Research On The Preparation Of Mircrowave Transparent SiBN Fiber

Combining the advantages of Si3N4 fiber and BN fiber, continuous SiBN fiber is the ideal candidate as reinforcement in the ceramic matrix composites for high temperature microwave transparent application because of its high thermal stability, mechanical and dielectric properties. The study on SiBN fiber is a newly developing field, and polymer-derived technique is the most effective way for the preparation of SiBN fibers. There is long way to go before its application. The synthesis scale of the precursor is low and the process for the preparation of SiBN fiber should be improved. Focused on such problems, this study has laid a sound basis for the continuous preparation of polymer-derived SiBN fiber.Polyborosilazane(PBSZ) was synthesized by using boron trichloride(BTC), dichloromethylsilane(DCMS) and hexamethyldisilazane(HMDZ) as the monomers through condensation route. The softening point of the achieved PBSZ can be adjusted by changing the molar ratio of the monomers. It can be adjusted in the range of RT~107℃by changing the molar percent of BTC proportionally from 9.4% to10.5%. However, when we ranged the molar percent of DCMS from 0 to 22.5%, the precursor with the soften point of about 75℃~40℃could thus be obtained. In addtion, it can also be found that more molar ratio of HMDZ resulted lower soten point precursor. The molar ratio of BTC:DCMS:HMDZ was optimized to be 0.9:2:6, under which the synthesis of the PBSZ can be scaled up from 15 g/batch to 200 g/batch without changing its composition and melt-spinnability. All the works mentioned above greatly contribute to the continuous spinning of PBSZ precursor and the preparation of SiBN ceramic fiber.Reaction mechanism study show that the preparation of PBSZ can be divided into three stages. 1) BTC and DCMS react with HMDZ to form low molecular-weight intermediate by releasing chlorotrimethylsilane. 2) the molecule weight of the intermediate molecule grow through intermolecular condensation with the removal of HMDZ. 3) the final PBSZ was obtained by dehydrogenation reaction. The backbone of PBSZ is composed of Si-NH-Si, Si-NH-B, and borazinic rings, in which, Si exists as SiN3C, SiN2CH and SiNC3 structure and B exists only as BN3 bonds. In addition, it should be noted that Si-(CH3)3 is the end group of the precuror, and the existance of Si-H and N-H bonds endows the precuror great activity.The study of rheological property of PBSZ shows that the PBSZ melt is a pseudo-plasticity liquid and the apparent active energy is about 142kJ/mol. The achived precuror exibits a very good spinnability when the apparent viscosity and the flow index are in the range of 700~800Pa·s and 0.84~0.87, respectively. Typically, the spinning process was carried out with the achieved large amount of precursor from the optimum techinique, and a optimum spinning process was obtained.Prior to pyrolysis, chemical vapor curing and thermal curing were combined to render the green PBSZ fibers infusible. It was found in the optimum curing process that the green fibers were cured first in a vapor of DCMS for 30min at room temperature, and then they were thermally cured at the temperature up to 300℃with a heating rate of 0.5℃/min. The cured fibers had a gel content of about 90.3% with carbon decrease of about 18.6%. Pyrolysis of the sufficiently cured PBSZ fibers was performed directly in an NH3/N2 atmosphere and SiBN fiber with the tensile strength of 1.0GPa could be obtained when the cured fiber was pyrolysed at 1200℃. The as-pyrolyzed fibers with carbon content of only about 0.1wt% are amorphous as shown by XRD pattern. The effectively controlling of the low carbon content endows SiBN fiber excellent wave-transparent property with the dielectric constant and dielectric loss tangent of about 3.6 and 0.0011, respectively.