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High energy density in P(VDF-HFP) nanocomposite with paraffin engineered BaTiO3 nanoparticles
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High energy density in P(VDF-HFP) nanocomposite with paraffin engineered BaTiO3 nanoparticles Dou Zhang a , Zhong Wu a , Xue-fan Zhou a , An-qi Wei a , Chao Chen a , Hang Luo a,b,∗ a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China
a r t i c l e
i n f o
Article history: Received 7 December 2016 Received in revised form 23 February 2017 Accepted 14 March 2017 Available online xxx Keywords: Paraffin Interface modification Energy density Dielectric nanocomposites BaTiO3
a b s t r a c t Dielectric materials with high energy storage density are attracting scientific and commercial interest due to the prospect of application in advanced electronic devices and electric power systems. Ceramic/polymer composite is one of the most promising materials to obtain high energy density because it combines high permittivity from ceramic and high breakdown strength as well as flexibility from polymer matrix. However, the homogeneous dispersion and compatibility of nanoparticles in polymers matrix are still of great challenges due to the different surface energy between inorganic fillers and organic polymer matrix. Interfaces modification for ceramic fillers is an effective way to ease the mentioned problems. Herein, we proposed a low-cost and environmentally friendly route to prepare a core–shell structure by paraffin engineer the BaTiO3 nanoparticles. The modified BaTiO3 nanoparticles exhibited homogeneous dispersion in the ferroelectric polymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix and strong interfacial adhesion with the polymer matrix. The nanocomposites prepared in this work showed notably enhanced energy storage density. The maximum storage energy density of the nanocomposite with 50 vol% BaTiO3 increased to 21.1 J/cm3 at 150 kV/mm. These results indicate that paraffin is an effective modifier to prepare high energy storage density capacitor. © 2017 Elsevier B.V. All rights reserved.
1. Introduction High energy density capacitors are widely applied in electronic devices with pulsed power such as lasers and rail guns [1–8]. As energy density of linear dielectric materials is defined by Eq. (1): Ue =
1 ε0 εE 2 2
(1)
where ε0 is the permittivity of vacuum, ε is the relative permittivity of the materials, and E is the applied electric field [9,10]. Thus, the maximum energy density depends on the permittivity and breakdown strength of the material. It is not surprising that numerous efforts have been made in the past few years to combine high breakdown strength from the polymers with high permittivity from ceramic nanoparticles [11–13]. In general, to achieve ceramic/polymer nanocomposites with high energy densities, the fillers should possess high permittivity [11]. Thus, ferroelectric ceramics with high permittivity, such as BaTiO3 [14], Pb(Zr, Ti)O3 (PZT) [15], Ba0.2 Sr0.8 TiO3 [16], TiO2 [17],
∗ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China. E-mail address: [email protected] (H. Luo).
Pb(Mg1/3 Nb2/3 )O3 –PBaTiO3 iO3 (PMN–PT) [18], etc. were usually selected to introduce to polymer matrix. However, due to the different surface energy between inorganic fillers and polymer matrix, the ceramic fillers were easily accumulated in the nanocomposites and hardly tightly adhered with the polymer matrix, which greatly limited the increase of energy density of the nanocomposites. As proved, engineering the interface was one of the most potential strategies to improve the mentioned problems. In this regard, fabrication of core–shell structured ceramics by various modifiers has attracted lots of attention. Yang etc. reported a core–shell structured BaTiO3 @polymer nanodielectrics through reversible addition–fragmentation chain transfer (RAFT) polymerization and Thiol–Ene click reaction method [19]. Xie etc. prepared a new core–shell structure of barium titanate hybrid nanofillers (BaTiO3 HBP) via grafting hyperbranched aromatic polyamide to the surface of BaTiO3 [20]. As results, fillers with the controllable core–shell structures and the composites with regulated dielectric constant as well as superior low dielectric loss were obtained. However, as we know that in-suit surface-initiated polymerization methods are always accompanied by complex processes and harsh conditions, which are difficult to realize in the lab, thus it is needless to say for large-scale practical application [21,22]. Paraffin is an excellent insulating material with low cost, stable, nontoxic, non-corrosive and other advantages, which
http://dx.doi.org/10.1016/j.sna.2017.03.018 0924-4247/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: D. Zhang, et al., High energy density in P(VDF-HFP) nanocomposite with paraffin engineered BaTiO3 nanoparticles, Sens. Actuators A: Phys. (2017), http://dx.doi.org/10.1016/j.sna.2017.03.018
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can effectively prevent space charge from accumulating in the nanocomposites [23]. In addition, paraffin possesses low melting points, as we know that high-temperature processing is not favorable for the formation of core–shell structured nanoparticles [24]. In this regard, paraffin was selected as the modifier of BaTiO3 nanoparticles via facile method, and the modified fillers were then introduced into ferroelectric polymer poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) to prepare high energy density nanocomposites. Ferroelectric BaTiO3 ceramics were selected as the fillers owing to the high permittivity and relatively low dielectric loss [10,25–28], P(VDF-HFP) was employed as the polymer matrix due to higher permittivity than other polymers [14,29–31]. As results, the Paraffin engineered BaTiO3 nanoparticles exhibited homogeneous dispersion in P(VDF-HFP) matrix and strong interfacial adhesion with the matrix. The nanocomposites showed obviously enhanced permittivity and low dielectric loss compared with the pure P(VDF-HFP). The permittivity of the nanocomposite with 50 vol.% paraffin engineered BaTiO3 increased to 49 at 1 kHz, which was higher than those with untreated BaTiO3 . It was notably that the nanocomposites possessed super-high polarization due to the insulated paraffin shell on the surface of BaTiO3 nanoparticles, which can prevent the space charge from transmission. Therefore, a large stored energy density of 21.1 J/cm3 was achieved in the nanocomposite with 50 vol.% BaTiO3 @Paraffin, which is great larger than lots of reported results.
Fig. 1. Schematic illustration of the formation of BaTiO3 @Paraffin core–shell NPs, and BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites.
Fig. 2. The XRD patterns of untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles.
2. Experimental sections
BaTiO3 nanoparticles with the average size of approximate 100 nm and the purity of 99.9%, and Poly(vinylidene fluoride-cohexafluoropropylene) P(VDF-HFP) polymer were purchased from Aladdin Co., Shanghai, China. Paraffin and H2 O2 (30 wt.%) were purchased from Guoyao, China.
The square metal mask plates with a side length of 30 mm and circular holes were designed. The diameter of circular holes was 2 mm. The as-prepared nanocomposite films were sandwiched between two metal mask plates to sputter the gold electrode on both sides. The sputtering time was 10 min to ensure the electrodes were enough thickness. As a comparison, pure P(VDF-HFP) film and nanocomposites with untreated BaTiO3 nanoparticles were also prepared via the similar approach.
