Advancements in spinnable CNT arrays
Vesselin Shanov and Mark Schulz, U. of Cincinnati
Most natural fibers and nanofibers are produced only in relatively short lengths, and most applications require a bulk or continuous material–but there is no effective method for using short fibers or carbon nanotube (CNT) powders to achieve breakthrough properties in bulk materials. The most promising approach to use nano-fibers in bulk material is to form an intermediate material by spinning CNT fibers into yarn, which can displace conventional fiber in composite materials and other applications. This article examines recent advances that are allowing spinning smaller CNT fibers, and their use in new applications.
Prior to discussing the recent advances, it is important to give due credit to antecedent work. The length and diameter of the fiber play critical roles in the success of spinning; diameters of fibers such as cotton used for spinning since the 16th century are in the micron range, whereas carbon nanotube diameters are much smaller, in the 10nm range. Spinning small diameter fibers also increases twist by about the same factor. Short and long CNTs can be compared to cotton fibers . For traditional spinning technologies such as rotor and ring spinning, there is a separate process called combing where short-length cotton fibers (less than ½ inch long) are removed from the raw material mass before spinning is performed. Typically, as much as 16% of the cotton raw material mass is short fiber, and removed; premium cottons such as Pima or Egyptian have less short fiber but still require combing where typically 8%-10% of the raw material is removed.
These factors and subsequent analyses indicate long CNTs will improve strength and electrical properties, and we have focused research at the University of Cincinnati [2-15] to produce long and strong CNT, spinning carbon nanotube yarns that have superior properties, are economically and commercially viable, and will meet the long-range needs of defense and commercial sectors.
Approaches to spinning CNTs
Spinning CNTs into thread is a relatively new topic of research. There are two main approaches: spinning thread from substrate grown forests of CNT, and direct spinning of CNTs into thread from a vertical reactor that uses the floating catalyst method of synthesis. Spinning from the array is done by a handful of research groups around the world [2-4, 5-9], and direct spinning from a vertical reactor using the floating catalyst method is done by just a few research groups around the world [10, 15]. Mechanical measurements indicate that yarn produced using both methods has a uniform strength of about 0.5N/Tex, which is equivalent to about 1.0GPa; short sections of yarn and special cases have shown higher strength. The electrical resistivity of thread is about 1x10-4 ohm cm and the current density is about 1×109 amp/m2. If the properties are divided by the density of the yarn, the specific properties are competitive with existing materials and the combination of properties can exceed those of existing materials. The mechanical and electrical properties are improving as the number of defects in the thread are reduced through improving the synthesis and spinning processes. The goal is to produce yarns that are strong, creep resistant, highly conducting, and reversibly deformable over relatively large strains to absorb energy.
Fiber properties for spinning. Understanding fiber spinning is important to move CNT out of the lab. Fibers must have certain particular properties to be able to be spun into thread, including strength, stiffness, and pliability–in other words, an openness and ease of fiber separation and toughness, and appropriate bending and radial stiffness. These aspects, along with quality and reproducibility, are of extreme importance in producing yarn. Spinning can be done using different approaches, the details of which are partly confidential; dry spinning from an array is discussed here. The relative size of the yarn being made commercially and the twist uniformity of a strand are important. Our initial target CNT yarn is a 10Tex size yarn (10g/1000m of yarn). CNT length is important in spinning yarn. The most important property of a CNT forest that is required for solid-state processing is that whenever the CNTs at the edge of the array are pulled away from the forest, the CNTs cling together (due to van der Waals forces) to form a continuous strand .
