Ceramics

Traditional ceramics

Hydraulic cement

Glass

Modern ceramics

Aluminum oxide

Magnesium oxide

Silicon carbide

Silicon nitride

Processing

Forming processes

Sintering

Machining

Design considerations

Resources

The term ceramic refers to an inorganic mineral that becomes hard and brittle after being subjected to high temperatures. The process of ceramics (which is Greek for “potter’s clay,” or burned material, keramos ) is historically defined as the art and science of making and decorating pottery; another name for any ceramic (earthy) material that is pliable when worked but becomes rigid when subjected to heat. Ceramic materials are usually understood to be compounds of metallic and nonmetallic elements, though some are actually ionic salts, and others are insulators. These materials can be very complicated, as are, for example, clays, spinels, and common window glass. Many ceramic compounds have very high melting points.

Ceramics have a wide range of applications. They have been used as refractories, abrasives, ferroelectrics, piezoelectric transducers, magnets, building materials, and surface finishes. Humans still use ceramics. They find their way into the kitchen cupboard as bowls, plates, and cups, and on shelves as flower vases. Indeed, a ceramic cup is still the most environmentally friendly method for holding one’s morning coffee. Bricks have been used for years by the construction industry, and for lining of ore smelting furnaces. Ceramic blades are being tested in jet engine turbines and automobile engines. Ceramics stovetops are common. Perhaps the most famous use of ceramics is the ceramic tiles that coat the space shuttle’s outside frame, which are designed to absorb the intense heat inflicted upon the spacecraft during re-entry into the Earth’s atmosphere.

Unlike metals, there are really no heat treatments that can be used to modify the properties of ceramics, but their properties can be altered by changes in chemical composition. By carefully considering the choice of chemical composition; purity; particle size; uniformity, arrangement, and packing of atoms, high quality ceramics can be synthesized in a wide variety.

Traditional ceramics

Arguably, ceramics is one of the pillars upon which modern civilization was built, both literally and figuratively. Ceramics have been used by humans since antiquity. The earliest ceramic articles were made from naturally occurring materials such as clay minerals. It was discovered in prehistoric times that clay materials become malleable when water is added to them, and that a molded object can then be dried in the sun and hardened in a high-temperature

fire. Many of the same raw materials that were used by the ancients are still used today in the production of traditional ceramics. Traditional ceramic applications include whitewares, heavy clay products, refractories, construction materials, abrasive products, and glass.

Clay minerals are hydrated compounds of aluminum oxide and silica. These materials have layered structures. Examples include kaolinite, halloysite, pyrophillite, and montmorillonite. They are all formed by the weathering of igneous rocks under the influence of water, dissolved CO₂, and organic acids. The largest sources of these clays were formed when feldspar was eroded from granite and deposited in lake beds, where it became altered to a clay.

Silica is a major ingredient in glass, glazes, enamels, refractories, abrasives, and whiteware. Its major source is quartz, which is made up primarily of sand, sandstone, and quartzite.

Feldspar is also used in the manufacture of glass, pottery, enamel, and other ceramic products. Other naturally occurring minerals used directly in ceramic production include talc, asbestos, wollastonite, and sillimanite.

Hydraulic cement

Hydraulic cements are set by interaction with water. Portland cement, the most common hydraulic cement, is primarily a water-free calcium silicate. It is slightly soluble in water and sets by a combination of solution precipitation and chemical reaction with water to form a hydrated composition. The ratio of water to cement in the initial mix greatly influences the strength of the final concrete: the lower the water-to-cement ratio, the higher the strength.

Glass

Glass is a ceramic material consisting of uniformly dispersed mixtures of silica, soda ash, and lime, that is often combined with metallic oxides of calcium, lead, lithium, cerium, etc. Glass is distinguished from solid ceramics by its lack of crystallinity. It is, in fact, a supercooled liquid. The atoms in glass remain disordered in the solid state, much as they are in the liquid state. Glasses are, thus, rigid structures whose atomic arrangements and properties depend on both composition and thermal history.

Group 16 elements (oxygen, sulphur, selenium, tellurium) are especially good candidates for glass formation. Oxygen is able to form stable bonds with silicon, boron, phosphorous, and arsenic and thereby form stable structures having oxygen atoms at the corners and one of the other atoms at the center. Pure oxide glasses are very stable because each oxygen atom is linked by electron bonds to two other atoms.