2.2. Functionalization of the BaTiO3 nanoparticles
2.4. Characterization
10 g of BaTiO3 nanoparticles were added into 100 mL 30 wt.% H2 O2 aqueous solution in a round-bottomed flask. The mixture was sonicated for 30 min and refluxed at 106 ◦ C for 6 h. The nanoparticles were recovered by centrifugation at 9000 rpm for 5 min. The BaTiO3 nanoparticles were washed with deionized water and then dried under vacuum at 80 ◦ C for 12 h. 8 g BaTiO3 nanoparticles treated by H2 O2 were dispersed into petroleum ether. The mixture was sonicated for 10 min and then 0.8 g paraffin was introduced into the solution at 40 ◦ C, followed by stirring for 2 h. Subsequently, the mixture was cooled to room temperature, and the nanoparticles were recovered by centrifugation at 1000 rpm for 5 min and then dried under vacuum at 40 ◦ C for 24 h.
The crystalline phases of the products were tested by Xray diffraction (XRD, D/max 2550, Japan) with Cu-K␣ radiation ˚ at room temperature. The size and morphology ( = 1.5406 A) of the nanoparticles and nanocomposites were observed using a scanning electron microscope (SEM, Nova NanoSEM230, USA). Transmission electron microscopy (HRTEM) images were taken with a Titan (G260-300) transmission electron microscope, using an accelerating voltage of 300 kV. Fourier-transform infrared (FTIR) spectroscopy was performed on Nicolet 6700 instrument over the range of 4000–450 cm−1 to determine the functionalization of the BaTiO3 nanoparticles. Thermogravimetric analysis (TGA, NETZSCH STA 449) was conducted at a heating rate of 10 ◦ C min−1 in a nitrogen flow (20 mL min−1 ). Frequency-dependent permittivity and dielectric loss were measured using an Agilent 4294A LCR meter with the frequency ranging from 1 kHz to 10 MHz. Electric displacement-electric field loops and leakage current were measured by a Precision Premier II ferroelectric polarization tester (Radiant, Inc.) at room temperature and 100 Hz.
2.1. Materials
2.3. Preparation of nanocomposites with BaTiO3 @Paraffin Firstly, 8 g P(VDF-HFP) were added into 92 g as-prepared component solvent (Vacetone :VDMF = 7:3) and the solution was stirred at 60 ◦ C until it became transparent. Then, the BaTiO3 @Paraffin particles were added into the P(VDF-HFP) solution to prepare the suspensions with 10, 20, 30, 40 and 50 vol.% BaTiO3 @Paraffin nanoparticles respectively. The suspensions were all stirred for another 24 h. Subsequently, the nanocomposite films were fabricated via tape-casting the as-stirred suspension on a glass substrate and dried at 80 ◦ C. The films were further hot-pressed at 200 ◦ C and 15 MPa. The schematic illustration of the formation of BaTiO3 @Paraffin core–shell NPs and BaTiO3 @Paraffin/P(VDF-HFP) nanocomposite was shown in Fig. 1.
3. Results and discussions 3.1. Characterization of the BaTiO3 @Paraffin and the nanocomposites The crystal structures of the untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles are characterized by XRD, respectively, which are shown in Fig. 2. As can be seen that the
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Fig. 3. (a) TEM images and (b) high-resolution TEM images of BaTiO3 @Paraffin nanoparticles.
nanoparticles showed sharp diffraction peaks at 22.8◦ , 31.4◦ , 38.7◦ , 45.0◦ , 52.4◦ and 55.9◦ , which were corresponded to the lattice plane of (100), (110), (111), (200), (210) and (211), respectively. It can be well matched with the PDF#05-0626 card and showed a typical tetragonal structure. The BaTiO3 @Paraffin nanoparticles showed the same strong absorption peaks as the pure BaTiO3 nanoparticles without any new diffraction peaks. TEM measurement was used to observe the morphology of BaTiO3 @Paraffin nanoparticles. Fig. 3 displays the TEM images in bright field and high-resolution TEM images of BaTiO3 @Paraffin nanoparticles respectively. The surface of the BaTiO3 nanoparticles was rough with clear irregularity. This kind of structure can enhance the adhesion between the fillers and polymer matrix. The diameter of the BaTiO3 nanoparticles was approximately 100 nm. In Fig. 3a, a resin layer can be observed on the surface of BaTiO3 nanoparticles, which was clearly proved by the high-resolution TEM images, as shown in Fig. 3b. The different lattice fringes in the two sides of the BaTiO3 @Paraffin can further prove the exit of the resin layer on the surfaces of the BaTiO3 nanoparticles. The SEM images shown in Fig. S1 (see Supporting Information) are also provided to investigate the morphology of untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles, which is agree with the results of TEM. To prove the exited resin on the surface of BT nanoparticles, the FT-IR spectra of BaTiO3 @Paraffinm nanoparticles, pure Paraffin and untreated BaTiO3 nanoparticles are provided in Fig. 4a–c, respectively. Compared with initial BaTiO3 , BaTiO3 @Paraffinm nanoparticles showed obvious new absorption bands. Among them, the strong absorption bands appearing at about 2918 and 2849 cm−1 were from typical C-H stretching vibration from alkyl. What’s more, the obvious absorption band at 1473 cm−1 was originated from the asymmetric in-plane rocking vibration of methyl, which was observed in the spectra of pure Paraffin. However, it was not detected in pure BaTiO3 nanoparticles. These results proved that Paraffin has successfully modified the surface of BaTiO3 nanoparticles. Ceramic nanoparticles possess excellent thermostability as they will not show significant weight loss above 1000 ◦ C ordinarily. However, the organic modifier coated on the surface of them or the organic functional groups grafted on them will yield weight loss at high temperatures. Thus, the weight loss can be used to characterize the content of organic on the surface of BaTiO3 nanoparticles. As shown in Fig. 5, the weight losses of untreated BaTiO3 nanoparticles, BaTiO3 @Paraffin nanoparticles and Paraffin at 650 ◦ C are 2.2%, 31.2% and 99.79%, respectively. The TGA curve of the BaTiO3 nanoparticles tended to be horizontal without obvious weight loss at 650 ◦ C. (the weight loss was mainly caused by water, hydroxyl, or other impurities). On the contrary, the residual mass of pure Paraffin was only 0.21% of the initial value at 400 ◦ C, meaning that the
Fig. 4. The FT-IR spectra of (a) BaTiO3 @Paraffinm nanoparticles, (b) pure Paraffin and (c) untreated BaTiO3 nanoparticles.
Fig. 5. The TGA curves of untreated BaTiO3 nanoparticles, BaTiO3 @Paraffin nanoparticles and Paraffin.
weight loss of BaTiO3 @Paraffin nanoparticles was just the content of the coated Paraffin (31.2%) [32]. The above results further indicated that Paraffin has been successfully coated on the surface of BaTiO3 nanoparticles.
