Page 158 - Polymer-based Nanocomposites for Energy and Environmental Applications

Polymer-based nanocomposites 133

2
U ¼ kE b =2 (5.1)

An enhanced permittivity and/or DBS is necessary for improved energy storage den-
sity. While an improvement in the composite dielectric constant can be acquired by
enhancing the volume fraction of high-permittivity fillers such as metal oxides, an
increase in DBS can play a more significant role in improved energy storage density
[30-32]. Significant increase in dielectric strength and other dielectric properties in
nanocomposite systems is imperative to replace existing energy storage and insulation
devices [29,33,34].
The interfacial volume of the nanocomposites is larger [35] than of micro-
composites, and the properties of the interfaces between the polymer and the par-
ticles in nanorange are thought to be responsible for improvement in nanodielectric
properties. Interfaces at the nanometric level are believed to play both active and
passive control over dielectric properties of the composites [33,35] though we
know that the bare nanoparticle surfaces are inherently quite conductive. Improve-
ment in the dielectric properties of nanodielectrics [33-36] could be due to several
factors, namely, variation in polymer morphology at the interface and local charge
distributions due to nanoparticle surface; variation in density and the energy depth
of trap sites due to change in local structure at the interface, which influences the
charge mobility and trapped state lifetime; and improvement in probability for
scattering mechanism. Thus, improved dielectric materials could be fabricated
by engineering the interface between the filler and matrix without harming the
dielectric characteristics of the bulk material [37]. Surface engineering of
nanoparticles through application of self-assembled monolayer (SAM) and rele-
vance to dispersion quality and dielectric properties along with the DBS were
investigated previously [13,38-44].
Generally, the energy density of a diphasic composite is the addition of the energy
density of each constituent. Though the volume fractions of the two constituents are on
the same order of magnitude in the ceramic polymer nanocomposite, hence, to obtain a
high energy density of a composite, a considerable energy density from each consti-
tute must be required [45].

5.2 Polymer nanocomposite dielectrics

Tremendous efforts were put to combine the efficient ferroelectric properties and high
dielectric permittivity of ceramics with the flexibility and easy handling of polymers.
The resultant polymer-based composites are believed to be promising candidates for
capacitors and charge-storage applications because of their flexibility, controllable
dielectric permittivity and better dielectric breakdown strength [46-48]. Pure poly-
mers have a small intrinsic dielectric constants (ε of 2–3), although their dielectric
1
breakdown strength is rather satisfactory (200–300 kV mm ). One common app-
roach to improve the dielectric constants of polymer composites is to add ceramic par-
ticles with high dielectric constants, such as BaTiO 3 [49],CaCu 3 Ti 4 O 12 (CCTO) [50],