Enhanced dielectric properties of poly(vinylidene fluoride) composites filled with nano iron oxide-deposited barium titanate hybrid particles

We report enhancement of the dielectric permittivity of poly(vinylidene fluoride) (PVDF) generated by depositing magnetic iron oxide (Fe3O4) nanoparticles on the surface of barium titanate (BT) to fabricate BT–Fe3O4/PVDF composites. This process introduced an external magnetic field and the influences of external magnetic field on dielectric properties of composites were investigated systematically. The composites subjected to magnetic field treatment for 30 min at 60 °C exhibited the largest dielectric permittivity (385 at 100 Hz) when the BT–Fe3O4 concentration is approximately 33 vol.%. The BT–Fe3O4 suppressed the formation of a conducting path in the composite and induced low dielectric loss (0.3) and low conductivity (4.12 × 10−9 S/cm) in the composite. Series-parallel model suggested that the enhanced dielectric permittivity of BT–Fe3O4/PVDF composites should arise from the ultrahigh permittivity of BT–Fe3O4 hybrid particles. However, the experimental results of the BT–Fe3O4/PVDF composites treated by magnetic field agree with percolation theory, which indicates that the enhanced dielectric properties of the BT–Fe3O4/PVDF composites originate from the interfacial polarization induced by the external magnetic field. This work provides a simple and effective way for preparing nanocomposites with enhanced dielectric properties for use in the electronics industry.

Dielectric materials that possess high dielectric permittivity (ε) and dielectric field strength without excessive dielectric loss are necessary to meet the miniaturization requirements of microelectronic device-structures, including gate dielectrics, high charge-storage capacitors and electro-active materials [1][2][3] . Polymer materials are currently of considerable interest as high-permittivity materials for electronics applications. However, their low dielectric permittivity limits their application. Many strategies to increase the dielectric permittivity of polymers have been reported 4 and the introduction of high dielectric permittivity nanoparticles (e.g., CaCu 3 Ti 4 O 12 (CCTO) and BT nanoparticles) into a polymer matrix has been widely adopted [5][6][7][8][9] . Unfortunately, the dielectric permittivity improvement for two-phase composite materials is still limited and often requires a large filler loading (> 60 vol.%) to enhance the dielectric permittivity, which causes the materials to lose their flexibility and uniformity. Investigations of percolative materials have been carried out by incorporating metal powders or other conductive fillers into a polymer matrix 10 . Ultra-high dielectric constant values can be achieved; however, a high conductivity and dielectric loss also result when the filler content approaches the percolation threshold.
To overcome these limitations, researchers have focused on improving the dielectric properties of the materials via surfactant treatment of the filler by a coupling agent [10][11][12][13][14] . Improving the physical compatibility at a novel interface can guarantee good dispersion of the ceramic particles. Luo et al. 12 modified BT nanoparticles using hydantoin epoxy resin and found that hydantoin/BT-P(VDF-HFP) (P(VDF-HFP): poly(vinylidene fluoride-co-hexafluoropropylene) nanocomposites had a high dielectric permittivity (ε = 48.9) and a low dielectric loss (0.06) with 50 vol.% filler loading at 1 kHz. Fu et al. 13 modified BT particles using polyvinyl pyrrolidone (PVP) fillers to realize composites with high dielectric permittivity (ε ≈ 120) and low loss tangent (tan δ ≈ 0.3) with 60 vol.% filler loading at 100 Hz. Another promising strategy is to fabricate three-phase polymeric composites containing conductive fillers 15,16 . Yang and co-workers 15 prepared Ni/CCTO/PVDF composites with a dielectric constant (140) and a dielectric loss of 0.5 near the percolation threshold when the filler content of Ni and CCTO was 60 vol.%. Many researchers have reported that nano-sized Ag particles discretely deposited on the surface of the ceramic can efficiently enhance the dielectric permittivity of the composites [17][18][19] . Luo et al. 17 prepared PVDF embedded with BT-Ag nanoparticles and found that the BT-Ag/PVDF composites with 56.8 vol.% filler loading presented a high dielectric permittivity (ε = 160) and a low dielectric loss (0.11) at 1 kHz. However, even with the high-volume fraction of inorganic compounds in the composite, the dielectric permittivity of the composite was not high enough for the practical application.
