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Enhanced energy storage properties of fine-crystalline Ba0.4Sr0.6TiO3 ceramics by coating powders with B2O3–Al2O3–SiO2
Journal Pre-proof Enhanced energy storage properties of fine-crystalline Ba0.4Sr0.6TiO3 ceramics by coating powders with B2O3–Al2O3–SiO2 Hongye Wang, Minghe Cao, Miao Liu, Hua Hao, Zhonghua Yao, Hanxing Liu PII:
S0925-8388(20)30254-1
DOI:
https://doi.org/10.1016/j.jallcom.2020.153891
Reference:
JALCOM 153891
To appear in:
Journal of Alloys and Compounds
Received Date: 2 December 2019 Revised Date:
11 January 2020
Accepted Date: 15 January 2020
Please cite this article as: H. Wang, M. Cao, M. Liu, H. Hao, Z. Yao, H. Liu, Enhanced energy storage properties of fine-crystalline Ba0.4Sr0.6TiO3 ceramics by coating powders with B2O3–Al2O3–SiO2, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153891. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Enhanced Energy Storage Properties of Fine-Crystalline Ba0.4Sr0.6TiO3 Ceramics by Coating Powders with B2O3-Al2O3-SiO2 Wang Hongye1, Cao Minghe1*, Liu Miao1, Hao Hua1, Yao Zhonghua1, Liu Hanxing2 1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China 2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Material Science and Engineering. Wuhan University of Technology, Wuhan 430070, Hubei, PR China *E-mail: [email protected] (M.Cao) Tel: +86 2787885812; Fax: +86 2787885813
Abstract A core–shell mixing technique was utilized to modify the Ba0.4Sr0.6TiO3 (BST)particles in order to obtain the fine- crystalline Ba0.4Sr0.6TiO3 ceramics and improve ceramics energy storage capability. Coating layers of (9B2O3-13Al2O3-78SiO2, mol%) (BAS)were deposited onto Ba0.4Sr0.6TiO3 nanoparticles by a sol-precipitation method. The microstructures, phase composition and dielectric properties were investigated. And fine-crystalline BST ceramics with an average grain size below 150 nm by coating BAS has been prepared. The dielectric breakdown strength and energy storage density significantly improved due to the addition of BAS. The maximum energy storage density obtained from coating 3 wt% BAS was 1.8 J/cm3 at 410 kV/cm, which was higher than pure BST ceramics. Thus, the core-shell architecture is an effective strategy to improve the energy storage performance.
Keywords: Chemical coating method; Fine-grained ceramics; Energy storage; Dielectric
breakdown strength
1. Introduction Renewable energy storage has been actively investigation because of the exhausting trend of fossil fuel and the recycling renewable energy [1]. Dielectric capacitors possess high power density and fast charge-discharge rate to suitable renewable energy storage [2,3]. Among the various technologies, such as electronic circuits, microwave communications and high-power applications, dielectric capacitors have been widely used in many fields [4]. For dielectric capacitors, the discharge energy storage density was acquired for the following equation [5]:
Jcharge= PdE= ౩ EdP Jdischarge=ηJcharge Where E is the electric field, Ps is the saturated polarization and η is the energy storage efficiency. Obviously, based on the equation, the important factors for the energy storage capacitor are high dielectric breakdown strength (BDS), large polarization and high energy storage efficiency. A larger dielectric constant value generates more polarization to achieve higher energy density [6-8]. Moreover, high breakdown strength and low dielectric loss are significant for improvement energy storage performance, these two parameters could improve energy density and decrease energy losses [9-11]. As we know, Ba0.4Sr0.6TiO3 (BST) as a linear dielectric was investigated widely in energy storage owing to the low dielectric loss, high dielectric constant and high energy storage efficiency [12]. However, low breakdown strength (BDS) of pure BST ceramics determine low energy storage density [13]. Breakdown strength as an important element effects to energy storage density performance. In general, the BDS of ceramics was improved by reducing the grain size, increasing
densification and adding the material with high BDS [14-17]. For BST ceramics, the sintering temperature is above 1300°C, and which is leads to the coarsening of grain size under high temperature [18]. Low sintering temperature is an excellent method to reduce the grain size effectively, improve BDS of ceramics, and improve energy storage performance of ceramics. Adding sintering aids, such as SiO2, B2O3 and glass [19-21], is an effective way to decrease sintering temperature due to the low melting point. Rhim et al. reported that the sintering temperature of Ba0.7Sr0.3TiO3 ceramics decreased from 1350°C to 1150°C by adding B2O3 and which lead to decrease the grain size and increase the BDS [22]. Zhang et al. found that the sintering temperature of BST ceramics decreased from 1350°C to 1100°C by adding BaO-SiO2-B2O3 glass while the energy storage density was 0.89 J/cm3 at 200 kV/cm [23]. Wang et al.
