Novel Ca doped Sr0.7Bi0.2TiO3 lead-free relaxor ferroelectrics with high energy density and efficiency

Journal Pre-proof Novel Ca doped Sr0.7 Bi0.2 TiO3 Lead-Free Relaxor Ferroelectrics with High Energy Density and Efficiency Peng Zhao, Bin Tang, Feng Si, Chengtao Yang, Hao Li, Shuren Zhang

PII:

S0955-2219(20)30006-6

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2020.01.006

Reference:

JECS 12983

To appear in:

Journal of the European Ceramic Society

Received Date:

11 October 2019

Revised Date:

2 January 2020

Accepted Date:

2 January 2020

Please cite this article as: Zhao P, Tang B, Si F, Yang C, Li H, Zhang S, Novel Ca doped Sr0.7 Bi0.2 TiO3 Lead-Free Relaxor Ferroelectrics with High Energy Density and Efficiency, Journal of the European Ceramic Society (2020), doi: https://doi.org/10.1016/j.jeurceramsoc.2020.01.006

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Novel Ca doped Sr0.7Bi0.2TiO3 Lead-Free Relaxor Ferroelectrics with High Energy Density and Efficiency Peng Zhaoa,b, Bin Tanga,b,, Feng Sia,b, Chengtao Yanga,b,, Hao Lic,, Shuren Zhanga,b a

National Engineering Center of Electromagnetic Radiation Control Materials, University of

Electronic Science and Technology of China, Chengdu 610054, P. R. China. b

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic

Science and Engineering, University of Electronic Science and Technology of China, Chengdu

c

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610054, P. R. China. College of Electrical and Information Engineering, Hunan University, Changsha, 410082, China

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Highlights

The introduction of Ca inhibits the grain growth and reduces oxygen vacancies.



The optimal composition exhibits ultra-high dielectric breakdown strength.



The optimal composition exhibits ultra-high energy storage density and efficiency.



The optimal composition shows excellent stability of temperature and frequency.



The optimal composition exhibits high power density.

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Abstract

Sr0.7Bi0.2TiO3 with high relaxor behavior and energy storage efficiency () is

expected to be applied in power energy storage capacitors. However, its energy storage density is limited by the relatively low dielectric breakdown strength (DBS). Herein, 

Corresponding author. Corresponding author.  Corresponding author. Email addresses: [email protected] (B. Tang), [email protected] (C.T. Yang), [email protected] (H. Li) 1 

Sr0.7Bi0.2CaxTiO3 (SBT-xC, x = 0 ~ 0.15) was prepared to decrease the average grain size of Sr0.7Bi0.2TiO3. This can effectively eliminate the oxygen vacancy and decrease the electrical conductivity and leakage current, which result in the enhanced DBS. Meanwhile, Ca doping increases the relaxor behavior and dielectric constant. When x = 0.1, the composition exhibits high DBS of 480.2 kV/cm and excellent energy storage properties, such as high energy storage density of 2.1 J/cm3 with high  of 97.6% at

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290 kV/cm, considerable thermal stability and great frequency stability. Moreover, SBT-0.1C shows high power density of 50.1 MW/cm3. These results suggest that SBT-

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0.1C is a potential candidate for high performance dielectric energy storage applications.

Keywords: energy storage, Sr0.7Bi0.2TiO3, lead-free, relaxor ferroelectrics, dielectric

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1. Introduction

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breakdown strength

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Energy storage components are critical for modern electronic and electrical power systems[1]. Currently, dielectric capacitors, supercapacitors and batteries are the three

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main energy storage devices. In advanced pulsed power systems, such as power electronics, electromagnetic devices, pulsed power weapons and hybrid electric

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vehicles, dielectric capacitors are superior to other energy storage devices due to its high power density, fast charge-discharge rate and long lifetime [1, 2]. The dielectric material is the major factor to determine the performance of dielectric capacitors. To develop dielectric materials with excellent properties, the following requirements should be satisfied synchronously: high recoverable energy density (Wrec) and 2

efficiency (), large maximum polarization (Pmax), low remnant polarization (Pr) and high dielectric breakdown strength (DBS) [3]. Currently, polymer and ceramics are the dominating dielectric energy storage materials. Compared to polymer, dielectric ceramics with superior mechanical property and outstanding thermal stability are regarded as promising candidates for the advanced dielectric capacitors[4]. Dielectric ceramics can be classified into two categories: bulk ceramics and thin films. The thin

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films possess large Wrec and high DBS resulting from their few defect and the thin thickness. However, their applications are limited by the low absolute energy stored in

the thin films. Therefore, bulk dielectric ceramics with high mass loading can be widely

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used[5].

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In general, there are four types of bulk dielectric ceramics used in energy storage:

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linear dielectric, ferroelectric, relaxor ferroelectric and anti-ferroelectric ceramics. Among them, relaxor ferroelectric and anti-ferroelectric ceramics have attracted great

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attentions as the advanced energy storage materials in high-performance dielectric capacitors owing to their high Pmax, low Pr, and moderate DBS [6]. However, it is

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difficult for anti-ferroelectric ceramics to tolerate continuous cycles because that the

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antiferroelectric-ferroelectric transition causes a large volume change and cracks during the charge-discharge process [5]. Meanwhile, anti-ferroelectric ceramic shows a quite low , which causes the stored electric energy to leak in heat, thus leads to a low DBS and the damage of the device [1]. Moreover, most of the anti-ferroelectric ceramics are lead-based

materials

(eg.

