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Ceramics: Properties and Applications

What is a Ceramic?

The word is traced back to the Greek term keramos, meaning potter‘s clay or pottery.

Keramos in turn is related to an older Sanskrit root meaning ―to burn.‖ Ceramus or Keramos

was also an ancient city on the north coast of the Aegean Sea in what is currently Turkey.

In the most simple of terms, ceramics are inorganic, nonmetallic materials. They are typically

crystalline in nature (have an ordered structure) and are compounds formed between metallic

and nonmetallic elements such as aluminum and oxygen (alumina, Al2O3), calcium and

oxygen (calcia, CaO), and silicon and nitrogen (silicon nitride, Si3N4).

In broader terms ceramics also include glass (which has a non-crystalline or amorphous

random structure), enamel (a type of glassy coating), glass-ceramics (a glass containing

ceramic crystals), and inorganic cement-type materials (cement, plaster and lime). However,

as ceramic technology has developed over time, the definition has expanded to include a

much wider range of other compositions used in a variety of applications.

Ceramic Classifications (or types)

There are two major categories of glasses and ceramics: traditional and advanced.

Traditional applications include consumer products like dinnerware or ovenware and

construction products like tile or windows. Most of these applications have been in use for

many years.

Advanced applications take advantage of specific mechanical/electrical/

optical/biomedical/chemical properties of glass or ceramic materials and have entered the

scene over the last several decades or so.

The two major categories can be further broken down into more specific product

classifications or market segments as seen in the tables below. One category that is

sometimes hard to define is refractories. The production of most ceramics and glasses, as well

as other commodities like metals, metal alloys, and cement, would not be possible without

these materials. Refractories are critical materials that resist aggressive conditions, including

high temperature (up to 2000°C), chemical and acid attack, abrasion, mechanical impact, and

more.

Hence, refractory ceramics are little known, enabling materials that play a crucial role in

allowing all commodity manufacturers to operate efficiently and profitably, thus supporting

the global economy. Although refractories are typically categorized as a traditional

application since the market is mature, their high performance makes them more of an

advanced material. In fact, one global market estimate for advanced ceramics includes kiln

furniture, which is estimated at around $211 million.

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Traditional Segments

Segment Products

Structural clay

products

Brick, sewer pipe, roofing tile, clay floor and wall tile (i.e., quarry tile),

flue linings

Whitewares

Dinnerware, floor and wall tile, sanitaryware (vitreous china plumbing

fixtures), electrical porcelain, decorative ceramics

Refractories

Brick and monolithic products used in iron and steel, non-ferrous metals,

glass, cements, ceramics, energy conversion, petroleum, and chemicals

industries, kiln furniture used in various industries

Glasses Flat glass (windows), container glass (bottles), pressed and blown glass

(dinnerware), glass fibers (home insulation)

Abrasives Natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond,

fused alumina, etc.) abrasives are used for grinding

Cements Concrete roads, bridges, buildings, dams, residential sidewalks,

bricks/blocks

Advanced Segments

Segment Products

Automotive

Diesel engine cam rollers, fuel pump rollers, brakes, clutches,

spark plugs, sensors, filters, windows, thermal insulation,

emissions control, heaters, igniters, glass fiber composites for

door chassis and other components

Aerospace

Thermal insulation, space shuttle tiles, wear components,

combustor liners, turbine blades/rotors, fire detection feedthrus,

thermocouple housings, aircraft instrumentation and control

systems, satellite positioning equipment, ignition systems,

instrument displays and engine monitoring equipment, nose

caps, nozzle jet vanes, engine flaps

Chemical/petrochemical Thermocouple protection tubes, tube sheet boiler ferrules,

catalysts, catalyst supports, pumping components, rotary seals

Coatings

Engine components, cutting tools, industrial wear parts,

biomedical implants, anti-reflection, optical, self-cleaning

coatings for building materials

Electrical/electronic

Capacitors, insulators, substrates, integrated circuit packages,

piezoelectrics, transistor dielectrics, magnets, cathodes,

superconductors, high voltage bushings, antennas, sensors,

accelerator tubes for electronic microscopes, substrates for hard

disk drives

Environmental Solid oxide fuel cells, gas turbine components, measuring

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wheels/balls for check valves (oilfields), nuclear fuel storage,

hot gas filters (coal plants), solar cells, heat exchangers, isolator

flanges for nuclear fusion energy research, solar-hydrogen

technology, glass fiber reinforcements for wind turbine blades

Homeland

security/military

Particulate/gas filters, water purification membranes, catalysts,

catalyst supports, sulfur removal/recovery, molecular sieves

Ceramic Applications in 20th

Century

The last century has seen an exponential explosion in engineering developments that would

not have been possible without ceramics. The following list of the National Academy of

Engineering‘s Top 20 Engineering Achievements (www.greatachievements.org/) that have

had the greatest impact on the quality of life in the 20th century shows how ceramic materials

have made life easier for all humanity.

Achievement How Ceramics Contribute:

1. Electrification

Electrical insulators for power lines,

insulators for industrial/household

applications, glass light bulbs

2. Automobile

Engine sensors, catalytic converters, spark plugs,

windows, engine components, electrical devices

3. Airplane

Anti-fogging/freezing glass windows, jet engine components,

electronic components

4. Safe water supply

and treatment

Filters/membranes

5. Electronics

Substrates and IC packages,

capacitors, piezoelectrics, insulators, magnets, superconductors

6. Radio and

television

Glass tubes (CRTs), glass

faceplate, phosphor coatings,

electrical components, magnets

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7. Agricultural

mechanization

Refractories for melting and

forming of ferrous and non-ferrous metal components

8. Computers

Electrical components, magnetic

storage, glass for computer monitors

9. Telephone

Electrical components, glass optical fibers

10. Air conditioning

and refrigeration

Glass fiber insulation, ceramic

magnets

11. Interstate

highways

Cement for roads and bridges, glass microspheres

used to produce reflective paints for signs and road lines.

