Improving Dielectric Properties of PVDF Composites by Employing

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Improving Dielectric Properties of PVDF Composites by Employing Surface Modified Strong Polarized BaTiO3 Particles Derived by Molten Salt Method Jing Fu, Yudong Hou,* Mupeng Zheng, Qiaoyi Wei, Mankang Zhu, and Hui Yan College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China S Supporting Information *

ABSTRACT: BaTiO3/polyvinylidene fluoride (BT/PVDF) is the extensive reported composite material for application in modern electric devices. However, there still exists some obstacles prohibiting the further improvement of dielectric performance, such as poor interfacial compatibility and low dielectric constant. Therefore, in depth study of the size dependent polarization and surface modification of BT particle is of technological importance in developing high performance BT/PVDF composites. Here, a facile molten-salt synthetic method has been applied to prepare different grain sized BT particles through tailoring the calcination temperature. The size dependent spontaneous polarizationof BT particle was thoroughly investigated by theoretical calculation based on powder X-ray diffraction Rietveld refinement data. The results revealed that 600 nm sized BT particles possess the strong polarization, ascribing to the ferroelectric size effect. Furthermore, the surface of optimal BT particles has been modified by watersoluble polyvinylprrolidone (PVP) agent, and the coated particles exhibited fine core−shell structure and homogeneous dispersion in the PVDF matrix. The dielectric constant of the resulted composites increased significantly, especially, the prepared composite with 40 vol % BT loading exhibited the largest dielectric constant (65, 25 °C, 1 kHz) compared with the literature values of BT/PVDF at the same concentration of filler. Moreover, the energy storage density of the composites with tailored structure was largely enhanced at the low electric field, showing promising application as dielectric material in energy storage device. Our work suggested that introduction of strong polarized ferroelectric particles with optimal size and construction of core−shell structured coated fillers by PVP in the PVDF matrix are efficacious in improving dielectric performance of composites. The demonstrated approach can also be applied to the design and preparation of other polymers-based nanocomposites filled with ferroelectric particles to achieve desirable dielectric properties. KEYWORDS: BaTiO3, nanocomposites,ferroelectric polymers, core−shell structure, energy storage, dielectric constant, dielectric loss, molten salt method

1. INTRODUCTION Dielectric materials play critical roles in many fields for charge control and energy storage, and many material systems have attracted much attention because of their special performance, in which ceramic materials usually have high constant but are limited by their relatively low breakdown strength while polymer usually enjoys higher breakdown strength but is affected by much smaller permittivity.1 In the past decade, great efforts have been made to develop flexible ceramic/polymer nanocomposites for the aim to acquire the dielectric materials that have both high dielectric constant, low dielectric loss, and good processing properties.2,3 These ceramics filled into polymers would be easily prepared in a form of flexible films and a variety of shapes.2 Because of the high dielectric constant, the ceramic fillers used into the polymer matrix are normally polar oxides, such as BaTiO3 (BT),3,4 Pb(Zr,Ti)O3 (PZT),5 CaCu3Ti4O12 (CCTO),6 etc. BT, as a typical lead free perovskite material which is environmentally friendly, plays © XXXX American Chemical Society

an important role in multilayer ceramic capacitors (MLCCs) production.7,8 Recently, for the merits of lower manufacturing costs and fairly high dielectric constant, commercial BT nanoparticles, usually in 100 nm size, have been widely used as filler to incorporate with ferroelectric polymer.9,10 Though so many excellent researches have been done, there still exists some limitations. First, the dielectric constant of 100 nm sized commercial BT particle is not optimal because of the size dependent spontaneous polarization characteristics of ferroelectric material. Second, phase separation and agglomeration of BT fillers in the polymer matrix are still the key barrier to achieving the practical composites for dielectric devices application. Received: June 16, 2015 Accepted: October 21, 2015

A

DOI: 10.1021/acsami.5b05344 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration for (a) synthesis of BT nanoparticles via molten salt route, (b) fabrication process, and (c) modified mechanism of core−shell structured PVP/BT-PVDF nanocomposites.

