Replacing Copper Wires with Carbon Nanotube Wires in Electrical Transformers

Carbon nanotubes, with their unique physical properties, have the potential to outperform conventionally used electrical wiring metals. Any improvement in this area of technology would be of great importance to industry, the economy, and the environment, as the global need for electrical energy and its efficient transfer and conversion rapidly increases. Carbon nanotube fibers, which are assemblies made purely of carbon nanotubes, can uniquely be used in macroscopic electrical applications including electrical wires and devices where the operation is enabled by these conductors. This paper presents details of the working prototype of an electrical machine, a transformer, where conventional copper wires have been replaced with conducting wires made purely of carbon nanotube fibers.     

Shaping Carbon Nanotubes and the Effects on Their Electrical and Mechanical Properties

A method is developed and shown to be able to shape a carbon nanotube (CNT) into a desired morphology while maintaining its excellent electrical and mechanical properties. Single, freestanding nanotubes are bent by a scanning tunneling microscopy probe, and their morphology is fixed by electron‐beam‐induced deposition (inside a transmission electron microscope) of amorphous carbon on the bent area. It is shown that the mechanical strength of the bent CNT may be greatly enhanced by increasing the amount of carbon glue or the deposition area, and the electrical conduction of the nanotube shows hardly any dependence on the bending deformation or on the deposition of amorphous carbon. Our findings suggest that CNTs might be manipulated and processed as interconnections between electronic devices without much degradation in their electrical conductance, and be used in areas requiring complex morphology, such as nanometer‐scale transport carriers and nanoelectromechanical systems.     

Carbon‐Nanotube–PDMS Composite Coatings on Optical Fibers for All‐Optical Ultrasound Imaging

Polymer–carbon nanotube composite coatings have properties that are desirable for a wide range of applications. However, fabrication of these coatings onto submillimeter structures with the efficient use of nanotubes has been challenging. Polydimethylsiloxane (PDMS)–carbon nanotube composite coatings are of particular interest for optical ultrasound transmission, which shows promise for biomedical imaging and therapeutic applications. In this study, methods for fabricating composite coatings comprising PDMS and multiwalled carbon nanotubes (MWCNTs) with submicrometer thickness are developed and used to coat the distal ends of optical fibers. These methods include creating a MWCNT organogel using two solvents, dip coating of this organogel, and subsequent overcoating with PDMS. These coated fibers are used as all‐optical ultrasound transmitters that achieve high ultrasound pressures (up to 21.5 MPa peak‐to‐peak) and broad frequency bandwidths (up to 39.8 MHz). Their clinical potential is demonstrated with all‐optical pulse‐echo ultrasound imaging of an aorta. The fabrication methods in this paper allow for the creation of thin, uniform carbon nanotube composites on miniature or temperature‐sensitive surfaces, to enable a wide range of advanced sensing capabilities.     

Single‐Walled Carbon Nanotube Networks: The Influence of Individual Tube–Tube Contacts on the Large‐Scale Conductivity of Polymer Composites

Over two decades after carbon nanotubes started to attract interest for their seemingly huge prospects, their electrical properties are far from being used to the maximum potential. Composite materials based on carbon nanotubes still have conductivities several orders of magnitude below those of the tubes themselves. This study aims at understanding the reason for these limitations and the possibilities to overcome them. Based on and validated by real single‐walled carbon nanotube (SWCNT) networks, a simple model is developed, which can bridge the gap between macroscale and nanoscale down to individual tube–tube contacts. The model is used to calculate the electrical properties of the SWCNT networks, both as‐prepared and impregnated with an epoxy‐amine polymer. The experimental results show that the polymer has a small effect on the large‐scale network resistance. From the model results it is concluded that the main contribution to the conductivity of the network results from direct contacts, and that in their presence tunneling contacts contribute insignificantly to the conductivity. Preparing highly conductive polymer composites is only possible if the number of direct, low‐resistance contacts in the network is sufficiently large and therefore these direct contacts play the key role.     

Carbon Nanotube Fiber Based Stretchable Conductor

Carbon nanotube (CNT) based continuous fiber, a CNT assembly that could potentially retain the superb properties of individual CNTs on a macroscopic scale, belongs to a fascinating new class of electronic materials with potential applications in electronics, sensing, and conducting wires. Here, the fabrication of CNT fiber based stretchable conductors by a simple prestraining‐then‐buckling approach is reported. To enhance the interfacial bonding between the fibers and the poly(dimethylsiloxane) (PDMS) substrate and thus facilitate the buckling formation, CNT fibers are first coated with a thin layer of liquid PDMS before being transferred to the prestrained substrate. The CNT fibers are deformed into massive buckles, resulting from the compressive force generated upon releasing the fiber/substrate assembly from prestrain. This buckling shape is quite different from the sinusoidal shape observed previously in otherwise analogous systems. Similar experiments performed on carbon fiber/PDMS composite film, on the other hand, result in extensive fiber fracture due to the higher fiber flexural modulus. Furthermore, the CNT fiber/PDMS composite film shows very little variation in resistance (≈1%) under multiple stretching‐and‐releasing cycles up to a prestrain level of 40%, indicating the outstanding stability and repeatability in performance as stretchable conductors.     

