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Applications of Polymer, Composite, and Coating Materials

Achieving High Energy Density and Low Loss in PVDF/ BST Nanodielectrics with Enhanced Structural Homogeneity Yunchuan Xie, Wanrong Jiang, Tao Fu, Jingjing Liu, Zhicheng Zhang, and Shengnan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10354 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Materials & Interfaces

Achieving High Energy Density and Low Loss in PVDF/BST Nanodielectrics with Enhanced Structural Homogeneity ＊

Yunchuan Xie†, Wanrong Jiang†, Tao Fu‡, Jingjing Liu†, Zhicheng Zhang† , Shengnan Wang‡

＊

† Department of Materials Chemistry, School of Science, Xi’an Key Laboratory of Sustainable Energy Materials Chemistry, Xi’an Jiaotong University, Xi'an, Shaanxi 710049, China ‡ Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuang 621999, China

ABSTRACT: Poor compatibility of polymer/ceramic composites used as high-pulse capacitors with high permittivity suffers from the reduced breakdown strength (Eb) and lowered energy density (Ue). Herein, mussels-inspired poly(dopamine) (PDA) modified BaSrTiO3 nanoparticle (mBST) and PVDF matrix are bonded together to fabricate nanocomposites with crosslinked network and enhanced compatibility. The significantly improved Eb of 466 MV/m and the highest Ue of 11.0 J/cm3 for PVDF-based polymer/BST composites have been obtained. By comparing the properties of three series of composites with different structure, the contribution of ferroelectric relaxation, interface polarization and leakage conduction to the dielectric loss has been well addressed. Notably, the surface modification of BST with PDA could remarkably enhance the compatibility of the two components and structural homogeneity of the composite. The improved bonding between polymer matrix and filler chemically or physically is responsible for the reduced dielectric loss from both conduction loss and interfacial polarization, which is the key to improve the Eb, Ue and η of the composite. It has been revealed that enhancing the homogeneity of the composites by modifying ceramics and constructing crosslinked networks between polymer matrix and filler might be a facile strategy to achieve high energy storage performance in polymer composite. KEYWORDS: PVDF; nanodielectrics; surface modification; low energy loss; energy storage capability.

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1. INTRODUCTION The rapid development of smart grid and high-energy weapon equipment, such as electric energy storage, high power laser and electromagnetic ejection system, has brought continuously increased demand for high-performance energy storage units with high energy density

1-3

. Metallized polymeric

film capacitors have been widely used for their fast charging and discharging speed of electrostatic energy along with their low cost and facile processing performance. However, the low energy storage density (Ue) (e.g. 1-3 J/cm3) due to their low dielectric permittivity (εr) has seriously limited the application of current dielectric polymers. As the most widely used metalized film, bi-axially oriented polypropylene (BOPP) film possesses a Ue of 2-3 J/cm3 even under a high electric field of 600 MV/m 4, which could hardly meet the growing demand for high electric energy storage equipment. Further increasing Ue of BOPP by improving Eb is seriously limited by the film processing technique, which leaves a great challenge for the scientists to explore new polymer dielectrics with excellent energy storage performance 5-7. D

Theoretically, energy storage density of film capacitor can be calculated from U e = ∫ EdD , where E 0

is the applied electric field, and D is the displacement of the dielectrics 8-9. For linear dielectric materials with consistent permittivity under varied electric field, the above equation can be simplified as

U e = 1 2 ε r ε 0 (Eb ) , where ε0 is the vacuum permittivity (8.85×10-12 F/m). Apparently, dielectrics with 2

excellent energy storage capability should possess both high εr and Eb. However, that could hardly be achieved in practice since Eb is in inverse proportion to the εr1/2 as indicated in equation Eb = 0.6(Y / ε 0ε r )1/2 . Usually, polymer dielectrics possess high Eb (102~103 MV/m) but rather low εr (10 wt%) leads to more aggregation of particles in the matrix, which may damage the Eb seriously. After coated with PDA, mBST could be well dispersed in PVDF matrix and the possibility of forming defects would be dramatically reduced. That is responsible for the larger Eb of c-PVDF/mBST composite than the neat matrix, which means Eb of composites is possibly higher than polymer matrix if 10 / 28


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the compatibility of two components and the homogeneity of the composites could be finely assured.

