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eliminated the failure prone skin-core structure that was identified in literature as a structural weakness of this class of nanofibers. The tensile strength increased monotonically reaching its maximum at 1400°C, while the elastic modulus increased steadily until 1700°C. The formation of turbostratic carbon crystallites with 3 - 8 layers thickness was among the reasons for increased modulus but also the source of failure initiation at high carbonization temperatures. The random orientation of the crystallites pointed to the necessity for stronger molecular alignment in the PAN precursor, to improve both the strength and the modulus. As the characteristic strength increased from 2.2 GPa to 3.6 GPa for fibers produced at 800°C and 1400°C, the Weibull modulus also increased from 3 to 6, which indicates that the higher processing temperature removed the major defects in the nanofibers. Carbonization at the higher temperature of 1700°C reduced the Weibull modulus to about 3 due to the formation of large and randomly distributed turbostratic carbon crystallites which acted as stress concentrations and sites for failure initiation. Finally, it should be noted that the versatile MEMS-based experimental tools made nanoscale tension experiments possible at the single nanofiber level, which proved instrumental in establishing the processing-structure-property relationships presented in this dissertation. 40PDF Image | HIGH STRENGTH CARBON NANOFIBERS DERIVED FROM ELECTROSPUN POLYACRYLONITRILE
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