Applications
Industrial applications of this new material include: polymer and elastomer fillers, commercial hydrogen storage systems, radio wave-absorbing composites, lithium battery electrodes, construction composites, oil additives, and gas-distribution layers for fuel cells, filters and absorbents
Properties
Carbon nanofibers (CNFs) have excellent mechanical properties, high electrical conductivity, and high thermal conductivity, which can be imparted to a wide range of matrices including thermoplastics, thermosets, elastomers, ceramics and metals. Carbon nanofibers also have a unique surface state, which facilitates functionalization and other surface modification techniques to tailor/engineer the nanofiber to the host polymer or application. Carbon nanofibers (CNFs) are discontinuous, highly graphitic, highly compatible with most polymer processing techniques, and they can be dispersed in an isotropic or anisotropic mode. Carbon nanofibers are available in a free-flowing powder form with typically 99% of mass in a fibrous form. Carbon nanofiber can be used for filler in composite materials or electron emitters like CNTs.
Nano fiber architecture
The individual nanofiber is precipitated from a catalyst particle, and has a hollow core that is surrounded by a cylindrical fiber comprised of highly crystalline, graphite basal planes stacked at about 25 degrees from the longitudinal axis of the fiber. This morphology, termed "stacked cup" or "herringbone", generates a fiber with exposed edge planes along the entire interior and exterior surfaces of the nanofiber. These edge sites are reactive, relative to the basal plane of graphite, and facilitate chemical modification of the fiber surface for maximum incorporation and mechanical reinforcement in polymer composites. This open architecture also facilitates rapid intercalation and de-intercalation by heterogeneous atoms, useful for tuning conductivities. Carbon fibers have an innermost diameter that is smaller than independently produced MWCNTs, the radius of the curvature at the tip is less than 3 nm, and the diameter of the innermost layer is less than 1 nm.
Synthesis
Due to their ultra-high internal surface area, nanostructured carbon materials have received considerable interest in the fields of gas separation, water purification, catalyst support, sensors and the anode materials in lithium-ion batteries.
Arc discharge deposition with hydrogen gas is used to produce carbon nanofibers. Carbon nanofibers have a very narrow inner cavity or practically no hollow channel in their center, and the graphitic layers at the tips are defective or partially broken.
Carbon nanofibers also can be fabricated by thermal CVD using templates. Anodic aluminum oxide (Al2O3) with a thickness of 30 μm and pores of about 200 nm in diameter is used as the template. After deposition of carbon materials through the pores, carbon nanofibers remain after removal of the template.
Vapor-grown carbon nanofibers are within the class of materials termed multi-walled carbon nanotubes (MWCNTs), and are produced by the floating catalyst method. Vapor-grown carbon nanofibers possess a unique morphology.
A transmission electron microscopy (TEM) image of nanofiber produced by hydrogen gas arc discharge shows an extremely narrow channel in the center. The diameter of the innermost layer is only about 1 nm.
Porous carbon nanofibers
Porous carbon nanofibers can be used as anode materials in lithium-ion batteries. The process involves the electro spinning of polyacrylonitrile (PAN) solutions in N, N-dimethylformamide (DMF), which contain silica nanoparticles. The electro spun composite nanofibers is subsequently carbonized at 700 C. Finally, the silica nanoparticles are removed using hydrofluoric acid, thereby resulting in nanoporous structures.
Floating catalyst technique
In the floating catalyst technique, metal catalysts are introduced in a continuous way by the upper end of a reaction chamber.
The catalysts descend through the furnace (temperature: 1050 - 1100ºC) and the hydrocarbons used are decomposed on the surface of the catalysts growing and thickening the nanofibres. In this process catalytic particles can be elements of group VIII of the periodic chart like iron, nickel or cobalt, or alloys of them. Carbon source may be hydrocarbons like benzene, n-hexane, methane and acetylene.
Several companies around the globe are actively involved in the commercial scale production of carbon nanofibers and new engineering applications are being developed for these materials intensively, the latest being a carbon nanofiber bearing porous composite for oil spill remediation