Properties of CNT yarn. The mechanical and electrical properties of CNT yarn depend on the number of defects in the CNT and in the yarn. Each gap or junction at the end of the nanotube can be considered to be a defect in the yarn. It can be considered that each CNT has one defect, which is the gap or junction between the next nanotube. Thus there are N nanotubes and N gap defects in the thread, besides defects in the walls that can cause the CNT to become wavy and weak. The gap interrupts the load transfer from CNT to CNT and requires that friction between CNTs carry the load. The effect of length of the CNT on strength of the thread can predicted based on a conventional thread-spinning model which was discussed by R. Baughman [5, 9]. For the CNT thread, a shorter migration length gives better strength–i.e. more fibers must run from the surface to the inside of the yarn in a short interval of length to make the fiber strong. A higher friction coefficient gives better fiber strength. However, the number of turns per unit length–the helix angle–plays an important role in fiber strength; when the number of turns increases the fiber strength decreases drastically. Similar to mechanical load transfer, electrical conduction is interrupted by the gaps between nanotubes, and thus electrical conduction must occur laterally from nanotube to nanotube probably by electron hopping. The resistance of the thread is equal to the longitudinal resistance of the nanotube plus the lateral resistance of the nanotube. The CNT yarn also has resistance, super-inductance, and super-capacitance properties, which are being studied to develop carbon electronics or “carbotronics” that have superior properties in certain applications compared to conventional copper components.
Direct spinning from the array. Centimeter-long “Black Cotton” [a type of CNT trademarked by UC spinoff General Nano] can be spun into thread for electrical wire and as fiber to reinforce composite materials supplementing or replacing carbon fiber. The long CNTs allow dry spinning, which is an advantage in terms of strength, cost, electrical conductivity, and scale-up to manufacturing commodity levels. The U. of Cincinnati’s spinning machine, specially designed to spin Black Cotton into thread, has two DC motors to independently control the twisting and winding while drawing thread directly from the CNT array. The spinner has independent orthogonal control of winding and twisting using a yoke assembly. The thread is twisted and wound onto a spindle. A post-treatment stage allows further processing of the thread, such as thermal annealing or coating with an insulating material.
Catalyst and substrates for growing of spinnable CNT arrays
It is observed that dense and aligned arrays are more spinnable, and the thread obtained from such an array is stronger. Continuous thread can be drawn from dense aligned arrays of nanotubes. In order to achieve this goal, increased catalyst particle density on the substrate is required. There is consensus that double-wall carbon nanotubes (DWCNT) are very appropriate for spinning into threads. A procedure developed for CVD of CNT was modified for synthesis of well-aligned and high-purity DWCNT arrays. A new catalyst based on an iron alloy with increased catalyst particle density was introduced, deposited on a Si/SiO2/Al2O3 substrate by e-beam deposition. After thermal annealing a uniform distribution of high-density catalyst particles was achieved, which was proven by AFM. The growth was performed at 750°C in a First Nano EasyTube 3000 reactor using a gas system consisting of ethylene (C2H4), water vapor, hydrogen, and argon with optimized concentrations, deposition temperature, and flow rates. Critical for this study was to maintain low-carbon partial pressure in the reaction zone. Two-hour growth with this catalyst produced a 1.1mm long array with excellent properties for spinning. Extremely long (up to 18mm) CNT arrays have been made; in one example an 11mm long CNT array was peeled completely off the substrate, which with no additional processing was reused to grow and yield an 8mm long CNT array.
By using multiple spools, University of Cincinnati doctoral student Chaminda Jayasinghe is able to spin bigger-diameter threads from long (4-5.6mm) CNT arrays.
Magnetron sputtering is a further improvement in substrate preparation being evaluated by North Carolina A&T and produces very uniform catalyst deposition and highly spinnable arrays.
