Bewise Inc. www.tool-tool.com Reference source from the internet. Silicon carbide (
SiC), also known as
carborundum, is a
compound of
silicon and
carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral
moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an
abrasive. Grains of silicon carbide can be bonded together by
sintering to form very hard
ceramics which are widely used in applications requiring high endurance, such as car brakes and
ceramic plates in
bulletproof vests. Electronic applications of silicon carbide as
light emitting diodes and
detectorsin early radios were first demonstrated around 1907, and nowadays SiC
is widely used in high-temperature/high-voltage semiconductor
electronics. Large single crystals of silicon carbide can be grown by
the Lely method; they can be cut into gems known as "synthetic
moissanite". Silicon carbide with high surface area can be produced from
SiO
2 contained in plant material.
Early,
non-systematic and often non-recognized syntheses of silicon carbide had
been reported by Despretz (1849), Marsden (1880) and Colson
(1882).Wide-scale production is credited to
Edward Goodrich Achesonaround 1893. He patented the method for making silicon carbide powder
on February 28, 1893.Acheson also developed the electric batch
furnaceby which SiC is still made today and formed The Carborundum Company to
manufacture bulk SiC, initially for use as an abrasive.In 1900 the
company settled with the
Electric Smelting and Aluminum Companywhen a judge's decision gave "priority broadly" to its founders "for
reducing ores and other substances by the incandescent method"It is said
that Acheson was trying to dissolve carbon in molten
corundum (
alumina)
and discovered the presence of hard, blue-black crystals which he
believed to be a compound of carbon and corundum: hence carborundum. Or,
he named the material "carborundum" by analogy to corundum, which is
another very hard substance (9 on the
Mohs scale).
Historically,
the first use of SiC was as an abrasive. This was followed by
electronic applications. In the beginning of the 20th century, silicon
carbide was used as a detector in the first radios,and in 1907
Henry Joseph Roundproduced the first LED by applying a voltage to a SiC crystal and
observing yellow, green and orange emission at the cathode. Those
experiments were later repeated by
O. V. Losev in the
Soviet Union in 1923.
Naturally occurring moissanite is found only in minute quantities in certain types of meteorite and in corundum deposits and
kimberlite. Virtually all the silicon carbide sold in the world, including moissanite jewels, is
synthetic. Natural moissanite was first found in 1893 as a small component of the
Canyon Diablo meteorite in
Arizona by Dr.
Ferdinand Henri Moissan,
after whom the material was named in 1905. Moissan's discovery of
naturally occurring SiC was initially disputed because his sample may
have been contaminated by silicon carbide
saw blades that were already on the market at that time.
While
rare on Earth, silicon carbide is remarkably common in space. It is a
common form of stardust found around carbon-rich stars, and examples of
this stardust have been found in pristine condition in primitive
(unaltered) meteorites. The silicon carbide found in space and in
meteorites is almost exclusively the beta-polymorph. Analysis of SiC
grains found in the Murchison carbonaceous chondrite meteorite has
revealed anomalous isotopic ratios of carbon and silicon, indicating an
origin from outside the solar system; 99% of these SiC grains originate
around carbon-rich
asymptotic giant branch stars.SiC is commonly found around these stars as deduced from their infrared spectra.
Due
to the rarity of natural moissanite, silicon carbide is typically
man-made. Most often it is used as an abrasive, and more recently as a
semiconductor and
diamond simulant of gem quality. The simplest manufacturing process is to combine
silica sand and carbon in an Acheson graphite electric resistance furnace at a high temperature, between 1600 and 2500 °C. Fine SiO
2 particles in plant material (e.g. rice husks) can be converted to SiC by heating in the excess carbon from the organic material
[13].
Synthetic SiC crystals ~3 mm in diameter
The material formed in the Acheson furnace varies in purity, according to its distance from the
graphite resistor heat source.
Colorless, pale yellow and green crystals have the highest purity and
are found closest to the resistor. The color changes to blue and black
at greater distance from the resistor, and these darker crystals are
less pure. Nitrogen and aluminium are common impurities, and they affect
the electrical conductivity of SiC.
