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Ceramics can be defined as heat-resistant, nonmetallic, inorganic solids
that are (generally) made up of compounds formed from metallic and
nonmetallic elements. Although different types of ceramics can have very
different properties, in general ceramics are corrosion-resistant and
hard, but brittle. Most ceramics are also good insulators and can
withstand high temperatures. These properties have led to their use in
virtually every aspect of modern life.
The two main categories of ceramics are traditional and advanced.
Traditional ceramics include objects made of clay and cements that have
been hardened by heating at high temperatures. Traditional ceramics are
used in dishes, crockery, flowerpots, and roof and wall tiles. Advanced
ceramics include carbides, such as silicon carbide, SiC; oxides, such as
aluminum oxide, Al
; nitrides, such as silicon nitride, Si
; and many other materials, including the mixed oxide ceramics that can
act as superconductors. Advanced ceramics require modern processing
techniques, and the development of these techniques has led to advances in
medicine and engineering.
Glass is sometimes considered a type of ceramic. However, glasses and
ceramics differ in that ceramics have a crystalline structure while
glasses contain impurities that prevent
. The structure of glasses is amorphous, like that of liquids. Ceramics
tend to have high, well-defined melting points, while glasses tend to
soften over a range of temperatures before becoming liquids. In addition,
most ceramics are opaque to visible light, and glasses tend to be
translucent. Glass ceramics have a structure that consists of many tiny
crystalline regions within a noncrystalline matrix. This structure gives
them some properties of ceramics and some of glasses. In general, glass
ceramics expand less when heated than most glasses, making them useful in
windows, for wood stoves, or as radiant glass-ceramic cooktop surfaces.
Some ceramics are composed of only two elements. For example, alumina is
aluminum oxide, Al
; zirconia is zirconium oxide, ZrO
; and quartz is
Ceramics are good insulators and can withstand high temperatures. A
popular use of ceramics is in artwork.
silicon dioxide, SiO
. Other ceramic materials, including many minerals, have complex and even
variable compositions. For example, the ceramic mineral feldspar, one of
the components of granite, has the formula KAlSi
The chemical bonds in ceramics can be covalent, ionic, or polar covalent,
depending on the chemical composition of the ceramic. When the components
of the ceramic are a
and a nonmetal, the bonding is primarily ionic; examples are magnesium
oxide (magnesia), MgO, and barium titanate, BaTiO
. In ceramics composed of a
and a nonmetal, bonding is primarily covalent; examples are boron
nitride, BN, and silicon carbide, SiC. Most ceramics have a highly
crystalline structure, in which a three-dimensional unit, called a unit
cell, is repeated throughout the material. For example, magnesium oxide
crystallizes in the rock salt structure. In this structure, Mg
ions alternate with O
ions along each
Manufacture of Traditional Ceramics
Traditional ceramics are made from natural materials such as clay that
have been hardened by heating at high temperatures (driving out water and
allowing strong chemical bonds to form between the flakes of clay). In
fact, the word “ceramic” comes from the Greek
, whose original meaning was “burnt earth.” When artists
make ceramic works of art, they first mold clay, often mixed with other
raw materials, into the desired shape. Special ovens called kilns are used
to “fire” (heat) the shaped object until it hardens.
Clay consists of a large number of very tiny flat plates, stacked together
but separated by thin layers of water. The water allows the plates to
cling together, but also acts as a lubricant, allowing the plates to slide
past one another. As a result, clay is easily molded into shapes. High
temperatures drive out water and allow bonds to form between plates,
holding them in place and promoting the formation of a hard solid. Binders
such as bone
ash are sometimes added to the clay to promote strong bond formation,
which makes the ceramic resistant to breakage. The common clay used to
make flowerpots and roof tiles is usually red-orange because of the
presence of iron oxides. White ceramics are made from rarer (and thus more
expensive) white clays, primarily kaolin.
The oldest known ceramics made by humans are figurines found in the former
Czechoslovakia that are thought to date from around 27,000
It was determined that the figurines were made by mixing clay with bone,
animal fat, earth, and bone ash (the ash that results when animal bones
are heated to a high temperature), molding the mixture into a desired
shape, and heating it in a domed pit. The manufacture of functional
objects such as pots, dishes, and storage vessels, was developed in
ancient Greece and Egypt during the period 9000 to 6000
An important advance was the development of white porcelain. Porcelain is
a hard, tough ceramic that is less brittle than the ceramics that preceded
it. Its strength allows it to be fashioned into beautiful vessels with
walls so thin they can even be translucent. It is made from kaolin mixed
with china stone, and the mixture is heated to a very high temperature
(1,300°C, or 2,372°F). Porcelain was developed in China
600 during the T’ang dynasty and was perfected during the Ming
dynasty, famous for its blue and white porcelain. The porcelain process
was introduced to the Arab world in the ninth century; later Arabs brought
porcelain to Spain, from where the process spread throughout Europe.
Bone china has a composition similar to that of porcelain, but at least 50
percent of the material is finely powdered bone ash. Like porcelain, bone
china is strong and can be formed into dishes with very thin, translucent
walls. Stoneware is a dense, hard, gray or tan ceramic that is less
expensive than bone china and porcelain, but it is not as strong. As a
result, stoneware dishes are usually thicker and heavier than bone china
or porcelain dishes.
Manufacture of Advanced Ceramics
The preparation of an advanced ceramic material usually begins with a
finely divided powder that is mixed with an organic binder to help the
powder consolidate, so that it can be molded into the desired shape.
