Carbon nanofibers hold promise for technologies ranging from medical imaging devices to precise scientific measurement tools, but the time and expense associated with uniformly creating nanofibers of the correct size has been an obstacle. Vapor-grown carbon nanofibers are within the class of multi-walled carbon nanotubes (MWCNTs), and are produced by the floating catalyst method. 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.
Researchers have shown that nickel nanoparticles coated with a ligand shell can be used to grow carbon nanofibers that are uniform in diameter. Using nanoparticles to grow nanofibers is useful, because the fibers tend to have the same diameter as the nanoparticles they are growing from and allow to define where the nanofibers grow and how they are arranged by arranged in that pattern before growing the fibers.
Manufacturing technologies
Carbon nanofibres can be manufactured by the following CVD techniques:
1. Growing on metal catalysts seeded on a substrate.
2. Floating catalyst technique, and
3. Carbon Nanofibres growing on a substrate: The process is non continuous and needs separation from the substrate.
Floating catalyst technique
In the floating catalyst technique, metal catalysts are introduced in a continuous way by the upper end of a reaction chamber kept at a temperature of 1050 - 1100ºC. The catalysts descend through the furnace and the hydrocarbons used are decomposed on the surface of the catalysts growing and thickening the nanofibres. Catalytic particles used are elements of group VIII of the periodic chart like iron, nickel or cobalt, or alloys of them. The carbon sources are hydrocarbons like benzene, n-hexane, methane and acetylene.
Continuous fabrication
Researchers at the University of Illinois have developed continuous fabrication process of complex, three-dimensional nanoscale structures to grow individual nanowires of unlimited length. To draw longer nanowires, the researchers developed a precision spinning process that simultaneously draws and winds a nanofiber on a spool that is of millimeters in diameter.
Based on the rapid evaporation of solvent from simple "inks," another process has been used to fabricate freestanding nanofibers, stacked arrays of nanofibers and continuously wound spools of nanowires.
Making nanofiber anode
The nanofibers can be created by dispersing nickel nanoparticles consistently on a fused silicon substrate covered with a fine chromium grid which acts as an electrode. The set up is then kept in a chamber at 700°C, and later filled with ammonia and acetylene gas. The chromium grid acts as a negatively charged electrode, and the upper part of the chamber comprises a positively charged electrode.
For preparing porous carbon nanofibers used as anode materials in lithium-ion batteries, the procedure involves electro spinning of polyacrylonitrile (PAN) solutions in N, N-dimethylformamide (DMF) which contains silica nanoparticles. The electro spun composite nanofibers are subsequently carbonized at 700° C. Finally, the silica nanoparticles are removed using hydrofluoric acid, thereby resulting in nanoporous structures.
Applications
The commercial nanofibers are 10 – 80 nm thick Multi-Wall NanoTube (MWNT) carbon structures with less than 1% (mass) of Ni. These have excellent mechanical properties, high electrical conductivity, and high thermal conductivity which can be imparted to a wide range of matrices including thermoplastics, thermo sets, 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 are available in a free-flowing powder form (typically 99% mass is in a fibrous form).
Potential applications include electronic interconnects, biocompatible scaffolds and nanofluidic networks. The industrial applications of this material include: polymer and elastomer fillers, lightweight bulletproof uniforms, 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.
The carbon nanofibers in thermoplastic materials have fire retardant properties. Composites loaded with carbon nanofibers and exposed to a flame exhibit delayed and lower peak heat release rates, lower smoke emissions, and no dripping or pooling of molten polymer.
The nanofiber coatings are used for applications in water repellent coatings, solar cells, biomedical research tools, and many others.
The carbon nanofiber-filled coatings devised by researchers from the National Institute of Standards and Technology (NIST) and Texas A&M University outperformed conventional flame retardants used in the polyurethane foam of upholstered furniture and mattresses by at least 160 percent and perhaps by as much as 1,130 percent.
Among many possibilities, nanofiber applications are as follows:
• Soft protective vests stronger than Kevlar
• Bandages that can contract to put pressure on
• Artificial muscles powered by electricity - much lighter than current hydraulics
• making easier to incorporation of electronic sensors and actuators into clothing
Sector wise nanofiber applications are listed below:
Automotive sector
Thermoplastic reinforcement (glass fibre replacement), under hood components subjected to mechanical efforts at high temperatures (thermal stability). Fuel system, Paintable parts, Exterior panels, Embedded Electronics.
Electronic sector
Packaging, materials for ESD, sensitive items, Hard Disk Drive manufacturing, EMI shielding, Semiconductor, manufacturing, Clean room Equipment.
Energy sector
Bipolar and end plates, Electrode catalyst, support in PEM fuel cells, Hydrogen Storage
Aerospace sector
High Performance, Conductive Adhesives, Microelectronics, Sensors, Enhanced Thermal Management, Broadband EMI Shielding, Nanocomposite, Rocket Ablative Materials, Conductive Coatings and
Paints, Light Weight Antennas and Ground Planes.
Chemistry sector
Catalyst Support.
Conductive plastic resins filled with graphite fibril nanofibers can be used in molded clean room tools such as wafer trays, bar code scanners, and tweezers. The resins produce a non sloughing, glossy surface with low out gassing levels.
In electronics, polycarbonate and polyetherimide (GE’s Ultem) components of computer hard drives have been reinforced with nanotubes to render them conductive and very smooth
Other applications are:
ESD Control of Isolated Metallic Standoffs
Electrically Bonded Structural Joints for
Inter Panel Power/Signal Return Currents
Electrically Conductive NanoAdhesive Advantages
Improved ESD of metallic components w/o added adhesive bead
Elimination of inter panel jumper cabling
Nanofiber coatings
North Carolina State University researchers have identified a novel technique to grow straight carbon nanofibers atop a transparent substrate. The technique uses a grid made of charged chromium and with the help of ions ensures that the developed nanofibers do not get curled because curling of the nanofibers will hinder its usage.
In particular, genetic material can be coated on the nanofibers and introduced into the cell’s nucleus, for instance, to enhance the research of gene therapy. Scientists can pass light through the transparent substrate, which results in better contrast and better visibility of the process taking place.
Hydrogen storage
Carbon nanofibers used for hydrogen storage consist of coil like fibers made up of very small graphite sheets that are stacked in specific configurations and separated by distances of 0.335 – 0.342 nm.
Carbon nanofibers are grown from the decomposition of carbon-containing gases such as hydrocarbons over metal or alloy surfaces which act as catalysts to the sheets’ formation. During the reaction, the carbon-containing gas molecules are adsorbed to certain faces of the catalyst’s surface and are subsequently decomposed. Following this, the carbon atoms diffuse through the catalyst particle and precipitate at one or more other surfaces and form successive sheets that stack on one another to form the carbon nanofibers.
When carbon nanofibers are placed in a vessel and exposed to hydrogen under pressures of 120-130 atm at room temperature, the hydrogen slips between the graphite sheets of the carbon nanofibers and adsorbs to surface of the carbon layers. To prepare the carbon nanofibers for hydrogen adsorption, they are carefully pretreated to remove any metal impurities and chemisorbed gases that may be present.
Carbon Nanofibers scaffold for heart
Engineers at Brown University and the India Institute of Technology in India have created a carbon nanofiber based scaffold that promotes regeneration of heart tissue. The researchers developed a mesh by stitching the carbon nanofibers together using a PLA polymer. The resulting mesh is also elastic, durable and enables the product to expand and contract with the heart tissue.