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Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles
Author’s Accepted Manuscript Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles Manwen Yao, Siyuan You, Yong Peng www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)32126-5 http://dx.doi.org/10.1016/j.ceramint.2016.11.128 CERI14216
To appear in: Ceramics International Received date: 24 October 2016 Revised date: 15 November 2016 Accepted date: 18 November 2016 Cite this article as: Manwen Yao, Siyuan You and Yong Peng, Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.11.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles Manwen Yao*, Siyuan You, Yong Peng Functional Materials Research Laboratory, School of Materials Science and Engineering, Tongji University, No.4800 Caoan Road, Shanghai 201804, China *
Corresponding author: Tel.: +86 21 65983130; fax: +86 21 659855179.
[email protected]
Abstract Ceramics-polymer
nanocomposites
consisting
of
core-shell
structured
BaTiO3@Al2O3 (BT@Al2O3) nanoparticles as the filler and poly(vinylidene fluoride) (PVDF) as the polymer matrix were fabricated by solution casting. At the same volume fraction, the BT@Al2O3/PVDF nanocomposites, with larger dielectric constant and higher energy density, outperformed the BT/PVDF nanocomposites. The 2.5 vol.% BT@Al2O3/PVDF nanocomposites at 360 MV/m had a double more energy density than pure PVDF at 400 MV/m (6.19 vs. 2.30 J/cm3), and a remarkably 42% lower remnant polarization than the 2.5 vol.% BT/PVDF nanocomposites (0.99 vs. 1.69 μC/cm2 at 300 MV/m). Such significant enhancement was closely related to the surface modification by Al2O3, which improved the insulation of BT nanoparticles and reduced the contrast of dielectric constant between the filler and the PVDF matrix. Keywords: BaTiO3 nanoparticles; poly(vinylidene fluoride); nanocomposites; energy density
1. Introduction Ceramics-polymer nanocomposites have attracted wide attention in the past decade due to their potential for high-energy storage applications [1-5]. These nanocomposites can be broadly applied in electrical and electronic fields (e.g. embedded capacitors, high charge-storage capacitors, and memory devices) owing to the combination of large dielectric constant from the ceramic fillers and high
breakdown strength and mechanical flexibility from the polymer matrix [6-8]. The energy storage density of dielectric materials is defined by the integral U=∫EdP (where E is the applied electric field and P is the polarization) implies that the high energy storage density strongly depends on a large dielectric constant and high breakdown strength. At present, the main focus is to enhance the dielectric constant of nanocomposites by incorporating large-dielectric-constant conductive fillers or ceramic fillers. As reported, the 20 vol.% Ag nanowires embedded into a PVDF matrix improved the dielectric constant at 100 Hz to 800, and remained at 379 up to 1000 Hz [9]. As the load of BaTiO3 particles as a dopant into a polyimide matrix rose from 0 to 67.5 vol.%, the dielectric constant of the films increased from 3.53 to 46.50 [10]. The filler-matrix interface is critical in determining the energy storage density of nanocomposites [11-13]. On one hand, the incorporation of conductive or ceramic fillers results in the agglomeration of fillers, air voids, inorganic-organic interfaces and other defects, increasing the leakage current and decreasing the breakdown strength of the nanocomposites [14]. On the other hand, the contrast of dielectric constants between fillers and matrix produces an inhomogeneous local electric field, which further declines the breakdown strength [15]. Therefore, to synthesize homogeneous nanocomposites, some researchers aim to reduce the negative influences due to the incorporation of ceramic fillers. For instance, Ba0.3Sr0.7TiO3 nanofibers prepared via electrospinning were modified by dopamine and evenly dispersed
in
the
poly(vinylidene
fluoride-trifluoroethylene)
matrix
[16].
SiO2-layer-coated BaTiO3 nanoparticles were homogeneously distributed in PVDF, and the energy density was increased to 6.28 J/cm3 by a load of 2 vol.% SiO2-layer-coated BaTiO3 nanoparticles [17]. Here we report an approach to synthesize core-shell structured BT@Al2O3 through precipitation. Homogeneous nanocomposites were prepared starting from core-shell structured BT@Al2O3 nanoparticles as the filler and PVDF as the matrix. Dielectric responses and energy storage properties of the nanocomposites were studied.