(Pb,La)(Zr,Ti)O3[7],

(Pb,La)(Zr,Sn,Ti)O3[8]

and

(Pb,La,Ba)(Zr,Sn,Ti)O3[9]), which are harmful to the environment. Considering the 3

protection of the environment, it is necessary to develop novel lead-free dielectric energy storage materials with good performance. Therefore, lead-free relaxor ferroelectrics with high  are considered as a type of appropriate energy storage materials for dielectric ceramics capacitors [10].

Sr0.7Bi0.2TiO3 is a type of relaxor ferroelectric with the perovskite structure, which

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possesses several advantages for energy storage: (1) SBT exhibits good ferroelectric relaxor behavior and diffused dielectric maximum in a wide range of temperatures, which come from the Sr site vacancy and Bi3+ ion off centering[1, 11]; (2) SBT shows

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a high dielectric constant which is derived from the dipole polarization associated with

dipole fluctuation of polar nanoregions (PNRs)[11, 12]; (3) High  can be obtained in

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SBT owing to low remnant polarization and coercive field[13]. In recent years, great

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attentions have been paid to study the energy storage properties of (Sr,Bi)TiO3-based ceramics have been investigated. Zhang et al.[12] investigated the relaxor behavior of

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Sr(1-1.5x)BixTiO3 ceramics and reported the energy storage density of 1.63 J/cm3 with the

 of 61.4% at 217.6 kV/cm. Chao et al.[13] reported that the relaxor ferroelectric

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ceramics (Sr,Pb,Bi)TiO3 possess an energy storage density of 0.228 J/cm3 with  of

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94.2% at 50 kV/cm. However, the low DBS value in (Sr,Bi)TiO3-based ceramics restricts further improvement of the energy storage performance. CaTiO3, a typical linear dielectric material, possesses low dielectric loss and high intrinsic dielectric strength (up to 4.2 MV/cm)[14]. Introducing a moderate amount of Ca can decrease the grain size, which is beneficial for the high DBS[15]. Based on these considerations, some researchers tried to introduce Ca to increase the DBS and to enhance the energy 4

storage density. Zhang et al.[15] increased the DBS from 239 kV/cm to 313 kV/cm by doping Ca into SrTiO3 and achieved an improved energy storage density of 1.95 J/cm3. Cao et al.[16] added CaTiO3 into Sr0.7Ba0.3Nb2O6 ceramics and increased the DBS from 137 kV/cm to 181 kV/cm. In addition, burying sintering process can inhibit the volatilization of Bi and contribute to form dense microstructure, which is beneficial

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for improving the DBS.

In this work, novel Sr0.7Bi0.2CaxTiO3 (SBT-xC, x = 0 ~ 0.15) lead-free relaxor ferroelectric ceramics were fabricated by the conventional solid-state reaction method

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and burying sintering process. As anticipated, Ca doped SBT shows a dense

morphology and pseudo-cubic perovskite structure, and the introduction of Ca

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decreases the average grain size of SBT, resulting in an enhanced DBS. Meanwhile, the

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relaxor behavior and dielectric constant of SBT are increased by the virtue of Ca doping. Based on these, the highest DBS (480.2 kV/cm) and Wrec of 2.1 J/cm3 with high  of

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97.6% at 290 kV/cm were achieved in SBT-0.1C with remarkable thermal stability (the variation of Wrec is less than 9% within -20  120 ºC with a high  of ~98% at 180

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kV/cm) and frequency stability (almost unchanged in energy storage properties within

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10  250 Hz). Besides, SBT-0.1C also exhibits favorable pulsed charge-discharge performance (high power density (PD) of 50.1 MW/cm3 at 300 kV/cm with short first discharge period (T1) of ~124 ns). These advantages indicate a broad prospect of the application in electrical energy storage.

2. Experimental section 5

2.1 Fabrication of Ca doped Sr0.7Bi0.2TiO3 ceramics

Sr0.7Bi0.2CaxTiO3 (SBT-xC, x = 0, 0.05, 0.075, 0.1, 0.125, 0.15, for weighing) were synthesized via the solid state reaction method using raw materials. The raw powders, SrCO3 (Chengdu Kelong Chemical Co., Ltd., Chengdu, China, 99.5%), Bi2O3 (Chengdu Shudu Nanomaterials Technology Development Co., Ltd., Chengdu, China,

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99.0%), TiO2 (Xiantao Zhongxing Electronic Materials Co., Ltd., Xiantao, China, 99.0%) and CaCO3 (Foshan Songbao Electronic Functional Materials Co., Ltd., Foshan, China, 99.5%) powders were weighed based on the stoichiometric ratios and mixed

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with ethyl alcohol and zirconia balls by ball milling for 6 h. Then, the dried mixtures were calcined at 950 C for 3 h. The calcined powders were ball milled again for 6 h.

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The dried slurries were mixed with 10 wt% solution of polyvinyl alcohol. The obtained

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samples were pressed into disks with a diameter of 12 mm and a thickness of 1 mm under 10 MPa. At last, the samples that covered with ZrO2 powders were placed in an

at 600 C.

