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Flexible regenerated cellulose/boron nitride nanosheet high-temperature dielectric nanocomposite films with high energy density and breakdown strength Jiaping Lao, Haian Xie, Zhuqun Shi, Gang Li, Bei Li, Guo-Hua Hu, Quanling Yang, and Chuanxi Xiong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01219 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Sustainable Chemistry & Engineering

Flexible Regenerated Cellulose/Boron Nitride Nanosheet High-Temperature Dielectric Nanocomposite Films with High Energy Density and Breakdown Strength Jiaping Lao†, Haian Xie†, Zhuqun Shi‡, Gang Li†, Bei Li†, Guo-Hua Hu#, Quanling Yang*,†,§ and Chuanxi Xiong*,† †

School of Materials Science and Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China

‡

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou 510640, China §

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China

#

Laboratory of Reactions and Process Engineering (LRGP, CNRS UMR 7274), CNRSUniversity of Lorraine, ENSIC, 1 rue Grandville, BP 20451, 54001 Nancy, France

*E-mail: [email protected] (Quanling Yang); *E-mail: [email protected] (Chuanxi Xiong)

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KEYWORDS: regenerated cellulose, boron nitride nanosheet, nanocomposite film, dielectric energy storage, thermal stability

ABSTRACT: Traditional dielectric polymers are non-biodegradable and non-renewable petroleum-derived synthetic ones. Here, we report on flexible and 2D nanolayer-structured nanocomposite dielectric films which are easily fabricated from aqueous NaOH/urea solutions of biodegradable and renewable cellulose and boron nitride (BN). Cellulose molecules serve as a stabilizer for exfoliated boron nitride nanosheet (BNNS) and provide the nanocomposites with a high dielectric constant, while BNNS significantly improves their breakdown voltage. 10 wt% of BNNS in the nanocomposite leads to the film with energy storage density of 4.1 J cm−3 and breakdown voltage of 370 MV m−1. The energy storage density is much higher than that of any commercial dielectric polymer as well as any biomass-based materials reported in the literature. Concurrently, these nanocomposites possess high thermal conductivity of 2.97 W m−1K−1 and excellent thermal stability of dielectric properties. This work provides a new route toward fabricating environmentally friendly biomass-based thermally stable dielectric energy storage devices.

INTRODUCTION

Nowadays, sustainability of resource and energy has become among the biggest problems on earth.1–3 Developing simple, efficient, sustainable and environment-friendly pathways to prepare flexible high-temperature energy storage materials is in great demand. Modern energy storage technologies in the fields of hybrid and electric vehicles, electronics and electric power systems,


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etc., require electrostatic capacitors which are able to store electrical energy in dielectric materials while possessing outstanding electric capability, high charge-discharge efficiency, and long-term stability.4–6 However, with the tendency of highly integrated, lightweight portable, wearable, implantable of electronic devices, it’s urgent to exploit the high-performance flexible energy storage devices. Therefore, flexible polymer materials with high dielectric strength, great process ability and reliability have been traditionally considered to be an ideal choice to store dielectric energy. Some polymer materials have been used for energy storage, e.g. biaxially oriented polypropylene (BOPP), poly(vinylidene fluoride) (PVDF).7–9 In general, the electrostatic energy storage density Ue is calculated from the equation Ue = ʃEdD, where E and D are the applied electric field and produced electric displacement, respectively. For linear dielectrics, Ue is directly related to dielectric constants of vacuum (ɛ0) and the dielectric material (k), and also E, following the equation Ue = 1/2kɛ0E2. So far, tremendous efforts have been made to achieve high Ue through improving the dielectric constant of polymer materials by adding high dielectric constant inorganic materials or conducting materials,10,11 such as barium titanate (BaTiO3),12,13 and carbon nanotubes (CNTs).14 However, the large quantities of fillers may aggregate and introduce more structural defects, e.g. microvoids, therefore they couldn’t obstruct breakdown paths present in polymer matrix, resulting in lower breakdown strength (Eb). Consequently, the energy densities of the composites are usually difficult to show more than a two-fold improvement in comparison with those neat polymer materials, and charge–discharge efficiencies are not ideal either. Therefore, the enhancement of Ue is substantially limited by the compromised Eb. Recently, a novel 2D layered structure which consists of polymer and insulative inorganic nanosheets with high Eb was used to improve the Ue of the polymer.15 The inorganic nanosheets provided uniform layered insulation


