Enhanced Dielectric Performance of P(VDF-HFP) Composites with Satellite-Core-Structured Fe2O3@BaTiO3 Nanofillers.

Polymer dielectric materials are extensively used in electronic devices. To enhance the dielectric constant, ceramic fillers with high dielectric constant have been widely introduced into polymer matrices. However, to obtain high permittivity, a large added amount (>50 vol%) is usually needed. With the aim of improving dielectric properties with low filler content, satellite-core-structured Fe2O3@BaTiO3 (Fe2O3@BT) nanoparticles were fabricated as fillers for a poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix. The interfacial polarization effect is increased by Fe2O3 nanoparticles, and thus, composite permittivity is enhanced. Besides, the satellite-core structure prevents Fe2O3 particles from directly contacting each other, so that the dielectric loss remains relatively low. Typically, with 20 vol% Fe2O3@BT nanoparticle fillers, the permittivity of the composite is 31.7 (1 kHz), nearly 1.8 and 3.0 times that of 20 vol% BT composites and pure polymers, respectively. Nanocomposites also achieve high breakdown strength (>150 KV/mm) and low loss tangent (~0.05). Moreover, the composites exhibited excellent flexibility and maintained good dielectric properties after bending. These results demonstrate that composite films possess broad application prospects in flexible electronics.

1. Introduction

Polymer dielectric materials are extensively applied in flexible electronics and energy storage devices owing to their merits of outstanding flexibility, ease of processing, light weight, and low cost [1,2,3,4]. Despite these merits, the dielectric permittivity (ε) of most polymers is quite low (<10). Two main strategies have been developed by researchers to enhance the dielectric permittivity [5,6,7,8,9,10,11,12]. One is incorporating ceramic fillers with intrinsically high dielectric constants (e.g., BaTiO3, BaxSr1-xTiO3, CaCu3Ti4O12) [13,14,15,16,17,18,19,20] into the polymer matrix; the other strategy is employing conductive fillers, including metals (e.g., Ag, Ni, Al) [21,22,23,24,25], carbon materials (e.g., carbon nanotubes, graphene) [26,27,28,29,30,31,32], semiconductors (e.g., ZnO) [33], and conductive polymers (e.g., polyaniline (PANI)) [34,35,36,37]. With ceramic/polymer composites, the merits of high ε from ceramic fillers and high breakdown strength from polymers are combined. However, ceramic fillers with a large amount of additions (>50 vol%) are usually needed to obtain a high ε, which can seriously affect flexibility and mechanical properties. Therefore, it is worth studying this problem to further improve the dielectric properties of composites with low filler content.

One type of n-type semiconductor is α-Fe2O3 (band gap: 2.1 eV). It has been studied extensively in pigments, lithium-ion batteries, gas sensors, and photoelectrochemical water splitting [38,39,40]. It has also attracted the attention of researchers for optimizing the dielectric performance of pure polymers by utilizing α-Fe2O3. Thakur et al. [41] reported in-situ-synthesized Fe2O3 in poly(vinylidene fluoride) (PVDF), which promotes the formation of β-PVDF and enhances ε through interfacial polarization. Hayashida [42] incorporated α-Fe2O3 into ten kinds of polymer matrices and studied its influences on the dielectric properties at 40–160 °C. The research showed that the ε of composites could be raised due to interfacial polarization induced by free electrons in α-Fe2O3 particles. In addition, constructing satellite–core-structured fillers for the polymer matrix was considered to be an effective approach for enhancing the dielectric performance. This structure combines two kinds of fillers by loading one filler onto the surface of another. For example, Ag@BT fillers [43] and SnO2@BT [44] fillers were fabricated by former researchers and enhanced dielectric properties were obtained compared with pristine BT fillers.

In this work, with the aim of improving dielectric properties with low filler content, Fe2O3@BT nanoparticles were fabricated as fillers to prepare Fe2O3@BT/P(VDF-HFP) and (FB/P(VDF-HFP)) composites. Satellite–core-structured Fe2O3@BT introduces extra interfaces, so the interfacial polarization and ε of composites are enhanced. Besides, the satellite–core structure of Fe2O3@BT prevents the direct contact of Fe2O3 particles with each other in the polymer matrix, so the loss tangent remains relatively low.

2. Materials and Methods

2.1. Materials

N, N-dimethylformamide (DMF) and Barium titanate (BaTiO3, BT) were bought from Aladdin (Shanghai, China). P(VDF-HFP) and Ferric nitrate nonahydrate were supplied by Sinopharm (Shanghai, China) and Sigma-Aldrich ( Shanghai, China), respectively.

