Preparation Of Polyacrylonitrile High Strength And Modulus Carbon Fibers By Graphitization Using Boron

Graphitization is a process of the disordered carbon structure transformed into ordered structure of hexagonal graphite by heat treatment at high temperature, at the same time the defects within or between the graphene layers are partially removed through atomic displacement. Graphifization is an essential procedure to produce high modulus carbon fibers. However, polyacrylonitrile based carbon fibers are mainly composed of amorphous carbon which is difficult to be transformed into graphite crystallite structure even through heat treatment above 3000℃. Furthermore, the tensile strength of the fibers would degrade sharply due to the excessive weight loss caused by displacement and escapement of carbon atoms at such a high temperature.Boron is a unique catalyst that can substitute the positions of carbon atoms in graphite-like structure, and accelerated the graphitization process by preventing formation of separate distinct carbon components. Although the catalytic graphitization effects of boron on carbon fibers have been reported and are able to enhance the tensile strength as well as the modulus in the process of graphitization, the effects of boron and boron content on fibers tensile strength are far more than clear. Therefore, more detailed work is needed to clarify the effects of boron and boron content on mechanical properties of carbon fibers. In this paper, the catalytic graphitization of polyacrylonitrile based carbon fibers by boric acid and nano-B4C was studied, and the effects of boron on mechanical properties of carbon fibers, and the relationship between microstructure and mechanical properties was taken further efforts to clarify.It is obvious that boron has significant catalytic graphitization effect on polyacrylonitrile based carbon fibers, which resulting in lower d002 and higher preferred orientation degree. The mechanical properties of the fibers depend on boron content and the temperature. It is found that there existed a key temperature point for the boron to take effect. When the fibers were modified with 7.0 wt% boric acid solution, with increasing temperature in a range of 500℃to 2300℃, the tensile strength was lower than that of the unmodified ones, but a reverse situation happened above 2300℃. Moreover, when being heated at 2500℃, the modified fibers showed an increasing tensile modulus and strength with increasing boron content to maximums of 404 GPa and 2.46 GPa,26% and 16% higher than those of unmodified ones. The mechanical properties of the fibers were affected by the interaction of carbon and boron, and also related with boron states. The decomposition of boron acid and its interaction with carbon brought defects on fiber surface, degrading the mechanical properties from 500℃to 1300℃. With further heat treatment, the boron diffused into the fibers and divided into two states:substitutional and interstitial. At a temperature over 2300℃with an appreciate boron content, the substitutional would be the dominant one, which removed the structural defects and relaxed the distortions, and benefit the mechanical properties. When nano-B4C used as catalyst, it not only increases the fibers modulus monotonously, but also enhances the fibers tensile strength from 1600℃to 2700℃with an appreciate boron content. The modified ones show an increasing tensile strength and modulus with increasing boron content till maximums of 3.08 GPa and 394 GPa,18% and 13% higher than those of unmodified ones when being heated at 2700℃.