| Grafting carbon nanotubes(CNTs)on the surface of carbon fibers(CF)by chemical vapor deposition(CVD)is one of the most common methods for preparing CNTs/CF multi-scale reinforcements.In this paper,the continuous CVD equipment designed by ourselves was used to realize the continuous preparation of CNTs/CF multi-scale reinforcements,and the further optimization of the continuous preparation process was completed.This paper focused on the following four aspects:(1)The effects of electrochemical treatment on the functional group of the pretreated carbon fiber surface,the single-filament tensile strength,and the surface morphology of the CNTs/CF multi-scale reinforcement were studied;(2)The elemental and phase composition of catalyst particles were analyzed,and the doping form of sulfur element was explored;(3)The catalytic system was optimized and thiourea was introduced into the catalyst precursor as a kind of catalyst promoter.The effect of growth temperature and thiourea concentration on the surface morphology,the single-filaments tensile strength,the surface wettability of CNTs/CF multi-scale enhancement and the microstructure of CNTs were explored;(4)The carbon fiber wire speed and the ratio of the growth atmosphere composed of H2 and C2H2 were adjusted.And the effect of deposition duration and atmosphere on the surface morphology,degree of graphitization,single-filament tensile strength of CNTs/CF multi-scale enhancement were explored.According to the research on the surface structure and mechanical properties of pretreated carbon fibers,the following experiment results would be found.The chemical composition of the carbon fiber surface would be changed and the content of polar functional groups on the surface got increased with the electrochemical anodization treatment(EAO).High-strength electrochemical surface treatment would severely etch the surface of carbon fiber,a large number of defects on the carbon fiber surface and the relatively loose random layer graphite structure would appear,which could reduce the degree of graphitization of the carbon fiber surface and decrease the single-filament tensile strength of the carbon fiber.With the electrochemical treatment intensity of 50C/g?100C/g and 150C/g,the carbon on the surface of the carbon fiber was reduced from 93.16%to 77.66%,73.64%and 70.21%;the polar functional groups was increased from 19.82%to 27.59%,34.48%and 35.67%;the single-filament tensile strength of the carbon fiber was reduced from 4.23GPa to 4.15GPa,4.10GPa and 3.91GPa.After electrochemical surface treatment,a gradually uniform distribution of CNTs on the surface of carbon fibers could be achieved.With the electrochemical pretreatment intensity of 100C/g and 150C/g,CNTs layer on the surface of CNTs/CF multi-scale reinforcement was uniform.The research on the catalyst of CVD process showed that the catalyst activity was greatly affected by deposition temperature.In the range of 500?-700?,the catalyst activity gradually is enhanced with the increase of temperature.At 600? the small-aspect-ratio CNTs appeared on the surface of carbon fiber,the wall of CNTs was tortuous because of the large amount of defects,and the degree of graphitization is low.At 650? and 700?,large-aspect-ratio CNTs was deposited,and the surface graphitization was improved.With adding thiourea to the catalyst precursor,the catalyst activity was significantly improved.At 550?,580?,and 600?,CNTs with large aspect ratio could be deposited on the surface of carbon fiber;CNTs deposited at 500? was small-aspect-ratio.The concentration of thiourea had a significant effect on the activity of the catalyst.More CNTs would be deposited on the surface of carbon fibers with the increase of thiourea concentration.With the thiourea concentrations of 0.01 mol/L,0.02 mol/L,and 0.03 mol/L,the tensile strengths of the CNTs/CF multi-scale reinforcements prepared at 580? were 4.33 GPa,4.53 GPa,and 4.51 GPa,respectively.According to the research on the conditions of CVD process,the following experiment result would be found.Reducing the wire speed to prolong the deposition time of the CNTs,the thickness and density of the CNTs layer would be increased.Under the condition that the wire speed is faster than 3cm/min,the degree of graphitization and single-filament tensile strength of the CNTs/CF multi-scale reinforcement would be increased with the decrease of the wire speed gradually.At a wire speed of 3cm/min,the lowest R(ID/IG)value and the highest degree of graphitization of the surface appeared.With the wire speed reduced to 2 cm/min,a large amount of amorphous carbon would deposited on the surface of the carbon fiber,so that the degree of graphitization of the surface of the CNTs/CF multi-scale reinforcement decreased to a lesser extent.At the same time,a large amount of amorphous carbon filled in the gaps of the CNTs layer made the CNTs layer to harden.H2 in the growth atmosphere had a certain inhibitory effect on the cracking of C2H2.It could extend the catalyst life and increase the degree of surface graphitization of the CNTs/CF multi-scale enhancement to increase the flow of H2.As the flow rate of H2 in the growing atmosphere gradually increased from 0 to 0.9 L/min,the degree of graphitization of the CNTs/CF multi-scale reinforcement gradually increased.With the H2 flow rate less than 0.3 L/min,it was difficult to obtain a uniform CNTs layer on the surface of carbon fiber.Apart from optimizing the continuous CVD process,this paper studied the growth mechanism of CNTs,the thermal oxidation properties and wettability of CNTs/CF multi-scale reinforcements.Through observation of transmission electron microscope images,it was found that most of CNTs possessed an open ending on the end of CNTs away from carbon fiber,and a small amount of catalyst particles located at the upper end of CNTs,indicating that growth mechanism of CNTs was the top growth mechanism under the experimental conditions in this paper.Compared with the desized carbon fiber,the TG curve of CNTs/CF multi-scale reinforcement under the air heating conditions showed two obvious weight-loss process.The first was the oxidative decomposition of amorphous carbon at 400?-450?.The second occurred at 600?-650?,which was caused by the oxidative decomposition of carbon fiber.With the CNTs deposited on the surface of carbon fibers,the wettability of carbon fiber to epoxy resin was significantly enhanced.With the increase of temperature,the number and aspect ratio of CNTs on the surface of carbon fiber increased,which increased the specific surface area of CNTs/CF multi-scale reinforcement.Then the resin adsorption amount increased.Under the thiourea assisted conditions,as the concentration of thiourea increased,the wettability of CNTs/CF multi-scale reinforcement to epoxy resin increased first and then decreased.The sulfur could increase the activity of Co catalyst,and higher active catalyst increased the number and the aspect ratio of CNTs,which would increase the specific surface area of the multi-scale reinforcements.However,with the increase of the thiourea concentration,the amorphous carbon on the surface of the multi-scale reinforcement increased,and the amorphous carbon filled in the gaps of the CNTs layer,which weakened the capillary action and reduces the wettability of the CNTs/CF multi-scale reinforcement. |
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