Environmental Engineering Reference
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from a liquid crystalline suspension of CNTs or they can be spun directly from
the reactor by drawing them out of the hot-zone during CNT growth by chemical
vapor deposition (CVD). Considerable attention has been devoted to optimizing
the structure of CNT fibers at the different stages of their production: controlling
the synthesis of specific nanotubes, the assembly of CNTs into a fully dense fiber
and using post-spin treatments to obtain specific properties. However, the integra-
tion of CNT fibers into composites and the properties of these composites have
received comparatively less attention, in spite of these aspects being fundamental
for many potential applications of this new high-performance fiber. A previous
study on the mechanical properties of CNT fiber/epoxy composites showed ef-
fective reinforcement of the thermoset matrix both in tension and compression
without the need for additional treatments on the CNT fiber. Large increases in
stiffness, energy absorption, tensile strength and compressive yield stress were
observed for composite with fiber mass fractions in the range 10-30%. CNT fi-
bers have an unusual yarn-like structure with an accessible surface area several
orders of magnitude higher than that of a traditional fiber. The free space between
bundles in the CNT fiber is able to take up non- cross-linked resins by capillary
action, which wick into the fiber and appear to fill the observable free space. As
a consequence, the composite develops a hierarchical structure, with each CNT
fiber, being an infiltrated composite itself. Measurements of CTE of CNT fiber
composites show good stress transfer between matrix and fiber due to the good
adhesion of the thermoset to the porous fibers aided by a significant level of struc-
tural 'keying.' The adding polymer into the CNT fibers does not disrupt the CNT
bundle network; hence the electrical conductivity of the fiber is largely preserved
and the electrical conductivity of the composite is increased. On the other hand,
the thermal conductivity of the CNT fiber composite increases more rapidly than
before with added fiber loading up to the maximum used here of 38%. Others
suggest that the infiltration of the polymer into the fiber improves the thermal
coupling between the nanotube bundles by filling the spaces between them, which
is a characteristic of the as-spun condition. Addition of CNT fiber to the matrix
produces an effective increase in composite thermal conductivity of 157 W/m K
per unit fiber mass fraction. Results show easy integration of CNT fibers into axi-
ally aligned composites and rather effective exploitation of the electrical, thermal
and mechanical axial properties of the CNT fibers thanks to the infiltration of
polymer into them.
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