2. Experimental Substances used here included BaTiO3 nanoparticles (diameter 100 nm, Guoteng Co. Ltd, China), PVDF (3F New Materials Co. Ltd, China), ammonia, aluminum sulfate (Al2(SO4)3), N,N-dimethylformamide (DMF), and polyvinylpyrrolidone (PVP) (Sinopharm Chemical Reagent Co. Ltd, China). The Al2O3 layer was synthesized through the precipitation reaction of Al2(SO4)3 on the surface of BT nanoparticles. The BT nanoparticles were dispersed in an aqueous PVP solution and then ultrasonicated for 1h. Then the solution was stirred at room temperature for 24 h and centrifuged at 4000 rpm for 20 min. The sediment was redispersed in a Al2(SO4)3 aqueous solution, which was slowly dropped with a 1M dilution ammonia. The new mixture was stirred for 24 h and centrifuged at 4000 rpm for 20 min. The powders were collected, washed with distilled water and ethanol, and sintered at 600 C for 2 h to form Al2O3-modified BT (BT@Al2O3) nanoparticles. PVDF powder was dissolved in DMF first and then the BT@Al2O3 nanoparticles were dispersed in the solution by ultrasonication for 1 h to form a stable suspension. The volume fraction of BT@Al2O3 nanoparticles in the composites varied from 0 to 10 vol.%. The suspensions were then stirred at 50 C for 8 h and cast onto indium tin oxides (ITO) glass. The PVDF composite films as-prepared were cured at 110 C for 10 h, heated at 200 C for 10 min, and quenched in ice-water. The composite films were about 10 μm thick. The crystal structures of nanoparticles were characterized on an X-ray diffractometer (XRD) (D8 Advance, Bruker, Germany) using CuKα radiation. Their chemical state were verified on an X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi). Microstructures of the samples were observed on a field-emission scanning electron microscope (FESEM) (S-4700, HITACHI, Japan) and a high-resolution transmission electron microscope (HRTEM) (JEOL-2100F, Japan). Broadband frequency dielectric properties were measured by an E4980A LCR meter (Agilent, Palo Alto, CA, USA) from 100 Hz to 2 MHz with a voltage amplitude of 500 mV at room temperature. Polarization-electric field (P-E) loops and breakdown
strength were detected by a Premier II ferroelectric test system (Radiant Technologies, Inc.) in silicon oil to avoid electrical discharge. The composite films were square-shaped (1.2 × 1.2 cm2) and about 10 μm thick. Gold electrodes (diameter in 2 mm, 50 nm thick) were sputtered with a shadow mask on the surface of the composites. The bottom electrode was ITO glass.
3. Results and discussion Al2O3 layers were continuously coated onto the surfaces of BT nanoparticles, as evidenced by the HRTEM of the core-shell structure (Fig. 1a). The amorphous layers are about 5 nm thick. As showed in Fig. 1b, the XRD patterns of the BT@Al2O3 nanoparticles and untreated BT particles both show the pure tetragonal phase, without obvious change in the crystal structures. These results indicate that Al2O3 existed as an amorphous structure. The amorphous layers were further verified as Al2O3 layers by XPS. As shown in Fig. 2a, the XPS curve of BT@Al2O3 nanoparticles shows the peaks of Al2s and Al2p corresponding to Al2O3. Obviously, the BT@Al2O3 nanoparticles have a stronger oxygen-to-barium (O/Ba) signal ratio than the BT nanoparticles, which is ascribed to the existence of Al2O3 layers atop the BT@Al2O3 nanoparticles. The high-resolution XPS spectra of O1s of BT and BT@Al2O3 nanoparticles are presented in Fig. 2b and c, respectively. Clearly, the deconvolution of the O1s peak gives two components: O1 at 529.96 eV and O2 at 531.33 eV (Fig. 2b), which are nearly equal to the reported values of BT (529.5 and 531.5 eV) [18]. Similar results detected in BT@Al2O3 nanoparticles (O1 at 529.90 eV and O2 at 531.23 eV) are shown in Fig. 2c, which belong to O1s signals of BT. The deconvolution of the O1s peak also gives a peak around 531.41 eV (Fig. 2c) corresponding to O2- in the Al2O3 framework [19], which is accompanied with a shoulder around 532.71 eV. [20]. The surface and cross-section morphologies of the 10 vol.% BT/PVDF or BT@Al2O3/PVDF nanocomposites observed by FESEM are showed in Fig. 3. Clearly, the BT nanoparticles serious agglomerated due to the large difference in surface energy between the fillers and matrix (Fig. 3a and c). The nanoparticle-matrix
