| Due to the weak interface between fibers and matrix resulting from the smooth and inert surface of carbon fibers, it is strongly required to modify carbon fibers before the preparation of carbon fiber reinforced composites for enhancing the interfacial cohesive force. This study focused on the in situ growing carbon nanotubes/carbon nanofibers (CNTs/CNFs) on carbon fibers to modify their surface by chemical vapor deposition (CVD), and the major contents include:(1) the effect of pretreatment process prior to growth of CNTs on the morphology and tensile strength of CNT/CNF-grafted carbon fibers, (2) the influence of CVD process on the morphology and tensile strength of CNT/CNF-grafted carbon fibers, and (3) the effect of the morphology, microstructure and yield of CNTs/CNFs on interfacial performance of carbon fiber reinforced composites.Based on electrochemical anodic oxidation, an innovative technique has been developed to efficiently obtain the uniform catalyst coating on continuously moving carbon fibers. To probe the optimum electrochemical modification process, the effect of electrochemical treatment intensity (ETI) on the morphology and performance of CNT/CNF-grafted carbon fibers and its reinforced composites has been systematically investigated. It was found that a very good and uniform coverage of the fiber surface by the catalyst is not only necessary to achieve a uniform growth of CNTs/CNFs on carbon fibers, but also critical for the fabrication of CNT/CNF-grafted carbon fibers with high tensile strength and its reinforced composites with high interlaminar shear strength (ILSS). The inhomogeneous distribution of catalyst precursor would cause the formaotion of larger catalyst particles and aggregation of catalyst particles, thus leading to the inhomogeneous growth of CNTs/CNF as well as the serious etching of carbon fiber surface by catalyst particles. Obviously, to achieve a fine modification and a low degradation of mechanical properties of carbon fibers simultaneously, the optimum ETI is 100 C/g. Low ETI (50 C/g or lower) brings about an insufficient surface modification, leading to the inhomogeneous distribution of catalyst coating, however, a high ETI of 150 C/g can give rise to the over oxidation of graphite layers occurring on the surface of carbon fiber and make the graphite layers easily peeled off the surface of carbon fiber under shear stress, hence reuslting in the low tensile strength of CNT/CNF-grafted carbon fibers and weak interface between carbon fibers and epoxy resin.According to the systematical study on the effect of various process parameters, such as the type and concentration of catalyst, CVD temperature and atmospheric pressure, on the morphology of catalyst particles and CNTs/CNFs, catalytic efficiency of catalyst particles (R) and yeild of CNTs/CNFs (Y), a mathematic model for CNT growth was established to explain the experimental results successfully. It was found that the gas pressure has a pronounced influence on the growth of CNTs/CNFs. The low pressure results in the low concentration of reactive gas leading to the low growth rate and yeild of CNTs/CNFs, whereas the high pressure brings about the high concentration of reactive gas leading to the high decomposition rate of reactive gas and so the formation of amorphous carbon. The increase of the concentration of catalyst precursor will gives rise to the enlarging of average size, the broadening of size distribution and the worsening of homogeneous distribution of catalyst particles, which will lead to the increased mean diameter and inhomogeneous distribution of CNTs/CNFs grown on carbon fibers. In the case of the CVD temperatures lower than 600℃, as the concentration of catalyst precursor, c, increases, the catalytic efficiency of catalyst particles, R, decreases whilst the yeild of CNTs/CNFs Y increases. Based on the CNT/CNF growth establised in this study it could be deduced that R is approximately inversely proportional to c(k+1)/3 and Y is approximately proportional to c(2-k)/3,0<k<1. However, in the case of temperature higher than or equal to the 600℃, R has a low value for the low c and increases with the increase of c. It reaches a maximum when the c is increased to a critical value (c0). When the c is increased further, the variations of R and Y with c are consistent with the cases lower than 600℃. In addition, c0 increases with the CVD temperature increasing. The CVD temperatre also has a pronounced influence on the yeild, morpholoy and microstructrure of CNTs/CNFs grown on carbon fibers. For the Fe catalyst, Y increases with the increase of CVD tempereture and reachs at 600℃, as the CVD temperature increases further, Y will decrease significantly. However, for the Ni catalyst, the curve of the variation of Y with temperature has two peaks appearing at 500℃ and 600℃, respectively. Through the study on dependence of the microstructure of carbon nano products on temperature varied from 500～600℃, it was found that for the