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Large energy density at high-temperature and excellent thermal stability in polyimide nanocomposite contained with small loading of BaTiO3 nanofibers
Accepted Manuscript Large energy density at high-temperature and excellent thermal stability in polyimide nanocomposite contained with small loading of BaTiO3 nanofibers Penghao Hu, Weidong Sun, Mingzhi Fan, Jianfeng Qian, Jianyong Jiang, Zhenkang Dan, Yuanhua Lin, Ce-Wen Nan, Ming Li, Yang Shen PII: DOI: Reference:
S0169-4332(18)32026-9 https://doi.org/10.1016/j.apsusc.2018.07.128 APSUSC 39943
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
9 March 2018 18 July 2018 19 July 2018
Please cite this article as: P. Hu, W. Sun, M. Fan, J. Qian, J. Jiang, Z. Dan, Y. Lin, C-W. Nan, M. Li, Y. Shen, Large energy density at high-temperature and excellent thermal stability in polyimide nanocomposite contained with small loading of BaTiO3 nanofibers, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.128
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Large energy density at high-temperature and excellent thermal stability in polyimide nanocomposite contained with small loading of BaTiO3 nanofibers Penghao Hua,*, Weidong Sunb, Mingzhi Fana, Jianfeng Qianb, Jianyong Jiangb, Zhenkang Danb, Yuanhua Linb, Ce-Wen Nanb, Ming Lib, Yang Shenb,c,* a
Institute for Advanced Materials and Technology, University of Science & Technology Beijing, Beijing, 100083
China. b
State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua
University, Beijing, 100084 China. c
Centre for Flexible Electronics Technology, Tsinghua University, Beijing, China, 100084.
Abstract: Dielectric capacitors are always faced with high temperature in many application cases, so the applicability of high temperature is highly desired for dielectrics besides large energy storage density. Polyimide is a widely used engineering polymer with excellent thermotolerance and ceramic nanofillers with large aspect ratio are effective in improving the dielectric properties of polymer nanocomposite. In this work, PI nanocomposite films contained with BaTiO 3 nanofibers prepared via electrospinning were fabricated by solution casting and thermal imidization process. The dielectric properties of the nanocomposites were investigated from room temperature to 200 oC. With the introduction of BaTiO3 nanofibers, the dielectric permittivity was enlarged with weak increase on dielectric loss in nanocomposites. The breakdown strength (550 kV/mm) as well as discharged energy density (5.82 J/cm3) was largely enhanced in BaTiO3/PI nanocomposite contained with 1 vol% nanofibers. Benefited from the decreased leakage current and improved thermal conduction induced by BaTiO3 nanofibers, the 1 vol% BaTiO3/PI nanocomposite also represented high energy efficiency and excellent thermal stability. The discharged energy density of above 2.1 J/cm3 and near 4 J/cm3 with efficiency larger than 90% are stable from room temperature to 150 oC and to 100 oC respectively. The high temperature applicability of the present material *Author to whom correspondence should be addressed. Electronic mail: [email protected] (P. Hu), shyang_mse@ tsinghua.edu.cn (Y. Shen) 1
makes it much promising in fabricating dielectric energy storage devices applied in hot environment. Keywords: nanofibers; nanocomposite; thermal stability; energy density; high-temperature 1. Introduction As energy storage devices, dielectric capacitors are significant for their excellent features of all solid state, low loss and ultrafast charging and discharging rate. Besides the classical inorganic capacitors, polymers are now adopted to fabricate thin-film capacitors representing advantages of flexibility, high operating voltage and large power density which are much promising in portable electronic products, hybrid electric vehicles, smart grid and pulsed power sources [1-4]. The energy density (Ue) of dielectric material is determined by both electric displacement (D) and electric field (E) as: Ue EdD . In case of linear dielectrics, Ue = 1/2εrε0Eb2, where εr and Eb are the relative permittivity and dielectric breakdown strength, respectively, and ε0 is the permittivity of free space. Although beneficial from the high breakdown strength, the energy density of polymers is much limited by their intrinsically low dielectric permittivity. On the purpose of increasing dielectric permittivity, ceramic nanofillers with high permittivity were introduced into polymer matrix to fabricate nanocomposites [5-8]. The addition of ceramic nanoparticles (such as BaTiO3, TiO2, Ba1-xSrxTiO3 and so on) was effective in achieving increased ε in nanocomposites but hard to obtain higher Ue, because the embedded ceramic fillers always dramatically reduced the Eb of polymer matrix. Compared with nanoparticles, the 1-dimensional nanofillers such as nanorods, nanowires and nanofibers possess smaller specific surface which reduces the surface energy and decreases aggregation to improve interface bonding. Moreover, the large aspect ratio of 1-dimensional nanofillers lengthens the migration path of charge carriers to maintain breakdown strength inside the nanocomposite [9-13]. Enhancement on both of εr and Eb have been observed in the nanocomposites contained with small amount of nanofillers of large aspect ratio, and representatively, larger energy density has been achieved in some of PVDF-based 2
nanocomposites contained with 1-dimensional nanofillers [14-19]. Besides the energy density available at room temperature, thermal stability has also been a critical property for dielectric capacitors to meet challenge from temperature rise in practical application. For example, both in service environment of hybrid electric vehicles and for power dissipation in embedded devices, the capacitors are always confronted with temperature up to 150 o
C, which commonly leads to terrible deterioration on dielectric performances [20]. However,
biaxially oriented polypropylene (BOPP), as the most widely used commercial dielectric polymer can operate only below 105 oC, and the energy efficiency of which reduce seriously from 70 oC [21,22]. Recently, the polymer-based dielectric nanocomposites with adaptability for high temperature have been received much attention [23-27]. Polyimide (PI) is a widely used engineering polymer with excellent mechanical and electrical insulation performance. Benefited from its superior thermal and chemical stability, PI is now supposed to be promising candidate of polymer matrix for high-temperature composite dielectrics [28-33]. For example, Dang et al. fabricated BaTiO3/PI nanocomposite films by in-situ polymerization, and reached increased εr of 20 and Eb of 67 kV/mm in composite with 40 vol% content of BaTiO 3 (BT) nanoparticles. They also reported a calcium copper titanate (CCTO)/PI hybrid film with giant dielectric permittivity (49.1) and good thermal stability [34,35]. Beier et al. prepared homogenous composites via in-situ polymerization method, where Ba0.7Sr0.3TiO3 (BST) nanocrystals were blended with PI [36]. However, none of the nanofillers mentioned above achieved considerable energy density, because they inevitably compromised the breakdown strength of PI nanocomposites. The 1-dimensional ceramic nanofillers may be able to obtain enhanced energy density in PI nanocomposite, similarly as they performed in PVDF polymer-based nanocomposites. In the present work, BT nanofibers (BTNFs) with large aspect ratio were prepared via electrospinning and the PI nanocomposite films contained with BTNFs were fabricated by solution casting and thermal imidization process. The dielectric properties of the nanocomposites were investigated in detail from room temperature to 200 oC. The introduction of BTNFs induced improvement on both 3