12. Space

exploration

Space shuttle tile, high-temperature

resistant components, ceramic ablation

materials, electromagnetic and

transparent windows, electrical

components, telescope lenses

13. Internet

Electronic components, magnetic storage,

computer monitor glass

14. Imaging: X-rays

to film

Piezoceramic transducers for

ultrasound diagnostics, sonar

detection, ocean floor mapping and

more, ceramic scintillator for X-ray

computed tomography (CT scans),

telescope lenses, glass monitors, phosphor coatings for radar and

sonar screens

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15. Household

appliances

Porcelain enamel coatings for major appliances,

glass fiber insulation for stoves and refrigerators,

electrical components, glass-ceramic stove tops,

spiral resistance heaters for toasters, ovens and ranges

16. Health

technologies

Replacement joints, heart valves, bone

substitutes, hearing aids, pacemakers,

teeth replacements/braces, transducers

for ultrasound diagnostics, scintillators for X-ray computed

tomography (CT scans), cancer treatments

17. Petroleum and

natural gas

technologies

Ceramic catalysts, refractories, packing media for

petroleum and gas refinement, cement and drill bits for well

drilling

18. Lasers and fiber

optics

Glass optical fibers, fiber amplifiers, laser materials, electronic

components

19. Nuclear

technologies

Fuel pellets, control rods, high-

reliability seats and valves, nuclear

waste containment

20. High-

performance

materials

Including advanced ceramics for their excellent wear, corrosion

and high temperature resistance; high stiffness; high melting

point; high compressive strength and hardness; and wide range

of electrical, magnetic, and optical properties

Ceramic Microstructures and Properties

The properties of ceramic materials, like all materials, are dictated (dependent) by the types

of atoms present, the types of bonding between the atoms, and the way the atoms are packed

together. The type of bonding and structure helps determine what type of properties a

material will have.

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Ionic Bonding

Ionic bonding occurs between two elements with a large difference in their

electronegativities (metallic and non-metallic), which become ions (negative and positive) as

a result of transfer of the valence electron from the element with low electronegativity to the

element with high electronegativity.

The typical example of a material with Ionic Bonding is sodium chloride (NaCl).

Electropositive sodium atom donates its valence electron to the electronegative chlorine

atom, completing its outer electron level (eight electrons):

As a result of the electron transfer the sodium atom becomes a positively charged ion (cation)

and the chlorine atom becomes a negatively charged ion (anion). The two ions attract to each

other by Coulomb force, forming a compound (sodium chloride) with ionic bonding.

Ionic bonding is non-directional.

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Covalent Bonding

Covalent bonding occurs between two elements with low difference in their

electronegativities (usually non-metallics), outer electrons of which are shared between the

four neighboring atoms.

Covalent Bonding is strongly directional.

Ionic-Covalent (mixed) Bonding

Ionic-covalent (mixed) bonding with various ratios of the two fractions (ionic and covalent)

occurs in most of ceramic materials.

Ceramics (ceramic materials) are therefore non-metallic inorganic compounds formed from

metallic (e.g. Al, Mg, Na, Ti, W) or semi-metallic (Si, B) and non-metallic (O, N, C)

elements.

Atoms of the elements are held together in a ceramic structure by one of the following

bonding mechanism: Ionic Bonding, Covalent Bonding, Mixed Bonding (Ionic-Covalent).

Most of ceramic materials have a mixed bonding structure with various ratios between Ionic

and Covalent components. This ratio is dependent on the difference in the electronegativities

of the elements and determines which of the bonding mechanisms is dominating ionic or

covalent.

Electronegativity

Electronegativity is an ability of atoms of the element to attract electrons of atoms of

another element. Electronegativity is measured in a relative dimensionless unit (Pauling

scale) varying in a range between 0.7 (francium) to 3.98 (fluorine).

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Non-metallic elements are strongly electronegative. Metallic elements are characterized by

low electronegativity or high electropositivity – ability of the element to lose electrons.

Ceramics usually have a combination of stronger bonds called ionic (occurs between a metal

and nonmetal and involves the attraction of opposite charges when electrons are transferred

from the metal to the nonmetal); and covalent (occurs between two nonmetals and involves

sharing of atoms). The strength of an ionic bond depends on the size of the charge on each

ion and on the radius of each ion. These types of bonds result in high elastic modulus and

hardness, high melting points, low thermal expansion, and good chemical resistance. On the

other hand, ceramics are also hard and often brittle which leads to fracture.

The greater the number of electrons being shared, the greater the force of attraction, or the

stronger the covalent bond.

Characterization of ceramics properties

In contrast to metallic bonding neither ionic nor covalent bonding form free electrons,

therefore ceramic materials have very low electric conductivity and thermal conductivity.

Since both ionic and covalent bonds are stronger than metallic bond, ceramic materials are

stronger and harder than metals.

Strength of ionic and covalent bonds also determines high melting point, modulus of

elasticity (rigidity), temperature and chemical stability of ceramic materials.

Motion of dislocations through a ceramic structure is impeded therefore ceramics are

generally brittle that limits their use as structural materials.

Ceramics may have either crystalline or amorphous structure. There are also ceramic

materials, consisting of two constituents: crystalline and amorphous.

In general, metals have weaker bonds than ceramics, which allows the electrons to move

freely between atoms. Think of a box containing marbles surrounded by water. The marbles

can be pushed anywhere within the box and the water will follow them, always surrounding

the marbles. This type of bond results in the property called ductility, where the metal can be

easily bent without breaking, allowing it to be drawn into wire. The free movement of

electrons also explains why metals tend to be conductors of electricity and heat.

Plastics or polymers of the organic type consist of long chains of molecules which are either

tangled or ordered at room temperature. Because the forces (known as van der Waals)

between the molecules are very weak, polymers are very elastic (like a rubber band), can be

easily melted, and have low strength. Like ceramics, polymers have good chemical

resistance, electrical and thermal insulation properties. They are also brittle at low

temperatures. The following table provides a general comparison of the properties between

the three types of materials.