2.2. Synthesis of Satellite-Core-Structured Fe2O3@BT Nanoparticles

The 0.303 g ferric nitrate nonahydrate was first dissolved in deionized water (100 mL). Then, 0.700 g BT nanoparticles were dispersed into this solution via sonicating and stirring. The molar ratio of BT/Fe was 4:1. The solution was stirred at 75 °C for 5 h, and cleaned with deionized water. After drying under vacuum, FeOOH@BT nanoparticles were obtained. The generated powder was heated at 550 °C for 2 h in air. Satellite–core-structured Fe2O3@BT nanoparticles were then generated.

2.3. Fabrication of Fe2O3@BT/P(VDF-HFP) Composites

A stoichiometric amount of Fe2O3@BT nanoparticles were distributed into dimethylformamide (DMF) via stirring and ultrasound. P(VDF-HFP) was then added and vigorously stirred for 12 h. The feeding ratio of P(VDF-HFP)/DMF was 1 g:15 mL. The composite films were then prepared through drop casting onto clean glass plates. The composites were kept at 60 °C to eliminate DMF, and then heated to 200 °C (5 min) and quenched in ice water. BT/P(VDF-HFP) and pure polymer were also generated.

2.4. Characterization

Scanning electron microscopy (SEM) (SU-8010, Hitachi, Japan) and transmission electron microscopy (TEM) using a Tecnai G2 F20 (FEI, Hillsboro, OR, USA) (accelerating voltage: 200 kV) with energy dispersive spectroscopy (EDS) were applied to examine the morphology of composites and particles. The elemental composition of nanoparticles was observed using X-ray photoelectron spectroscopy (XPS) with an Escalab 250Xi. XRD (X’ Pert PRO, PANalytical, Netherlands) using Cu Kα radiation was performed to identify the components of particles and composites. Differential scanning calorimetry (DSC) was tested by TA-Q200 at 90–190 °C (10 °C/min, nitrogen atmosphere). Dielectric performances were measured with an 4294 impedance analyzer (Agilent, Palo Alto, CA, USA) from 102–106 Hz (silver electrode, diameter: 4 mm, thickness: 100 nm). A dielectric strength tester (CS2674AX, Nanjing Changsheng, Nanjing, China) was employed to test the Direct Current (DC) breakdown strength under a direct current voltage ramp of 200 V s−1 at 25 °C.

3. Results and Discussion

3.1. Morphology and Structure of Fe2O3@BT Nanoparticles

Figure 1 presents the TEM photos of Fe2O3@BT nanoparticles, as well as the EDS elemental mapping photos. The pure BT nanoparticles are spherical, with a diameter of about 50–100 nm. Fe2O3 nanoparticles (5–10 nm) decorated on BT and the satellite–core-structured Fe2O3@BT nanoparticles are formed. As shown in the High Resolution Transmission Electron Microscope (HRTEM) image, the lattice fringe areas with 0.221 nm and 0.282 nm spacing are assigned to (113) and (110) planes of α-Fe2O3 and BT (JCPDS 75-0462, 33-0664), respectively [45,46]. The structure of Fe2O3@BT nanoparticles is illustrated in Figure 1d. EDS results further reveal the distribution of Fe2O3. It is shown that Ba, Ti, and O are homogenously distributed on the surface of nanoparticles. However, the amount of Fe is much less and its distribution is locally concentrated, corresponding to the satellite–core structure.

Figure 1

(a) Transmission electron microscopy (TEM) photo of a BT nanoparticle. (b) TEM and (c) HRTEM photos of a Fe2O3@BT nanoparticle. (c) Partially enlarged image of the blue square area in image (b). (d) Schematic illustration of a Fe2O3@BT nanoparticle. (e–i) HAADF-STEM image with mapping images of a Fe2O3@BT nanoparticle. The scale of the images (f–i) is the same with that of image (e).

Figure 2 presents XRD patterns of BT and hybrid particles. Characteristic peaks of BaTiO3 (Joint Committee on Powder Diffraction Standards (JCPDS) 75-0462) are obviously shown in hybrid particles. Moreover, some weak peaks at 24.1°, 33.2°, 35.6°, 49.5°, and 54.1° are also observed, corresponding to the (012), (104), (110), (024), and (116) planes of α-Fe2O3, respectively. No other phases of Fe2O3 are shown, which indicates that only α-Fe2O3 is obtained after calcination at 550 °C [47,48,49].

Figure 2

X-ray diffraction (XRD) patterns of Fe2O3@BT and BT nanoparticles.

To further analyze the elemental composition, XPS is conducted on Fe2O3@BT nanoparticles. As shown in Figure 3a, characteristic peaks of Ba, O, Fe, C, and Ti are shown in survey scan spectra. In Figure 3b, the peak at 724.6 eV and 710.9 eV correspond to Fe3+ 2p1/2 and Fe3+ 2p3/2 peaks, together with two satellite peaks at 733.5 eV and 719.2 eV. The binding energy difference between 2p1/2 and 2p3/2 is 13.7 eV. Besides, characteristic peaks are not observed for Fe2+ [50,51,52,53]. These results indicate that the element Fe in nanoparticles exists in the form of Fe3+, which means Fe2O3 is synthesized. In addition, the color of the powders is red-brown, which is consistent with that of Fe2O3.

