Ferroelectric, Piezoelectric and Dielectric Properties of Novel Polymer Nanocomposites

In this chapter, the Ferroelectric, Piezoelectric and Dielectric behavior of novel polymer/ceramic nano-composite (PCC) based on ferroelectric polymer [polyvinyledene fluoride (PVDF)] & nano Barium Titanate (n-BaTiO3) with different volume fractions of n-BaTiO3 (fBaTiO3), prepared through the novel cold pressing method has been discussed. The ferroelectric parameters of PCC are attributed to spherulites of PVDF, the increase of n-BaTiO3 and the ordered homogenous structure due to the novel cold pressing. The clustering of ceramic fillers is responsible for randomization of the structures of these composite ferroelectrics for some samples, leading to decrease of electrical polarisations. The piezoelectricity and piezoelectric coefficients of these composites ferroelectrics, increases with increase of ceramic filer content and remains constant beyond a certain ratio. However, the dielectric properties increase linearly as a function of ceramic content due to increase of interfaces/interfacial polarisations. The enhancement of effective dielectric constant (ɛeff) is attributed to the large interfacial polarization arising due to the charge storage at the spherulites of PVDF and at the polymer/filler interfaces of PCC and have been explained on the basis of sum effect with the help of the standard models. The achieved lower loss tangent (Tan δ) for the PCC as compared to the polymer/metal composites (PMC) is attributed to the highly insulating nature of PVDF & semiconducting n-BaTiO3. The thermal stability of the composites is also maintained due to the higher melting temperature (170°C) of PVDF. The cold pressed PCC based on PVDF are going to act as better polymer ferroelectric/dielectrics for memory and electrical energy storage applications.

To overcome these problems, PF are undergoing development based on ferroelectric polymer as well as ferroelectric ceramics. Among the ceramic fillers, BaTiO3 is a very good ferroelectric material and comparably better as others have harmful lead content [16,17]. Among the various polymers, very few polymers, such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride trifluoroethylene [PVDF Tr(FE)] and Teflon show ferroelectric/piezoelectric behavior and have high dielectric constants [18,19]. They are also of high breakdown strength, lightweight, flexible and having permanent dipolar polarization. Among the ferroelectric polymers, PVDF shows high piezo-electric coefficients, good ferroelectric behavior. Due to these advantages, PVDF is used as piezo-electric sensors/actuators/memory devices. But the major problem with PVDF is that, the magnitudes of ferroelectric parameters aren't as good as the parameters, obtained from the conventional ceramics, which limit them from direct applications. Similarly the PD are having higher flexibility, non-toxicity, bio-compatibility, low cost, higher visco-elastic properties, etc. [20][21][22][23][24][25][26][27][28][29][30][31]. Due to their higher energy density/lower loss tangent (Tan δ) with higher breakdown field strength, they are going to be the emerging materials of future for electrostatic energy storage applications. The PD composed of polymers with conductor/ceramic nanoparticles are considered recently to be the demanded materials for electrical energy storage applications [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. For energy storage applications, the maximum stored energy per unit volume is U K E ε = 2 0 max 1 2 , where K is the relative dielectric constant and E max is the maximum electric field, which can be applied to the material (proportional to the breakdown field of the material). Over the last 20 years of research, the ferroelectric polymers e.g PVDF matrix also have been preferred due to it's high static dielectric constant (~15)/higher visco-elastic properties/higher insulating nature as compared to other non-polar polymers. The preferred fillers are high dielectric constant ferroelectric ceramics in development of these PD. However, the development of the polymer-ceramic composites (PCC) have been slowed down, as the effective dielectric constant (ε eff ) for these composites were found be very low i.e. ε eff ~ 100 at low frequencies due to the low dielectric constant of the polymers as well as due to the conventional hot molding process conditions. In preparing these PCC, ferroelectric ceramic, such as, PMN-PT, BaTiO3, PbTiO3, etc. with varying particle size are introduced into the PVDF matrix through hot molding and partially the approach becomes effective in order to get better PD [5][6][7][8][9][10]. For preparing PCC based on PVDF, the traditional mixed technique (solution casting followed by hot molding) is used, during which the spherulites of PVDF get lost [2,33,34], which lowers the value of ε eff . Recently Panda et al. [23-30, 32, 33] has shown the importance of spherulites by following the cold pressing technique in preparing the PD based on PVDF, due to which the spherulites of PVDF are retained. The spherulites are responsible in the additional storage of electrical charge due to their additional interfaces, resulting higher interfacial polarization/ higher value of ε eff [2,33,34].
With the objective of achieving flexibility with low cost/easy processing and higher value of electrical parameters, for device applications, the traditional process condition (hot molding of the thick films prepared from solution casting) is changed to cold pressing developed by our group in which the spherulites of PVDF will be retained for the case of PCC. Hence, PCC of good ferroelectric/ piezoelectric/dielectric properties, are developed from good ferroelectric ceramics/good ferroelectric polymers. Since in the PZT/PVDF composites, lead is a toxic component, hence the PCC based on PVDF/n-BaTiO 3 , with different volume fraction of n-BaTiO 3 (f n-BaTiO3 ), were prepared with the help of cold pressing method. The prepared composites have shown the interesting ferroelectric/piezoelectric/ dielectric/conductivity properties and finds suitability for various applications.