incompatibility also led to the formation of more pores or voids. The introduction of an Al2O3 layer onto the surface of BT nanoparticles improved the filler-matrix compatibility, giving rise to good dispersion of BT@Al2O3 nanoparticles in the PVDF matrix (Fig 3b and d). Figure 4 shows the dielectric constant and dielectric loss of BT@Al2O3/PVDF nanocomposites and BT/PVDF nanocomposites as a function of the volume fraction of the fillers. The dielectric constant of the nanocomposites increases steadily as the volume fraction of the fillers rises. The dielectric constants of BT@Al2O3/PVDF nanocomposites with a load of 0, 2.5, 5, 7.5 and 10 vol.% are 9.9, 10.56, 12.83, 15.12 and 16.27 at 1kHz respectively. The BT@Al2O3/PVDF nanocomposites have larger dielectric constant than the BT/PVDF nanocomposites at the same load fraction. This is because the Al2O3 layer gives rise to good dispersion of BT@Al2O3 nanoparticles in the PVDF matrix. In addition, the BT@Al2O3/PVDF nanocomposites exhibit lower dielectric loss than the BT/PVDF nanocomposites, because Al2O3 has moderate dielectric constant and alleviates the local electric field concentration [21]. Especially, the dielectric losses of the BT@Al2O3/PVDF remain lower than 0.025 at all volume fractions, which is beneficial for the breakdown strength and energy density. Figure 5 shows the breakdown strengths of BT@Al2O3/PVDF and BT/PVDF nanocomposites
with
different
volume
fractions
of
fillers.
Clearly,
the
BT@Al2O3/PVDF nanocomposites maintain relatively high breakdown strengths, which are higher than the BT/PVDF nanocomposites at the same volume fraction. The breakdown strength was improved because the surface-modified procedure by Al2O3 reduced the contrast of the dielectric constant between ceramic fillers and matrix. Owing to the large contrast of dielectric constant, the filler-matrix interface is susceptible to Maxwell-Wagner-Sillars (MWS) polarization and then local charge transfer. The charge aggregation in the interface could led to the formation of an inhomogeneous local electric field, which decreased the breakdown strength. The presence of Al2O3 layers atop BT nanoparticles narrowed down the contrast of dielectric constant between fillers and matrix, and further reduced the MWS interfacial polarization and the electric field concentration in the nanocomposites [16].
To calculate the energy density of the nanocomposites, we measured P-E loops from the PVDF-based BT@Al2O3 nanocomposites at 100 Hz and room temperature (Fig. 6). The incorporation of the BT@Al2O3 particles enhanced the Pmax, which was attributed to the large dielectric constant of BT particles. The maximum electric field was 360 MV/m found in the 2.5 vol.% BT@Al2O3/PVDF nanocomposites. From the P-E loops, the energy storage density of the nanocomposites can be calculated based on U=∫EdP. The incorporation of the BT@Al2O3 nanoparticles into the PVDF matrix improved the maximum polarization, which was beneficial to the energy
density
of
the
nanocomposites.
The
energy
storage
density
of
BT@Al2O3/PVDF nanocomposites as a function of electric field is presented in Fig. 7. The maximum energy density of 6.19 J/cm3 at 360 MV/m was found in the 2.5 vol.% BT@Al2O3/PVDF nanocomposites, which is double more than pure PVDF of 2.30 J/cm3 at 400 MV/m. The practical application of energy storage capacitors requires both a high energy storage density and high discharge efficiency (η), since energy loss induces heating and then detrimentally affects the performance and reliability of the capacitor. Figure 7 also gives the efficiency (discharge energy/charge energy) of the BT@Al2O3/PVDF nanocomposites as a function of the electric field. Clearly, the efficiency decreases with the increased applied electric field, which is highly related to the conduction loss [4]. As the volume fraction of the ceramic fillers increases, the capacitor efficiency decreases due to the larger hysteresis in the polarization. However, the efficiency of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites is greater than 70% at electric fields below 100 MV/m and still higher than 60% at 360 MV/m. The P-E loops of the 2.5 vol.% BT/PVDF nanocomposites and 2.5 vol.% BT@Al2O3/PVDF nanocomposites under a series of applied electric fields are shown in Fig. 8. The BT@Al2O3/PVDF nanocomposites exhibit narrower P-E loops, lower maximum polarization and remarkably lower remnant polarization in comparison with the BT/PVDF nanocomposites. The maximum polarization of BT@Al2O3/PVDF nanocomposites is slightly smaller than the BT/PVDF nanocomposites under the same electric field. For instance, the maximum polarization of 2.5 vol.% BT@Al2O3/PVDF
