STRUCTURE AND PROPERTIES OF CNT YARNS AND CNT/CNF REINFORCED PAN-BASED CARBON FIBERS

There is continuing effort to enhance the strength and modulus of carbon fibers by various combinations of materials and processing. Carbon fibers are produced from various precursors, and the strength of the CFs are directly related to the type of precursor used to make them. Carbon Nanotubes (CNTs) have received a great deal of attention due to their unique structure and properties. Major focus of this research is on the evaluation of processing, structure and properties of CNT based yarns and composite fibers.

High strength and low cost carbon fibers (CFs) are needed for today’s applicatio ns. A low cost and low molecular weight textile grade PAN is studied as the precursor polymer with CNT/ carbon nanofibers (CNFs) as the filler material to enhance the strength of the carbon fibers. Efforts by several researchers have shown that incorporation of CNTs into carbon fibers is a challenging task and only a small percent could be introduced successfully. Various concentrations of modified CNTs or CNFs are used as reinforceme nt and an effort to increase the percentage of CNTs or CNFs in PAN precursor is attempted. The tensile strength of the precursor fibers is 150 MPa for 3.2 wt% CNFs in 12 wt% PAN and 430 MPa for carbonized fibers of the same precursor. Compared to pristine PAN, the reinforcement resulted in 187% increase in strength and 74% increase in modulus.

Commercially available CNT yarns have shown that their tensile properties are much lower than the calculated values. Various characterization techniques such as scanning electron microscopy (SEM), focused ion beam (FIB), transmission electron microscopy (TEM), tensile testing, and X-ray diffraction (XRD) are used to investigate the morphology of the fibers/yarns. Structural analysis indicated relatively poor packing/ orientation of nanotubes compared to density of CNT and yarn axis, respectively. Possible approaches to further enhance the properties of CNT yarns are investigated. Some of these techniques demonstrated the increase in strength by more than 330 % and modulus by more than 360 %. These observed increases are due to enhanced packing and interaction between the nanotube bundles in the CNT yarn.