Electrical conductivity of pure carbon nanotube yarns

The porosity of multi-walled carbon nanotube yarns can be varied over a wide range by adjusting the yarn construction, resulting in a dramatic change in yarn electrical conductivity. When the yarn electrical conductivity is converted into specific conductivity, its value remains approximately constant irrespective of the changes in yarn construction and porosity. The process of carbon nanotube yarn production involves two key steps, the formation of a network of carbon nanotube bundles spliced together by the entanglement of individual nanotubes and the compaction of the network into a cylindrical yarn. The splices formed from entangled individual nanotubes play a much greater role in electrical conduction than the cross-over contact formed between CNT bundles by compaction during spinning.     

Effect of gamma-irradiation on the mechanical properties of carbon nanotube yarns

Gamma-irradiation of carbon nanotube yarns in air has significantly improved the tensile strength and modulus of the yarns, presumably because of an increased interaction between the individual nanotubes. The improvement has been much greater for tightly structured yarns than for loosely structured yarns. Sonic pulse tests have also shown increased sound velocity and dynamic modulus in the carbon nanotube yarns as a result of gamma-irradiation treatment. X-ray photoelectron spectroscopic analyses on progenitor carbon nanotube forests show that gamma-irradiation treatment in air has dramatically increased the concentration of oxygen, for example as carboxyl groups, in the carbon nanotube assemblies in proportion to radiation dose, indicating that carbon nanotubes were oxidized under the ionizing effect of the gamma-irradiation. Such oxygen species are thought to contribute to the interaction between carbon nanotubes and thus to the improvement of carbon nanotube yarn mechanical properties.     

Production,structure and properties of twistless carbon nanotube yarns with a high density sheath

Carbon nanotube web drawn from a vertically aligned carbon nanotube forest is converted into a twistless yarn by a rubbing roller system. The yarn consists of a high packing density sheath with carbon nanotubes lying straight and parallel to the yarn axis and a low density core with many microscopic voids. The rubbing motion causes carbon nanotubes in the surface layers of the yarn to move in opposite directions and consequently large shear strains in the intermediate region tear the partially densified carbon nanotube assembly apart, resulting in the formation of large voids in the yarn core. The specific tensile modulus and sonic velocity of the core-sheath structured, twistless carbon nanotube yarns are significantly higher than that of their corresponding twist-densified yarns. These improvements have been attributed to increased nanotube-to-nanotube contact length in the high packing density sheath and very few carbon nanotubes lying at an angle to the yarn axis.     

Fabrication of a multifunctional carbon nanotube "cotton" yarn by the direct chemical vapor deposition spinning process

A continuous cotton-like carbon nanotube fiber yarn, consisting of multiple threads of high purity double walled carbon nanotubes, was fabricated in a horizontal CVD gas flow reactor with water vapor densification by the direct chemical vapor deposition spinning process. The water vapor interaction leads to homogeneous shrinking of the CNT sock-like assembly in the gas flow. This allows well controlled continuous winding of the dense thread inside the reactor. The CNT yarn is quite thick (1-3 mm), has a highly porous structure (99%) while being mechanically strong and electrically conductive. The water vapor interaction leads to homogeneous oxidation of the CNTs, offering the yarn oxygen-functionalized surfaces. The unique structure and surface of the CNT yarn provide it multiple processing advantages and properties. It can be mechanically engineered into a dense yarn, infiltrated with polymers to form a composite and mixed with other yarns to form a blend, as demonstrated in this research. Therefore, this CNT yarn can be used as a "basic yarn" for various CNT based structural and functional applications.     

Focused ion beam milling of carbon nanotube yarns to study the relationship between structure and strength

The effect of twist and solvent densification on the internal structure of carbon nanotube yarns was revealed using focused ion beam milling and related to yarn strength through tensile testing. Denser carbon nanotube yarns with smaller diameters were produced either through solvent densification or with increasing twist densities from 5 to ～15 turns/mm, but led to only minor improvement in yarn tenacity. At twist densities greater than ～15 turns/mm, a core-sheath structure developed and was correlated with a decline in strength. The implications of bonding between the nanotubes in the twisted yarn are briefly considered. These results have implications for the future development of high strength carbon nanotube yarn.     

The production of soft, durable, and electrically conductive polyester multifilament yarns by dye-printing them with carbon nanotubes

Carbon nanotube based dyestuffs were prepared by dispersing aggregates of multiwalled carbon nanotubes in water using a blend of zwitterionic surfactants with anionic surfactants. Using a dye-printing approach, the carbon nanotubes were directly applied to polyester multifilament yarns to form an electrically conductive layer over each filament of the multifilament yarn. Yarns having electrical resistivity ranging from 10³ to 10⁹ ohm/cm were obtained. Yarn with a resistivity of 10³ ohm/cm could be used to form flat, soft, and portable electrical heaters by vertically weaving the yarns into fabrics. The 10⁵ ohm/cm yarns could be used for anti-static clothing, and the 10⁹ ohm/cm level yarns for brushes for photocopying machines.     

