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 2 O 3 ; nitrides, such as silicon nitride, Si 3 N 4 ; 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 crystallization . 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 2 O 3 ; zirconia is zirconium oxide, ZrO 2 ; and quartz is
silicon dioxide, SiO 2 . 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 3 O 8 .
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 metal and a nonmetal, the bonding is primarily ionic; examples are magnesium oxide (magnesia), MgO, and barium titanate, BaTiO 3 . In ceramics composed of a metalloid 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 2+ ions alternate with O 2− ions along each perpendicular axis.
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 keramos , 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 B.C.E. 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 B.C.E.
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 around C.E. 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 precise shape.
Sol-gel technology allows better mixing of the ceramic components at the molecular level, and hence yields more homogeneous ceramics, because the ions are mixed while in solution. In the sol-gel process, a solution of an organometallic compound 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 networks). 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 −6 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 ceramics.
Most ceramics are hard, chemically inert , 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 transfer.
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 temperature applications.
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 2 Cu 3 O 7− x , 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 voltage 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 electrical signals.
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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