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Tape casting and hot pressing were used to prepare the BaTiO3 @Paraffin/P(VDF-HFP) composites. It is needed to sputter gold electrodes on both sides of BaTiO3 @Paraffin/P(VDF-HFP) composites film before the electric measurements. Metal mask plates, a key auxiliary tool was used as shown in Fig. S2a (see Supporting Information). Two pieces of metal mask plates sandwiched the composites films and gold electrodes were sputtered subsequently. Fig. S2b (see Supporting Information) shows the image of real products, where the golden small dots are electrodes with diameter of 2 mm. Each dot is symmetrical with the relevant dot on the other side with same shape and size. The surface of top and bottom electrodes were contacted by appropriate fixtures during electric measurements. In order to indicated the improved dispersibility and compatibility of BaTiO3 @Paraffin nanoparticles in P(VDF-HFP) matrix, the cross section SEM images of BaTiO3 @Paraffin/P(VDFHFP) and BaTiO3 /P(VDF-HFP) composites films were observed, respectively. Liquid nitrogen was used to freeze the composite films to obtain the brittle failure cross section. As can be seen in Fig. 6a and b, BaTiO3 @Paraffin nanoparticles shows excellent dispersibility and compatibility in P(VDF-HFP) matrix. It was known that the non-polar Paraffin possessed good compatibility with nonpolar polymer matrix P(VDF-HFP) and Paraffin layer on the surface of the BaTiO3 nanoparticles can prevent the BaTiO3 particles from gathering each other. Therefore, BaTiO3 @Paraffin nanoparticles were evenly distributed and embedded in the polymer matrix, without obvious particle reunion, visible cracks, holes and other defects. By contrast, the untreated BaTiO3 was difficult to disperse in the polymer matrix. The untreated BaTiO3 nanoparticles reunited together and the composites films combined with the polymer substrate badly with obvious defects such as holes and cracks. Thus, it can prove that Paraffin was a kind of effective modification that can improve the dispersibility and compatibility properties of BT nanoparticles in polymer matrix. Fig. S3 (see Supporting Information) is the cross section SEM images at high resolution of the nanocomposites with 10 vol% and 50 vol% BaTiO3 @Paraffin nanoparticles, respectively. It can be more clearly seen that Paraffin modified BaTiO3 nanoparticles dispersed in the P(VDF-HFP) polymer matrix uniformly. In particular, the nanoparticles were nearly monodispersed at a low content and no obvious agglomeration existed even the content was increased to 50 vol.%. All of the BaTiO3 nanoparticles were embedded in polymer matrix firmly, which future illustrated that Paraffin significantly improved the dispersibility and compatibility of BaTiO3 nanoparticles in P(VDF-HFP) polymer matrix. 3.2. The dielectric properties of the nanocomposites Fig. 7a and b shows The frequency dependence of permittivity and dielectric loss of the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites with different BaTiO3 contents, respectively. It can be seen that the permittivity increased significantly with the increase of BaTiO3 @Paraffin nanoparticles. Specially, when the content of BaTiO3 @Paraffin/P(VDF-HFP) reached 50 vol.%, the nanocomposites possessed the permittivity of 60.8 at 40 Hz while the pure P(VDF-HFP) polymer at the same situation only had a permittivity of 8.6. Besides, the permittivity of the nanocomposites reduced with the increase of frequency. The permittivity of the nanocomposites with 50 vol.% BaTiO3 @Paraffin nanoparticles decreased from 49.0 to 43.0 with frequency increasing from 1 kHz to 1 MHz. It can be attributed to that the effect of interfacial polarization decreased with the increase of frequency which resulted in decreased permittivity [33–36]. From Fig. 7b, it can be seen that all of samples exhibited a relatively low dielectric loss. The largest dielectric loss was about 0.06 at 1 kHz. The modification of Paraffin obviously improved the dispersibility of BaTiO3 nanoparticles in composites and the compatibility with polymer matrix. Thus, the
nanocomposites exhibited favorable homogeneity without defects such as agglomeration, holes or cracks. On the other hand, the Paraffin coating layer on the surface of BaTiO3 nanoparticles had excellent insulativity, which effectively prevented the accumulation and transmission of charges in composites. As a comparison, the frequency dependence of permittivity and loss of the untreated BaTiO3 /P(VDF-HFP) nanocomposites were also characterized, as shown in Fig. 7c and d. When the untreated BaTiO3 nanoparticles were added into P(VDF-HFP) matrix, the permittivity of the nanocomposites also showed a significant increase compared with the polymer matrix while still smaller obviously compared with the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites. For example, the nanocomposites with 50 vol.% untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles respectively possessed the permittivity of 34.9 and 49.0 at 1 kHz. As we know, the permittivity of nanocomposites is mainly contributed to the interface polarization inside nanocomposites. The untreated BaTiO3 nanoparticles agglomerated together in polymer matrix which would lead to the reduction of interface areas and weakening of the interfacial polarization. Moreover, the permittivity of nanocomposites with 50 vol.% untreated BaTiO3 nanoparticles was lower than these with 40 vol% untreated BaTiO3 nanoparticles. The agglomeration of untreated BaTiO3 nanoparticles in polymer matrix obviously reduced the percolation threshold of the nanocomposites. It can be concluded that the percolation threshold of untreated BaTiO3 /P(VDF-HFP) nanocomposites was lower than 50 vol.% while that of BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites was higher than 50 vol.%. The rall dielectric loss of untreated BaTiO3 /P(VDFHFP) nanocomposites maintained at a relatively low level but it is significantly higher than that of BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites. The value abruptly increased to 0.27 when the untreated BaTiO3 content increased to 50 vol.%, which was 12 times larger than pure P(VDF-HFP). It was still due to the uneven dispersion and bad compatibility of untreated BaTiO3 nanoparticles in the polymer matrix which induced many defects such as boles and cracks in nanocomposites. The corresponding space charge polarization became stronger and lad to the increase of dielectric loss [37–39]. What’s more, the percolation threshold of untreated BaTiO3 /P(VDF-HFP) nanocomposites was lower than 50 vol% as indicated by the results of permittivity. Thus, when the untreated BaTiO3 content reached to 50 vol%, local connected islands have formed inside the nanocomposites, and resulted in the significant increase of dielectric loss. According to Eq. (1) the breakdown strength is the key parameter to achieve a high discharged energy density for the composite. The breakdown strength of the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites with various volume fractions of BaTiO3 @Paraffin is shown in Fig. 8a, which is analyzed with a two-parameter Weibull distribution function. The equation is as:
ˇ E
P(E) = 1 − exp −
E0
.