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alumina crucible and sintered at 1250 ºC for 3 h after the removal of polyvinyl alcohol

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2.2 Characterization

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The crystal structure and phase composition were studied using the X-ray diffraction

(XRD) with the Cu K radiation (PANalytical, Netherlands). The Refined lattice parameters were analyzed using the GSAS program with the EXPGUI suite. The SEM images of the samples were obtained by the field-emission scanning electron microscopy (FESEM, FEI Inspect F50). The frequency dependence of the dielectric 6

characteristics were tested at room temperature by the LCR meter (Agilent 4284A, U.S.A.) with the frequency ranging from 100 Hz to 1 MHz. The temperature dependence of the dielectric characteristics was determined from -55 C to 150 C by the LCR meter (Agilent 4284A, U.S.A.) with the frequency ranging from 1 kHz to 1 MHz. The impedance of the samples were tested by the impedance analyzer (E4980a, Agilent, Palo Alto, CA). The insulation resistivity of the samples was conducted on the

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high temperature insulation resistance measurement system (HRMS-900, Partulab,

China) at 100 V (direct current (DC) field) under the temperature ranging from 380 C to 500 C. The dielectric breakdown strength (DBS) was carried out on the voltage

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withstand test instrument (RK2671AM, China) under DC field at room temperature.

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The polarization-electric field (P-E) hysteresis loops, I-E loops and leakage current densities of the samples were tested by the ferroelectric test system (Radiant Precision,

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U.S.A.) at room temperature under 10 Hz. The charge-discharge properties were

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obtained using an oscilloscope and a high voltage source. The P-E loops, I-E loops, leakage current densities and charge-discharge properties were tested from the polished

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samples with average thickness of 0.2 mm. All the samples used for electrical measurement were coated with silver on both sides and co-fired at 800 ºC. The area of

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the electrode for P-E loops, I-E loops and leakage current densities is about 0.75 cm2. The energy density and energy efficiency are calculated by the equations as follows:

W=∫

𝑃𝑚𝑎𝑥

𝐸𝑑𝑃

(1)

0

7

𝑊𝑟𝑒𝑐 = ∫

𝑃𝑚𝑎𝑥

𝐸𝑑𝑃

(2)

𝑊𝑟𝑒𝑐 × 100% 𝑊

(3)

𝑃𝑟

η=

where Pmax is the maximum polarization, Pr is the remnant polarization. W is the energy storage density, Wrec is the recoverable energy storage density, and E is the external

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electric field.

3. Results and discussion

3.1 Crystal structure and micro morphology analyses of Ca doped SBT

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Fig. 1a shows the XRD patterns of SBT-xC (x = 0 ~ 0.15). All major peaks of the

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samples are indexed with the SrTiO3 (PDF: 84-0444) and the sharp peaks with high

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intensity indicate the high crystallinity. Meanwhile, no splitting of (111) and (200) peaks corresponds to a pseudo-cubic phase for all samples (Fig. 1b)[17]. The samples

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with x ≤ 0.1 show a single perovskite phase without any impurities. It elucidates that Ca2+ completely diffused into the matrix lattice of SBT, which was due to similar

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equivalent ionic radii of Ca2+ (1.34 Å, 12 coordination number (C.N.)) and Sr2+ (1.44 Å, 12 C.N.)[18]. However, when x > 0.1, the peaks of the secondary phase (TiO2) were

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observed, indicating a limited solubility of Ca in SBT. In addition, the diffraction peaks (111) and (200) shift to higher degree with the increasing Ca doping content when x ≤ 0.1, and then shift to low degree with the increasing Ca doping content when x > 0.1 (Fig. 1b). This phenomenon indicates that the lattice parameters (a) and cell volume (V) decrease firstly and increase later with increase of Ca doping content. This result can 8

be confirmed by the values of a and V that are calculated from the refined XRD data using the Rietveld refinements method (Fig. 1c). When x ≤ 0.1, the gradually decreasing cell volume is due to Ca2+ with the equivalent ionic radius (1.34 Å, 12 C.N.) smaller than that of the Sr2+ (1.44 Å, 12 C.N.) mainly occupying A site[18]. But when x > 0.1, because of the limited solid solubility of A site, Ti4+ with the smaller equivalent ionic radius (0.605 Å, 6 C.N.) than that of the Ca2+ (1.00 Å, 6 C.N.) is substituted by

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redundant Ca2+ in B site[18], resulting in the formation of secondary phase (TiO2, Fig.

S1) and a cell volume larger than that of SBT-0.1C. In addition, the change of diffraction

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peak angles reveals the degree of structural distortion of Ca doped SBT ceramics[19].

9

(200)

(111)

(310)

(224)





(b)

TiO2

(220)

(211)

(200)

(111)

(110) 



(210)

(100)

x = 0.15 x = 0.125 x = 0.1

x = 0.05 x=0

20

SrTiO3 PDF#84-0444

30

40

50

60

70

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3.905

a V

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Lattice parameters (Å)

(c)

3.915

3.910

40.0

46.5

2Theta ()

2Theta () 3.920

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x = 0.075

60.0

59.8

59.6

Unit cell volume (Å3)

Relative intensity

(a)

59.4

0.000 0.025 0.050 0.075 0.100 0.125 0.150

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x

Fig. 1. (a) XRD patterns of Ca doped SBT ceramics and (b) the magnified (111) and

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(200) diffraction peaks. (c) Lattice parameters and unit cell volumes of Ca doped SBT ceramics as a function of Ca doping content.

