Advanced Materials series: Carbon Nanotubes
Latest update: April 2019
The scope of applications for Carbon Nanotubes has increased in recent years, the medical industry is considering the effectiveness of the material for various forms of immunotherapy, for example. Throughout all of this there has been little developments in the usage of Carbon Nanotubes with specific rail applications, although the material is being more widely researched in adjacent transport industries such as Aerospace.
What are Carbon Nanotubes
Carbon Nanotubes (CNTs) are a type of nanomaterial, consisting of tubular structures which are formed by one-atom-thick sheets of carbon, called graphene. The structure of graphene has a far higher surface to volume ratio than conventional materials, which gives the material distinct properties. CNTs exhibit high tensile strength, high thermal and electrical conductivity, and are highly flexible with strong elasticity properties, all with a relatively lower mass. CNTs can be stacked together to form multiwalled carbon nanotubes which enhance their conductive qualities.
What industries already use Carbon Nanotubes?
In the defence industry, Lockheed Martin has invented a process that significantly reduces the cost of manufacturing composites for aircraft structures. The structure of F-35 Lighting II fighters now incorporate CNT-reinforced polymers with a high strength-to-weight ratio. Within the aerospace industry, silicon carbide nanotubes are used as they add strength to ceramic composite materials. NASA will incorporate these in the next generation of rocket engines, with the nanotubes strengthening the composite materials beneath at high temperatures due to their high thermal conductivity, preventing exposure to oxygen which can lead to the formation of cracks within the structure. In the medical industry, CNTs are being used as a self-healing vascular system which not only strengthens materials but can also deliver a replenishable supply of self-healing material. CNTs are also being investigated for their ability to deliver immunotherapy, with their high thermal and electrical conductivity making them excellent candidates as photothermal therapy agents.
How can Carbon Nanotubes impact the rail industry?
Due to their high thermal conductivity, Carbon Nanotubes have great energy transfer and dissipation rates/properties, therefore composites could be used within the brakes or engines to dissipate heat more efficiently. The high surface area per unit of mass means that CNTs can be incorporated into different energy storage applications. This would not inherently affect rolling stock due to electrification but could mean that other modes of transport such as electric/hydrogen cars become more popular as their energy storage and efficiency improves. Furthermore, CNTs could be used in conductor rails due their low resistivity and would therefore will save on energy used. Their high strength-to-weight property means that they can be used in nanocomposites for the train body, reducing weight & energy consumption, as well as in track infrastructure to reduce the wear caused by rolling stock wheels. CNTs dispersed in paints form nanopaints. These can be used for their wear resistance, hydrophobic and de-icing effect properties; which would mean that less maintenance on the train body would be required, saving on maintenance costs.
What should the rail indutry do?
Rail could evaluate the feasibility of replacing current paints with nanopaints and conduct a cost benefit analysis on the overall cost saved by reduced maintenance costs. It may also be beneficial to research new cost-effective methods for reducing the cost of mass-producing high-quality nanotubes for composite structures for train bodies as well as improved thermal dissipating brakes. Further research the self-sensing/self-healing properties of CNTs could reveal applications for detecting/preventing structural weakness, reducing maintenance and inspection costs. Incorporating nanocomposites into rolling stock bodies to reduce weight and energy consumption could also be considered.
What R&D is underway and what uncertainties remain?
Over £117 million worth of research grants for graphene and carbon nanomaterials are currently being funded by the ESPRC. A slight downward trend following a period of high investment. In 2010, the US Air Force Research Laboratory developed a Carbon Nanotube reinforced material for the trailing wing edge of the B2 stealth bomber, which would theoretically allow the aircraft to remain undetectable by radar. Carbon nanopaints have been designed with specific nanotubes able to show large, predictable wavelength shifts (associated with the fluorescence property of CNTs) to measure the magnitude that the tubes are deformed by either tension or compression. Remote measuring devices such as laser and infrared spectrometers can then be used to identify strained parts of the structure. The U.S. Department of Energy’s Fuel Cell Technologies Office is researching the potential of using nanotubes for hydrogen storage systems. A recent advance achieved hydrogen storage and release inside nanocontainers made from single-walled nanotubes. Research is ongoing into the manufacturing of CNTs notably using chemical vapour deposition, which will lower manufacturing costs. Uncertainties remain in the manufacturing process as the quality of the produced nanotubes varies. Most mass-produced nanotubes are highly defective. Having multiple misplaced atoms within the structure causes fracture points, easily breaking the normally strong covalent bonds in the tubes. This causes adjacent tubes to then break which leads to the nanotube ‘unzipping’. This causes it’s practical strength to decrease greatly. Currently, high-quality CNTs are hard to produce in large quantities and are costly, and so further research into manufacturing process is be required.
Banner image author: Jynto [CC0], via Wikimedia Commons.