Properties Of Carbon Nanotube Fibers Research Articles

(Invited) Comprehensive Analysis of Electronic Properties of Carbon Nanotube Fibers

The electronic properties of various carbon nanotube (CNT) fibers that were produced by dry spinning from multi-walled CNT (MWCNT) forests, direct spinning from a CVD furnace and wet spinning of CNTs, are systematically investigated. The electrical conductivity of the fibers is significantly correlated with the effective CNT length [1], yarn density, and CNT alignment in the yarns. In contrast, the tensile strength depends more on the yarn density and the CNT alignment. The temperature dependence of the electronical properties of the CNT fibers are also investigated. Even though the constituent CNTs are very similar, the CNT fibers produced by the wet spinning of the acid treated CNT dispersion and the micelle CNT solution show different temperature dependences in the lower temperature region. The difference can be explained by the different conduction paths in the two wet-spinning fibers. Further, the iodide doping effect on the electronic property of the wet-spinning fiber produced by the acid treated dispersion is also discussed. This work was supported by a project (JPNP16010) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).[1] T. Morimoto et al., ACS Nano, 8, 9897–9904 (2014).

(Digital Presentation) Strategy to Enhance the Power Factor in Carbon Nanotubes

Flexible thermoelectrics, which can convert waste heat into electricity at surfaces with various shapes and moving parts, is one of important techniques for efficient use of our limited energy resources. Carbon nanotubes are one of possible candidates for flexible thermoelectrics, and then we have investigated the relationships between electronic structure, location of Fermi-energy level, morphology, and thermoelectric performance of the carbon nanotubes. Particularly, we have clarified the one-dimensional characteristics in thermoelectric properties of single walled carbon nanotubes (SWCNTs). In the case of metallic SWCNTs, the thermoelectric trade-off is violated when the Fermi-energy level is tuned around van hove-singularity [1]. Thus, simultaneous enhancement of Seebeck coefficient and electrical conductivity is possible in the metallic type. In the case of semiconducting SWCNTs, we theoretically clarified that it is very difficult to identify the 1D characters in the properties of their Seebeck coefficient, power factor, and electrical conductivity. We found it is necessary to clarify the relationships between thermoelectrical conductivity (L12 term) and electrical conductivity (σ) to discuss the dimensionality in thermoelectric properties of semiconducting nanomaterials. We observed clear peak structure in the L12-σ plot in high-purity (6,5) SWCNT thin films, reflecting the 1D traces in semiconducting SWCNTs[2]. Morphology control is also important to enhance power factor of carbon nanotubes. The Seebeck coefficient does not depend on the directions of heat flow and nanotube axis, but the electrical conductivity can be enhanced when the current direction is parallel to the nanotube axis [1, 3]. On the basis of above characteristics, we determined the following strategy to enhance the power factor (PF) of SWCNTs. Fine tuning of Fermi-energy level in the metallic type of nanotube can enhance PFAligned nanotubes with good electrical conductivity can enhance PF. Thus, we started to study the thermoelectric properties of carbon nanotube fibers with extremely good electrical conductivity, which are produced by Prof. Pasquali Group in Rice Univ [4]. As a result, we found the gigantic power factor of the nanotube fiber, which is 14 mWm-1K-2, with proper Fermi-level tuning [5]. Flexible thermoelectrics with such large PF will be useful for active cooling application.

Simultaneous reinforcement of the electrical and mechanical properties of carbon nanotube fibers by using a natural cross-linkable thermosetting polymer

Urushiol is infiltrated into carbon nanotube (CNT) fibers, and the urushiol infiltrated CNT (U-CNT) fibers are cross-linked by thermal or photochemical treatment. The cross-linked U-CNT (CL-U-CNT) fibers exhibit considerably enhanced properties.

