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SEMINAR ON MATERIAL ENGINEERING ROLL NO. 16
Properties processing and application of:o Aluminium oxideo Silicon carbideo Diamond
1) Aluminum oxide (Al2O3)Aluminum oxide is an amphoteric oxide with the chemical
formula Al2O3. It is commonly referred to as alumina (α-alumina), aloxide,
or corundum in its crystalline form, as well as many other names, reflecting
its widespread occurrence in nature and industry. Its most significant use is
in the production of aluminum metal, although it is also used as an abrasive
owing to its hardness and as a refractory material owing to its high melting
point. There is also a cubic γ-alumina with important technical applications.
FIGURE: ALUMINIUM OXIDE ( α-Al 2O3) CRYSTAL STRUCTURE
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PROPERTIES
Aluminium oxide is an electrical insulator but has a relatively high
thermal Conductivity (30 Wm−1K−1[1]) for a ceramic material. In its most
commonly occurring crystalline form, called corundum or α-aluminium
oxide, its hardness makes it suitable for use as an abrasive and as a
component in cutting tools.
Aluminium oxide is responsible for the resistance of metallic
aluminium to weathering. Metallic aluminium is very reactive with
atmospheric oxygen, and a thin passivation layer of alumina (4 nm
thickness) forms on any exposed aluminium surface. This layer protects the
metal from further oxidation. The thickness and properties of this oxide
layer can be enhanced using a process called anodising. A number of alloys,
such as aluminium bronzes, exploit this property by including a proportion
of aluminium in the alloy to enhance corrosion resistance. The alumina
generated by anodising is typically amorphous, but discharge assisted
oxidation processes such as plasma electrolytic oxidation result in a
significant proportion of crystalline alumina in the coating, enhancing it
shardness.
Aluminium oxide is completely insoluble in water. However it is
an amphoteric substance, meaning it can react with both acids and bases,
such as hydrochloric acid and sodium hydroxide.
Al2O3 + 6 HCl → 2 AlCl3 + 3 H2O
Al2O3 + 6 NaOH + 3 H2O → 2 Na3Al(OH)6
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Aluminium oxide was taken off the United States Environmental
Protection Agency's chemicals lists in 1988. Aluminium oxide is on
EPA's Toxics Release Inventory list if it is a fibrous form.
KEY PROPERTIES
Hard, wear-resistant
Excellent dielectric properties from DC to GHz frequencies
Resists strong acid and alkali attack at elevated temperatures
Good thermal conductivity
Excellent size and shape capability
High strength and stiffness
Available in purity ranges from 94%, an easily metallizable composition, to 99.5% for the most demanding high temperature applications.
PROCESSING:
Aluminium hydroxide minerals are the main component of bauxite, the
principal ore of aluminium. A mixture of the minerals comprise bauxite ore,
including gibbsite (Al(OH)3), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)),
along with impurities of iron oxides and hydroxides, quartz and clay minerals.
Bauxites are found in laterites. Bauxite is purified by the Bayer process:
Al2O3 + 3 H2O → 2 Al(OH)3
Except for SiO2, the other components of bauxite do not dissolve in base.
Upon filtering the basic mixture, Fe2O3 is removed. When the Bayer liquor is
cooled, Al(OH)3 precipitates, leaving the silicates in solution. The solid is
then calcined (heated strongly) to give aluminium oxide:
2 Al(OH)3 → Al2O3 + 3 H2O
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The product alumina tends to be multi-phase, i.e., consisting of several
phases of alumina rather than solelycorundum. The production process can
therefore be optimized to produce a tailored product. The type of phases present
affects, for example, the solubility and pore structure of the alumina product
which, in turn, affects the cost of aluminium production and pollution control.[8]
Known as alundum (in fused form) or aloxitein the mining, ceramic,
and materials science communities, alumina finds wide use. Annual world
production of alumina is approximately 45 million tonnes, over 90% of which is
used in the manufacture of aluminium metal. The major uses of specialty
aluminium oxides are in refractories, ceramics, and polishing and abrasive
applications. Large tonnages are also used in the manufacture of zeolites, coating
titania pigments, and as a fire retardant/smoke suppressant.
APPLICATION:
As a filler
Being fairly chemically inert and white, alumina is a favored filler for plastics.
Alumina is a common ingredient in sunscreen and is sometimes present in
cosmetics such as blush, lipstick, and nail polish.
As a catalyst and catalyst support
Alumina catalyses a variety of reactions that are useful industrially. In its largest
scale application, alumina is the catalyst in the Claus process for converting
hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for
dehydration of alcohols to alkenes.
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Alumina serves as a catalyst support for many industrial catalysts, such as those
used in hydrodesulfurization and some Ziegler-Natta polymerizations.Zeolites are
produced from alumina
Gas purification and related absorption applications
Alumina is widely used to remove water from gas streams. Other major
applications are described below.
