High-Temperature Properties Of Continuous Sic Fibers And The Improved Process Based On Thermal Crosslinking

The fact that the high-temperature properties of conventional continuous SiC fibers are much lower than the ideal values was systematically studied. The high-temperature properties of SiC fibers were improved by controlling the composition and structure through improving the processes of curing and pyrolysis of the precursor polycarbosilane (PCS) fibers. The studies demonstrate that the high oxygen and free carbon content of domestic manufactured KD-SiC fibers are the key factors of the lower high-temperature properties. The oxygen content of SiC fibers was significantly decreased using a low-degree air-curing followed by thermal crosslinking (TC) process, by which the tensile strength of the SiC fibers was also improved. While the free carbon content of these fibers was reduced by a one-step pyrolysis in the mixed atmosphere of H2 and N2. The continuous SiC fibers with low oxygen and free carbon content were prepared in the pilot plant, and the high-temperature properties of these fibers are similar to the commercial Nicalon fibers (NL-202).The KD-SiC fibers are characterized with about 20 wt% of oxygen content, carbon-rich in the surface and rather low in crystallinity. As comparison, NL-202 fibers are with about 12 wt% of oxygen content, oxygen-rich in the surface and higher crystallinity. Both types of fiber are multiphase fibers consisted of P-SiC crystalline, Si-C-0 amorphous and free carbon phase.The excess oxygen of KD-SiC fibers leads to a large range of Si-C-0 amorphous phase which decomposed rapidly in heat-treating at 1600℃ in Ar, and the tensile strength of KD-SiC fibers declined sharply. The free carbon in KD-SiC fibers was oxidized in air from 800℃, forming a thick oxide layer, and the tensile strength of the fiber decreased rapidly. As an example, the tensile strength residue ratio of KD-SiC was 20.4% after heat-treating at 1200℃ for 1 hr in air. So it is necessary to decrease the oxygen and free carbon content of continuous SiC fibers to improve the high-temperature properties.The mechanism of the deterioration of SiC fibers in high temperature heat-treatment in Ar was studied in details by XRD, Raman, SEM, EMPA and Surface Energy Spectrum. In the process, large amount of CO gas escaped from the fibers that caused a large weight loss. The most part of the Si-C-O amorphous phase became silicon-rich (C/Si =0.55) with almost no oxygen remained. Meanwhile, the p-SiC crystallites grew fast to form large grains until being restricted by the surrounding free carbon. Therefore the surface of grain was carbon-rich (C/Si =1.79). The huge difference and the boundary in both phases lead the fibers collapse completely.Before thermal crosslinking in N2? it was necessary for PCS fibers to be cured in air to a low degree to withstanding the temperature of the process. The oxygen content of TC-PCS fibers thus resulted was about 11 wt%, obviously lower than the HO-PCS fibers by high-degree air-cured (about 15 wt%, by which the KD-SiC fibers prepared). The optimum condition of thermal crosslinking was 420- 480℃, 4-8hr, and the elevated rate was 30-60癈/hr. The mechanism of thermal crosslinking was mainly thermal condensation reaction between Si-H and Si-CH3 bonds to form Si-CH2-Si bridge as well as the condensation of Si-OHbonds to form Si-O-Si bridge, those make the PCS molecular becoming three dimensional crosslink structure. The kinetics of gel content was investigated and a first order reaction was found with the activation energy 17 KJ/mol. The reaction degree of Si-H in TC-PCS fibers was linear to gel content.The pyrolysis of TC-PCS fibers were carried out by one-step and two-steps processes, and TC-SiC-I and TC-SiC-II fibers were thus prepared respectively. Similar to HO-PCS fibers to prepare KD-SiC fibers, the optimum condition for TC-SiC-I fibers were that the pyrolysis temperature 1350癈, N2 flow rate 300-400L/hr, winding rate 26-30m/hr. TC-SiC-I fibers were characterized with about 14 wt% of oxygen content (obviously lower than KD-SiC fibers), higher crystallinity than KD-fibers and carbon-rich on the su...