(NBT)-poly(vinylidene fluoride) (PVDF)

JOURNAL OF ADVANCED DIELECTRICS Vol. 8, No. 3 (2018) 1850017 (10 pages) © The Author(s) DOI: 10.1142/S2010135X18500170

Enhanced dielectric and ferroelectric properties of surface hydroxylated Na0:5 Bi0:5 TiO3 (NBT)-poly(vinylidene fluoride) (PVDF) composites Srikanta Moharana*, Shraddhakara Sai* and Ram Naresh Mahaling*,y,z *Laboratory of Polymeric and Materials Chemistry

J. Adv. Dielect. 2018.08. Downloaded from www.worldscientific.com by 139.81.202.88 on 07/13/18. Re-use and distribution is strictly not permitted, except for Open Access articles.

School of Chemistry, Sambalpur University Jyoti Vihar, Burla 768019, Odisha, India y

Nano Research Centre, Sambalpur University Jyoti Vihar, Burla 768019, Odisha, India z [email protected]

Received 24 March 2018; Revised 3 May 2018; Accepted 4 May 2018; Published 8 June 2018 The surface hydroxylation treatment has been carried out by using hydrogen peroxide (H2O2) to modify the surface of Na0:5 Bi0:5 TiO3 (NBT) particles in a ferroelectric polymer (PVDF) via solution casting technique. The FTIR study confirms the presence of hydroxyl groups on the surface of NBT. The FE-SEM analysis reveals that h-NBT particles are dispersed homogeneously within the polymer matrix. The surface hydroxylation treatment plays an important role in high dielectric constant and also reduced loss by conducting the material surface with OH functional groups. The prepared composite with 40 wt.% of h-NBT showed enhanced dielectric constant ( … 114), negligible loss (0.22) and high AC conductivity as compared to that of the unmodified NBT. Such significant enhancement in dielectric properties may be due to the strong interaction between h-NBT particles and PVDF matrix at the interface. The percolation theory is used to explain the dielectric properties of h-NBT-PVDF composite. Furthermore, the remnant polarization of the un-poled h-NBT-PVDF composites (2 Pr–1.19 C/cm2 for 40 wt.% of h-NBT) is also improved. The present findings give an idea of high dielectric constant and relatively low loss composite materials as a promising candidate for electronic and energy storage devices. Keywords: Poly(vinylidene fluoride); dielectric properties; surface hydroxylation.

1. Introduction In recent years, polymer-based ceramic composites have received considerable interest in academic research and industry due to their good process ability and flexibility.1,2 The combination of polymer/ceramics gets benefitted from both the ends for the development of new composite materials with high dielectric constant and is widely used in energystorage devices and bypass capacitors in microelectronics.2,3 Nowadays, various research groups are trying to replace leadbased materials by lead-free materials due to their environmental impact. The lead-free ferroelectric materials (NaBi0:5 Ti0:5 O3) (NBT) have interesting properties such as higher Curie temperature and excellent dielectric characteristics.4 Generally, ferroelectric ceramics have relatively high dielectric constant, they are brittle in nature and requires sintering at high temperature, whereas the polymers possess low dielectric constant values (< 10), flexibility, ease of processing and low cost.5,6 Polymer-based ceramic composites are prepared by dispersing ceramic particles into the polymer matrix to form a new route in combining the advantages of polymers and ceramics, and represent a novel type of flexible materials with relatively high dielectric

constant and high breakdown strength.5–7 According to the literature so far, much work has been carried out in the development of the polymer-ceramic composites.8–14 The most common widely used ceramic particles are BaTiO3,8–10 SrTiO3,11 Ba0:55 Sr0:45 TiO312,13 and Pb(ZrTi)O3.14 In order to achieve high dielectric constant of polymer-based ceramic systems, the weight percentage of ceramic particle is typical (> 50 wt.%), Such a high weight percentage of ceramics brings out a number of shortcomings such as high weight, poor dispersion and aggregation of particles in a polymer matrix.6,15 To improve their dispersion of ceramic particles, a few types of surface treatment are carried out on the surface of the ceramic particles into the polymer matrix.16–18 In the present investigation, we prepared homogeneous composites consisting of surface hydroxylated NBT particles with ferroelectric polymer (PVDF) matrix, which exhibit high dielectric constant and relatively low dielectric loss. However, for the ceramic particles, most of the organic modification agents are only adsorbed on the surface of the particles by van der Waals forces or electrostatic forces due to the lack of reactive functional groups. Hydroxylation is an effective way of improving the surface reactivity of the particles.16,19–22

This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited.

