Study On Low Temperature Growth Of Carbon Nanotubes On Carbon Fiber Surfaces And The Interfacial Properties Of Their Reinforced Composites

The failure of carbon fiber reinforced polymer composites is often caused by the weak interface between the carbon fiber and the matrix.The construction of an excellent interface can ensure the continuous and effective propagation of stress between the fiber and the matrix,which is of great significance to improve the mechanical properties of the composites.The insitu growth of carbon nanotubes(CNTs)on the surface of carbon fibers by chemical vapor deposition(CVD)can increase the surface roughness of carbon fibers,which is beneficial to improve the interfacial properties of composites.In this thesis,the low-temperature growth of CNTs on the surface of carbon fibers was realized,the etching effect of the catalyst on the fibers was reduced,the macroscopic mechanical properties of the composites were improved,and the enhancement mechanism of CNTs on the interfacial properties was revealed.The hydrogen peroxide(H2O2)treatment can activate the fiber surface,which was beneficial to the loading of the catalyst precursor on the fiber surface.It was found that the fiber strength and the morphology of CNTs were related to the treatment time.If the treatment time was too short,the activation of the fiber surface was insufficient,and the catalyst cannot be uniformly loaded on the fiber surface,resulting in uneven growth of CNTs;if the treatment time was too long,the fiber strength will be reduced.The experimental study showed that the uniform growth of CNTs on the fiber surface can be achieved when the treatment time was 90min,and the fiber strength was reduced by 1.2%at this time.The Fex-Co1-x(0≤x≤1)catalyst system was used to grow CNTs on the surface of carbon fibers to explore efficient catalyst compositions.It was found that the bimetallic catalyst had better catalytic effect at low temperature.When x=0.5,the catalyst can form uniformly distributed catalyst particles on the surface of the fiber,and the degree of damage to the fiber was smaller.The tensile strength of the fiber after reduction at 500℃ decreased by 7.63%.After the repairing effect of CVD process,the tensile strength of carbon fiber/CNTs reinforcement increased by 11.53%.The "droplet" phenomenon in the growth process of CNTs was found by high-resolution transmission electron microscopy(HRTEM),indicating that the catalyst particles were in liquid state during the CVD.The diffusion mode of carbon atoms in the catalyst was liquid diffusion or liquid diffusion and surface diffusion at the same time,which reduced the activation energy of CNTs growth and enabled CNTs to grow at lower temperatures.Through the characterization of the interfacial properties of the composites,it was found that the reinforcements prepared at different temperatures had different reinforcement effects.CNTs grown at high temperature had large density and serious entanglement,which was not conducive to the infiltration between CNTs and resin,and easy to produce defects at the interface.At 400℃,CNTs on the fiber surface can be fully infiltrated with the resin,which had the best reinforcement effect,and the ILSS and IFSS of the composites were increased by 32.29%and 30.73%,respectively.The microstructure of the fracture surface of the composites was observed by scanning electron microscopy(SEM)and the elastic modulus of the interface was measured by atomic force microscope(AFM).It was found that CNTs could induce an interphase with a thickness of about 0.41 μm between carbon fiber and resin,so that the stress was continuously propagated between them,and the energy was consumed by crack bridging and pulling out,so as to improve the damage resistance of the composites.There were three failure modes of CNTs in composites,namely,the fracture of CNTs and carbon fiber joints,the pull-out of CNTs from resin matrix and the fracture of CNTs themselves.The three failure modes depended on the binding force between CNTs and carbon fiber,CNTs and resin matrix.The two failure modes of CNTs pulling out from resin and self-fracture can increase the difficulty of separation between fiber and resin,which was beneficial to improve the interfacial properties of composites.The effects of catalyst precursor concentration,growth time and gas ratio on the growth of CNTs at low temperature were studied in detail.The morphology and loading amount of CNTs,the structure and tensile properties of carbon fiber/CNTs reinforcement and the ILSS of the composites were characterized.It was clarified how the change of CVD process conditions affected the morphology of CNTs,the strength of carbon fiber/CNTs reinforcement and the interfacial properties of the composites step by step.The