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: q_e=K_Lq_mC_e/(1+K_L+C_e)[24]

Freundlich: q_e=K_FC_e^1/n[25]

Redlich-Peterson qe=K_RC_e(1+αC_e^β)[26]

Temkin: q_e=(RT/b_T)ln(K_TC_e)[27]

Dubinin-Radushkevich: q_e=q_m exp(-K_DRε²)[28]

where q_e and q_m (mg g¹) signify the equilibrium and maximum adsorptive capacities.

C_e (mg L¹) is the equilibrium aqueous concentration.

K_L (L mg¹) and K_F (mg g¹ (mg L¹)ⁿ) represent the adsorptive capacity and the affinity between adsorbate and adsorbent.

k_R (L g¹), a (L g¹), and b (dimensionless) are constants.

k_T (L g¹) is the equilibrium binding constant.

 b_T (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.

k_DR (mol² kJ²) is related to the mean free energy (kJ mol¹) of adsorption and is equal to 1/(2k_DR)^1/2, and ε¼ RTln(1 /1+C_e).

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.