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A large number of TEM images of PAN derived carbon nanofibers were obtained to measure the average crystallite thickness. This approach provided an estimate of the change of crystallite thickness, Lc, and length, La, with increasing carbonization temperature. Lc and La both increased with increasing carbonization temperature: As listed in Table I, the average crystallite thickness increased from an average of 3.3 ± 0.9 layers at 800°C, which is in good agreement with previous reports for micron size diameter [27,29,31], commercial (T-300) [31], and nanoscale fibers [18], but higher than those reported before by Zhou et al. for similar size nanofibers processed between 800- 1400°C [27], to an average of 7.9 ± 1.9 layers at 1700°C. The average crystallite thickness of microscale PAN derived carbon fibers carbonized at 1800°C has been reported to be 8-10 layers [29], which is similar to the average crystallite thickness reported here suggesting that the nanoscale size of the fibers does not affect the growth of turbostratic carbon crystallites. Furthermore, the crystallite size for the carbonization temperature of 1100°C is very comparable to that reported for PAN derived carbon nanofibers with significantly lower tensile strength and modulus implying that the dramatic improvement in properties reported in this work is owed to other structural sources such as the nanofiber radial material homogeneity. In some cases, thin nanofibers with diameters of the order of 50 nm, not tested for their mechanical properties, were found to have significant crystallite content and larger crystallites, as shown in Figure 3.9, suggesting faster growth kinetics than in larger diameter (>150 nm) nanofibers. It should be noted however, that even in the case of large crystallite density, the crystallites were not aligned along the nanofiber axis which implies a limiting structure in terms of achieving properties significantly higher than those reported in this work. In general, the crystallite interlayer spacing, d002, in conventional carbon fibers decreases with increasing carbonization temperature [27-29] The value of d002 for carbon fibers heat treated at 2800°C is larger than that of highly oriented pyrolytic graphite (HOPG) [29], and the degree of orientation is less than that of HOPG, suggesting that PAN derived carbon fibers are very difficult to fully graphitize. Prior works reported on preferred alignment of turbostratic carbon crystallites at the nanofiber surface [18,65], potentially denoting a more compact and orderly skin, which was not present in the nanofibers produced in this work. 34PDF Image | HIGH STRENGTH CARBON NANOFIBERS DERIVED FROM ELECTROSPUN POLYACRYLONITRILE
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