nanocomposite is less than of the 2.5 vol.% BT/PVDF (4.32 vs. 4.51 μC/cm2) under the electric field of 300 MV/m. The remnant polarization at 300 MV/m is reduced remarkably by ~ 42% from 1.69 to 0.99 μC/cm2, which should be attributed to the reduction of the MWS interfacial polarization and the space charge polarization [22,23]. As the MWS interfacial polarization is strongly depending on the contrast of dielectric constant between the filler and matrix [24], Al2O3 which acts as a buffer layer between BT nanoparticles and PVDF matrix would reduce the concentration and charge carrier mobility normally associated with the layer, which reduces the MWS interfacial polarization. The Al2O3 layer also reduces the space charge polarization by enhancing the insulation and reducing the number of residual ions atop the BT nanoparticles. As the reduction of the two polarization types, the Al2O3 layer greatly reduces the remnant polarization of the nanocomposites. Figure 9 gives the energy discharge and energy loss of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites and BT/PVDF nanocomposites under a series of applied electric fields. The energy discharge is strongly depending on the maximum polarization and remnant polarization. The energy discharge of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites at 360 MV/m is about 98% higher than the 2.5 vol.% BT/PVDF nanocomposites at 300 MV/m (6.19 vs. 3.13 J/cm3). The energy losses of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites and 2.5 vol.% BT/PVDF nanocomposites at 250 MV/m are 1.59 and 2.43 J/cm3, respectively. These results indicate that the BT@Al2O3/PVDF nanocomposites exhibit much higher energy discharge and much lower energy loss than the BT/PVDF nanocomposites. This phenomenon should be attributed to the surface modification by Al2O3, which enhances the insulation of the BT nanoparticles and reduces the contrast of dielectric constant between ceramic fillers and polymer matrix.
4. Conclusions Core-shell structured BT@Al2O3 nanoparticles were prepared by the precipitation
method,
and
then
combined
with
PVDF
to
synthesize
high-energy-density nanocomposites through a solution casting method. TEM and
XPS reveal that Al2O3 was continuously coated atop the BT nanoparticles with an average thickness of 5 nm. The BT@Al2O3/PVDF nanocomposites exhibited much higher energy discharge and much lower energy loss than the BT/PVDF nanocomposites, which was because the Al2O3 layer enhanced the insulation of nanoparticles and reduced the contrast of dielectric constants between fillers and matrix. Thus, BT@Al2O3 core-shell structured nanoparticles as a dielectric filler are an effective solution improving the dielectric properties and also a promising candidate for application in energy-storage capacitors in the future.
Acknowledgment This work was supported by the Ministry of Science and Technology of China through 973-project (Grant no.2015CB654601). International Science & Technology Cooperation Program of China (Grant no. 2013DFR50470) and National Natural Science Foundation of China (Grant no.51272177).
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Fig. 1 (a) HRTEM morphology of the core-shell structured BT@Al2O3 nanoparticles and (b) XRD patterns of BT and BT@Al2O3 nanoparticles.
Fig. 2 (a) XPS spectra of BT nanoparticles and BT@Al2O3 nanoparticles; high-resolution XPS spectra of O1s for (b) BT nanoparticles and (c) BT@Al2O3 nanoparticles.
Fig. 3 Surface SEM morphologies of nanocomposite filled with 10 vol.% of (a) BT and (b) BT@Al2O3, and cross-section morphologies of nanocomposite filled with 10 vol.% of (c) BT and (d) BT@Al2O3.
Fig. 4 Dielectric constant and dielectric loss of the nanocomposites as a function of the volume fraction of ceramic fillers.
Fig. 5 Breakdown strength of the nanocomposites filled with different volume fraction of ceramic fillers.
Fig. 6 P-E loops of the nanocomposites filled with different volume fraction of BT@Al2O3.
Fig. 7 Energy storage density and efficiency of the nanocomposites as a function of the volume fraction of BT@Al2O3.
Fig. 8 P-E loops of 2.5 vol.% BT/PVDF and 2.5 vol.% BT@Al2O3/PVDF under different applied electric fields.
Fig. 9 Energy discharged and energy loss of 2.5 vol.% BT/PVDF and 2.5 vol.% BT@Al2O3/PVDF as a function of the applied electric field.
