Carbon nanotubes, what they are and how they are made
A carbon nanotube (CNT) is a tubular carbon structure with hollow cylindrical graphene walls capped by fullerene-type hemispheres. A carbon nanotube comprising a singular graphene tube (Fig. 1) is called a 'single wall carbon nanotube' (SWCNT), and concentric graphene tubes nested within each other like "Russian dolls" are called 'multi-wall carbon nanotubes', (MWCNT). Other carbon structures with herringbone or cup-stacked graphene layers, which form an angle with the longitudinal axis, are often called carbon nanofibres (CNFs). The carbon nanotube diameter can vary from only few nanometers for a SWCNT up to few tens of nanometers for MWCNTs, and more than hundreds of nanometers for CNFs; and tube length can vary from microns up to millimeters and even centimeters.
Carbon nanotubes and nanofibres (filaments) have been made successfully for more than two decades using a method known as chemical vapour deposition (CVD). In the CVD of carbon nanomaterials, a hydrocarbon gas is passed over a heated catalyst. The actions of the catalyst cause the hydrocarbon to decompose into hydrogen and carbon atoms, which provide the "feedstock" for carbon nanofibre/nanotube growth. Carbon nanofibres and nanotubes grown by the CVD method usually have catalyst particles attached to one end. Multi-walled nanotubes grown this way tend to have a large number of structural defects.
Carbon nanotubes – a brief background
The discovery of the C60 molecule in 1985 (also known as fullerene or buckminsterfullerene) by Sir Harry Kroto from University of Sussex, Brighton, with a team led by Richard Smalley from Rice University, Houston, marked the beginning of a new era in carbon science. The Nobel Prize for Chemistry in 1996 was awarded for this research to Kroto, Smalley and Ropbert Curl (also from Rice). In 1991, Sumio Iijima, using high-resolution transmission electron microscopy at the NEC in Tsukuba Japan, reported the first observation of structures that consisted of several concentric tubes of carbon nested inside each other like "Russian dolls". He called them microtubules of graphitic carbon, but from 1992 Iijima and other researchers began to call them carbon nanotubes. Iijima observed MWCNTs in the soot produced by the electric arc discharge between graphite electrodes in a helium atmosphere.
Carbon Nanotubes – Exploiting their Properties
Due to the high aspect ratio, the quasi-one-dimensional structure, and the graphite-like arrangement of the carbon atoms in the shells, nanotubes exhibit a very broad range of unique chemical, mechanical and electronic properties. The properties of nanotubes can change depending on their different structures and quality. Large increases in strength, toughness, and superior electrical and thermal properties, are some of the potential benefits of using nanotubes as the filler material in polymer-based composites, when compared with traditional carbon, glass or metal fibres.
The remarkable electrical and mechanical properties of carbon nanotubes make them excellent candidates for a range of electrical, mechanical and electro-mechanical applications. Carbon fibres have already been used to strengthen a wide range of materials, and the special properties of carbon nanotubes mean that they could be the ultimate-strength fibre. Since they are composed entirely of carbon, nanotubes also have a low specific weight.
Three-dimensional (3D) nano-carbon structures that can transfer the exceptional properties of carbon nanomaterials to meso- and micro-scale engineering materials are essential for the development of many applications. Well known engineering materials like carbon, ceramic or glass fibre could be exploited as a support for the formation of 3D nano-structures (Fig. 2). Carbon fibre bundles, woven and non-woven carbon fibre cloth can be used as three-dimensional scaffolds for carbon nanotube synthesis on the surface of fibres and in the empty spaces between them, to improve the mechanical, electrical and thermal properties of the composite material. Growing CNTs and CNFs on the surface of carbon fibres could also improve composite shear strength and load transfer at the fibre/matrix interface.
The high surface area of carbon and ceramic fibres coated with nanotubes and nanofibres is important for use in electrochemical applications. The high thermal conductivity of these materials may also be of use in automotive and aerospace applications, and for heat distribution or hot spot control. The first attempts to develop carbon-carbon composites containing carbon nanotubes have already been reported. The high electrical conductivity of these materials could be used, for example, in electronic components packaging, as gas diffusion layers in fuel cells or in electromagnetic shielding. Carbon fabric impregnated with carbon nanotubes could be used for applications as varied as lightweight-yet-strong structures, brake discs and bullet-proof vests.
Carbon nanotubes in cars and planes
The mechanical and electrical properties of carbon nanotubes can be exploited in applications, such as aircraft bodies with in-situ 'health' monitoring and self healing properties; superior brakes with carbon-carbon composite discs that could dissipate a heat more efficiently; strong and interactive windscreens with de-icing properties (Fig. 3). Even a few percentage loading of carbon nanotubes in a polymer matrix could make non-conductive polymers conductive, solving many problems with static electricity that could spark a fire within a vehicle. In aircraft wings, the conductivity of carbon nanotubes could provide de-icing and lighting strike protection, combined with weight reduction. Importantly, they could improve the strength of vehicle bodies, decrease their weight and make army vehicles or military airplanes electromagnetically invisible. Carbon nanotubes and nanofibres could be added to metals in order to improve their properties and make lighter engines, they could be used in tyres instead of carbon black to improve wear properties - and provide in situ pressure sensing!
Realising the promise of CNTs
There are a number of problems to overcome in the development process of carbon nanomaterials application. The properties of nanotubes need to be optimised for specific applications. The nanotubes must be efficiently dispersed and bonded to the material they are reinforcing (the matrix), in order to maximize load transfer. Many companies manage to solve some of these problems, with more or less success, and have developed new composite materials from various polymers and carbon nanotubes with improved mechanical, thermal or electrical properties. There are still many opportunities for the improvement of properties of traditional engineering materials and their replacement with use of carbon nanomaterials in transport and a wide range of other applications.
Looking to the future
Carbon nanotube and nanofibre related research and development has led to new production routes and the suggestion and realisation of many new exciting applications, steadily growing in number around the world. Carbon nanotube and nanofibre composites have already reached the market with the first sports applications including tennis rackets, baseball bats, and racing bikes. It is expected that carbon nanotube composite materials will have many more applications including automotive and aerospace components, structures and brakes.
Chemical vapour deposition has proven to be an effective technique for synthesis of carbon nanomaterials and nanostructures. It offers controlled carbon nanomaterials and nanostructures fabrication that is easily scalable and can be adopted for mass production at an industrial level. Similar temperature and gas composition environments needed for carbon nanomaterials synthesis exist in vehicle engines, and catalysts have already been used in modern vehicles in order to reduce green house gases emission. We can imagine a possibility where vehicles could be used to produce valuable carbon nanomaterials and further reduce green house gas emissions.
Intensive research in recent decades has already set up solid foundations for exploitation of the many properties that carbon nanomaterials have for the benefit of humanity. However, significant advances in understanding, synthesis and applications of carbon nanomaterials are still ahead of us.