Mechanical properties of carbon nanotube fibers reinforced epoxy resin composite films prepared by wet winding

Carbon nanotubes (CNTs) have a unique bond structure, high tensile strength, and high elongation at break, and thus are considered as excellent reinforcements for the preparation of high performance composites. However, the composites prepared by conventional methods suffer from poor CNT dispersion, low CNT alignment and low homogeneity of CNT-resin mixing. In this study, a new method is demonstrated to overcome these deficiencies. This method involves the preparation of CNT/epoxy (EP) resin composite films by wet winding using CNT macroscopic fibers as the reinforcements. Composite films with different EP contents are prepared by adjusting the EP concentration in the resin solution used for impregnation, the length of the resin container and the further impregnation time after winding. Thermogravimetric analysis, Raman spectroscopy and tensile testing are used to characterize the composite materials. Results show that the EP content and CNT alignment in the composite material can be controlled, and the composite film has a high tensile strength of 2 GPa and elongation at break of 4.5% when the EP content is optimized to be 44.62 wt% under the premise of a good degree of CNT alignment. The present study may provide a new technical route for the preparation of high-performance CNT composite materials.

Understanding the Mechanical and Conductive Properties of Carbon Nanotube Fibers for Smart Electronics.

The development of fiber-based smart electronics has provoked increasing demand for high-performance and multifunctional fiber materials. Carbon nanotube (CNT) fibers, the 1D macroassembly of CNTs, have extensively been utilized to construct wearable electronics due to their unique integration of high porosity/surface area, desirable mechanical/physical properties, and extraordinary structural flexibility, as well as their novel corrosion/oxidation resistivity. To take full advantage of CNT fibers, it is essential to understand their mechanical and conductive properties. Herein, the recent progress regarding the intrinsic structure-property relationship of CNT fibers, as well as the strategies of enhancing their mechanical and conductive properties are briefly summarized, providing helpful guidance for scouting ideally structured CNT fibers for specific flexible electronic applications.

Simultaneous enhancement of mechanical and electrical properties of carbon nanotube fiber by infiltration and subsequent carbonization of resorcinol-formaldehyde resin

An efficient strategy to enhance mechanical and electrical properties of carbon nanotube (CNT) fiber is developed by using resorcinol-formaldehyde (RF) resin as a functional infiltrating agent that can be carbonized under a mild condition. This strategy is a sequential process including infiltration of RF resin into CNT fiber, thermal stabilization and carbonization. The selection of resin and additive, and enhancement mechanism are elucidated by theoretical and experimental approaches. By using this strategy, the tensile strength, modulus, specific tensile strength and electrical conductivity of CNT fiber are enhanced by 8.2, 16.4, 2.1 and 3.6 fold, respectively.

Mechanical properties of carbon nanotube fibers at extreme temperatures

Carbon nanotube (CNT) fibers are strong, flexible, and multifunctional, which makes them promising candidates for use at extreme temperatures. However, the current reported mechanical properties of CNT fibers were commonly obtained at room temperature. Here, we report the measurement of the mechanical properties of CNT fibers at temperatures ranging from -196 °C to 2400 °C. Compared with the room temperature strength and modulus, CNT fibers tested at 1000 °C and 2400 °C retained 82% and 54% of the strength, and 71% and 50% of the modulus, respectively, while 68% and 220% increases in the strength and modulus, respectively, were observed for CNT fibers tested at -196 °C. We attributed the decay in the mechanical properties at high temperatures to the weakening of individual nanotubes and intertube interactions, and the strength enhancement at low temperature to the increased activation energy to break the nanotubes. The present study provides the fundamental mechanical properties of CNT fibers at extreme temperatures, which could facilitate the applications of CNT fibers in aeronautics and astronautics where extreme temperature conditions commonly exist.

Effects of Wet-Pressing and Cross-Linking on the Tensile Properties of Carbon Nanotube Fibers.

To increase the strength of carbon nanotube (CNT) fibers (CNTFs), the mean size of voids between bundles of CNTs was reduced by wet-pressing, and the CNTs were cross-linked. Separate and simultaneous physical (roller pressing) and chemical methods (cross-linking) were tested to confirm each method’s effects on the CNTF strength. By reducing the fraction of pores, roller pressing decreased the cross-sectional area from 160 μm2 to 66 μm2 and increased the average load-at-break from 2.83 ± 0.25 cN to 4.41 ± 0.16 cN. Simultaneous injection of crosslinker and roller pressing augmented the cross-linking effect by increasing the infiltration of the crosslinker solution into the CNTF, so the specific strength increased from 0.40 ± 0.05 N/tex to 0.67 ± 0.04 N/tex. To increase the strength by cross-linking, it was necessary that the size of the pores inside the CNTF were reduced, and the infiltration of the solution was increased. These results suggest that combined physical and chemical treatment is effective to increase the strength of CNTFs.