dielectric properties and thermal stability of the nanocomposite. Benefited from increased dielectric permittivity and enhanced breakdown strength, larger energy density was realized in the composite films at evaluated temperature. 2. Experimental 2.1. Sample Preparation The BaTiO3 nanofibers with large aspect ratio were prepared by a mature electrospinning technique reported in our previous study [12]. The BTNFs/PI nanocomposite films were fabricated by direct mixing process [37]. Pyromellitic dianhydride (PMDA, 98.5%), 4, 4’-oxydianiline (ODA, 98%), and N, N’-dimethyl-acetamide (DMAc, 99%) were purchased from China National Chemicals Corporation Ltd. Before preparation, the BTNFs, PMDA, and ODA were dried for 10 h at 60 oC. First, ODA was stirred for 15 min in DMAc and then PMDA was added in few times to make sure the complete dissolution. The mixture was stirred vigorously for 4 h at room temperature to form poly(amic acid) (PAA) solution. Second, BT nanofibers with target contents (1, 3, 5, 7, 9 of volume fraction, vol%) were respectively dispersed into DMAc by ultrasonication for 30 min and then stirred for 12 h to obtain BTNFs suspensions. Third, the BTNFs suspensions were added into PAA solution and stirred vigorously for 12 h. Finally, the mixtures were cast on glass substrates and then dried at 50 oC for 1 h in vacuum to completely remove the solvents. The as-prepared composite films were then subjected to the following thermal treatment procedures for thermal imidization: 100 oC for 1 h, 200 oC for 2 h, and 300 oC for 1 h, to generate PI nanocomposite films. The products of BTNFs/PI nanocomposite films were then dried at 50 oC for 12 h for further characterization. The thickness of the BTNFs/PI nanocomposite films was about 11~12 μm. 2.2. Characterization Microstructural morphology of BT nanofibers and BTNFs/PI nanocomposites were observed by scanning electron microscopy (SEM, Hitachi S-4500, Japan). X-ray powder diffraction instrument (X’Pert Powder PW3040/60, PANalytical, Holland) was used to identify the crystallinity 4
of the BT nanofibers and BTNFs/PI nanocomposites. Thermogravimetric analysis (TGA) on the nanocomposites were carried out by thermal gravimetric analysis (TGA, TGA/DSC1, Metter -Toledo, Switzerland). For the measurement of electric properties, Au electrodes of ~50 nm in thickness and ~3 mm in diameter were thermally evaporated on both sides of the nanocomposite films. The dielectric permittivity and loss were measured using an impedance analyzer (Agilent E4990A) in frequency range from 103 Hz to 107 Hz, and in temperature range from 25 oC to 150 oC. Dielectric breakdown strength was measured by Dielectric Withstand Voltage Test (Beijing Electro-Mechanical Research Institute Supervoltage Technique) at a ramping rate of 200 V S -1 and a limit current of 5 mA. Dielectric displacement-electric field (D-E) loops at 100 Hz and leakage current density-electric field curves were measured by a Premier Ⅱ ferroelectric test system (Radiant Technologies, Inc.) within temperature range of 25 oC to 200 oC. 3. Results and Discussion The BT nanofibers calcined from the electrospun ones represent uniform morphology with high density and large aspect ratio, which are shown in the SEM image in Fig. 1a. The XRD pattern in Fig. 1b indicates high crystallinity of BTNFs, which is well-indexed with pure cubic phase of barium titanate. The direct mixing process for preparation of BTNFs/PI nanocomposite films is schematically exhibited in Fig. 2a. The BTNFs were dispersed into PAA solution in DMAc by ultrasonication and then the mixed suspension was cast on glass substrate and dried. After thermal imidization and drying, the BTNFs/PI nanocomposite films were obtained. The SEM images of the surface morphology without any pores or voids shown in Fig. 2b reveal high quality of the nanocomposite films. The BT nanofibers kept high aspect ratio and are well dispersed in the matrix, where no obvious aggregation exists even in Fig. 2b6 for the 9 vol% loading sample. In addition, it can be observed from the images (See cross-sectional morphology in Fig. S2 in supplementary information) that all nanofibers represent in-plane distribution with random orientation, which is beneficial to maintaining high breakdown strength [9,12]. 5
FT-IR spectra of neat PI films and BTNFs/PI nanocomposite films as shown in Fig. 3a. The characteristic imide absorption peaks of symmetric C=O stretching, asymmetric C=O stretching and C-N stretching can be clearly observed at about 1720 cm-1, 1780 cm-1 and 1380 cm-1 respectively in all curves [38]. There is no characteristic absorption band of amide at 1660 cm-1 in the spectra, indicating the complete imidization from PAA to PI in the films. The amorphous phase of neat PI was formed with a broad peak at around 20o observed in the XRD pattern in Fig. 3b. The XRD patterns of BTNFs/PI nanocomposites with BTNFs loading of 1, 3, 5, 7, 9 volume fractions are also arranged exhibited in Fig. 3b. Both the characteristic peaks of PI and perovskite BT can be well indexed in the patterns, and the peaks of BT become more distinct with the content of BTNFs increasing, which demonstrates that BT nanofibers are well physically composited with PI. All the BTNFs/PI nanocomposites have ultrahigh thermal decomposition temperature above 450 oC, as shown in Fig. 4. The introduction of high ratio BT nanofibers has some positive effect on the thermal stability of the PI based films, which indicates good thermal adaptability of the films in application. Fig. 4 exhibits the dielectric permittivity and loss of BTNFs/PI nanocomposite films as function of frequency and temperature, respectively. As shown in Fig. 5a, the dielectric permittivity increases continuously with content of BTNFs rising from 3.1 for neat PI to 8.3 for BTNFs/PI contained with 9 vol% BTNFs at 1 kHz. The enhancement on dielectric permittivity is obviously attributed to the addition of BTNFs. Both the permanent dipole polarization induced by ferroelectric BT and the additional interfacial polarization generated at the interface between BTNFs and matrix promote the total polarization to substantially increase the apparent dielectric permittivity of nanocomposite. The dielectric permittivity is stable with frequency variation, the value of which decreases within 10% from 103 to 107 Hz. Although the dielectric loss is slightly increased with content of BTNFs rising, the value is below 0.04 for all samples, as seen in Figure 5b. The additional charge carriers trapped in interfacial region in nanocomposite may be responsible for the content-dependent increase of dielectric loss at low frequency. The charge carriers are unable to 6
