Carbon nanofiber
Encyclopedia
Carbon nanofibers, vapor grown carbon fibers (VGCFs), or vapor grown carbon nanofibers (VGCNFs) are cylindric nanostructures with graphene
layers arranged as stacked cone
s, cups or plates. Carbon nanofibers with graphene layers wrapped into perfect cylinder
s are called carbon nanotube
s.
is the building block of a myriad of organic and inorganic matter around us. It is a versatile atom capable of joining to other atoms in sp, sp2, and sp3 hybridized structures giving rise to millions of stable molecules. In its single element form, it has a number of allotropes (polymorphs) like diamond
, graphite
, and fullerene
s with different properties ranging from extremely hard to very soft scope. Carbon can be made to form tubular microstructure called filament or fiber. The unique properties of carbon fibers have expanded the science and technology of composite material
s in recent decades.
VGCFs and their smaller size variant, VGCNFs are among short carbon fibers that have drawn lots of attention for their potential thermal, electrical, frequency shielding, and mechanical property enhancements . They are being more and more utilized in different material systems like composites thanks to their exceptional properties and low cost.
(CVD) with variants like thermal and plasma-assisted is the dominant commercial technique for the fabrication of VGCF and VGCNF. Here, gas-phase molecules are decomposed at high temperatures and carbon
is deposited in the presence of a transition metal
catalyst on a substrate where subsequent growth of the fiber around the catalyst particles is realized. In general, this process involves separate stages such as gas decomposition, carbon deposition, fiber growth, fiber thickening, graphitization, and purification and results in hollow fibers. The nanofiber diameter depends on the catalyst size. The CVD process for the fabrication of VGCF generally falls into two categories : 1) fixed-catalyst process (batch), and 2) floating-catalyst process (continuous).
In the batch process developed by Tibbetts , a mixture of hydrocarbon/hydrogen/helium was passed over a mullite
(crystalline aluminum silicate) with fine iron catalyst particle deposits maintained at 1000⁰C. The hydrocarbon
used was methane
in the concentration of 15% by volume. Fiber growth in several centimeters was achieved in just 10 minutes with a gas residence time of 20 seconds. In general, fiber length can be controlled by the gas residence time in the reactor. Gravity and direction of the gas flow typically affects the direction of the fiber growth .
The continuous or floating-catalyst process was patented earlier by Koyama and Endo and was later modified by Hatano and coworkers . This process typically yields VGCF with submicron diameters and lengths of a few to 100 microns, which accords with the definition of carbon nanofibers. They utilized organometallic compounds dissolved in a volatile solvent like benzene
that would yield a mixture of ultrafine catalyst particles (5-25 nm in diameter) in hydrocarbon gas as the temperature rose to 1100⁰C. In the furnace, the fiber growth initiates on the surface of the catalyst particles and continues until catalyst poisoning occurs by impurities in the system. In the fiber growth mechanism described by Baker and coworkers , only the part of catalyst particle exposed to the gas mixture contributes to the fiber growth and the growth stops as soon as the exposed part is covered, i.e. the catalyst is poisoned. The catalyst particle remains buried in the growth tip of the fiber at a final concentration of about a few parts per million. At this stage, fiber thickening takes place.
The most commonly used catalyst is iron
, often treated with sulfur
, hydrogen sulfide
, etc. to lower the melting point
and facilitate its penetration into the pores of carbon and hence, to produce more growth sites . Fe/Ni, Ni, Co, Mn, Cu, V, Cr, Mo and Pd are also used as catalyst . Acetylene
, ethylene
, methane
, natural gas
, and benzene
are the most commonly used carbonaceous gases. Often carbon monoxide
(CO) is introduced in the gas flow to increase the carbon yield through reduction of possible iron oxides in the system.