PII: DOI: Reference:
S0272-8842(16)32126-5 http://dx.doi.org/10.1016/j.ceramint.2016.11.128 CERI14216
To appear in: Ceramics International Received date: 24 October 2016 Revised date: 15 November 2016 Accepted date: 18 November 2016 Cite this article as: Manwen Yao, Siyuan You and Yong Peng, Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.11.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles Manwen Yao*, Siyuan You, Yong Peng Functional Materials Research Laboratory, School of Materials Science and Engineering, Tongji University, No.4800 Caoan Road, Shanghai 201804, China *
Corresponding author: Tel.: +86 21 65983130; fax: +86 21 659855179.
[email protected]
Abstract Ceramics-polymer
nanocomposites
consisting
of
core-shell
structured
BaTiO3@Al2O3 (BT@Al2O3) nanoparticles as the filler and poly(vinylidene fluoride) (PVDF) as the polymer matrix were fabricated by solution casting. At the same volume fraction, the BT@Al2O3/PVDF nanocomposites, with larger dielectric constant and higher energy density, outperformed the BT/PVDF nanocomposites. The 2.5 vol.% BT@Al2O3/PVDF nanocomposites at 360 MV/m had a double more energy density than pure PVDF at 400 MV/m (6.19 vs. 2.30 J/cm3), and a remarkably 42% lower remnant polarization than the 2.5 vol.% BT/PVDF nanocomposites (0.99 vs. 1.69 μC/cm2 at 300 MV/m). Such significant enhancement was closely related to the surface modification by Al2O3, which improved the insulation of BT nanoparticles and reduced the contrast of dielectric constant between the filler and the PVDF matrix. Keywords: BaTiO3 nanoparticles; poly(vinylidene fluoride); nanocomposites; energy density
1. Introduction Ceramics-polymer nanocomposites have attracted wide attention in the past decade due to their potential for high-energy storage applications [1-5]. These nanocomposites can be broadly applied in electrical and electronic fields (e.g. embedded capacitors, high charge-storage capacitors, and memory devices) owing to the combination of large dielectric constant from the ceramic fillers and high
breakdown strength and mechanical flexibility from the polymer matrix [6-8]. The energy storage density of dielectric materials is defined by the integral U=∫EdP (where E is the applied electric field and P is the polarization) implies that the high energy storage density strongly depends on a large dielectric constant and high breakdown strength. At present, the main focus is to enhance the dielectric constant of nanocomposites by incorporating large-dielectric-constant conductive fillers or ceramic fillers. As reported, the 20 vol.% Ag nanowires embedded into a PVDF matrix improved the dielectric constant at 100 Hz to 800, and remained at 379 up to 1000 Hz [9]. As the load of BaTiO3 particles as a dopant into a polyimide matrix rose from 0 to 67.5 vol.%, the dielectric constant of the films increased from 3.53 to 46.50 [10]. The filler-matrix interface is critical in determining the energy storage density of nanocomposites [11-13]. On one hand, the incorporation of conductive or ceramic fillers results in the agglomeration of fillers, air voids, inorganic-organic interfaces and other defects, increasing the leakage current and decreasing the breakdown strength of the nanocomposites [14]. On the other hand, the contrast of dielectric constants between fillers and matrix produces an inhomogeneous local electric field, which further declines the breakdown strength [15]. Therefore, to synthesize homogeneous nanocomposites, some researchers aim to reduce the negative influences due to the incorporation of ceramic fillers. For instance, Ba0.3Sr0.7TiO3 nanofibers prepared via electrospinning were modified by dopamine and evenly dispersed
in
the
poly(vinylidene
fluoride-trifluoroethylene)
matrix
[16].
SiO2-layer-coated BaTiO3 nanoparticles were homogeneously distributed in PVDF, and the energy density was increased to 6.28 J/cm3 by a load of 2 vol.% SiO2-layer-coated BaTiO3 nanoparticles [17]. Here we report an approach to synthesize core-shell structured BT@Al2O3 through precipitation. Homogeneous nanocomposites were prepared starting from core-shell structured BT@Al2O3 nanoparticles as the filler and PVDF as the matrix. Dielectric responses and energy storage properties of the nanocomposites were studied.
