Page 181 - Polymer-based Nanocomposites for Energy and Environmental Applications

154 Polymer-based Nanocomposites for Energy and Environmental Applications

decreased, it caused shrinkage of elastomer. Therefore, the Maxwell-Wagner-Sillars
interfacial polarization and space charge polarization were decreased and lead to
reduced remnant polarization and hence improved discharge energy density. Hu
et al. prepared novel three-phase nanocomposites by combining three different layers,
top and bottom layer with 30 vol% of TiO 2 nanoparticles (for achieving high electric
displacement) and middle layer with PDOPA-functionalized 0–5 vol% of 5 mol%
Bi 2 O 3 -doped Ba 0.3 Sr 0.7 TiO 3 nanofibers (for achieving high breakdown strength)
employing layer-by-layer tape casting approach [218]. The dielectric constant was
found to rise from 12.5 to 14.5 (at 1 kHz) on varying the content of the sandwiched
layer from 0 to 5 vol%. Besides, the loss tangent was first decreased from 0.05 to 0.03
at 0–1 vol% and thereafter did not vary with sandwich-layer concentration. Hence, the
middle layer had a positive influence on the breakdown strength of the studied poly-
mer nanocomposites. The breakdown strength of two-phase nanocomposites con-
taining 3 vol% nanofibers was 3850 kV/cm, which eventually decreased in case of
three-phase nanocomposites containing 30 vol% of TiO 2 , having a value of
3000 kV/cm. The enhanced breakdown strength in presence of the sandwiched layer
was attributed to the nanofibers present in the middle layer that acted as the barrier for
charge flow from the two sides. In addition, the discharged energy density of 3 vol%
3
sandwiched layer was also more for the three-phase nanocomposites (7.99 J/cm )as
3
compared with the two-phase nanocomposites ( 4.5 J/cm ).
The benefits of smaller filler loadings were increased flexibility of the
nanocomposites along with the lower dielectric loss. Thus, from the literature of non-
conducting nanoparticles and nanofibers, it can be suggested that exploiting the
dielectric properties of both gives improved dielectric properties in comparison with
two-phase nanocomposites. Moreover, PVDF-based polymer blends also showed
improved dielectric properties.

5.3.2.2 Polymer nanocomposites based on conducting fillers
Apart from the nonconducting fillers, conducting fillers, namely, graphene, Zn, Ag,
and Cu, have also been explored to be employed in the dielectric nanocomposites.

Two-phase composites based on spherical fillers
The conducting filler-based polymer nanocomposites containing spherical fillers
and PVDF as polymer matrix are discussed here. Wang et al. used titanium carbide
(TiC) as fillers[219] in view of that they have better thermal stability, resistance to
erosion, and good conductivity and are relatively inert to oxide formation. The
dielectric constants at 11.58 vol% loading were 540 and 150 at 100 Hz and
1 kHz, respectively. The strong dielectric dependence at smaller frequency was
due to surface charge reorientation. The loss tangent of 0.48 (at 100 Hz) was found
at the percolation threshold. Other fillers like copper titanium oxide (CaCu 3 Ti 4 O 12 )
arealsoinfocusduetotheir high dielectric constant (2500 at 10 kHz) and lower
toxicity [220]. Thomas et al. synthesized CaCu 3 Ti 4 O 12 (1000–7000 nm) fillers by
the conventional solid-state reaction process [221]. The dielectric constant and
the tangent loss at 55 vol% filler loading were 80 and 0.11 (at 1 kHz),