Experimental details
Polymer composite based on PVDF/n-BaTiO3 from 0.0 to 0.60 of volume fraction of nano filler n-BaTiO3 (f BaTiO3 ) were prepared by mechanical hand mixing with Agate mortar/Pestle for 2 hours. The final pellets under room temperature consolidation at a pressure of 30 MPa with the help of a Hydraulic press [2,33] were prepared. The microstructure investigation on the samples was carried out with the help of FESEM. The ferroelectric hysteresis, i.e. the polarization versus electric field (P ~ E) measurement, is done with the help of a P ~ E hysteresis loop tracer. The piezoelectric coefficient (d 33 ) for the samples is measured with the help of Piezoelectric measurement instrument. The electrical measurements were made on all the PCC in the frequency range of 50 Hz to 5 MHz and in the temperature range of room temperature to 100 C. The dielectric results of the PCC have been understood by fitting with the help of the software Mathematica.

Microstructure
The FESEM micrographs of pure PVDF are given in Figure 1a and b. Figure 1a and b shows the presence of spherulites (the spherical semi-crystalline regions of the polymer). The micrographs of PVDF/n-BaTiO3 composites with different f BaTiO3 = 0.2 and f BaTiO3 = 0.60 are shown (Figure 1c-f). The ordered homogenous structures are also observable and is attributed to the recent novel method of cold pressing as evident from the sample with f BaTiO3 = 0.2 (Figure 1c and d). The spherulites present in the polymer are of diameter of the order of ~0.1 μm (Figure 1a and b). The n-BaTiO3 are also of the order of diameter of the spherulites as they are of size 100 nm, i.e. 0.1 μm, During cold pressing, the n-BaTiO3 clusters (Figure 1c and d) inside the polymer matrix, may have taken the typical shapes. For the sample with f BaTiO3 = 0.6 (Figure 1e and f), shows high level of heterogeneity, as lot of defects and dislocations has emerged in the structure and is responsible for giving a decrease in the ferroelectric & piezoelectric properties of the PF. Figure 1 reveal slight agglomeration of BaTiO3 nanoparticles in the nanocomposites. The average filler size in the nanocomposites are of ~100 nm. The nano-dispersion of filler in the polymer matrix is well observed. It is also obvious that a variety of interfaces have occurred into the composites, which will be always useful in the storage of electrical charge at the interfaces The large amount of n-BaTiO3 into the PCC will also be responsible for giving better ferroelectric/piezoelectric/dielectric properties [2,[34][35][36].

Ferroelectric hysteresis
The ferroelectric properties of the PF, prepared from PVDF/n-BaTiO3, the polarization versus electric field (P ~ E) of both the pure materials are given (Figure 2) at different voltages.  (Figure 1a-d). On the other hand, beyond f BaTiO3 = 0.30, the heterogeneity/disordered structure is accountable for the decrement of ferroelectric properties and that is also accredited to the clustering of n-BaTiO3 into the polymer medium (Figure 1e and f). It is also experiential that with rising the field, the saturation polarization (P s ), remnant polarization (P r ) and the coercive field (E c ) also increases as a function of f BaTiO3 and the finest result is obtained for the sample with f BaTiO3 = 0.3.
To have a thorough analysis and cross examination of the ferroelectric behavior of the PF, the P-E hysteresis loop of all the samples as a function of f BaTiO3 for different fields from 5 kV/cm to 8 kV/cm is shown in Figure 4. At all the fields, the ferroelectric behavior from the P-E hysteresis loops, are characterized by the change of P r , P s and E c .  The experimental observation is that with increasing of the f BaTiO3 in the PF, the P r , P s and E c also increases. This obviously indicates that the addition of n-BaTiO3 enhances the ferroelectric nature of the polymer material. But when the amount of n-BaTiO3 filler content improved, there is trivial agglomeration of filler in the PVDF matrix. The agglomeration of n-BaTiO3 act as hindrances, which eliminate PVDF Polymer from flowing into the BaTiO3 agglomerates and the aggregated filler causes poor enhancement in ferroelectric nature (decrement in   (Figure 5), i.e. the static ε eff enhances from 10 for pure PVDF to 400 for f BaTiO3 = 0.6, whereas the loss tangent increases from 0.09 for pure PVDF to 0.9 for f BaTiO3 = 0.6 [14]. The variation in the dielectric and ferroelectric behavior is credited to the different types of structures responsible for the two altered electrical properties respectively. The dielectric properties are connected with the more interfaces in the PF, hence ε eff & Tan δ enhances linearly with f BaTiO3 and the ferroelectric properties are associated with the ordered structure of the PF. Figure 6 give you an idea about the variation of P s as a function of f BaTiO3 for all the PF, at changed electric fields from 5 kV/cm to 8 kV/cm. It is practical that on increasing the concentration of n-BaTiO3, and also on rising the electric fields, the value of P s raises up to f BaTiO3 = 0.30, but for f BaTiO3 > 0.30, P s decreases, due to the aggregation of n-BaTiO3 causing poor improvement of the ferroelectric properties. At 5 kV/cm, the value of P s increases from 0.1 μC/cm 2 for f BaTiO3 = 0.2 up to 0.24 μC/ cm 2 for f BaTiO3 = 0.30, but when f BaTiO3 > 0.30, P s decreases to 0.24 μC/cm 2 .

