The following figure shows the three orientations that are possible: armchair, zigzag, and chiral. Electrical properties depend on the orientation of the hexagons.
Carbon nanotubes with the hexagons orientated in the configuration labeled armchair hexagons are lined up parallel to the axis of the nanotube have electrical properties similar to metals. When you apply a voltage between two ends of an armchair nanotube, a current will flow. An armchair carbon nanotube is, in fact, a better conductor than the copper normally used in electrical wire, or any other metal. Researchers are developing methods to spin carbon nanotubes together to make low-resistance electrical wires that could transform the electrical power grid, as we discuss in Chapter 5, as well as reduce the power consumed and weight of wiring in such power- and weight-sensitive uses as spacecraft and airplanes.
Development of flexible electronics has recently focused on organic molecules because, unlike silicon , they are compatible with bendable plastic substrates.
Flexible electronics have potential in such applications as low-power electronic newspapers or PDAs that roll up into the size and shape of a pen. Carbon-nanotube networks, on the other hand, combine the performance of silicon with the flexibility of organic films on plastic.
Rogers says that the speed of the nanotube device compares favorably with the speed of commercially used single-crystal silicon circuits. The transistors can also switch between on and off states in the range of several kilohertz, which is similar to the range of those used for liquid crystal displays and radio frequency identification RFID sensors.
However, the on-off current ratio for carbon nanotubes is still a few orders of magnitude lower than that for silicon transistors. The researchers made the networks by depositing nanotubes onto plastic by standard printing methods, which could lead to low-cost, large-scale fabrication.
And the printed circuits can bend to a radius of about five millimeters without compromising the electrical performance of the device. This needs to be used within three months of purchasing the report. If you have any questions about this report, please do not hesitate to contact our report team at research IDTechEx.
Subscription Enquiry. The chiral vector is represented by a pair of indices n, m. T denotes the tube axis, and a1 and a2 are the unit vectors of graphene in real space. Charge carrier mobility of carbon nanotubes compared with alternatives. Atomic Force Microscope image of carbon nanotubes before and after processing. Potential applications are flexible solar cells, displays and touch screens. Conductance in ohms per square for the different printable conductive materials, at typical thicknesses used, compared with bulk metal.
Illustrating how the many manufacturing techniques affect CNT quality, cost, scalability and accessible market. New printable elastic conductors made of carbon nanotubes are used to connect OLEDs in a stretchable display that can be spread over a curved surface. Main applications of conductive inks and some major suppliers today. Comparison of some of the main options for the semiconductors in printed and potentially printed transistors. Comparison of the three types of capacitor when storing one kilojoule of energy.
The carbon nanotube supercapacitor versus batteries and traditional capacitors. The process. The same fibers displayed a thermal conductivity of c. The same authors determined the effect of doping with iodine. From these, we can conclude that optimal morphology and structure i. As far as graphene fibers are concerned, the two-dimensional structure and crystal domains with finite dimensions dictate a few considerations. First of all, layered structures are very flexible, but their structural organization in a yarn of indefinite length, implies a stacking Figure 5.
A full scale approach to minimize defects from atomic to macroscale levels was attempted by Xu et al. The authors used large GO liquid crystals and the wet-spinning method with continuous stretching during the process to obtain the ordered orientation of graphene sheets along the axis of fibers.
The obtained fibers demonstrated a series of record properties, including ultrahigh stiffness GPa , tensile strength 1.
Figure 5. Structural models of: defects in GFs from the macroscale to the atomic scale of chemically reduced GO top panels ; high-quality defect-free GFs bottom panels. Adapted from Xu et al. Conventional and unconventional materials for electrical wiring are listed in Table 1.
In almost all cases, the Drude model of electrical conduction is a good approximation. Among metals, the best conductors have one electron in their outermost shell layer metal Cu: 2, 8, 18, 1; metal Ag: 2, 8, 18, 18, 1; and Au: 2, 8, 18, 32, 18, 1. It is well known that conventional electrical conductors operating nearly room temperature are commonly based on copper 5. Both metals require only a weak field to strip one electron as compared to other metals or elements.
Depending on the precursor type, the preparation process and the heating temperature, carbon fibers exhibit a relatively lower conductivity than graphite. Table 1. In principle, an assembled carbon yarn could virtually transport electricity like a ballistic conductor and the behavior could be well interpreted in terms of electrons and holes with the Drude model of electrical conduction Fujita and Suzuki, However, fibers made exclusively of the same metallic armchair SWCNT type or graphene strips, defect-free and infinitely long, are by now technologically unfeasible.
