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Page 1: Properties Processing and Application Of

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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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