Investitation On The Synthesis,performance And Application Of Carbon Nanotube Based Conducting Fibers

As an assembly of carbon nanotubes(CNTs),the CNT fiber was strong,robust,flexible,highly conductive and light-weight.All these features meet with the developing trend of the next-generation conducting materials,making CNT fiber a popular material in the fields of wires/cables,flexible electrodes,wearable smart devices and fiber-like functional devices.However,the giant intertube contact resistance significantly degrade the electrical performance of CNT fibers,e.g.their conductivity and ampacity are far below the prospective value.So,CNT fibers' performance need to be further modified and improved.On one hand,coating CNT fiber with metals,like Cu,is a feasible way to fabricate strong,highly conducting and lightweight fibers.On the other hand,the unique assembly structure together with CNT's special electrical properties endow CNT fiber with many multi-function characteristics,but little attentions have been paid on this aspect.In this dissertation,the electrical properties of CNT fibers are detailly studied.Several unique electrical behaviors were discovered,including the nonlinear resistance,negative differential resistance and conductivity hopping phenomena.In addition,CNT fibers can not be welded by existing soldering techniques.So,a new technique was developed to solder this new conductor material to construct all-carbon-based electrical circuits.The major research contents and innovations are summarized as follows:(1)Through the array-spun technique and the direct floating catalyst technique(FCCVD or iCVD),a continuous fabrication of CNT fiber was achieved.The array-spun fibers show a super-aligned structure while the CNT bundles are highly entangled in the floating fibers.The array-spun fibers are strong but brittle with a low elongation at break(2-3%),and their conductivity are only 400-500S/m.On the contrary,the floating fibers are much weaker,but their fracture elongation and conductivity can exceed 20%and 4000-6000 S/m.The postprocessings,e.g.twisting,solvent densification,acidizing and anodization,can modify both the electrical and mechanical properties of CNT fibers.(2)During the floating growth process,dioctyl phthalate(DOP)molecules were spontaneously formed with CNTs.On one hand,DOP can significantly increase the ductility but weaken the strengthe of CNT fibers,acting as plasticizer.Accordingly,a high-tough CNT fiber was designed by introducing polyvinylidene fluoride(PVDF)into the purified CNT fiber.Both the strength and plasticity were significantly improved by strapping CNT cross-links with long-chain PVDF molecules.On the other hand,DOP can also impro,ve CNT fibers' electrical conductivity,and cause a unique nonlinear resistance(NLR)and a negative differential resistance(NDR)phenomenon.Through a comparative study based on various different organics,it was confirmed that the benzene ring,oxygen-containing functional groups and long molecule chain caused a strong interaction between CNTs and organic molecules,which will improve the fibers' conductivity and induce the NLR and NDR phenomena.Throgh the molecular dynamics(MD)simulation,an organic-CNT interaction model was carried out and it was believed to be responsible for all these aforementioned phenomena.The intertube contact area was changed as the organic molecules can cause the collapse of CNTs.(3)The ampacity of CNT fiber is only 104 A/cm2 due to the giant intertube contact resistance.However,after the large-current-treatment,CNTs regraphitized and collapsed.Then a special S shape nonlinear electrical phenomenon took place due to the intertube interface structure changed as the collapsed CNTs would swell again under a twisting-caused confined situation when carrying current again.With the help of the quick heat dissipation effect of liquid nitrogen,the aforementioned nonlinear phenomenon turned into a unique hopping phenomenon.(4)An in-situ growth of fiber-like carbon nanostructures was realized on the CNT fiber surface with the help of CNT fibers' excellent electrothermal performance and the iron catalyst particles.The nanostructures are composed of carbon nanofibers and nano walls,which are well crystallized with good mechanical and electrical performance.Further,by taking advantage of the inter-fiber contact resistance,a targeted carbon deposition was realized to build load and electron transfer pathes between the overlapped fibers.Thus,a CNT fiber soldering technique was developed,which will promote the development of all-carbon-based circuits and devices.(5)CNT/Cu composite fiber was continuous prepared by depositing copper on CNT fiber surface.In such a composite fiber,electrons and heat could hop from the Cu layer into CNTs and then transfer ballistically without any consumption.Then,the electrical and thermal conductivity,ampacity,high frequcy performance of the CNT/Cu composite fiber were remarkably improved.A thin Ni nano buffer layer was introduced to improve CNT-Cu interfacial interaction.As a result,a strong,highly conducting and flexible composite fiber with high ampacity and excellent GHz signal transmission performance was obtained.