Thess et al. The authors measured at room temperature a resistivity of c. The conductivity of individual tubes is often two orders of magnitude larger than their assemblies, thus highlighting the role played by the contact resistances at the interfaces of the single nanotubes Li et al. As the overlap of nanoconductors make possible electrical connections and that an infinite number of junctions is present in the fiber, Xu F. Lekawa-Raus et al. Interestingly, the authors have also demonstrated that the conductivity scales inversely proportionally to the CNT diameter for an ideal fiber.
Barnett et al. Interestingly, it was shown that when there is a large mismatch i. The authors also showed that, in agreement with the unit cell dimensions Meunier et al.
Moreover, the overall properties of the assembled fibers are determined in a large extent by composition, morphology and structure i. The fibers containing carbon nanostructures can have a very wide range of electrical conductivity. In principle, both CNTs and graphene exhibit semimetal characteristics and their conductivities are few-orders of magnitude higher than that of their macroscopic assemblies, which behave as semiconductors Li et al.
Commonly, the temperature dependence with electrical properties indicates that the conductivity of all-carbon fibers increases from low to room temperature semiconducting behavior Li et al. In graphene and CNT fibers the electron transport properties are dominated by hopping mechanism Li et al. Electrical transport properties, investigated by Dini et al. In the same experiments, it was also observed at room temperature that the electrical transport was not affected by densification of CNT fibers and the contacts between CNTs.
As reported by some authors, properties of carbon ropes made of individual CNTs or graphene sheets, can be increased by the heat treatment. Xin et al. Wang et al. The selective thermal dehalogenation of the organic precursor, for a fact, may lead to the covalent bonding formation.
In another recent paper, Hada et al. Liu et al. The laser light absorbed from the black CNTs, may promote a local heating Cesano et al. On the other hands, the addition of graphene flakes within a CNT wire to constitute a hybrid composition has been shown to increase the electrical conductivity, current-carrying capacity or ampacity, which can be defined as the maximum current in amperes, that a conductor can carry continuously under the conditions of use and without exceeding its temperature rating and doping properties Li and Pandey, ; Lepak-Kuc et al.
Besides the remarked relevance of the graphene flake characteristics, of the fiber purity and of the solvent properties, the authors explained that graphene flakes constitute effective bridges by means of a theoretical model. Such bridges create Fermi level states inside the CNTs bands, with the consequent increase of current and electrical conductance in the system, while doping can enhance the conductance of the hybrid assembly.
The doping of the carbon and the related electrical conductivity is discussed in the next section. Along with CNT fibers, some important concepts for achieving higher electrical properties can be summarized.
Firstly, metallic nanotubes may exhibit ballistic transport, but the defects and impurities i. Lastly, the control over structure and defects of fibers i. However, in some of these preparations, CNTs are often in the presence of strong acids, raising the doubt that acid doping could be the main responsible for the high fiber conductivity. As far as graphene fibers are concerned, some important concepts for achieving higher electrical properties can be summarized.
Firstly, polymer based-graphene fibers exhibit a limited conductivity. Lastly, due to the polycrystalline nature of the graphene domains and the three-dimensionality of the microfiber backbone, the control over the orientation of graphene sheets and concentration of defects i. Surprisingly, Liu et al. All the previous considerations are valid with direct current DC , while for the conductivity in alternating current AC other factors should be also considered, because the signal transport through a conductor becomes difficult at the high frequency, due to skin and proximity effects.
Skin effect occurs in a conductor at high frequency with a distribution of the current density, leading to a higher resistance. The alternating flux in a conductor, caused by the current of the other near conductors, results an apparent increase in the resistance and is called proximity effect. It is expected that the tubular nature at the nanoscale of CNTs should mitigate both the skin and proximity effects, thus enabling a high frequency transport superior to typical metals Lekawa-Raus et al.
However, the literature on this subject is still very limited. In a recent study, Lee J. The authors reported that the highly packed and aligned CNTs confer high mechanical and electrical properties to the fiber among the record observed for the undoped CNT fibers reported in Table 1. Molecular assembly strategies alone Aboutalebi et al.
On the other hand, it is generally observed that the infiltration with polymers resulted in a significant decrease in electrical conductivity Lu et al.