The molten salt synthesis (MSS) method is one of the simplest, most versatile, and highly cost-effective methodology to prepare crystalline, chemically pure, single-phase oxides nanostructures.19,20 Although the MSS method has been applied to build bulk materials for a long time, the preparation of uniform oxide nanostructures using this technique has only arisen within the current century.20,21 Many research groups, including our laboratory, have applied this generalized methodology to the fabrication of various single-crystalline perovskite oxide nanostructures. Examples have been presented for preparing BaTiO3 nanowires,22,23 BaTiO3 nanostrips,24 BaZrO3 nanospheres,25 (K0.5Bi0.5)TiO3 nanowires,26 (La, Bi)AlO3nanoplates,27 and more. Previous works suggested that the calcination temperature is an important andreadily controllable factor that influences the size, as well as the crystallinity of the as-prepared particles, because the viscosity in the molten growth medium decreases when the temperature increases, which has an effect on the initial nucleation and subsequent diffusion of the growth species. Therefore, this temperature controllable process is promising for producing size-controllable BT single-crystalline nanostructures and this capability should enable future investigations of the sizedependent spontaneous polarization of these materials. In this work, we successfully synthesized the single-phase BT particles with a relatively wide range of sizes (100, 300, 400, 450, and 600 nm) by MSS route using the NaCl-KCl eutectic salts, heated from 600 to 950 °C. It should be noted that the pure BT particles with sizes above 650 nm could not be obtained in this case because of the formation of BaTi2O5 impurities in MSS process at higher temperature above 1000 °C. Through the theoretical calculation based on X-ray diffraction Rietveld refinement data, a strong correlation among the spontaneous

Ferroelectric size effect is of fundamental importance and plays a crucial role in the functionality of devices at increasingly smaller length scales.11 In polycrystalline ferroelectrics, such as unary BT,12 binary Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN− PT),13 and ternary Pb(Zn1/3 Nb 2/3 )O 3 −PbZrO 3 −PbTiO 3 (PZN−PZT),14,15 recent works revealed that both the internal stresses and the domain wall motion play a major role in determining a change of the dielectric and piezoelectric properties with decreasing grain size. For ferroelectric particles, theoretical models predict a gradual decrease of the Curie temperature, lattice strain, and polarization until the critical size corresponding to nonferroelectric cubic phase is reached.11 Recently, Ge et al. have performed detailed investigation on the relationship between particle size and dielectric properties of KNbO3 nanoparticles.16 They revealed that the large lattice distortion induced by grain size contributes mainly to the increase of spontaneous polarization Ps, which in turn enhanced the dielectric constant of KNbO3 particles. Similar to the former, BT undergoes the similar phase transition sequence as the temperature increases. Arlt et al. have studied the dielectric properties of fine-grained BT ceramics and found that dielectric constant is at its maximum of 5000 when grain size is about 1 μm and decreases dramatically as grain size is reduced.17 However, it should be noted that this study gives only the size effect of the BT ceramic body, which includes the contribution of grain boundaries, while for the single crystal particle without grain boundaries, the size-dependent dielectric properties will change and its nature is still not well understood.18 So, it is important for us to study the size effects in BT particles for developing high dielectric ferroelectric polymers-based composites. B


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Specimens for the TEM studies were prepared by dropping the sample solutions onto carbon-coated copper grids and air-drying before measurement. The crystal structures of samples were examined by Xray diffraction (XRD, Bruker D8 Advance, Karlsruhe, Germany) in the θ−2θ configuration using Cu Kα radiation. The fine scanning pattern was collected over a range of 20−80° with a 0.01° step width and a 2.5 s per step counting rate at room-temperature. The morphology of composites was detected on a fracture surface by a scanning electron microscope (SEM, Hitachi S4800, Japan). The characterization of the surface functionalization in the BT particles was investigated by the thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) (STA449C, NETZSCH Group, Selb, Germany). Alternating current ac dielectric properties of the pure PVDF, BT and their composites were determined with Novocontrol broadband dielectric spectrometer (Germany) in the frequency ranges of 1−107 Hz from −50 to 130 °C as an increase in temperature. Electric displacement−electric field (D−E) loops were measured by a ferroelectric tester (Premier II, Radiant Technologies Inc., Albuquerque, NM) at room temperature and 10 Hz using the same samples prepared for dielectric properties testing. Electrodes were painted with silver paste prior to measurement.

polarization, lattice distortion, and particle size has been revealed for MSS synthesized BT particles. Afterward, because of the strongest polarization, the 600 nm sized BT particles have been chosen as the optimal inorganic filler to incorporate with PVDF host matrix. Meanwhile, in order to modulate the interface between fillers and polymer matrix, the BT particles have been coated with polymer chains to construct core−shell structured composites.4,28−30 According to the hypothetical multilayered core model, building the intermediate layer tightly bonded to both inorganic filler and organic matrix is advantageous to improve the interface compatibility.31 Here, the surface of optimal BT particles was chemically modified by PVP, which possesses low toxicity because of the similar structural features like protein.32,33 It is expected that the groups of the PVP and PVDF matrix can interact with each other, improving the interface compatibility and reducing the defect between the fillers and matrix.34,35 Our work presents that PVP is effective to the construction of core−shell structured composites, resulting in uniform dispersion of ferroelectric particles in matrix. Eventually, the dielectric properties and the energy storage capability of the prepared PVP/BT-PVDF were investigated and analyzed with a comparison with untreated BT filled composites.