keep pace with electric field alternating so that leads to gradually decrease on loss with frequency rising. The temperature-dependent dielectric permittivity at 1 kHz of the nanocomposites are shown in Figure 5c. Similar with neat PI, the dielectric permittivity is almost unchanged in 1 vol% BTNFs/PI in the whole range from room temperature to 150 oC. With the content of BTNFs rising, the εr of BTNFs/PI represents a more and more evident increase tendency with temperature rising above 100 oC. The dielectric response of the BTNFs/PI nanocomposite is mainly contributed by the rotation of dipoles and the migration of charge carriers. The magnitude of dipoles is determined by the constituent, so the dielectric permittivity increases with the content of BTNFs rising. The charge carriers prefer to aggregate at the interfaces between nanofibers and matrix in the nanocomposite, where energy is relatively low (See AFM image in Fig. S4 in supplementary information), and the potential barrier will prevent the charge carriers from shifting. With temperature rising, the increasingly thermal motion facilitates the migration of charge carriers, results in enhancement on dielectric permittivity. The hydrogen bonds at the interface are beneficial for restraining charge carriers at relatively low temperature, however, the disruption of hydrogen bonds under high temperature will also generate charge carriers. So, the increment on dielectric permittivity is larger in the high content (5~9 vol%) samples where more nanofillers inducing more interfaces. The electric displacement of 9 vol% BTNFs/PI nanocomposite keeps rising with temperature increase (See D-E loops in Fig. S5 in supplementary information), which is identical with the dielectric permittivity vs temperature and indicates the charge carriers’ migration mechanism. With the content of BTNFs rising, there are bigger number of interfaces existing inside the nanocomposite which would change with temperature, so the increase tendency of dielectric permittivity become more distinct. The fluctuation of dielectric loss is slight with temperature variation in the BTNFs/PT nanocomposite, as shown in Figure 5d. Generally, the BTNFs/PI nanocomposites exhibit excellent dielectric performance of both frequency-dependent and temperature-dependent stability, which is desirable for application. The breakdown strengths of the nanocomposites contained with BTNFs and BT nanoparticles 7
(BTNPs) are comparatively shown in Figure 6. With the introduction of nanofillers, the Eb is well maintained by BT nanofibers in BTNFs/PI, which is larger than the decreased one induced by BT nanoparticles in BTNPs/PI. For example, the Eb is 300 kV/mm for 9 vol% BTNFs/PI which is twice as the Eb of 152 kV/mm for BTNPs/PI with similar content loading. Especially in BTNFs/PI nanocomposite contained with 1 vol% BTNFs, the breakdown strength is much increased from 451 kV/mm for neat PI to 553 kV/mm. Although with low amount of only 1 vol%, the in-plane separately distributed BT nanofibers contribute much to the breakdown strength of the nanocomposite. The hetero interfaces between nanofillers and matrix will trap mobile charge carriers to reduce charge shifting inside nanocomposite, while under electric field, the in-plane distributed nanofibers induce charge carriers shifting along the in-plane direction to decrease the out-plane electric leakage current [39,40]. However, aggregation is inevitable for high amount of BT nanofibers which leads to interfacial defects and decreases breakdown strength in nanocomposite. Consequently, the Eb in BTNFs/PI nanocomposite represent a nonmonotonic evolution with content loading that increased in 1 vol% and continuously decreased from 3 vol% to 9 vol%. The discharged energy density (Udis) and stored-discharged energy efficiency (η) of BTNFs/PI nanocomposites calculated from D-E loops (See D-E loops and calculation details in Fig. S6&S7 in Supplementary Information) are exhibited in Fig. 7. Although the maximal electric displacement is enlarged by introduction of BT nanofibers, the increased remnant displacement restricts the improvement on the Udis. The η continuously decreases with the increase of electric field due to the rising of Dr. Benefited from enhanced Eb and relatively low Dr, the maximal Udis of 5.82 J/cm3 is obtained in 1 vol% BTNFs/PI at 500 kV/mm with high η of 72.8. In the nanocomposites contained with more BTNFs, the energy density is limited by the deterioration of breakdown and enlarged remnant displacement. The leakage current density of 1 vol% BTNFs/PI as function of applied electric field at different temperature are exhibited in Fig. 8. The leakage current density shows distinct nonlinear 8
variation with electric field. The values of leakage currents are extremely low at a relatively low electric field range but begin to rise sharply when electric field attains a certain level. For example, the leakage current density of 1 vol% BTNFs/PI at 25 oC is less than 1 μC/cm2 below 400 kV/mm, and reaches up to 25 μC/cm2 at 500 kV/mm. The increase of leakage current density with electric field reveals the enhanced mobility of charge carriers in nanocomposite. The electric field force acting on the charge carriers is enlarged with the electric field rising, when the force is larger than the energy barriers, the localized or trapped charge carriers will begin to form directional migration orienting the applied electric field and create leakage current in nanocomposite [41]. The thermal motion is an important driving force to activate particles as well as charge carriers in material, so the charge carriers’ migration will be realized in nanocomposite at lower applied electric field under higher temperature. As seen in Fig. 8, the leakage current rapidly grows from near 400 kV/mm under 50 oC while sharply increases from about 300 kV/mm under 100 oC, and the electric field at which the leakage current begins to rise is 200 kV/mm under 200 oC. Although the dielectric properties of BTNFs/PI represent stable in the range of room temperature to 150 oC (See Fig. 5), the breakdown strength is sensitive with temperature. It’s doubtful that whether the 1 vol% BTNFs/PI which possesses the largest breakdown strength at room temperature would behave good energy storage performance at higher temperature. Fig. 9 shows the discharged energy density and energy efficiency of 1 vol% BTNFs/PI at evaluated temperature (See D-E loops of 1 vol% BTNFs/PI at different temperature in Fig. S8 in Supplementary Information). Due to gradually increased leakage current and remnant displacement, the maximal Udis and η gradually decrease with temperature rising. However, the maximal Udis of 1 vol% BTNFs/PI at states above 100 oC (4.24 J/cm3 at 100 oC, 2.98 at 150 oC, 1.74 at 200 oC, respectively) is significantly improved compared with that of neat PI (3.82 J/cm3 at 100 oC, 1.51 at 150 oC, 0.88 at 200 oC, respectively) [34]. Compared with other polymer-based nanocomposites for energy storage at high temperature (listed in Table 1), the discharged energy density is also comparatively high. Besides the contribution on the electric breakdown strength mentioned above, 9
the large aspect ratio of BT nanofibers with higher intrinsic thermal conductivity is beneficial to form thermal conductive pathways inside the nanocomposite to decrease heat accumulation and will retard thermal breakdown. Moreover, not only the discharged energy density in 1 vol% BTNFs/PI is enlarged, but the thermal stability for energy storage is also highly improved, as revealed in Fig. 10. The discharged energy density in 1 vol% BTNFs/PI and the energy efficiency are stable with temperature rising in 1 vol% BTNFs/PI. At 200 kV/mm, the 1 vol% BTNFs/PI represents a stable energy density near 1 J/cm3 from room temperature to 200 oC. At 300 kV/mm, the energy density is above 2.1 J/cm3 and changed a little from room temperature to 150 oC, with tiny decrease on energy efficiency keeping larger than 90%, makes the present nanocomposite promising in some hybrid electric vehicles and pulsed power sources. And the large energy density near 4 J/cm3 and high energy efficiency above 90% up to 100 oC at 400 kV/mm are useful for some embedded devices. Table 1 Energy storage density in polymer-based nanocomposites at high temperature No.