As an abrasive
Aluminium oxide is used for its hardness and strength. It is widely used as
an abrasive, including as a much less expensive substitute for industrial diamond.
Many types of sandpaper use aluminium oxide crystals. In addition, its low heat
retention and low specific heat make it widely used in grinding operations,
particularly cutoff tools. As the powdery abrasive mineral aloxite, it is a major
component, along with silica, of the cue tip "chalk" used in billiards. Aluminium
oxide powder is used in some CD/DVD polishing and scratch-repair kits. Its
polishing qualities are also behind its use in toothpaste. Alumina can be grown as a
coating on aluminium by anodising or by plasma electrolytic oxidation (see the
"Properties" above). Both its strength and abrasive characteristics originate from
the high hardness (9 on the Mohs scale of mineral hardness) of aluminium oxide.
As an effect pigment
Aluminium oxide flakes are base material for effect pigments. These pigments are
widely used for decorative applications e.g. in the automotive or cosmetic industry.
See main article Alumina effect pigment
As a fiber for composite materials
Alumina has been used in a few experimental and commercial fiber materials for
high-performance applications.
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MISCELLANEOUS USES
Gas laser tubes
Wear pads
Seal rings
High temperature electrical insulators
High voltage insulators
Furnace liner tubes
Thread and wire guides
Electronic substrates
Ballistic armor
Abrasion resistant tube and elbow liners
Thermometry sensors
Laboratory instrument tubes and sample holders
Instrumentation parts for thermal property test machines
Grinding media
2) Silicon carbide
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, car clutches and ceramic
plates in bulletproof vests. Electronic applications of silicon carbide as light
emitting diodes (LEDs) and detectors in early radios were first demonstrated
around 1907, and today 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
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carbide with high surface area can be produced from SiO2 contained in plant
material.
Structure of major SiC polytypes.
(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 Quartzite). 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.
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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 bearings and 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.[29] SiC also has a very
low coefficient of thermal expansion (4.0 × 10−6/K) and experiences no phase that
would cause discontinuities in thermal expansion.
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Properties of major SiC polytypes
Polytype 3C (β) 4H 6H (α)
Crystal structureZinc blende
(cubic)Hexagonal Hexagonal
Space group T2d-F43m C4
6v-P63mc C46v-P63mc
Pearson symbol cF8 hP8 hP12
Lattice constants (Å) 4.35963.0730; 10.053
3.0730; 15.11
Density (g/cm3) 3.21 3.21 3.21
Bandgap (eV) 2.36 3.23 3.05
Bulk modulus (GPa) 250 220 220
Thermal conductivity (W cm-1K-1)
@ 300K (see [28] for temp. dependence)
3.6 3.7 4.9
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PROCESSING:
Because of the rarity of natural moissanite, most silicon carbide is synthetic.
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 SiO2particles in
plant material (e.g. rice husks) can be converted to SiC by heating in the excess
carbon from the organic material. The silica fume, which is a byproduct of
producing silicon metal and ferrosilicon alloys, also can be converted to SiC by
heating with graphite at 1500 °C
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 redeposit into flake-like
single crystals, sized up to 2×2 cm2, at a slightly colder substrate. This process
yields high-quality single crystals, mostly of 6H-SiC phase (because of high
growth temperature). A modified Lely process involving induction in
graphite crucibles yields even larger single crystals of 4 inches (10 cm) in
diameter, having a section 81 times larger compared to the conventional Lely
process. Cubic SiC is usually grown by the more expensive process of chemical
vapor deposition(CVD). Homoepitaxial and heteroepitaxial SiC layers can be
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grown employing both gas and liquid phase approaches. Pure silicon carbide can
also be prepared by the thermal decomposition of a polymer, poly (methylsilyne),
under an inert atmosphere at 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
APPLICATION:
1) Abrasive and cutting tools:
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.
2) Structural material:
It is used for high temperature gas turbines. Also, 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.
Silicon carbide is used as a support and shelving material in high
temperature kilns such as for firing ceramics, glass fusing, or glass casting. SiC
kiln shelves are considerably lighter and more durable than traditional alumina
shelves.
3) Automobile parts:
Silicon-infiltrated carbon-carbon composite is used for high performance
"ceramic" brake discs, as 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
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cars, including the Porsche Carrera GT, theBugatti Veyron, the Chevrolet Corvette
ZR1, Bentleys, Ferraris, Lamborghinis, and some specific high performanceAudis.
Silicon carbide is also used in a sintered form for diesel particulate filters.
4) Electric systems:
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 strike to 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 ionized and 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-gapvaristors that use columns of zinc
oxide pellets.
5) Power electronic devices:
Silicon carbide is a semiconductor in research and early mass-production
providing advantages for fast, high-temperature and/or high-voltage devices. First
devices available were Schottky diodes, followed by Junction-gate
FETs and MOSFETs for high-power switching.