Fiber and fabric solar cells by directly weaving carbon nanotube yarns with CdSe nanowire-based electrodes

Electrode materials are key components for fiber solar cells, and when combined with active layers (for light absorption and charge generation) in appropriate ways, they enable design and fabrication of efficient and innovative device structures. Here, we apply carbon nanotube yarns as counter electrodes in combination with CdSe nanowire-grafted primary electrodes (Ti wire) for making fiber and fabric-shaped photoelectrochemical cells with power conversion efficiencies in the range 1% to 2.9%. The spun-twist long nanotube yarns possess both good electrical conductivity and mechanical flexibility compared to conventional metal wires or carbon fibers, which facilitate fabrication of solar cells with versatile configurations. A unique feature of our process is that instead of making individual fiber cells, we directly weave single or multiple nanotube yarns with primary electrodes into a functional fabric. Our results demonstrate promising applications of semiconducting nanowires and carbon nanotubes in woven photovoltaics.     

Preparation and characterization of hybrid conducting polymer-carbon nanotube yarn

Hybrid polypyrrole (PPy)-multi walled carbon nanotube (MWNT) yarns were obtained by chemical and electrochemical polymerization of pyrrole on the surface and within the porous interior of twisted MWNT yarns. The material was characterized by scanning electron microscopy, electrochemical, mechanical and electrical measurements. It was found that the hybrid PPy-MWNT yarns possessed significantly higher mechanical strength (over 740 MPa) and Young's modulus (over 54 GPa) than the pristine MWNT yarn. The hybrid yarns also exhibited substantially higher electrical conductivity (over 235 S cm(-1)) and their specific capacitance was found to be in excess of 60 F g(-1). Measurements of temperature dependence of electrical conductivity revealed semiconducting behaviour, with a large increase of band gap near 100 K. The collected low temperature data are in good agreement with a three-dimensional variable range hopping model (3D-VRH). The improved durability of the yarns is important for electrical applications. The composite yarns can be produced in commercial quantities and used for applications where the electrical conductivity and good mechanical properties are of primary importance.     

Towards the development of carbon nanotube based wires

The excellent electrical and mechanical performance of individual carbon nanotubes combined with their extremely low weight make these structures highly interesting materials for electrical wiring applications. The recent manufacture of macroscopic wire-like assemblies made purely of carbon nanotubes – carbon nanotube fibres – has opened up new prospects in this area. The extensive research on the optimization of the morphology of the fibres indicates that it will be soon possible to produce carbon nanotube fibres exceeding both the electrical and mechanical performance of conductive metals currently used in electrical engineering. To enable the application carbon nanotube fibres as wires in everyday electrical circuits it is necessary to provide them with electrical insulation. This paper proposes, for the first time, a method of insulation of the fibres and analyses the parameters which control the successful development of insulating coating on the surface of these highly porous carbon materials. It is shown that the applied insulation does not compromise either the electrical or mechanical performance of the fibres. The proposed insulation method is inexpensive, easy to integrate into a production process for the carbon nanotube fibres and it can be scaled up.     

Mechanical and electrical properties of the PA6/SWNTs nanofiber yarn by electrospinning

In this article, continuous PA6/single‐wall nanotubes (SWNTs) nanofiber yarns were obtained by a special electrospinning method; the mechanical and electrical properties and the electric resistance‐tensile strain sensitivity of the as‐spun yarns were specially studied. The main parameters in the spinning process were systematically studied. Scanning electron microscope images and mechanical tests indicated that the optimum parameters for the electrospinning process were operation voltage = 20 kV, spinning flow rate = 0.09 ml/h, and winding speed = 150 rpm. Transmission electron microscopy images showed that the SWNTs have aligned along the axis of the nanofibers and thus formed a continuous conductive network which greatly improved the electrical conductivity of the PA6 nanofiber yarn and the percolation threshold was about 0.8 wt%. The electric conductivities of the yarns at different stretching ratios were also measured with a custom‐made fixture attached to the high‐resistance meter, and for a given carbon nanotube concentration, the conductivity changes almost linearly with the tensile strain applied on the yarns. POLYM. ENG. SCI., 54:1618–1624, 2014. © 2013 Society of Plastics Engineers     

