Enhanced dielectric properties of surface hydroxylated bismuth ferrite–Poly (vinylidene fluoride-co-hexafluoropropylene) composites for energy storage devices

Abstract Dielectric properties of Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) based composites with surface hydroxylated BiFeO 3 (h-BFO) particles were prepared by solution casting techniques. The h-BFO fillers were synthesized from BiFeO 3 in aqueous solution of H 2 O 2 . The result showed that the dielectric properties of the h-BFO-PVDF-HFP composite exhibits better dielectric properties than that of the unmodified BFO-PVDF-HFP composites. Meanwhile, the 30 wt% of h-BFO-PVDF-HFP composite showed higher dielectric constant, better suppressed dielectric loss, high remnant polarization and high electrical conductivity. It is suggested that the strong interaction between h-BFO particles and PVDF-HFP matrix at the interface is the key role in the enhancement of the dielectric properties. It is helpful to understand the influence of surface hydroxylation on the interfaces between the filler and the polymer matrix. The outcome of this study may be exploited in the progress of high energy storage device applications.


Introduction
Polymer composites (PNCs) with high dielectric constant and low dielectric loss have attracted considerable interest recently owing to their potential applications in electrical and electronic industries such as gate dielectrics [1], embedded capacitors [2], aerospace and power industries [3][4]. In the past few decades, much effort has been devoted to an improvement of flexible ceramic filled polymer composites with high dielectric constant, which can be applicable for electronic industries to meet the rigorous requirements of advanced capacitors [5][6]. The traditional multiferroic material such as BiFeO 3 (BFO) has good dielectric constant but it has poor moldability, low breakdown strength, high processing temperature and it is very brittle and shock resistive in nature [7][8] have certain qualities such as they have (i) good electrical resistance (ii) easy process-ability (iii) high breakdown strength and (iv) low cost which may be useful for the composites regarding their dielectric performances [9][10][11][12][13] but, they have low dielectric constants (< 5) which restrict their future applications in the energy storage device [14]. To overcome this issues the combination of both fillers and polymer can fabricate novel type of ceramic based polymer composite to get high energy storage materials [15][16].
As many studies have been done by the addition of different types of polymer with various kinds of filler containing high dielectric constant, low dielectric loss and good thermal stability, to date much research work has been focused on the interface between the ceramic filler and the polymer matrix for dielectric applications of the polymer based composites [17][18][19][20].
Moreover, the effect of interface can remarkably improve the dielectric properties of the composites [21].
In this paper, we report a novel idea of preparing a surface hydroxylated BiFeO 3 polarization as compared to that of unmodified BFO-PVDF-HFP composite. This may be due to the hydrogen bond which leads to stronger interaction between the h-BFO fillers and the PVDF-HFP matrix [21]. We also have investigated the consequence of such surface hydroxylation on the dielectric properties of the composites with respect to interface zone.

Preparation of Bismuth ferrite (BFO) particles
The preparation of BFO particles were prepared by the conventional solid state reaction method.
The equi-molar quantities of Bi 2 O 3 and Fe 2 O 3 were first thoroughly mixed by an agate mortar and pestle in the presence of air for half an hour and then in methanol for another 2 hours. Then the mixed powders were calcined in a high purity alumina crucible at an optimized temperature of 700 0 C for 2 hours.

Surface Hydroxylation of Bismuth ferrite (h-BFO)
BFO particles (5g) were dispersed in aqueous solution of H 2 O 2 (30 wt%) were combined in a round bottomed flask and sonicated for 30 min. The solution was refluxed at 106 0 C for 6 h and then the particles were recovered by centrifugation. The obtained BFO particles were washed with de-ionized water and then were dried under oven at 80 0 C for 24 h. The hydroxylated BFO particles were named as h-BFO.

M A N U S C R I P T
A C C E P T E D ACCEPTED MANUSCRIPT 4 the solution was ultra-sonicated for 1 h and again stirred for 2 h to obtain a homogeneous mixture. Then the resulting mixture was poured into a clean glass petri-dish and dried in an oven at 80 0 C for overnight to remove any traces of the solvent. The total preparation process is shown in the Fig. 1. ). An external electric field of 3kV/cm was applied at a frequency of 50 Hz to measure the P-E loop. The leakage current of the composites were measured by using an electrometer (Keithley 6517B).

