Study Of Carbon Nanotubes Reinforced SiC Fiber And Composites

To prepare carbon nanotubes (CNT) reinforced ceramic composites is one of the most important application of CNT. However, several key issues must be considered. Firstly, the CNT should be dispersed uniformly in the matrices without any agglomeration. Secondly, a compatible interface between the CNT and the matrices is needed to achieve an enhanced stress transfer capability from the matrices to the CNT. Thirdly, the harsh conditions required in the traditional preparation of ceramic composites need to be avoided as such high temperature and high pressure may destroy the CNT. Up to date, chemical modification of CNT is one of most effective way to obtain ceramic/CNT composites with uniformly dispersed CNT and compatible interface.In this study, vinyl modified carbon nanotube (vCNT) was made by chemically modifying CNT and used in the preparation of ceramic/CNT composites. The premade CNT was treated with concentrated HNO3at ambient temperature for8h, and then reacted with acryloyl chloride at66℃for12h. The length of the CNT decreased from20-100μm to about8-12μm after the reactions. It was found that the polymerization of acryloyl chloride occurred on the surface of CNT and the product with ethylene groups encapsulated the surface. The chemical modification of CNT effectively promoted its dispersivity in the toluene solution of polycarbosilane (PCS). The vCNT was homogeneously dispersed in the obtained hybrid precursor without any entanglement and agglomeration.The vCNT-reinforced SiC ceramic fibers were successfully prepared by mixing vCNT with PCS, followed by melt spinning, curing, and pyrolysis. The vCNT/PCS precursors were melted and spun into continuous green fibers in N2stream at about305-325℃. The vCNT were aligned to the axis-orientation of the fibers after melt spinning. Oxidative cross-linking of the CNT/PCS green fibers was carried out in hot air to make these fibers infusible prior to pyrolysis. Then the cured fibers were heated in N2at1300℃and the CNT/SiC ceramic fibers were obtained. It was observed that the aligned vCNT remained in the ceramic fibers after curing and pyrolysis. Significant improvement in Young’s modulus and tensile strength was achieved by incorporating the vCNT into the SiC ceramic fibers. The addition of0.5wt%CNT led to an increase of93.6%in the Young’s modulus and an increase of38.5%in the tensile strength.Besides, vCNT reinforced porous SiC ceramics were successfully obtained by milling, molding and pyrolyzing the pretreated CNT/PCS precursor through the polymer derived ceramic (PDC) process. The vCNT was dispersed well in the precursors, and the flexural strength of the obtained porous CNT/SiC ceramics was improved. The addition of0.5wt%vCNT resulted in an increase of76.5%in the flexural strength (60.9MPa). The increase in mechanical properties can be attributed to the homogenous dispersion of vCNT within the matrix, the strong interface connections and the enhanced stress transfer capability from the matrix to the vCNT.Moreover, the vCNT reinforced Cf/SiC composites were fabricated by PIP method using antimony substituted polymethlysilane (A-PMS) as the precursor. It was found that CNT was well-dispersed in the precursor after ultrasonic treatment, and remain in the obtained ceramic composites after the PIP process. The vCNT embedded in the composites increased the pullout resistance, bridged the crack gaps, and caused crack deflection in the final composites. The vCNT broke in the sword-in-sheath fracture mode, in which the outer shells broke and the inner core was then pulled away, leaving fragments of the outer shells in the matrix. These fragments enabled the vCNT in the composites to have a much higher load carrying capacity. As a result, the mechanical properties of the obtained composites were improved. The addition of1.5wt%of CNT resulted in29.7%and27.9%increases in the flexural strength and the fracture toughness, respectively. Large-scale CNT/Cf/SiC/composites can be conveniently prepared by using this method.