Polymer matrix
Nanofillers (content)
1
c-BCB
Boron Nitride nanosheets (10 vol%)
2
PEI
Hexagonal Boron Nitride (19 layers)
3
PMMA
4
PEI
5
PC
BNNS (12 wt%) Al2O3 nanoparticle (20 nm, 0.32 vol%) None
6
FPE
None
7
PI
None
8
PEI
None
9 10
PEEK BOPP
None None
11
P(TFE-HFP)
None
12
PPEK
None
Eb (kV/mm) 421 416 403
480 350 300 300 350 200 250 200 400 400 250 420 550 450 450 450 450 450 10
Udis (J/cm3) 2.25 2 1.8 2.9 2.2 1.3 3.5 2.75 1.6 0.8 1.25 0.3 0.5 0.25 1.6 1.2 0.6 2.1 6.5 2.5 3.5 3.1 2.4 2.1
T (oC) 150 200 250 100 150 200 70 100 150 150 150 200 150 200 150 200 150 70 90 130 70 100 160 190
Ref.
[23]
[42] [43] [44]
[23]
[22]
[41]
13
PI
BaTiO3 nanoparticle (3 vol%)
14
PI
BaTiO3 nanofibers (1 vol%)
275 250 200 500 400 350
1.3 0.5 0.3 4.24 2.98 1.74
100 150 200 100 150 200
[37]
This work
4. Conclusion BTNFs/PI nanocomposite films aim to large energy density and high-temperature applicability were fabricated by direct mixing and thermal imidization process. The dielectric properties of the nanocomposites were investigated from room temperature to 200 oC. With the introduction of BaTiO3 nanofibers, the dielectric permittivity was highly enlarged (8.3 for 9 vol% BTNFs/PI) due to permanent dipole polarization and additional interfacial displacement in nanocomposite. The breakdown strength was largely enhanced in 1 vol% BTNFs/PI because the small loading of BT nanofibers decreases charges migration in nanocomposite. Benefited from the Eb of 550 kV/mm, the maximal discharged energy density of 5.82 J/cm3 was achieved in 1 vol% BaTiO3/PI nanocomposite. The small amount of BaTiO3 nanofibers were also effective in decreasing leakage current and improving thermal conduction, which contributes to energy storage performance under high temperature and thermal stability of the nanocomposite. The discharged energy density of above 2.1 J/cm3 and near 4 J/cm3 with energy efficiency larger than 90% in 1 vol% BTNFs/PI nanocomposite film are quite stable from room temperature to 150 oC and to 100 oC respectively. The present nanocomposite film is much promising in fabricating dielectric energy storage devices applied in hot environment. Acknowledgments This work was supported by the NSF of China (Grant No. 51402015, 51625202, 51532003), the National Basic Research Program of China (Grant No. 2015CB654603), National Key Research & Development Program (Grant No. 2017YFB0701603). Supplementary Information High-revolution TEM image of BaTiO3 nanofiber, cross-sectional SEM images of the neat PI 11
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Figure caption Fig. 1. (a) SEM image and (b) XRD of BT nanofibers. Fig. 2. (a) Schematic diagram of the preparation of PI nanocomposite films contained with BTNFs nanofibers. SEM images of (b1) neat PI film and BTNFs/PI nanocomposite films contained with (b2) 1 vol%, (b3) 3 vol%, (b4) 5 vol%, (b5) 7 vol%, (b6) 9 vol% of BTNFs. Fig. 3. (a) FT-IR spectra and (b) XRD patterns of neat PI film and BTNFs/PI nanocomposite films contained with BTNFs volume fractions of 1, 3, 5, 7, 9. Fig. 4. TGA curves of BTNFs/PI nanocomposite films contained with 0~9 vol% BT nanofibers. Fig. 5. Frequency-dependent of (a) dielectric permittivity and (b) dielectric loss and temperature-dependent of (c) dielectric permittivity and (d) dielectric loss of BTNFs/PI nanocomposite films contained with 0~9 vol% BTNFs. Fig. 6. Breakdown strength of BTNFs/PI nanocomposite films as function of content loading. The BTNPs/PI values are cited from Ref. 37. Fig. 7. (a) Discharged energy density and (b) energy efficiency of BTNFs/PI nanocomposite films as function of electric field. Fig. 8. Variations of leakage current density of 1 vol% BTO/PI nanocomposite with electric field under evaluated temperature from 25 oC to 200 oC. Fig. 9. (a) Discharged energy density and (b) energy efficiency of 1 vol% BTNFs/PI nanocomposite film as function of electric field under evaluated temperature from 25 oC to 200 oC. Fig. 10. Variations of discharged energy density and energy efficiency of 1 vol% BTNFs/PI at certain electric field with temperature.
17
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Table 1 Energy storage density in polymer-based nanocomposites at high temperature No.
Polymer matrix
Nanofillers (content)
1
c-BCB
Boron Nitride nanosheets (10 vol%)
2
PEI
Hexagonal Boron Nitride (19 layers)
3
PMMA
4
PEI
5
PC
BNNS (12 wt%) Al2O3 nanoparticle (20 nm, 0.32 vol%) None
6
FPE
None
7
PI
None
8
PEI
None
9 10
PEEK BOPP
None None
11
P(TFE-HFP)
None
12
PPEK
None
13
PI
BaTiO3 nanoparticle (3 vol%)
14
PI
BaTiO3 nanofibers (1 vol%)
18
Eb (kV/mm) 421 416 403
480 350 300 300 350 200 250 200 400 400 250 420 550 450 450 450 450 450 275 250 200 500 400 350
Udis (J/cm3) 2.25 2 1.8 2.9 2.2 1.3 3.5 2.75 1.6 0.8 1.25 0.3 0.5 0.25 1.6 1.2 0.6 2.1 6.5 2.5 3.5 3.1 2.4 2.1 1.3 0.5 0.3 4.24 2.98 1.74
T (oC) 150 200 250 100 150 200 70 100 150 150 150 200 150 200 150 200 150 70 90 130 70 100 160 190 100 150 200 100 150 200
Ref.
[23]
[42] [43] [44]
[23]
[22]
[41]
[37]
This work
Graphical abstract
19
Highlights: Fabricates polyimide nanocomposite contained with BaTiO3 nanofibers by portable method. The improvement on the dielectric properties induced by small loading of nanofibers is effective. Obtains enhanced energy density in the nanocomposite at high temperature. Realize better energy storage performances with high thermal stability than BOPP.
20
S0169-4332(18)32026-9 https://doi.org/10.1016/j.apsusc.2018.07.128 APSUSC 39943
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
9 March 2018 18 July 2018 19 July 2018
Please cite this article as: P. Hu, W. Sun, M. Fan, J. Qian, J. Jiang, Z. Dan, Y. Lin, C-W. Nan, M. Li, Y. Shen, Large energy density at high-temperature and excellent thermal stability in polyimide nanocomposite contained with small loading of BaTiO3 nanofibers, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.128
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Large energy density at high-temperature and excellent thermal stability in polyimide nanocomposite contained with small loading of BaTiO3 nanofibers Penghao Hua,*, Weidong Sunb, Mingzhi Fana, Jianfeng Qianb, Jianyong Jiangb, Zhenkang Danb, Yuanhua Linb, Ce-Wen Nanb, Ming Lib, Yang Shenb,c,* a
Institute for Advanced Materials and Technology, University of Science & Technology Beijing, Beijing, 100083
China. b
State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua
University, Beijing, 100084 China. c
Centre for Flexible Electronics Technology, Tsinghua University, Beijing, China, 100084.