3. RESULTS AND DISCUSSION 3.1. Size Dependence of the Polarization of the BT Particles. The phase and the purity of the as-prepared powders generated from molten salt route were characterized by X-ray diffraction (XRD) [Figure 2a]. As can be seen, pure BT

2. EXPERIMENTAL SECTION 2.1. Molten Salt Synthesis of BT Nanoparticles. In our work, the MSS method was employed to prepare BT particles with different sizes ranging from 100 to 600 nm. The fabrication process was schemed in Figure 1a. Analytical grade BaCO3 (99.0%) and TiO2 (99.5%) were used as starting materials. The molten salt medium NaCl−KCl (50 mol % NaCl + 50 mol % KCl) was chosen because of its relative nontoxicity and low eutectic melting temperature. In the synthesis process of BT nanoparticle, BaCO3, TiO2, and NaCl−KCl were mixed in an overall stoichiometric ratio of 1:1:20 and ground for 30 min by ball milling. The mixture was then transferred into a corundum crucible and calcined at 50 °C temperature intervals between 600 and 1000 °C for 5 h. In this stage, reactants diffuse and rearrange in the molten salt and BT was formed after nucleation and growth. Subsequently, samples were collected and purified with deionized water several times until no chloride ions were detected by silver nitrate solution. Then, the resulted particles were dried in an oven at 120 °C for 2 h. This MSS process can be easily and routinely scaled up to produce grams of single-crystalline BT nanoparticles. 2.2. Preparation of Composites with PVP Coated BT. For the composite process, 600 nm sized BT particle has been chosen as filler for its strong spontaneous polarization. The reason would be given in discussion part. The schematic illustration of fabrication process and modified mechanism of PVP/BT-PVDF nanocomposites are shown in Figure 1b and c, respectively. The fabrication process contains mainly two steps. The first step is functionalization of the BT nanoparticles by PVP coating. BT particles were added into ethanol solvent and sonicated for 1 h. Then, 1 wt % PVP was introduced into the solution, and sonicated for another 1 h. Subsequently, the solution was stirred for 2 h at room temperature, and centrifugated for 3 min at 5000 rpm, then the sediment was dried at 100 °C for 24 h to acquire the core− shell structured PVP/BT particles. The second step is the preparation of PVDF based nanocomposite filled with PVP-coated BT. Powdered PVDF polymer was dissolved in the N,N-dimethylformamide (DMF) solvent, after that, the functional PVP/BT nanoparticles were added into the solution with dissolved PVDF. The solution was sonicated for 1 h and stirred for 4 h at 50 °C. Then, DMF solvent was evaporated at 100 °C for 24 h. The mixture of BT fillers and PVDF at the volumetric fraction of 0−80% were molded by hot pressing at ∼190 °C, 20 MPa for 10 min. As a result, compact composites samples were produced. 2.3. Characterization. Transmission electron microscopy (TEM) images were carried out using an instrument (TEM, Model JEM2000F, JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV.

Figure 2. (a) XRD patterns of BT particles synthesized at 600−1000 °C by molten salt method and (b) the corresponding fine scanning XRD patterns of 2θ= 44.5−46°.

particles can be obtained at a wide temperature range between 650 and 950 °C. Due to the low melting point of 670 °C for NaCl−KCl,36 the real eutectic point of the reaction multicomposition containing a molten salt would be further reduced. In the molten salt liquid environment, because of the small diffusion distances of oxide mixtures, the high reactivity of salts, and the high mobility of species, completed reactions can be achieved in a relatively short time. Compared to traditional solid state synthesis, the molten salt method decreases the synthesis temperature of BT greatly.37,38 However, higher than 1000 °C, BaTi2O5 impurities appeared in the product inevitably, which is mainly due to the enhanced volatility of molten salt at high temperature.36 Further observation of the XRD patterns reveals obvious changes in diffraction peaks near 2θ = 45°, indicating that calcination temperature induces a structural transition.39 To determine the phase evolution, fine scanning was carried out in the diffraction angle range 2θ = 44.5−46°, and the results are shown in Figure 2b. It is clear that the phase structure shows the changing trend from the pseudocubic to the tetragonal structure as increasing calcination C