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6) Astronomy:
The low thermal expansion coefficient, high hardness, rigidity and thermal
conductivity make silicon carbide a desirable mirror material for astronomical
telescopes.
Following are some other uses of silicon carbide:
Thin filament pyrometry
Heating elements
Nuclear fuel particles Nuclear fuel cladding Jewelry Steel production Catalyst support
3) DIAMONDDiamond is the allotrope of carbon in which the carbon atoms are arranged in
the specific type of cubic lattice called diamond cubic. Diamond is an optically
isotropic crystal that is transparent to opaque. Owing to its strong covalent
bonding, diamond is the hardest naturally occurring material known. Yet, due to
important structural weaknesses, diamond's toughness is only fair to good. The
precise tensile strength of diamond is unknown, however strength up to 60 GPa has
been observed, and it could be as high as 90–225 GPa depending on the crystal
orientation. The anisotropy of diamond hardness is carefully considered
during diamond cutting. Diamond has a high refractive index (2.417) and
moderate dispersion (0.044) properties which give cut diamonds their brilliance.
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Scientists classify diamonds into four main types according to the nature of
crystallographic defects present. Trace impurities substitutionally replacing carbon
atoms in a diamond's crystal lattice, and in some cases structural defects, are
responsible for the wide range of colors seen in diamond. Most diamonds
are electrical insulators but extremely efficient thermal conductors. Unlike many
other minerals, these of diamond crystals (3.52) has rather small variation from
diamond to diamond.
PROPERTIES:
1) HARDNESS AND CRYSTAL STRUCTURE
Diamond is the hardest known naturally occurring material, scoring 10 on
the Mohs scale of mineral hardness. Diamond is extremely strong owing to the
structure of its carbon atoms, where each carbon atom has four neighbors joined to
it with covalent bonds. A surface perpendicular to the [111] crystallographic
direction (that is the longest diagonal of a cube) of a pure diamond has a hardness
value of 167 GPa when scratched with an nano diamond tip, while the nano
diamond sample itself has a value of 310 GPa when tested with another
nanodiamond tip. Because the test only works properly with a tip made of harder
material than the sample being tested, the true value for nano diamond is likely
somewhat lower than 310 GPa.
Diamonds crystallize in the diamond cubic crystal system and consist of
tetrahedrally, covalently bonded carbon atoms. A second form called lonsdaleite,
with hexagonal symmetry, has also been found, but it is extremely rare and forms
only in meteorites or in laboratory synthesis. n terms of crystal habit, diamonds
occur most often as euhedral (well-formed) or rounded octahedra and twinned,
flattened octahedra with a triangular outline.
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2) TOUGHNESS:
Unlike hardness, which only denotes resistance to scratching,
diamond's toughness or tenacity is only fair to good. Toughness relates to the
ability to resist breakage from falls or impacts. Because of diamond's perfect and
easy cleavage, it is vulnerable to breakage. A diamond will shatter if hit with an
ordinary hammer. The toughness of natural diamond has been measured as 2.0
MPa m1/2, which is good compared to other gemstones, but poor compared to most
engineering materials. As with any material, the macroscopic geometry of a
diamond contributes to its resistance to breakage. Diamond has a cleavage plane
and is therefore more fragile in some orientations than others. Diamond cutters use
this attribute to cleave some stones, prior to faceting.
3) OPTICAL PROPERTIES:
Color
Diamonds occur in various colors — black, brown, yellow, gray, white, blue,
orange, purple to pink and red. Colored diamonds containcrystallographic defects,
including substitutional impurities and structural defects, that cause the coloration.
Luster
The luster of a diamond is described as 'adamantine', which simply means
diamond-like. Reflections on a properly cut diamond's facets are undistorted, due
to their flatness. The refractive index of diamond is 2.417. Because it is cubic in
structure, diamond is also isotropic. Its highdispersion of 0.044 manifests in the
perceptible fire of cut diamonds.
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Fluorescence
Diamonds exhibit fluorescence, that is, they emit light of various colors and
intensities under long-wave ultra-violet light (365 nm): Cape series stones (type Ia)
usually fluoresce blue, and these stones may alsophosphoresce yellow, a unique
property among gemstones. Other possible long-wave fluorescence colors are
green (usually in brown stones), yellow, mauve, or red (in type IIb diamonds). n
natural diamonds, there is typically little if any response to short-wave ultraviolet,
but the reverse is true of synthetic diamonds. Some natural type IIb diamonds
phosphoresce blue after exposure to short-wave ultraviolet. In natural diamonds,
fluorescence under X-rays is generally bluish-white, yellowish or greenish. Some
diamonds, particularly Canadian diamonds, show no fluorescence.