2. Experimental Substances used here included BaTiO3 nanoparticles (diameter 100 nm, Guoteng Co. Ltd, China), PVDF (3F New Materials Co. Ltd, China), ammonia, aluminum sulfate (Al2(SO4)3), N,N-dimethylformamide (DMF), and polyvinylpyrrolidone (PVP) (Sinopharm Chemical Reagent Co. Ltd, China). The Al2O3 layer was synthesized through the precipitation reaction of Al2(SO4)3 on the surface of BT nanoparticles. The BT nanoparticles were dispersed in an aqueous PVP solution and then ultrasonicated for 1h. Then the solution was stirred at room temperature for 24 h and centrifuged at 4000 rpm for 20 min. The sediment was redispersed in a Al2(SO4)3 aqueous solution, which was slowly dropped with a 1M dilution ammonia. The new mixture was stirred for 24 h and centrifuged at 4000 rpm for 20 min. The powders were collected, washed with distilled water and ethanol, and sintered at 600 C for 2 h to form Al2O3-modified BT (BT@Al2O3) nanoparticles. PVDF powder was dissolved in DMF first and then the BT@Al2O3 nanoparticles were dispersed in the solution by ultrasonication for 1 h to form a stable suspension. The volume fraction of BT@Al2O3 nanoparticles in the composites varied from 0 to 10 vol.%. The suspensions were then stirred at 50 C for 8 h and cast onto indium tin oxides (ITO) glass. The PVDF composite films as-prepared were cured at 110 C for 10 h, heated at 200 C for 10 min, and quenched in ice-water. The composite films were about 10 μm thick. The crystal structures of nanoparticles were characterized on an X-ray diffractometer (XRD) (D8 Advance, Bruker, Germany) using CuKα radiation. Their chemical state were verified on an X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi). Microstructures of the samples were observed on a field-emission scanning electron microscope (FESEM) (S-4700, HITACHI, Japan) and a high-resolution transmission electron microscope (HRTEM) (JEOL-2100F, Japan). Broadband frequency dielectric properties were measured by an E4980A LCR meter (Agilent, Palo Alto, CA, USA) from 100 Hz to 2 MHz with a voltage amplitude of 500 mV at room temperature. Polarization-electric field (P-E) loops and breakdown
strength were detected by a Premier II ferroelectric test system (Radiant Technologies, Inc.) in silicon oil to avoid electrical discharge. The composite films were square-shaped (1.2 × 1.2 cm2) and about 10 μm thick. Gold electrodes (diameter in 2 mm, 50 nm thick) were sputtered with a shadow mask on the surface of the composites. The bottom electrode was ITO glass.
3. Results and discussion Al2O3 layers were continuously coated onto the surfaces of BT nanoparticles, as evidenced by the HRTEM of the core-shell structure (Fig. 1a). The amorphous layers are about 5 nm thick. As showed in Fig. 1b, the XRD patterns of the BT@Al2O3 nanoparticles and untreated BT particles both show the pure tetragonal phase, without obvious change in the crystal structures. These results indicate that Al2O3 existed as an amorphous structure. The amorphous layers were further verified as Al2O3 layers by XPS. As shown in Fig. 2a, the XPS curve of BT@Al2O3 nanoparticles shows the peaks of Al2s and Al2p corresponding to Al2O3. Obviously, the BT@Al2O3 nanoparticles have a stronger oxygen-to-barium (O/Ba) signal ratio than the BT nanoparticles, which is ascribed to the existence of Al2O3 layers atop the BT@Al2O3 nanoparticles. The high-resolution XPS spectra of O1s of BT and BT@Al2O3 nanoparticles are presented in Fig. 2b and c, respectively. Clearly, the deconvolution of the O1s peak gives two components: O1 at 529.96 eV and O2 at 531.33 eV (Fig. 2b), which are nearly equal to the reported values of BT (529.5 and 531.5 eV) [18]. Similar results detected in BT@Al2O3 nanoparticles (O1 at 529.90 eV and O2 at 531.23 eV) are shown in Fig. 2c, which belong to O1s signals of BT. The deconvolution of the O1s peak also gives a peak around 531.41 eV (Fig. 2c) corresponding to O2- in the Al2O3 framework [19], which is accompanied with a shoulder around 532.71 eV. [20]. The surface and cross-section morphologies of the 10 vol.% BT/PVDF or BT@Al2O3/PVDF nanocomposites observed by FESEM are showed in Fig. 3. Clearly, the BT nanoparticles serious agglomerated due to the large difference in surface energy between the fillers and matrix (Fig. 3a and c). The nanoparticle-matrix