Device-quality CNT thread, yarn, and ribbon
Our team has developed techniques for spinning long CNT directly from the array into thread, yarn, and ribbons (figure 1). This technique has produced CNT thread with strength of 1 GPa and electrical conductivity of 0.8×104 (ohm-cm)-1. This strength and electrical conductivity are at least 10X lower than the corresponding properties of perfect individual nanotubes. We assume that the lower properties of thread are due to: (i) defects in the walls of the nanotubes; (ii) large number of gaps in the thread at the ends of the CNT; and (iii) considerable open volume between the nanotubes in the threads. We are developing techniques that we believe will reduce these problems. One technique is to use electric current to fuse the ends of nanotubes together while the nanotubes are being spun into thread. This technique has been shown to work under a microscope and the challenge is to scale it up for mass production. Another technique is to perform secondary annealing/welding of the nanotube thread to reduce the number of defects. Other post-spinning treatments of the nanotube thread, such as ion irradiation and ozone or UV exposure [16-19], also have potential to improve the thread’s electrical and mechanical properties. The strategic importance of the research is to produce CNT thread that surpasses the properties of any existing material in terms of strength, weight, and electrical current carrying capability.
Figure 1. Four types of CNT materials fabricated by the UC team: a) as grown CNT bundles, b) single thread, (c) two strand yarn, and (d) ribbon.
Preparing thread and ribbon from CNT arrays
At the present time CNTs cannot be grown beyond about 2cm in length [20-37]. At the U. of Cincinnati, DWCNTs several millimeters in length are being spun into thread (with micron-range diameter) to produce a strong and tough bulk material with novel properties. Multiple threads have been woven together to form a yarn, which can be used to form tows and unidirectional plies; they also can be woven into a “smart fabric” with two-directional properties, which can then be used to fabricate strong, electrically conductive composite materials, or used as a wearable sensor embedded in clothing. Nanotube thread can also be used to form carbon electronic components (“carbotronics”), or electromagnetic devices such as an antenna to communicate with sensors inside the body. Thin narrow sheets of nanotubes called ribbon have also been drawn from the array [8, 11].
Spinning thread from DWCNT arrays
Closely aligned CNTs (spaced <100nm apart) are weakly held together by van der Waals intermolecular forces. In this forest configuration, they can be harvested by pulling a small bundle of CNTs away from an edge of the array in the direction that keeps the “centerlines” parallel and maintains the close spacing. The CNTs next to the first bundle will be pulled along also by van der Waals forces. As these CNT bundles are pulled away from the “forest” they form a long line in which all of the CNT centerlines are aligned in parallel. In the spinning process (pulling and twisting), CNTs are pulled from the array and held together by twisting around neighboring nanotubes [38-42], which prevents the CNTs from slipping along the lengths of their neighbors when axial force is applied. As the CNTs are twisted and pulled, more nanotubes are added to form a long, strong thread. CNT arrays that we have used for spinning range in length from 1mm to 0.5cm–and threads with lengths of 100m have been spun in our facility. The diameter of the thread can be controlled by the length of the CNT array and by the spinning parameters [43-53]. Figure 2a shows CNT thread being wound onto a spool.
Figure 2. Manufacturing of CNT thread and ribbon at UC: (a) thread being wound onto a spool, (b) pulling and winding ribbon.
As the quality of the CNTs improves, progressively longer CNTs will be used to spin thread. Long CNTs can improve thread properties by reducing the number of gaps in the thread at the nanotube ends, and by providing a longer length for mechanical interlocking each nanotube with its neighbors. If the quality and spin-ability of the 1.5-2cm long CNT that we currently produce are improved, thread properties could improve by an order of magnitude which would open up many applications. The excitement of this research is that the properties of thread are being continuously improved and are getting closer to the properties of individual nanotubes. If CNT thread reaches a strength of 10GPa (20% of the strength of individual nanotubes), this would be a large breakthrough in nanotechnology. Electrical conductivity would also be expected increase roughly in proportion to the strength increase.
Pulling ribbon from CNT arrays
A material called carbon nanotube ribbon (~200nm thick, 5mm wide) was also produced from our arrays, possessing a different morphology from threads. Winding CNT ribbon is shown in Figure 2b. The width of the ribbon is limited only by the lateral size of the array .
