Carbon Nanotubes: Structure, Properties, Manufacturing, And Applications

Nano technology has been a hot topic in recent times. These nanotubes were first discovered in June, 1991, by Sumio Lijima, a Japanese physicist who specializes in Nanotechnology. The above mentioned nano-technologies range from medical use to industrial use in composites. Amongst others, carbon nanotubes and their properties and material characteristics are finding great attraction amongst researches and industry throughout. The reason for their attraction is not only their uniqueness when being compared to conventional materials, but their promising material properties that can be used in future nanotechnology. The reason for the wide interest in this material is due to their unique thermal and mechanical properties. They are found to have a tensile strength of 400 times and density one sixth of that of steel, their thermal conductivity better than that of diamonds and they have a very high aspect ratio greater than 1000. These carbon nanotubes have found to absorb 99.96% of all incoming radiation which proves very useful in some applications.

Material Structure

When Sumio Lijima first discovered these carbon nanotubes (CNT) he describes them as extremely thin needle-like material whilst examining carbon materials under a microscope. The microstructure if these materials have large similarities to graphite in their carbon structure. The main difference is that CNTs are carbon atoms bonded covalently in a tubular honeycomb-like structure; whereas graphite is composed of these covalent carbon atomic bonds in a flat plate-like structure; the difference can be seen below.

These CNTs are categorized into three main categories according to their structure. The CNT that was first discovered was the single walled CNT. After much research it was found that they exist in other forms including CNT with multiple walls. Figure 2 above shows two types of CNTs found. On the left is a single walled CNT whereas on the right is a model of a double walled CNT.

Single walled CNTs have dimensions on 1 nano-meter in diameter and ranging in lengths of up to 20 micro-meters. These nanotubes can be stranded together to form yarns which can measure up to meters in lengths. As seen below, the yarns are formed by removing a single strand of nanotube from the nanotube forest. These nanoforests are formed by the methods in which they are synthesized. The structural dimensions of CNTs lend themselves to some surprising mechanical properties which are discussed below.

Basic mechanical properties

When looking at the mechanical properties of a material, these properties depend ultimately on the interatomic forces and special arrangements of the atoms. The stiffness which is seen by the modulus of elasticity is directly dependent of the stiffness of the bonds present in the material. We can introduce the bond strength to the spring constant k. It is seen that the spring constant for metals and ionic olids vary between 15 and 100N/m, whereas covalent bonds are found to range from 20N/m to 200n/m. The largest of these bonds known are namely the carbon-carbon bond which is between 500-1000N/m. Most polymers existing of carbon have a low density but are found to have weak bonds between their chains. This is where nanotubes are added to the polymers in order to increase their stiffness. During the linear-elastic state of a material, the arrangement of atoms in the material play a large role. A slight deformation or inconsistency in these atomic arrangements may have large implications and thus can cause a non-linear behavior in the elastic state of the material. This may ultimately lead to a plastic yield or brittle failure. Typically, the yield stress would be at 10% strain or Y/10, however due to these imperfections, the yield stress is typically found to be Y/104 in common materials. These imperfections could be led by the presence of dislocations, voids, defects and grain boundaries. Due to the structural formation of carbon nanotubes, they are considered to have a near perfect structure which makes them a favouable material when being compared to conventional materials due to the above discussion.

Non-elastic behavior

When large deformations are considered which lie outside the range of the Hookean behavior, these carbon nanotubes behave in a non-linear elastic behavior. This surprising behavior can be seen by the structure of the material. Both theoretical as well as practical results show that the material can greatly absorb the energy from large external forces by having large degrees of formation without plastically deforming. These CNTs develop kinds or ripple when put under a bending or axial force. When addressed to a torsional force, the CNTs are found to flatted into deflated “ribbins”. However after these deformations, the CNTs restore largely back to their original shape. These behaviors lead to large resilience which is not expected to exist for graphite-like materials.

Strength and Fracture

Properties such as yield strength are known as the plastic properties are quite predictable when it comes to single crystal graphite. However due to the non-linear material properties of CNT, these properties such as yield strength are difficult to determine. This is mainly due to the fact they CNTs can deform greatly under loads and thus absorb much or the external energy as discussed above. However, due to the known strength of carbon-carbon bonds, these CNTs are expected to have tensile strengths which far surpass other known fibres. Due to the size of these single CNTs, it further complicate methods of achieving these results from known practical tests such as tensile tests. The reason for this is that these CNTs are too small for conventional testing machinery and the strength if these CNTs are too large for tiny “optical tweezers”. This leaves the need to newly developed methods of measuring the strengths of these CNTs. Tensile tests were however achieved and they resulted in multi-walled CNTs having tensile strengths ranging from 11 to 6 GPa. These results correspond to a Youngs Modulus ranging between 270 and 950 GPa. Furthermore, the measured strain was measure to be well over 12% change in length.