1850017-1


S. Moharana, S. Sai & R. N. Mahaling

J. Adv. Dielect. 8, 1850017 (2018)

In addition, there have been many studies on the hydroxyl groups of ceramic particles with better compatibility with the PVDF-based polymers. To the best our knowledge, this study concerning the preparation of improved dielectric properties of surface hydroxylated NBT-based PVDF composites has not been reported in the literature so far. The NBT particles were treated with hydrogen peroxide (H2O2) to produce OH groups on the surfaces of ceramic particles. The modified NBT (h-NBT) and unmodified NBT-based PVDF composite films were prepared and modified by (OH) NBT which was evaluated using FTIR. Further, the dielectric properties of unmodified (NBT)-PVDF and h-NBT-PVDF composite films were investigated using an impedance analyzer at various frequency ranges.

2.4. Preparation of NBT-PVDF composite ﬁlms

2. Experimental

3. Characterization

2.1. Materials

The X-ray diffraction [XRD, (Mini Flex II, Rigaku, Japan)] measurement on the prepared NBT ceramics was performed using Cu Kα (λ ¼ 0:15405 nm) radiation. The morphology of the composite films was characterized using a field emission scanning electron microscopy (FE-SEM, Carl Zeiss SUPRA). Fourier-transform infrared spectroscopy (FTIR) was measured using a 5700 FTIR (Nicolet). The melting behavior of composites was investigated by a differential scanning calorimeter (DSC, PT-100, LINSEIS). The dielectric properties of the composite films were measured using an impedance analyzer (HIOKI 3532 LCR HiTESTER) at a frequency range (100 Hz–1 MHz) at room temperature.

PVDF polymer was purchased from Himedia Laboratories Pvt. Ltd, India. Sodium nitrate, bismuth nitrate, tetrabutyl titanate and Ethylene diamine tetra acetic acid (EDTA) (Merck, India) were used as a precursor. The aqueous solution of H2O2 (30%) and N, N-dimethylformamide (DMF) was obtained from Merck, India. All the chemicals were used without any further purification.

2.2. Preparation of Na0:5 Bi0:5 TiO3 (NBT ) particles The (Na0:5 Bi0:5 TiO3: NBT) particles were synthesized with EDTA precursor-based chemical route. First, an aqueous solution of EDTA was prepared by dissolving it in hot water with dropwise addition of dilute NH4OH solution. After complete dissolution of EDTA, the solution was boiled to remove excess NH3. Then, appropriate amounts of sodium nitrate, bismuth nitrate and tetrabutyl titanate were added to EDTA solution. The pH of the solution was found to be 6 and stirred for 1 h at room temperature using a magnetic stirrer. Subsequent to this process, the reaction mixture was evaporated to dryness on a hot plate at 110 ○ C. This was then calcined in air for 2 h at specified temperatures from 850 ○ C to obtain NBT powders.

The NBT-PVDF composite film was prepared via solution casting technique. Firstly, the required amounts of NBT and PVDF powders were proportionally dispersed in N, N-DMF under vigorous stirring and ultra-sonicated for 2 h to form a stable suspension. Then, the NBT-DMF solution with different weight percentage of NBT was added to the PVDFDMF solution. The NBT-PVDF composite solution was again stirred and ultra-sonication for 1 h to achieve uniform dispersion. Then, the resulting mixture was cast onto a plane glass Petridis and dried under vacuum at 80 ○ C for 12 h to remove the organic solvent of DMF. Scheme 1 shows the schematic illustration of the fabrication of h-NBT-PVDF composites.