Wet‐Spun Stimuli‐Responsive Composite Fibers with Tunable Electrical Conductivity

Wet‐spun stimuli‐responsive composite fibers made of covalently crosslinked alginate with a high concentration of single‐walled carbon nanotubes (SWCNTs) are electroconductive and sensitive to humidity, pH, and ionic strength, due to pH‐tunable water absorbing properties of the covalently crosslinked alginate. The conductivity depends on the material swelling in humid atmosphere and aqueous solutions: the greater the swelling, the smaller is the electrical conductivity. The covalently crosslinked fibers reversibly deform during the swelling/shrinking. In the swollen state, the fibers are less conductive, while they return to the same level of conductivity after shrinking. This unique reversible change of electroconductivity of the SWCNT‐alginate fibers is due to the elastic deformation of the alginate network in the area of electrical contacts between SWCNT bundles arrested in the alginate matrix. Fibers of this kind can be used as a simple, robust, disposable, and biocompatible platform for electrotextiles, biosensors, and flexible electronics in biomedical and biotechnological applications.     

Fabrication and Electrical and Mechanical Properties of Carbon Nanotube Interconnections

Individual carbon nanotubes (CNTs) have been cut, manipulated, and soldered via electron‐beam‐induced deposition of amorphous carbon (a‐C) and using a scanning tunneling microscope inside a transmission electron microscope. All CNT structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, have been successfully constructed with a high degree of control, and their electrical and mechanical properties have been measured in situ inside the transmission electron microscope. It is found that multiple CNTs may be readily soldered together with moderate junction resistance and excellent mechanical resilience and strength, and the junction resistance may be further reduced by current‐induced graphitization of the deposited a‐C on the junction.     

碳纳米管/石墨烯复合结构的第一性原理计算

为了研究碳纳米管/石墨烯复合结构的电学性质,采用密度泛函理论(DFT)下的第一性原理,对四种T型复合结构进行了几何结构优化,分析了该复合结构的结合能,能带结构,电子态密度,Mulliken电荷分布及功函数.结果表明复合结构均表现出半导体性质,其稳定性及电子结构取决于碳纳米管类型和复合结构的连接方式,而且复合材料的功函数...     

Properties of Carbon Nanotube Fibers Spun from DNA‐Stabilized Dispersions

The fabrication of single‐walled carbon nanotube (CNT) fibers containing (salmon) DNA has been demonstrated. The DNA material has been found to be adequate for dispersing relatively large concentrations (up to 1 % by weight) of carbon nanotubes. These dispersions are better suited for fiber spinning than previously studied dispersions based on conventional surfactants, such as sodium dodecyl sulfate (SDS). The DNA‐containing fibers were less conductive than the fibers based on SDS, but they were significantly stronger. Considerably increased conductivity was obtained by thermally annealing the CNT/DNA fibers, a process accompanied by a loss in mechanical strength. Smaller improvements in conductivity could be introduced by annealing the carbon nanotubes before fiber production, with no alteration of the fiber mechanical properties. Those CNT/DNA fibers that were mechanically strong and conductive also exhibited good electrochemical behavior and useful capacitance values (up to 7.2 F g^–1).     

Tuning Electrical Conductance of Serpentine Single‐Walled Carbon Nanotubes

Changes in resistivity of serpentine single‐walled carbon nanotubes are presented as a function of bending radius, r_b, in the range of 100–2000 nm. Resistivity (ρ) is observed to increase with curvature (1/r_b), which is consistent with theoretical speculation on strain‐induced bandgap increment. Furthermore, a sharp bend (r_b < 50nm) in the nanotubes results in a drastic change in the field‐effect behavior, i.e., from ambipolar to p type across the bend. Local Raman spectra show that the G‐band Raman frequencies shift along the curvature, which may be attributed to local deformation and broken cylindrical symmetry in the nanotubes. The results suggest the possibility to tune the electrical properties by bending nanotubes and to build an all‐nanotube device by modulating the structure of the same tube.     