Pure silicon carbide can be made by the so-called Lely process, in which SiC powder is sublimated in
argon atmosphere at 2500 °C and redeposited into flake-like single crystals, sized up to 2x2 cm
2,
at a slightly colder substrate. This process yields high-quality single
crystals, mostly of 6H-SiC phase (because of high growth temperature).
Cubic SiC is usually grown by the more expensive process of
chemical vapor deposition (CVD).Pure silicon carbide can also be prepared by the
thermal decomposition of a polymer,
poly(methylsilyne), under an
inert atmosphereat low temperatures. Relative to the CVD process, the pyrolysis method
is advantageous because the polymer can be formed into various shapes
prior to thermalization into the ceramic
(β)3C-SiC
4H-SiC
(α)6H-SiC
Silicon
carbide exists in about 250 crystalline forms. The polymorphism of SiC
is characterized by a large family of similar crystalline structures
called polytypes. They are variations of the same chemical compound that
are identical in two dimensions and differ in the third. Thus, they can
be viewed as layers stacked in a certain sequence.
Alpha silicon carbide (α-SiC) is the most commonly encountered
polymorph; it is formed at temperatures greater than 1700 °C and has a
hexagonal crystal structure (similar to
Wurtzite). The beta modification (β-SiC), with a
zinc blende crystal structure (similar to
diamond),
is formed at temperatures below 1700 °C. Until recently, the beta form
has had relatively few commercial uses, although there is now increasing
interest in its use as a support for heterogeneous catalysts, owing to
its higher surface area compared to the alpha form.
Properties of major SiC polytypes
Polytype
3C (β)
4H
6H (α)
Crystal structure
Zinc blende (cubic)
Hexagonal
Hexagonal
Space group
T
2d-F43m
C
46v-P6
3mc
C
46v-P6
3mc
Pearson symbolcF8
hP8
hP12
Lattice constants (Å)
4.3596
3.0730; 10.053
3.0730; 15.11
Density (g/cm
3)
3.21
3.21
3.21
Bandgap (eV)
2.36
3.23
3.05
Bulk modulus (GPa)
250
220
220
Thermal conductivity (W/(cm·K))
3.6
3.7
4.9
Pure SiC is colorless. The brown to black color of industrial product results from
iron impurities. The rainbow-like luster of the crystals is caused by a
passivation layer of
silicon dioxide that forms on the surface.
The high sublimation temperature of SiC (approximately 2700 °C) makes it useful for
bearingsand furnace parts. Silicon carbide does not melt at any known pressure.
It is also highly inert chemically. There is currently much interest in
its use as a
semiconductor material in electronics, where its high
thermal conductivity, high
electric field breakdown strength and high maximum
current density make it more promising than silicon for high-powered devices.
[23] SiC also has a very low
coefficient of thermal expansion (4.0 × 10
−6/K) and experiences no
phase transitions that would cause discontinuities in thermal expansion.
Silicon carbide is a semiconductor, which can be doped n-type by
nitrogen or
phosphorus and p-type by
aluminium,
boron,
gallium or
beryllium.
Metallic conductivity has been achieved by heavy doping with boron,
aluminium or nitrogen. Superconductivity has been detected in 3C-SiC:Al,
3C-SiC:B and 6H-SiC:B at the same temperature of 1.5 K. or aluminum.A
crucial difference is however observed for the magnetic field behavior
between aluminum and boron doping: SiC:Al is
type-II, same as Si:B. On the contrary, SiC:B is
type-I.
In attempt to explain this difference, it was noted that Si sites are
more important than carbon sites for superconductivity in SiC. Whereas
boron substitutes carbon in SiC, Al substitutes Si sites. Therefore, Al
and B "see" different environment that might explain different
properties of SiC:Al and SiC:B.
In the arts, silicon carbide is a popular abrasive in modern
lapidary due to the durability and low cost of the material. In manufacturing, it is used for its hardness in
abrasive machining processes such as
grinding,
honing,
water-jet cutting and
sandblasting. Particles of silicon carbide are laminated to paper to create
sandpapers and the grip tape on
skateboards.
In 1982 an exceptionally strong composite of
aluminium oxide and silicon carbide
whiskerswas discovered. Development of this laboratory-produced composite to a
commercial product took only three years. In 1985, the first commercial
cutting tools made from this alumina and silicon carbide
whisker-reinforced composite were introduced by the Advanced Composite
Materials Corporation (ACMC) and Greenleaf Corporation.