reported
that
sintering
temperature
of
BST
ceramics
with
addition
BaO-B2O3-SiO2-Na2CO3-K2CO3 glasses was lowered about 150°C while the energy storage density was 0.72 J/cm3 at 280 kV/cm [24]. Yang et al. found that the addition of BAS glass-ceramics could enhance the breakdown strength of the ceramics [25]. Hence, adding low melting point glass was a promising procedure to improve the energy storage performance. When preparing BST /glass ceramics, ball milling is an acquainted method to fabricate ceramics materials. However, this method is difficult to achieve uniform distribution of glass powder around ceramics particles [26]. And compared with high energy ball milling, which is difficult to introduce impurities by chemical coating method. Herein we use core–shell method to fabricate to BST/BAS core–shell structures [27]. The core–shell method ensures homogeneous mixing of BST and BAS glass. The glass coating layer of BAS is appropriate to a variety of glass compositions. Al2O3 and SiO2 are common coating layer for BaxSr1-xTiO3 particles because of the
high BDS, and B2O3 is an excellent sintering aids because of its low melting point (460°C) [28-30]. Here, we report the B2O3-Al2O3-SiO2 (BAS) as coating layer, which is an effective additive for decreasing sintering temperature, suppressing grain growth and improving energy storage performance of BST ceramics. An oxalate co-precipitation mothed was applied to prepare the BST powders [31], and the BST particles prepared by this method obtain characteristics of fine spherical shape and good uniformity. The sol-precipitation method was applied to fabricate the BAS-coated BST powders to obtain the fine-grained BST- x wt% BAS ceramic materials with highly energy storage density during low sintering temperature.
2. Experimental procedure The oxalate co-precipitation mothed applied to prepare the Ba0.4Sr0.6TiO3 (BST) powders. The xwt% B2O3-Al2O3-SiO2 (BAS)-coated Ba0.4Sr0.6TiO3(BST) powders (x= 1, 2, 3, 4, 5) were fabricated by sol-precipitation method. Starting materials including C8H2O4Si (TEOS, ≥28.4%), C12H27BO3 (≥99.0%), Al(NO3)3•9H2O (≥99.9%), C2H5OH (≥99.5%), C2H4O2 (≥99.5%) and NH4OH (25~28%). 2.1 Synthesis Ba0.4Sr0.6TiO3 nanoparticles The Ba0.4Sr0.6TiO3 nanoparticles were fabricated by oxalate co-precipitation mothed. In the fabrication procedure, oxalic acid was dissolved in 200 mL ethanol and DI water mixture to acquire oxalic acid solution. And acetylacetone added to titanium tetrabutoxide due to inhibit the hydrolysis, and dissolved in 100 mL ethanol. And the mixture was mixed with oxalic acid during magnetic stirring to fabricate H2TiO(C2O4)2 (HTO) precursor for 6h. Then, strontium oxalate and barium oxalate were dissolved in DI water, and polyethylene glycol was added in solution as a
dispersant. And the clear HTO solution added to the suspension under magnetic stirring to produce Ba0.4Sr0.6TiO(C2O4)2·4H2O (BSTO) precursor. And the precursor calcines at 600 °C for 2 h to manufacture the Ba0.4Sr0.6TiO3 (BST) powders. 2.2 B2O3-Al2O3-SiO2 coating In a typical fabrication procedure, BST particles were added to absolute ethyl alcohol, then the slurry was treated with the magnetic stirring and the ultrasonic dispersion to obtain the dispersion of BST particles in absolute ethyl alcohol. The stable BST suspension is the precondition for the uniform coating. And then, the TEOS and tributyl borate were dissolved in ethyl alcohol with a magnetic stirrer for 10 min. And DI water added to solution during water bath at 65 °C for 1h. And Al(NO3)3•9H2O added to the mixed solution during magnetic stirring to produce BAS precursor solution. And The precursor solution was added to suspension during magnetic stirring and adjusted pH value of suspension to 9.5 by adding acetic acid and ammonia water. And the suspension stirred at 65°C for 12 h to ensure BAS completely precipitated. The mixture solution dries at 150 ºC for 24h and calcines at 600 °C to eliminate crystal water and residual organic compounds. 2.3 Sintering The particles pressed into pellet at 3 Mpa with 5% PVA as a bind. After calcining at 600°C, the samples were fired in air at 1350°C -1180°C for 2 hr. 2.4 Characterization The bulk density of ceramics measured by Archimedes method. And the microstructure of BST-x wt%BAS powders and pure BST powders was examined by field emission transmission electron microscope (Talos F200s, FSTEM). The crystal structure of ceramics determined by
X-ray diffractometer (XRD, PANalytical X'PertPRO). The microstructure of ceramics measured by scanning electron microscopy (SEM, JSM-7100F). And the dielectric properties of ceramics measured by precision impedance analyser (E4980A, Agilent Tech). And the P-E hysteresis loop of ceramics measured by ferroelectric test system (PK-CPE1701, PolyK Technologies).