Figure 3

X-ray photoelectron spectroscopy (XPS) spectra of Fe2O3@BT nanoparticles: (a) survey scan, (b) Fe 2p.

3.2. Structure and Morphology of Fe2O3@BT/P(VDF-HFP) Composites

Figure 4 presents cross-section morphologies of composites. Numerous nanoparticle fillers are shown in the polymer. According to the XRD results of the composites, it can be seen that these nanoparticles are Fe2O3@BT. The nanoparticles are distributed well in P(VDF-HFP) and no apparent void or pore can be observed. In addition, the inset shows the digital photograph of 20 vol% composites, which can still be easily bent and rolled.

Figure 4

SEM images of cross-sectional film of: (a) pristine P(VDF-HFP), (b) 5 vol%, (c) 10 vol%, (d) 15 vol%, and (e) 20 vol% composites. Inset is the digital photograph of 20 vol% film.

Figure 5 demonstrates XRD patterns of composites. The three peaks at 18.2°, 19.9°, and 26.5° correspond to the (020), (110), and (021) planes of α-P(VDF-HFP), respectively [54,55]. The hybrid nanofillers peaks can be observed, as well as the matrix peaks. The relative intensity of the matrix peaks decreases as the Fe2O3@BT increases.

Figure 5

XRD patterns of P(VDF-HFP), Fe2O3@BT nanoparticles, and FB/P(VDF-HFP) composites.

3.3. Melting and Crystallization Behavior of Fe2O3@BT/P(VDF-HFP) Composites

Differential scanning calorimetry (DSC) was performed to analyze the crystallization of the polymer. As is shown in Figure 6a, a melting peak appears in the heating curve for each film, corresponding to the melting process of the polymer. The melting temperature (Tm) and crystallization temperature (Tc) decrease as the filler increases. The crystallinity (χc) can be calculated through the formula below: where ΔHm and represent the melting enthalpy of samples and 100% crystallized α-P(VDF-HFP) (93.07 J/g), respectively. Here, ω is the weight fraction of Fe2O3@BT nanoparticles in composites.

Figure 6

DSC of polymer and FB/P(VDF-HFP) composite (a) heating curves and (b) cooling curves.

As shown in Table 1, when the filler content increases, the crystallization peak moves towards lower temperatures and Tc decreases gradually. This phenomenon is mainly attributed to the hindering effect of nanoparticle fillers [56,57]. During the crystallization process, the Fe2O3@BT nanoparticles retard the movement of the polymer chain and impede the progress of crystallization, leading to the decrease of Tc. Fe2O3@BT can also act as a heterogeneous nucleation site, facilitating the crystallization. However, the hinderance effect dominates the crystallization process and the influences of heterogeneous nucleation are covered up. When more Fe2O3@BT nanoparticles are added, the hinderance effect is further enhanced and Tc continues to decrease. The final χc also reduces gradually because of the accumulation of the hinderance effect during crystallization.

Table 1

Melting Temperature (Tm), crystallization temperature (Tc), and crystallinity (χc) of polymer and FB/P(VDF-HFP) composites.

Sample	P(VDF-HFP)	5 vol% Fe2O3@BT	10 vol% Fe2O3@BT	15 vol% Fe2O3@BT	20 vol% Fe2O3@BT
Tm (°C)	155.5	155.0	153.9	153.7	153.0
Tc (°C)	127.0	126.6	126.3	125.4	124.7
χc (%)	37.3	35.4	33.4	33.2	30.7

3.4. Dielectric Properties of Fe2O3@BT/P(VDF-HFP) Composites

Figure 7 presents the dielectric characteristics of a pristine polymer, FB/P(VDF-HFP), and 20 vol% BT/P(VDF-HFP). In Figure 7a, the ε of each composite decreases as the frequency gets higher. This phenomenon is due to the interfacial polarization relaxation and dipole polarization relaxation at low and high frequencies. To further analyze the influences of Fe2O3@BT nanoparticles on the dielectric performance of composites, ε and tan δ values at 1 kHz of all samples are compared in Figure 8 (left axis). As the content of nanoparticles increases, the ε of FB/P(VDF-HFP) is increased notably. The enhancement is larger than in BT/P(VDF-HFP) at the same concentration, which is caused by the interfacial polarization induced by Fe2O3@BT particles, an important polarization mechanism that occurs in low frequency ranges because of its relatively long time of establishment. When a dielectric is placed in an electric field, the internal free electrons and holes migrate under the electric field and gather at the interfacial area containing two phases, impurities, and defects. Then, dipole moments are generated, and thus, interfacial polarization is induced. In FB/P(VDF-HFP) composites, the satellite–core-structured Fe2O3@BT nanoparticles introduce extra interfaces, including the Fe2O3/BT interface, Fe2O3/P(VDF-HFP) interface, and BT/P(VDF-HFP) interface; semi-conductive Fe2O3 brings about more charge carriers. Therefore, the interfacial polarization is enhanced by Fe2O3@BT nanoparticles and the dielectric permittivity of composites is raised. Figure S1 exhibits the dielectric performances of BT/P(VDF-HFP) composites. With 20 vol% nanoparticles added, the ε value of Fe2O3@BT/P(VDF-HFP) is 31.7 at 1 kHz, nearly 1.8 and 3.0 times that of 20 vol% BT/P(VDF-HFP) (18.0) and pure polymer (10.6), respectively. Figure S2 shows that the composite maintains good dielectric performances after bending, which proves the potential application in flexible electronics.