Silicon carbide is used for inner plates of ballistic vests
In the 1980s and 1990s, silicon carbide was studied in several research programs for high-temperature gas turbines in
Europe,
Japan and the
United States. The components were intended to replace
nickel superalloy turbineblades or nozzle vanes. However, none of these projects resulted in a
production quantity, mainly because of its low impact resistance and its
low fracture
toughness.
Like other hard ceramics (namely alumina and
boron carbide), silicon carbide is used in
composite armor (e.g.,
Chobham armor), and in ceramic plates in bulletproof vests.
Dragon Skin, which is produced by
Pinnacle Armor, uses disks of silicon carbide.
The Porsche Carrera GT's carbon-ceramic (silicon carbide) disc brake
Silicon-infiltrated
carbon-carbon composite is used for high performance "ceramic"
brake discsas it is able to withstand extreme temperatures. The silicon reacts
with the graphite in the carbon-carbon composite to become carbon fiber
reinforced silicon carbide (C/SiC). These discs are used on some road
going sports cars, including the
Porsche Carrera GT, the
Bugatti Veyron, the Chevrolet Corvette ZR1,
Bentleys,
Ferraris,
Lamborghinis, and some specific high performance
Audis.Silicon carbide is also used in a
sintered form for
diesel particulate filters.
The earliest electrical application of SiC was in
lightning arresters in electric power systems. These devices must exhibit high
resistance until the
voltage across them reaches a certain threshold
VT, at which point their resistance must drop to a lower level and maintain this level until the applied voltage drops below
VT.
It
was recognized early on that SiC had such a voltage-dependent
resistance, and so columns of SiC pellets were connected between
high-voltage
power lines and the earth. When a
lightning striketo the line raises the line voltage sufficiently, the SiC column will
conduct, allowing strike current to pass harmlessly to the earth instead
of along the power line. Such SiC columns proved to conduct
significantly at normal power-line operating voltages and thus had to be
placed
in series with a
spark gap. This spark gap is
ionizedand rendered conductive when lightning raises the voltage of the power
line conductor, thus effectively connecting the SiC column between the
power conductor and the earth. Spark gaps used in lightning arresters
are unreliable, either failing to strike an arc when needed or failing
to turn off afterwards, in the latter case due to material failure or
contamination by dust or salt. Usage of SiC columns was originally
intended to eliminate the need for the spark gap in a lightning
arrester. Gapped SiC lightning arresters were used as
lightning-protection tool and sold under
GE and
Westinghouse brand names, among others. The gapped SiC arrester has been largely displaced by no-gap
varistors that use columns of
zinc oxide pellets.
Ultraviolet LED
Silicon carbide is used for ultrafast, high-voltage
Schottky diodes,
MOSFETs and high temperature
thyristorsfor high-power switching.A major problem with SiC crystals is the
formation of different types of defect like edge dislocation, screw
dislocation (both hollow and closed core), triagular defect and basal
plane dislocation.As a result, devices made of SiC crystals show poor
breakdown performance though researchers have been tentatively finding
solutions to improving the breakdown performance.Apart from crystal
quality, problems with the interface of SiC with silicon dioxide have
hampered the development of SiC-based power MOSFETs and
insulated-gate bipolar transistors.
The
history of SiC LEDs is quite remarkable: the first LED action was
demonstrated in 1907 using SiC and the first commercial LEDs were again
based on SiC. Yellow LEDs made from 3C-SiC were manufactured in the
Soviet Union in the 1970s,and blue ones (6H-SiC) worldwide in the
1980s.The production was soon stopped because
gallium nitride showed 10-100 times brighter emission. This difference in efficiency is due to the unfavorable
indirect bandgap of SiC whereas GaN has a
direct bandgapwhich favors light emission. However, SiC is still one of the important
LED components – it is a popular substrate for growing GaN devices, and
it also serves as a heat spreader in high-power LEDs.
The low thermal expansion coefficient, high hardness, rigidity and thermal conductivity make silicon carbide a desirable
mirror material for
astronomicaltelescopes. The growth technology (chemical vapor deposition) has been
scaled up to produce disks of polycrystalline silicon carbide up to 3.5
meters in diameter, and several telescopes (like the
Herschel Space Telescope) are already equipped with SiC optics.