3. Results and discussion The BST particle coated with BAS were successfully prepared with various precursor concentrations. The microstructure of pure BST and 5wt% BAS-coated BST powders observed by TEM as shown in Fig. 1. A careful observation that homogenous and smooth coating layer were prepared by sol-precipitation method. The uniform BAS shell is obtained with a thickness from BST@3wt% BAS for ~3nm to BST@5wt% BAS for ~5nm. It was confirmed that the sol-precipitation method is a viable technique to acquire BST@BAS core-shell nanoparticles. It is known that the glass phase could facilitate ceramics sintering, increase densifications, and inhibit grain growth of ceramics. The sintering temperature for the BST@BAS ceramics were researched at a series of temperature and measured bulk densities of sintered pellets. Fig. 2 shows the density of the BST-x wt% BAS ceramics under different sintering temperature. The optimal sintering temperature of the BST-x wt% BAS ceramics was determined to be 1180ºC, and the sintering temperature of pure BST was obtained to 1320 ºC. The sintering temperature of BST xwt%BAS ceramics decreased obviously with addition BAS. The low sintering temperature achieved to decrease the production cost. After the optimum sintering treatment, the fracture surface of pure BST ceramics and BST-x wt% BAS ceramics were observed by SEM as shown in Fig. 3. The pure BST ceramics in Fig.3(a) revealed considerable grain growth with some grains growing up to ~3 µm. Fig. 4 shows the
average grain size of the BST- BAS ceramics was acquired by analysis software (Nano Measurer). And the tiny grain growth was observed for BST- BAS ceramics with all the grains maintained below 500nm. The fine grains were acquired at BST-3wt% BAS ceramics for 150nm. And the BAS suppress the grain growth of the BST ceramics. This grain growth inhibition in BST-BAS ceramics indicates that a thin diffusion barrier layer was formed between grains, and which inhibited the grain boundary migration during the sintering process [32]. The fracture surface of pure BST indicates prominent grain growth due to the absence of a coating barrier. The appropriate liquid content reduces sintering temperature, however, excessive liquid phase lead to an abnormal grain growth and abatement densification [33]. Excessive liquid content caused the abnormal grain growth when the BAS content was more than 3 wt%. Thus, the BAS glass coating on BST exhibits to inhibit grain growth and promotes densification. High densification was achieved after sintering from coating BAS layer. Fig.5 shows XRD patterns of sintered samples. It is apparent that the main phase of ceramics was perovskite structure. And a glass sintering agent cause small amounts of second phase after sintering. For the BST-BAS ceramics, it is observed that the secondary phase Ba1.6Sr2.4Ti2Si4O16 was formed during the sintering process. The secondary phase could be considered by the interfacial reaction between BST core and the SiO2 for the BAS glass shell as shown in eqn (1). 4Ba0.4Sr0.6TiO3 + 4SiO2 → Ba1.6Sr2.4Ti2Si4O16 + 2TiO2
(1)
As shown in eqn (1), the secondary phase is formed by the reaction between BAS and BST phases during sintering process. The second phase Ba1.6Sr2.4Ti2Si4O16 together with the remnant BAS phase between grain boundaries of BST had a significant role in the grain growth inhibition [32]. And the second phase around the grains boundaries could increase breakdown strength and
improve energy storage performances [34]. Overall, BAS glass is a superior sintering agent to inhibit grain growth and promote sintering densification. Fig.7(a) shows the dielectric constant of the BST-BAS ceramics from room temperature to 240ºC. The dielectric constants of the BST-BAS ceramics are lower than the coarse-grained BST due to the dilution effect of glass addition and the intrinsic size effects of BST [35,36]. The dielectric constant peak shifted to high temperature, and it is broadened and round, which indicate that better temperature stability compared to the pure BST. The variation of dielectric constants from room temperature to 240ºC is within ±10%. Fig.7(b) shows the dielectric loss of the BST-BAS ceramics from room temperature to 240ºC. As can be seen in fig.7(b), the BST-BAS ceramics reveals tiny dielectric loss with most values ≤1%. And it exhibits fine dielectric quality of ceramics [36]. The frequency dependence of dielectric constants and dielectric loss of the BST-BAS ceramics as shown in Fig. 8(a). The dielectric constant of the BST-BAS ceramics shows excellent frequency stability. The variance of dielectric constant from low frequency to high frequency is within ±1%. And Fig. 8(b) shows the dielectric loss of the BST-BAS ceramics with difference frequency. And the BST-BAS ceramics exhibits dielectric loss within 1% at high frequency. For the linear dielectrics or ferroelectrics, the dielectric constant varies with applied electric field, and this phenomenon called dielectric nonlinearity [37]. The dielectric constant of the BST ceramics reduced with increasing electric field, which damage the energy storage performance of the BST ceramics. Fig.8 shows dielectric constants of the BST-BAS ceramics for difference DC electric field. The dielectric constant of the BST-BAS ceramics exhibits good electric field stability than pure BST ceramics, which attributed to the fine grain size of BST-BAS ceramics. In
the BST ceramics, the grain exhibits nonlinear behavior with high dielectric constant, and grain boundary remains linear with low dielectric constant. The relative content of grain boundary increases with reducing grain size, as “dilution effect”, and the electric field stability of ceramics is improved [38]. The energy storage density capability of the BST-BAS ceramics was assessed by P-E text. P(E) hysteresis loops of the BST-BAS ceramics as shown in Fig. 9(a). As can be seen, the BST-BAS ceramics shows normal ferroelectric switching behavior. The breakdown strength(BDS) is a critical element for evaluating energy storage capability. The BDS of the ceramics is related to grain size. And the BDS of ceramics is increased with reducing the grain size [39]. Table 1 shows the BDS, grain size, discharge energy storage density (W) and energy storage efficiency (η) of BST-BAS ceramics. The BDS is enhanced from 210 kV/cm for pure BST ceramics to 410 kV/cm for BST-3 wt% BAS ceramics, which lead to improve energy storage density. And the energy storage density of BST-3wt% BAS ceramics reaches up 1.8 J/cm3, which is improved by 2.5 times as compared with that of the pure BST ceramics. The fine-grained ceramics show enormous energy storage density compared with coarse-grained ceramics at high electric field [40]. Fig. 9(b) exhibits the energy storage density and energy storage efficiency of BST-BAS ceramics under different electric fields. The energy storage density of BST-BAS ceramics was increasing approximate parabolically with the electric field. And the BST-3wt% BAS ceramics remained high energy storage efficiency (81.8%) at 410 kV/cm, which are mainly attributed to the fine grain and compact structure. The BST-BAS ceramics exhibits high breakdown strength and the energy storage density is confirmed to promise material system for energy storage capacitor applications.