Figure 7

Frequency dependence of (a) dielectric constant, (b) dielectric loss tangent, and (c) conductivity of pristine polymer, BT/P(VDF-HFP), and FB/P(VDF-HFP) composites.

Figure 8

Dielectric properties of the composites filled with Fe2O3@BT and BT as a function of filler content at 1 kHz.

Figure 7b shows the dielectric loss of composites. Tan δ declines at first and then increases for each sample as the frequency gets higher. The increase of tan δ is attributed to dipole polarization relaxation at high frequency. In this range, the establishment of dipole polarization cannot follow the electric field, so the relaxation leads to enhanced loss. The tan δ of composites is lower than the pristine polymer and it continues to decrease when the filler content increases. This phenomenon probably occurs because the Fe2O3@BT nanoparticles retard the movement of polymer chains, which can decrease the dipole polarization relaxation loss [58,59]. The loss tangent is derived from electric conduction loss and interfacial polarization relaxation at low frequencies. The tan δ values of all samples at 1 kHz are also compared in Figure 8 (right axis). With the increase of nanofiller content, the tan δ is slightly increased, because Fe2O3 generates many charge carriers. However, the satellite–core structure of Fe2O3@BT could prevent the direct contact of Fe2O3 particles with each other in the polymer matrix and suppress the long-range movement of charge carriers; therefore, the tan δ remains low (<0.06). With the addition of 20 vol% Fe2O3@BT nanoparticles, the tan δ of composites maintains a rather low value of 0.05. The tan δ values of BT/P(VDF-HFP) (20 vol%) and pure polymer are 0.03 and 0.02, respectively (Figure S1). And compared with other BT-based/polymer nanocomposites reported in the previous literature (Table S1), the results of the FB/P(VDF-HFP) nanocomposites reported herein are comparable or better. Figure 7c shows that the conductivity of composites increases when more nanofillers are added. Nevertheless, the conductivity of all composites is lower than 2 × 10−8 S/m, proving that the film provides good insulation.

Breakdown strength (E) is also a significant characteristic and determines the energy density and work voltage of composites. Due to the randomness of breakdown events, measured data of E is usually further processed by a two-parameter Weibull distribution function [60,61]: where P is the cumulative probability of electrical failure, E represents breakdown strength, E0 is the characteristic breakdown strength (cumulative failure probability: 0.632), and β is the shape parameter. As shown in Figure 9, breakdown strength decreases as the nanofiller content increases. This phenomenon results from the electrical mismatch between the polymer and the nanoparticles. However, the satellite–core structure of Fe2O3@BT nanoparticles suppresses the rise of dielectric loss and impedes the formation of conductive paths, so E still remains at a relatively high level. The E value of the 20 vol% Fe2O3@BT-filled composite is 152.7 MV/m.

Figure 9

(a) Weibull distribution for breakdown strength and (b) characteristic breakdown strength of composites.

4. Conclusions

Satellite–core-structured Fe2O3@BT nanoparticles were fabricated as fillers to prepare FB/P(VDF-HFP) composites. Fe2O3@BT nanoparticles show a hinderance effect on the crystallization process of polymers and the crystallization temperature and crystallinity of composite films both decrease as the content of the filler increases. The interfacial polarization effect is enhanced by Fe2O3 nanoparticles, and thus, the dielectric permittivity of composites is enhanced. The satellite–core structure prevents Fe2O3 particles from directly contacting each other, so the dielectric loss remains low. With the addition of 20 vol% Fe2O3@BT nanoparticles, the permittivity value of the composite is 31.7 at 1 kHz, nearly 1.8 and 3.0 times that of the 20 vol% BT and pristine polymer, respectively. Nanocomposites also demonstrate low loss tangent (~0.05) and high breakdown strength (>150 KV/mm). In addition, the composites also exhibit excellent flexibility and maintains good dielectric performances after bending.