| Laminates are the most widely used form of carbon fiber reinforced polymer composites.The common laminated carbon fiber reinforced polymer(CFRP)composites lack fiber reinforcement between the plies,and the matrix bares the load between the plies.Since that conventional thermosetting polymer matrix has low strength and poor fracture toughness,the interlaminar shear strength and fracture toughness of CFRP composites are low,which restricts the exertion of the superior mechanical properties of the laminates and the advantage of carbon fiber composites to reduce the structural weight.In addition,because of the good thermal and electrical properties of carbon fiber,the problems in thermal and electrical conductivity of the CFRP laminate are mainly caused by the polymer matrix,which brings poor thermal and electrical conductivity between the plies.Therefore,improving the interlaminar electrical and thermal conductivity of CFRP laminates is one of the most effective ways to realize the structure-function integration of CFRP.To solve the above problems,this research adopted the method of introducing interlaminar reinforcement of carbon nanotubes to construct carbon nanotubes-carbon fiber multi-scale composite.In this paper,the study on the carbon nanotube’ orientation between the plies of CFRP composite,its influence on the mechanical,thermal and electrical properties of the composites and the reinforcement mechanism were carried out.Through the driving of resin flow,the process of migration and orientation of carbon nanotubes in the micro-channels between carbon fibers was specifically studied.The preparation of the epoxy resin film containing carbon nanotubes was also studied.The fabrication of carbon nanotubes oriented along through-thickness direction of laminates within the multiscale composites based on the multi-layer resin film infusion were studied The orientation of the carbon nanotubes within the composites were characterized.The bending property,interlaminar shear property,interlaminar fracture toughness,thermal property and electrical conductivity of multiscale composites in thickness direction were investigated.The fluid-structure coupling simulation on the migration-orientation behavior of carbon nanotubes in the carbon-fibers microchannels revealed that the migrationorientation of carbon nanotubes mainly depends on the narrowing of the fiber channels.Due to the resultant moment of dynamic pressure and viscous resistance,the orientation of carbon nanotubes decreased when the flow channel became wider.The research on the molding pressure and resin viscosity found that it is feasible to realize the migrationorientation of carbon nanotubes under the conditions of resin viscosity and driving pressure in the actual process conditions.Carbon nanotubes with straight,thick and moderate length can improve their orientation effect.The curing behavior,rheological properties and mechanical properties of different hot-melt reactive epoxy resin systems were studied.The 3121 resin with low-thermalactivity,low-melt-viscosity,long-gelation-time,good-mechanical-properties and excellent-film-formability was selected.Through characterizing the structure and morphology of carbon nanotubes and the tensile properties of the carbon nanotubes/epoxy nanocomposites,two types of carbon nanotubes with significant morphological differences,namely CNT and CNF were selected.The preparation method of carbon nanotubes/epoxy resin film was explored.It was found that under 80 ℃ film-forming temperature,10 min film-forming time and 4wt.% ethanol content in the initial blend,the morphology of the film was perfect without islands and pore defects,and the quality of the film was the best.A multi-layer resin film infusion process based on a self-sealing and deformation controllable vacuum bag-compressive molding process was designed,which realized the control of resin impregnation along the through-thickness direction of the fiber preform and the high-fiber-volume-fraction and low-internal-defects preparation of multi-scale composite.By controlling the vacuum-assisted Z-infusion time,the orientation of carbon nanotubes inside the composite was tuned.By prolonging the vacuum-assisted Z-infusion time the orientation of carbon nanotubes along the thickness direction was improved.Through scanning electron microscopy characterization,it was found that CNT have a higher degree of orientation along through-thickness direction in the interlaminar region of the composite than CNF.Carbon nanotubes reinforced multi-scale composites with through-thickness orientation were prepared by MLRFI-CM process.With the extension of the vacuumassisted Z-infusion time,the flexural modulus of the multi-scale composite gradually increased,and the flexural strength of the composite decreased first and then increased.The flexural strength and flexural modulus of the composite increased by 10.8% and 16.2%after adding 0.5 wt.% of CNTs.The study on the interlaminar shear property of multiscale composite found that after incorporating carbon nanotube,the interlaminar shear strength of the multi-scale unidirectional composite and the fabric composite were improved.Under 10 min vacuum-assisted Z-infusion,the interlaminar shear strength of the unidirectional 0.3 wt.% CNT incorporated composite increased by 18.5%,and the interlaminar shear strength of the fabric composite increased by 12.0%.The study of interlaminar fracture toughness of multi-scale composites found that the addition of CNFs did not improve the Mode Ⅰ fracture toughness of the composites.However,the addition of CNT significantly improved the Mode Ⅰ fracture toughness of the composite.Under0.3 wt.% CNTs content,the initial value of Mode Ⅰ fracture toughness of the composite increases by 47.3%,the propagation value increases by 50.0%,and the maximum fracture toughness showed an increase of 62.1%.The toughening effect of CNTs on the interlaminar fracture toughness of composite is mainly manifested in toughening of the interlaminar matrix and triggering more fiber bridging caused by the branching of the main cracks and the cross-laminar propagation of branched cracks.The study of the through-thickness thermal conductivity of the carbon nanotube incorporated multi-scale composite found that the increasement of the orientation degree of the carbon nanotubes had a positive effect on the composites’ thermal diffusivity and conductivity.After adding 0.5 wt.% of CNTs,the thermal conductivity of the composites increased most significantly.Compared with the baseline specimens without vacuumassisted Z-infusion,the thermal conductivity increased by 52%.The addition of CNF had no obvious effect on the thermal conductivity of composites which is mainly due to poorer intrinsic thermal conductivity and lower degree of orientation between plies compared with CNT.Incorporating carbon nanotubes can effectively improve the through-thickness electrical conductivity of the composites.Compared with CNF,the addition of CNT showed more significant increase in the through-thickness electrical conductivity.This is because the CNT demonstrated higher intrinsic conductivity.On the other hand,CNT are more likely to be orientated via resin fluid field which is more favorable to form a conductive network across the interlaminar region.The through-thickness conductivity of the 0.5 wt.% CNT multi-scale composite increased by 40%.However the addition of carbon nanotubes had a limited contribution to the transvers conductivity of the multiscale composites. |
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