The micrographs of the cross-section of Ca doped SBT ceramics are shown in Fig. 2a-f, and all samples show dense microstructures. The histograms of the grain size distribution of Ca doped SBT ceramics are provided in the insets of Fig. 2a-f. As the 10

Ca doping content increases, the average grain size of the Ca doped SBT decreases from 2.47 μm for pure SBT to 1.43 μm for x = 0.1 and then gradually increases to 1.86 μm. This phenomenon suggests that a moderate amount Ca doping in SBT can inhibit the grain growth, and then the inhibition effect disappears with dopant increase. This is because that Ti is substituted by redundant Ca2+ in B site when x > 0.1. When Ca2+ occupies B site, the oxygen vacancies increase due to the substitution of redundant Ca2+

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for Ti4+, which is beneficial to mass transportation during sintering. As a result, it

improves the sintering behavior, and leads to an increase in the grain size [20]. In

addition, the EDS results (Fig. S2) of SBT-0.1C exhibit an even distribution of Sr, Ca,

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Bi, Ti and O elements. The small grain size, dense microstructure and uniform chemical

(a)

(c)

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(b)

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compositions of SBT-0.1C are believed to contribute to high DBS.

(f)

(e)

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(d)

Fig. 2 SEM images of the cross-section of Ca doped SBT with different doping content: (a) x = 0, (b) x = 0.05, (c) x = 0.075, (d) x = 0.1, (e) x = 0.125 and (f) x = 0.15. The grain size distributions and average grain sizes of Ca doped SBT are shown in the insets of SEM images. 11

3.2 Dielectric properties and relaxor behavior analyses of Ca doped SBT

The temperature dependence of the dielectric characteristics of Ca doped SBT are shown in Fig. S3a-f. The maximum dielectric constant temperature (Tm) is related to the ferroelectric-paraelectric phase transition temperature[10], which is far below the room temperature (-25 °C ~ -45 °C at 1 MHz) for Ca doped SBT. This phenomenon

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indicates that all samples presented the paraelectric cubic phase at room temperature, which agrees with the XRD results. All samples exhibit the frequency dispersion

tendency and the dielectric loss (less than 0.1) with frequency dispersion characteristics

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are also observed. The frequency dispersion characteristics are usually considered as a

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typical characteristic of relaxor ferroelectrics[21].

To further confirm the relaxor behavior of Ca doped SBT ceramics, the dielectric

(4)

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(𝑇 − 𝑇𝑚 )𝛾 1 1 − = 𝜀 𝜀𝑚 𝐶

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dispersion is evaluated by the modified Curie-Weiss law:

where C is the modified Curie-Weiss constant, γ reveals the degree of relaxor. γ = 1

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indicates the ideal ferroelectric, and γ = 2 indicates the ideal relaxor. Fig. 3a illustrates

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the value of γ at 1 MHz with different Ca doping content by fitting to Equation (4). The values of the samples are within the range of 1.702 ~ 1.823 for SBT-xC (x = 0 ~ 0.15) ceramics, suggesting strong relaxor behavior. For pure SBT ceramics, the strontium vacancies cause the distortion of oxygen octahedron around them, leading to relaxant movement of titanium ions[12]. Moreover, Bi3+ can drive a large displacement at the off-centered A-site owing to the stereochemical activity of Bi lone pair [3]. These result 12

in the formation of the PNRs, so SBT exhibits good relaxor ferroelectric behavior [11]. After doping with Ca in SBT, the cation disordering and the distortion of lattice are enhanced because of the difference of ionic radii and valence between the added Ca2+ and the host A or B site cations, which leads to a stronger relaxor behavior[13]. This is reflected by the higher γ of Ca doped SBT than that of pure SBT. The SBT-0.1C shows a largest γ value of 1.832, indicating the maximum relaxation degree. The frequency

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dependence of the dielectric characteristics of Ca doped SBT is illustrated in Fig. 3b.

All samples exhibit stable dielectric constant and low loss (less than 1%), which

contribute to energy storage. SBT-0.1C possesses the highest dielectric constant and

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the lowest dielectric loss probably due to the structural distortion and the compensation

1

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x=0

 = 1.765

x = 0.05

 = 1.723

x = 0.075

 = 1.823

x = 0.1

 = 1.776

x = 0.125

 = 1.809

x = 0.15

2

3

4

5

0.05

1100 0.04

1000

x=0 x = 0.05 x = 0.075 x = 0.1 x = 0.125 x = 0.15

900 800 700 600

6

ln(T-Tm)

0.06

(b)

0.03 0.02

Tanδ (a. u.)

 = 1.702

Dielectric constant (a. u.)

(a)

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-6 -12 -6 -12 -6 -12 -6 -12 -6 -12 -6 -12

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ln(1/-1/m)

of the strontium vacancy, which are conductive to achieve high DBS and Wrec.

0.01 0.00

100

1k

10k

100k

1M

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Frequency (Hz)

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Fig. 3 (a) The diffuseness parameter γ which reveals the degree of relaxor for SBT-xC (x = 0 ~ 0.15) ceramics by fitting with the modified Curie-Weiss law (Equation (4)) recorded at 1 MHz. (b) The frequency dependence of the dielectric constant and loss for SBT-xC (x = 0 ~ 0.15) from 100 Hz to 1 MHz at room temperature.

