Page 180 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 153

up to 30 vol% and thereafter enhanced with loading, which was attributed to the pres-
2
ence of residual PVP at higher loading. Dc leakage current (1.8 10 4 A/m ) and dc
conductivity (1.9 10 10 S/m) were also maximum at 55 vol%. The breakdown
strength (3360 kV/cm at 10 vol% loading) was 38% higher than that of untreated
3
BaTiO 3 nanocomposite. The energy density was improved to 6.9 J/cm at 55 vol%,
mainly due to enhanced permittivity of the resulting PVDF nanocomposites.
Xie et al. synthesized core-satellite nanoassemblies comprising of BaTiO 3 and sil-
ver nanoparticles (<10 nm) [73]. There was an increase in breakdown strength and
decreased dielectric loss. However, permittivity was suppressed on addition of Ag
particles in comparison with the pristine PVDF. The dielectric constants of 20 vol
% BaTiO 3 with 1 and 0% Ag were found to be 16 and 20 (1 kHz), respectively,
while the loss tangents obtained for both was 0.02. The lower dielectric constant
obtained was due to the higher activation energy of PVDF 1%Ag-BaTiO 3 , which
indicated that higher energy was necessary for space charges to flow due to accumu-
lation of charges at the interface between different regions of PVDF. Low dielectric
loss was attributed to the quantum confinement effect, which suggested that Ag par-
ticles were not in the metal state at room temperature and gave rise to the coulomb
blockade effect [215]. The discharge energy densities of 20 vol% Ag@BaTiO 3 /PVDF
3
at 600 and 1000 kV/cm were found to be 0.335 and 0.83 J/cm , while that of BaTiO 3 /
3
PVDF at 600 kV/cm was 0.242 J/cm and breakdown occurred at 1000 kV/cm. It sug-
gests that use of ultrasmall Ag nanoparticles changed the electric properties of
nanocomposites that lead to improved energy density.

Three-phase composites
The various combinations of fillers and polymers comprising of three-phase polymer
nanocomposites are discussed in this section. Yu et al. studied BaTiO 3 @SiO 2 -based
fillers [216] for improving the maximum polarization as it involved the decrease of
Maxwell-Wagner and space charge polarization, thereby decreasing the maximum
polarization too. Hence, one more component, (polyacrylate elastomer (AR71))
was introduced with BaTiO 3 nanoparticles in the PVDF matrix [217]. The BaTiO 3
content was fixed at 5 vol%, while the AR71 concentration was changed from 0 to
7 vol%. The dc conductivity of the nanocomposites was enhanced abruptly with
the increase in AR71 amount to 7.0 10 12 S/m at 3 vol% due to the rise in residual
ions. The permittivity was improved slightly with the addition of fillers, mostly in the
range of 12–13, while the loss tangent was enhanced from 0.03 to 0.05 as the filler
loading was increased from 0 to 7 vol% at 1 kHz. The change in maximum polariza-
tion with and without 3 vol% AR71 was enhanced gradually under the influence of
electric field. The maximum polarization and the discharge energy density obtained
2
3
for 3 vol% AR71 at 4000 kV/cm were 7.3 μC/cm and 8.8 J/cm , respectively.
AR71 was also considered as dielectric particles as it was also associated with inter-
facial and spatial charges under the influence of electric field. The maximum polar-
ization was enhanced with an increase in applied electric field due to the rise in the
volume of elastomers, which further leads to expansion and inhomogeneity to local
effective electric field in nanocomposites. However, as the electric field was