Nanotechnologies draw the intense attention of scientists as nanomaterials possess properties that significantly differ from other materials. The potential applications of nanotube fibers and carbon nanotubes (CNT) are structural reinforcement materials (active heating/cooling, electrical conductivity shielding), biochemical, atmospheric and thermal sensors, actuators and artificial muscles, and electrodes. The CNTs have remarkable mechanical, electrical, thermal, and optical properties. In 2010, the global consumption of carbon fiber reached 34,200 tons.
The research of CNT concentrates on two areas. These are the development of the reliable and replicable methods of obtaining CNT with the well-controlled properties and study of morphologies and properties.
The most common precursors of CNT are polyacrylonitrile, isotropic and mesophase pitch and lignin due to high yield. Cellulose is also a promising raw material for CNT as it has nicely ordered crystalline structure and undergoes thermal decomposition without melting. Bacterial, algal, animal and tunicate cellulose can be incorporated to CNT.
The length of carbon fiber is about ten nanometres. For practical application, it is necessary to align the individual CNTs along the %uFB01ber axis. There are four methods: spinning from CNT solutions, spinning from a vertically aligned CNT array previously grown from a substrate, spinning from a CNT aerogel and twisting (rolling) from a CNT film. The first method is a wet spinning, the others are dry spinning. After CNTs are obtained, they require stabilization. Stabilization is performed as chemical (polyacrylonitrile CNT), thermal treatment (pitch-based CNT), or incorporation of dehydrated salmon DNA into CNT.
The single-walled CNT (SWCNT) and multi-walled (MWCNT) can by synthesized. A MWCNT has a number of concentric inter-wall spacing of 0.34 nm. Various modifications of techniques are developed to improve one or more characteristics of the CNT. For example, the array-spun preparation of MWCNT results in the electrical conductivity of the samples as high as conductivity of CNT %uFB01bers. The thermal conductivity of the samples is the highest value among the reported in literature. SWCNTs have relatively high modulus (130 GPa), but moderate strength (0.2 GPa), despite the well-aligned microstructure, while MWCNT possess 0.5 GPa tensile strength. Polyvinyl alcohol SWCNTs have tensile strength of 1.8 GPa. Addition of the polymers (polyvinyl alcohol or epoxy) increases the Young’s module of polyacrylonitrile CNT to 120 GPa, tensile strength to 2GPa, and electric conductivity of 9.2%u2219104 S/m10. Double-walled CNT exhibit promising mechanical strength (~1.4 GPa) and energy-to-failure (100 J/g)11.
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Cellulose based CNT reveals a collapse of the tensile strength during the carbonization of CNT, at any heating rate. The introduction of the organosilicon into the structure of cellulose CNT results in increase of a tensile strength to 1100 MPa. Similar effect has been reported for pan-based fibers.The morphology of CNT is clear when investigating using SEM and HRTEM. The images of the CNT samples clearly depict fibers9,10,14.
Although the preparation and characterization of CNT is important, for practical application price is the significant factor. For today, the cost of CNT remains high. Industrial grade carbon fiber is $22/kg, 10 times the price of steel. The research should be continued for inexpensive precursors and preparation techniques.
The research of CNT is essential for the modern society as CNT materials are prospective for usage in various areas. For today, the challenge for the CNT research is the price reduction.