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centers together with tortuous paths for electrons, enhancing the breakdown strength. Especially, boron nitride nanosheets (BNNSs) were used to composite with polymers for dielectric energy storage materials, such as cross-linked bisbenzocyclobutene (c-BCB),16 poly(vinylidene fluorideter-trifluoroethylene-ter-chlorotrifluoroethylene)

(P(VDF-TrFE-CTFE))17,

polymethylmethacrylate (PMMA)18, etc. Unlike traditional dielectric nanocomposites based on high dielectric constant ceramics and conductive fillers, polymer/BNNS nanocomposites could have high Ue and Eb, meanwhile maintaining the low dielectric loss. Moreover, because boron nitride possesses excellent thermal conductivity, the composites possess good high-temperature dielectric stability. However, synthesis routes of c-BCB and P(VDF-TrFE-CTFE) are complex and costly. Furthermore, PVDF based polymers with high dielectric constant, BCB and polyimide (PI) with good thermal stability, and commercially used BOPP are all nonbiodegradable and non-renewable petroleum-derived synthetic polymers. As the most abundant biopolymer on earth, cellulose is cheap and possesses good biodegradability, mechanical property, and thermal dimensitional stability. It has been used to make novel functional materials19−25 including energy storage materials.26−29 Moreover, in the last few years, biopolymer dielectrics are considered to be one of the most potential candidates.30 A film made of cellulose nanofibers and carbon nanotubes was prepared and its dielectric constant, breakdown strength and energy density were 3198 at 1.0 kHz, 61.38 MV m−1 and 0.81 J cm−3, respectively.21 However, it is challenging to prepare cellulose-based dielectrics with high energy storage capabilities, and reports on such materials are scarce. For example, regenerated cellulose (RC)/polypyrrole composite films showed k of 20000 but high dielectric loss of 1900, restricting their practical application as energy storage materials.31 Therefore, development of


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cellulose materials with high Ue, Eb and low dielectric loss are promising for dielectric energy storage. Layered-structure nanocomposite from BNNS and RC are prepared in the present work through a facile, low cost, and ‘green’ pathway in NaOH/urea aqueous solution.20,22 A schematic of the structure of RC/BNNSs nanocomposites is shown in Figure 1. The dramatic enhancement in thermal conductive property mainly results from excellent in-plane thermal conductive property of BNNS together with forming 2D layered structure in the composite, in which a large contact area between BNNSs minimizes the interfacial thermal resistance when heat transfers along the RC−BN film. BNNSs are thermally conducting and electrically insulating. Their inplane thermal conductivity and Eb are as high as 2000 W m−1K−1 and 700 MV m−1,32,33 respectively. Furthermore, dielectric loss of BNNS is only 0.00034 at 1 MHz.33 Owing to the above inherent features of BNNS and the layer-structuration of the resulting nanocomposite, Eb and Ue reached as high as 370 MVm−1 and 4.1 J cm−3, respectively. Such enhancement in Eb was rarely seen in normal composites, where incorporation of fillers usually lowered Eb. Furthermore, the Ue is much better than that of any commercial dielectric polymer as well as any biomass-based materials reported in the literature. Low dielectric loss and outstanding discharge efficiency were also achieved. Besides, the nanocomposites displayed excellent thermal conductivity and good thermal stability.

EXPERIMENTAL

Materials. Hexagonal BN powder was purchased from Zibo jingyi Co. Ltd. (Zibo, China). Cellulose pulp with a weight average molecular weight of 8×104 g/mol was from Hubei


