Carbon-Based Nanotechnology Realizing the Promise and Overcoming the Challenges
As early as 1970, there was speculation that carbon fullerenes* existed in addition to the well known allotropes** of carbon found in the forms of coal, soot, diamond and graphite.1,2 The existence of C60 fullerenes, or “buckyballs,” was first demonstrated by Kroto, Curl and Smalley of Rice University in 1985. For their work, they were awarded the 1996 Nobel Prize in Chemistry. With the discovery of this new class of carbon allotropes, research interest in this family of materials exploded. In addition to buckyballs, fullerene structures include other spherical, ellipsoidal and tubular shapes, all of which display a hollow, cage-like structure formed by each carbon atom being covalently bonded to three others. The first carbon nanotubes (CNTs) were synthesized in 1991, and they have attracted increased attention since then as a result of their unique and tailorable properties.3
The structure of a CNT is shown in Figure 1. The figure depicts a single-wall carbon nanotube (SWNT). Note that CNTs are composed entirely of sp2 bonds, which are stronger than the sp3 bonds found in the diamond form of carbon.4 Multiwalled nanotubes (MWNT) also exist, and they are essentially the equivalent of concentric SWNTs.
A number of CNT-based nanomaterials are under development based on their unique properties, which make them attractive alternatives to traditional materials. CNTs can be formed into a thin sheet. “Buckypaper” is a particular type of CNT sheet. Figure 2 shows a scanning electron microscopy image of buckypaper on the left, as well as a large sheet of CNT paper on the right (produced at Nanocomp Technologies, Inc.). Due to advances in the manufacturability of buckypaper, it is becoming increasingly common to find this form of CNTs being used in structural, electromagnetic interference (EMI) shielding and thermal applications.
Graphene is another carbon-based nanomaterial that is receiving a lot of attention since Geim and Novoselov were awarded the 2010 Nobel Prize in Physics for its discovery. Graphene is a single atomic layer of carbon, equivalent to a CNT that has been “unrolled” into a two-dimensional structure as shown in Figure 3. Graphene is highly transparent yet conductive, making it an excellent candidate for photovoltaic applications, liquid crystal displays (LCDs) and light-emitting diodes (LEDs).5
CNTs can behave as semiconductors or metals, depending on their structure, and they can support high current densities. The thermal conductivity of CNTs is elevated along the nanotube axis and is approximately ten times that of copper. Yet, CNTs are excellent thermal insulators along their radial axis.
In addition to their electrical and thermal characteristics, CNTs are promising for structural applications due to their high strength and stiffness. With diameters on the order on 1−10 nanometers (nm), and lengths ranging from the submicron scale to several millimeters or more, CNTs exhibit tensile strengths along their axes approximately ten times that of Kevlar®.
At Raytheon, CNTs are being developed for use in EMI shielding and high-strength applications where lightweight materials are required. They are also being developed for use as thermally conducting interfaces in high-power devices. Additional uses for CNTs are being investigated through collaborations with university laboratories. However, challenges still exist in the development of CNT technologies for real-world applications. The behavior of bulk CNT materials often falls short for that of a single nanotube. Novel CNT growth techniques, as well as CNT alignment and integration into bulk materials, are ongoing and critical research areas. In fact, twenty years after they were first synthesized, the potential for CNTs is just now being realized.
