24 Dielectric Properties and AC Conductivity of Epoxy/Hybrid Nanocarbon. . .
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the creation of microcapacitors which prevail over structural formation of the filler
network. In other words, the percolating network also induces a capacitive effect
that is stronger than the loss of capacitance associated with the reduction of the
correlation length. In addition, the part of filler clusters is not directly integrated
to the percolation network and is likely to participate in the permittivity of the
composite [29].
So, near the percolation threshold, conductive GNP or CNT particles are almost
touching each other but still remain isolated due to the existence of epoxy space
layer. That is why we observed the broad maximum on ε
r (C) dependence around
percolation threshold and slight decrease of dielectric permittivity with increase of
nanocarbon content. It is important to note that the position of maximum on ε
r (C)
dependence (see Fig. 24.9a) fully correlates with the percolation threshold in σ dc
conductivity for these composites (the percolation threshold C cr for each series of
nanocarbon/L285 composites is indicated in Fig. 24.11 by dashed lines).
The dielectric constant of the composites decreases with increasing frequency.
The decrease of relative permittivity with frequency could be attributed to the
insufficient time for dipoles to align before the field changes direction; in other
words incapability of the dipole to follow the field variations at high frequency and
the displacement or orientation of bound charge carriers may also play a role in this
dispersion. At low frequency, the energy gap is wider than at high frequency; thus,
the capacitance at low frequency is higher than at high frequency. The smaller is the
distance between conductive particles, the lower is the frequency for the conduction.
Therefore, with the increase of frequency, the capacitive resistance will greatly
decrease, and some microcapacitors even become conductive, leading to decreased
dielectric constant.
Apparently, the dielectric permittivity of CMs with higher content of carbon filler
shows an even more serious frequency dependence behavior, suggesting that there
should be a stronger MWS polarization.
With regard to a conductor/polymer composite, its dielectric loss mainly consists
of the loss of electric conduction, dipolar loss, and the loss of interfacial polarization
[32]. As the volume fraction of nanocarbon filler increases, the number of conductive paths will increase, leading to increased loss of electric conduction; therefore,
the dielectric loss of GNP- and GNP/CNT/L285 composites increases as the content
of nanocarbon increases. On the other hand, at relatively low frequency, the loss of
electric conduction makes more contribution to dielectric loss than that of interfacial
polarization, so the dielectric loss of composites decreases with the increase of frequency as shown in Figs. 24.7 and 24.8. As we can see from Fig. 24.9, the increase
of CNT in hybrid filler promotes the high increase of dielectric permittivity and
dissipation factor tanδ (compare samples of series 1:0; 3:1; 1:1; 0.5:x with hybrid
filler content) and, accordingly, shift of the maximum of permittivity and dielectric
loss into lower concentration of filler. So, maximum dielectric loss was observed for
CMs series 0.5:x with 0.5wt.%GNP/1.5wt.%CNT, while the dielectric permittivity
for this composite is the same as for samples filled with 1wt.%GNP/1wt.%CNT,
3wt.%GNP/1wt.%CNT, and 6wt.%GNP with much lower values of dielectric loss.
Such behavior agreed with lower percolation threshold for CNT-based CMs related
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the creation of microcapacitors which prevail over structural formation of the filler
network. In other words, the percolating network also induces a capacitive effect
that is stronger than the loss of capacitance associated with the reduction of the
correlation length. In addition, the part of filler clusters is not directly integrated
to the percolation network and is likely to participate in the permittivity of the
composite [29].
So, near the percolation threshold, conductive GNP or CNT particles are almost
touching each other but still remain isolated due to the existence of epoxy space
layer. That is why we observed the broad maximum on ε
r (C) dependence around
percolation threshold and slight decrease of dielectric permittivity with increase of
nanocarbon content. It is important to note that the position of maximum on ε
r (C)
dependence (see Fig. 24.9a) fully correlates with the percolation threshold in σ dc
conductivity for these composites (the percolation threshold C cr for each series of
nanocarbon/L285 composites is indicated in Fig. 24.11 by dashed lines).
The dielectric constant of the composites decreases with increasing frequency.
The decrease of relative permittivity with frequency could be attributed to the
insufficient time for dipoles to align before the field changes direction; in other
words incapability of the dipole to follow the field variations at high frequency and
the displacement or orientation of bound charge carriers may also play a role in this
dispersion. At low frequency, the energy gap is wider than at high frequency; thus,
the capacitance at low frequency is higher than at high frequency. The smaller is the
distance between conductive particles, the lower is the frequency for the conduction.
Therefore, with the increase of frequency, the capacitive resistance will greatly
decrease, and some microcapacitors even become conductive, leading to decreased
dielectric constant.
Apparently, the dielectric permittivity of CMs with higher content of carbon filler
shows an even more serious frequency dependence behavior, suggesting that there
should be a stronger MWS polarization.
With regard to a conductor/polymer composite, its dielectric loss mainly consists
of the loss of electric conduction, dipolar loss, and the loss of interfacial polarization
[32]. As the volume fraction of nanocarbon filler increases, the number of conductive paths will increase, leading to increased loss of electric conduction; therefore,
the dielectric loss of GNP- and GNP/CNT/L285 composites increases as the content
of nanocarbon increases. On the other hand, at relatively low frequency, the loss of
electric conduction makes more contribution to dielectric loss than that of interfacial
polarization, so the dielectric loss of composites decreases with the increase of frequency as shown in Figs. 24.7 and 24.8. As we can see from Fig. 24.9, the increase
of CNT in hybrid filler promotes the high increase of dielectric permittivity and
dissipation factor tanδ (compare samples of series 1:0; 3:1; 1:1; 0.5:x with hybrid
filler content) and, accordingly, shift of the maximum of permittivity and dielectric
loss into lower concentration of filler. So, maximum dielectric loss was observed for
CMs series 0.5:x with 0.5wt.%GNP/1.5wt.%CNT, while the dielectric permittivity
for this composite is the same as for samples filled with 1wt.%GNP/1wt.%CNT,
3wt.%GNP/1wt.%CNT, and 6wt.%GNP with much lower values of dielectric loss.
Such behavior agreed with lower percolation threshold for CNT-based CMs related