(2)
The characteristic breakdown strength (E0 ) is obtained when the cumulative probability of electric failure (P(E)) equaled at 63.2%, ˇ is the shape parameter [39]. To guarantee the reliability of breakdown strength, nine samples were provided for every nanocomposites film. As can been seen that the breakdown strength of the nanocomposites were decreased with the increase of BT contents, which was due to the inevitable defects in the nanocomposites and the BT nanoparticles were closer and closer to each other. Anyway, all of the nanocomposites showed excellent insulating property, the value of the breakdown strength reached 156 MV/m even when the contents of the BT was up to 50 vol.%. It was worth to note that, the nanocomposites obtained superhigh energy storage density at a relatively low electric field. For
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Fig. 6. The cross-section SEM images of composites films (a) with 30 vol% BaTiO3 @Paraffin nanoparticles and (b) 30 vol% BaTiO3 nanoparticles.
example, the nanocomposite with 50 vol.% BaTiO3 @Paraffin fillers obtained a high energy storage density of 21.1 J/cm3 at an electric field of 150 MV/m. These were agreed with the results in Figs. S4 and S5 (see in Supporting Information), which showed that the nanocomposites possessed high polarization, especially when the BaTiO3 @Paraffin content was high. The interesting results may be mainly attributed from the insulated shell layer, which prevented the free charge from transporting in the nanocomposites in the electric field. The ceaselessly accumulated charge on the surface between Paraffin layer and P(VDF-HFP) matrix enhanced the interfacial polarization [40,41]. Thus, the nanocomposites achieved super-high energy storage density, while the nanocomposites also possessed a high loss energy density due to the high residual polarization. It is necessary to discuss the discharged energy density and efficiency of the nanocomposites, which are added in Fig. 8c and d, respectively. The discharged energy density Udis can be cal-
culated from the D–E loops via formula Udis =
D max Dr
EdD, where
Dmax and Dr are the maximum electric displacement and remnant electric displacement, respectively [42]. As can be seen from Fig. 8c, the discharged energy density increased with the increase of electric field. For example, the discharged energy density of the sample with 10 vol% BaTiO3 @Paraffin increased from 0.26 J/cm3 at the electric field of 60 MV/m to 1.49 J/cm3 at 180 MV/m. Meanwhile, the discharged energy density increased with the increase of the BaTiO3 @Paraffin content at the same electric field. The energy density of the composite with 50 vol% BaTiO3 @Paraffin was 0.62 J/cm3 at 60 MV/m, which was increased by 170% compared with the nanocomposites with 10 vol% BaTiO3 @Paraffin (0.23 J/cm3 at 60 MV/m). The energy efficiency = Udis /(Udis + Uloss ), where Uloss is the energy loss of the composites, the result is shown in Fig. 8d. As can be seen that the energy efficiency significantly decreased
Fig. 7. The frequency dependence of (a) permittivity and (b) dielectric loss of the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites at room temperature. The frequency dependence of (c) permittivity and (d) dielectric loss of the untreated BaTiO3 /P(VDF-HFP) nanocomposites at room temperature.
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Fig. 8. (a) Weibull distribution of the dielectric breakdown strength of nanocomposites filled with various BNT nanofibers, (b) storage energy density, (c) discharged energy density, and (d) energy efficiency of the nanocomposites with the electric field.
with the increase of BaTiO3 @Paraffin loading. The reasons maybe include that the filler with high loadings will bring increased voids and defects to the nanocomposites, and the accumulated charge on the surface between paraffin layer and P(VDF-HFP) matrix enhanced the interfacial polarization, therefore much stored energy is loss. The nanocomposites with low BaTiO3 @Paraffin loading still possess outstanding performance, e.g. the nanocomposite with 10 vol% BaTiO3 @Paraffin obtains a high discharged energy density of 3.67 J/cm3 and an energy efficiency of 45%.
4. Conclusions A fail method was introduced to prepare high energy storage density nanocomposites. An obvious paraffin shell with the thickness of approximately 5 nm can be observed on the surface of BaTiO3 nanoparticles from the TEM images. The modified BaTiO3 nanoparticles were homogeneously dispersed in the nanocomposites and tightly adhered with the P(VDF-HFP) matrix. Due to excellent insulating property of the paraffin shell, the nanocomposites achieved high energy storage density at a relatively low electric field. The maximum energy storage density of the nanocomposite with 50 vol.% Paraffin@BaTiO3 increased to 21.1 J/cm3 at 150 kV/mm. It was demonstrated that paraffin was an effective modifier to prepare dielectric nanocomposites with high energy storage density.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (no. 51672311), Science and Technology Project of Hunan Province, China (no. 2016WK2022), and Supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.sna.2017.03.018.
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Biographies
Dou Zhang is a professor from Powder Metallurgy Research Institute, Central South University, and a honorary professor at University of Birmingham. He got his Ph.D. degree from school of Metallurgy & Materials, University of Birmingham at 2009. His research interests focus on the areas of ferro/piezoelectric ceramics, composites and intelligent device, energy harvesting and storage. He published more than 100 journals and held over 30 Chinese Patents. Now he is an editorial board member of Journal of Electroceramics.
Zhong Wu received his B.E. Degree from Central South University, China in 2015. Since 2015, he continued his study on ferro/piezoelectric ceramics and energy storage application under the supervision of Prof. Dou Zhang at Central South University. She published 2 papers on the field of dielectric nanocomposites for the application on energy storage.
Xuefan Zhou received her B.E. Degree from Central South University, China in 2014. Since 2014, she continued her study on the morphology control of lead-free piezoelectric nanostructures and preparation of ferroelectric ceramics with high performance under the supervision of Prof. Dou Zhang. She published 6 papers on the field of lead-free piezoelectric ceramics and dielectric nanocomposites.
Anqi Wei received her B.E. Degree from Central South University, China in 2015. Since 2015, she continued her study on ferroelectric/dielectric thin film under the supervision of Prof. Dou Zhang at Central South University. She published 2 papers on the field of ferro-/piezoelectric ceramics.
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Chao Chen received his B.E. degree in Materials Science and Engineering from Central South University in 2009 and the Ph.D. degree in dielectric thin film from Tsinghua University in 2014. Now he is a postdoctoral research fellow at Central South University. His current research interests include Ferroelectric thin films.
Hang Luo received his B.E. degree and M.E. degree from Xiang Tan University in 2010 and 2013, majored in Chemistry. Since 2013, he continued his study on ferro/piezoelectric ceramics and energy storage application under the supervision of Prof. Dou Zhang at Central South University, and now he is a postdoctoral research fellow at the group of Prof. Xiao Bo Ji at College of Chemistry and Chemical Engineering, Central South University. He published 16 papers on the field of ferroelectric and dielectric nanocomposites and energy storage materials.