Conclusion
The BST-x wt% BAS core-shell particles were successful fabricated by sol-precipitation method. Low temperature sintering of the BST-BAS ceramics exhibited high densification. And fine grain size was obtained of BST-3wt% BAS ceramics for 150 nm. Then, the BST-3wt% BAS ceramics exhibited good electric field stability and excellent energy storage properties. A high dielectric breakdown strength (410 kV/cm) is beneficial to improve energy storage. And the highest energy storage density acquired by BST-3wt% BAS ceramics was ~1.8 J/cm3 at 410 kV/cm and high energy storage efficiency (81.8%). The result indicated that the BST-BAS ceramics is a promising procedure to improve the energy storage performance. In the end, the promising technology to fabricate new functional materials suggest that the core-shell structure creates a new way to acquire high energy storage density ceramics materials.
Acknowledgements This work was financially supported by the Natural Science Foundation of China (51872213), the NSFC-Guangdong Joint Funds of the Natural Science Foundation of China (No. U1601209), the National Key Basic Research Program of China (973 Program) (No. 2015CB654601), the Technical Innovation Special Program of Hubei Province (2017AHB055), and the Major Program of the Natural Science Foundation of China (51790490).
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Table 1 the BDS, grain size, discharge energy storage density (W) and energy storage efficiency (η) of pure BST ceramics and BST-x wt% BAS ceramics x wt%
BDS (kV/cm)
Grain size (µm)
W (J/cm3)
η
0
210
1.2
0.74
76.3%
1
280
0.33
1.17
83.6%
2
320
0.20
1.47
77.8%
3
410
0.15
1.80
81.8%
4
260
0.37
0.92
74.8%
5
280
0.46
1.06
74.1%
Fig.1 the TEM images of (a) the pure BST, (b) the 3wt% BAS-BST powders (c) the 5wt% BAS- BST powders
Fig.2 the bulk density of BST-x wt% BAS ceramics (x=0, 1, 2, 3, 4, 5) under different sintering temperature
Fig.3 the SEM photographs of fracture surface of BST-x wt% BAS ceramics (a) pure BST ceramics, (b) x=1, (c) x=2, (d) x=3, (e) x=4, (f) x=5
Fig.4 the average grain size and sintering temperature of BST-x wt% BAS ceramics
Fig.5 the XRD patterns of BST-x wt% BAS ceramics (x=0, 1, 2, 3, 4, 5)
Fig.6 the temperature dependence of (a) dielectric constant and (b) dielectric loss for BST-x wt% BAS ceramics in a temperature range of 50 ºC -240ºC;
Fig.7 the frequency dependence of (a) dielectric constant and (b) dielectric loss for BST-x wt% BAS ceramics in a frequency range of 20 Hz-2 MHz;
Fig.8 the DC electric field dependence of the dielectric constant for BST-xwt%BAS ceramics
Fig.9 (a) the P-E loops for BST-x wt% BAS ceramics under the maximum applied electric field; (b) the electric field dependence of the energy storage density for BST-x wt% BAS ceramics.
1. the B2O3-Al2O3-SiO2-coated Ba0.4Sr0.6TiO3 nanoparticles were prepared by the sol-precipitation mothed. 2. the average grain size of BST-3 wt% BAS ceramics was 150 nm, almost like the size of pristine BST powders (100 nm). 3. the BST-3 wt% BAS ceramics showed the optimal energy storage performance with the energy storage density of 1.8 J/cm3 and the energy storage efficiency of 81.8% at 410 kV/cm which were both higher than that of pure BST ceramics (0.74 J/cm3, 76.3%).
Author statement I have made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; And I have drafted the work or revised it critically for important intellectual content; And I have approved the final version to be published; And I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
All persons who have made substantial contributions to the work reported in the manuscript, including those who provided editing and writing assistance but who are not authors, are named in the Acknowledgments section of the manuscript and have given their written permission to be named. If the manuscript does not include Acknowledgments, it is because the authors have not received substantial contributions from nonauthors.
Author group information Hongye Wang1, Minghe Cao1*, Miao Liu1, Hanxing Liu2, Hua Hao1, Zhonghua Yao1 1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China 2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Material Science and Engineering. Wuhan University of Technology, Wuhan 430070, Hubei, PR China The role(s) of all authors is listed, as followed: Hongye Wang1: Software, Validation, Formal analysis, Investigation, Data Curation, Writing-Original Draft Minghe Cao1*: Conceptualization, Methodology, Writing-Review & Editing, Visualization Miao Liu1: Validation, Formal analysis, Investigation, Data Curation Hanxing Liu2: Supervision, Project administration, Funding acquisition Hua Hao1: Resources, Supervision, Project administration
Zhonghua Yao1: Resources, Supervision
The article is original and has not been published previously. The article has been written by the stated authors who are all aware of its content and approve its submission. The article is not under consideration for publication elsewhere. No conflict of interest exists, or if such conflict exists. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.