The doping paradigm to increase the electrical properties of carbon-based conductors is not new. Since the last century the graphite intercalation compounds GICs with halogens, alkali compounds were studied Hennig, ; Ottmers and Rase, It was found that a remarkable variation of the free-carrier concentration allows the enhancements of the electrical, thermal and magnetic properties of the host material Dresselhaus and Dresselhaus, Most of the recent studies on conductors based on doped carbons have examined the case of carbon nanotube fibers, while the literature on graphene fibers appears moderately more limited.
Randeniya et al. Other used dopants with molecular structure are H-carbazole p-dopant and phosphine derivatives n-dopants Lee T. A general result of the doping process is the change in the color close to the saturation percentages: pure all-carbon fibers are silver gray i. The increased carrier density and the mobility in doped fibers may allow a higher electrical conductivity, which was found to show the same temperature dependency with small variation than pure all-carbon fibers.
All these elements imply that the band structure evolves from a two-carrier semimetal model of graphite to one-carrier free-electron-like metal model with the increasing of the dopant concentration Fischer and Thompson, Recent studies suggest that the electrical properties increase with the amount of dopant due to the shifting of the Fermi level, although there is an intrinsic limit of the doping loading Zhang et al.
Zhao et al. With the doping with iodine, polyiodide chains are formed. Together with I 3 - and I 5 - species, the hole concentration was found to be one order of magnitude higher from c. Together with improved carrier mobilities, a similar increased charge carrier concentrations holes i. Along the dopant series, K and I appear to be the most promising, but they have been also found somehow instable in air and the fiber structure may undergo an immediate degradation by moderate temperature exposures.
In the paper, DWCNT yarns with smaller diameter were found to have larger resistivity compared to larger diameter fibers, thus indicating the role played by voids, other defects and skin effects on the conductivity.
The electrical conductivity boosted to a metallic level has been observed for doped-graphene and carbon nanotube fibers Xu and Gao, ; Fang et al. In a recent paper, Lee T. Interestingly, the authors reported that by optimizing the alignment degree of CNTs and doping the nanotubes with H-carbazole and phosphine derivatives p- and n-dopant, respectively the carrier mobility increased with an improvement of the power factor of the generator. Liu Y. The room temperature conductivity of the film reached the record value for the graphene series of 1.
In summary, the chemical doping has the potential to improve the thermal and electrical conductivities of CNT and graphene fibers.
Notwithstanding the increase observed for doped systems, the electrical conductivity of carbon yarns becomes comparable or higher than that of metal filaments. Another advantage of carbon-based filaments over metallic filaments is that they allow a remarkable chemical resistance.
However, the temperature and time stability under working conditions still needs further investigations and improvements. The method could be applied to examine structure, porosity, cracks phase distribution, external habit of fibers Figure 6 , thus highlighting also interfaces within the structure of the assembled fiber Headrick et al.
Figure 6. Phase contrast X-ray computed tomography images of CNT fiber cross-sections for three type of fibers prepared by twisting A thick, B thin, or C by solution-spinning from coagulation bath. SEM images of the same fibers are reported in the insets. Adapted from Headrick et al.
X-ray scattering techniques can provide useful insights of the fiber scaffold and of its components. Although, from X-ray diffraction XRD patterns, the structure of CNTs appears close to those of graphitic materials due to their intrinsic nature, 3D XRD analysis can further provide the fiber crystallite sizes along both, longitudinal and transversal directions Xin et al.
In addition, XRD measurements can be also utilized to study the nature of intercalated compounds e. For example, Zhou et al. The same authors reported also interlayer spacings of 0. Ca-doped carbon-assembled fibers was reported to show a series of 00 l reflections with d-spacings of 0. Wide-angle X-ray scattering WAXS technique can be informative about the structure and orientation degree along the fiber axis Xu et al.
Lamellar structural information is of particular interest when GO or liquid crystals are used in presence of nematic, lamellar and chiral compounds Kumar et al. This fact is however not surprisingly, due to the familiar concept in polymer science.
Figure 7. A XRD patterns of the pure and doped graphene fibers. Reproduced from Liu et al. Raman spectroscopy is one of the main analytical techniques that provide a detailed picture about chemical structure, phase, crystallinity and molecular interactions, not only of the CNTs and graphene sheets, but also of the final assembled fiber.