3.3. Energy Storage Performances. The high field dielectric properties of three sets of dielectric

composites are evaluated using unipolar and dipolar D-E loops under the increased electric fields, where the neat PVDF and c-PVDF are presented for comparison (Figure S6 and Figure S7). The unipolar D-E loops of three composites with 10 wt% BST or mBST are compared in Figure S7 because of their highest Eb among the composites. As indicated in Figure 5a, the displacements of PVDF/BST and c-PVDF/BST are observed as about 4 µC/cm2 under the maximum electric field of 300-325 MV/m. However, the maximum displacement of c-PVDF/mBST is about 7 µC/cm2 thanks to its much higher electric field (c.a. 466 MV/m) allowed to be applied. Besides, rather slim D-E loops with low remnant polarization are observed in all composites (Figure S6). The excellent relaxor ferroelectric performance of BST with low Pr may address the rather close D-E loops observed in composites to that of neat PVDF. Pr of c-PVDF/mBST is smaller than c-PVDF/BST, which means the energy stored could be more completely released once the electric field is removed. Pm

PVDF/BST c-PVDF/BST c-PVDF/mBST

80 15 Ue and η of

3

6

100

(b)20

Pr

Ue (J/cm )

2

Displacement (µC/cm )

(a) 8

4

10

2

60

PVDF c-PVDF/BST c-PVDF/mBST

40

5

0 0

100

200

300

400

500

0

η (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

0

Electric field (MV/m)

100

200

300

400

500

0

Electric filed (MV/m)

Figure 5. Energy storage performance of the three composites with 10 wt% particle content. (a) The maximum polarization (Pm) and remnant polarization (Pr) in unipolar D-E loops, (b) comparison of Ue and η as a function of electric field.

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Figure 5b shows the discharged energy density (Ue) and charge-discharging efficiency (η) of PVDF matrix and the two composites. As electric field increases, Ue of all the materials is increasing against the square of electric field. Under the same electric field, rather close Ue of PVDF, c-PVDF, PVDF/BST and c-PVDF/BST is observed. For example, Ue is calculated to be about 4.2 J/cm3 at the breakdown field (about 300 MV/m) as shown in Figure 5b and Figure S7. Ue of c-PVDF/mBST is slightly larger than the other materials under the consistent field, which might be ascribed to the lowered polarization in the discharging circle. Most strikingly, the maximum Ue of c-PVDF/mBST is obtained as 11.0 J/cm3 under 466 MV/m, which is 50% larger than the neat polymer matrix and the other two composites. Apparently, that should be mostly ascribed to the much higher Eb of c-PVDF/mBST than the other materials. Meanwhile, as presented in Figure 5b, η of all the composites is decreasing as electric field increases, which may be ascribed to increased conduction loss and remnant polarization. However, η of c-PVDF/mBST filled with 10 wt% particle content is about 72% under 300 MV/m, which is 12% higher than the other composites under the same electric field. Even at 466 MV/m, η as high as 63% could be well maintained, which is even better than neat PVDF ferroelectric polymers. The fair comparison between three composites strongly indicates that the surface modification of BST is playing the dominant role by improving Eb thus Ue. The enhanced η of c-PVDF/mBST is due to the reduced displacement during discharging circle, which is related to the conduction loss, ferroelectric relaxation of polymer matrix, and the interfacial polarization between two components. Table 2. Dielectric and energy storage performance of PVDF-based polymer/BST composites. Formulation

Shape (sizea))

Loading (wt%) K

Eb (MV/m)

Ue (J/cm3)

Ul (%)

Ref.

