” into the Al2O3-coating layer, and fewer electric fields are distributed on the grain boundaries and the PLZST particles, which corresponds to the absorption of the electric field from original crystal phase by Al2O3. For PLZST particles, Al2O3 is equivalent to an electric field buffer layer that
Fig. 8. Impedance spectra and equivalent circuits of (a) A0 and (b) A0.5. 727
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Fig. 9. (a) The polarization-electric field hysteresis loops of ceramics measured at 10 Hz with different applied fields at room temperature; inset shows the variation of Pmax for A0-A2 and D0.5; (b) The recoverable energy density versus electric field for A0-A2 and D0.5, inset shows the electric field dependence of the corresponding energy efficiency; (c) The Wst, Wre and ŋ values for A0-A2 and D0.5; (d) Energy storage performance of recently reported lead-based and lead-free AFE ceramics, in which the two PLZST are (Pb0.97La0.02)(Zr0.58Sn0.35Ti0.07)O3 [15] and (Pb0.94La0.06)(Zr0.66Sn0.23Ti0.11)O3 [16], respectively; PLZT is (Pb0.65La0.35) (Zr0.90Ti0.10)0.1625O3 [26]; PBLZST is (Pb0.90Ba0.04La0.04)[(Zr0.7Sn0.3)0.88Ti0.12]O3 [5]; ACN is Ag0.92Ca0.04NbO3 [27]; AN is AgNbO3 [28]; BNT-BKT is Bi0.5Na0.5TiO3– Bi0.5K0.5TiO3 [29]; NN-CZ is NaNbO3–CaZrO3 [30].
weakens the field strength. Furthermore, the breakdown strength of αAl2O3 is higher than that of PLZST and thus the presence of a small amount of Al2O3 crystallites at the grain boundaries does not affect the breakdown field strength. However, an excessive addition of Al2O3 sol will lead to more Al2O3 crystallites (confirmed in Fig. 3), which prevent the formation of converged grain boundaries and leave a few pores unremoved. Thus, the breakdown strength of A2 is significantly lower than that of other components due to the failure of the microstructure to be completely grown into ceramics. Of great importance is that the characteristic BDS for D0.5 (28.2 kV/ mm) is not as high as that of A0.5 (31.5 kV/mm). The result can be ascribed to the following two factors: uniformity of the Al2O3 dispersion and the existence of pores. (1) Compared with conventional solid-state doping, sol-gel coating method can help to disperse PLZST particles so as to avoid aggregation and make the applied voltage distribute evenly. (2) There exist certain amounts of pores on the ceramic surface for D0.5, which result in a drop in BDS due to their low breakdown strength. Dielectric properties as a function of temperature for ceramics at 100 kHz are given in Fig. 7(a). All ceramics are initially in an AFE state and display only one dielectric anomaly at the temperature Tm, corresponding to the Curie temperature (Tc) for the AFE to paraelectric (PE) phase transition. The value of Tc is about 150 °C and varies slight. Additionally, the Al2O3 additive, both in the form of sol and solid-state particle, can impair the value of dielectric constant yet maintaining the dielectric loss (tanδ) at a low level. As is discussed before, the dielectric constant of Al2O3 is much lower than that of PLZST. In this light, the
introduction of Al2O3 additive (A0-A2, D0.5) can lead to the suppressed dielectric constant. The more the Al2O3, the lower the dielectric constant, which accords with the data in the figure. The frequency dependent dielectric constant and dielectric loss (tanδ) measured at room temperature from 1 kHz to 2000 kHz are presented in Fig. 7(b). The values of dielectric constant drop gradually as the frequency increases, because long-time polarization process, like space charges, stops responding to the external electric field at high frequency [25]. With the aim of further understanding the effect of Al2O3 buffer on the outstanding electrical properties, impedance spectroscopy at 4 Hz–3600 kHz is observed for A0 and A0.5 and the corresponding equivalent circuits are (RPCP)(RB1CBRB2) and (RACA)(RPCP)(RB1CBRB2), respectively. RP and CP represent the resistance and the capacitance of the PLZST grains, while RB and CB refer to the resistance and the capacitance of the grain boundary. RA describes the resistance of the Al2O3 buffer and CA represents the corresponding capacitance. The fitting curves are shown in Fig. 8 and the fitting parameters are collected in the following tables. The errors between the fitting and measured data from the impedance spectra are quite small, indicating the reasonably good compatibility of the presented model. Apparently, the equivalent circuit of A0 is formed by the PLZST grains and the grain boundary in series; for A0.5, due to the introduction of Al2O3, the equivalent circuit is formed by the PLZST grains, the grain boundary and the Al2O3 grains in series. As is showed in tables, the resistance of PLZST grains (RP) and boundary (RB1 and RB2) are increased owing to 728
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sintered ceramics, there are α-Al2O3 particles intercalated at the grain boundary. The Al2O3-coating layer and the α-Al2O3 particles inhibited grain boundary movement to obtain small particles and high proportion of grain boundary. When a voltage is applied to the ceramic, the Al2O3-coating buffer layer, coupled with high proportion of grain boundary, enabled the sample to bear more electric field and was confirmed by the simulations of the electric field. Hence, the breakdown strength was enhanced significantly from 22.2 kV/mm to 35.5 kV/mm. A better insulation of ceramics with Al2O3 buffer was exhibited by impedance spectroscopy. In addition, the maximum energy storage density was up to 5.3 J/cm3, more than twice that of pure counterpart, with 88.6% in energy efficiency. As a result, this work provides a prospective method to enhance energy storage of antiferroelectric ceramics.