Improving mechanical properties of carbon nanotube fibers through simultaneous solid-state cycloaddition and crosslinking

Individual carbon nanotubes (CNTs) exhibit exceptional mechanical properties. However, difficulties remain in fully realizing these properties in CNT macro-assemblies, because the weak inter-tube forces result in the CNTs sliding past one another. Herein, a simple solid-state reaction is presented that enhances the mechanical properties of carbon nanotube fibers (CNTFs) through simultaneous covalent functionalization and crosslinking. This is the first chemical crosslinking proposed without the involvement of a catalyst or byproducts. The specific tensile strength of CNTFs obtained from the treatment employing a benzocyclobutene-based polymer is improved by 40%. Such improvement can be attributed to a reduced number of voids, impregnation of the polymer, and the formation of covalent crosslinks. This methodology is confirmed using both multiwalled nanotube (MWNT) powders and CNTFs. Thermogravimetric analysis, differential scanning calorimetry, x-ray photoelectron spectroscopy, and transmission electron microscopy of the treated MWNT powders confirm the covalent functionalization and formation of inter-tube crosslinks. This simple one-step reaction can be applied to industrial-scale production of high-strength CNTFs.

Mechanical and electrical properties of thermochemically cross-linked polymer carbon nanotube fibers

We report an effective post-treatment method to improve the mechanical and electrical properties of carbon nanotube fibers (CNTF) by using an aryl radical coupling reaction of polymers. The modified CNTF were analyzed using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The results demonstrated that chemical cross-links formed by Poly(p-iodostyrene) (PIS) inside the CNTF affected the mechanical and electrical properties. Compared with pristine CNTF, the cross-linked CNTF using PIS exhibited about 160%, 200% and 53% increased tensile strength (1.4N/tex), elastic modulus (103N/tex) and electrical conductivity (1480S/cm), respectively. This post-treatment method using polymer impregnation followed by thermochemical reaction may provide a general strategy for enhancing both mechanical and electrical properties of directly spun CNTFs.

Self-densified microstructure and enhanced properties of carbon nanotube fiber by infiltrating polymer

Polymer infiltration is an efficient way to strengthen carbon nanotube fibers in the process of spinning. Given the carbon nanotube/polymer composite fibers obtained from this approach possess higher mechanical properties than the ones of their components, the general rule of mixtures for a composite material is not applicable. Accordingly, a novel coarse-grained molecular dynamics model is proposed to investigate the mechanical behavior and microstructural evolution of the composite fibers, in which the carbon nanotubes and polymer chains are represented by mesoscale coarse-grained beads. Then the reinforcing mechanism of the composite fibers is illustrated. Dependence of the mechanical behavior on the microstructure of the composite fiber under different strain rates is revealed as well. Furthermore, the variations of mechanical properties of the composite fibers with cycle numbers under relatively low load are predicted.

Mechanical and electrical properties of carbon nanotube fibers from impregnation with poly(vinyl alcohol)/poly(acrylic acid) and subsequent thermal condensation

Sequential impregnation of a composite of poly(vinyl alcohol) and poly(acrylic acid) (PVA–PAA) into carbon‐nanotube fibers (CNTFs) and thermal condensation were investigated as a simple and efficient continuous textile engineering technique to prepare mechanically and electrically reinforced CNTFs. The CNTFs that were physically impregnated with PVA–PAA (PI‐CNTF–(PVA/PAA)) and the chemically crosslinked CNTF (CL‐CNTF–(PVA/PAA)) exhibited tensile strengths that were ∼1.6 and ∼2.1 times higher, respectively, and Young's moduli that were ∼1.3 and ∼2.3 times higher, respectively, than those of direct‐spun CNTFs. The electrical conductivity of PI‐CNTF–(PVA/PAA) and CL‐CNTF–(PVA/PAA) had values that were ∼1.5 and ∼1.7 times higher, respectively, than that of raw CNTFs. Moreover, the chemically crosslinked networks of CL‐CNTF–(PVA/PAA) resulting from the heat treatment enhanced the water resistance of the composite. POLYM. COMPOS., 39:971–977, 2018. © 2016 Society of Plastics Engineers

