Study Of Laser-induced Graphitization Of Carbon Fiber

PAN-based carbon fiber has been widely applied in many high-tech fields such as aerospace and so on.However,the cost of carbon fiber's preparation is so high that many civil uses are limited.PAN-based carbon fiber's cost originates from two parts,one is the PAN precursor and the other is the investment of complex devices and high-power consumption.The preparation of carbon fiber is a continuous process,during which the graphitization needs the highest power input.This is because the temperature of graphitization is more than 2000? in most cases.While the traditional graphitization furnace heats carbon fiber indirectly via graphite tube's Joule heat.Its heated special range is far beyond the volume of carbon fiber tow.Therefore,the efficiency of energy utilization is extremely low.Many factors including traditional graphitization furnace's complex configuration and slow heating rate lead to graphite fiber's high cost.According to our knowledge,laser is a powerful tool which can generate concentrated energy and heat substrates to a very high temperature within a small period.In addition,the graphitization furnace based on laser does not need heat-insulating materials with good quality.Hence,this technique is very suitable to be applied in the graphitization of carbon fiber.The feasibility of laser graphitization was confirmed through initial experiments.After continuous improvements of devices,uninterrupted graphite fibers were obtained.Furthermore,this thesis also investigated the effect of laser parameters on multi-scale structures of resultant graphite fibers and revealed the mechanism of laser graphitization.The results are shown as below:1.Raman spectroscopy was applied to study the evolution of chemical structures under different laser power densities.As the power density increases,D band and G band separate more obviously and the ratio of intensities between D band and G band decrease gradually.Which indicates that the degree of graphitization shows an upward tendency with power density.Moreover,the distribution of graphitization degree(AD/AG value)along the radial direction exhibits a more obvious skin-core difference with power density.2.Wide Angel X-ray Diffraction(WAXD)was used to study the effect of power density on the microcrystal structures of resultant carbon fibers.Carbon dioxide laser generates heating effect during graphitization process,which is same as traditional graphitization furnace.Hence the crystal sizes and orientation are improved with the increase of power density.While the increase of La is not obvious.This is because the graphite layers are peeled off from the carbon fiber surface as the function of laser irradiation.3.SEM was used to observe the morphological changes of carbon fiber's surface after laser graphitization.HRTEM was applied to investigate the formation mechanism of sheet-like structures on the surface of carbon fiber.As the power density increased,the thickness of sheet-like structures increased and sheets transformed into granular structure finally.Sheet-like structures are actually nano graphite sheets,and they are connected to fiber via amorphous carbon.4.The investigation of kinetics of laser graphitization of carbon fibers was conducted due to laser's characteristics such as fast heating and highly controllable time periods.With the increase of power density,carbon fiber's graphitization degree was improved and the time needed was reduced profoundly.