Microstructure, which is too small to be seen with the naked eye, plays an important factor in

the final property of a material. For ceramics, the microstructure is made up of small crystals

called grains. In general, the smaller the grain size, the stronger and denser is the ceramic

material. In the case of a glass material, the microstructure is non-crystalline. When these two

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materials are combined (glass-ceramics), the glassy phase usually surrounds small crystals,

bonding them together.

The wide variety of applications for ceramic materials results from their unique properties. In

many respects, these properties cannot be achieved by other materials. Among the many

properties that ceramic products take advantage of include:

high hardness

high mechanical strength

dimensional stability

resistance to wear

resistance to corrosion or chemical attack

weathering resistance

high working temperature

low or high thermal conductivity

good electrical insulation

dielectric and ferroelectric properties

Depending on the composition and the processing of the raw materials, as well as the

fabrication and firing conditions, the properties of the material can often be closely tailored to

the desired application.

General Comparison between Ceramics, Metals and Polymers:

Property Ceramic Metal Polymer

Hardness Very High Low Very

Low

Elastic modulus Very High High Low

High

temperature

strength

Thermal

expansion High Low

Very

Low

Ductility Low High High

Corrosion

resistance High Low Low

Wear resistance High Low Low

Electrical

conductivity

Depends on

material High Low

Density Low High Very

Low

Thermal

conductivity

Depends on

material High Low

Magnetic Depends on

material High

Very

Low

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Note: For general comparison only; specific properties depend on the material‘s specific

composition and how it is made.

These three material types can also be combined in various ways to form composites to take

advantage of each material‘s properties. For instance, ceramic particles or fibers can be added

to a ceramic or metal matrix to improve the mechanical properties and/or produce a special

property the matrix by itself generally would not have. Polymers are also reinforced with

glass fibers for a wide range of construction and structural applications.

Ceramics for Engineering Applications

Engineered ceramics are increasingly being used in commercial and military aircraft, and

have been used in the space shuttle and its equipment for many years. Ceramic applications

include thermal protection systems in rocket exhaust cones, insulating tiles for the space

shuttle, engine components, and ceramic coatings that are embedded into the windshield glass

of many airplanes. These coatings are transparent and conduct electricity for keeping the

glass clear from fog and ice.

Ceramic fibers are used as heat shields for fire protection and thermal insulation in aircraft

and space shuttles because they resist heat, are lightweight and do not corrode. Other

significant characteristics include high melting temperatures, resiliency, tensile strength and

chemical inertness.

A non-oxide ceramic called silicon nitride has excellent high temperature strength, excellent

fracture toughness, high hardness and unique tribological properties. Silicon nitride aerospace

applications result in superior mechanical reliability and wear resistance allowing

components to be used under minimal lubrication without wear. These include jet engine

igniters, bearings, bushings, and other wear components.

Making Space Travel Possible

Advanced ceramics are playing a critical role in the development of highly-efficient and cost-

effective new technologies for space travel. Morgan Technical Ceramics‘ division in

Erlangen, Germany has been working with a European space development program for a

number of years to support its research of ion propulsion systems. A lightweight alternative

to traditional chemical propulsion, ion engines have the potential to push spacecraft up to ten

times faster with the same fuel consumption, thereby significantly decreasing vehicle size and

increasing travel distance.

Ion propulsion technology, which uses electricity to charge heavy gas atoms that accelerate

from the spacecraft at high velocity and push it forwards, traditionally incorporated quartz

discharge vessels. Quartz has now been replaced by a ceramic oxide called alumina because

of the need for a material with the same dielectric properties but with higher structural

stability. Alumina is easier to fabricate and offers good thermal shock resistance, ensuring

that the chamber can withstand the extremes of temperature that occur during plasma

ignition. It is also lighter, which reduces the costs associated with each launch.

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Providing Accurate Fuel Measurements

One of the most successful commercial aircrafts in recent times, the Boeing 777, uses

piezoelectric ceramic material within the 60 ultrasonic fuel tank probes located on each

aircraft. The ultrasonic transducers are installed at a variety of locations in each fuel tank. A

pulsed electric field is applied to the ceramic material, which then responds by oscillating.

The resulting sound waves are reflected off the surface of the fuel and picked up by the

piezoelectric ceramic transducer. A digital signal processor interprets the ‗time of flight‘

measurement of the sound waves in order to continually indicate the amount of fuel present.

Similar ultrasonic fuel probes are also used in fighter aircraft and other level sensing

applications because of their ability to provide highly accurate readings, regardless of the

orientation of the aircraft.

Ceramic Applications in Construction

Ceramics are ‘The Building Blocks of Construction’

Ceramic products like floor, wall and roofing tile, cement, brick, gypsum, sewer pipe, and

glass are a major part of the multi-billion dollar construction industry. Approximately three

billion square feet of glass is produced each year to make various types of windows, which is

enough to build a 200-foot wide glass highway stretching from New York to Los Angeles.

Additionally, glass fibers are used for insulation, ceiling panels and roofing shingles to

protect humans from the elements.

Ceramic tile is used in applications such as flooring, walls, countertops, and fireplaces. Tile

also is a very durable and hygienic construction product that adds beauty to any application.

Bathrooms are furnished with sanitary ware (toilets, sinks, and sometimes bathtubs) that are

made of a similar material to that of some tile.

Clay Brick Withstands Hurricanes

Clay brick is used to build homes and commercial buildings because of its strength,

durability, and beauty. Brick is the only building product that will not burn, melt, dent, peel,

warp, rot, rust or be eaten by termites. A new study by the Brick Industry Association also

shows that homes built with brick offer dramatically more protection from wind-blown debris

than homes built with vinyl or fiber-cement siding. The study demonstrated that a medium-

sized wind-blown object, such as a 7.5-foot long 2 x 4, would penetrate homes built with

vinyl or fiber-cement siding at a speed of 25 miles per hour (mph). By comparison, the same

object would need to travel at a speed exceeding 80 mph in order to penetrate the wall of a

brick home

In a test that simulated wind-blown debris traveling at a speed of 34 mph, the 2 x 4 bounced

off the brick veneer with no damage to the interior wall. When the same test was conducted

on a vinyl or fiber-cement sided wall, the 2 x 4 easily penetrated the wall, with more than five

feet of the timber passing through the interior wall. The test was representative of weather

that would generate wind speeds of between 100 and 140 mph.