Table 1 Characteristic electrical properties for ceramics in this work. Sample
BDS (kV/mm )
Pmax ( µC / cm2 )
Wst
A0 A0.5 A1 A2 D0.5
22.4 31.5 35.5 26.2 28.2
22.2 36 30.7 26.1 30.1
2.8 6 5.4 4 5
( J / cm3 )
Wre
( J / cm3 )
2.6 5.3 4.6 3.6 4.3
92.7 88.3 85.2 90 86
the buffering of Al2O3. Moreover, Al2O3 itself has an extremely high resistance, which is the highest among the resistance parameters. As a result, the total resistance of A0.5 is much higher than that of A0, which further explains the enhancement of breakdown strength for PLZST ceramics with Al2O3 additive. Obviously, the values of CP and CB drop down with Al2O3 additive, indicating a decrease in dielectric constant which is consistent with previous discussions. The polarization-electric field hysteresis loops of ceramics measured at 10 Hz with different applied fields (breakdown electric fields at alternating voltage) at room temperature are illustrated in Fig. 9(a). As shown, all ceramics display slim double loop characteristics of AFE phase, and the residual polarization is close to zero after driving electric field is removed. To investigate the effect of Al2O3 buffer on the maximum polarization, the values of Pmax are plotted in Fig. 9(a) inset. The value of Pmax shares similar variation with that of BDS and the highest value can reach 36 µ C/cm2. The recoverable energy densities (Wre) of the ceramics versus the electric field are shown in Fig. 9(b), and the inset exhibits the corresponding energy efficiencies (ŋ). The energy storage densities grow with the increase of the electric field. In particular, when the applied electric field is increased from 15 kV/mm to 20 kV/mm, the increment of energy storage density is most significant. This result is related to the forward transition electric field (EAF). When the applied electric field reaches the value of EAF, the ceramic changes from the antiferroelectric state to the ferroelectric state, and the polarization surges. Therefore, the energy density increases apparently. It can be inferred from Fig. 9(a) and (b) that the values of EAF for different ceramics vary little and are about 18 kV/mm. The energy efficiencies show an upward trend at the EAF. Due to the sudden increase of the storage energy density, the proportion of the energy loss reduces and the energy storage efficiency is increased. However, the higher the applied electric field, the greater the dielectric loss and the leakage current caused by charge migration, so the lower the energy storage efficiency. The calculated Wst, Wre and ŋ of all ceramics from polarization-electric loops are given in Fig. 9(c). The Al2O3 buffer successfully improved the energy density of PLZST ceramics with relative high efficiency. The maximum Wre is up to 5.3 J/cm3 for PLZST ceramic with 0.5%wt coating Al2O3 buffer (A0.5), an 204% enhancement over the pure counterpart (2.6 J/cm3), and the corresponding efficiency is as high as 88.3%. In addition, by comparing the data of A0.5 and D0.5, the higher recoverable energy density of A0.5 provides an evidence that the Al2O3 buffer introduced by sol-gel coating method is more effective than traditional solid-state doping in improving the energy density of PLZST ceramics. The characteristic electrical properties of ceramics in this work are listed in Table 1. Furthermore, the Wre of recently studied AFE ceramics [5,15,16,26–30] are summarized in Fig. 9(d). It proves that the recoverable energy storage in this study achieved at an electric field of 31 kV/mm is extremely high, demonstrating that the core-shell structured PLZST ceramic with Al2O3 buffer is a promising candidate for energy storage.
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