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3.5. Conclusions An optimization process was pursued to establish fabrication-structure-properties relationships in order to realize strong carbon nanofibers from PAN precursors and to identify factors that are limiting the ultimately possible tensile properties of this class of nanofibers. The tensile strength and the elastic modulus of the carbon nanofibers were 6 and 3 times larger than previously reported as a result of selecting appropriate conditions for PAN electrospinning, stabilization and carbonization. The homogenized fiber cross- section eliminated the failure prone skin-core structure that was identified before as a structural weakness of these fibers. The tensile strength increased monotonically with a maximum value at 1400°C, while the elastic modulus increased steadily until 1700°C. The formation of turbostratic carbon crystallites with 3 - 8 layers in thickness was among the reasons for increased modulus but also the source of failure at high carbonization temperatures. The random orientation of the crystallites pointed out to the necessity for better molecular orientation in the PAN precursor to improve both the strength and the modulus. Compared to existing strong VGCNFs, the present nanofibers can provide immediate load transfer because of their wire-like geometry as opposed to the wavy structure of VGCNFs. The improved mechanical properties reported here were due to the smooth fiber surface and the homogeneous cross-section that eliminate the skin-core fiber structure, thus reaching the properties of commercial grade carbon microscale fibers. 38PDF Image | HIGH STRENGTH CARBON NANOFIBERS DERIVED FROM ELECTROSPUN POLYACRYLONITRILE
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