S0925-8388(20)30254-1
DOI:
https://doi.org/10.1016/j.jallcom.2020.153891
Reference:
JALCOM 153891
To appear in:
Journal of Alloys and Compounds
Received Date: 2 December 2019 Revised Date:
11 January 2020
Accepted Date: 15 January 2020
Please cite this article as: H. Wang, M. Cao, M. Liu, H. Hao, Z. Yao, H. Liu, Enhanced energy storage properties of fine-crystalline Ba0.4Sr0.6TiO3 ceramics by coating powders with B2O3–Al2O3–SiO2, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153891. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Enhanced Energy Storage Properties of Fine-Crystalline Ba0.4Sr0.6TiO3 Ceramics by Coating Powders with B2O3-Al2O3-SiO2 Wang Hongye1, Cao Minghe1*, Liu Miao1, Hao Hua1, Yao Zhonghua1, Liu Hanxing2 1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China 2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Material Science and Engineering. Wuhan University of Technology, Wuhan 430070, Hubei, PR China *E-mail: [email protected] (M.Cao) Tel: +86 2787885812; Fax: +86 2787885813
Abstract A core–shell mixing technique was utilized to modify the Ba0.4Sr0.6TiO3 (BST)particles in order to obtain the fine- crystalline Ba0.4Sr0.6TiO3 ceramics and improve ceramics energy storage capability. Coating layers of (9B2O3-13Al2O3-78SiO2, mol%) (BAS)were deposited onto Ba0.4Sr0.6TiO3 nanoparticles by a sol-precipitation method. The microstructures, phase composition and dielectric properties were investigated. And fine-crystalline BST ceramics with an average grain size below 150 nm by coating BAS has been prepared. The dielectric breakdown strength and energy storage density significantly improved due to the addition of BAS. The maximum energy storage density obtained from coating 3 wt% BAS was 1.8 J/cm3 at 410 kV/cm, which was higher than pure BST ceramics. Thus, the core-shell architecture is an effective strategy to improve the energy storage performance.
Keywords: Chemical coating method; Fine-grained ceramics; Energy storage; Dielectric
breakdown strength
1. Introduction Renewable energy storage has been actively investigation because of the exhausting trend of fossil fuel and the recycling renewable energy [1]. Dielectric capacitors possess high power density and fast charge-discharge rate to suitable renewable energy storage [2,3]. Among the various technologies, such as electronic circuits, microwave communications and high-power applications, dielectric capacitors have been widely used in many fields [4]. For dielectric capacitors, the discharge energy storage density was acquired for the following equation [5]:
Jcharge= PdE= ౩ EdP Jdischarge=ηJcharge Where E is the electric field, Ps is the saturated polarization and η is the energy storage efficiency. Obviously, based on the equation, the important factors for the energy storage capacitor are high dielectric breakdown strength (BDS), large polarization and high energy storage efficiency. A larger dielectric constant value generates more polarization to achieve higher energy density [6-8]. Moreover, high breakdown strength and low dielectric loss are significant for improvement energy storage performance, these two parameters could improve energy density and decrease energy losses [9-11]. As we know, Ba0.4Sr0.6TiO3 (BST) as a linear dielectric was investigated widely in energy storage owing to the low dielectric loss, high dielectric constant and high energy storage efficiency [12]. However, low breakdown strength (BDS) of pure BST ceramics determine low energy storage density [13]. Breakdown strength as an important element effects to energy storage density performance. In general, the BDS of ceramics was improved by reducing the grain size, increasing
densification and adding the material with high BDS [14-17]. For BST ceramics, the sintering temperature is above 1300°C, and which is leads to the coarsening of grain size under high temperature [18]. Low sintering temperature is an excellent method to reduce the grain size effectively, improve BDS of ceramics, and improve energy storage performance of ceramics. Adding sintering aids, such as SiO2, B2O3 and glass [19-21], is an effective way to decrease sintering temperature due to the low melting point. Rhim et al. reported that the sintering temperature of Ba0.7Sr0.3TiO3 ceramics decreased from 1350°C to 1150°C by adding B2O3 and which lead to decrease the grain size and increase the BDS [22]. Zhang et al. found that the sintering temperature of BST ceramics decreased from 1350°C to 1100°C by adding BaO-SiO2-B2O3 glass while the energy storage density was 0.89 J/cm3 at 200 kV/cm [23]. Wang et al.