and subsequent carbon deposition and filament growth. The true appreciation of these fibers, however, came much later when their structure could be analyzed by electron microscope
. The first electron microscopy observations of carbon nanofibers were performed in the early 1950s by the Soviet scientists Radushkevich and Lukyanovich, who published a paper in the Soviet Journal of Physical Chemistry showing hollow graphitic carbon fibers that are 50 nanometers in diameter . Early in the 1970s, Japanese researchers Koyama and Endo succeeded in the manufacturing of VGCF with a diameter of 1 µm and length of above 1 mm. Later, in the early 1980s, Tibbetts in the USA and Benissad in France continued to perfect the VGCF fabrication process. In the USA, the deeper studies focusing on synthesis and properties of these materials for advanced applications were led by R. Terry K. Baker and were motivated by the need to inhibit the growth of carbon nanofibers because of the persistent problems caused by accumulation of the material in a variety of commercial processes especially in the particular field of petroleum processing. The first commercialization of VGCF was attempted by the Japanese company Nikosso in 1991 under the trade name Grasker® , the same year Sumio Iijima
published his famous paper introducing the discovery of Carbon Nanotube
s (CNTs) to the world. VGCNF is produced through essentially the same manufacturing process as VGCF, only the diameter is typically less than 200 nm. 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 .
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
layers arranged as stacked cone
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
s, cups or plates. Carbon nanofibers with graphene layers wrapped into perfect cylinder
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...
s are called carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s.
Introduction
CarbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
is the building block of a myriad of organic and inorganic matter around us. It is a versatile atom capable of joining to other atoms in sp, sp2, and sp3 hybridized structures giving rise to millions of stable molecules. In its single element form, it has a number of allotropes (polymorphs) like diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
, graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
, and fullerene
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
s with different properties ranging from extremely hard to very soft scope. Carbon can be made to form tubular microstructure called filament or fiber. The unique properties of carbon fibers have expanded the science and technology of composite material
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
s in recent decades.
VGCFs and their smaller size variant, VGCNFs are among short carbon fibers that have drawn lots of attention for their potential thermal, electrical, frequency shielding, and mechanical property enhancements . They are being more and more utilized in different material systems like composites thanks to their exceptional properties and low cost.
Synthesis
Catalytic Chemical Vapor Deposition (CCVD) or simply Chemical Vapor DepositionChemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
(CVD) with variants like thermal and plasma-assisted is the dominant commercial technique for the fabrication of VGCF and VGCNF. Here, gas-phase molecules are decomposed at high temperatures and carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
is deposited in the presence of a transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
catalyst on a substrate where subsequent growth of the fiber around the catalyst particles is realized. In general, this process involves separate stages such as gas decomposition, carbon deposition, fiber growth, fiber thickening, graphitization, and purification and results in hollow fibers. The nanofiber diameter depends on the catalyst size. The CVD process for the fabrication of VGCF generally falls into two categories : 1) fixed-catalyst process (batch), and 2) floating-catalyst process (continuous).
In the batch process developed by Tibbetts , a mixture of hydrocarbon/hydrogen/helium was passed over a mullite
Mullite
Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge balancing cations present...
(crystalline aluminum silicate) with fine iron catalyst particle deposits maintained at 1000⁰C. The hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
used was methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
in the concentration of 15% by volume. Fiber growth in several centimeters was achieved in just 10 minutes with a gas residence time of 20 seconds. In general, fiber length can be controlled by the gas residence time in the reactor. Gravity and direction of the gas flow typically affects the direction of the fiber growth .
The continuous or floating-catalyst process was patented earlier by Koyama and Endo and was later modified by Hatano and coworkers . This process typically yields VGCF with submicron diameters and lengths of a few to 100 microns, which accords with the definition of carbon nanofibers. They utilized organometallic compounds dissolved in a volatile solvent like benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
that would yield a mixture of ultrafine catalyst particles (5-25 nm in diameter) in hydrocarbon gas as the temperature rose to 1100⁰C. In the furnace, the fiber growth initiates on the surface of the catalyst particles and continues until catalyst poisoning occurs by impurities in the system. In the fiber growth mechanism described by Baker and coworkers , only the part of catalyst particle exposed to the gas mixture contributes to the fiber growth and the growth stops as soon as the exposed part is covered, i.e. the catalyst is poisoned. The catalyst particle remains buried in the growth tip of the fiber at a final concentration of about a few parts per million. At this stage, fiber thickening takes place.
