390
L. L. Vovchenko et al.
frequency range (up to 1.5 * 10 5 Hz) as seen the plateau region in Fig. 24.10b for the
sample 0.5wt.%GNP/1.5wt.%CNT/L285.
The frequency-independent conductivity is commonly regarded as direct current DC conductivity. As it was mentioned above, electron conduction in the
nanocarbon-filled composites occurred via two mechanisms: along carbon particles
contacting each other (leakage current) or carbon particles separated by very small
gaps in the composites (tunneling current) [35]. The gaps could be considered as
potential barriers for electrons to hop by the tunneling effect. Generally, being
higher the leakage current makes more contribution to conductivity than tunneling
current. Hence for the leakage current, the content of filler should be high so
that a network is formed in the matrix. Thus, the conduction mechanism of the
composites is closely related to both the dispersion and the content of conductors.
Specifically, when the content of GNP and CNT is very small, that is, the content
of nanocarbon particles in the composites is not sufficient to make the connecting
networks, hence, their conduction mechanisms are due to the tunneling of electron.
In the frequency range higher onset frequency f c , electrical conductivity increased
since the frequency was high enough, and some gaps in carbon pathways became
conductive, and the nanocarbon chains with small gaps became conductive first.
The smaller is the distance between conductors, the lower is the frequency for the
conduction. Therefore, with the increase of frequency, the capacitive resistance will
greatly decrease, and some microcapacitors (formed between conducting particles
or clusters) even become conductive, leading to decreased dielectric constant and
increased electrical conductivity.
24.4 Conclusion
It was found that dielectric permittivity and electrical conductivity increase with
increasing content of hybrid carbon filler and are characterized by percolative
behavior. In composites with high nanocarbon content, the Nyquist diagram
exhibits a semicircle dependence Z (Z
) indicating that conductive pathways
have been formed and there are currents and polarization among conductive filler
particles.
It was shown that substitution of GNP particles by carbon nanotubes promotes
the shift of percolation threshold into lower filler content and enhances the electrical
conductivity, permittivity, and dielectric loss (tanδ) of these CMs compared with
CMs filled only with GNPs. The increase of dielectric permittivity can be mainly
attributed to a gradual formation of microcapacitor networks in the epoxy matrix as
the volume fraction of conductive nanofiller increases. The microcapacitor network
is an additional contribution to interfacial permittivity of fillers, due to charge carrier
accumulation at the fillers’ interface. It was shown that dependence of dielectric
permittivity versus nanocarbon filler content ε
r (C) exhibits the broad maximum
around percolation threshold, and its position fully correlates with the percolation
L. L. Vovchenko et al.
frequency range (up to 1.5 * 10 5 Hz) as seen the plateau region in Fig. 24.10b for the
sample 0.5wt.%GNP/1.5wt.%CNT/L285.
The frequency-independent conductivity is commonly regarded as direct current DC conductivity. As it was mentioned above, electron conduction in the
nanocarbon-filled composites occurred via two mechanisms: along carbon particles
contacting each other (leakage current) or carbon particles separated by very small
gaps in the composites (tunneling current) [35]. The gaps could be considered as
potential barriers for electrons to hop by the tunneling effect. Generally, being
higher the leakage current makes more contribution to conductivity than tunneling
current. Hence for the leakage current, the content of filler should be high so
that a network is formed in the matrix. Thus, the conduction mechanism of the
composites is closely related to both the dispersion and the content of conductors.
Specifically, when the content of GNP and CNT is very small, that is, the content
of nanocarbon particles in the composites is not sufficient to make the connecting
networks, hence, their conduction mechanisms are due to the tunneling of electron.
In the frequency range higher onset frequency f c , electrical conductivity increased
since the frequency was high enough, and some gaps in carbon pathways became
conductive, and the nanocarbon chains with small gaps became conductive first.
The smaller is the distance between conductors, the lower is the frequency for the
conduction. Therefore, with the increase of frequency, the capacitive resistance will
greatly decrease, and some microcapacitors (formed between conducting particles
or clusters) even become conductive, leading to decreased dielectric constant and
increased electrical conductivity.
24.4 Conclusion
It was found that dielectric permittivity and electrical conductivity increase with
increasing content of hybrid carbon filler and are characterized by percolative
behavior. In composites with high nanocarbon content, the Nyquist diagram
exhibits a semicircle dependence Z (Z
) indicating that conductive pathways
have been formed and there are currents and polarization among conductive filler
particles.
It was shown that substitution of GNP particles by carbon nanotubes promotes
the shift of percolation threshold into lower filler content and enhances the electrical
conductivity, permittivity, and dielectric loss (tanδ) of these CMs compared with
CMs filled only with GNPs. The increase of dielectric permittivity can be mainly
attributed to a gradual formation of microcapacitor networks in the epoxy matrix as
the volume fraction of conductive nanofiller increases. The microcapacitor network
is an additional contribution to interfacial permittivity of fillers, due to charge carrier
accumulation at the fillers’ interface. It was shown that dependence of dielectric
permittivity versus nanocarbon filler content ε
r (C) exhibits the broad maximum
around percolation threshold, and its position fully correlates with the percolation