Effect Of Group Components On Structures And Properties Of Mesophase Pitch And Its Corresponding Carbon Fibers

Mesophase pitch is an excellent precursor of carbon materials.Its corresponding carbon fiber is widely used in many fields due to the excellent properties such as high modulus,high thermal conductivity and low thermal expansion coefficient.In this thesis,the roles of group components in the mesophase pitch were investigated,and further the components of mesophase pitch were adjusted to prepare carbon fibers.The effects of n-heptane soluble and toluene soluble components in mesophase pitch on the structures and properties of mesophase pitch and corresponding carbon fibers were investigated in detail.The compositions and structures of the pitch samples were studied by elemental analysis(EA),hot stage polarizing microscope,infrared spectroscopy(FTIR),X-ray diffraction(XRD)and Raman spectroscopy.The results show that the softening point of the mesophase pitch and aromatic index increase with the reduction of the n-heptane or toluene soluble components(HS and TS).Furthermore,the mesophase develops from domain-flow structure to mosaic structure.Of course the crystal structures also change.The stack spacing of polycyclic aromatic hydrocarbons gradually decreases,and the stacking thickness,plane size and corresponding flake number all increase significantly.The spinnability of pitch becomes worse.The relationship between HS and TS in mesophase pitch and fiber structure and properties was investigated by means of SEM,XRD,mechanical and conductivity testing methods.With the content of HS in mesophase pitch decreases from 5.31%to 0%,the radial structure in fiber section gradually changes to the random structure.The distance between the crystal planes of the fiber is reduced,and the size of the unit cell gradually increases,which is beneficial to the ordering of the crystal arrangement.The tensile strength of graphite fiber increases from 1438.2 MPa to 2048.2 MPa,while the thermal conductivity decreases from 880.6 W/(m·K)to 658.75 W/(m·K).With the decrease of TS content from 21.43%to 19.18%in mesophase-pitch,The cross section of the fibers develops from radial structure to onion skin structure.The size of graphite microcrystalline and the number of graphite lamellae increase obviously,and the crystal structure changes.The tensile strength of the fibers increases to 2252.6 MPa,while the thermal conductivity decreases to 613.9 W/(m·K).After the fibers spun from the original mesophase pitch were impregnated with n-heptane and toluene,the microstructure of the cross-section structure did not change significantly,but the mechanical properties increase to 1808 MPa,and the thermal conductivity decrease to 555.7 W/(m·k).After the fiber is immersed in pyridine,cracks appear,and as the treatment time increases,the cracks deepen and even cause fiber cracking.