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Ps ≈ (c /a)0.5

temperature. Double peaks of structure are significant when the calcination temperature is above 750 °C. The BT powders were further examined using the scanning electron microscopy (SEM) to reveal the morphology and size dispersion of the particles. Figure 3 shows the SEM images of

(1)

In eq 1, the spontaneous polarization is proportional to tetragonality (c/a), which is tightly related to the size of ceramic particles. Accordingly, to compare and characterize the structures of the prepared BT, XRD Rietveld refinements for the BT synthesized at 850 (450 nm), 900 (550 nm), and 950 °C (600 nm) respectively were executed with P4 mm space group, by using the Fullprof software.16 As shown in Figure 4,

Figure 4. Observed (circles), calculated (line), and differential (bottom of figure) X-ray powder diffraction profiles for BT synthesized at 850, 900, and 950 °C obtained after the Rietveld refinements at room temperature. The tick marks give the positions of all possible Bragg reflections.

the calculated diffraction profiles agree well with the observed ones for the BT synthesized at 850, 900, and 950 °C respectively. The associated detailed crystallographic data and structure refinement parameters are summarized in Table 1. As Table 1. Crystallographic Data and Structure Refinement Parameters of All Three BT Particles by X-ray Powder Diffraction Figure 3. SEM images of BT particles prepared by molten salt method at (a) 600, (b) 650, (c) 750, (d) 850, (e) 950, and (f) 1000 °C. Inset: Histograms of the particle size distributions of the asprepared nanoparticles.

temp (°C) cryst syst space group lattice parameters

BT powders calcined at 600−1000 °C. It is obvious that the products mainly consist of solid, crystalline particles, free of hard agglomeration. Using Image Pro Plus software, the distributions of particle sizes have been further analyzed. It can be seen that the average particle size of the BT particles enlarged with increasing calcinations temperature. For example, the meansize of BT particles calcined at 600 °C was 100 nm, and it was increased to 600 nm when calcined at 950 °C. Because of the acceleration of the crystal growth at high temperature molten salt media, the particle size gradually grows as the calcination temperature increases.16 In general, the dielectric properties of BT depend strongly on the size of the crystallites and the domain configuration. According to the previous analysis by Devonshire40 and the theoretical expression of Sun,41 the relationship between the spontaneous polarization Ps and tetragonality (c/a) is given by equation

cell volume agreement factors

a (Å) b (Å) c (Å) c/a V (Å3) Rwp (%) Rp (%) Rexp (%) χ2

850 tetragonal P4mm 3.99749 (16)



3.99749 (16) 4.02270 (20) 1.00630 64.282 11.9

3.99728 (16) 4.02252 (20) 1.00631 64.273 13.1

3.99596 (10) 4.02347(13) 1.00688 64.245 13.6

8.60 5.56

9.49 5.67

10.1 5.69

4.61

5.38

5.71

can be seen, tetragonality (c/a) is gradually increased with the increase of the calcination temperature. It is known that the ferroelectric characters of perovskite-type structure are mainly originated from the atom displacement along the ferroelectric axis and the increased tetragonality (c/a) is conducive to enhancing ferroelectric properties. Accordingly, the spontaneous polarization (Ps) because of the relative ion displaceD


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3.2. Characterization of the BT Coated with PVP and the Composite. In fact, the dielectric constant of particles would influence the particle polarization in a composite through its effect on the local electric field distribution experienced by particles. According to the Furukawa,46 the mean electric field, Er,cera, acting on particles in a composite can be described as

ments can be basically estimated by considering a purely ionic crystal. e Ps = ∑ Zi″Δi V i (2) where V is the unit cell volume, Δi represents the shifts of the ith atom along the ferroelectric axis, and Z″i is the apparent charge.42 In terms of the atoms displacements in Table 2, the

Er,cera =

Table 2. Atoms Displacements (Å) of All Three BT Particles by X-ray Powder Diffraction Δz(Ti) Δz(OI) Δz(OII)

850 °C

900 °C

950 °C

0.00229 0.05834 0.01620

0.00669 0.06423 0.01967

0.02905 0.07631 0.06116

3εr,poly 2εr,poly + εr,cera + fcera (εr,poly − εr,cera)