4. Result and Discussions 4.1. Structural and morphological studies Figure 1 shows the XRD patterns of the Na0:5 Bi0:5 TiO3 ceramic fillers prepared via EDTA precursor routes. It is observed that eight diffraction patterns at 2θ ¼ 23 ○ ; 32 ○ ; 40 ○ ; 47 ○ ; 54 ○ ; 59 ○ ; 68 ○ and 78 ○ correspond to (100), (110),

2.3. Surface hydroxylation of Na0:5 Bi0:5 TiO3 (NBT ) particles The NBT particles (2 g) were dispersed into an aqueous solution of H2O2 (30 wt.%) in a round bottomed flask and the mixture was sonicated for 30 min. The resultant solution was refluxed at 105 ○ C for 4 h and the mixture was collected by centrifugation. The obtained mixture was washed with distilled water and then dried under vacuum at 80 ○ C for 12 h. The product of the hydroxylated NBT particles was named as h-NBT. 1850017-2

Fig. 1. XRD pattern of pure NBT particles.



J. Adv. Dielect. 8, 1850017 (2018)

(a)

(b)

Fig. 2. FTIR spectra of (a) pure NBT and (b) h-NBT particles.

(111), (200), (210), (211), (220) and (310) are characteristic peaks of NBT and no other intermediate phase is found. Moreover, the strongest diffraction peak at 32.5 ○ corresponds to the (110) plane of NBT.23,24 Figure 2 shows the FTIR spectra of the unmodified NBT and h-NBT particles. In the FTIR spectra (Fig. 2(a)), the appearance of symmetric stretching vibration band at 1453 cm 1 is due to C–O of EDTA which is used as a precursor. On the other hand, the h-NBT particles (Fig. 2(b)) the absorption band at 3485 cm 1 is associated to the stretching mode of O–H groups, which confirms hydroxylation at the surface of NBT particles. Moreover, the band at 1453 cm 1 disappears, after hydroxylation. We also noticed that the Ti–O bond vibration of NBT indicates the characteristic absorption band at 590 and 533 cm 1 confirms M–O bond.25 SEM and FESEM are used to examine the morphological structure and dispersion of NBT particles in the polymer matrix. Figure 3 provides SEM and FESEM images of the pure NBT and composite films containing 10 and 40 wt.% of unmodified and modified NBT contents. It is observed that in case of NBT-PVDF composites, there is much smaller aggregation which is clearly seen in Figs. 3(b) and 3(c), respectively. However, the h-NBT particles are well dispersed in the PVDF matrix and also small voids are seen in the lower weight percentage of h-NBT [Fig. 3(d)]. Interestingly, the higher weight percentage of h-NBT (40 wt.%) [Fig. 3(e)] shows uniform dispersion in the polymer matrix with minimum porosity due to the molecule interaction of NBT and PVDF. This result shows that the surface hydroxylated h-NBT particles not only disperse in the PVDF matrix, but also have strong interaction between PVDF matrixes by hydroxyl bonds in the NBT particles, which is clearly visualized in the form of a fibrillar structure. Furthermore, the formation of hydrogen bonds between the h-NBT particles and PVDF matrix facilitates the dispersive

nature and pronounces the enhancement of dielectric performance of the resultant composite systems. As per the scheme, Fig. 4, functionalization leads to the introduction of OH groups on the surface of NBT (h-NBT), with the addition of h-NBT particles into PVDF matrix, hydrogen bond formed between the F atoms of PVDF molecular chains and the OH groups of the h-NBT particles and get dispersed uniformly in the polymer matrix.19,26,27 The schematic illustration of the distribution of different concentration of NBT and h-NBT particles in PVDF matrix is shown in Fig. 5. As shown in Fig. 5, the PVDF matrix network structures are blocked and discontinued by the NBT fillers, whereas the h-NBT particles are uniformly dispersed in the PVDF matrix without any kinds of agglomeration and particles get engulfed by the polymeric matrix. 4.2. Thermal properties of NBT-PVDF composites The thermal properties of pure PVDF and NBT-PVDF composites with 20 and 40 wt.% modified and unmodified fillers were investigated by using DSC, respectively. Figure 6 shows the DSC thermo grams under heating and cooling conditions of respective composites. It is observed that there is one endothermic peak in the range of 150–180 ○ C for NBT and h-NBT used composites, in the DSC curve. However, it is clearly revealed that in case of h-NBT-PVDF composites, the peak is slight shifted into higher temperature than that of the unmodified one, which may be attributed to the hydroxyl groups on the surface of NBT. Further, the endothermic peaks appear in the region of 150–180 ○ C, which correspond to the melting behavior of PVDF. However, there is one exothermic peak in the range of 110–130 ○ C, which represents the degradation process of modifier from NBT surfaces.