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Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
High energy density in P(VDF-HFP) nanocomposite with paraffin engineered BaTiO3 nanoparticles Dou Zhang a , Zhong Wu a , Xue-fan Zhou a , An-qi Wei a , Chao Chen a , Hang Luo a,b,∗ a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China
a r t i c l e
i n f o
Article history: Received 7 December 2016 Received in revised form 23 February 2017 Accepted 14 March 2017 Available online xxx Keywords: Paraffin Interface modification Energy density Dielectric nanocomposites BaTiO3
a b s t r a c t Dielectric materials with high energy storage density are attracting scientific and commercial interest due to the prospect of application in advanced electronic devices and electric power systems. Ceramic/polymer composite is one of the most promising materials to obtain high energy density because it combines high permittivity from ceramic and high breakdown strength as well as flexibility from polymer matrix. However, the homogeneous dispersion and compatibility of nanoparticles in polymers matrix are still of great challenges due to the different surface energy between inorganic fillers and organic polymer matrix. Interfaces modification for ceramic fillers is an effective way to ease the mentioned problems. Herein, we proposed a low-cost and environmentally friendly route to prepare a core–shell structure by paraffin engineer the BaTiO3 nanoparticles. The modified BaTiO3 nanoparticles exhibited homogeneous dispersion in the ferroelectric polymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix and strong interfacial adhesion with the polymer matrix. The nanocomposites prepared in this work showed notably enhanced energy storage density. The maximum storage energy density of the nanocomposite with 50 vol% BaTiO3 increased to 21.1 J/cm3 at 150 kV/mm. These results indicate that paraffin is an effective modifier to prepare high energy storage density capacitor. © 2017 Elsevier B.V. All rights reserved.
1. Introduction High energy density capacitors are widely applied in electronic devices with pulsed power such as lasers and rail guns [1–8]. As energy density of linear dielectric materials is defined by Eq. (1): Ue =
1 ε0 εE 2 2
(1)
where ε0 is the permittivity of vacuum, ε is the relative permittivity of the materials, and E is the applied electric field [9,10]. Thus, the maximum energy density depends on the permittivity and breakdown strength of the material. It is not surprising that numerous efforts have been made in the past few years to combine high breakdown strength from the polymers with high permittivity from ceramic nanoparticles [11–13]. In general, to achieve ceramic/polymer nanocomposites with high energy densities, the fillers should possess high permittivity [11]. Thus, ferroelectric ceramics with high permittivity, such as BaTiO3 [14], Pb(Zr, Ti)O3 (PZT) [15], Ba0.2 Sr0.8 TiO3 [16], TiO2 [17],
∗ Corresponding author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China. E-mail address: [email protected] (H. Luo).
Pb(Mg1/3 Nb2/3 )O3 –PBaTiO3 iO3 (PMN–PT) [18], etc. were usually selected to introduce to polymer matrix. However, due to the different surface energy between inorganic fillers and polymer matrix, the ceramic fillers were easily accumulated in the nanocomposites and hardly tightly adhered with the polymer matrix, which greatly limited the increase of energy density of the nanocomposites. As proved, engineering the interface was one of the most potential strategies to improve the mentioned problems. In this regard, fabrication of core–shell structured ceramics by various modifiers has attracted lots of attention. Yang etc. reported a core–shell structured BaTiO3 @polymer nanodielectrics through reversible addition–fragmentation chain transfer (RAFT) polymerization and Thiol–Ene click reaction method [19]. Xie etc. prepared a new core–shell structure of barium titanate hybrid nanofillers (BaTiO3 HBP) via grafting hyperbranched aromatic polyamide to the surface of BaTiO3 [20]. As results, fillers with the controllable core–shell structures and the composites with regulated dielectric constant as well as superior low dielectric loss were obtained. However, as we know that in-suit surface-initiated polymerization methods are always accompanied by complex processes and harsh conditions, which are difficult to realize in the lab, thus it is needless to say for large-scale practical application [21,22]. Paraffin is an excellent insulating material with low cost, stable, nontoxic, non-corrosive and other advantages, which
http://dx.doi.org/10.1016/j.sna.2017.03.018 0924-4247/© 2017 Elsevier B.V. All rights reserved.
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can effectively prevent space charge from accumulating in the nanocomposites [23]. In addition, paraffin possesses low melting points, as we know that high-temperature processing is not favorable for the formation of core–shell structured nanoparticles [24]. In this regard, paraffin was selected as the modifier of BaTiO3 nanoparticles via facile method, and the modified fillers were then introduced into ferroelectric polymer poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-HFP)) to prepare high energy density nanocomposites. Ferroelectric BaTiO3 ceramics were selected as the fillers owing to the high permittivity and relatively low dielectric loss [10,25–28], P(VDF-HFP) was employed as the polymer matrix due to higher permittivity than other polymers [14,29–31]. As results, the Paraffin engineered BaTiO3 nanoparticles exhibited homogeneous dispersion in P(VDF-HFP) matrix and strong interfacial adhesion with the matrix. The nanocomposites showed obviously enhanced permittivity and low dielectric loss compared with the pure P(VDF-HFP). The permittivity of the nanocomposite with 50 vol.% paraffin engineered BaTiO3 increased to 49 at 1 kHz, which was higher than those with untreated BaTiO3 . It was notably that the nanocomposites possessed super-high polarization due to the insulated paraffin shell on the surface of BaTiO3 nanoparticles, which can prevent the space charge from transmission. Therefore, a large stored energy density of 21.1 J/cm3 was achieved in the nanocomposite with 50 vol.% BaTiO3 @Paraffin, which is great larger than lots of reported results.
Fig. 1. Schematic illustration of the formation of BaTiO3 @Paraffin core–shell NPs, and BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites.
Fig. 2. The XRD patterns of untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles.
2. Experimental sections
BaTiO3 nanoparticles with the average size of approximate 100 nm and the purity of 99.9%, and Poly(vinylidene fluoride-cohexafluoropropylene) P(VDF-HFP) polymer were purchased from Aladdin Co., Shanghai, China. Paraffin and H2 O2 (30 wt.%) were purchased from Guoyao, China.
The square metal mask plates with a side length of 30 mm and circular holes were designed. The diameter of circular holes was 2 mm. The as-prepared nanocomposite films were sandwiched between two metal mask plates to sputter the gold electrode on both sides. The sputtering time was 10 min to ensure the electrodes were enough thickness. As a comparison, pure P(VDF-HFP) film and nanocomposites with untreated BaTiO3 nanoparticles were also prepared via the similar approach.
2.2. Functionalization of the BaTiO3 nanoparticles
2.4. Characterization
10 g of BaTiO3 nanoparticles were added into 100 mL 30 wt.% H2 O2 aqueous solution in a round-bottomed flask. The mixture was sonicated for 30 min and refluxed at 106 ◦ C for 6 h. The nanoparticles were recovered by centrifugation at 9000 rpm for 5 min. The BaTiO3 nanoparticles were washed with deionized water and then dried under vacuum at 80 ◦ C for 12 h. 8 g BaTiO3 nanoparticles treated by H2 O2 were dispersed into petroleum ether. The mixture was sonicated for 10 min and then 0.8 g paraffin was introduced into the solution at 40 ◦ C, followed by stirring for 2 h. Subsequently, the mixture was cooled to room temperature, and the nanoparticles were recovered by centrifugation at 1000 rpm for 5 min and then dried under vacuum at 40 ◦ C for 24 h.
