Page 179 - Polymer-based Nanocomposites for Energy and Environmental Applications

152 Polymer-based Nanocomposites for Energy and Environmental Applications

improved from 52 to 95, with an increase in diameter of BaTiO 3 from 50 to 100 nm
and subsequently decreased to 70 at 500 nm at 1 kHz. The increase in dielectric con-
stant with the size of BaTiO 3 was exhibited due to the change in phase from cubic to
tetragonal. It is understood that the tetragonal phase of BaTiO 3 exhibits comparatively
larger dielectric constant as compared with the cubic phase due to nonsymmetry of the
former phase. Hence, the overall permittivity of the polymer nanocomposites was also
enhanced [212]. However, comparatively lower values of permittivity for the diam-
eters above 100 nm were not clear; a systematic modeling is imperative to be done to
understand the clear mechanism. The loss tangent of the PVDF nanocomposite was
<0.1 for all the diameters at low frequency. For example, at 60 vol% loading the loss
tangent was 0.05 at 1 kHz. However, a relatively larger value of loss tangent for
PVDF (0.13 at 1 kHz) was obtained due to the conducting nature of PVDF along with
interface effects. The maximum breakdown strength of the polymer nanocomposite
was observed to be 55 kV/cm, which was limited to only 25 kV/cm for the pure PVDF.
It was observed that doping along with the surface modification of the nanoparticles
can be a useful procedure to further minimize the dielectric loss as discussed below.
Lin et al. doped neodymium oxide (Nd 2 O 3 ) with hollow BaTiO 3 (diameter¼75 nm)
fillers [213].Nd 2 O 3 was used in view of the fact that it can easily occupy the sites in
between titanium (0.0605 nm) and barium (0.135 nm), as it has an ionic radius of
0.0983 nm along with a higher dielectric constant of 300,000. The Nd 2 O 3 loading
was limited at 10 wt% because an increased concentration of Nd 3+ leads to the forma-
tion of Nd(OH) 3 phase (nanorod shape along (001) axis), which was detrimental to the
electric properties. The maximum dielectric constant of 500 (at 1 kHz) was obtained
at 40 wt% filler loading with 10 wt% Nd doping. The dielectric loss was 1.0; how-
ever, an increase in dielectric loss and a decrease in dielectric constant were observed
at higher loading that was attributed to the agglomerations of fillers in the PVDF
nanocomposites. The comparative analysis of the results reported by Lin et al.
[213] with Mao et al. [193] clearly infers that a higher value of dielectric constant
could be achieved at smaller diameter with doping but at the cost of comparatively
higher dielectric losses.
As a step forward, Yu et al. also modified the BaTiO 3 surface with poly-
vinylpyrrolidone (PVP) [214]. Dielectric analyses showed that the curves contained
two regions in the curve: 1 kHz to 1 MHz and 1–50 MHz, in coherence to the results
of tetrafluorophthalic acid modified BaTiO 3 -based polymer nanocomposites [215].
Up to 30 vol% of BaTiO 3 , the dielectric constant showed a small variation in the
whole frequency range. However, as the filler loading was further enhanced, the
dielectric constant declined gradually in between 1 kHz and 1 MHz. It suggested that
the Maxwell-Wagner interfacial polarizations and space charge polarizations were the
main factors behind the decrease in dielectric constant. Further, at the higher frequen-
cies, the dielectric constant decreased comparatively faster than that in the low-
frequency region. The percolation threshold of the nanocomposite was between
35 and 50 vol%, while the maximum dielectric constant was found at 55 vol%, having
a value of 77 at 1 kHz. Besides, the loss tangent was enhanced from 0.05 for pure poly-
mer to 0.1 for 55 vol% loading. The comparison of untreated and treated fillers
showed that the dielectric loss of modified fillers was same as that of untreated fillers