Cover Picture: Fabrication and Electrical and Mechanical Properties of Carbon Nanotube Interconnections (Adv. Funct. Mater. 11/2005)

The fabrication of carbon nanotube (CNT) structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, has been achieved by cutting and soldering CNTs using electron‐beam‐induced deposition of amorphous carbon (a‐C), as detailed in the work of Peng and co‐workers on p. 1825. These CNT structures have been constructed with a high degree of control, and it is found that the electric conductance and mechanical strength of the junctions can be improved by the deposition of a‐C and by increasing the contact area of the junctions. Individual carbon nanotubes (CNTs) have been cut, manipulated, and soldered via electron‐beam‐induced deposition of amorphous carbon (a‐C) and using a scanning tunneling microscope inside a transmission electron microscope. All CNT structures, including simple tube–tube connections, crossed junctions, T‐junctions, zigzag structures, and even nanotube networks, have been successfully constructed with a high degree of control, and their electrical and mechanical properties have been measured in situ inside the transmission electron microscope. It is found that multiple CNTs may be readily soldered together with moderate junction resistance and excellent mechanical resilience and strength, and the junction resistance may be further reduced by current‐induced graphitization of the deposited a‐C on the junction.     

Integrating Multiple Resistive Memory Devices on a Single Carbon Nanotube

Nano‐objects would be of great interest for the development of new types of electronic circuits if one could combine their nanometer scale with original functionalities beyond the conventional transistor action. However, the associated circuit architectures will have to handle the increasing variability and defect rate intrinsic to the nanoscale. In this context, there is a very fast growing interest for memory devices, and in particular resistive memory devices, used as building blocks in reconfigurable circuits tolerant to defects and variability. It was recently shown that optically gated carbon nanotube field effect transistors (OG‐CNTFETs) based on large assemblies of nanotubes covered by an organic photoconductive thin film can be operated as programmable resistors and thus used as artificial synapses in circuits with function‐learning capabilities. Here, the potential of such approach is evaluated in terms of scalability by integrating and addressing several individually programmable resistances on a single carbon nanotube. In addition, the charge storage mechanism can be controlled at a length scale smaller than the device length allowing to also program the direction in which the current flows. It thus demonstrates that a single nanotube section can combine all‐in‐one the properties of an analog resistive memory and of a rectifying diode with tunable polarity.     

Carbon Nanotube Alignment via Electrohydrodynamic Patterning of Nanocomposites

Electrohydrodynamic (EHD) pattern formation in carbon nanotube‐polymer composite films yields well‐defined patterns on the micrometer scale along with the alignment of carbon nanotubes (CNTs) within these patterns. Conductive pathways in nanotube networks formed during EHD patterning of nanocomposite films results in a substantial increase in the composites’ conductivity at loadings exceeding the percolation threshold. The degree of nanotube alignment can be tuned by adjusting the EHD parameters and the degree of alignment is mirrored by the conductivity across the film. Using etching techniques or by embedding relatively long nanotubes, patterned surfaces decorated by CNT brushes were generated.     

Ultra‐Lightweight and Highly Adaptive All‐Carbon Elastic Conductors with Stable Electrical Resistance

The rapid development of wearable electronics needs flexible conductive materials that have stable electrical properties, good mechanical reliability, and broad environmental tolerance. Herein, ultralow‐density all‐carbon conductors that show excellent elasticity and high electrical stability when subjected to bending, stretching, and compression at high strains, which are superior to previously reported elastic conductors, are demonstrated. These all‐carbon conductors are fabricated from carbon nanotube forms, with their nanotube joints being selectively welded by amorphous carbon. The joint‐welded foams have a robust 3D nanotube network with fixed nodes and mobile nanotube segments, and thus have excellent electrical and mechanical stabilities. They can readily scale up, presenting a new type of nonmetal elastic conductor for many possible applications.     

Temperature‐Dependent Electrical Transport in Polymer‐Sorted Semiconducting Carbon Nanotube Networks

The temperature dependence of the electrical characteristics of field‐effect transistors (FETs) based on polymer‐sorted, large‐diameter semiconducting carbon nanotube networks is investigated. The temperature dependences of both the carrier mobility and the source‐drain current in the range of 78 K to 293 K indicate thermally activated, but non‐Arrhenius, charge transport. The hysteresis in the transfer characteristics of FETs shows a simultaneous reduction with decreasing temperature. The hysteresis appears to stem from screening of charges that are transferred from the carbon nanotubes to traps at the surface of the gate dielectric. The temperature dependence of sheet resistance of the carbon nanotube networks, extracted from FET characteristics at constant carrier concentration, specifies fluctuation‐induced tunneling as the mechanism responsible for charge transport, with an activation energy that is dependent on film thickness. Our study indicates inter‐tube tunneling to be the bottleneck and implicates the role of the polymer coating in influencing charge transport in polymer‐sorted carbon nanotube networks.     