The crystalline phases of the products were tested by Xray diffraction (XRD, D/max 2550, Japan) with Cu-K␣ radiation ˚ at room temperature. The size and morphology ( = 1.5406 A) of the nanoparticles and nanocomposites were observed using a scanning electron microscope (SEM, Nova NanoSEM230, USA). Transmission electron microscopy (HRTEM) images were taken with a Titan (G260-300) transmission electron microscope, using an accelerating voltage of 300 kV. Fourier-transform infrared (FTIR) spectroscopy was performed on Nicolet 6700 instrument over the range of 4000–450 cm−1 to determine the functionalization of the BaTiO3 nanoparticles. Thermogravimetric analysis (TGA, NETZSCH STA 449) was conducted at a heating rate of 10 ◦ C min−1 in a nitrogen flow (20 mL min−1 ). Frequency-dependent permittivity and dielectric loss were measured using an Agilent 4294A LCR meter with the frequency ranging from 1 kHz to 10 MHz. Electric displacement-electric field loops and leakage current were measured by a Precision Premier II ferroelectric polarization tester (Radiant, Inc.) at room temperature and 100 Hz.
2.1. Materials
2.3. Preparation of nanocomposites with BaTiO3 @Paraffin Firstly, 8 g P(VDF-HFP) were added into 92 g as-prepared component solvent (Vacetone :VDMF = 7:3) and the solution was stirred at 60 ◦ C until it became transparent. Then, the BaTiO3 @Paraffin particles were added into the P(VDF-HFP) solution to prepare the suspensions with 10, 20, 30, 40 and 50 vol.% BaTiO3 @Paraffin nanoparticles respectively. The suspensions were all stirred for another 24 h. Subsequently, the nanocomposite films were fabricated via tape-casting the as-stirred suspension on a glass substrate and dried at 80 ◦ C. The films were further hot-pressed at 200 ◦ C and 15 MPa. The schematic illustration of the formation of BaTiO3 @Paraffin core–shell NPs and BaTiO3 @Paraffin/P(VDF-HFP) nanocomposite was shown in Fig. 1.
3. Results and discussions 3.1. Characterization of the BaTiO3 @Paraffin and the nanocomposites The crystal structures of the untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles are characterized by XRD, respectively, which are shown in Fig. 2. As can be seen that the
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Fig. 3. (a) TEM images and (b) high-resolution TEM images of BaTiO3 @Paraffin nanoparticles.
nanoparticles showed sharp diffraction peaks at 22.8◦ , 31.4◦ , 38.7◦ , 45.0◦ , 52.4◦ and 55.9◦ , which were corresponded to the lattice plane of (100), (110), (111), (200), (210) and (211), respectively. It can be well matched with the PDF#05-0626 card and showed a typical tetragonal structure. The BaTiO3 @Paraffin nanoparticles showed the same strong absorption peaks as the pure BaTiO3 nanoparticles without any new diffraction peaks. TEM measurement was used to observe the morphology of BaTiO3 @Paraffin nanoparticles. Fig. 3 displays the TEM images in bright field and high-resolution TEM images of BaTiO3 @Paraffin nanoparticles respectively. The surface of the BaTiO3 nanoparticles was rough with clear irregularity. This kind of structure can enhance the adhesion between the fillers and polymer matrix. The diameter of the BaTiO3 nanoparticles was approximately 100 nm. In Fig. 3a, a resin layer can be observed on the surface of BaTiO3 nanoparticles, which was clearly proved by the high-resolution TEM images, as shown in Fig. 3b. The different lattice fringes in the two sides of the BaTiO3 @Paraffin can further prove the exit of the resin layer on the surfaces of the BaTiO3 nanoparticles. The SEM images shown in Fig. S1 (see Supporting Information) are also provided to investigate the morphology of untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles, which is agree with the results of TEM. To prove the exited resin on the surface of BT nanoparticles, the FT-IR spectra of BaTiO3 @Paraffinm nanoparticles, pure Paraffin and untreated BaTiO3 nanoparticles are provided in Fig. 4a–c, respectively. Compared with initial BaTiO3 , BaTiO3 @Paraffinm nanoparticles showed obvious new absorption bands. Among them, the strong absorption bands appearing at about 2918 and 2849 cm−1 were from typical C-H stretching vibration from alkyl. What’s more, the obvious absorption band at 1473 cm−1 was originated from the asymmetric in-plane rocking vibration of methyl, which was observed in the spectra of pure Paraffin. However, it was not detected in pure BaTiO3 nanoparticles. These results proved that Paraffin has successfully modified the surface of BaTiO3 nanoparticles. Ceramic nanoparticles possess excellent thermostability as they will not show significant weight loss above 1000 ◦ C ordinarily. However, the organic modifier coated on the surface of them or the organic functional groups grafted on them will yield weight loss at high temperatures. Thus, the weight loss can be used to characterize the content of organic on the surface of BaTiO3 nanoparticles. As shown in Fig. 5, the weight losses of untreated BaTiO3 nanoparticles, BaTiO3 @Paraffin nanoparticles and Paraffin at 650 ◦ C are 2.2%, 31.2% and 99.79%, respectively. The TGA curve of the BaTiO3 nanoparticles tended to be horizontal without obvious weight loss at 650 ◦ C. (the weight loss was mainly caused by water, hydroxyl, or other impurities). On the contrary, the residual mass of pure Paraffin was only 0.21% of the initial value at 400 ◦ C, meaning that the
Fig. 4. The FT-IR spectra of (a) BaTiO3 @Paraffinm nanoparticles, (b) pure Paraffin and (c) untreated BaTiO3 nanoparticles.
Fig. 5. The TGA curves of untreated BaTiO3 nanoparticles, BaTiO3 @Paraffin nanoparticles and Paraffin.
weight loss of BaTiO3 @Paraffin nanoparticles was just the content of the coated Paraffin (31.2%) [32]. The above results further indicated that Paraffin has been successfully coated on the surface of BaTiO3 nanoparticles.