reported
that
sintering
temperature
of
BST
ceramics
with
addition
BaO-B2O3-SiO2-Na2CO3-K2CO3 glasses was lowered about 150°C while the energy storage density was 0.72 J/cm3 at 280 kV/cm [24]. Yang et al. found that the addition of BAS glass-ceramics could enhance the breakdown strength of the ceramics [25]. Hence, adding low melting point glass was a promising procedure to improve the energy storage performance. When preparing BST /glass ceramics, ball milling is an acquainted method to fabricate ceramics materials. However, this method is difficult to achieve uniform distribution of glass powder around ceramics particles [26]. And compared with high energy ball milling, which is difficult to introduce impurities by chemical coating method. Herein we use core–shell method to fabricate to BST/BAS core–shell structures [27]. The core–shell method ensures homogeneous mixing of BST and BAS glass. The glass coating layer of BAS is appropriate to a variety of glass compositions. Al2O3 and SiO2 are common coating layer for BaxSr1-xTiO3 particles because of the
high BDS, and B2O3 is an excellent sintering aids because of its low melting point (460°C) [28-30]. Here, we report the B2O3-Al2O3-SiO2 (BAS) as coating layer, which is an effective additive for decreasing sintering temperature, suppressing grain growth and improving energy storage performance of BST ceramics. An oxalate co-precipitation mothed was applied to prepare the BST powders [31], and the BST particles prepared by this method obtain characteristics of fine spherical shape and good uniformity. The sol-precipitation method was applied to fabricate the BAS-coated BST powders to obtain the fine-grained BST- x wt% BAS ceramic materials with highly energy storage density during low sintering temperature.
2. Experimental procedure The oxalate co-precipitation mothed applied to prepare the Ba0.4Sr0.6TiO3 (BST) powders. The xwt% B2O3-Al2O3-SiO2 (BAS)-coated Ba0.4Sr0.6TiO3(BST) powders (x= 1, 2, 3, 4, 5) were fabricated by sol-precipitation method. Starting materials including C8H2O4Si (TEOS, ≥28.4%), C12H27BO3 (≥99.0%), Al(NO3)3•9H2O (≥99.9%), C2H5OH (≥99.5%), C2H4O2 (≥99.5%) and NH4OH (25~28%). 2.1 Synthesis Ba0.4Sr0.6TiO3 nanoparticles The Ba0.4Sr0.6TiO3 nanoparticles were fabricated by oxalate co-precipitation mothed. In the fabrication procedure, oxalic acid was dissolved in 200 mL ethanol and DI water mixture to acquire oxalic acid solution. And acetylacetone added to titanium tetrabutoxide due to inhibit the hydrolysis, and dissolved in 100 mL ethanol. And the mixture was mixed with oxalic acid during magnetic stirring to fabricate H2TiO(C2O4)2 (HTO) precursor for 6h. Then, strontium oxalate and barium oxalate were dissolved in DI water, and polyethylene glycol was added in solution as a
dispersant. And the clear HTO solution added to the suspension under magnetic stirring to produce Ba0.4Sr0.6TiO(C2O4)2·4H2O (BSTO) precursor. And the precursor calcines at 600 °C for 2 h to manufacture the Ba0.4Sr0.6TiO3 (BST) powders. 2.2 B2O3-Al2O3-SiO2 coating In a typical fabrication procedure, BST particles were added to absolute ethyl alcohol, then the slurry was treated with the magnetic stirring and the ultrasonic dispersion to obtain the dispersion of BST particles in absolute ethyl alcohol. The stable BST suspension is the precondition for the uniform coating. And then, the TEOS and tributyl borate were dissolved in ethyl alcohol with a magnetic stirrer for 10 min. And DI water added to solution during water bath at 65 °C for 1h. And Al(NO3)3•9H2O added to the mixed solution during magnetic stirring to produce BAS precursor solution. And The precursor solution was added to suspension during magnetic stirring and adjusted pH value of suspension to 9.5 by adding acetic acid and ammonia water. And the suspension stirred at 65°C for 12 h to ensure BAS completely precipitated. The mixture solution dries at 150 ºC for 24h and calcines at 600 °C to eliminate crystal water and residual organic compounds. 2.3 Sintering The particles pressed into pellet at 3 Mpa with 5% PVA as a bind. After calcining at 600°C, the samples were fired in air at 1350°C -1180°C for 2 hr. 2.4 Characterization The bulk density of ceramics measured by Archimedes method. And the microstructure of BST-x wt%BAS powders and pure BST powders was examined by field emission transmission electron microscope (Talos F200s, FSTEM). The crystal structure of ceramics determined by
X-ray diffractometer (XRD, PANalytical X'PertPRO). The microstructure of ceramics measured by scanning electron microscopy (SEM, JSM-7100F). And the dielectric properties of ceramics measured by precision impedance analyser (E4980A, Agilent Tech). And the P-E hysteresis loop of ceramics measured by ferroelectric test system (PK-CPE1701, PolyK Technologies).