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The test also found that homes made with brick exceed the 34 mph impact resistance

requirement for high velocity hurricane zones in the Florida building code. Brick also

exceeds Florida‘s impact resistance requirements for essential facilities in hurricane areas.

Smart Windows Control Light and Heat

Windows have become more intelligent with the help of ceramic particles. Called suspended

particle device technology, a patented technology developed and licensed by Research

Frontiers Incorporated, it allows users to control the amount of light, glare and heat passing

through glass or plastic products such as windows, skylights, doors, sunroofs, visors, eyewear

and more.

Suspended particle devices, more commonly known as ―SPDs‖ or ―light valves,‖ use a film

within which droplets of liquid suspension are distributed. Light-absorbing microscopic

particles are dispersed within the liquid suspension. These randomly oriented microscopic

particles are typically made of a black ceramic pigment (chromium iron oxide, copper

chromium oxide, or bismuth manganese oxide). These particles are capable of reflecting heat,

especially near-infrared radiation.

The film is then enclosed between two glass or plastic plates coated with a transparent

conductive material. When no electrical voltage is present, the particles absorb light and

block it from passing through the film. When an electrical voltage is applied, the particles

align so that light can pass through. By regulating the voltage with a simple switch or other

control device, users instantly regulate the amount of light, glare and heat coming through

products such as windows. This gives users a range of transparency where light transmission

can be rapidly varied depending upon the amount of voltage applied.

Clay Roofing Tile Saves Energy

Working with the Oak Ridge National Laboratory, the Tile Roofing Institute reports that tile

roofing‘s mass, reflectivity and ventilation beneath the tiles contribute to a reduction of heat

transference of at least 50%, when compared to a traditional asphalt shingle roof. Coated clay

tiles can cut the transfer of heat by up to 70%. Tile roofing is very fire resistant and also very

durable, withstanding harsh weather conditions like high winds, snow, hail and even

earthquakes. In fact, tile roofs have lasted for centuries, thus making them also very

economical.

Ceramic Applications in Electronics:

Ceramics Make Electronic Devices Possible

The nearly $2-trillion global electronics industry would not exist without ceramics. Ceramics‘

wide range of electrical properties including insulating, semi-conducting, superconducting,

piezoelectric and magnetic are critical to products such as cell phones, computers, television,

and other consumer electronic products. The global market for electronic ceramics is

estimated at around $9 billion.

Ceramic spark plugs, which are electrical insulators, have had a large impact on society. They

were first invented in 1860 to ignite fuel for internal combustion engines and are still being

used for this purpose today. Applications include automobiles, boat engines, lawnmowers,

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and the like. High voltage insulators make it possible to safely carry electricity to houses and

businesses.

Ceramics Improve Antenna Performance

The next generation of mobile phone antennas would not be possible without special

ceramics developed by Morgan Advanced Ceramics for Sarantel, a leading miniature antenna

specialist. Sarantel‘s PowerHelix range uses a patented design in which copper tracks,

deposited onto a small ceramic cylinder, are individually and automatically laser trimmed for

optimum frequency response.

Currently, this type of antenna is used in GPS applications, where its zero ground plane

allows space saving in handheld and portable equipment. When mounted side by side, the

antennas can also be used in combined applications such as Bluetooth and GPS without loss

of performance. However, it is within the mobile phone market that the PowerHelix range

may be of most benefit. Under E-911 Legislation in the United States, it will soon be

mandatory for GPS receivers to be built in to mobile phone handsets, so that the technology

can be used to help emergency services respond more effectively to distress calls. 3G mobile

and Wi-Fi networks are also potential applications.

The continuing debate over the health implications of using mobile phones is another major

issue. International safety regulations defined in terms of the specific energy absorption rate

(SAR) encourage optimization of the ratio of radiated power versus absorbed power in the

user head. The patented PowerHelix antenna design significantly reduces the losses of current

that can cause an incident magnetic field at the user‘s skin. A specific ceramic is used that

enables the manufacture of antennas which yield just five per cent of the radiation emitted by

other systems.

Transistors Advance With Ceramic Material

Intel is combining new high dielectric ceramic and metal materials to build the insulating

walls and switching gates of its 45 nanometer transistors. Transistors are tiny switches that

process the ones and zeroes of the digital world. The gate turns the transistor on and off and

the gate dielectric is an insulator underneath it that separates it from the channel where

current flows.

Hundreds of millions of these microscopic transistors-or switches-will be inside the next

generation of multi-core processors, resulting in record-breaking PC, laptop and server

processor speeds. By replacing the conventional dielectric material with a thicker hafnium-

based oxide material, transistor gate leakage is reduced by more than 10 times and transistors

can be made smaller, increasing transistor density by approximately two times.

When the hafnium ceramic is combined with a compatible metal gate, the result is more than

a 20 percent increase in drive current (higher transistor performance) and more than a five

times reduction in source-drain leakage, thus improving the energy efficiency of the

transistor. The smaller transistor size means active switching power is reduced by

approximately 30 percent.

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Ceramic use in Medicine

A wide range of ceramic and glass materials are being used in biomedical applications;

ranging from bone implants to biomedical pumps. Dentistry has also advanced with ceramic

teeth that can be matched to a patient‘s natural ones and other applications for improving a

patient‘s smile. In the future, ceramics will find applications in gene therapy and tissue

engineering.

Glass Beads Offer Hope for Liver Cancer Patients

Currently used treatments for inoperable liver cancer can reduce symptoms of this disease but

require hospitalization and usually cause side effects that reduce the quality of life for

patients. For example, chemotherapy often produces nausea, vomiting and hair loss. For this

reason, there is a need for new treatments that offer the convenience of outpatient therapy and

fewer and milder side effects. More importantly, the life expectancy for persons with liver

cancer is short, usually less than a year.