Abstract: Dielectric capacitors are always faced with high temperature in many application cases, so the applicability of high temperature is highly desired for dielectrics besides large energy storage density. Polyimide is a widely used engineering polymer with excellent thermotolerance and ceramic nanofillers with large aspect ratio are effective in improving the dielectric properties of polymer nanocomposite. In this work, PI nanocomposite films contained with BaTiO 3 nanofibers prepared via electrospinning were fabricated by solution casting and thermal imidization process. The dielectric properties of the nanocomposites were investigated from room temperature to 200 oC. With the introduction of BaTiO3 nanofibers, the dielectric permittivity was enlarged with weak increase on dielectric loss in nanocomposites. The breakdown strength (550 kV/mm) as well as discharged energy density (5.82 J/cm3) was largely enhanced in BaTiO3/PI nanocomposite contained with 1 vol% nanofibers. Benefited from the decreased leakage current and improved thermal conduction induced by BaTiO3 nanofibers, the 1 vol% BaTiO3/PI nanocomposite also represented high energy efficiency and excellent thermal stability. The discharged energy density of above 2.1 J/cm3 and near 4 J/cm3 with efficiency larger than 90% are stable from room temperature to 150 oC and to 100 oC respectively. The high temperature applicability of the present material *Author to whom correspondence should be addressed. Electronic mail: [email protected] (P. Hu), shyang_mse@ tsinghua.edu.cn (Y. Shen) 1
makes it much promising in fabricating dielectric energy storage devices applied in hot environment. Keywords: nanofibers; nanocomposite; thermal stability; energy density; high-temperature 1. Introduction As energy storage devices, dielectric capacitors are significant for their excellent features of all solid state, low loss and ultrafast charging and discharging rate. Besides the classical inorganic capacitors, polymers are now adopted to fabricate thin-film capacitors representing advantages of flexibility, high operating voltage and large power density which are much promising in portable electronic products, hybrid electric vehicles, smart grid and pulsed power sources [1-4]. The energy density (Ue) of dielectric material is determined by both electric displacement (D) and electric field (E) as: Ue EdD . In case of linear dielectrics, Ue = 1/2εrε0Eb2, where εr and Eb are the relative permittivity and dielectric breakdown strength, respectively, and ε0 is the permittivity of free space. Although beneficial from the high breakdown strength, the energy density of polymers is much limited by their intrinsically low dielectric permittivity. On the purpose of increasing dielectric permittivity, ceramic nanofillers with high permittivity were introduced into polymer matrix to fabricate nanocomposites [5-8]. The addition of ceramic nanoparticles (such as BaTiO3, TiO2, Ba1-xSrxTiO3 and so on) was effective in achieving increased ε in nanocomposites but hard to obtain higher Ue, because the embedded ceramic fillers always dramatically reduced the Eb of polymer matrix. Compared with nanoparticles, the 1-dimensional nanofillers such as nanorods, nanowires and nanofibers possess smaller specific surface which reduces the surface energy and decreases aggregation to improve interface bonding. Moreover, the large aspect ratio of 1-dimensional nanofillers lengthens the migration path of charge carriers to maintain breakdown strength inside the nanocomposite [9-13]. Enhancement on both of εr and Eb have been observed in the nanocomposites contained with small amount of nanofillers of large aspect ratio, and representatively, larger energy density has been achieved in some of PVDF-based 2
nanocomposites contained with 1-dimensional nanofillers [14-19]. Besides the energy density available at room temperature, thermal stability has also been a critical property for dielectric capacitors to meet challenge from temperature rise in practical application. For example, both in service environment of hybrid electric vehicles and for power dissipation in embedded devices, the capacitors are always confronted with temperature up to 150 o
C, which commonly leads to terrible deterioration on dielectric performances [20]. However,
biaxially oriented polypropylene (BOPP), as the most widely used commercial dielectric polymer can operate only below 105 oC, and the energy efficiency of which reduce seriously from 70 oC [21,22]. Recently, the polymer-based dielectric nanocomposites with adaptability for high temperature have been received much attention [23-27]. Polyimide (PI) is a widely used engineering polymer with excellent mechanical and electrical insulation performance. Benefited from its superior thermal and chemical stability, PI is now supposed to be promising candidate of polymer matrix for high-temperature composite dielectrics [28-33]. For example, Dang et al. fabricated BaTiO3/PI nanocomposite films by in-situ polymerization, and reached increased εr of 20 and Eb of 67 kV/mm in composite with 40 vol% content of BaTiO 3 (BT) nanoparticles. They also reported a calcium copper titanate (CCTO)/PI hybrid film with giant dielectric permittivity (49.1) and good thermal stability [34,35]. Beier et al. prepared homogenous composites via in-situ polymerization method, where Ba0.7Sr0.3TiO3 (BST) nanocrystals were blended with PI [36]. However, none of the nanofillers mentioned above achieved considerable energy density, because they inevitably compromised the breakdown strength of PI nanocomposites. The 1-dimensional ceramic nanofillers may be able to obtain enhanced energy density in PI nanocomposite, similarly as they performed in PVDF polymer-based nanocomposites. In the present work, BT nanofibers (BTNFs) with large aspect ratio were prepared via electrospinning and the PI nanocomposite films contained with BTNFs were fabricated by solution casting and thermal imidization process. The dielectric properties of the nanocomposites were investigated in detail from room temperature to 200 oC. The introduction of BTNFs induced improvement on both 3
dielectric properties and thermal stability of the nanocomposite. Benefited from increased dielectric permittivity and enhanced breakdown strength, larger energy density was realized in the composite films at evaluated temperature. 2. Experimental 2.1. Sample Preparation The BaTiO3 nanofibers with large aspect ratio were prepared by a mature electrospinning technique reported in our previous study [12]. The BTNFs/PI nanocomposite films were fabricated by direct mixing process [37]. Pyromellitic dianhydride (PMDA, 98.5%), 4, 4’-oxydianiline (ODA, 98%), and N, N’-dimethyl-acetamide (DMAc, 99%) were purchased from China National Chemicals Corporation Ltd. Before preparation, the BTNFs, PMDA, and ODA were dried for 10 h at 60 oC. First, ODA was stirred for 15 min in DMAc and then PMDA was added in few times to make sure the complete dissolution. The mixture was stirred vigorously for 4 h at room temperature to form poly(amic acid) (PAA) solution. Second, BT nanofibers with target contents (1, 3, 5, 7, 9 of volume fraction, vol%) were respectively dispersed into DMAc by ultrasonication for 30 min and then stirred for 12 h to obtain BTNFs suspensions. Third, the BTNFs suspensions were added into PAA solution and stirred vigorously for 12 h. Finally, the mixtures were cast on glass substrates and then dried at 50 oC for 1 h in vacuum to completely remove the solvents. The as-prepared composite films were then subjected to the following thermal treatment procedures for thermal imidization: 100 oC for 1 h, 200 oC for 2 h, and 300 oC for 1 h, to generate PI nanocomposite films. The products of BTNFs/PI nanocomposite films were then dried at 50 oC for 12 h for further characterization. The thickness of the BTNFs/PI nanocomposite films was about 11~12 μm. 2.2. Characterization Microstructural morphology of BT nanofibers and BTNFs/PI nanocomposites were observed by scanning electron microscopy (SEM, Hitachi S-4500, Japan). X-ray powder diffraction instrument (X’Pert Powder PW3040/60, PANalytical, Holland) was used to identify the crystallinity 4