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Chemical Fiber Co. Ltd. (Xiangyang, China). Other chemical reagents were provided by Sinopharm chemical reagent Co. Ltd. Preparation o f RC−BN nanocomposite films. RC films were preparead according to a previous method by dissolution in aqueous NaOH/urea solution.34,35 A given amount of commercial BN powder was dispersed in above NaOH/urea solution, and then mechanical stirred 3 hours at room temperature. The resulting BN dispersion was further treated by sonication 1 hour. The exfoliated BN dispersion was cooled to –13 °C at which the cellulose could be best dissolved. Afterwards, cellulose was dispersed in it with 4 wt % cellulose concentration followed by stirring immediately 3 minutes for composite solution. It was spread on a glass plate after degassed by centrifugation, followed by immersion in different regeneration baths to allow regeneration (5% H2SO4 aqueous solution, 5 min; or acetone, 30 min ). The obtained hydrogel was washed in water to remove the inorganic molecules, and subjected to air drying or press-vacuum drying35 at 25 °C. Weight ratios between BN and cellulose were 0:100, 5:95, 10:90, 20:80 and 30:70, respectively, and the films were designated as RC, RC−BN5, RC−BN10, RC−BN20, and RC−BN30. Characterization. Atomic force microscope (AFM) was characterized on an atomic force microscope (Multimode 8, USA). Scanning electron microscopy (SEM) was characterized with a Hitachi S-4800 SEM microscope (Japan) with 5 kV accelerating voltage. JEOL JEM-2001F transmission electron microscope (TEM) with 200 kV accelerating voltage was used for TEM images. ESCALAB 250Xi Photoelectron spectrometer (Thermo Fisher, USA) was used for getting of X-ray photoelectron spectroscopy (XPS). Fourier transform infrared (FTIR) spectrometer (Nicolet6700, USA) was used for FTIR spectra from 400−4000 cm−1. The thermal stabilities of the composite films were measured via thermogravimetric analysis (TGA) in air


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(TA Instruments model STA449F3, Germany). The ultraviolet–visible (UV-Vis) spectrometer (Lambda 750 S, USA) was used to measure the optical transmittance. The tensile test was performed using a universal testing machine (SANS CMT6503, China). The tensile speed rate was set as 1 mm min-1 and the initial grip distance was 1 cm. Dielectric spectra were performed with an Agilent instrument (E4980A, USA). Before testing, both sides of the specimens were painted by silver electrodes. Ferroelectric tester (TRED MODEL 609B, USA) was used for dielectric breakdown strength and polarization–electric field (P–E) measurements. The energystorage performances were derived from the P–E results. Thermal conductivities were measured by thermal constant analyzers (Hot Disk TPS2500S, Sweden).

RESULTS AND DISCUSSION

SEM, AFM and TEM were used to research the morphology of the raw BN powders and exfoliated BN nanosheets in the NaOH/urea aqueous solution. As shown in Figure S1, the thickness of the raw BN powder was more than several hundred nanometers, exhibiting quantity of aggregation of BNNS in the powder. After facile mechanical stirring and sonication, the results in Figures 2a and S2, S3, S4 demonstrate that the obtained BNNSs exhibited wafery and lamellar morphology with a typical lateral diameter of ∼600 nm and a thickness of ∼1.3 nm, which was provided by height profile of BNNSs in AFM images and distinguishable curled edges of BNNSs in TEM images. Furthermore, the electron diffraction pattern in Figure S3 displayed a special sixfold symmetry owing to the hexagonal crystalline structure of BNNS, indicating the purity and good hexagonal crystalline structure of the exfoliated BN nanosheets.30 Figure 2b, c, d and Figure S5 show the SEM images of cross sections of RC and RC−BN composite films, respectively. The BN platelets are uniformly dispersed in the RC matrix and


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arranged in parallel in the plane direction of the films, while the cellulose wrapped the 2D BN platelets together. From another perspective, the BN platelet were parallel to each other and penetrated the entire plane, which provide ordered scattering centers and tortuous paths for electrons during breakdown hence leading to enhanced breakdown strength.15 Such an effective architecture should also contribute to improvement of the thermal conductivitive property.36 To study the chemical composition and bonding situation of BNNS, XPS was carried out and the relevant N 1s core-level spectra of BNNS were shown in Figure S6. Besides the characteristic peak of N–B, obvious characteristic peak of N–H was also observed, indicating the existence of amino groups, probably formed on the BNNS edges during exfoliation by sonication in NaOH/urea solution.30 The presence of amino groups on BNNS are possible to form hydrogen bonding with hydroxyl groups of cellulose. Figure S7 shows the FTIR spectra of the RC and RC−BN composite films. The spectra of composites show combination of characteristic peaks of both BN and RC film, suggesting successful integration of BN into the RC matrix. From the spectra of RC and BN, the broad absorption bands in 3440 and 3444 cm−1 respectively could be readily identified, which are attributed to the stretching vibration of –OH and –NH2. In comparison, the same position peaks of RC−BN nanocomposite films show smaller wavenumbers than these of RC and BN. It’s expected that the absorption peak shift to low wavenumber may be caused by the strong hydrogen bonding formed between BNNS and cellulose molecules. This implies the excellent interfacial interaction in the RC−BN nanocomposites, resulting in well interfacial compatibility. The dielectric constant and dielectric loss of the RC−BN composites are shown in Figure 3a and b, respectively. Dielectric polarization is mainly determined by dipole alignment of main/segmental polymer chains or migration of ions within the material.31 For RC−BN