Eapp

(3)

fcera represents volume fractions of the ceramic particles; εr,poly and εr,cera stand for dielectric constant of the polymer and ceramic particle, respectively; and Eapp is electric field applied on the composite. However, it should be noted that in fact the electric field is not only applied on the BT particles and polymer matrixes, but also the interface (even voids) between two of them, therefore the eq 3 should include εr,interface or εr,voids, which would weaken mean electric field acting on the ceramics. Thus, interface compatibility becomes the key to the problem. The ceramic fillers often show poor compatibility with polymer matrixes. Though the inclusion of ceramic particles with high dielectric constants increases the average dielectric constant of the composite, they also produce a highly inhomogeneous electric field with local hot spots (e.g., the defect). It does not only reduce the effective breakdown strength of the composite, but also disperse the electric field that should act on BT particles, which would weaken the polarization response of the BT particles.47,48 Additionally, the high surface energy of nanoparticles usually leads to phase separation and agglomeration from the polymer matrix, especially in an incompatible matrix, deteriorating the quality of composites and weakening dielectric properties, such as low dielectric strength and high dielectric loss. Normally, the use of dispersant and surfactant, coupling agent has been found to be an effective approach for enhancing the dispersion of the particles and improving interface compatibility. Here, the surface of optimal molten salt synthesized BT particle has been modified by environmentally friendly PVP to construct core− shell structure. Figure 6a displays the TEM images of the untreated BT particles prepared by MSS method. It indicates the formation of well-developed, single-crystalline BT particles. The interplanar spacings of about 0.2843 and 0.2891 nm correspond to (110) and (101), respectively. Figure 6b shows the TEM images of the BT particles coated with PVP. A stable and dense PVP shell almost 6 nm was clearly observed on the surfaces of the BT particles. This result indicated that the core− shell structured PVP modified BT particles were successfully constructed. Moreover, TGA and DSC measurement provide another proof that PVP were grafted onto the surface of BT particles. Figure 6c and d show the TGA and DSC curves of BT, BT/PVP, and pure PVP, respectively. For the treated BT particles, the corresponding TGA curve show a dramatic weight loss from 380 to 480 °C, which is caused by the decomposition of PVP undoubtedly.49 The DSC curve of the pure PVP gives three obvious peaks at 398, 411, and 435 °C, respectively. The exothermic peak at 398 °C indicates the crystallization of PVP while the endothermic peak at around 411 °C represents the process of dehydrogenation, carbonization, and decomposition of PVP. For the exothermic peak at higher temperature of 435 °C, it indicates the combustion of the carbon deposition. The endothermic process of the treated BT is more significant than untreated BT, which is caused by the decomposition of PVP.

spontaneous polarization as a function of particle size can be calculated according to eq 2. Figure 5a and b show tetragonality

Figure 5. (a) Variation of cell parameters c, a, c/a ratio, and (b) the spontaneous polarization (Ps) as a function of calcination temperature.

(c/a) and the spontaneous polarization (Ps) as a function of calcinations temperature. Apparently, the calculated spontaneous polarization (Ps) increases with increasing particle size from 450 (850 °C) to 600 nm (950 °C), exhibiting the similar variation trend as that of tetragonality. The maximum Ps value of 30 μC/cm2 is obtained at about 600 nm (950 °C), implying that the spontaneous polarization was strongly size dependent.43,44 Our result agrees well with the reports in the previous work that proper particle size for getting greater dielectric constant is 0.5−0.7 μm in diameter.39,44,45 Thus, in subsequent experiments, the 600 nm sized BT particles have been selected as an optimal inorganic filler to build nanocomposites. E


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Figure 6. TEM images of (a) untreated BT and (b) PVP/BT particles at low magnification. The inset shows the corresponding surface morphology at high magnification. (c) TGA and (d) DCS curves of untreated BT and PVP/BT particles and pure PVP.

