Researchers have created a new technique to produce the highest quality carbon nanotubes affordably from carbon dioxide in the atmosphere. This article looks at the new forms of carbon, such as nanotubes and graphene, and considers whether we may be on the verge of a carbon revolution.
The New Carbon Materials: Graphene and Fullerenes
There has been huge excitement in recent years about new forms of carbon such as graphene, carbon nanotubes and Buckminsterfullerene (buckyballs). Probably the most talked about in recent years has been graphene, a flat hexagonal lattice only one atom thick. Other pure carbon materials contain graphene in different arrangements. For example, the graphite found in pencils is made up of flat layers of graphene that can easily slide over each other. Carbon fibers on the other hand have crumpled layers of graphene that lock together, giving greater strength and stiffness. The weakest link in both of these materials is the connection between the sheets of graphene. When an individual sheet of graphene is isolated, it has incredible strength and other exceptional properties. Through the breakdown of graphite, graphene has existed for many years. Layers of graphene are separated every time you write with a pencil. However, graphene has only come to prominence since Andre Geim and Konstantin Novoselov isolated and studied it in 2004.
Graphene was not the first of the exotic new forms of carbon. It all began with Buckminsterfullerene, also known as buckyballs or C60. Buckyballs are similar in structure to graphene, but some of the hexagons are reduced to pentagons causing the lattice to curve into a sphere. This is similar to the way a football, made from flat pieces cut into hexagons and pentagons, curves into a ball. C60 was discovered by Harry Kroto, Jim Heath, Sean O'Brien, Robert Curl and Rick Smalley in 1985.
C60 was the first to be discovered, but many other hollow molecules can be achieved with different combinations of rings containing five, six and sometimes seven carbon atoms. These are collectively referred to as Fullerenes. Perhaps the most important Fullerenes are carbon nanotubes. These are essentially tubes of graphene that may vary in diameter. Different diameters of tubes may be arranged into multi walled nanotubes, while groups of nanotubes naturally form into bundles similar to rope. Individual carbon nanotubes have been observed which are over 185mm long while being just over one nanometer in diameter.
How Graphene and Fullerenes Might Change the World
The new carbon materials have a huge number of potential uses. It’s been suggested that graphene and carbon nanotubes are set to give us much stronger and lighter structures, more efficient electrical energy systems, nano-bots, cheap flexible solar cells, abundant fresh water and much more. C60 has a smaller number of uses, primarily as a lubricant, catalyst and for the delivery of pharmaceuticals into the body.
Structurally, individual sheets of graphene and individual carbon nanotubes both demonstrate extremely high strength and stiffness with low density. Carbon nanotubes have been demonstrated to have a strength to weight ratio almost 20 times better than the strongest carbon fibers. However, actual nanotube fibers, which typically contain over a million nanotubes, do not have these properties since the individual tubes slide past each other relatively easily. This is similar to the way in which the sheets of graphene with far less strength to slide. Initial research has shown that irradiation of nanotube bundles induces cross-linking between the tubes without causing significant damage to the tubes themselves. This may enable carbon nanotube fibers to achieve the impressive potential for strength suggested by the individual nanotubes. However, it has only been shown to work with bundles of a few nanotubes. There is quite a way to go before we will see carbon nanotube fibers replacing conventional carbon ones. Graphene also has similar issues with translating the theoretical performance seen in very small samples into actual material properties. So, don’t expect to see these materials replacing carbon fiber anytime soon.
Where the structural properties of graphene and carbon nanotubes are showing more immediate promise is as an unstructured additive to resins and plastics. According to a recent joint report by the National Graphene Institute at the University of Manchester, where graphene was first isolated, and the Aerospace Technology Institute, “In the near term, graphene will be added to the resin in thermoset CFRP to form a hybrid system, where the fiber provides the stiffness whereas graphene improves properties such inter-laminar shear strength and damage tolerance, allowing a reduction in ply-thickness. Graphene may be used as the main reinforcement for stiffness in high performance polymers, such as PEEK, to produce small parts and pipes, which replace metal equivalents.”
Has the Revolution Already Started?
