BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 33

Current trends toward the development of carbon fibers have been driven in two
directions; ultrahigh tensile strength fiber with a fairly high strain to failure (2%),
and ultrahigh modulus fiber with high thermal conductivity. Today, a number of
ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh
modulus pitch-based (more than 900 GPa) carbon fibers have been commercially
available. Carbon fibers with exceptionally high thermal conductivity are critical
for many thermal control applications in the aerospace and electronics industries.
The thermal conductivity of carbon fibers was found to increase asymptotically
as the degree of preferred orientation of the crystalline parts in the fiber increases.
However, further improvement of thermal conductivity over the existing highly
oriented pitch-based carbon fiber while retaining the desired mechanical property
has proven to be very challenging. One of the most effective approaches to further
increase the thermal conductivity is to graft carbon nanotubes (CNTs) on the car-
bon fibers. CNTs have an extremely high thermal conductivity in the axial direc-
tion, and the thermal conductivities of multi-walled CNTs had been reported to
be as high as 3000 W/m K. The grafting of CNTs on carbon fibers using chemical
vapor deposition and electro deposition has been reported in the literature. Some
researchers reported that the grafting of CNTs improves the mechanical proper-
ties and Weibull modulus of ultrahigh strength PAN-based and ultrahigh modulus
pitch-based carbon fibers. The effect of grafting CNTs on the thermal conductiv-
ity of T1000GB PAN-based and K13D pitch-based carbon fibers were investi-
gated (Fig. 1.3). Recently reported a method for the self-assembled fabrication
of a single suspended amorphous carbon nanowire on a carbon- MEMS platform
by electro spinning and pyrolysis of PAN and polymers. Here, we explore this
technique's potential to fabricate the CNT/PAN composite nanofibers anchored
to electrodes and thus, investigate the graphitic and electrical properties of single
suspended CNT/carbon composite nanofibers. The conductivity of electro spun
carbonized CNT/PAN nano fiber is measured at four different concentrations of
MWCNT in the PAN electro spinning solution. In order to understand the struc-
tural changes that are responsible for the increase in conductivity of these nano-
fibers, micro Raman spectroscopy, X-ray diffraction (XRD) and high-resolution
transmission electron microscopy (HRTEM) are used. Results indicate that the
crystallinity and electrical conductivity of these composite nanofibers increase
with increase in concentration of CNTs.