PVDF/BST

Nanofiber (~50)

17 (4.4 vol%)

13

310

5.2

-

48

PVDF/BST

Nanofiber (~100)

20 (2.5 vol%)

12

380

7.5

35

49

PVDF/BST

Particle (110nm)

20 (5.0 vol%)

10

250

3.9

-

50

P(VDF-HFP)/BST

Nanowire (~20)

35 (10.0 vol%)

20

200

2.3

40

26

P(VDF-CTFE)/BST

particle (200nm)

10 (2.0 vol%)

35

466

11.0

37

This

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study a)

Diameter (particle) or aspect ratio (fiber/wire).

To show the advantages of the c-PVDF/mBST composites in energy storage properties, the reported Ue and Ul detected at breakdown field of composites fabricated from PVDF-based polymers filled with BST fillers are compared in Table 2. Among the reported composites, the present c-PVDF/mBST with 10 wt% mBST possesses the largest Ue owing to its highest Eb. The Ul of c-PVDF/mBST is still comparable with other composites even under much higher electric field. The larger Ue of c-PVDF/mBST than PVDF matrix strongly indicates that the dielectric composites with high Ue and low Ul could be fabricated by finely treat the interface between two different components.

3.4. Dielectric Properties under Low Electric Field. As discussed above, εr is another important

factor affecting the energy storage performance besides Eb. To show the influence of surface modification of BST, the frequency dependence of dielectric constant and loss tangent (tanδ) of three series of composites measured on a BDS (broadband dielectric spectroscopy) C80 instrument at room temperature are shown in Figure 6. As frequency increases, dielectric constant of all three sets of composites is gradually decreased for their reduced dielectric response. The dielectric constant of composites is enhanced from 10 to 35 (@100Hz) with the increasing content of BST or mBST nanoparticle from 0 wt% to 15 wt% as indicated in Figure S8. The maximum dielectric constant is ~35 at 100 Hz when 15 wt% either BST or mBST is loaded, which is 3.5 times that of the pristine PVDF. The remarkable enhancement should be attributed to the considerably high dielectric constant of the BST or mBST nanoparticle (~ 103@100Hz), which can be well predicted with the series or parallel models

51-52

. Comparing with PVDF/BST, dielectric constant of c-PVDF/BST and c-PVDF/mBST

composite shows little difference. In addition to electronic conduction loss, it has been well accepted that the loss tangent (tanδ) of polymer materials is mainly originating from ionic and interfacial polarization at lower frequency (~10-2 13 / 28



Hz), and the loss tangent at higher frequency (~106 Hz) is corresponding to the dielectric response hysteresis of dipoles 53.

250

(b) PVDF/BST 0 wt% 5 wt% 10 wt%

200 150

2 wt% 8 wt% 15 wt%

4 PVDF/BST 0 wt% 5 wt% 10 wt%

3

tanδ

Dielectric Constant

(a)

100

2

0 -2

-1

10 10

0

1

2

3

4

5

6

7

-2

10 10 10 10 10 10 10 10

10 10

-1

0

3

4

5

6

7

Frequency (Hz)

(d) 4

c-PVDF/BST 0 wt% 5 wt% 10 wt%

2

2 wt% 8 wt% 15 wt%

c-PVDF/BST 0 wt% 5 wt% 10 wt%

3

2 wt% 8 wt% 15 wt%

ions

150

2

tanδ

Dielectric Constant

200

1

10 10 10 10 10 10 10 10

Frequency (Hz)

(c)250

2 wt% 8 wt% 15 wt%

1

50 0

100

1 dipole

50 0

0 -2 -1 0 1 2 3 4 5 6 7 10 10 10 10 10 10 10 10 10 10

-2

10 10

-1

0

200

2

3

4

5

6

7

Frequency (Hz)

(e)250 c-PVDF/mBST 0 wt% 5 wt% 10 wt%

1

10 10 10 10 10 10 10 10

Frequency(Hz)

