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CARBON NANOTUBES DISPERSIONS
Carbonnanotubes are strong and flexible but very cohesive. They are difficult to disperse into liquids, such as water, ethanol, oil, polymer or epoxy resin. Carbonnanotubes (CNT) are used in adhesives, coatings and polymers and as electrically conductive fillers in plastics to dissipate static charges in electrical equipment and in electrostatically paintable automobile body panels. By the use of nanotubes, polymers can be made more resistant against temperatures, harsh chemicals, corrosive environments, extreme pressures and abrasion. There are two categories of carbon nanotubes: Single-wall nanotubes (SWNT) and multi-wall nanotubes (MWNT).
Currently two approaches are widely used in nanotube dispersion—the mechanical approach and the chemical approach. Themechanical approach includes ultrasonication and high-shear mixing. These processes are time-consuming and less efficient. Furthermore, ultrasonication canresult in fragmentation of CNTs, in turn, decreasing their aspect ratio. Besides this, the stability of the dispersion is poor. On the otherhand, the chemical approach includes both covalent and non covalent methods. Covalent methods involve functionalization with various chemical moieties to improve solubility in solvents.
Howeveraggressive chemical functionalization at high temperature createsdefects at the nanotube surface, consequently altering the electrical properties of carbon nanotubes. In contrast, a noncovalentapproach involves adsorption of the chemical moieties onto thenanotube surface, either via π–π stacking interaction such as in DNA, uncharged surfactants, etc., or through coulomb attraction in the case of charged chemical moieties. The non covalent approach is superior in the sense that it does not alter the π-electron cloud of graphene, in turn preserving the electrical properties of carbon nanotubes.
Currently two approaches are widely used in nanotube dispersion—the mechanical approach and the chemical approach. Themechanical approach includes ultrasonication and high-shear mixing. These processes are time-consuming and less efficient. Furthermore, ultrasonication canresult in fragmentation of CNTs, in turn, decreasing their aspect ratio. Besides this, the stability of the dispersion is poor. On the otherhand, the chemical approach includes both covalent and non covalent methods. Covalent methods involve functionalization with various chemical moieties to improve solubility in solvents.
Howeveraggressive chemical functionalization at high temperature createsdefects at the nanotube surface, consequently altering the electrical properties of carbon nanotubes. In contrast, a noncovalentapproach involves adsorption of the chemical moieties onto thenanotube surface, either via π–π stacking interaction such as in DNA, uncharged surfactants, etc., or through coulomb attraction in the case of charged chemical moieties. The non covalent approach is superior in the sense that it does not alter the π-electron cloud of graphene, in turn preserving the electrical properties of carbon nanotubes.
Properties of CNTs
The chemical bonding of CNTs is composed entirely of sp2 carbon– carbon bonds. This bonding structure – stronger than the sp3 bonds found in diamond – provides CNTs with extremely high mechanical properties. It is well known that the mechanical properties of CNTs exceed those of any existing materials. Although there is no consensus on the exact mechanical properties of CNTs, theoretical and experimental results have shown unusual mechanical properties of CNTs with Young’s modulus as high as 1.2 TPa and tensile strength of 50–200 GPa. These make CNTs the strongest and stiffest materials on earth. In addition to the exceptional mechanical properties associated with CNTs, they also possess other useful physical properties and chemical properties.
Applications of CNTs dispersions
CNTs can be used as a flame-retardant additive to plastics; this effect is mainly attributed to changes in rheology by nanotube loading. These nanotube additives are commercially attractive as a replacement for halogenated flame retardants, which have restricted use because of environmental regulations. Some other commercial applications of CNTs dispersions are:
Coatings and Films
Leveraging CNT dispersion, functionalization, and large-area deposition techniques, CNTs are emerging as a multifunctional coating material. For example, MWNT-containing paints reduce biofouling of ship hulls by discouraging attachment of algae and barnacles. They are a possible alternative to environmentally hazardous biocide-containing paints. Incorporation of CNTs in anticorrosion coatings for metals can enhance coating stiffness and strength while providing an electric pathway for cathodic protection. Widespread development continues on CNT based transparent conducting films as an alternative to indium tin oxide (ITO). A concern is that ITO is becoming more expensive because of the scarcity of indium, compounded by growing demand for displays, touch-screen devices, and photovoltaic’s. Besides cost, the flexibility of CNT transparent conductors is a major advantage over brittle ITO coatings for flexible displays. Further, transparent CNT conductors can be deposited from solution (e.g., slot-die coating, ultrasonic spraying) and patterned by cost-effective non lithographic methods (e.g., screen printing, micro plotting).
Energy Storage and Environment
CNTs are widely used in lithium ion batteries for notebook computers and mobile phones, marking a major commercial success. In these batteries, small amounts of MWNT powder are blended with active materials and a polymer binder, such as 1 wt % CNT loading in LiCoO2 cathodes and graphite anodes. CNTs provide increased electrical connectivity and mechanical integrity, which enhances rate capability and cycle life. Many publications report gravimetric energy storage and power densities for unpackaged batteries and super capacitors, where normalization is with respect to the weight of active electrode materials. The frequent use of low areal densities for active materials makes it difficult to assess how such gravimetric performance metrics relate to those for packaged cells, where high areal energy storage and power densities are needed for realizing high performance based on total cell weight or volume.
Biotechnology
Ongoing interest in CNTs as components of biosensors and medical devices is motivated by the dimensional and chemical compatibility of CNTs with biomolecules, such as DNA and proteins. At the same time, CNTs enable fluorescent and photo acoustic imaging, as well as localized heating using near-infrared radiation. SWNT biosensors can exhibit large changes in electrical impedance and optical properties in response to the surrounding environment, which is typically modulated by adsorption of a target on the CNT surface. Low detection limits and high selectivity require engineering the CNT surface (e.g., functional groups and coatings) and appropriate sensor design (e.g., field effects, capacitance, Raman spectral shifts, and photoluminescence).
