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Carbon nanotubes are cylindrical tubes
composed of graphene and can be categorized as either single-walled nanotubes (SWNT)
or multi-walled nanotubes (MWNT). SWNT’s have only a single layered cylindrical
form (similar to a straw) while MWNT’s consist of more than one nanotube that
are nested. Carbon nanotubes can also have three specific carbon atom
arrangements: armchair, zig-zag, and chiral, which are indicated in Image I. The complex structure of carbon nanotubes
allows it to maintain certain properties such as electrical and thermal
conductivity as well as mechanical strength. In order to obtain these complex
structures and properties, however, the carbon tubes need to be synthesized into
their according structures. Chemical vapor deposition is the most common
technique, although other synthesis methods such as arc discharge and laser ablation
can be used. In chemical vapor deposition, vaporized hydrocarbons are heated
and decomposed by a metal catalytic substrate which serves as a template for
nanotube formation. The uniqueness of carbon nanotubes extends far beyond their
structures and properties, as their level of compliance with different fields
is vast. From building the darkest color to providing a medium for tissue
regrowth, there seems to be few boundaries for these nanomaterials. Vantablack
is a material created by Surrey Nanosystems which was originally created for
satellite-borne blackbody calibration systems6. Vantablack is
currently being recognized as one of the darkest man made materials as it
reflects only 0.036% of light6. 
When it is applied to a 3D surface, the object appears to lose its
dimension to the darkness and resembles a black hole in space (Image 26). Its high
absorption extends from the UV spectrum, into the visible, and also the
infrared (>16microns)6. The reason it absorbs such a large range
of light is because of vertically aligned carbon nanotube arrays (VANTA). The
arrays contain spacing which permit the entrance of a photon, however, the light/radiation
reflects between the tubes and eventually becomes absorbed as it is converted
to heat. The heat is then transferred to the nanotubes’ substrate which can be made
of aluminum alloys, cobalt, copper, nickel, sapphire and more. Besides optical
properties, Vantablack contains other properties make it an extraordinary
material. For example, it is very hydrophobic and therefore prevents water from
interfering with optical properties and it also has high thermal and mechanical
shock resistance. Due to Vantablack’s properties, it can be applied to various
pieces of technology to enhance data collection and imaging. Some applications
include its incorporation into IR imaging, Spectroscopy, telescopes, sensors,
and calibration sources.Carbon nanotubes also contribute to the
development of high performance power sources. In one study, carbon nanotubes
were paired with carbon nanofibers to create a flexible supercapactior capable
of delivering high energy. The carbon nanotubes, which have the unique ability
to store charges, coupled with nanofibers largely increase the storage and
transfer of energy. The high capacity performance of these newly synthesized
materials is attributed to the branching of the nanotubes which can be seen in Image III3.The
creation of a flexible super capacitor first involved the synthesis of a
nonwoven fiber mat composed of polyacrylonitrile, polyvinylpyrrolidone(PVP),
and Ni(AC)2. The fiber mat was placed into a silica tube along with
more PVP powder before being placed into a furnace connected to a gas system.

The temperature was raised to 900 degrees Celsius for two hours before it was
cooled, resulting in the formation of carbon nanotubes. After, HNO3
was used to strip Ni from the activated VACNTs/CNFs. The VACNTs/CNFs were then
pressed into electrode films before capacitor assembly. The capacitor was
arranged by placing fiber filter paper between the electrode films before the
infiltration of electrolytes. The electrode terminals were connected to a VMP3
(for electrochemical characterization).

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Scientists found that the VACNTs/CNFs
nanostructure was capable of transporting a specific energy of 70.7 Wh/kg at 30
degrees celcius and 98.8 Wh/kg at 60 degrees celcius3. What was even
more impressive was the ability of the capacitor to retain 97% of its
capacitance after 20,000 charge/discharge cycles3. This super
capacitor property of carbon nanotubes is especially useful as they can be
incorporated into technology and
equipment as reservoirs for energy storage. The flexibility is also favorable
as they can complement the increasingly simplified sizes of modern day
technology.

 

            Not only are carbon nanotubes useful
for constructing and enhancing technological materials, but also for the
development of advanced biomaterials. Another recent discovery details one of
the many uses of carbon nanotubes as a biomineralization materials. Researchers
were able to observe polycrystalline biological apatites which formed on the VACNT
tips after being soaked in simulated body fluid. This discovery leads them to
believe that the VACNT have great potential for tissue regeneration and constructing
other bionanomaterials. The synthesis of these VACNT films was carried
out using a microwave plasma chamber as well as a substrate composed of
titanium covered by Iron (deposited using e-beam evaporator). The iron was then
treated with plasma N2/H2 in order to promote the formation of nanoclusters
before which polar groups were incorporated onto the ends of the nanotubes
(using pulse-direct current plasma reactor and oxygen). After the nanotubes
where properly synthesized, they were then allowed to soak in simulated body
fluid which mimicked the conditions of the human body relative to pH,
temperature, and chemical components. Once removed from incubation, the
apatites where observed using Raman spectroscopy and Fourier transform infrared
spectroscopy.

           

Scientists concluded that the polar
groups (specifically COOH) attached to the ends of the tubes permitted the cell
adhesion. The other factor that made these nanotubes favorable biomineralization
components is the large amount of sites that are available for attachment,
which subsequently favors the growth of new cells. The adhesion properties
specifically enhanced the binding of osteoblasts. These findings reveal the potential use of carbon
nanotubes as biological repair factors such as tissue and bone. These
properties demonstrate the great potential of carbon nanotubes in

 

Global warming as the result of the
increased output of greenhouse gasses is one of the most complicated environment
problems humanity faces today. Scientists currently researching this problem
have developed ways to decrease the amount of greenhouse gasses in the
atmosphere, one of which involves the use of carbon nanotubes. The scientists
in this next example aimed to optimize carbon dioxide absorption of carbon
nanotube arrays by applying electrical charges which yielded particularly
promising results. Researchers
investigated carbon dioxide absorption using bundles of zigzag and armchair
single walled carbon nanotubes arranged in a hexagonal lattice. The group
simulated carbon dioxide absorption isotherms using the grand-canonical Monte
Carlo method (GCMC). In this method, temperature, chemical potential, and
volume remained constant. Charges of 0.01 to 0.04e where applied to each carbon
to analyze the absorption effect.  

 

Overall, researchers found that carbon
dioxide absorption was highest when the carbon nanotubes were supplied with a
positive charge while a negative charge seemed to decrease the carbon dioxide
absorption relative to the neutral and positively charged nanotubes1.

This discovery provides a deeper look into carbon dioxide absorption and how
global warming could potentially be reversed or slowed. If researchers could
magnify this absorption effect of carbon nanotubes, it may be possible to
improve environmental conditions of the earth and lessen the effects of global
warming by significantly reducing the amount of carbon dioxide in the
atmosphere.

 

            The previously mentioned research is
only a small representation of the advances of carbon nanotubes. Much
development and research is needed to expand upon the potential advancements
that these nanomaterials can offer. As carbon nanotubes were only just
discovered in the 1990s, this research only documents the beginning of what
could be a largely impactful material. Much more has yet to be discovered about
carbon nanotubes and their applications.