of the BT nanofibers and BTNFs/PI nanocomposites. Thermogravimetric analysis (TGA) on the nanocomposites were carried out by thermal gravimetric analysis (TGA, TGA/DSC1, Metter -Toledo, Switzerland). For the measurement of electric properties, Au electrodes of ~50 nm in thickness and ~3 mm in diameter were thermally evaporated on both sides of the nanocomposite films. The dielectric permittivity and loss were measured using an impedance analyzer (Agilent E4990A) in frequency range from 103 Hz to 107 Hz, and in temperature range from 25 oC to 150 oC. Dielectric breakdown strength was measured by Dielectric Withstand Voltage Test (Beijing Electro-Mechanical Research Institute Supervoltage Technique) at a ramping rate of 200 V S -1 and a limit current of 5 mA. Dielectric displacement-electric field (D-E) loops at 100 Hz and leakage current density-electric field curves were measured by a Premier Ⅱ ferroelectric test system (Radiant Technologies, Inc.) within temperature range of 25 oC to 200 oC. 3. Results and Discussion The BT nanofibers calcined from the electrospun ones represent uniform morphology with high density and large aspect ratio, which are shown in the SEM image in Fig. 1a. The XRD pattern in Fig. 1b indicates high crystallinity of BTNFs, which is well-indexed with pure cubic phase of barium titanate. The direct mixing process for preparation of BTNFs/PI nanocomposite films is schematically exhibited in Fig. 2a. The BTNFs were dispersed into PAA solution in DMAc by ultrasonication and then the mixed suspension was cast on glass substrate and dried. After thermal imidization and drying, the BTNFs/PI nanocomposite films were obtained. The SEM images of the surface morphology without any pores or voids shown in Fig. 2b reveal high quality of the nanocomposite films. The BT nanofibers kept high aspect ratio and are well dispersed in the matrix, where no obvious aggregation exists even in Fig. 2b6 for the 9 vol% loading sample. In addition, it can be observed from the images (See cross-sectional morphology in Fig. S2 in supplementary information) that all nanofibers represent in-plane distribution with random orientation, which is beneficial to maintaining high breakdown strength [9,12]. 5
FT-IR spectra of neat PI films and BTNFs/PI nanocomposite films as shown in Fig. 3a. The characteristic imide absorption peaks of symmetric C=O stretching, asymmetric C=O stretching and C-N stretching can be clearly observed at about 1720 cm-1, 1780 cm-1 and 1380 cm-1 respectively in all curves [38]. There is no characteristic absorption band of amide at 1660 cm-1 in the spectra, indicating the complete imidization from PAA to PI in the films. The amorphous phase of neat PI was formed with a broad peak at around 20o observed in the XRD pattern in Fig. 3b. The XRD patterns of BTNFs/PI nanocomposites with BTNFs loading of 1, 3, 5, 7, 9 volume fractions are also arranged exhibited in Fig. 3b. Both the characteristic peaks of PI and perovskite BT can be well indexed in the patterns, and the peaks of BT become more distinct with the content of BTNFs increasing, which demonstrates that BT nanofibers are well physically composited with PI. All the BTNFs/PI nanocomposites have ultrahigh thermal decomposition temperature above 450 oC, as shown in Fig. 4. The introduction of high ratio BT nanofibers has some positive effect on the thermal stability of the PI based films, which indicates good thermal adaptability of the films in application. Fig. 4 exhibits the dielectric permittivity and loss of BTNFs/PI nanocomposite films as function of frequency and temperature, respectively. As shown in Fig. 5a, the dielectric permittivity increases continuously with content of BTNFs rising from 3.1 for neat PI to 8.3 for BTNFs/PI contained with 9 vol% BTNFs at 1 kHz. The enhancement on dielectric permittivity is obviously attributed to the addition of BTNFs. Both the permanent dipole polarization induced by ferroelectric BT and the additional interfacial polarization generated at the interface between BTNFs and matrix promote the total polarization to substantially increase the apparent dielectric permittivity of nanocomposite. The dielectric permittivity is stable with frequency variation, the value of which decreases within 10% from 103 to 107 Hz. Although the dielectric loss is slightly increased with content of BTNFs rising, the value is below 0.04 for all samples, as seen in Figure 5b. The additional charge carriers trapped in interfacial region in nanocomposite may be responsible for the content-dependent increase of dielectric loss at low frequency. The charge carriers are unable to 6
keep pace with electric field alternating so that leads to gradually decrease on loss with frequency rising. The temperature-dependent dielectric permittivity at 1 kHz of the nanocomposites are shown in Figure 5c. Similar with neat PI, the dielectric permittivity is almost unchanged in 1 vol% BTNFs/PI in the whole range from room temperature to 150 oC. With the content of BTNFs rising, the εr of BTNFs/PI represents a more and more evident increase tendency with temperature rising above 100 oC. The dielectric response of the BTNFs/PI nanocomposite is mainly contributed by the rotation of dipoles and the migration of charge carriers. The magnitude of dipoles is determined by the constituent, so the dielectric permittivity increases with the content of BTNFs rising. The charge carriers prefer to aggregate at the interfaces between nanofibers and matrix in the nanocomposite, where energy is relatively low (See AFM image in Fig. S4 in supplementary information), and the potential barrier will prevent the charge carriers from shifting. With temperature rising, the increasingly thermal motion facilitates the migration of charge carriers, results in enhancement on dielectric permittivity. The hydrogen bonds at the interface are beneficial for restraining charge carriers at relatively low temperature, however, the disruption of hydrogen bonds under high temperature will also generate charge carriers. So, the increment on dielectric permittivity is larger in the high content (5~9 vol%) samples where more nanofillers inducing more interfaces. The electric displacement of 9 vol% BTNFs/PI nanocomposite keeps rising with temperature increase (See D-E loops in Fig. S5 in supplementary information), which is identical with the dielectric permittivity vs temperature and indicates the charge carriers’ migration mechanism. With the content of BTNFs rising, there are bigger number of interfaces existing inside the nanocomposite which would change with temperature, so the increase tendency of dielectric permittivity become more distinct. The fluctuation of dielectric loss is slight with temperature variation in the BTNFs/PT nanocomposite, as shown in Figure 5d. Generally, the BTNFs/PI nanocomposites exhibit excellent dielectric performance of both frequency-dependent and temperature-dependent stability, which is desirable for application. The breakdown strengths of the nanocomposites contained with BTNFs and BT nanoparticles 7
