The Influence Of Precursor Modification On The Structure And Properties Of Carbon Fiber

With the continuous development of science and technology and increasing awareness of energy conservation, carbon fiber has become the key materials for national defense and essential civilian. Among the three commercialized carbon fibers, polyacrylonitrile (PAN) based carbon fiber has been predominantly developed for its combination properties. The characteristics of high performance carbon fiber include high tensile strength, high Young's modulus, and high carbon content, less porosity, less structure defects. However, in addition to improving the technology of pre-oxidation, carbonization and graphitization, the modification of PAN precursor has also been a means to increase the performance of carbon fiber.Potassium permanganate, NiSO4, COSO4 and other solvent have been used to impregnate the precursor by the researchers, which could reduce the temperature of pre-oxidation, ease the pre-oxidation heat and improve the performance of carbon fiber. At the same time, the researchers have also found that the modifiers would bring metal ions into fibers, which reduced the mechanical properties of carbon fiber. Therefore, non-metallic boron was proposed as a modifier. In the process of high-temperature heat treatment, boron can replace the carbon, also boron and carbon can form solid solution, which could catalyze the graphitization. The role of boron diffusion solution could make up the lattice defects of fibers, promote the growth of crystal and order of graphite layers, which improve the strength and modulus of carbon fibers and graphite fibers. However, these results were obtained through impregnating carbon fibers with boron before graphitization.Compared with the cross-link structure of carbon fiber, the precursor is the linear structure, so boric acid could be deep into the fibers more easily. Besides, boron oxide coating on the surface of fibers could depress the oxidation, which could give more time for oxygen to diffuse into the fibers. So we choose boric acid to impregnate the precursor of carbon fibers.A polyacrylonitrile fiber was dipped with boric acid, and then was oxidized, carbonized and graphitized successively. The structure and performance of the precursor fiber, the derived oxidized fiber, carbon fiber and graphite fiber were studied on the characterizations of elemental analysis (EA), X ray diffraction (XRD), infrared ray (IR), differential scanning calorimetry(DSC), scanning electron microscope (SEM), as well as fiber density and tensile strength tests. The results showed through impregnation, the boric acid could enter the interior of the precursor, but was only physically adsorbed without chemical reactions. However, the boric acid could improve the performance of precursor. The adsorption dynamics of boric acid to the precursor were fitted by the Bangham equation based on the experimental data. DSC and DMA analysis displayed that during the process of pre-oxidation, boric acid could deactivate the oxidation, which could interpret development of the strength and modulus of the pre-oxidized fiber. By the analysis of XRD, boric acid reduced the spacing distance of the microcrystal of the derived carbon fibers and improved the ordering of the fiber. SEM analysis showed that boric acid could make up the defects of fibers during carbonization and graphitization.Because the carbonization and graphitization were intermittent, during which the strain imposed on the fiber was not large enough and unchanged, the mechanical properties of the derived carbon fiber were not so good. However, after modification of boric acid, the strength of the derived carbon fiber carbonized at 1200℃was improved from 1.6GPa to 2.54GPa, increasing by 58.8%.