Glass microspheres, originally developed at the University of Missouri-Rolla, are now FDA

approved and are being used to treat patients with primary liver cancer at 29 hospital sites in

the U.S. Called TheraSpheresTM, these microspheres are made radioactive by neutron

activation in a nuclear reactor. The microspheres, which are about one third the diameter of a

human hair, are then inserted into the artery that supplies blood to the tumor using a catheter.

The radiation destroys malignant tumor with only minimum damage to the normal tissue.

The treatment usually takes less than an hour and patients can go home the same day. Side

effects are generally minimal, with some fatigue lasting for several weeks until the radiation

disappears. Most patients receive a single injection, but there are an increasing number of

patients which have been given multiple injections. There is a growing body of evidence

showing that life expectancy is increased with documented cases of patients surviving up to

eight years. Other potential uses for these radioactive beads include treating other forms of

cancer (kidney, brain, prostate) and treating rheumatoid arthritis.

Ceramic Braces for Dental Repair/Alignment

Traditionally, braces have consisted of metal brackets and wires. However, some people have

feared the idea of a ―metal mouth‖ so much that they refuse to wear braces altogether,

missing out on the possibility of a beautiful smile. For this reason, orthodontic research began

to focus on less visible options. This is how ceramic braces-the braces responsible for Tom

Cruise‘s straightened smile-came to be.

Transparent polycrystalline alumina (TPA) was originally identified by NASA and a ceramic

company called Ceradyne for helping track heat-seeking missiles. Ceradyne went on to

partner with Unitek Corporation/3M to develop Transcend brackets, made from TPA. These

orthodontic braces are as effective as metal braces, but are nearly invisible when viewed at

normal distances, thus providing a more attractive cosmetic option for the wearer. Because

this material is non-porous and 99.9 percent pure, it is extremely resistant to staining or

discoloration.

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Hip Replacements Become Stronger with Ceramic femoral head and acetabular cup

Over the last twenty years there has been a considerable increase in the use of ceramic

materials for implant devices. With an excellent combination of strength and toughness

together with bio-inert/biocompatible properties and low wear rates, special type of oxide

ceramics called zirconia and alumina are currently used for applications such as femoral

heads and acetabular cups for total hip replacements.

The zirconia heads have double the strength of comparable alumina heads and consequently

the diameter of the femoral head can be reduced to < 26 mm, leading to a reduction in patient

trauma during the hip replacement operation. Other applications for zirconia implants include

knee joints, shoulders, phalangeal joints and spinal implants. This material is also being used

for endoscopic components and pace maker covers.

Ceramic Coatings for Drug Release

MIV Therapeutics, Inc., a leading developer of new generation biocompatible coatings and

advanced drug delivery systems for cardiovascular stents and other implantable medical

devices, is developing coatings based on hydroxyapatite (HAp), a ceramic material that has a

similar composition to natural bone. These proprietary coatings show potential for

outperforming technologies and products currently in use that release drugs after stent

implantation. The microporous films are designed to remain highly biocompatible even after

all drug material is eluted from the coating. In this respect, HAp performance far exceeds

polymer-based coatings, wherein drugs are necessary to sustain acceptable coating

performance.

The ultra-thin films are designated as a surface modification of metallic implants, whereas the

micro-thin films are evaluated also as a potential vehicle for drug delivery purposes for

implantable medical devices. In the extremely demanding application on stents the coating

not only has to withstand deformation during manufacturing (i.e. stent crimping) and at the

implantation stage and remain un-damaged in such operation. If this was not enough, the

coating has to maintain its integrity and resist fatigue stresses in concert with the heart beat

over the years after deployment in human heart.

Composite Layers for Gene Therapy

An efficient and safe gene transferring system is a key technology in gene therapy and tissue

engineering. Particles of DNA/calcium phosphate complex have long been used for

facilitating gene transfer because of their low toxicity. The gene transferring efficiency of this

reagent is, however, insufficient compared with other reagents such as DNA/lipid complexes.

Recent research has shown that gene transfer on the surface of a DNA/apatite composite layer

is as efficient as an optimized commercial lipid-based reagent.

A laminin/DNA/apatite composite layer was successfully formed on the surface of an

ethylene/vinyl alcohol copolymer by Japanese researchers. The immobilized DNA was

transferred to the cells adhering onto the laminin/DNA/apatite composite layer more

efficiently than those adhering onto a laminin-free/DNA/apatite composite layer. It is

considered that laminin immobilized in the surface layer enhances cell adhesion and

spreading, and DNA locally released from the layer is effectively transferred into the

adhering cells, taking advantage of the large contact area. The present gene transferring

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system, which shows high efficiency and safety, would be useful in gene therapy and tissue

engineering.

Ceramic application in the Army

A variety of ceramic materials are finding their way into military vehicles and other

technologies. Engine components, missile radomes, and personal/vehicular armor are just a

few of the applications. For the last half century, ceramics have been used for personnel and

light vehicle protection against small arms and machine gun threats. Whether it is protecting

Americans at home or abroad, ceramics are a key enabler for this mission.

Ceramic Armor Protects Soldiers

The distinct characteristics of advanced ceramics, including light weight, the ability to

withstand extremely high temperatures, hardness, resistance to wear and corrosion, low

friction, and special electrical properties, offer major advantages over conventional materials

such as plastics and metals. Because of this, advanced ceramics are the foundation for the

lightest, most durable body armor used for small to medium caliber protection available. Hot

pressed boron carbide and silicon carbide ceramic is integrated with optimized composite

structures to produce rugged multi-hit body armor plates.

Complete aircraft armor systems that include ceramic armor seats, components, and panel

systems are found in the Apache, Gazelle, Super Puma, Super Cobra, Blackhawk, Chinook,

and other military helicopters. Armor tiles are also specified in many fixed wing applications

including the C-130 and C-17 aircraft. Single, double and triple curve plates and multi-hit

armor systems featuring boron carbide and silicon carbide advanced ceramics are also used

for special operations forces as body, side and shoulder armor. Advanced protection for other

vulnerable body areas including hips, legs and arms is under development.