FTIR study
The effect of H 2 O 2 on the surface behavior of BiFeO 3 (h-BFO) particles were analyzed by using FTIR. As shown in Fig. 3, it is observed that, the absorption band at ~3500 cm -1 which corresponds to h-BFO represents the stretching mode of surface hydroxyl groups and this clearly indicates that the hydroxyl groups are present in the BFO surfaces. Again, the absorption peak at 455 cm -1 is assigned to the FeO 6 octahedral of the perovskite structure. This absorption band is due to the Fe-O stretching and bending vibration of BFO. This represents the presence of metaloxygen bond.    Fig.6. As a result, the dielectric constant of such composite with surface hydroxylated BFO particles is much higher than that of unmodified BFO particles which is clearly shown in Fig. 5 (a,b).

Fig. 6 Plausible interaction between PVDF-HFP and h-BFO particles
The dependence of the dielectric constant of the composite on the volume fraction of BFO fillers in a frequency range of 100 to 1000 Hz at room temperature is shown in Fig. 5c. It is found that   (Table-1). The dielectric loss of the composites decreases with the increase in frequency (Fig. 7 a, b). However, there is an increase in the dielectric loss is observed in the low frequency range, which may be due to the relaxation mechanism present in the polymer. For comparative study, the 30 wt% of h-BFO-PVDF-HFP composite shows relatively low dielectric loss value because the h-BFO particles are uniformly distributed in the polymer matrix.
Meanwhile, in unmodified BFO-PVDF-HFP composites the dielectric loss value is quite high because the BFO particles are agglomerated in the polymer matrix. The high value of the dielectric loss at the lower frequency range can be attributed due to the relaxation of the space charge polarization [28][29].
The dependence of the dielectric loss of the composite on the volume fraction of BFO fillers in the frequency range 100 to 1000 Hz at room temperature is shown in Fig. 7c. It is observed that in case of h-BFO-PVDF-HFP composite, the dielectric loss value remains at a low level (< 1).
This phenomenon has been observed in many polymer/ceramic composite systems and can be commonly attributed to the small amount of defects at the interface and uniform dispersion of h-BFO particles in the PVDF-HFP matrix, which is clearly visualized from Fig. 4(b) and Fig.1.
Further, in case of unmodified BFO-PVDF-HFP composite the dielectric loss value is quite high because unmodified BFO particles are agglomerated in the PVDF-HFP matrix as a result the dielectric loss value remains high. Secondly, the higher loss value may be originated from their high electrical conductivities [30,31]. For comparison, the composite films with surface hydroxylated and unmodified BFO particles are prepared and their dielectric properties are given in Table-

AC electrical conductivity
The frequency dependence of AC electrical conductivity of h-BFO-PVDF-HFP and BFO-PVDF-HFP composite with different wt% of BFO fillers at room temperature is shown in Fig.8. The value of AC electrical conductivity (σ ac ) of the composites was evaluated by using the dielectric data and the following empirical formula: Where the symbols have their usual meaning: σ ac = AC conductivity, ε 0 = Permittivity in free space, ε r = Relative dielectric constant, ω = Angular frequency, tan δ = Loss tangent.
As shown in Fig. 8 (a), the AC electrical conductivity of h-BFO-PVDF-HFP composite increases with increase in frequency for all weight percentages of BFO content and slightly higher than that of the pure PVDF-HFP over the whole frequency range, indicating good insulating properties of the composites. It is observed that the composites with surface hydroxylated BFO particles have high electrical conductivity than those of unmodified one as shown in Fig. 8 (b).
This is because the particles are homogeneous dispersion in the polymer matrix in case of modified BFO due to which the electrons can easily move in presence of applied electric field [29,31].

Current -voltage characteristics
The current-voltage (I-V) characteristic curve for the unmodified BFO-PVDF-HFP and modified h-BFO-PVDF-HFP composites is shown in Fig.10. It is observed that in case of modified composite the leakage current is much lower than that of the unmodified composites. The reduced leakage current may be attributed to the surface modification by hydroxyl groups on the surface of the BFO particles and due to the formation of passivation layers in h-BFO [17]. The leakage current behavior leads the asymmetric PE loops [34] which have been shown in the