Carbon Nanotubes and its applications
Synthesis
Arc evaporation method, laser ablation, chemical vapor deposition, electrolysis, flame
synthesis are the various methods that are used for the synthesis of carbon
nanotubes in large numbers. Arc evaporation method, laser ablation and chemical
vapor deposition are the techniques
which are used broadly for synthesis of CNTs.
Arc
evaporation method
When hot plasma discharge is
created between two graphite electrodes connected by a power supply of 10-20 A
in the presence of inert gas such as helium gas carbon nanotubes are
produced. The yield of carbon nanotubes can be increased by increasing the
helium pressure inside the chamber to a certain value but beyond that value the
yield of carbon nanotubes are reduced. Better quality of carbon nanotubes can
be obtained by lowering the supply current. Instead of helium nitrogen and CF4
were also used.
Laser
ablation method
When the carbon source(graphite) is doped with
small amount of metallic catalyst (Co and Ni) which is then vaporised with the help of pulsed laser beam in the
presence of inert gas (argon) at high temperature and pressure of about 12000C
and 500 Torr respectively. The target material is impregnated by the laser
beam and vaporised and then it is condensed on the other end which is
comparatively at lower temperature. Due to the requirement of high power supply this method is
very costly affair.
Chemical vapor deposition
The decomposition of mixture of hydrocarbon
gases like methane and ethylene or volatile carbon compounds present in the
chamber on to a metallic substrate, where the metallic nanoparticles acts as a
catalyst and nucleation sites in the growth process of carbon nanotubes in the
temperature of 500-10000C and at atmospheric pressure. The
catalyst and substrate selection plays a key role in the production of carbon
nanotubes. Fe, Co and Ni are considered to be the suitable catalyst and porous
silicon is considered to be the substrate.
Application
Reinforcements in turbine blades of marine
turbines The high bending forces, cavitation, high
flow velocity fluctuations resulting from the waves and turbulence are the main
challenges in the marine current turbines. The body of the marine turbines is
to be designed in such a way that it should withstand in the ocean and it
should have high strength as the prototype broke into pieces while testing
it. The Carbon nanotubes the most studied reinforcement agent to strengthen
the composite material can be incorporated in the turbine blades to improve the
strength of the turbines. The large scale marine current turbine blades were
manufactured using epoxy resin and carbon
fibers which is also known as prepag material. The Researchers have
investigated many synthesis techniques to synthesize the carbon nanotubes
incorporated epoxy resin and the results obtained show a positive improvement
in the mechanical properties such as high shear strength, high fracture
toughness and high fatigue resistance and the electrical properties of the carbon nanotubes reinforced
epoxy resin are also increased. The fouling factor of carbon nanotubes incorporated epoxy resin is high and hence the turbine blades manufactured with the above material can be subjected to corrosion and deterioration. To avoid this the fouling resistance paint is coated with more durability. The researchers have also noted that instead of increasing the mechanical properties the carbon nanotubes increases the fouling resistance reasonably.
Detection of
atoms and molecules using vibration of graphene sheets
The recent studies and
research in the field of electromechanical resonator systems has given rise to
a new promising area of ultra sensitive sensors. This study also shows that the
nano electro mechanical systems fabricated with the single and multi walled
graphene sheets have high young’s modulus, extremely low mass and high surface
area and these make the nano resonator sensor may be useful for mass force and
charge sensors. From the tests it is noted that combination of applied high
tensile mechanical strain increases the Q factor of graphene sheets in
conjunction with two single atom vacancies and enhances the bonding between
absorbed atom and graphene monolayer. The
potential of the graphene resonator sensors in the sensing of the noble gases
randomly attached on the graphene by using molecular dynamics simulations.
The simulation results indicates that the resolution of a mass sensor made of
square graphene sheet with a size of 10nm can achieve an order of 10-6
fg and the mass sensitivity can be enhanced by decreasing the size of graphene
sheets. Owing to the unparalleled
properties of GSs, future potential studies on nano-resonator sensors made of
graphenes can involve detection of macromolecules
such as DNA molecules.
Mechanism of heavy metal removal by functionalized graphene nanomaterials and carbon nanotubes
Adsorption isotherms
The most important
indicator in evaluating the adsorbent is the adsorptive capacity. The
adsorption isotherm models are used to calculate the theoretical adsorptive
capacity. Langmuir, Freundlich, Redlich-Peterson, Temkin and
Dubinin-Radushkevich models are the isotherm models that are used to evaluate
the adsorption capacity of the heavy metals by functionalized carbon
nanotubes.
Langmuir: qe=KLqmCe/(1+KL+Ce)[24]
Freundlich: qe=KFCe1/n[25]
Redlich-Peterson qe=KRCe(1+αCeβ)[26]
Temkin: qe=(RT/bT)ln(KTCe)[27]
Dubinin-Radushkevich: qe=qm exp(-KDRε2)[28]
where qe and qm
(mg g1) signify the equilibrium and maximum adsorptive
capacities.
Ce (mg L1) is the equilibrium
aqueous concentration.
KL (L mg1)
and KF (mg g1 (mg L1)n) represent
the adsorptive capacity and the affinity between adsorbate and adsorbent.
kR (L g1), a (L g1),
and b (dimensionless) are constants.
kT (L g1) is the equilibrium
binding constant.
bT (J
mol1) is related to the heat of adsorption.
1/n (dimensionless) represents the heterogeneity of
the adsorbent sites and also indicates the affinity between adsorbate and
adsorbent.
kDR (mol2 kJ2) is
related to the mean free energy (kJ mol1) of adsorption and is equal
to 1/(2kDR)1/2, and ε¼ RTln(1 /1+Ce).
For Monolayer adsorption Langmuir isotherm is used in
which most of the adsorption sites have
equal affinities towards adsorbate. While the Freundlich isotherm is used for
the heterogeneous chemisorptions process. The Redlich-Peterson model is the combination of
Langmuir and Freundlich isotherms. Temkin isotherm is used for the heat of
adsorption and the interaction between adsorbates on adsorption isotherms. Most
of the carbon nanotubes shows better fit with the Langmuir isotherm.
Rubber - CNT based Applications
The
physical and mechanical properties of polymers were improved by addition of CNT
as the reinforcement, compared to the conventional fillers like silicas and carbon
black. The CNT – rubber nanocomposites is suitable for extensive
applications in numerous fields. They possess good stretch-ability,
flexibility, conductivity about the tube axis and CNT – rubber nanocomposites
have good strength makes them extremely relevant in capacitors, pressure
sensors, strain sensors, fuel hoses, oil seal. The other modified
nanotubes have widespread usage in fuel cell, aircraft structures, air
filtration, water filtration, solar plate, paper with less wettability,
improved corrosion resistance, damping property, smart fabrics, nano-robots,
drugs, improved battery charge, etc. The chemically modified
CNT applications are not limited, further research study may leads to an
applications with better interactions and bonding between the CNT and matrix
material.
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