3. Results and discussion The BST particle coated with BAS were successfully prepared with various precursor concentrations. The microstructure of pure BST and 5wt% BAS-coated BST powders observed by TEM as shown in Fig. 1. A careful observation that homogenous and smooth coating layer were prepared by sol-precipitation method. The uniform BAS shell is obtained with a thickness from BST@3wt% BAS for ~3nm to BST@5wt% BAS for ~5nm. It was confirmed that the sol-precipitation method is a viable technique to acquire BST@BAS core-shell nanoparticles. It is known that the glass phase could facilitate ceramics sintering, increase densifications, and inhibit grain growth of ceramics. The sintering temperature for the BST@BAS ceramics were researched at a series of temperature and measured bulk densities of sintered pellets. Fig. 2 shows the density of the BST-x wt% BAS ceramics under different sintering temperature. The optimal sintering temperature of the BST-x wt% BAS ceramics was determined to be 1180ºC, and the sintering temperature of pure BST was obtained to 1320 ºC. The sintering temperature of BST xwt%BAS ceramics decreased obviously with addition BAS. The low sintering temperature achieved to decrease the production cost. After the optimum sintering treatment, the fracture surface of pure BST ceramics and BST-x wt% BAS ceramics were observed by SEM as shown in Fig. 3. The pure BST ceramics in Fig.3(a) revealed considerable grain growth with some grains growing up to ~3 µm. Fig. 4 shows the
average grain size of the BST- BAS ceramics was acquired by analysis software (Nano Measurer). And the tiny grain growth was observed for BST- BAS ceramics with all the grains maintained below 500nm. The fine grains were acquired at BST-3wt% BAS ceramics for 150nm. And the BAS suppress the grain growth of the BST ceramics. This grain growth inhibition in BST-BAS ceramics indicates that a thin diffusion barrier layer was formed between grains, and which inhibited the grain boundary migration during the sintering process [32]. The fracture surface of pure BST indicates prominent grain growth due to the absence of a coating barrier. The appropriate liquid content reduces sintering temperature, however, excessive liquid phase lead to an abnormal grain growth and abatement densification [33]. Excessive liquid content caused the abnormal grain growth when the BAS content was more than 3 wt%. Thus, the BAS glass coating on BST exhibits to inhibit grain growth and promotes densification. High densification was achieved after sintering from coating BAS layer. Fig.5 shows XRD patterns of sintered samples. It is apparent that the main phase of ceramics was perovskite structure. And a glass sintering agent cause small amounts of second phase after sintering. For the BST-BAS ceramics, it is observed that the secondary phase Ba1.6Sr2.4Ti2Si4O16 was formed during the sintering process. The secondary phase could be considered by the interfacial reaction between BST core and the SiO2 for the BAS glass shell as shown in eqn (1). 4Ba0.4Sr0.6TiO3 + 4SiO2 → Ba1.6Sr2.4Ti2Si4O16 + 2TiO2
(1)
As shown in eqn (1), the secondary phase is formed by the reaction between BAS and BST phases during sintering process. The second phase Ba1.6Sr2.4Ti2Si4O16 together with the remnant BAS phase between grain boundaries of BST had a significant role in the grain growth inhibition [32]. And the second phase around the grains boundaries could increase breakdown strength and
improve energy storage performances [34]. Overall, BAS glass is a superior sintering agent to inhibit grain growth and promote sintering densification. Fig.7(a) shows the dielectric constant of the BST-BAS ceramics from room temperature to 240ºC. The dielectric constants of the BST-BAS ceramics are lower than the coarse-grained BST due to the dilution effect of glass addition and the intrinsic size effects of BST [35,36]. The dielectric constant peak shifted to high temperature, and it is broadened and round, which indicate that better temperature stability compared to the pure BST. The variation of dielectric constants from room temperature to 240ºC is within ±10%. Fig.7(b) shows the dielectric loss of the BST-BAS ceramics from room temperature to 240ºC. As can be seen in fig.7(b), the BST-BAS ceramics reveals tiny dielectric loss with most values ≤1%. And it exhibits fine dielectric quality of ceramics [36]. The frequency dependence of dielectric constants and dielectric loss of the BST-BAS ceramics as shown in Fig. 8(a). The dielectric constant of the BST-BAS ceramics shows excellent frequency stability. The variance of dielectric constant from low frequency to high frequency is within ±1%. And Fig. 8(b) shows the dielectric loss of the BST-BAS ceramics with difference frequency. And the BST-BAS ceramics exhibits dielectric loss within 1% at high frequency. For the linear dielectrics or ferroelectrics, the dielectric constant varies with applied electric field, and this phenomenon called dielectric nonlinearity [37]. The dielectric constant of the BST ceramics reduced with increasing electric field, which damage the energy storage performance of the BST ceramics. Fig.8 shows dielectric constants of the BST-BAS ceramics for difference DC electric field. The dielectric constant of the BST-BAS ceramics exhibits good electric field stability than pure BST ceramics, which attributed to the fine grain size of BST-BAS ceramics. In
the BST ceramics, the grain exhibits nonlinear behavior with high dielectric constant, and grain boundary remains linear with low dielectric constant. The relative content of grain boundary increases with reducing grain size, as “dilution effect”, and the electric field stability of ceramics is improved [38]. The energy storage density capability of the BST-BAS ceramics was assessed by P-E text. P(E) hysteresis loops of the BST-BAS ceramics as shown in Fig. 9(a). As can be seen, the BST-BAS ceramics shows normal ferroelectric switching behavior. The breakdown strength(BDS) is a critical element for evaluating energy storage capability. The BDS of the ceramics is related to grain size. And the BDS of ceramics is increased with reducing the grain size [39]. Table 1 shows the BDS, grain size, discharge energy storage density (W) and energy storage efficiency (η) of BST-BAS ceramics. The BDS is enhanced from 210 kV/cm for pure BST ceramics to 410 kV/cm for BST-3 wt% BAS ceramics, which lead to improve energy storage density. And the energy storage density of BST-3wt% BAS ceramics reaches up 1.8 J/cm3, which is improved by 2.5 times as compared with that of the pure BST ceramics. The fine-grained ceramics show enormous energy storage density compared with coarse-grained ceramics at high electric field [40]. Fig. 9(b) exhibits the energy storage density and energy storage efficiency of BST-BAS ceramics under different electric fields. The energy storage density of BST-BAS ceramics was increasing approximate parabolically with the electric field. And the BST-3wt% BAS ceramics remained high energy storage efficiency (81.8%) at 410 kV/cm, which are mainly attributed to the fine grain and compact structure. The BST-BAS ceramics exhibits high breakdown strength and the energy storage density is confirmed to promise material system for energy storage capacitor applications.
