Interfacial Properties And Strain Sensing Performance Of Carbon Nanotube Yarn Reinforced Composites

With rapid development of science and technology,fiber materials are play an important role in energy,military industry,aerospace and other fields.Carbon nanotubes with excellent properties,can meet the requirements of high conductivity and electrochemical activity in energy field;chemical stability and corrosion resistance in military field;high strength and low density in aerospace field.Carbon nanotube(CNT)yarn is a macro-scale struct ure of CNT assembly.As a result,it has great potential in mechanical,electrical and thermal application.At present,the preparation of continuous CNT yarns is based on the traditional solution spinning,array spinning and chemical vapor deposition(CVD)spinning.Among them,the direct chemical vapor deposition spinning is more convenient and has a continuous process,which has an industrialized prospect.But the yarn fabricated by CVD has an uneven diameter,more defects and low strength.Thus,the postprocessing during the fabrication is necessary.Moreover,current researches of CNT yarns are typically related to the intrinsic mechanical and electrical performance,rarely do in-depth research on interfacial properties of CNT yarn composites.And the interface is an important factor in composite performance.Hence,for further improve the industrial application of carbon nanotube yarn materials prepared by CVD method,investigate the interfacial properties of carbon nanotube yarn reinforced composites.This article explores the structure characteristics of CVD yarn,the mechanical properties of carbon nanotube yarns were improved by post-treatment.The interfacial properties between CNT yarn s and different resins(thermoplastic and thermosetting)were tested.Meanwhile,for further broaden the application of CNT yarn reinforced composites,In this study,the interface of yarn composites was improved by using environment-friendly cryogenic treatment and high-efficiency plasma treatment.Finally,the piezoelectric sensing properties of CNT yarn composites are also studied.Firstly,CNT yarn fabricated by floating catalyst CVD has poor structure since the loose and uneven distribution of CNT bundles in the yarn.In order to improve the structure and tensile strength of the CNT yarn,three postprocessing(shrinking,cyclic loading and twisting)were applied.Morphology of the CNT yarn was observed by scanning electron microscope(SEM)and mechanical properties of the CNT yarns were analyzed.the pristine CNT yarn was pretreated using a drafting and methylene dichloride shrinking processes.The mechanical properties of the treated CNT yarns improved due to a more compact and better aligned structure.The scale parameter and shape parameter of Weibull distribution confirmed the improvement;Different tensile strain cyclic loadings were applied to improve the yarn since the self-regulating of internal CNT bundles,meanwhile the micro-structure changing mechanism was investigated so as to identify the enhancement of the tensile strength;Twisting process deepen the tangles of the internal CNT bundles.as the twisted increased,the structure of the yarn became uniform and compact,showing a good helix-twisting structure,hence increased the strength.However,the over-twisting treatment decreased the strength and modulus,due to the increasing angle between yarn and CNT,which limit the mechanical bearing capacity.Secondly,the interfacial bonding properties between the CNT yarn and high-performance thermoplastic polyphenylene sulfide(PPS)resin are investigated using the micro-bond test.Morphology of the CNT yarn /PPS was observed by SEM and Polarization optical microscope(POM).CNT yarn fabricated by floating CVD was calculated with porosity of 70.5% and it has a high Poisson’s ratio of 3.5.The interfacial shear strength between CNT yarn and PPS matrix was determined using a micro-droplet test and the effective interfacial shear strength showed an average value of 13.1MPa.The mechanism for the interfacial failure was analyzed.Due to a large difference between the Poisson’s ratio of the yarn and that of the resin,a mixed mode fracture of opening crack and sliding shear lead to the interfacial failure.This phenomenon laid a foundation for further research on interfacial property of larger Poisson’s yarn reinforced composites.Thirdly,due to the weak bonding between resin matrix and nanotube bundles,the interfacial shear strength of CNT yarn is insufficiency.The effects of cryogenic treatment on properties of the CNT yarn/epoxy composites are investigated using a temperature-controlled cooling process with low cooling rate.The CNT yarn/epoxy single yarn composite fragmentation test is performed to compare the interfacial properties at ambient and cryogenic temperatures.Differences in the surface morphology of the treated and the untreated CNT yarn were observed using field emission scanning electron microscope(FESEM).The transmission electron microscope(TEM),Thermogravimetric analysis(TG)and Raman spectroscopy were conducted to evaluate the structure and properties of the CNT yarn.The Differential Scanning Calorimetry(DSC)and X-Ray Diffraction(XRD)were introduced to characterize the structure of epoxy.Mechanical results along with a Weibull distribution analysis showed that the strength of CNT yarn was not affected significantly.The tensile strength of the epoxy was increased by 27% after cryogenic treatment,due to the compact structure.An asymmetrical interfacial stress distribution is observed corresponding to the asymmetrical birefringence patterns of the fragmented yarn in the single yarn composite.The IFSS was tested using the fragmentation test and calculated by corrected formula.The interfacial shearing strength was found to be 31% higher than the pristine CNT yarn composite.Meanwhile,the SEM images explained the inner structure changing of embedded CNT yarn due to the differential shrinkage of CNT yarn and epoxy at cryogenic temperatures because of the big difference between their thermal expansion coefficients.In addition,plasma is an effective method to improve load transfer since functional group grafted on the CNTs.Here,atmospheric pressure radio frequency(RF)plasma with various times(0,1,2,3 and 4s)was applied to enhance the inner and interface of the CNT yarn.The interfacial shear strength(IFSS)between a CNT yarn and epoxy is measured using a micro-bond test.Surface functionalization was quantified by X-ray photoelectron spectroscopy(XPS).Raman spectroscopy was utilized to characterize the defects for the CNT yarn.The static contact test of infiltrating properties and the mechanical properties of CNT yarn were also investigated.In order to observe the morphologies of the CNT yarn,TEM and FESEM were used.As a result,the tensile strength of the CNT yarn increased 49.5%.Since the oxygenic groups generating,especially carboxyl groups which can achieve an effective combination with epoxy,the interfacial bonding strength was calculated to increase 84.6% from 17.37 MPa to 32.08 MPa.However,the linear and repeatable gauge factors of the embedded CNT yarn under cyclic loading are capable of evaluating the stability of interfacial bonding between plasma treated yarn and matrix.Hence,the enhanced-interfacial properties of the CNT yarn can be greatly pertained in composites with improved mechanical and electrical performance.Lastly,CNT yarn enjoyed specific electrical properties due to the excellent strain sensing property under stretching,which makes CNT yarn very sensitive to mechanical environments.In this study,the CNT yarn with the strain sensing gage factor of 1.75 was selected and embedded into Ethylene Vinyl Acetate(EVA)Elastomer.By measuring the electric resistance variation under the stretching,bending,longitudinally and transversely compressing of the CNT embedded EVA,the resistance changing curves and their linear correlations were investigated and analyzed.The result showed that gage factor under the tensile test is the highest(2.51).In the tensile process,the axial stress of EVA makes the yarn structure more compact,thus improving the sensing performance.The gage factor of the CNT yarn embedded into EVA elastomer under bending is 2.29 with worse linear correlation,since the complicated loading states.The gage factors under longitudinal and transverse compression are 1.55 and 0.79,respectively.The reason is the contact resistance changing,it depends on the internal structure change when the yarns are under different pressure.