The U.S. Army is developing metal-ceramic and metal-ceramic-composite hybrids for

improved performance. The first type is metal-encapsulated ceramic armor that allows delay

of ceramic failure for improved ballistic performance due to better metal-ceramic bonding.

The second type, found on the Stryker-Interim Armored Vehicle, uses a polymer matrix

composite to catch any spall off the back of the metal. Future applications for ceramic-based

armor include the US Marine Corps Expeditionary Fighting Vehicle (formerly known as the

Advanced Amphibious Assault Vehicle, or AAUV) and the US Army‘s Future Combat

System.

Seeing Better with Transparent Ceramics

Because many ceramic materials are transparent to certain types of energy, light or otherwise,

they can be used for infrared domes, sensor protection, and multi-spectral windows. In

addition to these optical properties, such ceramics have the desired abrasion resistance,

strength, and thermal stability. A special type of glass-ceramic material shows promise for

electromagnetic windows for use in artillery projectile because of its suitable electrical

properties and high temperature capability.

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Silicon nitride (a non-oxide ceramic) is used as radomes for missiles in the latest air defense

systems. It was specifically selected for missile radomes because of its mechanical strength

and dielectric properties. The material allows microwave or other energy to pass through to

locate incoming targets. Its mechanical strength allows the missile system to withstand

erosion and large temperature excursions while flying at hyper-velocity through the

atmosphere. Also under development are transparent infrared windows using nanocrystalline

yttria (a type of oxide ceramic) for missile applications.

Improved glasses and glass ceramics are also being considered for armor windows with the

desired ballistic performance. Glasses can be produced in large sizes with curved geometries,

and can be produced to provide incremental ballistic performance at incremental cost. A

fused silica glass is one material under development.

Other transparent materials are being considered for windshields, blast shields, and sensor

protection in aircraft. A ceramic material called spinel (magnesium aluminate) has superior

optical properties within the infrared region, which makes it attractive in sensor applications

where effective communication is impacted by the protective dome‘s absorption

characteristics.

Improving Turbine Engine Efficiency

Future Army helicopters will be able to fly farther and carry more payload thanks to

ceramics. Turbine engine operational efficiencies can be increased through the use of ceramic

matrix composites and ceramic thermal barrier coatings due to their high temperature

capability. Ceramics have the potential to operate at temperatures above 1100°C with

minimal or no cooling. Composites are also 30 to 50% lighter than the metallic alloys

currently in use. When composite combustor liner and turbine vane applications are coated

with ceramics, operating temperatures increase to 1650C and the components are protected

from the combustion environment. A multi-component ceramic coating based on hafnium

oxide has survived a 300 hour test at 1650°C.

Application of Ceramics in Sport

Whether to be NASCAR or the Olympics, ceramics are helping drivers cross the finish line

first and athletes win gold medals on the slopes. Sporting equipment manufacturers are taking

advantage of the unique properties of ceramics to make it easier for these competitors to win.

Ceramics Keep Race Cars Cool and Fast

Carbon/ceramic rotors have been used in race car brakes for more than 15 years. The

advantages of these rotors include light weight, durability, and fade resistance, which

improves steering precision and handling. Brake discs and pads last a lot longer as well.

Silicon nitride balls, which have excellent temperature and wear resistance, are also used in

Formula One Racing cars‘ wheel bearings and gear boxes to improve performance.

Besides improving the car‘s performance, ceramics are also helping NASCAR drivers keep

cool in the cockpit, which can reach 115ºF during the race, hot enough to melt the drivers‘

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shoes. The same type of lightweight ceramic textile used to protect space shuttle tiles, 3M‘s

Nextel, is being used as thermal barriers for the transmission/tunnel, header/collector, spark

plug boots, driver‘s floor panel and firewall. Nextel ceramic textiles maintain their strength

and flexibility at continuous temperatures in excess of 2000ºF and help keep race car cockpit

surfaces 40 percent cooler.

Make a Birdie with Ceramic Putters

High performance ceramic materials are helping to shave strokes off a golfer‘s handicap.

Ceramics are the perfect material for a putter head because it is lighter and softer than steel

and provides increased feel and control. Despite its lightness, ceramic materials have a

reputation for durability and toughness. When made of a solid one-piece construction,

ceramic putters provide ―one huge sweet spot‖, which translates into more successful putts

and better consistency.

Skiing Smarter With Piezoelectric Fibers

Used by reputable ski manufacturers, revolutionary ceramic disc technology developed by

Norton Industries is achieving the sharpest, smoothest ground edge while providing a near

mirror finish for skis. Thus, skis and snowboards turn and grip icy slopes better, improving

both performance and safety. Edges also remain sharper longer.

Head Sport, a leading sports equipment manufacturer, is also manufacturing ―smart‖ skis that

make use of a ceramic‘s piezoelectric properties. When skiing at high speeds, skis tend to

vibrate, lessening the contact area between the ski edge and snow surface. This results in

reduced stability and control and decreases the skiers speed.

The skis are embedded with piezoelectric fiber composites developed by Advanced

Cerametrics. These composites convert the unwanted vibrations into electrical energy, thus

keeping the skis on the snow. The skis continuously adjust to all conditions, with a maximum

reaction time of 5 thousandths of a second. Ceramic fiber technology also guarantees up to

6% more functional edge, helping several athletes at the 2006 Winter Olympics win two gold

medals and one silver. The skis are available at high end sporting goods stores and have been

the choice of ski instructors and ski patrollers for three years. The skis were one of Time

Magazine‘s ―Coolest Products for 2002″and won the 2003 R&D 100 Award.

Such ceramic fibers have two important qualities: on the one hand they feature exceptional

wear resistance and on the other they have far better torsional properties than many plastic

materials. However, the main advantage of ceramic fibers is their ability to change shape very

slightly when subjected to small electrical charges.