Properties of Carbon-Based Nanomaterials
In applications where strength or weight is an issue, CNTs and buckypaper can have a significant advantage over traditional materials. While individual CNTs have measured tensile strengths ranging from 10–150 gigapascals (GPa) in laboratory demonstrations, the tensile strength of mass-produced yarns of CNTs and buckypaper has fallen far short of those values. State-of-the-art CNT yarns and sheets have tensile strengths of 1.5–3.0 GPa and 0.4–1.2 GPa, respectively.6
While still lagging behind Kevlar (3.7 GPa), these values meet or exceed those of copper, aluminum and steel. Moreover, when the density of the materials is considered (typical densities < 1 g/cm3), the strength-to-weight ratio of CNT-based materials holds a dramatic advantage over these other materials, including Kevlar. The tensile strength and density of carbon-based and other common materials are shown in Figure 4.6
CNT Fabrication Techniques
Synthesizing CNTs remains a primary challenge because low yields, and often diverse and unpredictable properties, are obtained7 due to the assortment of material diameters, lengths and chiralities*** produced; yielding a mix of semiconducting and metallic material. CNTs can be fabricated using a number of techniques, including chemical vapor deposition (CVD), electric arc discharge and pulsed laser ablation. Each technique has numerous process variables that affect uniformity, defects and purity, thereby affecting ultimate quality and utilization.
Because of its potential for large-volume CNT production, CVD synthesis is the method most commonly employed in industry. In the CVD process, catalysts such as cobalt, molybdenum, iron and nickel are nucleated using thermal or chemical methods. Gaseous carbon compounds are introduced at high temperatures, typically on the order of 700–1,000 degrees Celsius (lower for plasma-enhanced CVD). Carbon migrates to the nucleation sites and nanotubes are grown on the catalysts. Electric arc discharge is probably the most common synthesis method used in research due to its relatively low equipment cost. Using this method, high current discharge between graphite electrodes generates individual carbon atoms that migrate to a cathode and crystallize, forming nanotubes. Laser ablation techniques similarly use high-power lasers (pulsed or continuous wave) to vaporize graphite, forming nanotubes as the carbon condenses onto a cool substrate. Table 1 summarizes some advantages and disadvantages of these techniques.
While optimal CNT synthesis remains a topic of much investigation, manufacturing processes for CNTs have matured significantly. CNTs are produced at many small and large companies throughout the industry. Annual global production is projected to reach ~1,000 tons in the coming years.9 Additionally, improvements in manufacturing methods have led to about a 75 percent drop in the cost of CNTs over the last ten years.10 Now that reliable sources of CNTs are available, the largest barrier to commercialization of CNT-based products is the integration of CNTs into composites and other material systems. By themselves, CNTs are typically sold suspended in a solvent; however, agglomeration often prevents uniform dispersion and inhibits the potential benefits of CNT use. In addition, lack of material standards and metrology capabili ties, as well as environmental, health and safety (EH&S) concerns, have limited the rate at which CNT technologies have been accepted and therefore can mature.
Compared to composites loaded with CNTs, the use of buckypaper circumvents many of the agglomeration and EH&S issues. CNT sheets or film (buckypaper) was first produced by Smalley et al in 1998, when suspensions of functionalized CNTs were vacuum dried on membranes.11 A similar technique is being used at the High Performance Materials Institute (HPMI) at Florida State University, where sonicated CNTs are being filtered through membranes to produce buckypaper in both batch and continuous processes. Aligned buckypaper can be fabricated from carbon nanotube “forests” by essentially flattening the CNTs in one direction against the substrate. The strong Van der Waals (intermolecular) forces between nanotubes create a structure that is then easily removed from the substrate. Alignment can also be achieved by post-processing randomly oriented buckypaper.
Nanocomp Technologies produces CNT sheets. They use the CVD-based nanotube growth process combined with a drum up-take to allow batch sheets to be fabricated in sizes up to 4.5 by 9 feet. These pieces can then be seamed into indefinitely sized roll stock (see Figure 2). Nanocomp also produces CNT yarns at a rate of ~15 km/week.
Depending on the processing, buckypaper can be malleable or brittle. Typical sheets are on the order of 25 microns thick and less than 2 grams/ft2.12 Some typical properties of CNT sheets are listed in Table 2.
