Multiwall Carbon Nanotube Carbon Composites

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Multiwall Carbon Nanotube - Carbon
Composites Carl W. Grinter, David Jacques, Adam
Berkovich and Rodney Andrews Center for Applied
Energy Research University of Kentucky
Abstract The synthesis and properties of pitch
derived carbon fibers reinforced with multiwall
carbon nanotubes have been investigated. In this
work, a coal extract and a petroleum pitch were
doped with multiwall carbon nanotubes and the
derived composite materials melt spun to produce
continuous filament. These composite pitch fibers
were stabilized and carbonized to yield carbon
fibers reinforced with carbon nanotubes. The
physical properties of the composite fibers have
been measured and the results compared to carbon
fibers derived from undoped isotropic pitch
feedstocks. Introduction The extraordinary
properties of carbon nanotubes (CNT) has prompted
intense research into a wide range of
applications where their deployment could have a
major impact. The development of advanced
engineering materials comprising multiwall carbon
nanotubes (MWNT) dispersed in selected matrices
is one area where they could play an important
role, provided their unique properties can be
realized. The tensile properties of a range of
traditional reinforcing carbon components is
compared with the predicted values of composite
fibers containing CNT, Figure 1.
Experimental Mixing and Dispersion Dispersion of
the MWNTs in the pitch matrix was achieved by
heating the pitch to a temperature close to its
softening point in a Haake Polylab Rheomix, where
high shear mixing could be applied. Tests were
performed to determine the optimum conditions
(temperature, mixing time and rotor speed)
required to adequately disperse the MWNTs and
yield a homogenous product, but without unduly
degrading the pitch. At the completion of each
test, the composite was allowed to cool,
recovered from the mixer, and crushed to yield a
granular product. Samples were set in resin,
sectioned and polished to assess dispersion by
optical microscopy. Spinning The pitch/MWNT
composite samples were transferred to a Wayne
bench scale extruder fitted with a 6.2mm diameter
screw and 0.3mm dia x 1mm capillary die, Figure
2. Tests were conducted to determine the
conditions under which the samples could be
successfully extruded to produce a continuous
thread. Feed size distribution and the
temperature profile along the barrel and nozzle
were crucial to this task. The extruded thread
was attached to a wind-up drum rotating at speeds
of up to 1700rpm (12m/s) to draw filament in the
range 15 to 30mm. The shear fields generated
within the extruder, capillary die and fiber
draw-down result in axial alignment of the MWNTs.
Results and Discussion Fabrication Good
dispersion was achieved by operating at a
temperature of no more than 5oC above the
softening point of the pitch. The pitch is then
mobile enough to allow processing while having
sufficient viscosity to produce high shearing
that promotes dispersion. The dispersion
efficiency can be related to the mechanical
energy input into the mix, approximating to a
logarithmic function, Figure 3. Higher energy
input was required with increasing MWNT
concentration due to an increase in melt
viscosity, Figure 4. Uniform dispersion with
alignment along the axis of the fiber is evident
in SEM images of the composite carbon fibers for
all of the MWNT concentrations examined,
Figure 5. There is no apparent effect on fiber
morphology as the loading is increased, although
the MWNT distribution density in the fracture
surface noticeably increases, Figures 5ab.
Properties The characteristics of the pitch
matrix apparently have a pronounced effect upon
fiber properties. With the coal-derived pitch
there is a significant increase in the tensile
strength and small increase in elastic modulus of
the carbon fibers compared with those produced
from the neat pitch. In contrast, with the
petroleum pitch, A500, there is no significant
change in performance as the concentration of
MWNTs is increased, Table 1. This may be
attributed to the aromaticity of the two pitches.
The highly aromatic coal-derived pitch should
have a much greater affinity for the MWNTs than
the aliphatic petroleum pitch allowing alignment
of the graphene crystallites derived from the
pitch with the curved graphene nanotube surface.
Surface treatment of the MWNTs to increase the
interfacial bonding with the matrix could show
even greater improvements in their physical
properties.
Realizing the potential offered by these new
materials is the rational behind the work carried
out here to develop methods for the production
of carbon/carbon composites with enhanced
physical properties. The use of pitch as the
matrix would seem to be advantageous due to the
compatibility between the graphene structure
within both materials. Three of the key elements
of the work are to
Figure 2 Wayne Benchtop Extruder Showing Fiber
Wind-up
Table 1 Properties of Composite fibers
Stabilization carbonization Tows of fiber cut
from the drum were stabilized by heating slowly
in air up to 310oC. During stabilization the
fibers undergo oxidative cross-linking reactions
that progressively increase the softening point
of the pitch until it becomes infusible. The
fibers are then rapidly carbonized by heating to
1000oC in nitrogen. During carbonization volatile
components and impurities are lost and the fibers
develop the crystallite structure that confers
their strength and stiffness. The tensile
properties of the fibers were measured using a
MTS QTest instrument for single filament tests
following ASTM D3379.
Figure 4 Mixing Energy as a function of MWNT
concentration.
Figure 3 Fiber Dispersion as a Function of Energy
Input
Conclusions Multiwall carbon nanotubes have been
successfully dispersed in pitch matrices by high
shear mixing. The derived composite materials
containing up to 4wt MWNTs were melt spun to
yield continuous filament and converted into
carbon fibers by heat treatment. The shear fields
generated during fiber spinning resulted in
alignment of the MWNTs with the axis of the
carbon fiber. This offers a practical route to
the fabrication composites with controlled
nanotube configuration. There was a significant
increase in tensile strength of the composite
fibers produced from the coal-derived pitch.
Figure 1 High performance composite fibers
(i) disperse the MWNTs in a pitch matrix, (ii)
convert the pitch/MWNT composite to a
carbon/carbon artifact and (iii) develop means to
ensure that load transfer across the MWNT- matrix
boundary can be achieved.
Figure 5a Pitch fiber with 1 MWNT
Figure 5b Pitch fiber with 2 MWNT
This work was funded by the National Science
Foundation, Division of Materials Research under
grant no. DMR-9809686