Improvements in the mechanical properties of carbon nanotube fibers through graphene oxide interlocking

Carbon nanotubes (CNTs) have attracted remarkable attention due to their high mechanical performance and low density which are desirable for application as lightweight materials in automotive and aerospace industries to improve fuel efficiency. However, CNT fibers generally exhibit far lower mechanical properties (strength, toughness, etc.) than individual CNTs due to the weak interfacial shear strength between adjacent shells, nanotubes, and nanotube bundles, causing insufficient load transfer. In this study, to improve the interfacial shear strength of the fibers, graphene oxide (GO) was infiltrated into the CNT fibers to interlock CNT bundles. GO was selected due to its high mechanical properties and similar carbon-based structure as the CNT fibers. In addition, GO interfaces are known to exhibit higher frictional forces for sliding contacts than those found for pristine graphene sheets, which makes it more desirable for enhancing the shear interactions. GO particles with a width of ∼40 nm, which match closely to the void size within fibers, were optimal for enhancing the mechanical properties of the CNT fibers. Tensile testing demonstrated optimized GO infiltrated CNT fibers exhibited improvement in: stiffness ∼100%, yield strength ∼110%, ultimate tensile strength ∼56%, and energy to failure ∼30% of pristine CNT fibers.

Effect of oxygen plasma treatment on the mechanical properties of carbon nanotube fibers

The effect of atmospheric-pressure plasma treatment on the mechanical properties of carbon nanotube (CNT) fibers was investigated. When the CNT fibers were irradiated with RF atmospheric-pressure plasma, it was confirmed that functional groups such as hydroxyl or carboxylic acid groups were generated on the surface of the CNT fibers. The mechanical properties of CNT fibers were improved as a result of the plasma treatment because the functional groups on individual CNTs were able to induce hydrogen bonds between them. After plasma treatment with gas flow of argon (10lpm) and oxygen (100sccm), the tensile strength of the pure CNT fiber increased by approximately 40%, from 0.62N/tex to 0.86N/tex, and the modulus increased by approximately 20%, from 27.9N/tex to 32.9N/tex.

Effect of acidification conditions on the properties of carbon nanotube fibers

Carbon nanotube (CNT) fibers prepared by dry-spun method were functionalized by mixture of nitric and sulfuric acids. The effects of acidification conditions on the electrical conductivity and tensile properties of CNT fibers were investigated. A strong, high conductive CNT fiber was obtained under the optimal mixture ratio and processing time, with a electrical conductivity and tensile strength up to 3.2×104S/m and 1103MPa, respectively. It showed that the acids densified the surface of the CNT fiber and introduced functional groups onto the tubes, both of which contributed to the conductivity improvement of the CNT fiber. The infrared spectroscopy, Raman and fracture analysis indicated that acidification process resulted in two competitive effects on the tensile properties of CNT fibers, one was the positive contribution by the enhancement of interactions between CNTs through the densification and functional groups, and the other was the negative effect due to the structural destruction of the tubes.

A damage mechanics model for twisted carbon nanotube fibers

Carbon nanotube fibers can be fabricated by the chemical vapor deposition spinning process. They are promising for a wide range of applications such as the building blocks of high-performance composite materials and micro-electrochemical sensors. Mechanical twisting is an effective means of enhancing the mechanical properties of carbon nanotube fibers during fabrication or by post processing. However, the effects of twisting on the mechanical properties remain an unsolved issue. In this paper, we present a two-scale damage mechanics model to quantitatively investigate the effects of twisting on the mechanical properties of carbon nanotube fibers. The numerical results demonstrate that the developed damage mechanics model can effectively describe the elastic and the plastic-like behaviors of carbon nanotube fibers during the tension process. A definite range of twisting which can effectively enhance the mechanical properties of carbon nanotube fiber is given. The results can be used to guide the mechanical twisting of carbon nanotube fibers to improve their properties and help optimize the mechanical performance of carbon nanotube-based materials.