Shear flow, magnetic field, electric field, or electric force can change the molecular arrangement of a polymer and the distribution of conductive particles in a host polymer, which influence the microstructure and macro-properties of composites [20][21][22][23][24] . In this study, we fabricated super-paramagnetic Fe 3 O 4 nanoparticles deposited on the surface of BT ceramic particles via a chemical precipitation method. On this basis, we have designed BT-Fe 3 O 4 /PVDF composites treated under a constant magnetic field for 30 min at 60 °C. The morphology of BT-Fe 3 O 4 particles was characterized by transmission electron microscopy (TEM), and the composites were studied by scanning electron microscopy (SEM). The effect of an external magnetic field on the dielectric properties of the composites filled by BT-Fe 3 O 4 and correlation with the structure and morphology of the composites are also discussed systematically.     Fig. 4. The dielectric permittivity of the composites showed a weak frequency dependence when the volume fraction of Fe 3 O 4 was less than 10 vol.%. When the volume fraction of Fe 3 O 4 was greater than 10 vol.%, the dielectric permittivity increased significantly and the frequency dependence of the dielectric permittivity of composites gradually increased as the volume fraction increased, especially at low frequency. The dielectric permittivity of the 20 vol.% BT-xFe 3 O 4 /PVDF composite was 42 when the volume fraction of Fe 3 O 4 reached 30 vol.%, which is 1.2 times higher than that of 20 vol.% BT/PVDF. This demonstrates that incorporating conducting fillers into the polymer matrix results in an increase in dielectric permittivity. The increased conductivity of the interlayer between the BT and PVDF matrix created by the Fe 3 O 4 enhances the space charge polarization and Maxwell-Wagner-Sillars effect, which play an important role in improving the dielectric permittivity 25,26 .

Results and Discussion
To understand the influence of the BT-   the dielectric permittivity was 138, which is 15 times higher than that of the pure PVDF matrix. However, the dielectric permittivity of the composites was still not high enough for embedded devices. On this basis, we designed BT-Fe 3 O 4 /PVDF composites by applying an external magnetic field. The frequency dependence of the dielectric properties of the BT-Fe 3 O 4 /PVDF# composites is shown in Fig. 6b. Dielectric permittivity increases with volume fraction up to 33 vol.%, and then decreases when volume fraction exceeds 33 vol.%. The dielectric permittivity of the BT-Fe 3 O 4 /PVDF# composites reached 385, which is 1.8 times higher than that of the 40 vol.% BT-Fe 3 O 4 / PVDF composites and this value is higher than that of many previous reports 12,17,[27][28][29][30][31][32] . For example, as shown in Table 1, this value was found to be significant larger than that of BT@SnO 2 /PVDF composites containing 45vol.% BT@SnO 2 (≈ 160) 29 . It should be noted that a high dielectric permittivity (280) of BT-Fe 3 O 4 /PVDF# composites was obtained at 1 kHz and this value is superior to that of BT-Ag/PVDF composites with higher filler loading. In that report, the highest dielectric permittivity reported by Luo et al. 17 for 56.8 vol.% BT-Ag hybrid particles filled into PVDF was 160 at 1 kHz. Moreover, the amount of filler in the BT-Fe 3 O 4 /PVDF# composites was smaller than that in other materials described in the literature, and displayed better flexibility. In addition, compared with BT-Fe 3 O 4 /PVDF composites, the dielectric permittivity of the BT-Fe 3 O 4 /PVDF# composites was increased  greatly, and this result also indicated that the applied magnetic field can greatly affect the dielectric properties of the BT-Fe 3 O 4 /PVDF composites. The structure of the BT-Fe 3 O 4 hybrid particles means that we can regard each particle as a unit. The classic percolation theory was used to predict the dielectric behavior of the BT-Fe 3 O 4 /PVDF composites 33 . The dielectric behavior of the BT-Fe 3 O 4 /PVDF composites yields to the classic percolation theory as below: where ε and ε 1 are the dielectric permittivity of the composites and PVDF matrix, respectively, f is the volume fraction of BT-Fe 3 O 4 and f c is the percolation threshold, q is a critical exponent. As shown in Fig. 7a, the experimental results agreed well with the percolation theory when the volume fraction of the filler was greater than 10 vol.