Research Of PAN Electrospinning Technology, Structure Of Electrospun PAN Fibers And Electrical Properties Of PAN-based Carbonized Fibers

As the unique technology to produce continuous nanofiber nowadays, electrospinning has been a hot spot for many scholars, because of its many particular characters like the low cost, controllable producing process, no impurity produced during the preparation and so on. At the same time, as the essential to produce available carbon nanofiber, polyacrylonitrile (PAN) nanofibers with high performance appear to be more and more important. "Size effect" demonstrates that the thinner the fibers diameters are, the fewer defects they may contain. Lots of deep and detailed researches have been expended around the electrospinning of PAN fibers. But in general there are some problems:the scale of experiment parameter is not large enough and the conclusions of the same experiment are different. Therefore the effects of electrospinning parameters on the diameter and its distribution of PAN nanofibers were intensively evaluated and the the scopes of experimental parameters were set as broadly as possible. The parameters include the concentration of PAN/DMF solution, the applied voltage and the gap distance (the distance from needle tip to the roller). The result of comparative experiment shows the solution concentration has the most significant influence on the morphology and diameters of fibers under the conditions. The diameter increases swiftly with the increasing PAN concentration; with the increasing voltage, the diameters increase primarily and decrease later; the same diameters trend like the voltage appears with prolonging gap distance. The electrospinning parameters to produce thinnest PAN fibers in a wider range were obtained through orthogonal test method:concentration:6wt%, applied voltage:7kV, gap distance:6cm.The structure of electrospun PAN fibers is different from the traditional ones. The research for the morphology and the inner structure of electrospun fibers is comparatively rare, so it needs further study and exploration. In this work a phase separation method "ultrasonic etching" was used to study the crystal and amorphous phases and their distribution of the fibers combined with atomic force microscopy (AFM), scanning electron microscope (SEM) and X-ray diffraction (XRD). The result shows there are many grooves arranging nearly perpendicular to the fiber axis on the surface of electrospun PAN fibers. The thickness of the grooves is about 20-35nm. The distance between two adjacent grooves is about 40-190nm. The hollow grooves are the disordered regions. The relative ordered regions locate between the disordered regions containing few ordered part and the disordered part. The two regions are stagger together arranging nearly perpendicular to the fiber axis. Under the experiment condition, the best parameters of ultrasonic etching have been got:ultrasonic time:5h, the solution concentration:85wt%. The morphology of the ordered region is most clear after the treatment under such condition.Based on previous work, the electrospun PAN fibers have been oxidized and carbonized under different temperature. Fourier transform infrared spectroscopy, elemental analysis and XRD were used to identify the transformation during the heating process. It is deduced the oxidization degree of electrospun PAN fibers is higher than traditional ones under the same oxidized conditions. The content of oxygen of the oxidized fibers would be higher with the rising temperature. But the functional groups transformations are same with the traditional ones. After the heated treatment of same carbonized temperature, electrospun carbonized fibers show smooth surface, smaller average grain size, larger interlayer spacing, more mean stack layers and higher apparent crystallinity. It seems that the structure of electrospun carbonized fibers is more regular. But the different oxidizing technologies make no differences for the crystal structure of electrospun carbonized fibers.The bulk resistivity of different carbonized electrospun fibers was measured. The bulk resistivity of the fibers decreases when the carbonization temperature rises and the oxidizing method has little influence on the carbonized fibers of same heating temperature. The carbonized fibers were grinded into pieces and mixed with the four fluorine resin to prepare composite coating. The surface resistivity decreases with the increasing amount of fibers. Compared with the coating mixed with short traditional carbon fibers, the electrospun fiber amount is too much and the surface resistivity is higher. Covering the whole single piece of carbonized fibers with resin layer directly can decrease the surface resistivity effectively and the fibers proportion is low. Finally the single piece of carbonized fibers was covered with the resin mixed with the grated fiber. The surface resistivity is lower than the coating consisted of resin and single piece of carbonized fibers. The surface resistivity decreases with the increasing tested voltage.