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composites, interfacial polarization also contributes significantly to the dielectric constant. The composites exhibit slightly decreased dielectric constant with increasing BN content, probably because of low dielectric constant of BN, about 4.2.33 However, RC−BN5 and RC−BN10 show a higher dielectric constant than RC at a high frequency, due probably to stronger interfacial polarization. Furthermore, BN improved frequency-dependent stability of dielectric constant. Meanwhile, the dielectric losses of the composites are much lower than RC. The significant decrease in dielectric loss may result from following two issues. Firstly, strong interaction between cellulose and BN restricts the dipole polarization. Secondly the inherent features of BN hinder the charge migration. Figure S9 summarizes the data of the dielectric constants and losses of dielectric polymers reported.37 It reveals that our RC−BN show better properties than common polymers. Eb of the nanocomposites is analyzed by Weibull distribution function: P(E) = 1−exp(−(E/Eb)β), where P(E) is cumulative probability of the electric failure, E is the experimental breakdown strength, Eb is the Weibull breakdown strength with a cumulative failure probability of 63.2%, and β is Weibull modulus associated with scatter of experimental data.38 As shown in Figure 3c, Eb increases significantly with increasing amount of BN platelets, from 98 MV m−1 of RC to 249 MV m−1 of the nanocomposite containing 10 wt% of BN. Such a significant improvement could first be attributed to the high Eb (700 MV m−1) and large aspect ratio of BNNS. More importantly, the layered structure of the BNNS increases the tortuous paths for electrons before breakdown.13 Meanwhile, the mobility of the cellulose molecules is restricted owing to strong interaction between BN and cellulose, leading to the constraint of charge transfer between cellulose molecules. Compared with RC, the values of β are higher for the nanocomposites, which indicate that RC−BN exhibits good comparability and contains little


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defect. However, a further increase in BN content from 10 to 30 wt% results in obvious defect, e.g. microvoids, thus leading to decrease of Eb from 249 to 217.4 MV m−1, and β from ∼22 to ∼15. To study the improvement in energy densities of RC−BN nanocomposite films, P–E loops of RC−BN films are obtained. As shown in Figure 4a, at 100 MV m−1, P slightly decreases with increasing BN loading. This trend coincides with the variation of the dielectric constant. The most striking feature is the very small areas of the P-E loops, which once again confirm the very small dielectric loss. Figure 4b shows the energy density of the RC and RC−BN nanocomposite films in different polarization–electric fields. RC−BN10 exhibits the highest energy density of ∼2.01 J cm−3 at 250 MV m−1, around 6 times of the pure RC film (0.35 J cm−3). This significant improvement in energy density is primarily resulted from high breakdown strength. Furthermore, Figure 4b shows the charge–discharge efficiency obtained based on P–E loops. η is affected by dielectric loss at high electric field, which results from conduction loss and hysteresis loss.39,40 Owing to the low dielectric loss of the RC−BN nanocomposites, the values of η of the nanocomposites are all higher than 90% when the electric field ranges between 50 and 250 MV m−1. RC−BN10 even possesses a remarkable η of 97% at 100 MV m−1. According to the previous work, the crystallinity and density of RC film could be significantly affected by the drying conditions as well as the regenerate baths for the cellulose.34, 35

The pyranose rings of cellulose molecules tend to be in parallel to the film surface, thus

probably also in parallel to the B-N ring, leading to a denser layered structure of RC-BN nanocomposite. Figure 5 shows a model of interaction between cellulose molecules (top) and BN nano-platelet (bottom) in the RC-BN nanocomposite. The denser layered structure is expected to be favourable for breakdown strength and energy density. For this reason, a RC−BN10


