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Kaerkitcha et al. Nanoscale Research Letters (2016) 11:186 Page 5 of 9 Fig. 5 TEM images of the carbon nanofibers after carbonization. a CNFs from single nozzle and HCNFs from b normal coaxial nozzle, c inward coaxial nozzle, and d outward coaxial nozzle cross-sectional SEM images. An aluminium electrode (thickness 100 nm) was thermally evaporated on the samples, and then the current density-voltage (J-V) char- acteristic (Bunkoukeiki, IV-2401) is measured to obtain the conductance (G, Siemens) values. Then the conduct- ance value was used to calculate the electrical conductiv- ity of the samples. Results and Discussion To confirm the effect of applied voltage on the PMMA/ PAN composite nanofibers obtained from three different nozzle-end configurations. The applied voltage was var- ied from 10 to 25 kV while the other factors including the collecting distance and solution flow rates were kept constant. The morphologies of obtained as spun com- posite nanofibers were shown in Table 1. As indicated in SEM images, the effect of voltage shows the same trend for all three configurations of the nozzle. The good morphologies of PMMA/PAN nanofibers (smooth and uniform in the distribution of the sizes) were obtained from the coaxial nozzle electrospinning at 15 kV. At a lower applied voltage of 10 kV, the injection was oc- curred after an electrospinning solution was agglomerated at the end of the nozzle, retarded for a while until the elec- trostatic force was strong enough to inject the solution jet. The initial agglomerated shape may cause some difficulties for the solution jet to form the continuous compound of Taylor’s cone at the end of the nozzle. Therefore, the ob- tained nanofibers show a non-uniform beaded fiber morphologies and some nanofibers were fused together as a bundle of fibers. For the coaxial electrospinning at higher voltage of 15–25 kV, composite nanofibers with no beads and more uniform morphologies were obtained. However, when the applied voltage was increased greater than 20 kV, the extraction rate of the precursor solution was too high and the solution was collected and accumu- lated at the collector before the evaporation of the solvent. Therefore, wet nanofibers were fused onto the aluminium foil collector or fused together resulting in the bundle morphologies. From the obtained results, we used the ap- plied voltage at 15 kV for further experiment. For comparison, the morphologies of as spun PAN nanofibers from single nozzle electrospinning were also Fig. 6 N2 adsorption isotherms of the HCNFs and the CNFs at −196 °CPDF Image | carbon nanofibers obtained from coaxial electrospinning
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