P r ~ f BaTiO3 for different fields
The variation of P r as a function of f BaTiO3 for all the PF (Figure 7), at different electric fields from 5 kV/cm to 8 kV/cm. It can be seen that on increasing the concentration of n-BaTiO3, P r increases up to f BaTiO3 = 0.30, but for f BaTiO3 > 0.30,

(color online) polarization (P) vs. applied electric field (E) hysteresis loop measured at a frequency of 1 Hz with different f BaTiO3 for different fields (a) 5 kV/cm (b) 6 kV/cm (c) 7 kV/cm (d) 8 kV/cm.
Ferroelectric, Piezoelectric and Dielectric Properties of Novel Polymer Nanocomposites DOI: http://dx.doi.org/10.5772/intechopen.96593 P r decreases, i.e. a parallel behavior as observed in the case of variation of P s ~ f BaTiO3 , is also observed which is attributed to the same origin. At 8 kV/cm, P r increases from 0.10 μC/cm 2 for f BaTiO3 = 0.2 up to 0.20 μC/cm 2 for f BaTiO3 = 0.30, but for f BaTiO3 > 0.30, P r decreases and approaches to much less than 0.20 μC/cm 2 .

E c ~ f BaTiO3 for different fields
The variation of E c as a function of f BaTiO3 for all the PF, at different electric fields from 5 kV/cm to 8 kV/cm is shown in Figure 8. The value of E c is preserved higher up to f BaTiO3 = 0.30, but for f BaTiO3 > 0.30, E c decreases, i.e. a similar conduct as observed in the case of variation of P s ~ f BaTiO3 , is also observed endorsed to the same origin. At 6 kV/cm, the value of E c increases from 2.5 kV/cm for f BaTiO3 = 0.2 up to 3.5 kV/cm for f BaTiO3 = 0.30, but for f BaTiO3 > 0.30, E c decreases and becomes much less than 3.5 kV/cm.

