Preparation Of Polymer-Derived Carbon Doped And Boron-Carbon Doped Silicon Carbide Fibers

Polymer-derived is the main method for preparing continuous SiC fibers.Precursor polycarbosilane of SiC fibers can be modified by physical or chemical methods to prepare doped silicon carbide composite fibers with excellent high temperature resistance and microwave-absorbing properties.Therefore,the researches and developments of modified SiC fibers have become the hot topics in recent years.In this dissertation,the preparation of carbon doped and boron-carbon doped SiC fibers were systematically investigated.FTIR ? XRD ? SEM ? TEM ? Raman and other characterization methods were used to analyze the composition and structure of the fibers.The mechanical properties,electromagnetic properties and oxidation resistance of the fibers were also tested.The main results and conclusions are as follows:?1?The C doped and B-C doped SiC precursors can be prepared by the dissolving and mixing with polycarbosilane,pitch xylene soluble fraction and polyborosilazane,the content of pitch xylene soluble fraction and boron have a significant effect on the spinning performance of the precursors.When the content of pitch xylene soluble is 20wt%,the spinning performance of C doped SiC precursor behaves best,and the B-C doped SiC precursor has the best spinning performance when the content of boron is 0.08wt%,and the spinning performance of the precursor becomes worse with the further increase of boron content.The prepared C doped and B-C doped SiC precursors were then melt spun into precursor fibers.The precursor fibers were subsequently cured in air.The C doped and B-C doped silicon carbide fibers were finally obtained by the heat treatment of the cured fibers.?2?The heat treatment temperature has an important influence on the structure and properties of C doped and B-C doped SiC fibers.The tensile strength of C doped and B-C doped SiC fibers increased with the increase of heat treatment temperature and the tensile strength of fibers was the highest when the heat treatment temperature reached 1300?.After that,the tensile strength of fibers decreased rapidly with the increase of heat treatment temperature.Doping a few boron in B-C doped SiC fibers can obviously inhibit the growth of SiC grains during high temperature treatment and improve the strength of fiber.?3?C doped SiC fibers have a certain frequency dispersion effect on electromagnetic waves,they have a reflectivity of less than-5d B in the X band?8.2¹2.4GHz?and a reflectivity of less than-10 d B when the frequency range is 8.7¹2.4GHz,with a maximum absorption value of-14.5d B.Thus,C doped SiC fibers broaden the absorption bandwidth of SiC fibers to electromagnetic wave and improve its microwave absorption performance.?4?Compared with C doped SiC fibers,B-C doped SiC fibers show better oxidation resistance and thermal stability due to the simultaneous generation of B₂O₃ and SiO₂ glass phases during high temperature oxidation,which could dissolve and react with each other to form a borosilicate protective film with good fluidity and low oxygen diffusion coefficient.