This thesis research established fabrication-structure-properties relationships to realize strong carbon nanofibers from PAN precursors and to identify factors that currently limit the ultimately possible tensile properties of this class of nanofibers. Chapter 2 presented the experimental methods and procedures while Chapter 3 discussed the results of this research following the objectives and experimental approaches outlined in Chapter 1.
Experiments for PAN nanofibers conducted in the past by this group were used to identify the optimum electrospinning conditions for PAN nanofibers with improved molecular orientation and homogeneous cross-section. With this information as the basis, carbon nanofibers derived from optimized PAN nanofibers were produced with smooth surfaces and uniform diameters along their length. These nanofibers were straight, which is an advantage compared to VGCNFs, which, due to their waviness, do not provide appreciable stiffening to stiff polymer matrices at strains less than 1 - 2%. Individual CNFs with diameters between 150 - 500 nm were tested for their mechanical properties and TEM images of CNFs were obtained to identify the formation of randomly oriented turbostratic carbon crystallites at different carbonization temperatures. It was found that the crystallite size increased with carbonization temperature and was key in tuning the mechanical properties of the CNFs.
The tensile strength and the elastic modulus of the CNFs were 6 and 3 times larger than previously reported as a result of selecting appropriate conditions for PAN electrospinning, stabilization and carbonization. The homogenous CNF cross-section
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.
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