Study On Carbonization Mechanisms Of Polyacrylonitrile Fibers During Microwave Heating

Polyacrylonitrile (PAN) fibers are one of the most important precursors for the production of high-performance carbon fibers (CFs). Although CFs have a superior tensile strength, the practical mechanical property, especially the tensile strength of CF is far lower than the theoretically predicted value. In order to further improve the mechanical properties and reduce the manufacturing cost of the CFs, extensive efforts have been focused on the development of manufacturing processes. Alternative method for the carbonization process such as microwave heating has also been considered as a more rapid and energy saving method compared with conventional heating. However, there has been little true development in understanding of the microwave effect on the CF carbonization, other than the empirical consensus that the microwave heating is more rapid and energy saving than the conventional heating method. To accurately understand the microwave heating mechanism of CFs, the electromagnetic properties and microwave absorption of CFs fabricated from different carbonization temperatures were systematically investigated. Moreover, the pre-oxidized fibers were carbonized by microwave heating and conventional heating, respectively, the temperature in the two processes was calibrated by the process temperature control ring that was wrapped by the CFs. The microstructure and composition evolution of the obtained fibers were monitored and compared in the two processes under different ring temperatures to get better understanding of the microwave effect on the CF carbonization. Finally, a two-step carbonization process including microwave or conventional pre-carbonization was conducted to verify the effect of pre-carbonization manner and temperature on the microstructure of CFs. The main conclusions are as follows:The pre-oxidized fibers were carbonized by conventional heating at the temperature ranging from 400 to 1300 ? for 1 h. The microwave absorption of CFs was examined in the frequency range of 2-18 GHz, and it is found that the reflection loss characteristics are highly sensitive to the carbonization temperature. The real (?') and imaginary part (?") of complex permittivity of CFs obtained below 710 ? are almost constant with nearly no variation throughout the whole frequency range, implying its very poor dielectric loss. With the carbonization temperture increasing, both ?' and ?" of CFs are simultaneously improved, which should be attributed to the formation of more nanocrystalline graphite domains at high carbonization temperature and the consequent increase of electrical conductivities. In addition,it is discovered that the real (?') and imaginary parts (?") of complex permeability of all the CFs are close to one and zero, respectively, indicating the negligible magnetic loss for incident electromagnetic wave. At a thickness of 2 mm, the CFs obtained at 710 ? exhibits the best microwave absorbing ability with a maximum reflection loss of -22.9 dB at 15 GHz,and a bandwidth exceeding -10 dB in the range 12.4-18 GHz. It is concluded that dielectric loss in cooperation with better matched characteristic impedance results in the excellent microwave absorption of CFs. It is apparent that the carbonization temperature is critical for microwave absorption of CFs. Low temperature will make ?' and ?" too small to consume the energy of microwave, while over high temperature will make ?' and ?" too large to transmit the microwave into the materials.The pre-oxidized fibers were carbonized by microwave heating and conventional heating,respectively. The temperature in the two processes was calibrated by a process temperature control ring that was wrapped by the fibers. The microwave carbonized fibers (MCFs)fabricated at 750-1000 ? possess the higher carbon content, larger average crystallite dimensions (La and Lc), higher porosity, and more homogeneous structure along cross-section than those of conventionally carbonized fibers (CCFs) obtained at the same temperature.However, when the temperature reaches 1000 ? and above, the crystallite sizes of MCFs are smaller than those of CCFs fabricated at the same temperature, and the difference between the skin and core structure becomes larger as the temperature increases for MCFs. The tensile strength and modulus of MCFs are higher than those of CCFs obtained at 750-1000 ?, while the mechanical properties for MCFs and CCFs obtained at temperature above 1000 ? is close.This phenomenon could be attributed to the temperature dependence of microwave adsorption ability of the as-obtained CFs.When the CFs are pre-carbonized by microwave below 700 ? prior to the final conventional carbonization process, the as-obtained CFs possess lower carbon content, higher d002 and lower Lc value than those of conventionally pre-carbonized CFs. The CFs pre-carbonized by microwave at 820 ? have the crystallite sizes of graphite domains (La and Lc)than those of conventionally pre-carbonized CFs,but the high-temperature conventional carbonization destroys the uniform cross-section structure of CFs.