Advanced Cerametrics‘ fibers are also used in several of Head‘s tennis rackets, adding up to

15% more power to a ball hit. The Intelligence, Protector and LiquidMetal lines of rackets

using these piezo ceramic fibers were the largest selling rackets in the world in the 2005/2006

season. They have been clinically proven to eliminate tennis elbow. The Protector racket was

also selected 2005 Racket of the Year by Tennis Magazine. In the summer of 2006, smart

pool cues made with these fibers (JossWest made by Hamson Industries) won the largest

purse ever in a pool tournament.

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Snowboards Get Tougher With Composites

The estimated 500,000 to 700,000 active snowboarders now have a stronger, tougher

snowboard thanks to special composite materials that combine innovative glass laminates and

newly developed carbon fiber materials. The laminates are made from E-glass (PPG or

Owens Corning), which has high strength and stiffness, good resistance to moisture, and the

ability to maintain strength properties over a wide range of conditions. When Never Summer

Snowboard Manufacturing used these composites for their snowboards, they were able to

extend their warranty to three years, from the typical one offered by other companies. Broken

or cracked sidewalls were also not a problem as with conventional materials.

Traditional ceramics, ceramic materials that are derived from common, naturally occurring

raw materials such as clay minerals and quartz sand. Through industrial processes that have

been practiced in some form for centuries, these materials are made into such familiar

products as china tableware, clay brick and tile, industrial abrasives and refractory linings,

and portland cement. This report describes the basic characteristics of the raw materials

commonly used in traditional ceramics, and it surveys the general processes that are followed

in the fabrication of most traditional ceramic objects.

Raw materials

Because of the large volumes of product involved, traditional ceramics tend to be

manufactured from naturally occurring raw materials. In most cases these materials are

silicates—that is, compounds based on silica (SiO2), and oxide form of the element silicon. In

fact, so common is the use of silicate minerals that traditional ceramics are often referred to

as silicate ceramics, and their manufacture is often called the silicate industry. Many of the

silicate materials are actually unmodified or chemically modified aluminosilicates (alumina

[Al2O3] plus silica), although silica is also used in its pure form. Altogether, the raw materials

employed in traditional ceramics fall into three commonly recognized groups: clay, silica,

and feldspar. These groups are described below.

Clay

Clay minerals such as kaolinite (Al2[Si2O5][OH]4) are secondary geologic deposits, having

been formed by the weathering of igneous rocks under the influence of water, dissolved

carbon dioxide, and organic acids. The largest deposits are believed to have formed when

feldspar (KAlSi3O8) was eroded from rocks such as granite and was deposited in lake beds,

where it was subsequently transformed into clay.

The importance of clay minerals to traditional ceramic development and processing cannot be

overemphasized. In addition to being the primary source of aluminosilicates, these minerals

have layered crystal structures that result in plate-shaped particles of extremely small

micrometre size. When these particles are suspended in or mixed with water, the mixture

exhibits unusual rheology, or flow under pressure. This behaviour allows for such diverse

processing methods as slip casting and plastic forming, which are described below. Clay

minerals are therefore considered to be formers, allowing the mixed ingredients to be formed

into the desired shape.

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Silica and feldspar

Other constituents of traditional ceramics are silica and feldspar. Silica is a major ingredient

in refractories and whitewares. It is usually added as quartz sand, sandstone, or flint pebbles.

The role of silica is that of a filler, used to impart ―green‖ (that is, unfired) strength to the

shaped object and to maintain that shape during firing. It also improves final properties.

Feldspars are aluminosilicates that contain sodium (Na), potassium (K), or calcium (Ca).

They range in composition from NaAlSi3O8 and KAlSi3O8 to CaAl2Si2O8. Feldspars act as

fluxing agents to reduce the melting temperatures of the aluminosilicate phases.

Processing

Beneficiation

Compared with other manufacturing industries, far less mineral beneficiation (e.g., washing,

concentrating, sizing of particulates) is employed for silicate ceramics. Clays going into

common structural brick and tile are often processed directly as dug out of the ground,

although there may be some blending, aging, and tempering for uniform distribution in water.

Such impure clays are workable in untreated form because they already contain fillers and

fluxes in association with the clay minerals. In the case of whitewares, for which the raw

materials must be in a purer state, the clays are washed, and impurities are either settled out

or floated off. Silicas are purified by washing and separating unwanted minerals by gravity

and by magnetic and electrostatic means. Feldspars are beneficiated by flotation separation,

a process in which a frothing agent is added to separate the desired material from impurities.

Blending

The calculation of amounts, weighing, and initial blending of raw materials prior to forming

operations is known as batching. Batching has always constituted much of the art of the

ceramic technologist. Formulas are traditionally jealously guarded secrets, involving the

selection of raw materials that confer the desired working characteristics and responses to

firing and that yield the sought-after character and properties. Clays must be selected on the

basis of workability, fusibility, fired colour, and other requirements. Silicas, likewise, must

meet criteria of chemical purity and particle size distribution.

Forming

The fine, platy morphology of clay particles is used to advantage in the forming of clay-based

ceramic products. Depending upon the amount of water added, clay-water bodies can be stiff

or plastic. Plasticity arises by virtue of the plate-shaped clay particles slipping over one

another during flow. (Nonclay ceramics can be similarly formed if plasticizers—usually

polymers—are added to their mixes. In many cases organic binders are used to help hold the

body together until it is fired.) With even higher water content and the addition of dispersing

agents to keep the clay particles in suspension, readily flowable suspensions can be produced.

These suspensions are called slips or slurries and are employed in the slip casting of clay

bodies. The mechanisms of plastic forming and slip casting are described below.

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Plastic forming

Plastic forming is the primary means of shaping clay-based ceramics. After the raw materials

are mixed and blended into a stiff mud or plastic mix, a variety of forming techniques are

employed to produce useful shapes, depending upon the ceramic involved and the type of

product desired. Foremost among these techniques are pressing and extrusion.

Pressing involves the application of pressure to eliminate porosity and achieve a specific

shape, depending upon the die employed. Refractory bricks, for example, are often made by

die presses that are either single-action (pressing from the top only) or dual-action

(simultaneously pressing from top and bottom). Structural clay products such as brick and tile

can be made in the same fashion. In pressing operations the feed material tends to have a

lower water content and is referred to as a stiff mud.

