388
L. L. Vovchenko et al.
to higher aspect ratio of CNT compared with GNPs and better ability to form carbon
network in epoxy matrix. The presence of GNP in hybrid filler GNP/CNT expands
the filler concentration range before percolation for increasing of the number of
carbon filler particles acting as “artificial dipoles” and leading to increased dielectric
permittivity. In addition, GNP particles with the large diameter/thickness ratio make
it difficult for CNT on one side to bypass the graphite sheet and tangle with another
CNT on the other side, consequently blocking the aggregation of CNTs [33]. This
may improve the dispersion of CNTs promoting their de-agglomeration that also
leads to increase of the number of formed microcapacitors and enhances dielectric
permittivity. The presence of aggregates and agglomerates with shape irregularities
in random packing causes the number of contacts to increase with increasing
inclusions concentration and will eventually affect the conduction threshold. As it
was shown in [34], ternary composites expanded graphite–carbon nanotube/cyanate
ester (EG–MWCNT/CE) with very small contents of conductors can simultaneously
have remarkably higher dielectric constant and lower dielectric loss, indicating that
the weight ratio between EG and MWCNT is an important factor of determining
the dielectric properties of the ternary composites. This is attractive for practical
application of these composites for storing energy and homogenizing electric field.
In our case it may be concluded that optimal ratio between GNP and CNT in hybrid
is 3:1, when material is characterized by relatively high value of permittivity and
low dielectric loss.
24.3.4 AC Conductivity of CMs GNP/CNT/L285
Figure 24.10 displays the frequency dependence of alternating current (AC) conductivity of developed nanocomposites GNP- and GNP/CNT/L285 with filler contents
1 and 2 wt.% at various GNP:CNT ratios. As expected, the variation of AC
conductivity with frequency is the reverse of dielectric constant ε
r . Figure 24.11
depicts the data on AC conductivity versus nanocarbon content at the frequency
10 kHz. It can be clearly seen that all composite systems show typical percolation
transition as CNT content in CMs increases. For GNP/L285 nanocomposites, the
conductivity shows dependence on frequency at 1 wt.% content owing to the
insulating nature.
The conductivity of CMs with increased content of CNT in hybrid GNP/CNT
displays a conducting characteristic which remains nearly frequency-independent
(up to onset frequency f c ) when the GNP/CNT loading exceeds the percolation
threshold (see Figs. 24.9a and 24.10b, sample 0.5GNP/1.5CNT). So, in this case
DC electrical conductivity is large, and overall conductivity σ (f ) may be expressed
as [15]:
σ (f ) = σ DC + σ AC = σ DC + A · ω
u
(24.6)
L. L. Vovchenko et al.
to higher aspect ratio of CNT compared with GNPs and better ability to form carbon
network in epoxy matrix. The presence of GNP in hybrid filler GNP/CNT expands
the filler concentration range before percolation for increasing of the number of
carbon filler particles acting as “artificial dipoles” and leading to increased dielectric
permittivity. In addition, GNP particles with the large diameter/thickness ratio make
it difficult for CNT on one side to bypass the graphite sheet and tangle with another
CNT on the other side, consequently blocking the aggregation of CNTs [33]. This
may improve the dispersion of CNTs promoting their de-agglomeration that also
leads to increase of the number of formed microcapacitors and enhances dielectric
permittivity. The presence of aggregates and agglomerates with shape irregularities
in random packing causes the number of contacts to increase with increasing
inclusions concentration and will eventually affect the conduction threshold. As it
was shown in [34], ternary composites expanded graphite–carbon nanotube/cyanate
ester (EG–MWCNT/CE) with very small contents of conductors can simultaneously
have remarkably higher dielectric constant and lower dielectric loss, indicating that
the weight ratio between EG and MWCNT is an important factor of determining
the dielectric properties of the ternary composites. This is attractive for practical
application of these composites for storing energy and homogenizing electric field.
In our case it may be concluded that optimal ratio between GNP and CNT in hybrid
is 3:1, when material is characterized by relatively high value of permittivity and
low dielectric loss.
24.3.4 AC Conductivity of CMs GNP/CNT/L285
Figure 24.10 displays the frequency dependence of alternating current (AC) conductivity of developed nanocomposites GNP- and GNP/CNT/L285 with filler contents
1 and 2 wt.% at various GNP:CNT ratios. As expected, the variation of AC
conductivity with frequency is the reverse of dielectric constant ε
r . Figure 24.11
depicts the data on AC conductivity versus nanocarbon content at the frequency
10 kHz. It can be clearly seen that all composite systems show typical percolation
transition as CNT content in CMs increases. For GNP/L285 nanocomposites, the
conductivity shows dependence on frequency at 1 wt.% content owing to the
insulating nature.
The conductivity of CMs with increased content of CNT in hybrid GNP/CNT
displays a conducting characteristic which remains nearly frequency-independent
(up to onset frequency f c ) when the GNP/CNT loading exceeds the percolation
threshold (see Figs. 24.9a and 24.10b, sample 0.5GNP/1.5CNT). So, in this case
DC electrical conductivity is large, and overall conductivity σ (f ) may be expressed
as [15]:
σ (f ) = σ DC + σ AC = σ DC + A · ω
u
(24.6)