(f) 2 wt% 8 wt% 15 wt%

4 c-PVDF/mBST 0 wt% 5 wt% 10 wt%

3

150

tanδ

Dielectric Constant

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

2 wt% 8 wt% 15 wt%

2 1

50

0 0 -2 -1 0 1 2 3 4 5 6 7 10 10 10 10 10 10 10 10 10 10

-2

-1

10 10

Frequency (Hz)

0

1

2

3

4

5

6

7

10 10 10 10 10 10 10 10

Frequency (Hz)

Figure 6. Frequency dependence of (a, c, e) dielectric constant and (b, d, f) dielectric loss of polymer 14 / 28


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nanocomposite with different particle contents under room temperature. As shown in Figure 6, the obvious interfacial polarization relaxation could be detected in both c-PVDF/BST and c-PVDF/mBST composites characterized with a loss tangent peak at ~10-2 Hz. However, the relaxation peak of PVDF/BST appears at lower frequency than 10-2 Hz. That could be attributed to the chemical crosslinking induced crystallinity reduction in the matrix. Loss tangent of all three sets of composites shows no obvious change at higher frequency (~106 Hz), which means the BST or mBST nanoparticles does not affect the dipole relaxation of PVDF matrix. In all three sets of composites, the addition of BST or mBST results into depressed dielectric loss at low frequency. Meanwhile, the relaxation peak is shifting to the lower frequency as the BST content increases. Notably, the modification of BST with PDA causes dramatically reduced dielectric loss comparing with the composites filled with the same content of neat BST. For example, loss tangent values of PVDF/BST and c-PVDF/BST with 10 wt% particle content are both ~2.5 at 10-2 Hz, while the value of c-PVDF/mBST is only 1.7. That is mainly attributed to the depressed interfacial polarization caused by PDA-coating. The improved compatibility of mBST with PVDF matrix can effectively reduce the ion displacement induced loss tangent. The conclusion could be further confirmed by the reduced conductivity in Figure S9 and the improvement of Eb values as discussed above. As shown in Figure S9, alternative current (AC) conductivity by a logarithm of three series of composites is presented against testing frequency by logarithm from 102 to 106 Hz. The conductivity of composites is 10-11~10-9 S/m, and is linearly increasing with the change of frequency from 102 to 106 Hz, which suggests that all composites show good insulating performance. AC conductivity of all composites is continuously increased with the increasing content of BST or mBST, and the conductivity rising rate of c-PVDF/mBST is slower than PVDF/BST and c-PVDF/BST when the filler content ranges from 2 wt% to 15 wt%. That means the conductivity of c-PVDF/mBST is almost independent onto the filler content. First of all, the PDA layer containing catechol groups could easily form chelation with free ions and thus depress the ion displacement loss as reported by Fu et al 15 / 28


54

. Secondly, the good


compatibility of PDA layer with matrix may improve the electric resistant capability and further decrease the leakage current. 3.5 Conduction Loss Induced Polarization under High Electric Field. As discussed above, the

high Ue and η of c-PVDF/mBST could be ascribed to the improved Eb thanks to PDA coating. In order to deeply and directly elucidate the enhancing mechanism for Eb values of c-PVDF/mBST samples, for the first time, a Sawyer-Tower (S-T) circuit under high electric field with a square voltage waveform is applied onto the composites instead of mostly utilized triangle waveform in D-E loops. It has been well reported that the measurement can identify the contribution of conduction loss onto the polarization in D-E loops 55. Through this D-E loops, the contribution of ferroelectric relaxation and conduction loss to the polarization of three sets of composites are precisely obtained as shown in Figure S10 of Supporting Information file. 1.5

(b)

PVDF/BST c-PVDF/BST c-PVDF/mBST

0.4 PVDF/BST c-PVDF/BST c-PVDF/mBST

0.3

1.0

2

Pr (µC/cm )

0.5

0.2

a

2

Pc100 (µC/cm )

(a)

0.1

0.0

0.0 100

150

200

100

250

(c)

a

2

6

150

200

250



Polarization (µC/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Pr

Pc100

Prelax

PIR

(d)

250 MV/m

4

2

0

c-PVDF/BST

c-PVDF/mBST


Figure 7. Polarization contributed by (a) conduction loss (Pc100) and (b) remnant polarization (Pra), (c) 16 / 28


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comparison of different polarization for the two composites with 10 wt% particle content under 250 MV/m, (d) proposed mechanism of reduced leakage current and depressed interfacial polarization.

