Page 182 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 155

respectively. The permittivity was enhanced with temperature obtaining a value
of 190 at 100 Hz at 150°C, while at higher frequencies (100 kHz 1MHz), it
declined to 65 due to smaller thermal coefficient of permittivity. Various theoretical
models were employed, and the results showed that EMT and Yamada models were
well in coherence at higher volume fractions.
Yuan et al. synthesized high-energy-density nanocomposites with PANI-based
fillers at smaller filler loading [222]. The percolation threshold was 4.2 vol%, and
the dielectric constant and the dielectric loss showed very small variation with fre-
quency down the percolation threshold. The dielectric constant was enhanced fairly
with loading and acquired a maximum value of 385 with a loss tangent of 0.85 (at
1 kHz) at 5 vol%. However, the breakdown strength of 600 kV/cm was still acquired
in the region of 4 vol%<f PANI <5 vol%. Further, the high energy density of 6.1 J/cm 3
at 5 vol% was attributed to the β-phase of PVDF and the presence of an insulating
PVDF layer surrounding PANI.

Two-phase composites based on one-dimensional fillers
One-dimensional fillers have a great role for high-energy-density applications as they
produce higher dielectric constant and energy density at relatively lower loading as
compared with nanoparticles. Here, we will try to present various one-dimensional
conducting filler-based polymer nanocomposites. CNTs are one of the most attractive
one-dimensional fillers for high-energy-density capacitor applications. The introduc-
tion of CNTs in the polymer brings interesting variations in dielectric properties [223-
226]. Baji et al. designed well-aligned CNTs/PVDF nanocomposites using
electrospinning, which actually decreased any pores or voids in the matrix [227].
CNTs induced β-phase transformation from α-phase in the polymers, thereby enhanc-
ing the ferroelectricity of the polymer nanocomposites. The dielectric constant and the
loss tangent at 3 wt% loading were 16 and 0.01 (at 1 kHz), respectively. Although
dielectric constant and the dielectric loss did not show significant variation with load-
ing, the mechanical properties of the nanocomposites were improved significantly
The strength and stiffness of 3 wt% CNT-based nanocomposites were improved to
276 MPa and 1.64 GPa, respectively, in comparison with 236 MPa and 1.33 GPa
for pure PVDF with 80% β-crystal phase.
The relatively higher values for copper nanowire/PVDF nanocomposites at lower
frequencies were due to the improved interfacial polarization with a rise in the number
of mobile charge carriers. The loss tangents were also enhanced on increasing loading
due to ohmic and polarization losses associated with the creation of conductive net-
works and interfacial polarization, respectively. The loss tangents of MWCNT/PVDF
nanocomposites at 0.4, 0.8, and 1.5 vol% loadings were 0.5, 200, and 1100, whereas
for copper nanowire/PVDF, the corresponding values were 1.05, 1.2, and 2.5, respec-
tively (at 1 kHz). Therefore, copper nanowires present sign for commercial applica-
tions with certain further processing to reduce the dielectric loss and hence opened a
wide area of research in the this field. Yuan et al. synthesized MWCNT-based
nanofillers in PVDF matrix by melt mixing [196]. The fillers were fairly bonded to
the polymer surface by acceptor-donor interaction, generally through delocalization