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Tape casting and hot pressing were used to prepare the BaTiO3 @Paraffin/P(VDF-HFP) composites. It is needed to sputter gold electrodes on both sides of BaTiO3 @Paraffin/P(VDF-HFP) composites film before the electric measurements. Metal mask plates, a key auxiliary tool was used as shown in Fig. S2a (see Supporting Information). Two pieces of metal mask plates sandwiched the composites films and gold electrodes were sputtered subsequently. Fig. S2b (see Supporting Information) shows the image of real products, where the golden small dots are electrodes with diameter of 2 mm. Each dot is symmetrical with the relevant dot on the other side with same shape and size. The surface of top and bottom electrodes were contacted by appropriate fixtures during electric measurements. In order to indicated the improved dispersibility and compatibility of BaTiO3 @Paraffin nanoparticles in P(VDF-HFP) matrix, the cross section SEM images of BaTiO3 @Paraffin/P(VDFHFP) and BaTiO3 /P(VDF-HFP) composites films were observed, respectively. Liquid nitrogen was used to freeze the composite films to obtain the brittle failure cross section. As can be seen in Fig. 6a and b, BaTiO3 @Paraffin nanoparticles shows excellent dispersibility and compatibility in P(VDF-HFP) matrix. It was known that the non-polar Paraffin possessed good compatibility with nonpolar polymer matrix P(VDF-HFP) and Paraffin layer on the surface of the BaTiO3 nanoparticles can prevent the BaTiO3 particles from gathering each other. Therefore, BaTiO3 @Paraffin nanoparticles were evenly distributed and embedded in the polymer matrix, without obvious particle reunion, visible cracks, holes and other defects. By contrast, the untreated BaTiO3 was difficult to disperse in the polymer matrix. The untreated BaTiO3 nanoparticles reunited together and the composites films combined with the polymer substrate badly with obvious defects such as holes and cracks. Thus, it can prove that Paraffin was a kind of effective modification that can improve the dispersibility and compatibility properties of BT nanoparticles in polymer matrix. Fig. S3 (see Supporting Information) is the cross section SEM images at high resolution of the nanocomposites with 10 vol% and 50 vol% BaTiO3 @Paraffin nanoparticles, respectively. It can be more clearly seen that Paraffin modified BaTiO3 nanoparticles dispersed in the P(VDF-HFP) polymer matrix uniformly. In particular, the nanoparticles were nearly monodispersed at a low content and no obvious agglomeration existed even the content was increased to 50 vol.%. All of the BaTiO3 nanoparticles were embedded in polymer matrix firmly, which future illustrated that Paraffin significantly improved the dispersibility and compatibility of BaTiO3 nanoparticles in P(VDF-HFP) polymer matrix. 3.2. The dielectric properties of the nanocomposites Fig. 7a and b shows The frequency dependence of permittivity and dielectric loss of the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites with different BaTiO3 contents, respectively. It can be seen that the permittivity increased significantly with the increase of BaTiO3 @Paraffin nanoparticles. Specially, when the content of BaTiO3 @Paraffin/P(VDF-HFP) reached 50 vol.%, the nanocomposites possessed the permittivity of 60.8 at 40 Hz while the pure P(VDF-HFP) polymer at the same situation only had a permittivity of 8.6. Besides, the permittivity of the nanocomposites reduced with the increase of frequency. The permittivity of the nanocomposites with 50 vol.% BaTiO3 @Paraffin nanoparticles decreased from 49.0 to 43.0 with frequency increasing from 1 kHz to 1 MHz. It can be attributed to that the effect of interfacial polarization decreased with the increase of frequency which resulted in decreased permittivity [33–36]. From Fig. 7b, it can be seen that all of samples exhibited a relatively low dielectric loss. The largest dielectric loss was about 0.06 at 1 kHz. The modification of Paraffin obviously improved the dispersibility of BaTiO3 nanoparticles in composites and the compatibility with polymer matrix. Thus, the
nanocomposites exhibited favorable homogeneity without defects such as agglomeration, holes or cracks. On the other hand, the Paraffin coating layer on the surface of BaTiO3 nanoparticles had excellent insulativity, which effectively prevented the accumulation and transmission of charges in composites. As a comparison, the frequency dependence of permittivity and loss of the untreated BaTiO3 /P(VDF-HFP) nanocomposites were also characterized, as shown in Fig. 7c and d. When the untreated BaTiO3 nanoparticles were added into P(VDF-HFP) matrix, the permittivity of the nanocomposites also showed a significant increase compared with the polymer matrix while still smaller obviously compared with the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites. For example, the nanocomposites with 50 vol.% untreated BaTiO3 and BaTiO3 @Paraffin nanoparticles respectively possessed the permittivity of 34.9 and 49.0 at 1 kHz. As we know, the permittivity of nanocomposites is mainly contributed to the interface polarization inside nanocomposites. The untreated BaTiO3 nanoparticles agglomerated together in polymer matrix which would lead to the reduction of interface areas and weakening of the interfacial polarization. Moreover, the permittivity of nanocomposites with 50 vol.% untreated BaTiO3 nanoparticles was lower than these with 40 vol% untreated BaTiO3 nanoparticles. The agglomeration of untreated BaTiO3 nanoparticles in polymer matrix obviously reduced the percolation threshold of the nanocomposites. It can be concluded that the percolation threshold of untreated BaTiO3 /P(VDF-HFP) nanocomposites was lower than 50 vol.% while that of BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites was higher than 50 vol.%. The rall dielectric loss of untreated BaTiO3 /P(VDFHFP) nanocomposites maintained at a relatively low level but it is significantly higher than that of BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites. The value abruptly increased to 0.27 when the untreated BaTiO3 content increased to 50 vol.%, which was 12 times larger than pure P(VDF-HFP). It was still due to the uneven dispersion and bad compatibility of untreated BaTiO3 nanoparticles in the polymer matrix which induced many defects such as boles and cracks in nanocomposites. The corresponding space charge polarization became stronger and lad to the increase of dielectric loss [37–39]. What’s more, the percolation threshold of untreated BaTiO3 /P(VDF-HFP) nanocomposites was lower than 50 vol% as indicated by the results of permittivity. Thus, when the untreated BaTiO3 content reached to 50 vol%, local connected islands have formed inside the nanocomposites, and resulted in the significant increase of dielectric loss. According to Eq. (1) the breakdown strength is the key parameter to achieve a high discharged energy density for the composite. The breakdown strength of the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites with various volume fractions of BaTiO3 @Paraffin is shown in Fig. 8a, which is analyzed with a two-parameter Weibull distribution function. The equation is as:
ˇ E
P(E) = 1 − exp −
E0
.