Plasma torch production of macroscopic carbon nanotube structures

Analysis of the many studies of carbon nanotube formation in high-temperature ovens clearly indicates the key requirements of nanotube formation are an ‘atomic’ carbon source and a source of nanometal particles. We adapted this formulation to the high temperature (>3000 K) environment found in a low-power (<1000 W) atmospheric pressure, microwave plasma torch, by simultaneously feeding carbon monoxide (carbon source), and (presumably) iron carbonyl (source of metal catalyst particles) through an argon stabilized plasma flame. This technique led to the relatively rapid (25 mg/h) formation of carbon nanotubes of a unique form: macro-sized ‘woven’ threads. Scanning electron microscopy and high-resolution transmission electron microscopy studies revealed that the woven threads consist entirely of carbon nanotubes (primarily carbon single-wall nanotube) and associated nano-iron particles. The structures appear ‘fractal’ in that each woven thread appears to be constructed of smaller threads that in turn are formed of yet smaller woven threads. Simple mechanical tests show the threads can be bent without breaking, and the thread will spring to its original shape when the force holding it is released. Threads of the size produced can be woven together to form actual cloth or ropes and thus this result represents a step toward the ultimate application of carbon nanotubes for super strong/light structures.     

An improved method for functionalisation of carbon nanotube spun yarns with aryldiazonium compounds

We report a method for modifying carbon nanotube (CNT) spun yarns with aryldiazonium salts that involves the pH controlled application of the diazonium salts to CNTs both during and after the yarn formation process. This largely facilitates the chemical accessibility to CNTs within the yarn, potentially enabling a more extensive and uniform modification. The modified CNT yarns were characterised by X-ray photoelectron spectroscopy, Raman spectroscopy and scanning electron microscopy, and also examined for their mechanical properties. The results demonstrated that a CNT spun yarn was effectively modified by this method without impairing the yarn integrity. The formation of oligomerised polyene structures on the CNT surfaces was observed. This modification resulted in an increase in tensile strength and Young’s modulus of the CNT yarn. The functional groups grafted on CNTs also provide opportunities to form crosslinks in the yarn to further improve mechanical properties.     

Investigation into microstructure of carbon nanotube multi-yarn

Structural analysis at the nano and micro scale was performed on a carbon nanotube (CNT) multi-yarn. The yarns were made by a process of drawing CNTs into a ribbon and twisting the ribbon into a yarn. Scanning electron microscopy (SEM) was used to view the exterior of the yarn. Polarized microscopy was used to examine details of the 1-yarn, and it also identified ribbon–ribbon boundaries. Further examination of interior structure was done by NanoCT scans which showed that folding of the ribbons had occurred causing complicated structures. The interior folding was found by milling into the yarn with a focus ion beam gun (FIB) and imaging with SEM. These different methods thus provided various microstructural details (structure, ribbon–ribbon boundary, folding and void fraction) of CNT multi-yarn which could be used to compare with other yarns fabricated with different procedures/sources as well to provide parameters for analytical tools. Further, these microstructural details can be related to macro mechanical and physical properties.     

Fabrication and processing of high-strength densely packed carbon nanotube yarns without solution processes

Defects of carbon nanotubes, weak tube-tube interactions, and weak carbon nanotube joints are bottlenecks for obtaining high-strength carbon nanotube yarns. Some solution processes are usually required to overcome these drawbacks. Here we fabricate ultra-long and densely packed pure carbon nanotube yarns by a two-rotator twisting setup with the aid of some tensioning rods. The densely packed structure enhances the tube-tube interactions, thus making high tensile strengths of carbon nanotube yarns up to 1.6 GPa. We further use a sweeping laser to thermally treat as-produced yarns for recovering defects of carbon nanotubes and possibly welding carbon nanotube joints, which improves their Young's modulus by up to ～70%. The spinning and laser sweeping processes are solution-free and capable of being assembled together to produce high-strength yarns continuously as desired.     

Evaluation of mechanical properties of untwisted carbon nanotube yarn for application to composite materials

Carbon nanotubes (CNTs) with superior mechanical properties have been of interest as reinforcement for polymer composites. However, the length of individual CNTs is limited. As a solution, yarns spun by twisting together multi-walled carbon nanotubes (MWCNTs) have been reported. In this study, untwisted CNT yarns were prepared by a non-conventional method drawing CNTs through a die. The MWCNTs in these yarns are held together by strong van der Waals forces that arise due to the interactions on the long and smooth surfaces of the MWCNTs. Here, mechanical properties of untwisted CNT yarn were studied by tensile tests. The strength of the CNT yarn was increased by increasing the apparent density of the yarn. The CNT yarns showed high tensile strength of 1 GPa and elastic modulus of 79 GPa at a yarn diameter of 35 μm. The interfacial shear strength between the CNT yarn and epoxy resin was studied by the microdroplet method, and it was very low. The wettability of the CNT yarn was affected by a type of curing agent. A unidirectional composite of epoxy resin and CNT yarn was prepared by the pultrusion molding method. Mechanical properties of the unidirectional composite were affected by the type of curing agent.     