(BTNPs) are comparatively shown in Figure 6. With the introduction of nanofillers, the Eb is well maintained by BT nanofibers in BTNFs/PI, which is larger than the decreased one induced by BT nanoparticles in BTNPs/PI. For example, the Eb is 300 kV/mm for 9 vol% BTNFs/PI which is twice as the Eb of 152 kV/mm for BTNPs/PI with similar content loading. Especially in BTNFs/PI nanocomposite contained with 1 vol% BTNFs, the breakdown strength is much increased from 451 kV/mm for neat PI to 553 kV/mm. Although with low amount of only 1 vol%, the in-plane separately distributed BT nanofibers contribute much to the breakdown strength of the nanocomposite. The hetero interfaces between nanofillers and matrix will trap mobile charge carriers to reduce charge shifting inside nanocomposite, while under electric field, the in-plane distributed nanofibers induce charge carriers shifting along the in-plane direction to decrease the out-plane electric leakage current [39,40]. However, aggregation is inevitable for high amount of BT nanofibers which leads to interfacial defects and decreases breakdown strength in nanocomposite. Consequently, the Eb in BTNFs/PI nanocomposite represent a nonmonotonic evolution with content loading that increased in 1 vol% and continuously decreased from 3 vol% to 9 vol%. The discharged energy density (Udis) and stored-discharged energy efficiency (η) of BTNFs/PI nanocomposites calculated from D-E loops (See D-E loops and calculation details in Fig. S6&S7 in Supplementary Information) are exhibited in Fig. 7. Although the maximal electric displacement is enlarged by introduction of BT nanofibers, the increased remnant displacement restricts the improvement on the Udis. The η continuously decreases with the increase of electric field due to the rising of Dr. Benefited from enhanced Eb and relatively low Dr, the maximal Udis of 5.82 J/cm3 is obtained in 1 vol% BTNFs/PI at 500 kV/mm with high η of 72.8. In the nanocomposites contained with more BTNFs, the energy density is limited by the deterioration of breakdown and enlarged remnant displacement. The leakage current density of 1 vol% BTNFs/PI as function of applied electric field at different temperature are exhibited in Fig. 8. The leakage current density shows distinct nonlinear 8
variation with electric field. The values of leakage currents are extremely low at a relatively low electric field range but begin to rise sharply when electric field attains a certain level. For example, the leakage current density of 1 vol% BTNFs/PI at 25 oC is less than 1 μC/cm2 below 400 kV/mm, and reaches up to 25 μC/cm2 at 500 kV/mm. The increase of leakage current density with electric field reveals the enhanced mobility of charge carriers in nanocomposite. The electric field force acting on the charge carriers is enlarged with the electric field rising, when the force is larger than the energy barriers, the localized or trapped charge carriers will begin to form directional migration orienting the applied electric field and create leakage current in nanocomposite [41]. The thermal motion is an important driving force to activate particles as well as charge carriers in material, so the charge carriers’ migration will be realized in nanocomposite at lower applied electric field under higher temperature. As seen in Fig. 8, the leakage current rapidly grows from near 400 kV/mm under 50 oC while sharply increases from about 300 kV/mm under 100 oC, and the electric field at which the leakage current begins to rise is 200 kV/mm under 200 oC. Although the dielectric properties of BTNFs/PI represent stable in the range of room temperature to 150 oC (See Fig. 5), the breakdown strength is sensitive with temperature. It’s doubtful that whether the 1 vol% BTNFs/PI which possesses the largest breakdown strength at room temperature would behave good energy storage performance at higher temperature. Fig. 9 shows the discharged energy density and energy efficiency of 1 vol% BTNFs/PI at evaluated temperature (See D-E loops of 1 vol% BTNFs/PI at different temperature in Fig. S8 in Supplementary Information). Due to gradually increased leakage current and remnant displacement, the maximal Udis and η gradually decrease with temperature rising. However, the maximal Udis of 1 vol% BTNFs/PI at states above 100 oC (4.24 J/cm3 at 100 oC, 2.98 at 150 oC, 1.74 at 200 oC, respectively) is significantly improved compared with that of neat PI (3.82 J/cm3 at 100 oC, 1.51 at 150 oC, 0.88 at 200 oC, respectively) [34]. Compared with other polymer-based nanocomposites for energy storage at high temperature (listed in Table 1), the discharged energy density is also comparatively high. Besides the contribution on the electric breakdown strength mentioned above, 9
the large aspect ratio of BT nanofibers with higher intrinsic thermal conductivity is beneficial to form thermal conductive pathways inside the nanocomposite to decrease heat accumulation and will retard thermal breakdown. Moreover, not only the discharged energy density in 1 vol% BTNFs/PI is enlarged, but the thermal stability for energy storage is also highly improved, as revealed in Fig. 10. The discharged energy density in 1 vol% BTNFs/PI and the energy efficiency are stable with temperature rising in 1 vol% BTNFs/PI. At 200 kV/mm, the 1 vol% BTNFs/PI represents a stable energy density near 1 J/cm3 from room temperature to 200 oC. At 300 kV/mm, the energy density is above 2.1 J/cm3 and changed a little from room temperature to 150 oC, with tiny decrease on energy efficiency keeping larger than 90%, makes the present nanocomposite promising in some hybrid electric vehicles and pulsed power sources. And the large energy density near 4 J/cm3 and high energy efficiency above 90% up to 100 oC at 400 kV/mm are useful for some embedded devices. Table 1 Energy storage density in polymer-based nanocomposites at high temperature No.
Polymer matrix
Nanofillers (content)
1
c-BCB
Boron Nitride nanosheets (10 vol%)
2
PEI
Hexagonal Boron Nitride (19 layers)
3
PMMA
4
PEI
5
PC
BNNS (12 wt%) Al2O3 nanoparticle (20 nm, 0.32 vol%) None
6
FPE
None
7
PI
None
8
PEI
None
9 10
PEEK BOPP
None None
11
P(TFE-HFP)
None
12
PPEK
None
Eb (kV/mm) 421 416 403
480 350 300 300 350 200 250 200 400 400 250 420 550 450 450 450 450 450 10
Udis (J/cm3) 2.25 2 1.8 2.9 2.2 1.3 3.5 2.75 1.6 0.8 1.25 0.3 0.5 0.25 1.6 1.2 0.6 2.1 6.5 2.5 3.5 3.1 2.4 2.1
T (oC) 150 200 250 100 150 200 70 100 150 150 150 200 150 200 150 200 150 70 90 130 70 100 160 190
Ref.