(2)
The characteristic breakdown strength (E0 ) is obtained when the cumulative probability of electric failure (P(E)) equaled at 63.2%, ˇ is the shape parameter [39]. To guarantee the reliability of breakdown strength, nine samples were provided for every nanocomposites film. As can been seen that the breakdown strength of the nanocomposites were decreased with the increase of BT contents, which was due to the inevitable defects in the nanocomposites and the BT nanoparticles were closer and closer to each other. Anyway, all of the nanocomposites showed excellent insulating property, the value of the breakdown strength reached 156 MV/m even when the contents of the BT was up to 50 vol.%. It was worth to note that, the nanocomposites obtained superhigh energy storage density at a relatively low electric field. For
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Fig. 6. The cross-section SEM images of composites films (a) with 30 vol% BaTiO3 @Paraffin nanoparticles and (b) 30 vol% BaTiO3 nanoparticles.
example, the nanocomposite with 50 vol.% BaTiO3 @Paraffin fillers obtained a high energy storage density of 21.1 J/cm3 at an electric field of 150 MV/m. These were agreed with the results in Figs. S4 and S5 (see in Supporting Information), which showed that the nanocomposites possessed high polarization, especially when the BaTiO3 @Paraffin content was high. The interesting results may be mainly attributed from the insulated shell layer, which prevented the free charge from transporting in the nanocomposites in the electric field. The ceaselessly accumulated charge on the surface between Paraffin layer and P(VDF-HFP) matrix enhanced the interfacial polarization [40,41]. Thus, the nanocomposites achieved super-high energy storage density, while the nanocomposites also possessed a high loss energy density due to the high residual polarization. It is necessary to discuss the discharged energy density and efficiency of the nanocomposites, which are added in Fig. 8c and d, respectively. The discharged energy density Udis can be cal-
culated from the D–E loops via formula Udis =
D max Dr
EdD, where
Dmax and Dr are the maximum electric displacement and remnant electric displacement, respectively [42]. As can be seen from Fig. 8c, the discharged energy density increased with the increase of electric field. For example, the discharged energy density of the sample with 10 vol% BaTiO3 @Paraffin increased from 0.26 J/cm3 at the electric field of 60 MV/m to 1.49 J/cm3 at 180 MV/m. Meanwhile, the discharged energy density increased with the increase of the BaTiO3 @Paraffin content at the same electric field. The energy density of the composite with 50 vol% BaTiO3 @Paraffin was 0.62 J/cm3 at 60 MV/m, which was increased by 170% compared with the nanocomposites with 10 vol% BaTiO3 @Paraffin (0.23 J/cm3 at 60 MV/m). The energy efficiency = Udis /(Udis + Uloss ), where Uloss is the energy loss of the composites, the result is shown in Fig. 8d. As can be seen that the energy efficiency significantly decreased
Fig. 7. The frequency dependence of (a) permittivity and (b) dielectric loss of the BaTiO3 @Paraffin/P(VDF-HFP) nanocomposites at room temperature. The frequency dependence of (c) permittivity and (d) dielectric loss of the untreated BaTiO3 /P(VDF-HFP) nanocomposites at room temperature.
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Fig. 8. (a) Weibull distribution of the dielectric breakdown strength of nanocomposites filled with various BNT nanofibers, (b) storage energy density, (c) discharged energy density, and (d) energy efficiency of the nanocomposites with the electric field.
with the increase of BaTiO3 @Paraffin loading. The reasons maybe include that the filler with high loadings will bring increased voids and defects to the nanocomposites, and the accumulated charge on the surface between paraffin layer and P(VDF-HFP) matrix enhanced the interfacial polarization, therefore much stored energy is loss. The nanocomposites with low BaTiO3 @Paraffin loading still possess outstanding performance, e.g. the nanocomposite with 10 vol% BaTiO3 @Paraffin obtains a high discharged energy density of 3.67 J/cm3 and an energy efficiency of 45%.
4. Conclusions A fail method was introduced to prepare high energy storage density nanocomposites. An obvious paraffin shell with the thickness of approximately 5 nm can be observed on the surface of BaTiO3 nanoparticles from the TEM images. The modified BaTiO3 nanoparticles were homogeneously dispersed in the nanocomposites and tightly adhered with the P(VDF-HFP) matrix. Due to excellent insulating property of the paraffin shell, the nanocomposites achieved high energy storage density at a relatively low electric field. The maximum energy storage density of the nanocomposite with 50 vol.% Paraffin@BaTiO3 increased to 21.1 J/cm3 at 150 kV/mm. It was demonstrated that paraffin was an effective modifier to prepare dielectric nanocomposites with high energy storage density.
Acknowledgements This work was financially supported by National Natural Science Foundation of China (no. 51672311), Science and Technology Project of Hunan Province, China (no. 2016WK2022), and Supported by State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.sna.2017.03.018.
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Biographies
Dou Zhang is a professor from Powder Metallurgy Research Institute, Central South University, and a honorary professor at University of Birmingham. He got his Ph.D. degree from school of Metallurgy & Materials, University of Birmingham at 2009. His research interests focus on the areas of ferro/piezoelectric ceramics, composites and intelligent device, energy harvesting and storage. He published more than 100 journals and held over 30 Chinese Patents. Now he is an editorial board member of Journal of Electroceramics.
Zhong Wu received his B.E. Degree from Central South University, China in 2015. Since 2015, he continued his study on ferro/piezoelectric ceramics and energy storage application under the supervision of Prof. Dou Zhang at Central South University. She published 2 papers on the field of dielectric nanocomposites for the application on energy storage.
Xuefan Zhou received her B.E. Degree from Central South University, China in 2014. Since 2014, she continued her study on the morphology control of lead-free piezoelectric nanostructures and preparation of ferroelectric ceramics with high performance under the supervision of Prof. Dou Zhang. She published 6 papers on the field of lead-free piezoelectric ceramics and dielectric nanocomposites.
Anqi Wei received her B.E. Degree from Central South University, China in 2015. Since 2015, she continued her study on ferroelectric/dielectric thin film under the supervision of Prof. Dou Zhang at Central South University. She published 2 papers on the field of ferro-/piezoelectric ceramics.
Please cite this article in press as: D. Zhang, et al., High energy density in P(VDF-HFP) nanocomposite with paraffin engineered BaTiO3 nanoparticles, Sens. Actuators A: Phys. (2017), http://dx.doi.org/10.1016/j.sna.2017.03.018
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Chao Chen received his B.E. degree in Materials Science and Engineering from Central South University in 2009 and the Ph.D. degree in dielectric thin film from Tsinghua University in 2014. Now he is a postdoctoral research fellow at Central South University. His current research interests include Ferroelectric thin films.
Hang Luo received his B.E. degree and M.E. degree from Xiang Tan University in 2010 and 2013, majored in Chemistry. Since 2013, he continued his study on ferro/piezoelectric ceramics and energy storage application under the supervision of Prof. Dou Zhang at Central South University, and now he is a postdoctoral research fellow at the group of Prof. Xiao Bo Ji at College of Chemistry and Chemical Engineering, Central South University. He published 16 papers on the field of ferroelectric and dielectric nanocomposites and energy storage materials.
Please cite this article in press as: D. Zhang, et al., High energy density in P(VDF-HFP) nanocomposite with paraffin engineered BaTiO3 nanoparticles, Sens. Actuators A: Phys. (2017), http://dx.doi.org/10.1016/j.sna.2017.03.018