The most commonly used catalyst is iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, often treated with sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, etc. to lower the melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
and facilitate its penetration into the pores of carbon and hence, to produce more growth sites . Fe/Ni, Ni, Co, Mn, Cu, V, Cr, Mo and Pd are also used as catalyst . Acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
, ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
, and benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
are the most commonly used carbonaceous gases. Often carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO) is introduced in the gas flow to increase the carbon yield through reduction of possible iron oxides in the system.
Applications
- Field electron emission sources
- Composite materials
- Scanning probe microscopyScanning probe microscopyScanning Probe Microscopy is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. An image of the surface is obtained by mechanically moving the probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a...
tips - Carrier material for various catalysts in petrochemistryPetrochemistryPetrochemistry is a branch of chemistry that studies the transformation of crude oil and natural gas into useful products or raw materials. These petrochemicals have become an essential part of the chemical industry today.-Origin of Petroleum:...
- In vertically-aligned arrays, a platform for gene deliveryGene deliveryGene delivery is the process of introducing foreign DNA into host cells. Gene delivery is, for example, one of the steps necessary for gene therapy and the genetic modification of crops. There are many different methods of gene delivery developed for a various types of cells and tissues, from...
. (See ImpalefectionImpalefectionImpalefection is a method of gene delivery using nanomaterials, such as carbon nanofibers, carbon nanotubes, nanowires Ref.1. Needle-like nanostructures are synthesized perpendicular to the surface of a substrate. Plasmid DNA containing the gene, intended for intracellular delivery, is attached to...
) - For electrode materials
- Oil spill remediation
History
One of the first technical records concerning carbon nanofibers is probably a patent dated 1889 on synthesis of filamentous carbon by Hughes and Chambers. They utilized a methane/hydrogen gaseous mixture and grew carbon filaments through gas pyrolysisPyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
and subsequent carbon deposition and filament growth. The true appreciation of these fibers, however, came much later when their structure could be analyzed by electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
. The first electron microscopy observations of carbon nanofibers were performed in the early 1950s by the Soviet scientists Radushkevich and Lukyanovich, who published a paper in the Soviet Journal of Physical Chemistry showing hollow graphitic carbon fibers that are 50 nanometers in diameter . Early in the 1970s, Japanese researchers Koyama and Endo succeeded in the manufacturing of VGCF with a diameter of 1 µm and length of above 1 mm. Later, in the early 1980s, Tibbetts in the USA and Benissad in France continued to perfect the VGCF fabrication process. In the USA, the deeper studies focusing on synthesis and properties of these materials for advanced applications were led by R. Terry K. Baker and were motivated by the need to inhibit the growth of carbon nanofibers because of the persistent problems caused by accumulation of the material in a variety of commercial processes especially in the particular field of petroleum processing. The first commercialization of VGCF was attempted by the Japanese company Nikosso in 1991 under the trade name Grasker® , the same year Sumio Iijima
Sumio Iijima
Sumio Iijima is a Japanese physicist, often cited as the discoverer of carbon nanotubes. Although carbon nanotubes had been observed prior to his "discovery", Iijima's 1991 paper generated unprecedented interest in the carbon nanostructures and has since fueled intense research in the area of...
published his famous paper introducing the discovery of Carbon Nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s (CNTs) to the world. VGCNF is produced through essentially the same manufacturing process as VGCF, only the diameter is typically less than 200 nm. 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 .
See also
- Allotropes of carbonAllotropes of carbonThis is a list of the allotropes of carbon.-Diamond:Diamond is one of the most well known allotropes of carbon. The hardness and high dispersion of light of diamond make it useful for both industrial applications and jewellery. Diamond is the hardest known natural mineral. This makes it an...
- Carbon nanotubes
- Carbon blackCarbon blackCarbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...
- Carbon nanoconeCarbon nanoconeCarbon nanocones are conical structures which are made predominantly from carbon and which have at least one dimension of the order one micrometer or smaller. Nanocones have height and base diameter of the same order of magnitude; this distinguishes them from tipped nanowires which are much longer...
- Carbon fiberCarbon fiberCarbon fiber, alternatively graphite fiber, carbon graphite or CF, is a material consisting of fibers about 5–10 μm in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in crystals that are more or less aligned parallel to the long axis of the fiber...