The relaxor ferroelectric behavior and the dielectric properties can also be reflected 13

by the P-E loops and I-E loops of Ca doped SBT. All samples show slim P-E loops at 180 kV/cm (Fig. S4a) with negligible Pr (0.006 ~ 0.053 μC/cm2, as can be seen in Fig. S4b and Table S1). This is regarded as a typical relaxor ferroelectric characteristic. Owing to the existence of PNRs, the response of aligning and back-switching for microdomains under external electric field is faster than that of macroscopic domains. It leads to slim P-E loops with negligible hysteresis characteristics[22], which gives

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rise to the high η (98.5% ~ 99.2%) of Ca doped SBT ceramics (Fig. S4c and Table S1).

Meanwhile, the values of Pmax for Ca doped SBT ceramics are similar and SBT-0.1C shows the largest Pmax (Fig. S4b), resulting in the highest Wrec (Fig. S4c). This

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phenomenon is related to the highest dielectric constant of SBT-0.1C. In addition, the

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I-E loops of all samples present a gentle slope with the bulge shape (Fig. S4d), which is a typical characteristic of relaxor ferroelectric. Meanwhile, the broad current peak is

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related to PNRs switching[23]. Only a flat current density platform can be detected in

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I-E loops of all samples. This indicates a pure ergodic state at room temperature and means that the polarization is mainly contributed by mutual interaction between PNRs

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[24, 25]. It can further demonstrate the relaxor ferroelectric behavior in Ca doped SBT

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ceramics.

3.3 DBS, electrical conductivity and leakage current density analyses of Ca doped SBT

For energy storage dielectric materials, the DBS, a critical factor deciding Wrec and

working voltage, is affected by many microstructural factors, such as grain size, porosity, defects and secondary phase, etc [4]. Among them, the grain size mainly affects the DBS of bulk ceramics[5]. Tunkasiri et al.[26] investigated the relationship 14

between the DBS and the grain size as follows: 𝐸𝐷𝐵𝑆 ∝

1

(5)

√𝐺

where EDBS is the DBS, and G is grain size. The relationship indicates that the dielectric materials with smaller grain size possess higher DBS. To obtain a reasonable value of the DBS of Ca doped SBT ceramics, the Weibull distribution was used to evaluate the

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characteristic breakdown strength by Equations (6) and (7): 𝑋𝑖 = 𝑙𝑛(𝐸𝑖 )

(6)

𝑖 )) 𝑛+1

(7)

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𝑌𝑖 = 𝑙𝑛 (−𝑙𝑛 (1 −

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where Ei is the breakdown voltage of each sample, i is the rank of the samples and n is

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the sum number of the samples. The Weibull distribution of the DBS dates are plotted in Fig. 4a. The slope of the linear fit (β) is in the range of 9.58 ~ 13.68, indicating high

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reliability of the fitting results. With the Ca doping content increasing, the BDS increases from 352.2 kV/cm to 480.2 kV/cm at the beginning when x ≤ 0.1 and

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decreases to 420.3 kV/cm afterwards when x > 0.1. This shows a negative relationship for the average grain size (Fig. 4b) because that the small grain size contributes to the

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decrease of the electrical conductivity (increasing the electrical resistivity) [14]. On the one hand, the grain boundary usually possesses higher resistance compared to the grain in bulk ceramics. Smaller grain size results in more grain boundary, and this enhances the electrical resistivity. Meanwhile, more grain boundaries can trap more charge carries and forming localized Schottky barriers[27]. On the other hand, for oxide 15

ceramics, during the cooling down procedure from the sintering temperature, it is easy for the re-oxidation process to penetrate the small grain to remove the oxygen vacancies that formed in the sintering process, leading to the decrease of the electrical conductivity [14]. Therefore, the SBT-0.1C ceramics with the smallest average grain size (1.43 μm) exhibit the highest DBS (480.2 kV/cm).

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The temperature dependence of the electrical conductivity, which is very important to determine the DBS, is shown in Fig. 4c. For each sample, the electrical conductivity increases with the increasing temperature because of the facilitated hopping and

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increased concentration of thermally activated charge carriers. Meanwhile, the

electrical conductivity decreases firstly and increases later with the increased Ca doping

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content. This agree with the result of the Nyquist plot at 400 ºC (Fig. S5). Besides, the

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SBT-0.1C ceramics exhibits the minimum electrical conductivity, resulting in a high DBS, which is consistent with the analysis results of the DBS. Furthermore, to

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determine the conduction mechanism and the carrier species, the activation energy (Ea) is calculated by the Arrhenius equation: −𝐸𝑎 σ = 𝜎0 𝑒𝑥𝑝 ( ) 𝑘𝐵 𝑇

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(8)

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where σ is the DC electrical conductivity, σ0 is the pre-exponential factor, kB is the Boltzman constant and Ea is the activation energy. The Ea values of the Ca doped SBT ceramics are shown in Fig. 4d and Fig. S6a-f. The Ea values of the SBT-xC for x = 0 and 0.05 are 0.86 eV and 1.08 eV, respectively, which are close to the Ea of the oxygen vacancies (~ 1 eV)[28]. This indicates that the oxygen vacancy is the main conduction 16

factor for the samples with x = 0 and 0.05. With the increasing doping content with 0.05 < x ≤ 0.1, the Ea value increases to 1.42 eV. The increased Ea indicates that the shortrange hopping of the oxygen vacancies is impeded, and higher energy is required for the oxygen vacancies transportation [23, 29]. This phenomenon results from the grain size decreasing with the Ca doping content increasing when x ≤ 0.1. The bulk ceramics with small grain size contribute to the removal of the oxygen vacancies, which leads to

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an enhanced Ea. The SBT-0.1C shows the largest Ea value, which leads to a low

electrical conductivity and a high DBS. When 0.1 < x ≤ 0.15, the Ea value slightly decreases with the Ca doping content increasing, which is due to the increase of the

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oxygen vacancies after Ti4+ is substituted by the redundant Ca2+ in B site.