A detailed description of CNT and graphene Raman features goes beyond the scope of this review and can be found elsewhere Dresselhaus et al. Among all fingerprints, the presence of the D-band at about cm —1 comes from double resonance Raman effect corresponding to disordered carbon structure, while the G-band c. This spectral feature associated with the curvature and appearing similar for nanotubes and graphene, reveals distinct characteristics allowing to distinguish a carbon structure from another one i.
A D-band overtone centered at about cm —1 and called 2D band can be found. This fingerprint can also provide a detailed description of the type, structure and diameters of the nanotubes. Interestingly and very recently, Park Y. Raman spectroscopy is also worth of a special mention in the case of doped systems.
As a matter of fact, the G band upshift is usually observed in doped CNTs and graphene also when the structural integrity of the structure is preserved. The reason is that G-band position, very sensitive to charge doping, shifts the Fermi level left from the neutrality point Liu et al.
The author observed a similar G-band upshifting of c. Additional peaks at cm —1 , cm —1 , and cm —1 can be found to verify the successful doping with FeCl 3 Liu et al.
This fact arises from the not negligible lattice expansion occurred after doping. Electron microscopies scanning and transmission electron microscopy: SEM and TEM, respectively are typically adopted techniques to investigate the morphology, structure and twisting properties of the assembled fibers Aboutalebi et al. Visual inspections of the fiber backbone, allows to identify multiscale defects, which include rough surface with wrinkles, degree of homogeneity of the structure, presence of larger pores and boundary regions at lower resolution SEM , crystallinity and defects of CNTs and of graphene sheets, including vacancy defects, dislocation edges, stacking faults, grain boundaries and other features HR-TEM , as well documented in some papers Behabtu et al.
Furthermore, elemental mapping images can demonstrate not only the doping, but also the element distributions along the fiber. Focused ion beam FIB milling can be used to cut down a fiber with sections perpendicular and parallel to the axis of the fiber. Such sections can be directly SEM imaged to reveal the yarn structure and correlate changes in morphology and structure with properties Sears et al.
X-ray photoelectron spectroscopy XPS measurements can be informative of the chemical state, and of the electronic state composition of the elements at the surface of fibers. Together with the characteristic peaks of both graphene and CNTs at It is also observed that the intensities of oxygen-reach peaks increased with the oxidation time Park H.
The high-resolution spectra indicated C—N sp 2 and sp 3 bonding at Often the assembled fibers contain pores among nanotubes or sheets due to the microcrystalline nature of the carbon building blocks.
A porosity would be beneficial for electrochemical and sensing applications Xu et al. For this reason, it would be convenient to investigate the porosity of the fiber by means of sorption techniques.
Zeng et al. The authors observed an increase of the electrical conductivity with the densification of the fiber of one order of magnitude. Contact angle measurements can provide important findings on the fiber-liquid interactions Zhang L. The non-polar and polar surface energy components can be obtained and other wetting parameters work of adhesion, spreading coefficient and wetting tension can be predicted.
In most of the synthesis methods to obtain nanocarbons a material with a large variety of properties is produced.
Such material contains also catalyst nanoparticles, amorphous carbon and other side-products. Among all, termogravimetric analysis TGA can be used to verify purity and thermal stability of the fibers Lepak-Kuc et al.
If the fiber is undoped, the residue of the thermal treatment performed in air can be easily associated with the inorganic content of the catalyst and of the metal oxide support. A decrease in the stability of materials with a reduction of the oxidation temperature can be also observed after an acid treatment e. Notice that, in general the strongly oxidized carbon materials are more reactive to the thermal treatments Cravotto et al. Behabtu et al. Interestingly, the authors showed a light-emitting diode LED suspended and electrically contacted by two CNT fibers 24 nm in diameter loaded with 30 mA current 6.
More interestingly, the same authors illustrated a field-emitting device fabricated with a CNT fiber working as a cathode. An emitted current density of 5. Surprisingly, Kim et al. These kinds of devices could be used perhaps in the sea to harvest wave energy, as well as with thermally driven artificial muscles to convert temperature fluctuations to electrical energy.
In other papers, Uddin et al. Beside other advantages flexibility, mechanical resistance, electrically and catalytically active , the authors reported a photovoltaic efficiency from the sunlight raising about the 7. Koziol and colleagues Lekawa-Raus et al. The authors found the characteristics of the transformer to be perfectly linear and are not dependent on the material in agreement with the classical theory Kurzepa et al.
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