The Catalytic Effect Of Boron Nitride On The Structure And Mechanical Properties Of Polyacrylonitrile-based Carbon Fiber

Polyacrylonitrile-based carbon fibers have been used in many fields, such as the aerospace and automotive industry, due to their excellent mechanical properties. However, the polyacrylonitrile-based carbon fibers is one of difficult graphitized materials. In order to obtain carbon fibers with Young’s modulus higher than 400 GPa, carbon fibers must be subjected to heat treatment at 3000℃, which makes continuous production difficult and increases the cost. What’s the most important, the tensile strength of the polyacrylonitrile-based carbon fibers would degrade sharply with increasing heating temperature. Therefore, it is necessary to add some chemicals to carbon fibers to accelerate their graphitization process at lower temperature.Among many catalysts, boron is a unique graphite catalyst that can replace carbon atoms in graphite-like lattice, and promote the graphitization process by preventing formation of separate distinct carbon components. Although the catalytic graphitization effects of boron on carbon fibers have been investigated and can increase the Young’s modulus during the graphitization process, the effects of boron and boron content on the tensile strength are not very clear. Furthermore, through the impregnation method for introducing boron into carbon fibers, the interaction force between carbon fibers and boride is very weak, leading to boride easily detached from carbon fibers during production process.Therefore, in this article, we applied a method using poly(vinyl alcohol)(PVA) as a binder and form a uniform coating with quantitative catalyst on the surface of carbon fibers. The effect of boron nitride catalytic graphitization on microstructure and mechanical properties of PAN-based carbon fibers was investigated. The results showed that: BN has a clear catalytic effect on the graphitization of carbon fibers. It was found that the Young’s modulus of the derived carbon fibers becomes higher with BN amounts increasing through heat-treatments at 1600-2350℃, while the tensile strength increases at 1600 and 2000℃ and decreases at 2200 and 2350℃with BN amounts increasing. The modulus and strength of carbon fibers are increased by 6.6% and 7.4% at maximum respectively. It can be interpreted that the modulus is mainly determined by the preferred orientation of graphene planes and the shear modulus between them, while the tensile strength is determined by not only the two factors but also defects. Through X-ray diffraction, Raman spectra and scanning electron microscope analysis, it is noted that BN catalysis enhances the preferred orientation of graphene planes and make little change on the shear modulus, however generates more defects above 2200℃ for intensive reaction.