is no evidence of particle−matrix debonding and PVP/BT nanoparticles exhibited the homogeneous dispersion in the polymer matrix. PVP, as a water-soluble and biocompatible polymer, can establish bindings with inorganic BT nanoparticles in ethanol solvent after sonicated and stirred.50 On the other hand, because of the presence of a strong specific dipolar interaction between the PVDF’s fluorine group (CF2) and the PVP’s carbonyl group (CO), the PVP/BT interface bonds tightly with the PVDF host, resulting in excellent interface compatibility between BT particles and the PVDF matrix.51,52 Uniform dispersion of the PVP/BT particles, less voids and defects in PVDF matrix are favored in enhancing the dielectric performance of the BT/PVDF composite for the merits of promoting the accumulation of charge carriers at an interface, offering stronger interfacial polarization, and enlarging the mean electric field acting on BT particles. Figure 8 presents the XRD patterns of BT, PVDF in two different conditions, the initial PVDF powder and the hot-press PVDF disk, and PVP/BT-PVDF composites with different volume of BT. Compared to the distinguished diffraction peaks of perovskite type BT, XRD patterns of PVDF before and after hot pressing exhibited a typical characteristic hump of amorphous structure. However, for the nanocomposites, the characteristics of amorphous scattering of PVDF can not be clearly observed, which could be ascribed to the shielding effect for high intensity of diffraction peaks in BT. Moreover, it can be found that the initial PVDF powder is single α-phase, however, part of α-phase transforms to β-phase after hot-press process. It is known that the neat PVDF has four different crystal

Comparing to the pure PVP, the peaks of the treated BT are not obvious, which may be due to the smaller amount of PVP coated on BT than the pure PVP. On the basis of the above characterization results, it can be established without doubt that PVP was successfully coated on the surface of BT particles. The PVDF-based nanocomposites filled with functional BT particles were obtained by hot compression. For comparison, the nanocomposites filled with untreated BT were also prepared in the same procedure. Figure 7a and (b) present SEM images of the freeze-fractured cross sections of the nanocomposites filled with crude BT and PVP modified BT, respectively. It suggests that the composite with the PVP/BT filler shows better combination than that with pure BT. There

Figure 7. SEM images of the freeze-fractured cross section of the composite filled with (a) the untreated 600 nm sized BT and (b) PVPmodified 600 nm sized BT at a concentration of 40 vol %. F


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Figure 10a and b shows the frequency dependence of dielectric constant and loss tangent of pure PVDF polymer and

Figure 8. XRD patterns of BT nanoparticles, pure PVDF powder, hotpress PVDF disk, and the composites with different volume fractions of 600 nm sized BT.

forms,10,53 in which the β-phase is the desirable phase due to its ferroelectric characteristic and therefore, the partially formation of β-phaseis beneficial for the enhancement of the dielectric properties of the composites. 3.3. Dielectric Properties of the PVP/BT-PVDF Composites. The dependence of the dielectric properties of the composites on the volume concentration of BT fillers at room temperature and 1 kHz is given in Figure 9. It can be seen

Figure 10. Frequency dependencies of (a) dielectric permittivity and (b) loss tangent of the pure PVDF and composites at varied volume fractions of 600 nm sized BT, measured at 25 °C from 0.1 Hz to 10 MHz.

the composites with different volume fraction of the treated BT, measured at room temperature over the range of 0.1 Hz to 10 MHz. It can be discovered in Figure 10a that the dielectric constant of all composites remained stable up in the frequency range between 1 Hz and 100 kHz. However, in the higher frequency range above 100 kHz, a distinct decrease with the frequency increasing present, especially for the composites filling with high amount of BT (>40 vol %). It is believed that the decrease of dielectric permittivity at high frequency is mainly caused by the reduction in Maxwell−Wagner−Sillars (MWS) polarization and space charge polarization, which are more pronounced for the highly filled dielectric composites.55,56 Additionally, compared with the variation trend of dielectric constant, the dielectric loss of the nanocomposites shows a different tendency, as seen in Figure 10b. The dielectric loss in the frequency range from 0.1 to 100 Hz was high due to the interfacial polarization, which derived from the significant different dielectric constant between BT fillers and PVDF matrix.57 80 vol % BT/PVDF composite exhibited larger dielectric constant and dielectric loss because of more defects than the others. Moreover, the loss curve maintained smooth in the frequency range from 100 Hz to 1 kHz and then an sharp increasewere noted around 10 MHz, which is a typical feature of the glass transition relaxation of the PVDF matrix.55,58 The frequency dependence of dielectric constant and loss tangent of

Figure 9. Dielectric permittivity, loss tangent of the composites as a function of the volume fraction of 600 nm sized BT, measured at 25 °C and 1 kHz.

that the dielectric constants increased sharply with the increase of the BT volume fraction in the initial stage. Specifically, the dielectric constant increased from 11 to 115 as the BT content increased from 0 to 60 vol %. Meanwhile, dielectric loss of composites in the corresponding filling range remained at a low level (

Improving Dielectric Properties of PVDF Composites by Employing

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