Specific properties of CNT and thread will be important for weight-critical applications and are calculated via dividing the property by the density of the material. However, since a single wall of each nanotube is one atom thick, defects can greatly reduce the strength and electrical conductivity of CNTs. Reducing intrinsic defects will greatly improve the properties of future CNT threads, ribbons, and yarns. Re-spinning, treating with solvent, and other post-processing is being done to improve the properties of thread. Several techniques to post-treat CNT yarn are being evaluated, mostly consisting of applying energy to the spun yarn in the form of electron flow or heat. This will be in a controlled atmosphere to prevent oxidation. The intent is to “fuse” the twisted CNT together so they will resist slipping when a lateral force is applied to the yarn.
Applications of CNT thread
CNTs’ high specific strength and stiffness, electrical and thermal conductivity, and compatibility with electronics and sensing applications are a key enabling medium for the convergence of textiles with fully integrated functions. Smart fabrics or interactive textiles have many potential applications such as physiological monitoring, power bus systems and communications, medical care, multifunctional exteriors, harvesting of energy and water, and passive and active thermal management. CNT yarns may eventually find applications in composite materials, in electrically conductive wire, bulletproof vests, light emitters by incandescence, antennas, and materials that block electromagnetic waves. The macroscopic CNT yarns may find application as mechanical actuators for artificial muscles, flexible conductors for textile sensors, power buses for communication, flexible batteries, and solar cells.
One example of new application is in signal communications. Researchers at the U. of Cincinnati have applied a 25µm spun carbon nanotube thread to create a dipole cell-phone antenna , with transmission close to that of copper but at a fraction of the weight . CNT yarns also could be woven into cloths for use as a very lightweight reflector or dish antenna–one that could be deformed to change the focus of the antenna, e.g. to be molded directly into electronic device casings or aircraft structures. Nanocomp Inc.  has demonstrated fabrication of coaxial cables using CNT threads as center conductors and CNT sheets for outer conductors or shields, which would be substantially lighter than similarly sized copper coaxial cables; USB cables using only CNT threads and yarns also have been made.
David Mast, U. of Cincinnati associate professor of physics, demonstrates new wireless applications of the spun carbon nanotubes.
The surface of CNT materials also can impart different functionalities–e.g., absorption of gas molecules to make ultrasensitive sensors for toxic gases or biological agents. Fabrication of such functionalized CNT sensors with integral CNT antenna for wireless sensor applications is currently being investigated. These threads and yarns can also be wound into small loops or spirals for micro-miniature antenna for possible bio-medical applications.
The CNT yarns are good electrical conductors and can carry enough current to act as an incandescent filament or to emit electrons to produce light from phosphorescent screens. Electrons field emitted from the side of a cold, negative nanotube yarn electrode hit a transparent fluorescence screen to provide light emission. Low voltages are possible for field emission because of both the field enhancing effect of the yarn shape and the high aspect ratio of nanotubes that protrude from the sides of the yarn. The CNT yarns can be used as electron field emitters for light sources (lighting and displays) and X-ray sources that could be in a micro-catheter used for medical applications. The individual nanotubes are anchored into the yarn by twist, which should enhance electron emission stability and device lifetime.
Steady progress in CNT synthesis and spinning by a small number of groups around the world is moving nanotube yarn technology ever closer to the point where it can be come a disruptive material that can offer multi-functional properties that cannot be achieved by any other materials on earth. CNT yarn may supplement or displace carbon, glass, and aramid fibers and copper wire in high-performance critical applications.
Dr. Vesselin Shanov is associate professor of chemical and materials engineering at the University of Cincinnati.
Dr. Mark Schulz is an associate professor of mechanical engineering at the University of Cincinnati, and deputy director of the National Science Foundation’s Engineering Research Center for Revolutionizing Metallic Biomaterials located at North Carolina A&T.
Shanov and Schulz are co-directors of the UC Nanoworld and Smart Materials and Devices Laboratories at the University of Cincinnati, and are affiliated with the start-up company General Nano LLC in Cincinnati that is commercializing the Black Cotton material.
References for this story are available online, at www.smalltimes.com.