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composite film with a denser structure (D−RC−BN10) was prepared via regeneration in acetone followed by press-vacuum drying. The SEM images of its cross section and dielectric properties are shown in Figure S10 & S11, respectively. The P–E loops of D-RC−BN are shown in Figure S12. As shown in Figure 6a, high break down strength of 370 MV m−1 and energy density of 4.1 J cm−3 are achieved. For traditional organic-inorganic systems, the energy density of composites can only be 2 times that of the pure polymer, because of the limited Eb. The value of 4.1 J cm−3 obtained in this work is ∼11 times that of the pure RC (∼0.35 J cm−3), which is extremely rare for polymer nanocomposites dielectrics. In fact, to our best knowledge, it’s the highest energy density obtained from biopolymer dielectric composites until now.21,29 Figure 6b compares the energy density vs dielectric loss of D−RC−BN10 with other dielectric polymers. The D−RC−BN10 film possesses much higher energy density compared with 2 J cm−3 of the best commercially available dielectric polymer BOPP.37 RC−BN nanocomposites are thus expected to potentially replace traditional fossil-based polymer materials. Preparation of high-temperature dielectric polymer-based materials with high temperature stability of dielectric properties is in great demand owing to the increasing requirement of dielectric at extreme environment.14 Thus the high-temperature dielectric properties of our RC−BN nanocompoistes are studied. Figure 7a and b shows their dielectric constant and dielectric loss at different temperature. A frequency of 104 Hz is interesting for the common power conditioning. Before 150°C, RC shows moderate variation in dielectric constant (22.7%) and a large variation in dielectric loss (from 0.031−0.085). By contrast, the variation of dielectric constant and loss are much smaller for RC−BN10. The variation in dielectric constant is merely 4.4% and the dielecttric loss at 104 Hz increases from 0.017 to 0.039 only. Such a high thermal


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stability is comparable to that of polycarbonate (PC), poly(ether ether ketone) (PEEK), and polyimide (PI).16 As a material to be used at high temperature, good thermal conductivity (TC) is very important and low TC would adversely influence its performance and reliability. Figure 8 show that RC possessed TC of 0.035 W m−1 K−1. In comparison, a high TC of 2.97 W m−1 K−1 was obtained after incorporation of 30 wt% BN. This is about 84 times that of the RC film. Besides high in-plane TC of BNNS, such enhancement is also because 2D layered structure forms in the composite, in which large contact areas among BNNSs minimizes interfacial thermal resistance when heat transfers along the RC-BN film, as shown in Figure 1. This mechanism is also evidenced by previous reports.

36,41–43

TGA curves (Figure S8) further support the excellent

thermal stability of the nanocomposites. There was almost no weight loss of BN even at 800 °C (Figure S8), indicating excellent thermal stability. The weight loss of RC and the RC−BN composites around 100 °C is owing to the moisture in the films. The moisture content of the composite films decreased from 10 wt% to 3 wt% with increasing BN content from 0 to 10 wt%. The decrease in moisture content resulted from the incorporation of BN in cellulose molecules may also contribute to the improvement of breakdown strength and decrease in dielectric loss for composites. Furthermore, these composites exhibited a high maximum decomposition rate temperature of around 330 °C. Moreover, because of the addition of BN, the residues of the composites were more compared with RC. These results further confirmed good thermal stability of the RC−BN composites. Combined with the above mentioned high temperature dielectric properties, RC−BN nanocomposites are potential candidates in the field of high-temperature energy storage materials.


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In addition to excellent dielectric energy storage performance and good thermal conductivity, the flexible RC−BN composite films also show interesting optical properties and good mechanical strength. Figure 9a shows that the pattern and letters behind RC-BN5 are clearly visible. Figure 9b shows the influence of BN loading on the transmittance. Moreover, the RC−BN30 film with 30 wt% of BN still exhibits good flexibility (Figure 9c). RC-BN films with adjustable transmittance and good flexibility are suficient for transparent substrates as flexible electronics. Stress–strain curves of films are shown in Figure 9d. Elongations at break of the RC−BN5, RC−BN10 and RC−BN20 films are better compared with the neat RC film, probably resulting from tightly packed layered structure of the films. RC exhibited improved tensile strength from 86 to 105 MPa when it contains 5 wt% of BN. However, as the BN content further increases to 30 wt%, it decreases to 70 MPa, which may be caused by severe aggregation of BN nanoplatelets in the composite.