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The leakage current characteristics of Ca doped SBT ceramics can also reflect the

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electrical conductivity and the DBS under high electric field. As shown in Fig. 4e, the leakage current densities of all samples increase linearly to the increased of the electric

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field from 0 kV/cm to 180 kV/cm, indicating that the charged carriers (e.g. oxygen vacancies) get enough energy to contribute to the electric conduction based on the

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Ohmic conduction mechanism in the specific range of the electric field [14, 19]. When

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x ≤ 0.1, the leakage current density decreases as the Ca doping content increasing due to the decreased oxygen vacancies concentration and the electrical conductivity, leading to a gradually increased DBS. When x > 0.1, the leakage current density gradually increases because of the increased oxygen vacancies and the increased defects resulting from the formation of the secondary phase, which contributes to the electrical conductivity. The increased leakage current density leads to a reduced DBS when x > 17

0.1. In addition, all samples exhibit low leakage current densities, resulting in the hysteresis-free loops and high  of Ca doped SBT. In summary, when x = 0.1, the introduction of Ca in the SBT ceramics significantly reduces the leakage current density and the electrical conductivity. It also increases the activation energy, thus enhancing

550

(a)

= 12.83 = 9.58

-0.6 x=0 x = 0.05 x = 0.075 x = 0.1 x = 0.125 x = 0.15

= 13.68

-1.2 -1.8

= 9.59

= 10.34

-2.4 3.4

3.5

3.6

3.7

3.8

3.9

DBS (kV/cm)

0.0

2.4

450

2.0

400

1.6

350 300

4.0

ln(Ei)

2.8

ro of

DBS Average grain size

500

0.6

ln(-ln(1-(i/(n+1))))

(b)

= 11.33

-p

1.2

Average grain size (m)

the DBS.

0.000 0.025 0.050 0.075 0.100 0.125 0.150

1.2

x

1.6

(d)

re

(c) 1E-6

1E-8 400

420

440

460

480

na

380

Ea (eV)

x=0 x = 0.05 x = 0.075 x = 0.1 x = 0.125 x = 0.15

1E-7

1.2

lP

(S/cm)

1.4

1.0

0.8

500

0.000 0.025 0.050 0.075 0.100 0.125 0.150

x

1E-6

(e)

Jo

ur

Leakage current density (A/cm2)

Temperature (C)

1E-7

x=0 x = 0.05 x = 0.075 x = 0.1 x = 0.125 x = 0.15

1E-8

1E-9 0

40

80

120

160

200

Electric field (kV/cm)

Fig. 4. (a) The Weibull distribution of DBS for Ca doped SBT with different Ca doping content, the inset shows a picture of SBT-0.1C ceramics sample. (b) DBS values and 18

average grain size of Ca doped SBT with different Ca doping content. (c) Temperature dependence of DC electrical conductivity (σ) of Ca doped SBT ceramics. (d) The variation of activation energy (Ea) calculated based on the DC electrical conductivity (σ) under different temperatures for Ca doped SBT ceramics with different Ca doping content. (e) Leakage current density of Ca doped SBT as a function of electric field

ro of

with different Ca doping content.

3.4 Energy storage properties and charge-discharge performance of Ca doped SBT

The unipolar P-E loops and the energy storage properties of the SBT-xC (x = 0 

-p

0.15) ceramics with maximum applied electric fields are shown in Fig. 5a and b, Fig.

re

S7a and Table S2a. All samples show ultra slim P-E loops with approximately zero Pr due to excellent relaxor behavior. The ultra-low Pr indicates low energy loss during the

lP

charge-discharge process, and all samples exhibits  higher than 97%, which contributes to energy storage. Furthermore, the slim P-E loops are conductive to high

na

charge density, electric displacement and fast discharge capacities[30]. The variation of the maximum applied electric field is consistent with that of the DC DBS, and the SBT-

ur

0.1C ceramics exhibit the highest Wrec owing to the enhanced maximum applied electric

Jo

field and Pmax. Figs. 5c and 5d show the slim P-E loops and the energy storage properties of the SBT-0.1C at 60 ~ 290 kV/cm. With low Pr, the Pmax increases linearly to the increasing electric field (Fig. S7b and Table S2b). The increasement of the Pmax is attributed to the enhanced interaction of PNRs with the increasing electric field[25], resulting in the Wrec becoming higher. For η, there is a slight decline originating from the fact that the energy loss increases with the electric field increasing, but it always 19

maintains an ultrahigh value of 97.6% at 290 kV/cm, indicating a weak dependence on the electric field. The small difference between the charge and discharge current of the highly symmetric I-E loops (Fig. S7c) of SBT-0.1C demonstrates the a low thermal loss, implying a high η[23]. The stable and high η can result in a widespread application, low waste heat, long lifetime and good reliability of capacitors for practical application. The SBT-0.1C exhibits a high value of Wrec at 2.1 J/cm3 with an ultrahigh η of 97.6% under

ro of

290 kV/cm.

