Page 185 - Polymer-based Nanocomposites for Energy and Environmental Applications

158 Polymer-based Nanocomposites for Energy and Environmental Applications

% (at 1 kHz). Different amounts of GNs (1–4 phr, where phr refers to pounds of GNs
added in 100 lb of PVDF) were added to a fixed weight ratio of 80/20 of BaTiO 3 /
PVDF. The percolation threshold for the GNs was observed to be 2.5 phr. The intro-
duction of GNs enhanced the dielectric constant up to 492.0 (at 1 kHz) at BaTiO 3 /GN/
PVDF¼20/3/80 with a loss tangent of 49.1, which was not suitable to use for high-
energy-density applications. Wang et al. explored BaTiO 3 and PANI-functionalized
GNs as fillers in the PVDF matrix [236]. For two-phase RGO/PVDF nanocomposites,
the permittivity and the loss tangent were 300, almost 30 times that of the pure PVDF
and 4, respectively, at the percolation threshold of 1.49 vol%. Also, the dielectric
constant was improved with BaTiO 3 content for a given RGO fraction. Like, the
dielectric constant was enhanced from 27 to 54 as the BaTiO 3 loading was
increased from 10 to 30 vol% for 0.63 vol% of RGO. Same way, at RGO loading
of 1.25 vol%, the dielectric constant was enhanced to 170 for 30 vol% BaTiO 3 loading
in comparison with 60 at 10 vol% loading. The loss tangent of three-phase
nanocomposite was found to be less than that of the two-phase nanocomposites,
namely, the loss tangent of RGO/PVDF was 0.7 (at 1 kHz) at an RGO loading of
1.40 vol%, while that of RGO/BaTiO 3 /PVDF was 0.25 (at 1 kHz) at RGO and
BaTiO 3 loadings of 1.25 and 30 vol%, respectively. Therefore, the dielectric proper-
ties could be tuned by changing the ratio of BaTiO 3 or RGO.
Hence, to acquire the optimized dielectric properties, a thorough knowledge
regarding the fillers, the matrix, and the preparation techniques is highly desired. From
this discussion, it could be realized that the dielectric properties of the three-phase
nanocomposites containing nanoparticles and nanofibers were improved as compared
with those of nanoparticles and nanofibers used individually. Therefore, a lot of effort
is imperative in the future to design three-phase dielectric nanocomposites and opti-
mize various dielectric parameters.

5.3.3 Titania-based polymer nanocomposites

Titanium dioxide (TiO 2 and TO) can be found in three crystalline forms at atmo-
spheric pressure, namely, anatase, rutile, and brookite. The rutile contains chains of
trans-edge-sharing TiO 6 octahedra, connected by sharing corners. Contrarily, the ana-
tase phase is a framework of distorted TiO 6 octahedra that share four edges. This struc-
tural difference is believed to be responsible for a higher dielectric permittivity in
rutile than in anatase phase. As mentioned earlier, it is believed that the introduction
of high-dielectric-permittivity fillers into relatively low dielectric permittivity poly-
mers may not be ideal to obtain an appreciable increase in the energy density of
the composite. This is based on the belief that if the filler has a much higher permit-
tivity than the polymer matrix and most of the increase in the effective dielectric per-
mittivity comes through an increase in the average field in the polymer matrix, then
very little of the energy will be accumulated in the high-permittivity filler phase [237].
Also, a huge difference in permittivity of the polymer and filler phases generates an
inhomogeneous electric field that substantially lowers the effective breakdown
strength of the nanocomposite. Thus, the titania got much attention as its average
dielectric permittivity of 47 is much closer to the permittivities of many polymer