Preparation Of BCN Fibers By Boronation Of PAN Fibers

BCN materials are attracting more and more peoples'interests and concerns for their excellent performance and potential applications. This paper reported the preparation of BCN solid and hollow fibers by curing and carbonization of green PAN fibers in different atmospheres. The influences of heat treatment parameters, fiber morphologies and atmosphere alternations were studied on the preparation of the BCN fibers. The chemical compositions, microstructures and permittivities of the final products were characterized and analyzed.When the green PAN fibers were cured in air and carbonized in BCl₃, the inorganic fibers obtained have been primarily consisting of B, C, N and O elements in addition to a trace percent of H. The B content of the fibers have been increased with the increasing of holding time at 400℃when the BCl₃ was introduced. BCN attached particles can be observed on the surface of the solid PAN fibers, as a result of the boronation of the released gas, HCN. The B content has been decreased gradually from the surface to the centre, which was found mainly in the form of B-N bonds. When the hollow PAN fibers were used instead, the inorganic fibers obtained have a quite smooth surface which was mainly consisting of B and C elements. Some white particles can also be observed on the surfaces.When the green PAN fibers were cured in BCl₃, the C≡N bonds of green PAN fibers reacted with BCl₃ and formed B-N crosslinking bonds between different moleculars, leading to the infusible state. The B content can increase up to 6.65% when the hollow green PAN fibers were cured at 230²90℃for 3h and 400℃for 3h. The fiber sufaces mainly consiste of B and C. Compared with the air-cured PAN fibers, the pyrolysis yield of BCl₃-cured PAN fibers increased, and at the same time, the amount of the virulent gas HCN have been released during carbonization reduced evidently.When the green PAN fibers were cured alternatively in BCl₃ and NH₃, a worm-like structure has been formed on the fiber surfaces due to the shrinkage variation in the diameter direction. This is a result of the gradual distribution of B-N bonds, which were confirmed by the XPS, FTIR and EDS analysis. After carbonization, the diameter increased from 10μm to13μm, which is consistent with the modeling results. When a tensile force was applied on the fibers in the process, the worm-like structure has been prevented and the diameter decreased to 9μm as well. When the hollow fibers were used, no worm-like structure has been formed as well.The electrical resistivities of the inorganic fibers has been increased in order level from 0.03Ω?cm to 0.36Ω?cm, 1.46Ω?cm and 26.2Ω?cm, corresponding to the fibers obtained from the air-cured PAN fibers carbonized in N2, the air-cured PAN fibers carbonized in BCl₃, the green PAN fibers cured and carbonized in BCl₃, the green PAN fibers cured and carbonized in BCl₃/NH₃, respectively. The permittivity of the fiber/paraffin composite has been reduced as well, especially for the imaginary parts of the permittivity. The permittivity of the hollow fibers is much lower than the solid fibers, especially for the imaginary parts.