Fig. 5e shows the energy storage properties of the SBT-0.1C compared with other

-p

lead-free energy storage ceramics [3-5, 10, 29, 31-55]. It is clear that (K0.5Na0.5)NbO3based and AgNbO3-based energy storage ceramics exhibit high Wrec values due to the

re

high DBS and large (Pmax-Pr), but the  is low. BiFeO3-based energy storage ceramics

lP

usually shows low DBS and , which limit the improvement of Wrec. Many studies were carried on the BaTiO3-based and Bi0.5Na0.5TiO3-based ceramics, so the energy storage

na

properties of those ceramics are widely distributed. Compared with BaTiO3-based and Bi0.5Na0.5TiO3-based ceramics, SBT-0.1C shows certain advantages in the DBS and .

ur

In this work, SBT-0.1C simultaneously possesses high Wrec,  and DBS. In particular,

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an ultrahigh η of 97.6% was achieved under high electric field of 290 kV/cm owing to the retention of near-zero Pr in SBT after Ca doping, which contributes to excellent reliability and wide application scope for capacitors. Therefore, x = 0.1 is the optimum doping content of Ca in SBT ceramics to improve the energy storage properties based on our study.

20

(a)

(b)

2.5

100

15

0

60

120

180

240

60 1.5

40

Wrec 1.0

300

0.000 0.025 0.050 0.075 0.100 0.125 0.150

Electric filed ( kV/cm) 2.5

(d)

100

15 5 0 60 kV/cm 120 kV/cm 180 kV/cm 240 kV/cm 290 kV/cm

-5 -15 -20

2.0

-100

0

100

200

Wrec



1.0

300

20

50

150

This work

[53] 240 kV/cm 240 kV/cm [52] 218 kV/cm 143.5 kV/cm 230 kV/cm [10] [38] 210 kV/cm [50] [54] [51] 310 kV/cm [31] 130 kV/cm [3] 93 kV/cm [45] 220 kV/cm [39] [49] 130 kV/cm 80 kV/cm [36] [46] 150 kV/cm 200 kV/cm 120 kV/cm [32] 135 kV/cm [34]

re

80

[47] 155 kV/cm [4] 180 kV/cm [48] 140 kV/cm

60

175 kV/cm [35] 120 kV/cm [33]

na

50 0

1

250

300

0

2

BT-based BNT-based BF-based KNN-based AN-based This work

[44] 273 kV/cm [37] [40] 230 kV/cm 290 kV/cm 280 kV/cm [5] 230 kV/cm [43] 300 kV/cm [29]

lP

70

200

-p

(e)[55]185 kV/cm

90

(%)

100

Electric field (kV/cm)

Electric filed ( kV/cm)

100

60 40

0.5 0.0

-300 -200

80

1.5

ro of

10

-10

0

x

(c) Wrec (J/cm3)

Polarization (C/cm2)

20

20



(%)

5

80

2.0

(%)

x=0 x = 0.05 x = 0.075 x = 0.1 x = 0.125 x = 0.15

10

0

Wrec (J/cm3)

Polarization (C/cm2)

20

3

Wrec (J/cm3)

[41] 400 kV/cm [42]

4

400 kV/cm

5

ur

Fig. 5. P-E loops (a) and energy storage performance (b) of SBT-xC (x = 0  0.15)

Jo

ceramics with maximum applied electric fields. P-E loops (c) and energy storage performance (d) of SBT-0.1C ceramics in different electric fields. (e) Comparisons of the energy storage properties of SBT-0.1C and other reported lead-free energy storage ceramics.

The temperature stability is a significant benchmark to evaluate the energy storage 21

properties. Therefore, the temperature dependent of P-E loops of SBT-0.1C ceramics were tested under the temperature ranging from -20 °C to 120 °C (Fig. 6a). The SBT0.1C ceramics show slim P-E loops with near-zero Pr (Fig. S7d and Table S3a) over the whole temperature range, indicating a great relaxor behavior. The Wrec is 1.16 J/cm3 at -20 °C and remains 0.98 J/cm3 at 120 °C with less than 9% variation in the whole temperature range (Table S3a). The slight drop of Wrec is attributed to the slightly

ro of

decreased Pmax (Fig. 6b), which is caused by the decreased dielectric constant and

enhanced thermal fluctuation as the temperature increases. Moreover, the η with a high value of ~98% is almost constant with variable temperatures. Therefore, the SBT-0.1C

-p

ceramics shows a great temperature stability. In addition, the frequency stability of the

re

energy storage properties is another crucial factor for capacitor applications. Fig. 6c and 6d illustrate the frequency stability of the SBT-0.1C ceramics within the frequency

lP

range from 10 Hz to 250 Hz. The SBT-0.1C ceramics show slim P-E loops in the whole

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frequency range, and there is no significant change of Pmax and Pr (Fig. S7e), resulting in a stable Wrec and η under different frequencies (Table S3b). The Wrec values slightly

ur

fluctuate between 1.13 J/cm3 and 1.14 J/cm3 and η is remain stable in the range of 97.4% ~ 98.3% (Fig. 6d and Table S3b), indicating an excellent frequency stability. The

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frequency-insensitive characteristic demonstrates a highly flexible domain wall motion owing to the small size of PNRs with large kinetic energy, resulting in a fast response to external electric field[24]. The excellent temperature and frequency stability of the SBT-0.1C ceramics are attractive to the commercial applications in harsh environment.