Exploitation of Carbon Nanotubes in High Performance Polyvinylidene Fluoride Matrix Composite: A Review

Among nanocarbon fillers, carbon nanotubes are considered to be an ideal reinforcement due to their miniscule size, and excellent electrical, thermal, and mechanical properties. However, carbon nanotubes can be utilized in polymer nanocomposites only if they are homogenously dispersed into polymer matrices. The multiwalled carbon nanotube has been concentrated as a reinforcement for an important type of thermoplastic polyvinylidene fluoride. This review initially focuses on carbon nanotubes modification both by mechanical methods and chemical functionalization to improve their dispersion. Moreover, the processing methods for polyvinylidene fluoride/carbon nanotubes nanocomposite have been discussed. Multiwalled carbon nanotubes facilitate the electrical conductivity, thermal, rheological, and mechanical properties of polyvinylidene fluoride.     

Spinnable carbon nanotube forests grown on thin, flexible metallic substrates

Towards the goal of providing a continuous process for the solid-state fabrication of carbon nanotube sheets and yarns from carbon nanotube forests, we report the growth of yarn-spinnable and sheet-drawable carbon nanotube forests on highly flexible stainless steel sheets, instead of the conventionally used silicon wafers. Sheets and yarns were fabricated from the 16 cm maximum demonstrated forest width, from both sides of a stainless steel sheet, and the catalyst layer was shown to be reusable, thereby decreasing the need for catalyst renewal during a proposed continuous or semi-continuous process where the stainless steel sheet serves as a moving belt to enable forest growth at one belt end and carbon nanotube yarn or sheet fabrication at an opposite belt end.     

Nanocarbon aerogel complexes inspired by the leaf structure

Inspired by the structure of a leaf, which is constituted of veins, midribs and laminas, we report the synthesis of aerogels based on nanocarbon complexes that exhibit good electrical conductivity, large internal surface area and stable structural integrity upon cyclic compression. These materials are prepared as monolithic solids from suspensions of unzipped and partially exfoliated multi-walled carbon nanotubes. Under optimized oxidation conditions, all the walls of the multi-walled carbon nanotubes are unzipped but only the outer tubes are exfoliated, creating nanoscale multi-layered graphene oxide sheets attached to inner trench-like structures. The exfoliated parts provide high surface area and functional groups, while the inner trench-like structures remain relatively intact and thus retain their electrical conductivity and mechanical properties, which facilitates charge transport and structural stability for the aerogel. The hydrophilic functional groups on the graphene oxide nanosheets make these structures highly soluble, and as a result, the density and mechanical properties can be adjusted to a large extent without sacrificing the porosity or cell wall uniformity. These nanocarbon aerogel complexes exhibit high damping capability with no significant change in piezoresistive properties after more than 4500 compressive cycles, and its original shape can be recovered quickly after compression release.     

Electrical and mechanical characteristics of buckypapers and evaporative cast films prepared using single and multi-walled carbon nanotubes and the biopolymer carrageenan

The electrical and mechanical characteristics of composite materials prepared using evaporative casting and vacuum filtration of carbon nanotubes (CNTs) dispersed in the biopolymer τ-carrageenan (IC) are reported. It is demonstrated that the contact angle of water with films is proportional to the CNT mass and volume fraction, which is used to compare the properties of buckypapers with those of evaporative cast films. Multi-walled carbon nanotube films were found to exhibit higher conductivity values compared to those observed for single-walled carbon nanotubes composites at comparable contact angle values up to true nanotube volume fraction of 0.12. Buckypapers prepared by varying the absolute amount of CNT mass while keeping the IC amount of mass constant, were found to be more robust and conducting compared to evaporative cast films. In contrast, buckypapers prepared by changing the amount of IC mass while keeping the CNT amount of mass constant were found to be more conducting, but less robust compared to evaporative cast films. It is suggested that the electrical characteristics of these gel-carbon nanotube materials are determined by the relative amounts of mass (or volume) of CNTs and polymer, while the mechanical characteristics are governed by the absolute amounts of mass (or volume).     

Poisson’s ratio and porosity of carbon nanotube dry-spun yarns

The Poisson’s ratio of carbon nanotube (CNT) dry-spun yarns can be tuned over an extremely wide range of values that are up to 20-30 times higher than common solid materials. This is a result of the highly variable porosity of the yarn structure, from 90% in very low twist yarns to 40% in high twist yarns. The change of CNT geometry during the conversion from forest to web also plays an important role in the formation of CNT bundles and consequently influences the CNT dry-spun yarn structure. The CNT dry-spun yarn achieved its maximum specific strength when the CNTs on the yarn surface formed a 20° angle to the yarn axis. These CNT dry-spun yarn structure-property relationships can be utilized in the design of different applications, such as tuning the sensitivity of sensors and the functional characteristics of CNT composites.