Study On Effect Factors Of Elastic Modulus And Improved Process Of SiC Fibers

The effect factors of elastic modulus of SiC fiber was studied in this paper which focused on the poor elastic modulus of domestic manufactured SiC fibers. The reasonable conditions for testing the elastic modulus of SiC fibers were discussed based on the current test. The effect of oxygen content on the elastic modulus of SiC fibers was studied by prepared the SiC fibers with different oxygen content; the relationship between carbon content and the elastic modulus was researched by pyrolyzed the SiC fibers with different carbon contents in the mixture of H₂ and N₂; The effect ofβ-SiC crystallite size on the elastic modulus of SiC fibers was studied after high temperature heat treatment. This relationship between the SiC fiber diameter, density and elastic modulus was also studied in the paper. On the basis of these studies, a higher modulus of SiC fibers was obtained by optimized the preparation process.A reasonable testing method for elastic modulus of the SiC fiber was established based on the choice of the binder, span, and the speed of tensile test.Series of PCS fibers with different Si-H reaction degree were prepared by different curing process. SiC fibers with different oxygen content were obtained by pyrolyzed at the same temperature. The results show that: The oxygen content increased the crystallization decreased the Si-C-O amorphous phase increased and the modulus of SiC fibers were decreased. The oxygen content changed from 11.7wt% to 21.5wt%, while the elastic modulus decreased from 167GPa to 131GPa. The obviously influence of the oxygen content on the elastic modulus of SiC fibers was found. After treated in the high temperature, the results show that the lower oxygen contents in the SiC fiber, the higher the elastic modulus at the same heat treatment temperature. The oxygen content of CVC-SiC and TC-SiC fibers were 8.5wt% and 9.6wt%, respectively, and the elastic modulus of 185GPa and 178GPa. So, lower oxygen content can promote the elastic modulus of SiC fibers.Compared with the carbon-riched SiC fiber (carbon content 16.4wt %) prepared in N₂, higher degree of regular carbon and more carbon crystallite were existed in the low carbon content fiber (carbon content 26.8wt %) pyrolyzed at 1000℃in mixed atmosphere of H₂ and N₂ by one-step process. Due to poor compaction and low crystallization, the tensile strength and elastic modulus of this fiber were low of 15% and 40% respectivly. The heat treatment of KD fibers in mixed atmosphere and N₂ was studied in this work. It was found that the free carbon content reduced 4.4%, and the elastic modulus increased by 6% in the fibers after heat-treated in the mixed atmosphere. While the free carbon content was reduced by 1.3%, and the elastic modulus changed hardly in the fiber after heat-treated in N₂. So, the reduction of carbon content can promote the elastic modulus of SiC fibers. Theβ-SiC crystallite grain size was changed by heat-treated in different high temperature with oxygen-rich KD-SiC fibers and CVC-SiC fibers. Theβ-SiC crystallite grain size increased from 0.5nm to 2.4nm while the heat treatment temperature increased from 800℃to 1400℃. The results show that: with the increase of crystallite size, the elastic modulus increased from 165GPa to 200GPa. It was declined when the crystallite size was larger than 2nm. After the high-temperature heat treatment with different oxygen content of SiC fibers, it was also found the elastic modulus increased with the increasing crystallite size while the crystallite size was less than 2nm. The elastic modulus was decreased when the defect appeared which caused by the excess growth of crystallite size. As a result, it was useful to promote the elastic modulus of SiC fibers by increasing the crystallite size in a certain range.The relationship between diameter, density, defects and elastic modulus of SiC fiber was researched in this paper. Series of PCS fibers with different diameters were prepared by different speed of spinning. SiC fibers which diameters distributed between 8μm to 21μm were obtained by pyrolyzed at the same temperature. The results show that: with the SiC fiber diameter increased, the tensile strength decreased from 2.0GPa to 0.6GPa, and the elastic modulus decreased from 190GPa to 110GPa. So, it is easy to obtain excellent tensile strength and modulus in finer SiC fibers. The density of SiC fiber was tested by suspension. The results showed that, the density of SiC fiber was increased with the increasing of crystallite size. The density increased, the elastic modulus increased too. It was found that the elastic modulus of SiC fiber was not only determined by the crystallite size but also the defects which studied. Based on the comprehensive study, an optimize preparation process was proposed to pyrolyzed SiC fiber with higher elastic modulus in tube-furnace by one-step process. Series of SiC fibers with tensile strength > 2.0GPa and elastic modulus >200GPa were prepared by preparation of small diameter fibers, pre-oxidation at 170-190℃, closed nitrogen atmosphere ,slowly heating , pyrolyzed at 1200℃, heat-treated at 1000℃in mixed atmosphere of H₂ and N₂.The primary formula used for accounting the elastic modulus of SiC fibers was studied in this paper finally.