1850017-3



J. Adv. Dielect. 8, 1850017 (2018)

(a)

(b)

(c)

(d)

(e) Fig. 3. (a) SEM images of pure NBT particles, FE-SEM images of NBT-PVDF, (b) 10 wt.%, (c) 40 wt.% and h-NBT-PVDF composite films with h-NBT contents (d) 10 wt.% and (e) 40 wt.%.

The crystallinity content (Xc ) of NBT-PVDF composites can be calculated by using the following relation: Xc ¼

ΔHm 100; Δ H m○ w

where Δ Hm is the melting enthalpy of the composites (Jg 1 ), Δ H m○ ¼ 104:7 J/g is the melting enthalpy of the 100% crystalline PVDF28,29 and w is the weight fraction of PVDF in the composites. The value of Xc , melting temperature (Tm) and

crystallization temperature (Tc) are listed in Table 1. It can be seen that the melting temperature and crystallization temperature gradually shift into the higher temperature in the composites by an increase in the filler content. Moreover, it is observed that the crystallinity percentage of h-NBT-PVDF composites is higher than that of the unmodified one. These observations indicate the addition of hydroxyl groups onto the surface of NBT and also some influence on the crystallization process of PVDF. The presence of OH on the

1850017-4



J. Adv. Dielect. 8, 1850017 (2018)

Fig. 4. Schematic diagrams of the hydroxylation of NBT particles and formation of hydrogen bond in h-NBT-PVDF composites.

Fig. 5. Schematic illustration of the distribution of NBT particles in PVDF matrix at (a) PVDF, (b) lower concentration of NBT fillers (10 wt.%), (c) higher concentration of NBT fillers (40 wt.%) and (d) schematic images of the composite for NBT fillers, interface and PVDF matrix.

(a)

(b)

Fig. 6. DSC thermo-grams under curves (a) heating and (b) cooling conditions of pure PVDF, NBT-PVDF and h-NBT-PVDF composite films at different filler loading. 1850017-5


J. Adv. Dielect. 8, 1850017 (2018)

Table 1. Tm, Tc and Xc values of PVDF, NBT-PVDF and h-NBT-PVDF composite film at 20 and 40 wt.% of filler loadings. Sample


Neat PVDF NBT-PVDF (20 wt.%) NBT-PVDF (40 wt.%) h-NBT-PVDF (20 wt.%) h-NBT-PVDF (40 wt.%)

Tm ( ○ C)

Tc ( ○ C)

Xc (%)

153.49 153.49 151.85 155.13 154.48

123.59 123.44 122.39 122.84 122.39

25.46 27.21 27.83 37.69 33.75

surface of NBT, which is capable of being attached with F atom of PVDF, results in entanglement. The formation of Hbonding confined the chain mobility of PVDF and free volume. The shifting of peaks towards higher temperature side refers to higher melting behavior with higher crystallite size. This may be of clear resemblance towards restriction of chain mobility in the melting zone with reference to lesser ΔH value. So, it’s a concern of lesser volume change. The abovementioned facts may be the reason why shifting of peak position takes place along with percentage of crystallinity. 4.3. Dielectric and electrical properties of NBT-PVDF composite ﬁlms The frequency dependence of dielectric constant and dielectric loss of the modified h-NBT-PVDF composites with the different weight percentage of h-NBT is shown in Fig. 7. As shown in Fig. 7(a), it is observed that the dielectric constant increases slowly with an increase in the h-NBT content and decreases with an increase in the frequency for all the composites. The dielectric constant of h-NBT-PVDF composite film can reach up to 114 at 100 Hz with a filler loading of 40 wt.% of h-NBT, which is nearly 19 times higher than that of the pure PVDF. It can be clearly observed that the dielectric constant of the composites at lower frequency