incompatibility also led to the formation of more pores or voids. The introduction of an Al2O3 layer onto the surface of BT nanoparticles improved the filler-matrix compatibility, giving rise to good dispersion of BT@Al2O3 nanoparticles in the PVDF matrix (Fig 3b and d). Figure 4 shows the dielectric constant and dielectric loss of BT@Al2O3/PVDF nanocomposites and BT/PVDF nanocomposites as a function of the volume fraction of the fillers. The dielectric constant of the nanocomposites increases steadily as the volume fraction of the fillers rises. The dielectric constants of BT@Al2O3/PVDF nanocomposites with a load of 0, 2.5, 5, 7.5 and 10 vol.% are 9.9, 10.56, 12.83, 15.12 and 16.27 at 1kHz respectively. The BT@Al2O3/PVDF nanocomposites have larger dielectric constant than the BT/PVDF nanocomposites at the same load fraction. This is because the Al2O3 layer gives rise to good dispersion of BT@Al2O3 nanoparticles in the PVDF matrix. In addition, the BT@Al2O3/PVDF nanocomposites exhibit lower dielectric loss than the BT/PVDF nanocomposites, because Al2O3 has moderate dielectric constant and alleviates the local electric field concentration [21]. Especially, the dielectric losses of the BT@Al2O3/PVDF remain lower than 0.025 at all volume fractions, which is beneficial for the breakdown strength and energy density. Figure 5 shows the breakdown strengths of BT@Al2O3/PVDF and BT/PVDF nanocomposites
with
different
volume
fractions
of
fillers.
Clearly,
the
BT@Al2O3/PVDF nanocomposites maintain relatively high breakdown strengths, which are higher than the BT/PVDF nanocomposites at the same volume fraction. The breakdown strength was improved because the surface-modified procedure by Al2O3 reduced the contrast of the dielectric constant between ceramic fillers and matrix. Owing to the large contrast of dielectric constant, the filler-matrix interface is susceptible to Maxwell-Wagner-Sillars (MWS) polarization and then local charge transfer. The charge aggregation in the interface could led to the formation of an inhomogeneous local electric field, which decreased the breakdown strength. The presence of Al2O3 layers atop BT nanoparticles narrowed down the contrast of dielectric constant between fillers and matrix, and further reduced the MWS interfacial polarization and the electric field concentration in the nanocomposites [16].
To calculate the energy density of the nanocomposites, we measured P-E loops from the PVDF-based BT@Al2O3 nanocomposites at 100 Hz and room temperature (Fig. 6). The incorporation of the BT@Al2O3 particles enhanced the Pmax, which was attributed to the large dielectric constant of BT particles. The maximum electric field was 360 MV/m found in the 2.5 vol.% BT@Al2O3/PVDF nanocomposites. From the P-E loops, the energy storage density of the nanocomposites can be calculated based on U=∫EdP. The incorporation of the BT@Al2O3 nanoparticles into the PVDF matrix improved the maximum polarization, which was beneficial to the energy
density
of
the
nanocomposites.
The
energy
storage
density
of
BT@Al2O3/PVDF nanocomposites as a function of electric field is presented in Fig. 7. The maximum energy density of 6.19 J/cm3 at 360 MV/m was found in the 2.5 vol.% BT@Al2O3/PVDF nanocomposites, which is double more than pure PVDF of 2.30 J/cm3 at 400 MV/m. The practical application of energy storage capacitors requires both a high energy storage density and high discharge efficiency (η), since energy loss induces heating and then detrimentally affects the performance and reliability of the capacitor. Figure 7 also gives the efficiency (discharge energy/charge energy) of the BT@Al2O3/PVDF nanocomposites as a function of the electric field. Clearly, the efficiency decreases with the increased applied electric field, which is highly related to the conduction loss [4]. As the volume fraction of the ceramic fillers increases, the capacitor efficiency decreases due to the larger hysteresis in the polarization. However, the efficiency of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites is greater than 70% at electric fields below 100 MV/m and still higher than 60% at 360 MV/m. The P-E loops of the 2.5 vol.% BT/PVDF nanocomposites and 2.5 vol.% BT@Al2O3/PVDF nanocomposites under a series of applied electric fields are shown in Fig. 8. The BT@Al2O3/PVDF nanocomposites exhibit narrower P-E loops, lower maximum polarization and remarkably lower remnant polarization in comparison with the BT/PVDF nanocomposites. The maximum polarization of BT@Al2O3/PVDF nanocomposites is slightly smaller than the BT/PVDF nanocomposites under the same electric field. For instance, the maximum polarization of 2.5 vol.% BT@Al2O3/PVDF