Cooperative deformation of carboxyl groups in functionalized carbon nanotubes

Functionalized carbon nanotubes have tremendous potential for nanotechnology applications such as in the fabrication of polymeric carbon fibers. However, approaches to design carbon nanotube structures by using functional groups as glue and carbon nanotubes as stiff building blocks to reach superior mechanical strength and toughness at the fiber level with limited amount of materials remains poorly understood. Inspired by the outstanding mechanical properties of spider silk, here we present a bio-inspired structural model of carbon nanotube based fibers connected by weak hydrogen bonds (H-bonds) formed between functional carboxyl groups as the molecular interface. By applying shear loading, we study how the deformation of H-bonds in functional groups is affected by the structural organization of the carboxyl groups, as well as by the geometry of constituting carbon nanotubes. The analysis of H-bond deformation fields is used to compute the extent of significant deformation of inter-CNT bonds, defining a region of cooperativity. We utilize an exponential function (exp (−x/ξ)) to fit the deformation of H-bonds, with the cooperative region defined by the parameter ξ, and where a higher value of ξ represents a weaker exponential decay of displacements of carboxyl groups from the point where the load is applied. Hence, the parameter ξ characterizes the number of carboxyl groups that participate in the deformation of CNTs under shear loading. The cooperativity of deformation is used as a measure for the utilization of the chemical bonds facilitated by the functional groups. We find that for ultra-small diameter CNTs below 1nm the external force deforms H-bonds significantly only within a relatively small region on the order of a few nanometers. We find that the mechanical properties of carbon nanotube fibers are affected by the organization of H-bonds in functional carboxyl groups. Both, the grouping of functional groups into clusters, and a specific variation of the clustering of functional groups along the CNT axis are shown to be potential strategies to improve the cooperativity of deformation. This allows for a more effective utilization of functional groups and hence, larger overlap lengths between CNTs in fibers. The effect of structural organization of functional groups is not only significant in very small diameter CNTs, but also in larger diameter CNTs as they are most commonly used for engineering applications. Notably larger-diameter CNTs naturally show a larger cooperative deformation range. Our model can be applied to other functional groups attached to CNTs, and could in principle also include strong bonds such as covalent or ionic bonds, or other weak bonds such van der Waals forces or dipole–dipole interactions.

Carbon nanotube fibers for electrochemical applications: effect of enhanced interfaces by an acid treatment

Chemical treatment using concentrated nitric acid (16 M) not only induced significant improvement of mechanical and electrical properties of carbon nanotube fibers due to the enhanced interfacial interaction but also allowed much more efficient deposition of polyaniline for developing fiber-shaped supercapacitors. After the 2 h treatment, the acidized fiber had a tensile strength of 1.52 GPa and an electrical conductivity of 1050 S cm(-1), increased by 52% and 128%, respectively, compared with the untreated one. By depositing polyaniline for 10 min around the fiber, the composite fiber had a volumetric capacitance of 239 F cm(-3), 17% higher than that without the acid treatment. For a long time treatment up to 6 h, although the strength and conductivity decreased slightly, the composite fiber had a super high volumetric capacitance up to 299 F cm(-3). The improvement of electrochemical performance is attributed to the increased deposition rate and structural change of polyaniline due to the existence of functional groups on the fiber surface.

Effect of potassium doping on electrical properties of carbon nanotube fibers

We investigate the effect of potassium (K) doping on the transport properties of aligned single-walled carbon nanotube fibers. The temperature dependence of the electrical resistance, the current-voltage characteristics, and the magnetoresistance vs external magnetic field of the fibers consistently show that doping enhances the metallic character of the fibers and that the response of the samples can be quantitatively explained in two thermal regimes separated by a characteristic temperature ${T}^{*}$. At temperatures higher than ${T}^{*}$, the data are interpreted in the framework of variable range hopping theory, suggesting that the increased conductance with potassium doping is due to the increase of the density of states, which enhances carriers hopping. For temperatures below ${T}^{*}$, experimental evidence of fluctuation temperature-induced tunneling mechanism suggests that the doping by K atoms affects the potential barriers established between adjacent carbon nanotubes, enhancing the metallic properties of the fibers.

Dependence of structures and properties of carbon nanotube fibers on heating treatment

The dependence of structures and mechanical and electrical properties of carbon nanotube fibers on heating treatments have been first investigated in both argon and air. The relationships between structures and properties have been also explored. These discoveries may be used to design and improve carbon nanotube fibers.