26 Carbon Nanotubes

S.S. islam

 

Carbon Nanotubes

 

Carbon nanotubes (CNTs) are seamless cylinders made by rolling up one or more layers of graphene (lat-tice) sheets to form tubular structures. Depending upon the number of graphene layers used to form the cylinder, CNTs can be either single-walled CNTs (SWCNTs) or multi-walled CNTs (MWCNTs). SWCNTs are produced by rolling up a single layer of graphene and its diameter varies from 0.8 to 2 nm. While, MWCNTs are prepared by rolling up more than one graphene layer, and its diameter can range from 5-20 nm. Additionally, the diameter of MWCNT can also exceed 100 nm. CNTs can be 0.1 µm to 0.5 m long. Individual CNT walls are either metallic or semiconducting depending upon the chirality which is defined as the orientation of the lattice with respect to the tube axis.

 

As of 2013, carbon nanotube production exceeded several thousand tons per year, used for its applications in the field of energy storage, electrical and electronics, sensors and actuators, water filters, coatings, and electromagnetic shields.

 

1.   Potential applications of carbon nanotubes

 

a.   Biological Applications

 

CNTs when added in low wt% to biodegradable polymeric matrices, can significantly enhance the mechan-ical properties of these polymers. Such nanocomposites have wide use in tissue engineering applications such as bone, cartilage, muscle and nerve tissues. Graphene, another carbon nanomaterial, when added in small percentages can increase the compressive as well as flexural strengths of polymers.

 

It is well recognized that carbon nanotubes are compatible with biological molecules (e.g., DNA, proteins, etc.) in terms of their dimensions and chemistry. Additionally, CNTs can also be used for fluorescent as well as photo-acoustic imaging applications. Besides, CNTs can be used to induce localised heating by making use of near-infrared (NIR) radiations.

 

In case of biosensing, either change in electrical impedance or optical properties is observed, where SWNT based biosensors exhibit large changes in both the cases. This is typically done by modulation of adsorption of a target species at the surface of CNTs. Some important sensing parameters such as low detection levels and high specificity require engineering various attributed of CNTs such as: surface modification, for sensor design. Some devices developed or being developed are: printed test strips for the detection of estrogen and progesterone levels, microarrays to detect DNA and protein and sensors for toxic environmental toxic gases such as NO2, NH3, CO, etc. Similarly, CNTs based sensing devices support food industry, military and organic or inorganic hazardous compounds detection applications.

 

b.    Composite Materials

 

Carbon nanotubes possess superior mechanical properties, and because of this fact the development of nu-merous structures such as smart clothes, sports gear, combat jackets, space elevators, etc., is expected to benefit tremendously from the use of CNTs. However, further improvement in the practical tensile strength of carbon nanotubes is necessary in the performance of space elevators; for this application, the CNT tech-nology needs further advancements.

Figure 1 Spinning a yarn from CNTs.

 

Owing to their excellent mechanical properties, CNTs are expected to be behave as building blocks for composite materials. Due to high strength, CNTs are being envisaged as promising materials to produce stab proof and bullet proof cloths. CNTs are supposed to prevent bullet from penetrating the body. However, the kinetic energy of the bullet may cause internal bleeding and some damage to bones. Figure 1 shows a yarn being spun from vertically aligned CNTs. The mechanical strength of these yarns is found to be very high.

 

c.   Mixtures

 

In the area of carbon nanotubes based composites, multi-walled carbon nanotubes (or MWCNTs) were the first to be exploited as electrically conducting fillers. MWCNTs were added to metal at high concentrations (~84 wt%). CNTs can also be used as fillers in insulating matrices. For example, a mere 10 % by weight addition of MWCNTs in polymer matrix increased the electrical conductivity upto 10,000S/m. CNTs based plastics are widely used in automotive industries for electrostatic painting of mirror housings, fuel lines as well as filters which dissipate electrostatic charge. Other uses of CNTs are in electromagnetic interference shielding.