Main article:
Thin filament pyrometry 
Image of the test flame and glowing SiC fibers. The flame is about 7 cm tall.
Silicon carbide fibers are used to measure gas temperatures in an optical technique called
thin filament pyrometry.
It involves the placement of a thin filament in a hot gas stream.
Radiative emissions from the filament can be correlated with filament
temperature. Filaments are SiC fibers with a diameter of 15 micrometers,
that is about 5 times thinner than human hair. Because the fibers are
so thin, they do little to disturb the flame and their temperature
remains close to that of the local gas. Temperatures of about 800–2500 K
can be measured.
References to silicon carbide heating elements
exist from the early 20th century when they were produced by Acheson's
Carborundum Co. in the U.S. and EKL in Berlin. Silicon carbide offered
increased
operating temperatures compared with metallic heaters. Silicon carbide elements are used today in the melting of non-ferrous metals and glasses,
heat treatment of metals,
float glass production, production of ceramics and electronics components, igniters in
pilot lights for gas heaters, etc.
Silicon carbide is often used as a layer of the
tristructural-isotropic coating for the
nuclear fuel elements of
high temperature gas cooled reactors or
very high temperature reactors such as the
Pebble Bed Reactor.
Silicon carbide provides the mechanical stability to the fuel and is
the main diffusion barrier to the release of fission products.
A moissanite ring
As a
gemstone used in
jewelry, silicon carbide is called "synthetic moissanite" or just "moissanite" after the mineral name. Moissanite is similar to
diamond in several important respects: it is transparent and hard (9-9.5) on the
Mohs scale (compared to 10 for diamond), with a
refractive index between 2.65 and 2.69 (compared to 2.42 for diamond). Moissanite is somewhat harder than common
cubic zirconia. Unlike diamond, moissanite can be strongly
birefringent.
This quality is desirable in some optical applications, but not in
gemstones. For this reason, moissanite jewels are cut along the
optic axis of the crystal to minimize birefringent effects. It is lighter (density 3.21 g/cm
3 vs. 3.53 g/cm
3), and much more resistant to heat than diamond. This results in a stone of higher
luster,
sharper facets and good resilience. Loose moissanite stones may be
placed directly into wax ring moulds for lost-wax casting; unlike
diamond, which burns at 800 °C, moissanite remains undamaged by
temperatures up to 1800 °C (cf. the 1064 °C melting point of pure
gold).
Moissanite has become popular as a diamond substitute, and may be
misidentified as diamond, since its thermal conductivity is much closer
to that of diamond than any other diamond substitute. Many thermal
diamond testing devices are fooled by moissanite, but the gem can be
distinguished from diamond by its
birefringenceand a very slight green or yellow fluorescence under ultraviolet light.
Some moissanite stones also have curved string-like inclusions, which
diamonds never have.
Piece of silicon carbide used in steel making
Silicon carbide dissolved in a
basic oxygen furnace used for making
steel acts as a
fuel.
It is cleaner than coal, allowing a reduction in scrap output. It can
also be used to raise tap temperatures and adjust the carbon content.
Use of silicon carbide costs less than of
ferrosilicon and carbon combination, produces cleaner steel due to low level of
trace elements, it has a low gas content and it does not lower the temperature of steel.
The
natural resistance to oxidation exhibited by silicon carbide, as well
as the discovery of new ways to synthesize the cubic β-SiC form, with
its larger surface area, has led to significant interest in its use as a
heterogeneous catalyst support. This form has already been employed as a
catalyst support for the oxidation of hydrocarbons, such as n-butane,
to maleic anhydride.
Silicon carbide is used in
carborundum printmaking – a
collagraph printmakingtechnique. Carborundum grit is applied in a paste to the surface of an
aluminium plate. When the paste is dry, ink is applied and trapped in
its granular surface, then wiped from the bare areas of the plate. The
ink plate is then printed onto paper in a rolling-bed press used for
intaglio printmaking. The result is a print of painted marks embossed
into the paper.
Silicon carbide is used to produce
grapheneby graphitization at high temperatures. This is considered as one of
the promising methods to synthesize graphene at large scale for
practical applications.
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