Before it is fired, the ceramic body is called “green.” The
green body is first heated at a low temperature in order to decompose or
oxidize the binder. It is then heated to a high temperature until it is
“sintered,” or hardened, into a dense, strong ceramic. At
this time, individual particles of the original powder fuse together as
chemical bonds form between them. During sintering the ceramic may shrink
by as much as 10 to 40 percent. Because shrinkage is not uniform,
additional machining of the ceramic may be required in order to obtain a
Sol-gel technology allows better mixing of the ceramic components at the
molecular level, and hence yields more
ceramics, because the ions are mixed while in solution. In the sol-gel
process, a solution of an
is hydrolyzed to produce a “sol,” a colloidal suspension of
a solid in a liquid. Typically the solution is a metal alkoxide such as
tetramethoxysilane in an alcohol solvent. The sol forms when the
individual formula units polymerize (link together to form chains and
The sol can then be spread into a thin film, precipitated into tiny
uniform spheres called microspheres, or further processed to form a gel
inside a mold that will yield a final ceramic object in the desired shape.
The many crosslinks between the formula units result in a ceramic that is
less brittle than typical ceramics.
Although the sol-gel process is very expensive, it has many advantages,
including low temperature requirements; the ceramist’s ability to
control porosity and to form films, spheres, and other structures that are
difficult to form in molds; and the attainment of specialized ceramic
compositions and high product purity.
Porous ceramics are made by the sol-gel process. These ceramics have
spongelike structures, with many porelike lacunae, or openings, that can
make up from 25 to 70 percent of the volume. The pore size can be large,
or as small as 50 nanometers (2 × 10
inches) in diameter. Because of the large number of pores, porous
ceramics have enormous surface areas (up to 500 square meters, or 5,382
square feet, per gram of ceramic), and so can make excellent catalysts.
For example, zirconium oxide is a ceramic oxygen sensor that monitors the
air-to-fuel ratio in the exhaust systems of automobiles.
Aerogels are solid foams prepared by removing the liquid from the gel
during a sol-gel process at high temperatures and low pressures. Because
aerogels are good insulators, have very low densities, and do not melt at
high temperatures, they are attractive for use in spacecraft.
Properties and Uses
For centuries ceramics were used by those who had little knowledge of
their structure. Today, understanding of the structure and properties of
ceramics is making it possible to design and engineer new kinds of
Most ceramics are hard, chemically
, refractory (can withstand very high heat without deformation), and poor
conductors of heat and electricity. Ceramics also have low densities.
These properties make ceramics attractive for many applications. Ceramics
are used as refractories in furnaces and as durable building materials (in
the form of bricks, tiles, cinder blocks, and other hard, strong solids).
They are also used as common electrical and thermal insulators in the
manufacture of spark plugs, telephone poles, electronic devices, and the
nose cones of spacecraft. However, ceramics also tend to be brittle. A
major difficulty with the use of ceramics is their tendency to acquire
tiny cracks that slowly become larger until the material falls apart. To
prevent ceramic materials from cracking, they are often applied as
coatings on inexpensive materials that are resistant to cracks. For
example, engine parts are sometimes coated with ceramics to reduce heat
Composite materials that contain ceramic fibers embedded in polymer
matrices possess many of the properties of ceramics; these materials have
low densities and are resistant to corrosion, yet are tough and flexible
rather than brittle. They are used in tennis rackets, bicycles, and
automobiles. Ceramic composites may also be made from two distinct ceramic
materials that exist as two separate ceramic phases in the composite
material. Cracks generated in one phase will not be transferred to the
other. As a result, the
resistance of the composite material to cracking is considerable.
Composite ceramics made from diborides and/or carbides of zirconium and
hafnium mixed with silicon carbide are used to create the nose cones of
spacecraft. Break-resistant cookware (with outstanding thermal shock
resistance) is also made from ceramic composites.
Although most ceramics are thermal and electrical insulators, some, such
as cubic boron nitride, are good conductors of heat, and others, such as
rhenium oxide, conduct electricity as well as metals. Indium tin oxide is
a transparent ceramic that conducts electricity and is used to make liquid
crystal calculator displays. Some ceramics are semiconductors, with
conductivities that become enhanced as the temperature increases. For
example, silicon carbide, SiC, is used as a semiconductor material in high
High temperature superconductors are ceramic materials consisting of
complex ionic oxides that become superconducting when cooled by liquid
nitrogen. That is, they lose all resistance to electrical current. One
example is the material YBa
, which crystallizes to form “sheets” of copper and oxygen
atoms that can carry electrical current in the planes of the sheets.
Some ceramics, such as barium ferrite or nickel zinc ferrites, are
magnetic materials that provide stronger magnetic fields, weigh less, and
cost less than metal magnets. They are made by heating powdered ferrite in
a magnetic field under high pressure until it hardens. Ceramic magnets are
brittle, but are often used in computers and microwave devices.
The properties of piezoelectric ceramics are modified when
is applied to them, making them useful as sensors and buzzers. For
example, lead zirconium titanate is a piezoelectric ceramic used to
provide “muscle action” in robot limbs in response to
Some ceramics are transparent to light of specific frequencies. These
optical ceramics are used as windows for infrared and ultraviolet sensors
and in radar installations. However, optical ceramics are not as widely
used as glass materials in applications in which visible light must be
transmitted. An electro-optic ceramic such as lead lanthanum zirconate
titanate is a material whose ability to transmit light is altered by an
applied voltage. These electro-optic materials are used in color filters
and protective goggles, as well as in memory-storage devices.
Still other ceramics are important in medicine. For example, they are used
to fabricate artificial bones and to crown damaged teeth. The fact that
many ceramics can be easily sterilized and are chemically inert makes
ceramic microspheres made of these materials useful as biosensors. Drugs
and other chemicals can be carried within microsphere pores to desired
sites in the body.
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