region is always greater than that of the higher frequency region. The frequency-dependent dielectric constant of the composite films can be explained by different types of polarization mechanisms with the application of electric fields such as electronic, ionic, dipolar and interfacial polarization.30,31 It is found that for all composites, the dielectric constant decreases in the low frequency region, and then becomes more or less constant at higher frequency regions representing dielectric dispersion of the materials. The dielectric performance at lower frequency region may be attributed to the interfacial polarization.32,33 Moreover, in the high frequency region, h-NBT particle shows a large dielectric relaxation and reduced dielectric constant values. The dielectric constant value increases with an increase in the filler content and results in higher polarization with the higher filler loading.10 Furthermore, the interface between the functionalized ceramic particles and polymer matrix plays a crucial role in the improvement of the dielectric performance of the composites.34 The addition of functionalized ceramic particles into the PVDF matrix results in chemical linkage, which leads to the enhancement of dielectric constant values, i.e., hydrogen bonding between the F atom of PVDF and OH groups of the NBT particles. Figure 7(b) shows the dielectric loss of h-NBT-PVDF composite films as a function of frequency at room temperature. The dielectric loss is the energy dissipation from the movement of rotation of the molecules by applying alternating electric field.34 It is observed that the dielectric loss plot of the composites shows similar features as compared to that of dielectric constant, as shown in Fig. 7(a). In the lowfrequency region, the value of dielectric loss is relatively high, while it has lower values in the higher-frequency regions for all weight percentage of filler loading and pure polymers. The increase in dielectric loss at low-frequency region (from 102 to 103 Hz) may be due to the relaxation mechanism present in the polymer matrix.35–37 The increase

(a)

(b)

Fig. 7. Frequency dependence of (a) dielectric constant and (b) dielectric loss of h-NBT-PVDF composite films at room temperature. 1850017-6


J. Adv. Dielect. 8, 1850017 (2018)

Table 2. Comparison of dielectric properties of h-NBT-PVDF and NBT-PVDF composite films (102 Hz).

Sample


NBT-PVDF NBT-PVDF NBT-PVDF NBT-PVDF

(10 wt.%) (20 wt.%) (30 wt.%) (40 wt.%)

Dielectric constant ("r ) (h-NBT/ unmodified)

Dielectric loss (tan δ) (h-NBT/ unmodified)

46.12/30.09 62.25/62.25 91.16/80.70 114.33/91.60

0.08/0.15 0.12/0.18 0.14/0.26 0.22/0.31

in dielectric loss with increase in h-NBT content can be explained on the basis of polarization of the sample. However, the composites with 10 and 20 wt.% of h-NBT do not follow the trend of other composite systems with the frequency region from 102 to 106 Hz with the increase of h-NBT content. This phenomenon arises due to the formation of voids and pores and thus such behavior referred to the porous nature of the composites.38,39 Further, the dielectric loss increases with increase in frequency, which is a typical feature of the glass transition relaxation of PVDF and it is attributed to α a relaxation associated with molecular motion in the crystalline regions of PVDF.40,41 Moreover, the h-NBTPVDF composites show relatively high dielectric constant (114 at 100 Hz) with relatively low loss (0.22) as compared to that of the unmodified NBT-PVDF composite (Table 2), which is the foundation of a material for use in devices for energy storage. For example, the dielectric loss of composite film is 0.22 at 100 Hz with a loading of 40 wt.% of h-NBT contents. Meanwhile, in unmodified NBT-PVDF composite films, the NBT particles are agglomerated in the polymer matrix with uneven distribution. Furthermore, the dielectric loss of the composite decreases (