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The stabilized PAN nanofibers are converted into CNFs by heating at temperatures larger than 800°C in an inert atmosphere. During this step, the carbon content increases dramatically maintaining an amorphous structure with partial crystallinity. Most of the non-carbon elements are eliminated during carbonization. High strength CNFs are expected to be produced after carbonization according to the mechanical behavior of microscale carbon fibers discussed in a previous section. If high modulus is desired, then a final graphitization treatment at very high temperatures (2000- 3000°C) increases the crystallite size and improves their orientation along the fiber axis. Although high temperature graphitization increases the elastic modulus rather monotonically, it does reduce the strength. Since an objective of this thesis was to produce high strength CNFs, no graphitization treatment was carried out once the temperature at which the CNFs attained a maximum value was identified. 1.4. Objectives of this Dissertation Research Continuous CNFs with diameters of the order of 100 nm, have not yet reached their potential due to molecular homogenization and defect reduction issues that limit their mechanical strength [23]. Therefore, the objectives of this research were the: Fabrication of continuous, aligned and smooth carbon nanofibers from PAN nanofiber precursors by heat treatment. Optimization of the heat treatment temperature for high strength carbon nanofibers conducted with the support of nanomechanical property experiments with single carbon nanofibers. Relations between the nanofiber crystallite size and structure, the heat treatment process and the mechanical strength. The research pursued in this dissertation benefited from the work by Naraghi et. al [41-43] to characterize the mechanical strength and modulus of PAN nanofibers in order to obtain improved molecular alignment. The same experimental methods were applied on individual CNFs to identify the optimum carbonization conditions for high modulus 7PDF Image | HIGH STRENGTH CARBON NANOFIBERS DERIVED FROM ELECTROSPUN POLYACRYLONITRILE
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