单根多壁碳纳米管的电学测试

提出了一种制备多壁碳纳米管的简单方法。以乙醇为碳源,利用催化化学气相沉积工艺制备了碳纳米管。用较为简单的设备在较低的反应温度下,在基底上生长了取向多壁碳纳米管阵列。利用扫描电子显微镜内部的纳米操纵仪对单根碳纳米管进行操纵,并测试了单根多壁碳纳米管的电学特性。     

Mechanical Properties of Functionalized Single‐Walled Carbon‐Nanotube/Poly(vinyl alcohol) Nanocomposites

Nanocomposites based on semi‐crystalline poly(vinyl alcohol) (PVA) and well‐dispersed chemically functionalized single‐walled carbon nanotubes are combined through simple mixing. The interaction between the nanotubes and the polymer matrix is studied using optical and thermal methods. Significant enhancement of the mechanical properties is obtained for the functionalized‐nanotube‐based composites. These results imply that promoting nanotube dispersion and strong interfacial bonding through adequate functionalization of nanotubes improves the load transfer from the matrix to the reinforcing phase.     

Hierarchical Self‐Assembly of Peptide‐Coated Carbon Nanotubes

Numerous applications, from molecular electronics to super‐strong composites, have been suggested for carbon nanotubes. Despite this promise, difficulty in assembling raw carbon nanotubes into functional structures is a deterrent for applications. In contrast, biological materials have evolved to self‐assemble, and the lessons of their self‐assembly can be applied to synthetic materials such as carbon nanotubes. Here we show that single‐walled carbon nanotubes, coated with a designed amphiphilic peptide, can be assembled into ordered hierarchical structures. This novel methodology offers a new route for controlling the physical properties of nanotube systems at all length scales from the nano‐ to the macroscale. Moreover, this technique is not limited to assembling carbon nanotubes, and could be modified to serve as a general procedure for controllably assembling other nanostructures into functional materials.     

Carbon Nanotube Wires Sheathed by Aramid Nanofibers

Coaxial fibers are the key elements in many optical, electrical, and biomedical applications. Recent success in materials synthesis has provided versatile choices for the core part, but the search of high‐performance sheath materials remains much less productive. These surface coatings are however as important as the core for their role as protection layers and interaction medium with the externals, thereby critically affecting the real performance of coaxial fibers. Here it is shown that aramid nanofibers (ANFs) with exceptional environmental stability and mechanical properties can be advanced coating materials for both wet‐ and dry‐spun carbon nanotube (CNT) wires. Co‐wet‐spinning ANFs with CNT aqueous dispersion can produce coaxial fibers with a compact sheath comprised of aligned ANFs, showing much enhanced mechanical properties by transferring stress to the sheath without sacrificing the conductivity. On the other hand, an immersion‐precipitation process is used to prepare a porous sheath made from randomly distributed nanofibers on dry‐spun CNT wires, which can be combined with ionic conductive gel electrolyte as a strong packaging layer for flexible solid‐state supercapacitors. The excellent intrinsic characteristics as well as variable ways of structural organizations make ANF‐based coatings an attractive tool for the design of multifunctional high‐performance hybrid materials.     

High‐Yield Fabrication,Activation, and Characterization of Carbon Nanotube Ion Channels by Repeated Voltage‐Ramping of Membrane‐Capillary Assembly

The interior channels of carbon nanotubes are promising for studying transport of individual molecules in a 1D confined space. However, experimental investigations of the interior transport have been limited by the extremely low yields of fabricated nanochannels and their characterization. Here, this challenge is addressed by assembling nanotube membranes on glass capillaries and employing a voltage‐ramping protocol. Centimeter‐long carbon nanotubes embedded in an epoxy matrix are sliced to hundreds of 10 µm‐thick membranes containing essentially identical nanotubes. The membrane is attached to glass capillaries and dipped into analyte solution. Repeated ramping of the transmembrane voltage gradually increases ion conductance and activates the nanotube ion channels in 90% of the membranes; 33% of the activated membranes exhibit stochastic pore‐blocking events caused by cation translocation through the interiors of the nanotubes. Since the membrane‐capillary assembly can be handled independently of the analyte solution, fluidic exchange can be carried out simply by dipping the capillary into a solution of another analyte. This capability is demonstrated by sequentially measuring the threshold transmembrane voltages and ion mobilities for K⁺, Na⁺, and Li⁺. This approach, validated with carbon nanotubes, will save significant time and effort when preparing and testing a broad range of solid‐state nanopores.