Fe catalyst, only slight change appears on the final carbon nano products which have been confirmed to be CNFs, while for the Ni catalyst, the increased temperature incurs the improvement of graphitic degree of the carbon nano products and the structural transformation from CNFs to CNTs.To probe the critical parameters for the fabrication of CNT/CNF-grafted carbon fibers with high tensile strength, the influences of the growth parameters including the heating rate of reactor, catalyst type and CVD temperature on the morphology and tensile strength of CNT-grafted carbon fibers have been systematically studied. Based on the observation from HRTEM images, the healing and strengthening mechanism has been studied. The critical requirements for preparation of CNT-grafted carbon fibers with high tensile strength have been found, mainly including (i) the obtainment of uniform coating of catalyst particles with small particle size, (ii) the low catalyst-induced and mechano-chemical degradation of carbon fibers, and (iii) the high catalyst activity which is beneficial for catalyst nano-particles to yield some excessive carbon atoms to repair the surface damage, enlarge the surface crystallites and form the crosslinks of neighboring crystals by CNTs on carbon fibers, the optimum growth temperature is found to be 500℃ under our CVD system. Lower temperature causes the low catalytic efficiency of catalyst, leading to the weak ability to repair and strengthen carbon fibers. However, the higher growth temperature causes serious catalyst-induced degradation and mechano-chemical degradation of carbon fibers, leading to only limited increase in tensile strength. By increasing the heating rate to reduce the interaction time between carbon fibers and catalyst, not only the degradation of mechanical properties of carbon fiber, but also the decrease of catalyst activity can be prevented, which leads to a pronounced increase of tensile strength of CNT-grafted carbon fibers. Between 450～600℃, due to its high catalyst activity compared to the Fe and Co catalysts, Ni is found to be the optimum catalyst for the fabrication of CNT-grafted carbon fibers with high tensile strength. There is a pronounced increase of 10%vin tensile strength of carbon fibers after CNT growth at 500℃ by using Ni catalyst. Trace addition of Cu can obviously enhance the catalytic activity of Fe and Ni catalysts at the low temperatures of 500℃ and 600℃, so it leads to the significant increase of the yield of CNTs/CNFs, the pronouced decrease of defects existing on carbon fiber surface, and the significant increase of the Weibull modulus for CNT/CNF-grafted carbon fibers, thus incurring the increase of tensile strength of CNT/CNF-grafted carbon fibers for this case. From the HRTEM images, it can be observed that many tube walls have been immersed into the graphitic layers below the surface of carbon fibers, however, there is no closed end with a hemispheric shape observed below the surface of carbon fibers, especially at the root of CNTs. Considering the visible increase of tensile strength of CNT-grafted carbon fibers and the strong interaction between CNTs and carbon fibers, it can be inferred that the roots of CNTs may have been bonded to the carbon crystals below the surface of carbon fibers. This makes the neighboring carbon crystals on the interface between carbon fibers and CNTs crosslinked by the root of CNTs, leading:to the pronounced increase of tensile strength of carbon fibers.The effect of density, diameter, length and microstructure of final carbon nano products on the interfacial shear strength (IFSS) of CNT/CNF-grafted carbon fibers reinforced composites was studied by varing the catalyst concentration, CVD temperature and time. It was found that the microstructure of carbon nano products depositted on carbon fibers has a remarkable influence on the interfacial properties of CNT/CNF-grafted carbon fiber reinforced composites. Compared with CNF-grafted carbon fibers, CNT-grafted carbon fiber reinforced composites possess a higher IFSS due to the excellent mechanical properties of CNTs and capillary effect of CNTs with epoxy resin. The concentration of catalyst precursor determines the density and diamter of CNTs/CNFs grown on carbon fibers, thus affecting the nterfacial properties between the fibers and epoxy markedly. The too-low density of CNTs/CNFs brings about the limitted modification whilst the too-high density of CNTs/CNFs resulting in the poor wettability between CNT/CNF-grafted carbon fibers and epoxy. The optimum catalyst concentration is found to be at range of 0.03-0.05 mol/L. The depositing time also has a prounced influence on the IFSS of composites. The short CNTs/CNFs cause the poor modification, whereas the long CNTs/CNFs are derimental to the wettability of CNT/CNF-grafted carbon fibers with epoxy. For 500℃ and 600℃, the oputimum growth times of CNTs/CNFs are 15 min and 10 min, respectively. |
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