[23]
[42] [43] [44]
[23]
[22]
[41]
13
PI
BaTiO3 nanoparticle (3 vol%)
14
PI
BaTiO3 nanofibers (1 vol%)
275 250 200 500 400 350
1.3 0.5 0.3 4.24 2.98 1.74
100 150 200 100 150 200
[37]
This work
4. Conclusion BTNFs/PI nanocomposite films aim to large energy density and high-temperature applicability were fabricated by direct mixing and thermal imidization process. The dielectric properties of the nanocomposites were investigated from room temperature to 200 oC. With the introduction of BaTiO3 nanofibers, the dielectric permittivity was highly enlarged (8.3 for 9 vol% BTNFs/PI) due to permanent dipole polarization and additional interfacial displacement in nanocomposite. The breakdown strength was largely enhanced in 1 vol% BTNFs/PI because the small loading of BT nanofibers decreases charges migration in nanocomposite. Benefited from the Eb of 550 kV/mm, the maximal discharged energy density of 5.82 J/cm3 was achieved in 1 vol% BaTiO3/PI nanocomposite. The small amount of BaTiO3 nanofibers were also effective in decreasing leakage current and improving thermal conduction, which contributes to energy storage performance under high temperature and thermal stability of the nanocomposite. The discharged energy density of above 2.1 J/cm3 and near 4 J/cm3 with energy efficiency larger than 90% in 1 vol% BTNFs/PI nanocomposite film are quite stable from room temperature to 150 oC and to 100 oC respectively. The present nanocomposite film is much promising in fabricating dielectric energy storage devices applied in hot environment. Acknowledgments This work was supported by the NSF of China (Grant No. 51402015, 51625202, 51532003), the National Basic Research Program of China (Grant No. 2015CB654603), National Key Research & Development Program (Grant No. 2017YFB0701603). Supplementary Information High-revolution TEM image of BaTiO3 nanofiber, cross-sectional SEM images of the neat PI 11
and BTNFs/PI nanocomposite films, elemental mapping measurement and atomic force microscopy images of 1 vol% BTNFs/PI nanocomposite film, unipolar D-E loops of 9 vol% BTNFs/PI nanocomposite, D-E loops of neat PI and BTNFs/PI nanocomposite films, calculation details of energy density, D-E loops of 1 vol% BTNFs/PI nanocomposite film at evaluated temperatures. References [1] B.J. Chu, X. Zhou, K.L. Ren, B. Neese, M.R. Lin, Q. Wang, F. Bauer, Q.M. Zhang, A Dielectric Polymer with High Electric Energy Density and Fast Discharge Speed, Science 313 (2006) 334-336. [2] P. Barber, S. Balasubramanian, Y. Anguchamy, S. Gong, A. Wibowo, H. Gao, H. J. Ploehn, H. C. zur Loye, Polymer Composite and Nanocomposite Dielectric Materials for Pulse Power Energy Storage, Materials 2 (2009) 1697-1733. [3] C. Liu, F. Li, L. P. Ma, H. M. Cheng, Advanced Materials for Energy Storage, Adv. Mater. 22 (2010) 28-62. [4] Q. Wang, L. Zhu, Polymer Nanocomposites for Electrical Energy Storage, J. Polym. Sci. Pt. B-Polym. Phys. 49 (2011) 1421-1429. [5] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage, Adv. Mater. 25 (2013) 6334-6365. [6] X. Y. Huang, P. K. Jiang, Core-Shell Structured High-k Polymer Nanocomposites for Energy Storage and Dielectric Applications, Adv. Mater. 27 (2015) 546-554. [7] Q. Chen, Y. Shen, S.H. Zhang, Q.M. Zhang, Polymer-Based Dielectrics with High Energy Storage Density, Annu. Rev. Mater. Res. 45 (2015) 433-458. [8] K. Meeporn, S. Maensiri, P. Thongbai, Abnormally enhanced dielectric permittivity in poly(vinylidene fluoride)/nanosized-La2NiO4-δ film, Appl. Surf. Sci. 380 (2016) 67-72. [9] Y. Song, Y. Shen, H. Liu, Y. Lin, M. Li, C.-W. Nan, Improving the dielectric constants and breakdown strength of polymer composites: effects of the shape of the BaTiO 3 nanoinclusions, surface modification and polymer matrix, J. Mater. Chem. 22 (2012) 16491-16498. [10] S.H. Liu, S.X. Xue, W.Q. Zhang, J.W. Zhai, G.H. Chen, Significantly enhanced dielectric property in PVDF nanocomposites flexible films through a small loading of surface-hydroxylated Ba0.6Sr0.4TiO3 nanotubes, J. Mater. Chem. A 2 (2014) 18040-18046. 12
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Figure caption Fig. 1. (a) SEM image and (b) XRD of BT nanofibers. Fig. 2. (a) Schematic diagram of the preparation of PI nanocomposite films contained with BTNFs nanofibers. SEM images of (b1) neat PI film and BTNFs/PI nanocomposite films contained with (b2) 1 vol%, (b3) 3 vol%, (b4) 5 vol%, (b5) 7 vol%, (b6) 9 vol% of BTNFs. Fig. 3. (a) FT-IR spectra and (b) XRD patterns of neat PI film and BTNFs/PI nanocomposite films contained with BTNFs volume fractions of 1, 3, 5, 7, 9. Fig. 4. TGA curves of BTNFs/PI nanocomposite films contained with 0~9 vol% BT nanofibers. Fig. 5. Frequency-dependent of (a) dielectric permittivity and (b) dielectric loss and temperature-dependent of (c) dielectric permittivity and (d) dielectric loss of BTNFs/PI nanocomposite films contained with 0~9 vol% BTNFs. Fig. 6. Breakdown strength of BTNFs/PI nanocomposite films as function of content loading. The BTNPs/PI values are cited from Ref. 37. Fig. 7. (a) Discharged energy density and (b) energy efficiency of BTNFs/PI nanocomposite films as function of electric field. Fig. 8. Variations of leakage current density of 1 vol% BTO/PI nanocomposite with electric field under evaluated temperature from 25 oC to 200 oC. Fig. 9. (a) Discharged energy density and (b) energy efficiency of 1 vol% BTNFs/PI nanocomposite film as function of electric field under evaluated temperature from 25 oC to 200 oC. Fig. 10. Variations of discharged energy density and energy efficiency of 1 vol% BTNFs/PI at certain electric field with temperature.
17
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Table 1 Energy storage density in polymer-based nanocomposites at high temperature No.
Polymer matrix
Nanofillers (content)
1
c-BCB
Boron Nitride nanosheets (10 vol%)
2
PEI
Hexagonal Boron Nitride (19 layers)
3
PMMA
4
PEI
5
PC
BNNS (12 wt%) Al2O3 nanoparticle (20 nm, 0.32 vol%) None
6
FPE
None
7
PI
None
8
PEI
None
9 10
PEEK BOPP
None None
11
P(TFE-HFP)
None
12
PPEK
None
13
PI
BaTiO3 nanoparticle (3 vol%)
14
PI
BaTiO3 nanofibers (1 vol%)
18
Eb (kV/mm) 421 416 403
480 350 300 300 350 200 250 200 400 400 250 420 550 450 450 450 450 450 275 250 200 500 400 350
Udis (J/cm3) 2.25 2 1.8 2.9 2.2 1.3 3.5 2.75 1.6 0.8 1.25 0.3 0.5 0.25 1.6 1.2 0.6 2.1 6.5 2.5 3.5 3.1 2.4 2.1 1.3 0.5 0.3 4.24 2.98 1.74
T (oC) 150 200 250 100 150 200 70 100 150 150 150 200 150 200 150 200 150 70 90 130 70 100 160 190 100 150 200 100 150 200
Ref.
[23]
[42] [43] [44]
[23]
[22]
[41]
[37]
This work
Graphical abstract
19
Highlights: Fabricates polyimide nanocomposite contained with BaTiO3 nanofibers by portable method. The improvement on the dielectric properties induced by small loading of nanofibers is effective. Obtains enhanced energy density in the nanocomposite at high temperature. Realize better energy storage performances with high thermal stability than BOPP.
20