Page 184 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 157

in parallel and perpendicular directions was found to be 176 and 57 (at 1 kHz), respec-
tively, at a filler loading of 21 vol%. The greater value for the parallel direction was
due to following reason: the dipole length in the parallel direction was more as com-
pared with that along the thickness. Therefore, polarization would be greater in the
parallel direction and hence resulted in enhanced permittivity. Moreover, the dielec-
tric loss and conductivity were improved with loading, although the highest dielectric
loss and conductivity were found to be 0.06 and <1 10 6 S/m, respectively. The loss
tangent (<0.06) is important for the realistic applications of the as-synthesized
advanced polymer composites. Many nanocomposites were synthesized with 0–3
or 1–3 type of dispersion, while other combinations were almost not even tried. Deng
et al. though prepared novel 2–3-type nanocomposites based on ZFs and compared the
results with 0–3-type PVDF-based nanocomposites [229]. The ZFs were prepared
from Zn powders by exploiting its ductility nature using a ball-milling synthesis
approach.
Graphene nitrides (GNs) have large electric conductivity and very high aspect
ratio, so percolation could be achieved at relatively smaller loading as compared with
that of the rolled CNTs fillers [198]. Fan et al. synthesized multilayered reduced GNs
with very low percolation thresholds using PVDF polymer matrix [230]. GNs were
synthesized by reduction of graphene oxide (GO) sheets using phenylhydrazine
and hydrazine hydrate as reducing agents. It was observed that the GNs obtained from
phenylhydrazine showed better dispersion stability in DMAC as compared with that in
hydrazine hydrate. This was attributed to the steric effect of covalently bonded phenyl
groups with the GNs. Thus, phenylhydrazine-based GNs were well chosen for the
dielectric study. The percolation threshold was 0.18 vol%, which was relatively very
small as compared with the other conducting fillers like MWCNTs and copper nano-
wires [231]. The dielectric constant and the loss tangent at the percolation threshold of
0.177 vol% were 180 and 0.98 (at 1 kHz) and further improved to 6000 and 4at
1.77 vol% (at 1 kHz). The larger dielectric loss of the nanocomposites with the
increase in loading was attributed to the generation of electric current, thereby gen-
erating thermal energy. Rahman et al. synthesized PVDF-based flexible
nanocomposites using GO and RGO as fillers [232]. It was found that the dielectric
constant and the dielectric loss of RGO/PVDF were relatively higher than those of
GO/PVDF and pure PVDF. The enhanced dielectric constant was due to a larger num-
ber of microcapacitors generated in the matrix.

Three-phase composites
As a step forward, there is also a possibility of improving the dielectric properties of
ceramic fillers by adding highly conducting materials, so that the dielectric properties
are enhanced at the percolation threshold of the metals. It was already known that
graphene is a 2-D planar molecule, known for its good conductivity and high mechan-
ical properties, which are widely explored in high-energy-density applications
[233,234]. Li et al. synthesized three-phase nanocomposites by doping BaTiO 3 /PVDF
with GNs using PVDF as polymer matrix [235]. In the case of BaTiO 3 /PVDF binary
composites, the dielectric constant was only 31.4 with a loss tangent of 0.026 at 60 wt