Carbon nanofibers
Carbon
nanofibers are steam-grown carbon fibers or steam-enhanced carbon nanofibers cylindrical nanostructures that are arranged as conifers, cups, or plates
decorated with graphene layers. Graphene-coated carbon nanofibers wrapped in a perfect cylinder are called carbon nanotubes.
History of carbon nanofibers
One
of the first technical records was probably patented by Hughes and Chambers in
the 1889 filamentous carbon synthesis. They use a methane/hydrogen gaseous
mixture and increase the carbon filament through gas pyrolysis and subsequent
carbon accumulation and filament growth. The actual realization of these fibers
came much later when their structure could be analyzed by electronic
microscopy. The first electronic microscopy observations of carbon nanofibers
were performed in the early 1950s by Soviet scientists Raduskevich and
Lukyanovich, who published a paper in the Soviet Journal of Physical Chemistry
stating that 50-nanometer blank graphic carbon fibers.
In
1991, Japanese researcher Sumio Ijima synthesized hollow carbon molecules to
determine their crystal structure. In the following years, these molecules were
called carbon nanotubes for the first time. VGCNF is produced through basically
the same manufacturing process as VGCF, only the diameter is usually less than
200 nm. A number of companies worldwide are actively involved in the
commercial-scale production of carbon nanofibers and new engineering
applications for these materials are being intensively developed.
Properties carbon nanofibers
1.
Carbon nanofibers have a high level of chemical bonding power.
2.
It has a number of allotropes including diamond, graphite, and fullerenes.
3.
This underscores the versatility of CNFs, which is notable for their thermal,
electrical, and electromagnetic protection and an increase in mechanical
properties.
4.
Since carbon is readily available at a low cost, CNFs are a popular addition to
composite materials.
5.
CNFs exist on very small, nanometer scales.
6.
Carbon nanofibers (CNFs) are clearly used as anode materials for the production
of numerous interstitial sites, they show great performance in sodium and
lithium storage.
7.
Carbon nanofibers (CNF) are produced catalytically in the CVD process.
8.
Diameter ranging between 150–300 nm.
The manufacturing process of Carbon nanofibers
Catalytic
chemical vapor deposition (CCVD) or only CVDs with thermal and plasma-assisted
variants are effective commercial strategies for the manufacture of VGCF and
VGCNF. Here, the gas-phase molecules decompose at high temperatures, and the
carbon is deposited in a layer in the presence of a metallic catalyst where the
subsequent growth of the fibers around the catalyst particles is observed. In
general, this process involves the results of individual stages such as gas
decomposition, carbon deposition, fiber growth, fiber density, graphitization
and purification, and hollow fibers. The nanofiber diameter depends on the
catalyst size. The CVD process for VGCF fabrication usually falls into two
categories: a. a static-catalytic mechanism, and b. Floating-catalytic
mechanism.
In
the batch process developed by the Tibetans, the hydrocarbon/hydrogen/helium
mixture passes over the mullite by depositing iron catalyst particles at 1000
°C. Methane was 15% concentrated according to the hydrocarbon volume used. With
a gas bus time of 20 seconds, the fiber growth is found to be several
centimeters at 10 centimeters. In general, the length of the fiber can be
controlled by the time of residence of the gas in the furnace. The direction of
gravity and gas flow usually affects the direction of fiber growth.
The
continuous or floating-catalytic process Koyama and Endo was first patented and
later modified by Hatano and colleagues. This process develops VGCF with
sub-micrometer diameters and lengths of a few to 100 mm that comply with the
definition of carbon nanofibers. They use orthometallic compounds dissolved in
volatile solvents such as benzene that obtain a mixture of ultrafine catalyst
particles of hydrocarbon gas with a temperature of 1100 °C. In the furnace, the
development of fibers begins on the surface of the catalyst particles and
continues until the catalyst is poisoned by the impurities of the system. In
the fiber growth system described by Bucker and colleagues, only a fraction of
the catalytic particles released into the gas mixture contribute to the
development of the fiber, i.e. the catalyst is poisoned. Catalyst particles are
buried at the final end of a few parts per million at the growing tip of the
fiber. At this stage, the fiber is dense.
Advantages of carbon nanofibers
1.
Carbon nanofibers are used to deliver therapeutic drugs and developed an
elastic material that is embedded with a needle.
2.
It works as a carrier material for various catalysts in petrochemistry.
3.
Field electron emission sources.
4. The unique structure of these porous carbon nanofibers has resulted in good
electrochemical performance and they are used as anodes in rechargeable
lithium-ion batteries.
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