As shown in Figure 7, the polarization contributed from conduction loss (Pc100) and remnant polarization (Pra) under different electric field of the three composites with 10 wt% BST are compared. For PVDF/BST composite, the conduction loss is increasing linearly against the electric field suggesting its increased contribution to the overall polarization. After crosslinked, c-PVDF/BST show even higher conduction loss than PVDF/BST composite, which might be ascribed to the reduced crystallinity and modulus of the polymer matrix. Most notably, the composites filled with mBST exhibit dramatically reduced conduction loss and its increasing speed against electric field is much lower than the other two sets of composites. Taking the results obtained under 250MV/m as an instance, Pc100 value of c-PVDF/mBST is reduced to about 0.5 µC/cm2 from 0.9 and 1.1 µC/cm2 of PVDF/BST and c-PVDF/BST composites containing 10 wt% BST, which is over 40% and 50% dropped, respectively. That means the surface modification of BST with PDA could effectively depress the conduction loss induced by the inhomogeneous interface especially under high electric field. Besides, after removing the conduction loss from the overall polarization, the remnant polarization (Pra) would be obtained from the square waveform S-T polarization, which may be ascribed to the ferroelectric relaxation, interfacial polarization and the irreversible polarization of the materials. As shown in Figure 7b, under electric field below 200 MV/m, Pra of all the composites is increasing slowly and Pra of c-PVDF/mBST is the lowest. Comparatively, when the electric field is 250 MV/m, Pra of PVDF/BST and c-PVDF/BST composites is increased to about 1.5 times of that obtained at 200 MV/m. However, Pra of c-PVDF/mBST at 250 MV/m is only about 50% larger than that of 200 MV/m, which agrees well with experimental result of our last work

41

. Apparently, the difference could only be

ascribed to the interfacial polarization since the ferroelectric and irreversible polarization associated with the composites should be more or less the same in the three sets of composites. Under high electric 17 / 28



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field, the interfacial polarization induced by different components would contribute a lot to the overall polarization. Unfortunately, this part of polarization is mainly related to the charge injection and charge trapped on the interfaces, which requires rather long period to be released as discussed in the dielectric loss at low frequency (~10-2 Hz). As shown in Figure 7b, the improved compatibility between two components could effectively reduce the remnant polarization by means of reducing charge injection speed as well as depressing the amount of the trapped charge in c-PVDF/mBST.

Figure 7c shows the polarization contributed from the instantaneous releasable polarization (PIR), relaxed polarization (Prelax), Pc100 and Pra during the energy discharging circle after charged under electric field of 250 MV/m for 100 ms. The crosslinked composite samples filled with BST and mBST are compared. From the energy discharging point of view, both PIR and Prelax are related to the discharged energy in 100 ms or under 10 Hz electric field. Pc100 and Pra are the major source of energy loss. Under 250MV/m, PIR and Prelax of c-PVDF/mBST and c-PVDF/BST are rather close, which agrees well with the consistent discharged energy density calculated from the D-E loops tested with triangle waveform as discussed above. Differently, after coated with PDA layer, both Pc100 and Pra values of c-PVDF/mBST are significantly depressed than the other two composites. Pra values of c-PVDF/mBST is reduced from ~0.3 µC/cm2 of c-PVDF/BST composite to ~0.1 µC/cm2, which is over 60% dropped. Especially, Pc100 of c-PVDF/mBST is reduced to 0.5 µC/cm2 from 1.1 µC/cm2 of c-PVDF/BST sample. Both are responsible for the reduced energy loss, enhanced discharging efficiency, even the improved Eb and Ue obtained in c-PVDF/mBST composite. Therefore, as illustrated in Figure 7d, the surface modification of BST with PDA is the key to increase the compatibility between the filler and matrix. The strong bonding force on the interface may significantly reduce the charge mobility along the interface thus the leakage current observed under high electric field, which usually leads to reduced Eb. Meanwhile, the closely coated PDA layer on BST may help to eliminate the undesired charge concentration on the interface owing to the high polar dipoles in PDA. The more evenly dispersed dipoles in the composites are responsible for the reduced space charge 18 / 28


