Study On The Fabrication And Mechanical And Electrical Properties Of Carbon Nanotube Reionforced SiC Composite Fibers

Carbon nanotubes?CNT?possess superhigh tensile strength?¹00GPa?and modulus?¹TPa?,and with excellent toughness,electrical and thermal properties.Nevertheless,owing to the CNTs have poor dispersing ability,their application has been considerably restricted.Assembling the CNTs into the macro-structure fibers can transmit the excellent properties of the CNTs.Moreover,the macro-structure fibers can be operated and applied easily,and have great potential to put them into applications,such as reinforcements for high-performance composites,conductive wires,supercapacitors and microelectrodes.However,due to the limitations of the current preparation techniques and the microstructures of the fibers,the remarkable mechanical and electrical properties of the invidual CNT haven't been fully transmitted.In this dissertation,we focused on the preparation of high-performance CNT fibers,firstly carried out the fabrication of CNT/SiC composite fibers by the precursor infiltration and pyrolysis?PIP?method.We successfully fabricated the CNT/SiC composite fibers by optimizing the process parameters,and the microstructures,mechanical and electrical properties of the composite fibers were systematically studied.In addition,the thermal and oxidation resistant and high-temperature electrical properties of the CNT/SiC composite fibers were studied and their applications in high temperature fields were also demonstrated.The study of high-temperature heat treatment of the CNT fibers was carried out in a wide temperature range?800²000oC?in order to provide guidence on the pyrolysis temperature of the preparation of CNT/SiC composite fibers,and the relationships between the microstructurs and properties of the fibers were obtained.The results showed that high-temperature exposure could increase the defect density and loosen the packing state of the fiber,but enhance the graphitization degree of the CNT.Accordingly,there was a ductile-to-brittle transition in the uniaxial tensile response as the heat treatment temperature increased,and this was mainly a consequence of the failure mode transitions from localized shear to defect induce fracture.The tensile strength and electrical conductivity of the heat treatment fibers were gradually decreased with increasing the heat treatment temperature.The tensile strength and electrical conductivity of the as-received CNT fibers were 257±12 MPa and 1273±20S/cm,and after heat treatment at 2000oC,the tensile strength and electrical conductivity of the fibers were decreased by 58%and 40%,respectively.Both were due to the increased defect density within the fibers after high-temperature heat treatment.The CNT/SiC composite fibers were fabricated by the PIP method using polycarbosilane?PCS?as SiC precursor,the effect of pyrolysis temperature on the mechanical and electrical properties of the composite fibers was studied.The results show that the tensile strength and conductivity of the CNT/SiC composite fibers reach the maximum at the pyrolysis temperature was 800oC,up to 498.3±39.6 MPa and1688±82.1 S/cm,⁹4%and³3%higher than the as-received CNT fibers,respectively.This should be a result of the lower microstructure defects at this pyrolysis temperature.Therfore,the pyrolysis temperature was selected as 800oC.The microstructures of the CNT/SiC fiber were characterized by EDS,XPS and TEM.The results show that an adequate infiltration of SiC into the interior of the CNT fiber after the PIP process and the SiC was uniformly distributed within the composite fiber.The mutual CNTs were effectively bridged by the SiC matrix that was in an amorphous structure,and the CNT/SiC/CNT structure was obtained.The mechanism of the mechanical properties enhancements of the CNT/SiC fibers by SiC was mainly due to the CNT/SiC/CNT microstructure was obtained,thus leading to better load transfer in response to the external tensile stress.The conductivity of the CNT/SiC composite fiber increased was mainly due to the denser microstructure of the composite fiber.Based on the determined pyrolysis temperature,the CNT/SiC composite fibers were fabricated at different PCS concentrations and PIP cycles,and the effects of PCS concentrations and PIP cycles on the mechanical and electrical properties of the composite fibers were studied,respectively.The results show that appropriate increasing of PCS concentration and PIP cycles may contribute to increase the density of the fibers,leading to the mechanical and electrical properties of the fibers increased.While,the higher concentration of the PCS solution and PIP cycles,both are unfavorable to the PCS solution infiltrates the fibers sufficiently,and causing adverse impact on the properties of the fibers.Finally,we fabricated the CNT/SiC composites fibers with excellent mechanical and electrical properties by 2 PIP cycles at the pyrolysis temperature was 800oC,the precursor concentration was 10 wt.%.The tensile strength and electrical conductivity of the CNT/SiC composite fiber were 713.8±88.5 MPa and2600±106.8 S/cm,respectively,¹.8 times and 1 time higher than the as-received CNT fiber,respectively.Due to the CNT/SiC composite fibers mainly used in high-temperatue fields,the thermal-resistant,oxidation-resistant and high-temperature conduction properties of the CNT/SiC composite fibers were studied.The results show that the CNT/SiC composite fibers exhibit better thermal and oxidation properties than the as-received CNT fibers,this were mainly due to the well protection of the CNT by the thermally stable and oxidation-resistant SiC ceramic.The laws of the conductivities of the CNT and CNT/SiC fibers varing with the temperature were firstly investigated in high-temperature,and the high-temperature conduction mechanisms were revealed.The results show that the electrical conductivity of a typical CNT structured fiber is dominated by the resistance within single CNTs(R_tube)and the contact resistance among CNTs(R_contact),both are temperature dependent.As the temperature increasing,the conduction mechanisms of the as-received and CNT/SiC fibers transmitted at the critical temperature,Tc.When T<Tc,the resistance of the individual CNT was very small/negligible,the conduction of the fibers was determined by the R_contact.At this temperature regime,the R_contact was expected less sensitive to the temperature at this low temperature regime.Therefore,the electrical conductivities of the fibers were stable.When T>Tc,the R_tube increased significantly due to the electron-phonon scattering.The conduction of the fibers was determined by both the R_tube and R_contact.Therefor,the conductivities of the fibers were decreased with increasing temperature.