%. However, for volume fractions less than 10 vol.%, the fitting results deviate from the experimental data apparently, indicating that the BT-Fe 3 O 4 hybrid particle is not a real conducting phase in the composites. In this study, a series-parallel model was employed to estimate the permittivity of the BT-Fe 3 O 4 hybrid particles 4,18 .  Fig. 7b). The fitting parameters f c and q are 31.5 vol.% and 0.90, respectively. The linear fit of the log value of the dielectric permittivity and volume fraction also indicates that the dielectric permittivity fits well with percolation theory (see Fig. 7b inset). The inter-particle distance would decrease as the volume fraction of the BT-Fe 3 O 4 hybrid particles increased, and that the probability of BT-Fe 3 O 4 hybrid particles coming into contact increased because of the high-intensity magnetic field (see Fig. 3b). Fe 3 O 4 with high conductivity can produce electrical current under an applied filed and the charges will move and accumulate at the interface between the Fe 3 O 4 and PVDF matrix. The charge accumulation will result in enhanced polarization and dielectric response under the electric filed. That is, the percolation effect was induced by the external magnetic field, which could effectively enhance the interfacial polarization of the BT-Fe 3 O 4 /PVDF composites.
The energy loss due to the consumption of a dielectric material can be determined by the following equation: 2 where ξ is the electric field strength and f is the frequency. For embedded capacitor applications, the dielectric loss is an essential parameter. The dielectric loss measured at a given frequency includes polarization loss and conduction loss 19 . The loss tangent as a function of frequency for the BT-Fe 3 O 4 /PVDF composites is shown in Fig. 8a. It can be found that the dielectric loss remained low (tanδ < 0.3) over the whole frequency range. The conduction loss is caused by charge flow through the composites, which depends on the electric conductivity of the composites. As shown in Fig. 8c, the conductivity of the composites with a filler loading of 5 vol.% remained low (5 × 10 −11 S/cm) because the absorbed insulating polymer chains act as the dielectric barrier governing the tunneling conduction and make it impossible for complete contact between the nanoparticle clusters 34,35 . The conductivity of the BT-Fe 3 O 4 /PVDF composites increased as the BT-Fe 3 O 4 loading increased. The conductivity increased from 5 × 10 −11 S/cm to 1.4 × 10 −9 S/cm at 100 Hz, indicating that a conducting path was not formed in the composites, in agreement with the low dielectric loss (shown in Fig. 8a). As shown in Fig. 8b,d, a relatively low dielectric loss (0.3) and a low conductivity (4.12 × 10 −9 S/cm) were obtained when the volume fraction of BT-Fe 3 O 4 was 33 vol.%. Compared with the BT-Fe 3 O 4 /PVDF composites, the BT-Fe 3 O 4 /PVDF# composites exhibited a substantial increase of dielectric permittivity, a slight increase of dielectric loss as well as a slight increase of conductivity. In general, percolative composites can exhibit very high dielectric constants at the proper filler loading. However, these composites also exhibit a relatively high conductivity due to the insulator-conductor transition near the percolation threshold. In the present study, the insulating BT particles lower the probability of the Fe 3 O 4 particles coming into contact because they are discontinuous and discretely fixed on the BT surface. The BT-Fe 3 O 4 hybrid particles made it difficult for the Fe 3 O 4 particles to form a complete conductive network throughout the whole system, resulting in composites with high dielectric permittivity, low dielectric loss, and low conductivity.   Characterization. The phase compositions of the BT-Fe 3 O 4 hybrid particles and the PVDF composites were analyzed using X-ray diffraction (XRD, Empyrean) using Cu Kα radiation at 40 kV and 40 mA. The microstructure of the PVDF composites was determined using SEM (Hitachi S-3400N) and the BT-Fe 3 O 4 hybrid particles were analyzed using TEM (JEOL JEM-2100F). Prior to performing dielectric measurements, a thin layer of Al paste (diameter of 25 mm) was applied to the sides of the composites. The dielectric properties of the PVDF composites were determined in the frequency range of 100 Hz to 1 MHz at room temperature using a broadband dielectric spectral instrument (Novocontrol Alpha-A).