CNT sheets and yarns can be used in cables to replace conventional EMI shielding and conductor materials. Nanocomp Technologies has demonstrated a weight reduction of 40–50 percent for coax cables, and as much as 70 percent for USB cables. In addition to cable applications, buckypaper is well suited for use as a pre-impregnated material. Composites that incorporate CNT sheet can provide EMI shielding, integrated de-icing heaters and lightning protection solutions for aircraft. A Nanocomp CNT sheet was recently deployed on the Materials International Space Station Experiment-8, and it was implemented on the Juno mission, launched in August 2011, to provide electrostatic discharge protection for the spacecraft. Completion of stringent qualification criteria for NASA, combined with Nanocomp’s high-volume capabilities, has proven that these technologies are poised to realize the potential of CNTs in real-world applications.
Graphene was recognized for years as a contaminant, which often formed on the surface of semiconductors or metals and interfered with electronic transport experiments.5 Mechanical exfoliation of graphene was first demonstrated in 2004, promoting the explosive growth of graphene research. With intrinsic charge carrier mobilities higher than any other known material,13 and better thermal conductivity at room temperature than diamond,14 graphene’s extraordinary properties are being investigated to provide next-generation solutions for complementary metal oxide semiconductors (CMOS), among possible applications.
While mechanical exfoliation continues to be the most common method for graphene synthesis, epitaxial growth is also being optimized to allow for oriented, large-scale production. As the name implies, mechanical exfoliation of graphene is achieved by rubbing graphite on a smooth surface.15 The resulting film has good electrical properties and can reliably be produced at the millimeter scale.5
Epitaxial growth is currently performed on silicon carbide (SiC) substrates, but growth on large-area polycrystalline materials such as nickel, copper and platinum has been demonstrated.16,17,18 Epitaxial growth on SiC is achieved by heating the substrate to temperatures of 1,200–1,800 degrees Celsius, where silicon desorbs and promotes the formation of graphene through the rearrangement of the remaining carbon atoms.19 Compared to exfoliation techniques, similarly high mobilities at room temperature have been achieved with epitaxial methods.20
Graphene is a remarkable electronic conductor, capable of achieving intrinsic charge carrier mobilities of 200,000 cm2/Volt-sec at room temperature and capable of sustaining current densities5 of 5 x 10 8M/ A/cm2. Its thermal conductivity (at room temperature) is as high as 5,000 watts/meter-Kelvin, more than double that of high-quality diamond and more than an order of magnitude greater than copper. Additionally, graphene has a high intrinsic strength of 130 GPa, a Young’s Modulus of 1 terapascal (TPa), and it can support strains in excess of 20 percent without breaking.21
Because of its single atomic layer thickness, the surface area to volume ratio of graphene is very high, making it potentially interesting for sensor applications and energy storage. Quantum confinement of a charge in graphene allows bandgap tuning, yielding essentially unlimited design possibilities for nanoscale transistors.22,23 Graphene applications for LCDs, organic LEDs and transparent conducting electrodes for solar cells are enabled by its unique combination of electronic and optical properties.
Raytheon continues to explore these promising allotropes of carbon to enable new functionalities, as well as to improve the performance and reduce the weight of our products. The use of CNT-loaded composites has been explored for light-weight body armor applications. We are currently partnering with Purdue and Georgia Tech to develop CNT-based interface materials to enable efficient thermal contact between electronic devices and heat spreaders. These projects and many others are fueling the evolution and ultimately the realization of advanced carbon-based materials solutions for the warfighter.
* Fullerine – Any of various cage-like, hollow molecules composed of hexagonal and pentagonal groups of carbon atoms (adapted from the American Heritage Dictionary, 2009).
** Allotrope – Any of two or more physical forms in which an element can exist (adapted from the Collins English Dictionary 2009).
*** Chirality – The configuration (or handedness) of an asymmetric structure (adapted from the Collins English Dictionary 2003).
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Mary K. Herndon, Stephanie Fernandezi
The authors would like to thank John Dorr and David Lashmore of Nanocomp Technologies, Inc., for contributing data and graphics used in this article