results showed that when the concentration of catalyst precursor was too low,the catalyst was easy to diffuse into the carbon fiber and lose its activity,which resulted in the low surface density of CNTs and the poor enhancement effect.When the concentration of catalyst precursor was too high,it was easy to aggregate into large catalyst particles on the surface of the fiber,which aggravated the etching of the fiber,and tended to form impurities on the surface of the fiber,affecting the wettability of the fiber and the resin.If the CVD time was too short,the loading amount of CNTs on the carbon fiber surface was limited,and the defects on the fiber surface cannot be effectively repaired,which was not conducive to the improvement of the mechanical properties of the composites.If the growth time was too long,the curled CNTs were easy to tangle with each other on the fiber surface,affecting the infiltration of fibers and resin.At the same time,too long growth time will also cause the inactivated catalyst particles to be wrapped by carbon atoms to form impurities,forming defects at the interface of composites,and becoming stress concentration points,leading to material failure.When the concentration of H2 was too low.it can not be reduced to produce enough catalyst nanoparticles for the growth of CNTs,resulting in a low loading amount.When the concentration of H2 was too high,the concentration of carbon source was diluted.At the same time,the activity of the catalyst can be maintained by etching effect of H2,which hindered the diffusion of carbon atoms in the catalyst.Also,the loading amount of CNTs was low,so that the enhancement effect was limited.When the growth temparature was 400℃,the concentration of catalyst precursor was 0.05mol/L,the growth time was l0min,the ratio of H2 and C2H2 was 3:1,and the loading amount of CNTs was controlled between 5-7%.CNTs had the best reinforcement effect on the interfacial properties of the composites.Fe-Co,Fe-Ni.Co-Ni and Fe-Co-Ni catalysts were loaded on the fiber surface.CNTs were grown at low temperatures of 350-400℃ through CVD process.The effects of different catalysts on the morphology and loading amount of CNTs,microstructure,and mechanical properties of carbon fiber/CNTs reinforcement at various temperatures were studied.It was found that the catalytic efficiency of different catalysts was Fe-Co<Co-Ni<Fe-Ni<Fe-Co-Ni,and the microstructure of CVD products was closely related to temperature and catalyst.The products catalyzed at 350℃ were mainly CNFs with a solid structure,and increasing the temperature was beneficial to the increase of the degree of graphitization of the products,which gradually transformed to CNTs with a hollow structure.For different catalysts,the graphitization degree of CVD products catalyzed by Fe-Co catalyst was higher,and the graphitization degree of CVD products catalyzed by Fe-Co-Ni catalyst was the lowest.The strength of carbon fiber/CNTs reinforcement prepared with Fe-Ni catalyst at 400℃ was the highest,reaching 3.99GPa,which was 11.14%higher than that of desized fibers.Both CNTs and CNFs can improve the interfacial properties of the composites,but their strengthening mechanisms were not completely the same.For CNFs,high surface energy can make the carbon fiber/CNFs reinforcement fully infiltrate with the resin,and improve the interface bonding.However,the strength of CNFs was low,and it was easy to fail due to the effect of shear force,which cannot play a good reinforcement effect.Although the surface energy of CNTs was low,the π electrons on the surface of CNTs can interact with epoxy resin.At the same time,the axial strength of CNTs was large.When subjected to shear force,the propagation path of force can be changed and the crack can be deflected,so as to improve the bearing capacity of composites.In order to further improve the mechanical properties of the composites,the carbon fiber/CNTs reinforcements were subjected to oxidation and amination post-treatment,which increased the number of polar functional groups on the surface of the fiber and improved the wettability of the fiber and resin.The functional groups introduced on the surface could react with the epoxy resin matrix to form a chemical bond between the two.The mechanical interlocking effect caused by CNTs and the chemical interaction caused by polar groups coexisted in the interphase.The two effects were synergistic and jointly improved the interfacial properties of the composites.The ILSS of carbon fiber/CNTs reinforcement prepared at 500℃reached 77.68MPa after oxidation,which was 24.45%higher than that before oxidation.The IFSS of carbon fiber/CNTs reinforcement prepared at 400℃ was increased to 74.52MPa after amination,which was 11.14%higher than that before amination.