nanocomposite is less than of the 2.5 vol.% BT/PVDF (4.32 vs. 4.51 μC/cm2) under the electric field of 300 MV/m. The remnant polarization at 300 MV/m is reduced remarkably by ~ 42% from 1.69 to 0.99 μC/cm2, which should be attributed to the reduction of the MWS interfacial polarization and the space charge polarization [22,23]. As the MWS interfacial polarization is strongly depending on the contrast of dielectric constant between the filler and matrix [24], Al2O3 which acts as a buffer layer between BT nanoparticles and PVDF matrix would reduce the concentration and charge carrier mobility normally associated with the layer, which reduces the MWS interfacial polarization. The Al2O3 layer also reduces the space charge polarization by enhancing the insulation and reducing the number of residual ions atop the BT nanoparticles. As the reduction of the two polarization types, the Al2O3 layer greatly reduces the remnant polarization of the nanocomposites. Figure 9 gives the energy discharge and energy loss of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites and BT/PVDF nanocomposites under a series of applied electric fields. The energy discharge is strongly depending on the maximum polarization and remnant polarization. The energy discharge of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites at 360 MV/m is about 98% higher than the 2.5 vol.% BT/PVDF nanocomposites at 300 MV/m (6.19 vs. 3.13 J/cm3). The energy losses of the 2.5 vol.% BT@Al2O3/PVDF nanocomposites and 2.5 vol.% BT/PVDF nanocomposites at 250 MV/m are 1.59 and 2.43 J/cm3, respectively. These results indicate that the BT@Al2O3/PVDF nanocomposites exhibit much higher energy discharge and much lower energy loss than the BT/PVDF nanocomposites. This phenomenon should be attributed to the surface modification by Al2O3, which enhances the insulation of the BT nanoparticles and reduces the contrast of dielectric constant between ceramic fillers and polymer matrix.
4. Conclusions Core-shell structured BT@Al2O3 nanoparticles were prepared by the precipitation
method,
and
then
combined
with
PVDF
to
synthesize
high-energy-density nanocomposites through a solution casting method. TEM and
XPS reveal that Al2O3 was continuously coated atop the BT nanoparticles with an average thickness of 5 nm. The BT@Al2O3/PVDF nanocomposites exhibited much higher energy discharge and much lower energy loss than the BT/PVDF nanocomposites, which was because the Al2O3 layer enhanced the insulation of nanoparticles and reduced the contrast of dielectric constants between fillers and matrix. Thus, BT@Al2O3 core-shell structured nanoparticles as a dielectric filler are an effective solution improving the dielectric properties and also a promising candidate for application in energy-storage capacitors in the future.
Acknowledgment This work was supported by the Ministry of Science and Technology of China through 973-project (Grant no.2015CB654601). International Science & Technology Cooperation Program of China (Grant no. 2013DFR50470) and National Natural Science Foundation of China (Grant no.51272177).
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Fig. 1 (a) HRTEM morphology of the core-shell structured BT@Al2O3 nanoparticles and (b) XRD patterns of BT and BT@Al2O3 nanoparticles.
Fig. 2 (a) XPS spectra of BT nanoparticles and BT@Al2O3 nanoparticles; high-resolution XPS spectra of O1s for (b) BT nanoparticles and (c) BT@Al2O3 nanoparticles.
Fig. 3 Surface SEM morphologies of nanocomposite filled with 10 vol.% of (a) BT and (b) BT@Al2O3, and cross-section morphologies of nanocomposite filled with 10 vol.% of (c) BT and (d) BT@Al2O3.
Fig. 4 Dielectric constant and dielectric loss of the nanocomposites as a function of the volume fraction of ceramic fillers.
Fig. 5 Breakdown strength of the nanocomposites filled with different volume fraction of ceramic fillers.
Fig. 6 P-E loops of the nanocomposites filled with different volume fraction of BT@Al2O3.
Fig. 7 Energy storage density and efficiency of the nanocomposites as a function of the volume fraction of BT@Al2O3.
Fig. 8 P-E loops of 2.5 vol.% BT/PVDF and 2.5 vol.% BT@Al2O3/PVDF under different applied electric fields.
Fig. 9 Energy discharged and energy loss of 2.5 vol.% BT/PVDF and 2.5 vol.% BT@Al2O3/PVDF as a function of the applied electric field.