| As a heat-resistant low-density material with high specific strength and specific modulus, SiC continuous fibers, particularly zirconium-containing ones, have excellent resistance to chemical attack at high temperature and are promising for wide applications in many industries such as aeronautical, aerospace, nuclear and chemical engineering. However, domestic research on zirconium-containing SiC fibers is scarce and limited to low zirconium contents below 5wt%. It is, therefore, necessary to study high-performance silicon carbide fibers with various zirconium contents.In this thesis, synthesis of polyzirconocarbosilane (PZCS) was carried out by using polycarbosilane and zirconium n-butoxiden-butyl alcohol as starting materials at high temperature under atmospheric pressure argon flow. Three types of PZCS containing, in theory,0.5wt%,3.8wt% and 9.5wt% zirconium were fabricated, which were then melt-spun into three green SiC fibers, respectively. These green fibers were then cured in ambient air and pyrolysed to ceramic fibers. Also investigated were effects of pyrolysis temperature, atomosphere, contents of zirconium and fiber string tension on fiber tensile strength. Fiber morphology, microstructure and chemical composition were characterized by Scanning Electron Microscope (SEM) and Tunnelling Electron Microscope (TEM) with attached Energy Spectrum Analysis (EDX). X-ray Diffraction (XRD) was used for identifying phase compositions. Tensile strength at room temperature and after exposing to high-temperature oxidation conditions was measured and correlated with the fiber morphology, composition and structure. The main results are summarized as follows.(1) The synthesis temperatures of about 300℃ with a hold time 2h,250℃ holding for 2h, and 230℃ for 7h were shown to be the optimum conditions for the PZCS precursors with 0.5,3.8 and 9.5 wt% zirconium, respectively.And the softing point of the precursors is 210℃,210℃ and 165℃, respectively.(2) For the PZCS precursor with 0.5 wt% zirconium, green SiC fibers were processed at the spinning temperature of about 310℃ with a spinning pressure 0.5MPa to attain a maximum spinning speed 15 r/s, while the precursor with 3.8wt% zirconium needed a spinning temperature of about 300℃ under a pressure 0.6MPa to give the maximum spinning speed 15 r/s. The precursor with 9.5wt% zirconium saw a best fit condition of the spinning temperature at about 210℃ and pressure at 0.8MPa to yield a maximum spinning speed 7.5 r/s.(3) The tensile strength of the three types of SiC fibers is 2.2-2.3GPa,2.6-2.7GPa and 2.4-2.5GPa, respectively. The tension during the pyrolysis process is very helpful to improve the fibers’tensile strength. The three types of SiC fibers are observed as no obvious defect on fiber surface by SEM. The SiC fibers with 0.5wt% zirconium content contain 9.16wt% oxygen and have average dimeters of 12～13μm. The SiC fibers with 3.8wt% zirconium content contain7.68wt% oxygen and have average dimeters of 13～14μm. The SiC fibers with 9.5wt% zirconium content contain 12.73wt% oxygen and have average dimeters of 13-14um.(4) The results of the high-temperature oxidation tests are as follows. After high temperature oxidation treatment at 1250℃ for 0.5h in ambient air, the residual tensile strength of the three types of fibers are 1.5GPa,1.9GPa and 1.7Gpa, respectively. The strength retention rates are 65.5%,71% and 68%. Moreover, after high temperature oxidation treatment in 1000℃ for 100h under air atmosphere, the strength retention rates of the three types of SiC fibers containing diferent zirconium are 65%,74% and 71%, respectively.(5) After the high temperature treatment under air atmosphere, the SiC fibers containing diferent zirconium have formed a dense oxidation layer of SiO2. The thickness of the oxidation layer will improve with the increase of the oxidation time and temperature. |
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