 

The use of CNTs in the blades of wind turbines and hulls in boats for maritime security are based on the enhanced strength of the fiber composites using CNTs. CNTs can be made into more stronger carbon finers having diameter of the order of few microns. In addition, the arrangement of carbon with pyrolyzed fibers is also affected by the presence of CNTs. Besides, the composites of concrete with CNTs enhance the tensile properties as well as crack performance of the resulting composites.

 

d.   Textiles

 

Originally, CNTs’ applications in textiles for enhancing the physical and mechanical behaviors of the fibers, only included spinning the fibers. However, recent investigations have proposed coating of the textile fibres with CNTs. The findings of these experiments can be summarised as follows:

 

–    Randomly oriented double-walled carbon nanotubes (DWCNTs) were coated with a thin film of polymer compounds to prepare DWCNTs/polymer composites. Afterwards, years were made from these compo-sites by twisting and stretching ribbons out of it.

 

–    These fibres were developed by Cambridge University and licensed for commercial production of body armors, e.g., combat jackets from them.

 

–    Synthetic muscles have been prepared by CNT/polymer composites. They produce high contraction and extension ratio for a given amount of electric current.

 

e.   CNTs Springs

 

Forests of stretched and aligned MWCNT springs have host of advantages than any other materials. The energy density obtained by these forests about 10 times higher than that achieved by the springs made from steel. Thus, MWCNTs based springs offer excellent cyclic durability, temperature tolerance, and no abrupt charge/discharge rates. Owing to their better properties than MWCNTs, SWCNTs are expected to perform better than MWCNTs in almost every application.

 

f.   Alloys

 

The incorporation of CNTs even in small proportions into metals leads to considerable improvements in the tensile strength and Young’s modulus. Such CNTs/metal composites find wide structural applications in aerospace as well as automotive industries. Commercially available composites of CNTs and aluminium demonstrate strengths which are comparable to steel (0.7-1 GPa), while their density is around one-third to that of steel. Additionally, they outperform their aluminium-lithium counterparts.

 

g.   Optical Power Detectors

 

Simple coatings of CNTs/ceramic composites are resistant to damage when exposed to high power lasers. Since these coating exhibit good absorption of high power laser light without any breaking down of the film, these coatings are frequently used in optical power detectors. These coatings can be used to defuse unexploded mines and thus can be used for military applications as well.

 

h.   Radar Absorption

 

Microwave frequencies at which radars operate, are readily absorbable by MWCNTs. If MWCNTs are coated on the surface of the aircraft, the radar cross-section is reduced to a great extent due to the absorption of radar by the nanotubes. Stealth technology in aircrafts can highly benefit from the use of MWCNTs. Additionally, since CNTs do not reflect of scatter light in visible range, they are practically invisible during night.

 

i.   Microelectronics

 

Carbon nanotube field-effect transistors (CNTFETs) are single electron transistors that can work at room temperature. First CNTs based logic circuit was fabricated in 2013. However, there are many obstacles for CNTs application to microelectronics, such as difficulties for mass production; circuit density; electrical contacts for individual tubes; controlling purity, length, chirality and alignment of CNTs; thermal budget and contact resistance.

 

Controlling the conductivity of a nanotube is quite challenging, since a CNT can be both a conductor or semiconductor depending upon subtle modifications in the structure of the tube, e.g., chirality.

 

An alternative approach to fabricate CNT based transistors is by using random network of nanotubes. In this approach, the electrical differences are averaged out and mass production of similar devices is possible.

 

As the mean free path for electrons can exceed 1 m in SWCNTs, long channel CNTFETs demonstrate

 

near ballistic conduction properties. For this reason, these devices are much faster than the conventional transistors, and CNT based devices are expected to be operable at gigahertz frequencies also.

 

j.   Thermal Management

 

Owing to large thermal conductivities, CNTs based structure (e.g., films) can be employed for thermal management in electronic circuits. In a study, ~1mm thick CNT layer was used to fabricate cooling pads. Though the performance of this structure was comparable to copper, its density and weight of this structure were very low than that made from copper.