Conclusion
The BST-x wt% BAS core-shell particles were successful fabricated by sol-precipitation method. Low temperature sintering of the BST-BAS ceramics exhibited high densification. And fine grain size was obtained of BST-3wt% BAS ceramics for 150 nm. Then, the BST-3wt% BAS ceramics exhibited good electric field stability and excellent energy storage properties. A high dielectric breakdown strength (410 kV/cm) is beneficial to improve energy storage. And the highest energy storage density acquired by BST-3wt% BAS ceramics was ~1.8 J/cm3 at 410 kV/cm and high energy storage efficiency (81.8%). The result indicated that the BST-BAS ceramics is a promising procedure to improve the energy storage performance. In the end, the promising technology to fabricate new functional materials suggest that the core-shell structure creates a new way to acquire high energy storage density ceramics materials.
Acknowledgements This work was financially supported by the Natural Science Foundation of China (51872213), the NSFC-Guangdong Joint Funds of the Natural Science Foundation of China (No. U1601209), the National Key Basic Research Program of China (973 Program) (No. 2015CB654601), the Technical Innovation Special Program of Hubei Province (2017AHB055), and the Major Program of the Natural Science Foundation of China (51790490).
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Table 1 the BDS, grain size, discharge energy storage density (W) and energy storage efficiency (η) of pure BST ceramics and BST-x wt% BAS ceramics x wt%
BDS (kV/cm)
Grain size (µm)
W (J/cm3)
η
0
210
1.2
0.74
76.3%
1
280
0.33
1.17
83.6%
2
320
0.20
1.47
77.8%
3
410
0.15
1.80
81.8%
4
260
0.37
0.92
74.8%
5
280
0.46
1.06
74.1%
Fig.1 the TEM images of (a) the pure BST, (b) the 3wt% BAS-BST powders (c) the 5wt% BAS- BST powders
Fig.2 the bulk density of BST-x wt% BAS ceramics (x=0, 1, 2, 3, 4, 5) under different sintering temperature
Fig.3 the SEM photographs of fracture surface of BST-x wt% BAS ceramics (a) pure BST ceramics, (b) x=1, (c) x=2, (d) x=3, (e) x=4, (f) x=5
Fig.4 the average grain size and sintering temperature of BST-x wt% BAS ceramics
Fig.5 the XRD patterns of BST-x wt% BAS ceramics (x=0, 1, 2, 3, 4, 5)
Fig.6 the temperature dependence of (a) dielectric constant and (b) dielectric loss for BST-x wt% BAS ceramics in a temperature range of 50 ºC -240ºC;
Fig.7 the frequency dependence of (a) dielectric constant and (b) dielectric loss for BST-x wt% BAS ceramics in a frequency range of 20 Hz-2 MHz;
Fig.8 the DC electric field dependence of the dielectric constant for BST-xwt%BAS ceramics
Fig.9 (a) the P-E loops for BST-x wt% BAS ceramics under the maximum applied electric field; (b) the electric field dependence of the energy storage density for BST-x wt% BAS ceramics.
1. the B2O3-Al2O3-SiO2-coated Ba0.4Sr0.6TiO3 nanoparticles were prepared by the sol-precipitation mothed. 2. the average grain size of BST-3 wt% BAS ceramics was 150 nm, almost like the size of pristine BST powders (100 nm). 3. the BST-3 wt% BAS ceramics showed the optimal energy storage performance with the energy storage density of 1.8 J/cm3 and the energy storage efficiency of 81.8% at 410 kV/cm which were both higher than that of pure BST ceramics (0.74 J/cm3, 76.3%).
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Author group information Hongye Wang1, Minghe Cao1*, Miao Liu1, Hanxing Liu2, Hua Hao1, Zhonghua Yao1 1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China 2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Material Science and Engineering. Wuhan University of Technology, Wuhan 430070, Hubei, PR China The role(s) of all authors is listed, as followed: Hongye Wang1: Software, Validation, Formal analysis, Investigation, Data Curation, Writing-Original Draft Minghe Cao1*: Conceptualization, Methodology, Writing-Review & Editing, Visualization Miao Liu1: Validation, Formal analysis, Investigation, Data Curation Hanxing Liu2: Supervision, Project administration, Funding acquisition Hua Hao1: Resources, Supervision, Project administration
Zhonghua Yao1: Resources, Supervision
The article is original and has not been published previously. The article has been written by the stated authors who are all aware of its content and approve its submission. The article is not under consideration for publication elsewhere. No conflict of interest exists, or if such conflict exists. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.