Strain dependence of electrical resistance in carbon nanotube yarns

Carbon nanotube forests or arrays can be drawn into a web and further twisted into threads. These carbon nanotube threads contain thousands of carbon nanotubes in their cross-sections and can be further composed into yarns that consist of one or more threads. The superior mechanical, thermal and electrical properties of carbon nanotubes are not translated into the carbon nanotube yarns. However, carbon nanotube yarns still exhibit relatively high mechanical stiffness and strength, and low electrical resistivity. More importantly, carbon nanotube yarns exhibit piezoimpedance that could be used for sensing purposes. In order to use carbon nanotube yarns as piezoimpedance-based sensors for structural health monitoring, it is necessary to determine the change in impedance of the yarn as a function of its mechanical strain or stress. This paper presents the results of an experimental study on the coupled mechanical response and electrical response in the direct current mode of the carbon nanotube yarn. A behavior consisting of a negative piezoresistive response was encountered during most of the deformation range of the yarn. This response was shown to exhibit a parabolic response and it was followed by a linear positive piezoresistive response that preceded the failure of the yarn.     

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.     

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.     

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.     

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.     

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.     

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     

Fabrication and characterization of hollow nanofibrous PA6 yarn reinforced with CNTs

Core-sheath nanofibrous yarns were obtained through electrospinning of polyamide 6 (PA6) solution containing different concentrations of multi-wall carbon nanotubes (MWNTs) as sheath and PVA multifilament as the yarn core. By dissolving PVA, for obtaining conductive hollow nanofibrous PA6/MWNTs yarn, two types of porosity could be obtained including hollow central tube due to the structure of hollow yarn and nano-porous areas embedded in electrospun nanofibers. SEM results showed that the diameters of nanofibers were varying in the range of 103–145 nm obeying MWNTs concentrations and TEM results revealed that the MWNTs were embedded in nanofiber matrix as straight and aligned form. DSC analysis showed that electrospinning process caused the formation of less-ordered γ phase in nanofibers. The electrical conductivity of yarns increased from 10^?13 S m^?1 to 2.4?×?10^?6 S m^?1 with increasing the concentration of nanotubes from 0 wt.% to 7 wt.%.     

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.     

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.     

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.     

Effects of Carbon Yarn Size on the Mechanical Properties of Plain Woven C/SiC Composites

The effects of yarn size on the mechanical properties of silicon carbide composites reinforced with a plain woven carbon cloth with two sizes of yarns (1 and 3 k) were investigated. The experimental results show that the composite fabricated with 1 k yarns exhibits greater stiffness and strength than the composite fabricated with 3 k yarns. Microstructural observations revealed the existence of matrix microcracks in both the composites under the as-processed condition due to the large difference of thermal expansion between the fibers and the matrix, which are more severe for the composite with 3 k yarns. The fractured surfaces of the composite with 1 k yarns showed extensive fiber pull-out in contrast to the yarn pull-out in the composite with 3 k yarns. The larger interyarn and intrayarn voids due to difficulties of matrix infiltration in the composite with 3 k yarns represent the primary contribution to the diminished mechanical properties. Unequal yarn sizes give rise to different yarn waviness, which may be another source of difference in the mechanical properties of composites.     

摩擦纺包缠纱纺织预型件及其复合材料的加工

本文研究了摩擦纺包缠纱纺织预型件及其复合材料的加工性能。借助地机织，纺织和针织等三种预型件加工方法，通过必种典型纺织结构的加工研究了摩擦纺包缠纱的预型件加工性能，并在平板热压机上成型了几种结构的复合材料，结果表明：摩擦纺包弹纱可以用于纺织预型件加工以成型纺织结构型的复合材料，灵活地运用纺织结构可以得到独特结构的复合材料，此外摩擦纺包缠纱经适当防剥离处理后，可加工出紧密结构机织物，以成型较高纤维体积     

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.     

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.     

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.     

PEG-PLA-PCL based electrospun yarns with curcumin control release property as suture

This work presents an interesting method using an electrospinning process to fabricate suture yarns loaded with curcumin to achieve reasonable mechanical properties as well as tunable drug release behavior. Different structures including different yarn counts and twists as well as core-sheath structures were used to adjust drug release properties along with improving the yarn's mechanical properties. The core parts were made of polycaprolactone and the sheath parts were made of polyethylene glycol, polylactic acid, and polycaprolactone. Drugs can be incorporated in both parts based on the required condition and application. Electrospun yarns were compared using both structural properties and their drug release profiles as metrics. The results of comparing drug release profiles of six electrospun yarns with different yarn counts and twists showed that yarns with finer fiber diameters in the core part have more drug release as well as more initial release. Overall evaluations showed that core-sheath drugloaded yarn with appropriate physical and mechanical properties can be a useful material as a drug delivery system to the site of damaged tissue. It can also be concluded that the amount and duration of drug release can be controlled using the structural parameters of electrospun yarns as an engineering tool for designing suture yarns with required properties.     

The effect of carbon nanotube properties on the degree of dispersion and reinforcement of high density polyethylene

The efficacy with which a range of nanotubes could reinforce a high density polyethylene (HDPE) matrix was investigated, in relation to nanotube diameter, purity, functionalization, alignment and nanotube bulk density. Composites were prepared by melt blending multiwall carbon nanotubes (MWNTs) with high density polyethylene (HDPE), followed by the injection molding of tensile specimens. At a 5 wt% loading, the most effective nanotubes were those of large diameter, received in an aligned form with low bulk density, producing a 66% increase in elastic modulus and a 69% improvement in yield stress. This was contradictory to theoretical mechanics calculations that predicted an increasing degree of reinforcement for nanotubes of reduced diameter. This difference was explained by the higher degree of dispersion observed in the composites with MWNTs of greater diameter.     

Enhanced ablation of small anodes in a carbon nanotube arc plasma

The ablation rate of a graphite anode is investigated as a function of anode diameter for a carbon nanotube arc plasma. It is found that anomalously high ablation occurs for small anode diameters. This result is explained by the formation of a positive anode sheath. The increased ablation rate due to this positive anode sheath could imply greater production rate for carbon nanotubes.     

Structure of flattened carbon nanotubes

A large number of flattened multi-walled carbon nanotubes have been synthesized using chemical vapor deposition. They are often capped with elongated crystalline cobalt catalyst nanoparticles. Sample rotation studies inside the transmission electron microscope show that the catalyst nanoparticles have a cylindrical rather than a rectangular shape, indicating that the flattened structure of the nanotubes is not templated by the shape of the catalyst particles. Observations reveal the existence of nanotube flattening parallel to the long axis and a spiraling of fully flattened nanotube around the long axis of the nanotube. The flattening of the nanotubes is attributed to the pressure difference between the inside and outside of the nanotubes which are sealed by the catalyst nanoparticles during their growth. The flattening and the spiraling are ascribed to axial compression and torsion, respectively.