The problem with die casting is that it is a piecemeal rather than a continuous process,

thereby limiting throughput. Many silicate ceramics are therefore manufactured by extrusion,

a process that allows a more efficient continuous production. In a commercial screw-type

extruder, a screw auger continuously forces the plastic feed material through an orifice or die,

resulting in simple shapes such as cylindrical rods and pipes, rectangular solid and hollow

bars, and long plates. These shapes can be cut upon extrusion into shorter pieces for bricks

and tiles.

Slip casting

A different approach to the forming of clay-based ceramics is taken in slip casting of

whiteware, as shown in Figure 1.

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As mentioned above, with sufficient water content and the addition of suitable dispersing agents,

clay-water mixtures can be made into suspensions called slurries or slips. These highly stable suspensions of clay particles in water arise from the careful manipulation of surface charges on the

platelike clay particles. Without a dispersing agent, oppositely charged edges and surfaces of the

particles would attract, leading to flocculation, a process in which groups of particles coagulate into

flocs with a characteristic house-of-cards structure. Dispersing agents neutralize some of the surface charges, so that the particles can be made to repel one another and remain in suspension indefinitely.

When the suspension is poured into a porous plaster mold, capillary forces suck the water into the

mold from the slip and cause a steady deposition of clay particles, in dense face-to-face packing, on

the inside surface of the mold. After a sufficient thickness of deposit has been obtained, the remaining

slip can be poured off or drained and the mold opened to reveal a freestanding clay piece that can be

dried and fired. Surprisingly complex shapes can be achieved through slip casting.

Firing

After careful drying to remove evaporable water, clay-based ceramics undergo gradual heating to remove structural water, to decompose and burn off any organic binders used in forming, and to

achieve consolidation of the ware. Batches of specialty products, produced in smaller volumes, are

cycled up and down in so-called batch furnaces. Most mass-produced traditional ceramics, on the other hand, are fired in tunnel kilns. These consist of continuous conveyor belt or railcar operations,

with the ware traversing the kiln and gradually being heated from room temperature, through a hot

zone, and back down to room temperature. Pyrometric cones, which deform and sag at specific temperatures, often ride with the ware to monitor the highest temperature seen in the traverse through

the kiln.

Vitrification

The ultimate purpose of firing is to achieve some measure of bonding of the particles (for strength) and consolidation or reduction in porosity (e.g., for impermeability to fluids). In silicate-based

ceramics, bonding and consolidation are accomplished by partial vitrification. Vitrification is the

formation of glass, accomplished in this case through the melting of crystalline silicate compounds

into the amorphous, noncrystalline atomic structure associated with glass. As the formed ware is heated in the kiln, the clay component turns into progressively larger amounts of glass. The partial

vitrification process can be analyzed through a phase diagram such as that shown in Figure 2.

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In this diagram three crystalline phases are shown: the end members cristobalite (one

crystallographic form of silica [SiO2]) and alumina (Al2O3) and an intermediate compound,

mullite (3Al2O3 · 2SiO2). The melting points of alumina and cristobalite, as shown on the left

and right edges of the diagram, are quite high. However, intermediate compositions begin to

melt at lower temperatures. As shown by the two horizontal lines on the diagram, melting

begins to occur at 1,828° C (3,322° F) for high alumina compositions and as low as 1,587° C

(2,889° F) for high silica compositions. (These temperatures can be lowered still further by

the addition of fluxing agents, such as alkali or alkaline-earth oxide feldspars.) Between the

two horizontal lines and the region of the diagram marked liquid, all compositions are only

partly liquid (e.g., mullite and liquid, alumina and liquid). This partial vitrification allows for

the retention of solid particles, which helps to maintain the rigidity of the ceramic piece

during firing in order to minimize sagging or warpage.

The role of the glassy liquid phase in the consolidation of fired clay objects is to facilitate

liquid-phase or reactive-liquid sintering. In these processes the liquid first brings about a

denser rearrangement of particles by viscous flow. Second, through solution-precipitation of

the solid phases, small particles and surfaces of larger particles dissolve and reprecipitate at

the growing ―necks‖ that connect large particles. Rearrangement and solution-precipitation

lead to bond formation and to progressive densification with reduction of porosity. A range of

glass contents and residual porosities can be obtained, depending on the ingredients and the

time the object is held at maximum temperature.

Finishing

If fired ceramic ware is porous and fluid impermeability is desired, or if a purely decorative

finish is desired, the product can be glazed. In glazing, a glass-forming formulation is

pulverized and suspended in an appropriate solvent. The fired ceramic body is dipped in or

painted with the glazing slurry, and it is refired at a temperature that is lower than its initial

firing temperature but high enough to vitrify the glaze formulation. Glazes can be coloured

by the addition of specific transition-metal or rare-earth elements to the glaze glass or by the

suspension of finely divided ceramic particles in the glaze.

Products

The raw materials and manufacturing processes outlined above produce a range of traditional

ceramic products. These products are described in some detail in separate articles on

whiteware, structural clay products, brick and tile, refractory, abrasive, and cement.

Further Reading

A good introduction to ceramics is provided by David W. Richerson, Modern Ceramic

Engineering: Properties, Processing, and Use in Design, 2nd ed., rev. and expanded (1992).

The processing of traditional ceramics is described in F.H. Norton, Elements of Ceramics,

2nd ed. (1974); James S. Reed, Introduction to the Principles of Ceramic Processing (1988);

George Y. Onoda, Jr., and Larry L. Hench, Ceramic Processing Before Firing (1978); and

four sections of Theodore J. Reinhart (ed.), Engineered Materials Handbook, vol. 4,

Ceramics and Glasses, ed. by Samuel J. Schneider (1991): ―Ceramic Powders and

Processing,‖ pp. 41–122; ―Forming and Predensification, and Nontraditional Densification

Processes,‖ pp. 123–241; ―Firing/Sintering: Densification,‖ pp. 242–312; and ―Final Shaping

and Surface Finishing,‖ pp. 313–376.

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