4) Thermal stability:
Being a form of carbon, diamond oxidizes in air if heated over 700 °C. In
absence of oxygen, e.g. in a flow of high-purity argon gas, diamond can be heated
up to about 1700 °C. Its surface blackens, but can be recovered by re-polishing. At
high pressure (~20 GPa) diamond can be heated up to 2500 °C, and a report
published in 2009 suggests that diamond can withstand temperatures of 3000 °C
and above.
PROCESSING:
CUTTING PROCESS
Diamond cutting has been a work in progress since the first diamonds were cut in
the mid 1300s. Cuts have evolved from the first and most simplistic point cut - a
basic four-sided cut that resembles the outline of rough octahedral crystals - to the
table cut, single cut, old mine cut, old European cut and finally the modern brilliant
cut. When the single cut was developed in the early 1600s it became apparent that
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more facets meant more brilliance and it was the single cut that laid the foundation
for the modern brilliant cut that is so popular today.
The modern round brilliant cut diamond consists of a round outline, symmetrical
triangular and kite-shaped facets, a table, and a small culet or no culet at all.
Verena Pagel-Theisen, Diamond Grading ABC: Handbook for Diamond Grading
Incredible amounts of patience, skill and consideration are required to transform
the world's hardest material into a beautiful and valuable polished gem. A cutter
must carefully consider the size, shape, cleavage planes and even inclusions when
preparing to fashion a rough diamond. The ultimate goal is to end up with the best
cut and the most carat weight possible at the lowest production cost. This results in
some difficult decisions.
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THE FOUR BASIC STEPS FOR DIAMOND CUTTING
Planning
Planning is a crucial step in diamond manufacturing because during this stage the
size and relative value of the cut stones that the rough will produce are determined.
A person called a planner decides where to mark the diamond rough for fashioning
into the most profitable polished gem(s). The planner must consider the size,
clarity and crystal direction when deciding where to mark the diamond rough.
Incorrectly marking a diamond by a fraction of a millimeter can make a difference
of thousands of dollars in some cases. In addition, if one attempts to cleave a
diamond in the wrong position, the diamond could shatter and become worthless.
Cleaving or sawing
Once the planner decides where the diamond should be cut, the diamond is either
manually cleaved or sawed. Sawing can be done with a diamond-coated rotary saw
or a laser.
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Bruting
Bruting forms the basic face-up outline of a round diamond to prepare it for
faceting. During the bruting phase the diamond being bruted is spun on a rotating
lathe while another diamond is forced against it, gradually forming the rounded
outline. Essentially, one diamond is used to shape the other.
Polishing
Polishing is the final stage of the cutting process, giving the diamond its finished
proportions. The first and perhaps most crucial polishing stage is blocking. This
step lays the foundation for the potential of the diamond's performance because it
establishes the diamond's basic symmetry. During the blocking stage, the first 17
or 18 facets are made, creating a single cut. For some very small diamonds, the
process stops here. Larger diamonds go on to the brillianteering stage. In this
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process a specialist called a brillianteer, polishes the final facets. It is this stage that
will determine how much brilliance and fire a diamond displays. Minor
inconsistencies in symmetry and proportions can make the difference between a
gorgeous diamond and a dull, lifeless stone. The Hearts and Arrows in our
beautiful diamonds are the result of a skilled and mastered brillianteer.
APPLICATION:
The diamond has certain very unique properties that enable it to be used in
these various applications. It is the hardest substance on earth, has an extremely
high thermal conductivity, is optically transparent and has high electrical
resistance.
The hardness property of the diamond makes it amenable to be used as a
cutting tool, especially for hard substances like marble, granite and hard wood. It is
embedded in mechanical tools to enable the shaping of engine blocks and
automotive components.
Once the process of developing synthetic diamonds was discovered, these
synthetic diamonds were obviously the most preferred option for use in machinery.
Other than the cost, there are other advantages of using synthetic diamonds in
tools. The growth of the synthetic diamond can be monitored, controlled and the
diamond can be shaped in the manner desired.
This is not the case with respect to natural diamonds where nature
determines the shape, size and contours of the diamond based on various random
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natural events. Since the development of the synthetic diamond takes place in a
laboratory, the level of impurities and mineral inclusions can be controlled. Due to
these reasons and more the synthetic diamond today is sturdier as compared to the
natural ones.
Another property that lends itself to use in mechanical work is that of
thermal conductivity. The type IIa diamond can conduct up to 5 times more heat
than the metal copper. The fact that it can absorb high levels of heat means that it
can be used to reduce the friction in many engineering parts. Including the
diamond as a 'heat sink' helps in extending the life of the machinery since it avoids
wear and tear due to friction and heat. 'Slices' of synthetic diamonds are also be
used for other industrial and surgical tools.
Much research is being done to use diamond chips instead of silicon chips in
computers and it is being said that such computers would be 1000 times faster than
the existing ones.
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