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in c-PVDF/mBST and the reduced remnant polarization. Both effects result into reduced dielectric loss and improved energy storage properties under high electric field.

4. CONCLUSIONS In summary, through BST modification and matrix cross-linking, composite films with dielectric constant of 35, dielectric loss of 0.06 (@100 Hz) and breakdown strength of 466 MV/m, have been successfully prepared. The significantly improved performance are mainly contributed to the uniformly dispersed mBST particle and strong hydrogen bonds between the two phases, which helps constructing a homogeneous interface with reduced defects. As a result, c-PVDF/mBST composite with 10 wt% particle content possesses a Ue of 11.0 J/cm3 under 466 MV/m, which is about 160% greater than the composites filled with neat BST particle. Meanwhile, η of c-PVDF/mBST is as high as 72% under 300 MV/m, which is 12% superior to the other composites. Our findings strongly suggest that the significantly improved interface compatibility between the matrix and filler, which may depress the contribution from both the conduction loss and the charges injection, is the crucial factor dominating the high Eb thus Ue and η of the dielectric composites. Therefore, improving the homogeneity of the composites by means of eliminating the interface defects might be the key to fabricate high performance dielectric composites, especially for high electric field application.

AUTHOR INFORMATION Corresponding

Authors.

Tel:

86-29-82663937.

E-mail:

[email protected];

[email protected]; Coauthors. E-mails: [email protected]; [email protected]; [email protected]; [email protected]; ORCID Yunchuan Xie: 0000-0003-0202-384X 19 / 28



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Zhicheng Zhang: 0000-0003-1871-117x Notes

The authors declare no conflict of interest.

ACKNOWLEDGMENT The authors are grateful for the support and funding from the National Natural Science Foundation of China (Grant Nos. 51773164, 51573146), Aeronautical Science Foundation of China (Grant No. 2016ZF53054), Natural Science Basic Research Plan in Shaanxi Province of China (Grant Nos. 2015JZ009, 2016JQ2010) and Fundamental Research Funds for the Central Universities (Grant No. XJJ2016063).

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Captions of all figures and tables: Figure 1. Schematic illustration for the preparation of PVDF-based composite film. Figure 2. (a) TEM image of mBST particles with mussel-inspired PDA layer, (b) FT-IR spectra and (c) XRD patterns of BST and mBST particles.

Figure 3. Cross-sectional SEM images of PVDF-based nanocomposite films with 10 wt% (a, b) BST and (c, d) mBST particles.

Figure 4. Weibull distribution of breakdown strength for three sets of composites with different particle contents under room temperature.

Figure 5. Energy storage performance of the three composites with 10 wt% particle content. (a) The maximum polarization (Pm) and remnant polarization (Pr) in unipolar D-E loops, (b) comparison of Ue and η as a function of electric field.

Figure 6. Frequency dependence of (a, c, e) dielectric constant and (b, d, f) dielectric loss of polymer nanocomposite with different particle contents under room temperature.

Figure 7. Polarization contributed by (a) conduction loss (Pc100) and (b) remnant polarization (Pra), (c) comparison of different polarization for the two composites with 10 wt% particle content under 250 MV/m, (d) proposed mechanism of reduced leakage current and depressed interfacial polarization..

Table 1. Abbreviation for the materials and composites. Table 2. Dielectric and energy storage performance of PVDF-based polymer/BST composites.

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