Although it has recently been discovered that the legendary Damascus swords contain small quantities of carbon nanotubes it has not generally been possible to produce products with graphene or Fullerenes until recently. After the initial excitement when the materials were first isolated and studied in the lab, the cost remained prohibitively high, amounting to thousands of dollars per gram. In recent years, this price has come down dramatically. Nanotubes are now readily available for just a few dollars per gram. This dramatic reduction in cost is leading to nanotubes and graphene appearing in a range of products typically marketed as simply “graphene.” In reality, they typically contain a small percentage of these materials and are not functioning as a replacement for carbon fibers.
In 2005, Zyvex Technologies supplied the first commercial carbon nanotube enhanced materials, working in a partnership with Easton Sports to produce a baseball bat featuring carbon nanotubes. Zyvex now produces prepreg, resins and adhesives enhanced with carbon nanotubes and graphene. They claim that the combination of both carbon nanotubes and graphene gives their composites improved strength across a range of properties when compared to carbon fiber without these additives. They claim that tensile strength is increased by 26 percent, compressive modulus by 12 percent, flexural modulus by 35 percent, interlaminar shear by 20 percent and fracture toughness is nearly doubled.
The first company to produce a graphene product was Head with a graphene tennis racquet. First released in 2013, this is claimed to allow Head to create a product 20 percent lighter and 30 percent stronger compared to a conventional racquet, although these figures are not fully explained.
Dassi manufactures a graphene bicycle frame that they claim is 30 percent lighter and twice as strong as a result of adding just 1 percent graphene to the carbon fiber reinforced plastic (CFRP). According to Dassi, it first created prepreg sheets that are then laid up to form the frame. “We mix graphene with an epoxy resin we developed ourselves that is then electronically functionalized to disperse the graphene evenly within the resin. The carbon weave is then introduced into the resin mix, which in turn forms the graphene carbon material in a prepreg that can be used for laying up components.” They also noted that adding graphene significantly reduces the cure time of the resin. This means that processes must be optimized to complete the lay up before the resin cures.
The first question that came to my mind was how adding a very small quantity of graphene could produce such dramatic performance improvements, considering that experimental work has failed to replicate the strength of graphene for larger samples. Dassi provided some explanation for this by saying that the material gives a 70 percent increase in inter-laminar shear strength, 50 percent increase in fracture toughness, retarded crack propagation and increased carbon-to-resin adhesion. So, the graphene doesn’t need to improve on the tensile strength of carbon fibers to achieve these advantages. This usage and reported benefits is in line with the recent report from the National Graphene Institute and the Aerospace Technology Institute, which I mentioned above. This is, therefore, the type of usage and performance we can expect from graphene enhanced structures appearing in the near future.
Another really exciting area for graphene and nanotube reinforcement is to strengthen plastic 3D-printed parts. A number of companies are producing print filaments that includes graphene (Directa Plus, Graphene 3D Lab and Haydale). The improved thermal conductivity enables increased deposition rates while strengths are also improved.
Evaluating the Quality of Carbon Nanotubes
Although carbon nanotubes are becoming accessible for use in consumer products, not all nanotubes are the same. They may be divided into single wall carbon nanotubes (SWCNT) and multiwall carbon nanotubes (MWCNT). Their properties are also determined by their diameter and chiral angle. The chiral angle can be understood by imagining a flat sheet of graphene rolled up. The geometric arrangement of the tube will depend on the direction in which the sheet of graphene is rolled. The chiral angle strongly effects the electrical properties causing the nanotubes to be either metallic or semiconducting. Defects and diameter strongly effect structural strength. Generally, MWCNTs have more defects and are more difficult to evaluate using common optical techniques.
Although many evaluation methods have been developed, Raman spectroscopy is most commonly used to characterize SWCNTs with a number of excitation modes used to identify tube diameter, purity, chiral angle and level of structural defects. The ratio of the G/D modes is the standard measure of structural quality with ratios over 100 regarded as high quality.
Carbon Nanotubes from the Air
Recent research published by staff at Vanderbilt University has created a technique to cost-effectively convert carbon dioxide from the air into high quality carbon nanotubes. Cary Pint explained, “We are able to use electrochemistry to pull apart carbon dioxide and we can literally just stich together those carbon atoms into new forms of matter that’s very valuable. These could potentially revolutionize the world that we live in.”
Techniques such as this one look set to deliver low cost nanomaterials. However, translating the incredible performance seen at the molecular level into actual bulk material properties is still some ways off.