 

k.    Solar Cells

 

Due to strong absorption in UV, visible, and NIR regions, SWCNTs have potential applications in solar panels. Latest findings also shows that they offer a considerable increase in efficiency even at their current unoptimized state.

 

l.  Hydrogen Storage

 

There has been a great deal of research in using carbon nanotubes as hydrogen storage device for its appli-cation as a fuel source. The nanometer range tube diameter gives CNTs an advantage for its use as a storage device by using its capillary effects. It is possible to condense gases, notably hydrogen (H2) in high density inside SWCNTs without being converted to liquid. This storage method can be employed in hydrogen-powered vehicles by replacing gas fuel tanks.

 

m.  Energy Storage

 

The use of carbon nanomaterials can reduce the platinum needed in fuel cells as catalysts by upto 60%. It is expected that the use of doped CNTs can completely eliminate the use of platinum.

 

n.    Batteries

 

Exciting electronic properties of carbon nanotubes (CNTs) have great potential for battery applications. CNTs are attracting interest as novel anode materials in the most popular lithium ion batteries (LIBs). An-odes in LIBs require high reversible capacity at a potential close to the lithium metal, and a moderate irre-versible capacity. CNTs as anode materials have demonstrated these capabilities by enhancing the electrical conductivity as well as improving mechanical robustness of the electrode. This leads to enhanced rate ca-pability and cyclic life of the battery.

 

o.   Chemical

 

The use of CNTs in chemical industries are many. Few of them are as follows-

–   CNTs have potential applications as desalinators, wherein water molecules are separated from salt by passing them via CNTs networks having adjustable porosity. The pressure involved in this process is very low in comparison to the present day RO process. In contrast to the plain membrane, CNT based devices operate at low temperatures (nearly 20 °C lower), and at six times higher flow rate. Aligned CNTs can act as straight passage to water flow through the interior of the nanotube. In this case, very narrow SWCNTs are required to block the salt molecules.

 

–   CNTs based meshes can be used to produce portable filters. These networks electrochemically oxidise organic contaminants, bacteria and viruses, thereby purifying the water.

 

–   CO2 is an environmental pollutant. CNT based membranes can be used to filter CO2 from power plat emissions.

 

–   CNTs when attached with biomolecules, become potential candidates for biotechnology applications.

 

p. Actuators

 

Owing to their marvellous electrical as well as mechanical properties, CNTs hold the potential to replace the traditional electrical actuators for both microscopic and macroscopic applications. CNTs are excellent conductors of electricity as well as heat. They demonstrate high stiffness and elastic modules in specific directions.

 

q.   Optical

 

CNTs have tremendous optical properties which make them potential candidates for diagnostic applications in material science research. CNT photoluminescence (fluorescence) can be used to observe semiconduct-ing SWCNTs species. Photoluminescence maps made by acquiring the emission and scanning the excitation energy, allow sample characterization. CNT fluorescence for biomedical imaging as well as biosensors is being investigated. CNTs can also work as pyroelectric infrared detectors.

 

r.   Environmental Remediation

 

Nanotubes based sponge (nanosponge) containing sulfur and iron can effectively soak water contaminants e.g., oil, fertilizers, pesticides, etc. Their magnetic properties make them easier to retrieve once the cleaning is done.

 

s.   Water Treatment

 

Carbon nanotubes are known to demonstrate strong adsorption affinities towards a wide range of aromatic and aliphatic contaminants in water, mainly because of their large and hydrophobic surfaces. They also show similar adsorption capacities when used as activated carbons in the presence of natural organic mat-ter. Consequently, they are regarded as potential adsorbents for removal of contaminant in water and wastewater treatment systems.

 

Furthermore, membranes made from CNT arrays can be used as switchable molecular sieves, wherein their sieving and permeation features can be dynamically controlled by either pore size distribution (passive control) or external electrostatic fields (active control).

 

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