Dielectric properties
The variation of dielectric properties of the PCC as a function of frequency at 300 K are shown in Figure 10a (Figure 11).
The static dielectric constant (ε r ) of the cold pressed pure PVDF is ~16 i.e. higher than the ε r of hot molded pure PVDF (~10) due to the loss of spherulites (Inset, Figure 11) of the polymer. ɛ eff decreases with increase of frequency due to the absence of contribution from interfacial polarization at higher frequencies, where only the involvement from dipolar and atomic polarization exists. A very low Tan δ for the PCC with f BaTiO3 = 0.6 (having highest ɛ eff = 420 observed at 50 Hz) was approached to 0.9 at 50 Hz and that also decreases with increase of frequency and the tendency of decrement is also observed for all the PCC. Nevertheless, in the cold pressed PMC [33], the Tan δ was reported to be 10 at 50 Hz (Inset, Figure 10b) for the percolative sample, with ɛ eff ~ 2000. The PMC at f c shows 10 times higher value of Tan δ in contrast to the result of PCC, although both type of polymer composites are prepared by the same cold pressing procedure. Figure 12 shows thebehaviour of ɛ eff , σ eff and Tan δ of the composites as a function of f BaTiO3 at different frequencies.
The ɛ eff rises linearly from 16 to 120 for f BaTiO3 rises from 0.0 to 0.50 at 100 Hz, due to the large interfacial polarization occurring due to the presence of spherulites. The interfaces formed at the PCC, increases ɛ eff largely from 120 to 350 & 420 for f BaTiO3 = 0.55 & 0.60 respectively. The expression developed by Yamada et al. (which is a model for explaining the sum properties of the composite) was fitted to the dielectric data (Figure 12(b)) at 1 kHz frequency. The model is given by where eff ε is the effective dielectric constant of the composite, PVDF K and BaTiO K 3 are the dielectric constants of the polymer matrix and the ceramic, respectively, BaTiO f 3 is the volume fraction of the ceramic and 'n' is a parameter related to the geometry of ceramic particles [2]. PVDF K , BaTiO K 3 and n found from the fitting of Eq. (1) to the dielectric data are 17, 1600 and 10, is in good agreement with the earlier literature [5].
The σ eff & Tan δ increases with the increase of BaTiO f 3 in the PCC slowly, suggesting the semiconducting nature of the BaTiO3 nano-ceramics. For BaTiO f 3 =0.6, the σ eff value varies within 10 −8 Ω −1 cm −1 to 10 −4 Ω −1 cm −1 for frequency varying between 50 Hz to 5 MHz, while the value of Tan δ is maintained in between 0.1 to 0.9.  σ eff & Tan δ are also found to be increasing with increase of frequency, suggesting conventional hopping conduction in the disordered PCC. Similarly, σ eff value was found to be very low i.e. less than 10 −4 Ω −1 for all the PCC and that value remains constant over the entire frequency range. The Tan δ raises slowly as a function of f BaTiO3 is found to be less than 0.9 even with the PCC having f BaTiO3 = 0.6.
The electrical parameters as a function of temperature of the PCC was confirmed by measuring and are given in Figure 13. For f BaTiO3 = 0.4 (Figure 13a) & f BaTiO3 = 0.50 (Figure 13b), the low frequency (50 Hz) value of ɛ eff is sustained at a thermal stabilized value of 90 & 130 (with their corresponding decrement as a function of frequency) as a function of temperature from 40-100°C. The stabilization of ɛ eff is ascribed due to the major effective contribution coming from the sum properties of the dielectric constant of both the components. Yet, for the samples with f BaTiO3 = 0.55 &0.6, the reached ɛ eff ~ 350 & 420 value arises due to the sum properties of the dielectric constant of both the components as well as also due to the major contribution of the spherulites. Hence with the rise of temperature, the ɛ eff decreases due to the deteriorating of the spherulites of the PCC (Figure 13c and d). Hence the spherulites are useful at room temperature in case of PCC for realizing high value of ɛ eff with lower Tan δ.

Electrical conductivity
Ac conductivity ( ac Tan σ ωε ε δ = 0 ) as a function of frequency at different f BaTiO3 is shown in Figure 14. The σ eff as a function of frequency was found to be the ac hoping conduction satisfying the Johnscher's fractional power law. The plot shows dispersion of ac conductivity with frequency corresponding to f BaTiO3 ≤ 0.20, be in agreement with Eq. (2) i.e.

( )
with the σ dc part becoming zero and the value of k ~ 1. The non-presence of dc conductivity for the samples with f BaTiO3 ≤ 0.20, can be understand as the nonpresence of percolating paths (being formed from the semiconducting BaTiO3 nano-ceramics in the PVDF matrix) due to insufficient fraction of BaTiO3. The long rage dc conduction starts to develop for f BaTiO3 = 0.3 to 0.5, but a good fit of Eq. (2) could not be resulted for them as the percolating paths were not sufficient. Interestingly, for f BaTiO3 ≥ 0.55, a mixed conductivity is found. The plateau due to the appearance of long range dc conductivity. At higher frequency the conductivity becomes more or less with f BaTiO3 dependent. This "hopping or critical frequency ω H ." at which the change in slope takes place can be observed to be increasing with the increase of f BaTiO3 , since the length of dc plateau increases with increase of f BaTiO3 from 0.3 to 0.6. On the other hand, the value of 'k' lies well within the Johnscher's universal regime [0,1] symptomatic of the validity of Johnscher's power law universally.

Conclusions
The micro-structural, ferroelectric, piezoelectric, dielectric and conductivity properties of the polymer composites have been analyzed and are correlated. The properties strongly depend on the novel cold pressing preparation techniques and the dispersion of n-BaTiO3 filler particles into the PVDF matrix and also on the nano-sizes of ceramics. The addition of n-BaTiO3 enhances the ferroelectric, piezoelectric and dielectric properties of the composites. It is also found that this cold pressing method is more suitable to the PCC based on PVDF matrix (since very low value of Tanδ is observed). The spherulites present in PVDF matrix are always helpful in maintaining the dielectric constant and increasing the ɛ eff of PCC. The enhancement of dielectric results are explained with the help of standard Yamada model. The mixed conductivity appears for the PD/PF and Jonschers universal fractional power law is well satisfied for all composites. The hoping conduction in these disordered materials have been confirmed in all PCC. The dynamics of