Thermal and electrical properties

Due to the material structure of carbon nanotubes, these properties yield great thermal and electrical conductivities. It has been seen that this material might just be the best heat-conductivity known to man and it has been seen to be a superconductor below 20 Kelvin. These carbon nanotubes can be used in polymetric materials in order to improve their thermal and thermomechanical properties. The cylindrical structure lends itself to great ability to store field emissions and thus lends itself to being used as so called ultra-capacitors. These properties can further be used to store energy in many different applications

Environmental impact and sustainability

A study named multiwalled carbon nanotubes in alfalfa and wheat: toxicology and uptake was done and published on 12 September 2012 regarding some of the environmental impacts and implications these nanotubes have. It was said that these nanotubes have been found to be used in biosensors, drug delivery vehicles and tumor imaging, electrically conductive polymers and concrete reinforcement; however, the interactions that this nano-techonology has with the environment and biological systems in not well understood. The species which were under examination, mentioned above in the study name, was found undamaged by the nano-technologies. Research by means of microscopy and other methods proved that CNTs enhanced the root development of these plant species. Though it was found that the CNT absorption was chemically bound to the plant cell walls; they proved to have no effect on altering the plant development or root tissue morphology. As seen above, there are some positive implications that CNTs have on the environment, however there are a few rising matters as well. Due to the size of this material, it has been found difficult to manage. They pollution of this materials into water systems are difficult to manage. Due to these materials absorbing the majority of light by means of its structural properties, it inhabits the growth of algae in water systems which could prove catastrophic to ecosystems. However, research has shown that heavy metals such as uranium which is abundant on earth and in water systems are strongly attracted to these carbon nanotubes. This could be beneficial in applying them to methods in order to remove these heavy metals from water systems. However, this method is not yet financially viable in applying the technology to water purification systems. It was mentioned that the implications of these CNTs in water should be further investigated before any conclusions are drawn.

One worrying characteristic of these carbon nanotubes are their ability to penetrate cell walls. This is due to their small structure being 50 000 times thinner that human hair. This may prove to be of a large health hazard as it would penetrate human cell walls and have negative implications on health. There is however much research that needs to be done before conclusive arguments can be drawn.

Processing and manufacturing

There are many methods in order to manufacture these CNTs. One of the major factors which play a role in the production of these materials are the associated high cost. Below we are introduced to a few methods in order to product these CNTs. The resulting composition of the CNTs after manufacturing are highly process dependent as well as have a direct correlation to the cost involves.

Arc Method: This method is the most common and seen as the easiest way to produce CNTs. One drawback from this method is that it produces a large array of components which require further purification methods in order to extract the CNTs.

The process starts by placing two carbon electrodes in an inert gas at low pressure. Current is sent through these rods where it then vaporized the surface of the one electrode and forms a quantity of deposit on the other electrode. The yield of CNT produced by this method relies largely on the uniformity of the plasma arc which is formed between the arcs during vaporization. This method has proven to product high quality material, however in small quantities. This is therefore now a viable method form commercial production.

Lazer Methods: In 1996, CNTs were first produced by means of a so-called laser method. This method entails the use of a dual-pulse laser which was seen to achieve yields of >70wt% purity. The preparation of this method included two graphite rods with a 50:50 catalyst mixture that composed of Cobalt and Nickel at 1200oC in a flowing argon liquid. These rods were vaporized and thereafter followed heat treatment in a vacuum at 1000oC in order to remove any unwanted fullerenes. Fullerenes is a sphere-like molecule consisting of carbon. Firstly, a laser pulse is used to vaporize the graphite and thereafter a second pulse in used to break up the larger particles which were formed by the initial pulse and thus feeds the particles into a growing nanotube structure. The outcome of this process is seen to be a bundle of single walled CNTs which were aligned along a common axis. The two methods above are currently the primary method of obtaining high quality single carbon nanotubes, however they suffer some drawbacks. One of these drawbacks are that the product of these CNT produced are aligned in a highly unorganized order. This makes it difficult to separate the CNTs and proves tricky to align them in an orderly fashion.

Catalyzed Chemical Vapor Deposition: This is the most common method for carbon nanotubes synthesis. The CNTs in this method are synthesized by means of vaporizing carbon over a metal catalyst. This method is not unique to CNTs but have been use to produce materials namely carbon fibers and filaments for over two centuries. More detail of this method can be seen in the video on the home page.

Applications

Bulletproof Vests

Bulletproof vests are designed to absorb the energy from the impact of a projectile. This energy is thus absorbed by the vest and thus less energy is dissipated into the human body which could cause severe damage. Due to the known material characteristics of carbon nanotubes, their ability to absorb energy without permanently deforming in of a great advantage. These characteristics make CNTs an ideal material in bulletproof technology. The bulletproof material is made by means of woven individual nanotubes into an extremely lightweight material. This material is then used to make bulletproof vests as well as other bulletproof material. Research from the Centre for Advanced Materials Technology at the University of Sydney published conducted a study which determined which CNT form would work best for this application. It was found that single walled CNTs worked best in absorbing energy and furthermore, large diameter CNTs were able to withstand higher bullet speeds and ones with longer lengths were able to absorb more energy. The figures below show graphic results on the above discussion.

Space Observations

Due to the ability of CNTs to absorb most light passing through them, research has been done by NASA in order to use this material on space observation devices such as telescopes. It was stated that the walls of space telescopes were previously painted with a black paint; however some light was still being reflected onto the lenses which decreased the quality of the observed image by adding noise to the image. These CNTs are therefore being used to replace this black paint as it reduces the amount of reflected light and thus results in a clearer observation. Investigation was done regarding the ability for these CNTs to withstand harsh conditions which are experienced in space travel. It was concluded that these CNTs were to be painted onto a metal. The reason for this is due to the excellent adhesive qualities of the CNT to certain metal surfaces.

There are numerous other applications for these nanotechnologies and thus opens various new doors into the unknown and solving problems which were once seen to be impossible to solve.