JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022 - 56

FEATURE functionalization
Figure 3 shows diff erent samples used to examine
the heating behaviour of rCFRP. On
the left side thermographic images of a sample
plate of about 20 x 30 cm size are shown,
with diff erent voltages applied. Th e sample
plate consists of rCF nonwoven with a surface
weight of 100 g/m² in a matrix of polyvinyl
ether resin. Th e electrical resistance between
the embedded contacts is 1 to 2 Ω [13].
Fig. 2: Left: Comparison of the mechanical properties of rCFRP with other materials [6, 7], right: Comparison of
the electrical conductivity of graphene, graphite and carbon fi bres with metallic materials [8, 10, 11, 12].
and 33 % weight reduction at a similar cost
structure (8-13 €/kg). rCFRPs are thus particularly
suitable for semi-structural components
[5, 6]. In addition to mechanical performance,
the fi bres have another advantage:
the ability to transport electrical currents.
Carbon fi bres consist of graphite layers folded
into one another. Graphite itself consists of
layers of graphene stacked on top of each other
(Figure 2 on the right). Due to their physical
properties, graphenes are excellent electrical
conductors. At around 100 · 106
S/m, the specifi
c conductivity of graphene is about 64 %
higher than the specifi c conductivity of copper,
as shown in Figure 2. Th e physical properties
between the graphene layers act as an
electrical insulator. Th erefore, conduction of
current only takes place in the direction of the
graphene layers [9]. Th e electrical conductivity
is thus 3 · 106
layers and 3 · 102
S/m parallel to the graphene
crosswise.
Due to the folding of the graphite layers, the
electrical conductivity of carbon fi bres is in in
the dimension of 6 · 104
S/m [9, 10, 11].
Carbon fi bres are thus to be considered as
electrical conductors (conductivity > 104
S
/m), but the conductivity is three magnitudes
lower than that of copper.
Despite the lower electrical conductivity compared
to metallic conductors, the combination
of physical and chemical properties leads to
interesting application potential for the electrical
functionalisation of CFRP components.
Compared to metallic components, rCFRPs
based on nonwovens made from recycled carbon
fi bres are lighter, corrosion-resistant and
allow individual component design thanks to
their excellent drapability.
In principle, there are several potential applications
for the use of rCFRPs for electrical func56
jec
composites magazine / N°
tionalisation. Examples are the use as:
- actuators (e. g. for generating heat),
- sensors (e. g. detection of electrical
resistance or capacitance between two
components),
- electrical conductors (e. g. for the
dissipation of electrostatic charges, signal
transmission or the supply of electronics
with low energy requirements),
- manipulators for electromagnetic waves (e.
g. for shielding or refl ection).
In the following, the above-mentioned potentials
are explained in more detail by presenting
selected project results.
Application of rCFRP for heat generation
If an electric conductor is passed by an electric
current, heat is dissipated in the amount of
the electric power loss. In the case of a direct
current source, the amount of dissipated heat
corresponds to the product of the applied
voltage and the fl owing current [8]. Th e heat is
released over the surface of the conductor. Th e
high electrical resistance of rCFRP compared
to metallic materials can be compensated by a
correspondingly higher voltage or shorter distances
between the contact points when used
for heat generation.
Th e thermographic images on the left visualise
a local fl uctuation of the temperature. Th is
fl uctuation is caused by the fact that the local
dissipation depends on the local electrical resistance.
Materials with inhomogeneous electrical
resistance therefore also have an inhomogeneous
distribution of local heat dissipation.
In case of the samples shown, the local change
in resistance is the result of non-uniform fi bre
deposition in the laboratory nonwoven formation
process. With correspondingly uniform
nonwovens, the local fl uctuations are smaller,
as can be seen in the same fi gure on the right.
Th e maximum heat density dissipated by the
test panels is 2,100 W/m² on average. Th e
power is limited by the fact that hotspots occurring
in the material at the contact points lead to
local degradation of the thermoset matrix [13].
Due to high thermal resistance of the carbon
fi bres, much higher dissipation powers are
possible with suitable contacting and/or the
selection of thermally more resistant materials.
Th e right-hand side of Figure 3 shows a
wrapped GRP composite pipe with an inner
layer of carbon fi bre nonwoven made in
the publicly funded project PolyTube (ZF4159001TA5).
GRP pipelines are used, for
example, for the transport of fl uid media in
chemical parks or in shipbuilding. Depending
on the application, there is a need to protect
the transported media from cooling down,
Fig. 3: Thermographic images of sample components made of rCFK conducting electrical currents [13].
146 June - July 2022

JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022

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