CONCLUSIONS

In summary, the RC−BN nanocomposite films with a 2D layered-structure prepared in this work display outstanding dielectric properties, thermal conductivity and mechanical properties. In particular, the RC-BN film containing 10 wt% of BN shows high electric strength of 370 MV m−1 and high energy density of 4.1 J cm−3. The latter is better than any other bio-polymer based composite ever reported in the literature and is also better than that of PVDF and BOPP. In addition, RC−BN nanocomposites show high thermal conductivity, good high-temperature dielectric properties, high flexibility, good mechanical properties and interesting optical properties. All these performances make them a potential candidate as biomass-based material for electric energy storage at high temperature.


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ASSOCIATED CONTENT

Supporting Information. Further details on analysis, including AFM image, FTIR spectra, SEM image, dielectric properties, polarization–electric field curves, and TGA curves are available free of charge. (PDF)

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Quanling Yang); *E-mail: [email protected] (Chuanxi Xiong)

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (NSFC) (51703177, 21704079), State Key Laboratory of Pulp and Paper Engineering of South China University of Technology (201765), and Fundamental Research Funds for the Central Universities (WUT: 2016IVA002).

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High temperature

RC

High dielectric constant layer

Page 22 of 29

Low temperature

BNNS

High breakdown strength layer

Thermal conduction

Figure 1. Schematic to show the layered structure of the RC−BN nanocomposite film to achieve high breakdown strength and thermal conduction.


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a

b

500 nm 500 nm

c

dd

500 nm

500 nm500 nm

Figure 2. a) AFM image of BN nanosheets. SEM cross section images of b) RC, c) RC−BN10, d) RC−BN30 after force loading for the tensile test (Red line in Fig. 2d represents the direction of BN nanosheets arranged in parallel).


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Figure 3. Frequency-dependent a) dielectric constant, b) dielectric loss of RC and RC−BN nanocomposites. c) Failure probability of dielectric breakdown deduced from the Weibull distribution for the RC-BN nanocomposites with different BN contents.


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Figure 4. a) Polarization–electric field (P–E) curves of the RC−BN films at 100 MV m−1. b) Discharge energy density and charge–discharge efficiency under an electric field for the RC−BN nanocomposites with different BN contents.


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Figure 5. A model of interaction between cellulose molecules (top) and BN nanoplate (bottom) in the composite, where the pyranose rings of cellulose molecules are parallel to the B-N ring of BN nano-platelet.

Figure 6. a) Energy density and charge-discharge efficiency of the D−RC−BN10 film prepared via regeneration in acetone followed by press-vacuum drying with various applied electric fields at 100 Hz. b) Energy density vs dielectric loss of D−RC−BN10 and other dielectric polymers.33


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Figure 7. a) Temperature (room temperature to 150 °C) dependence of the dielectric constant and the dielectric loss of RC−BN10 films measured from 104 to 106 Hz. b) Temperature dependence of the dielectric constant and dielectric loss of RC and RC−BN10 films measured at 104 Hz.

Figure 8. In-plane thermal conductivity of RC−BN composites with different BN contents.


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a

c

Figure 9. a) Appearance of a 35 µm thick RC−BN film with 5 wt% of BN. (Image credit: Trademark of Wuhan university of technology). b) Light transmittance of RC−BN composite films with different BN loadings. c) Flexibility of a 65 µm thick RC−BN film with 30 wt% of BN. d) Stress-strain curves of RC and RC−BN nanocomposite films.


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For Table of Contents Use Only

Regenerated cellulose (RC)/boron nitride nanosheet (BNNS) nanocomposites with high energy density and breakdown strength are promising in the fields of environment-friendly hightemperature dielectric energy storage devices.

Low temperature

High temperature

BNNS

RC

High dielectric constant layer

High breakdown strength layer

Thermal conduction


29

boron nitride nanosheet high

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