The charge-discharge performance is important to evaluate the practicability of the 22

ceramic capacitors in pulsed power system. Fig. 6e shows the underdamped pulsed discharge current waveforms of the SBT-0.1C ceramics tested by the RLC circuit. The SBT-0.1C ceramics exhibit similar discharging behaviors under different electric fields, and the amplitude of the first current peak gradually enhances. Meanwhile, the discharge period of the first current peak is almost a constant (124 ns) with the variation of electric field, which is lower than the values published in previous work

ro of

[13, 56, 57], indicating a weak pinning effect. This phenomenon implies a fast polarization reversal and excellent relaxor behavior of the SBT-0.1C, which results in

a low Pr and high . Fig. 6f reveals the maximum current (Imax) and the power density

-p

(PD) under different electric field from undamped discharging waveforms of the SBT-

𝐸𝐼𝑚𝑎𝑥 2𝑆

(9)

lP

𝑃𝐷 =

re

0.1C. The power density was calculated by Equation (9):

where S is the area of electrode, and E is the electric field strength. The amplitude of

na

the first peak current varies from 3.7 A at 50 kV/cm to 60.1 A at 300 kV/cm, and the PD reaches 50.1 MW/cm3 at 300 kV/cm, which is superior to many lead-free energy storage

ur

ceramics [10, 24, 25, 58-60]. This novel Ca doped SBT ceramics with high power

Jo

density exhibit great potential to be applied in advanced pulsed power systems.

23

-20 ºC 0 ºC 20 ºC 40 ºC

0 -5

60 ºC 80 ºC 100 ºC 120 ºC

-10 -15 -20

-200 -150 -100 -50

0

0.8

60

0.6 40 0.4

(c)

1.2

Wrec (J/cm3)

10 Hz 25 Hz 50 Hz 100 Hz

5 0 -5

125 Hz 200 Hz 250 Hz

-10 -15 -200 -150 -100 -50

0

20

0

20



1.0

60

0.6

40

0.4

20 0

0

50

150

200

250

-p

45

(f)

50 40

Imax

re

0

60

PD

30

30

20

15

lP

Curent (A)

20

-20

100

Frequency (Hz) 50 kV/cm 100 kV/cm 150 kV/cm 200 kV/cm 250 kV/cm 300 kV/cm

40

10 0

0

0

500

Time (ns)

1000

50

100

150

200

250

300

Electric field (kV/cm)

na

-40

100 80

0.8

0.0

Curent (A)

(e)

0 80 100 120 140 Wrec

0.2

50 100 150 200

60

60

(d)

Electric filed ( kV/cm) 80

40

Temperature (ºC) 1.4

10

Polarization (C/cm2)

80

0.0 -40 -20

Electric filed ( kV/cm) 15

1.0

0.2

50 100 150 200

100

(%)

5



1.2

PD (MW/cm3)

10

Wrec

(b)

(%)

1.4

(a)

ro of

Polarization (C/cm2)

15

Wrec (J/cm3)

20

Fig. 6. (a) P-E loops of SBT-0.1C ceramics at different temperatures under 10 Hz and

ur

180 kV/cm. (b) The calculated Wrec and η values from (a). (c) P-E loops of SBT-0.1C

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ceramics at different frequencies under room temperature and 180 kV/cm. (d) The calculated Wrec and η values from (c). (e) Pulsed discharge current waveforms of SBT0.1C ceramics under room temperature and different electric fields. (f) Variation of Imax and PD as a function of the electric field.

4. Conclusions 24

In conclusion, a novel Ca doped SBT lead-free relaxor ferroelectric ceramics with a pseudo-cubic perovskite structure was synthesized by the conventional solid-state reaction methods. It is found that the introduction of Ca increases the relaxor behavior and the dielectric constant of the SBT. Moreover, when x = 0.1, the introduction of Ca in SBT ceramics significantly inhibits the grain growth of SBT, therefore result in a reduced average grain size. The small grain gives rise to the reduction of the oxygen

ro of

vacancies, which decreases the electrical conductivity and the leakage current, leading to a high DBS (480.2 kV/cm). Benefiting from these advantages, the SBT-0.1C ceramics

exhibit outstanding energy storage properties with high Wrec value of 2.1 J/cm3 and high

-p

 of 97.6% at 290 kV/cm. Meanwhile, the SBT-0.1C ceramics show an excellent

re

temperature stability (the variation of Wrec is less than 9% from -20 ºC to 120 ºC with a high  of ~98% at 180 kV/cm) and frequency stability (almost unchanged Wrec and 

lP

over the range of 10  250 Hz). In addition, this composition also exhibits great pulsed

na

charge-discharge performance with a short T1 of ~124 ns and a PD of 50.1 MW/cm3 at 300 kV/cm. These features endow SBT-0.1C with enormous promising applications in

ur

the pulsed power systems and the electrical energy storage area.

Jo

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements We are grateful to the National Natural Science Foundation of China (Grant No. 51672038).

25

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