Various two-phase structures may exist as glasses. One such structure is a mixture of glass and crystal. These materials are converted into strong and durable ceramics that are part glass and part crystal by prolonged heat treatment.

Modern ceramics

In the twentieth century, scientists and engineers acquired a much better understanding of ceramics and their properties. They succeeded in producing ceramics with tailor-made properties. Modern ceramics include oxide ceramics, magnetic ceramics, ferroelectric ceramics, nuclear fuels, nitrides, carbides, and borides.

Aluminum oxide

Aluminum oxide (Al₂ O₃) occurs naturally in the mineral corundum, which in gem-quality form is known as the precious stones ruby and sapphire. Ruby and sapphire are known for their chemical inertness and hardness. Al₂ O₃ is produced in large quantities from the mineral bauxite. In the Bayer process, bauxite (primarily aluminum hydroxide mixed with iron hydroxide and other impurities) is selectively leached with caustic soda. Purified aluminum hydroxide is formed as a precipitate. This material is converted to aluminum oxide powder, which is used in the manufacture of aluminum-oxide-based ceramics. Aluminum oxide powder is used in the manufacture of porcelain, alumina laboratory ware, crucibles and metal casting molds, high temperature cements, wear-resistant parts (sleeves, tiles, seals), sandblast nozzles, and other products.

Magnesium oxide

Magnesium oxide (MgO) occurs naturally in the mineral periclase, but not in sufficient quantity to meet commercial demand. Most MgO powder is produced from MgCO₃ or from seawater. MgO is extracted from seawater as a hydroxide and, then, converted to the oxide. MgO powder finds extensive use in high temperature electrical insulation and in refractory brick.

Silicon carbide

Silicon carbide (SiC) has been found to occur naturally only as small green hexagonal plates in metallic iron. The same form of silicon carbide has been manufactured synthetically, however. In this process, SiO₂ sand is mixed with coke in a large elongated mound in which large carbon electrodes have been placed at either end. As electric current is passed between the electrodes, the coke is heated to about 3, 992°F (2, 200°C). The coke reacts with the SiO₂ to produce SiC plus CO gas. Heating continues until the reaction has completed in the mound. After cooling, the mound is broken up, and the green hexagonal SiC crystals, which are low in impurities and suitable for electronic applications, are removed. The lower purity material is used for abrasives. The outer layer of the mound is reused in the next batch. SiC can be formed from almost any source of silicon and carbon. It has been produced in the laboratory from silicon metal powder and sugar, and from rice hulls. SiC is used for high-temperature kiln furniture, electrical resistance heating elements, grinding wheels and abrasives, wear-resistance applications, and incinerator linings.

Silicon nitride

Silicon nitride does not occur naturally. Most of the powder commercially available has been produced by reacting silicon metal powder with nitrogen at temperatures between 2, 282°F (1, 250°C) and 2, 552°F (1, 400°C). The powder that is removed from the furnace is not ready to use. It is loosely bonded, and must be crushed and sized. The resulting powder contains impurities of Fe, Ca, and Al. Higher purity silicon nitride powder has been produced by reducing SiO₂ with carbon in a nitrogen environment, and by reaction of SiCl₄ with ammonia; these reactions produce a very fine powder. High purity silicon nitride powder has also been made by laser reactions in which a mixture of silane (SiH₄ ) and ammonia is exposed to laser light from a CO₂ laser. This produces spherical particles of silicon nitride of very fine size.

Processing

The raw materials for ceramics are chosen on the basis of desired purity, particle size distribution, reactivity, and form. Purity influences such high temperature properties as strength, stress rupture life, and oxidation resistance. Impurities may severely influence electrical, magnetic, and optical properties. Particle size distribution affects strength.

Binders may be added to the ceramic powder to add strength prior to densification. Lubricants reduce particle-tool friction during compaction. Other agents are added to promote flowability during shaping.

Forming processes

The ceramic powder along with suitable additives are placed in a die, to which pressure may be applied for compaction. Uniaxial pressing is often used for small shapes such as ceramics for electrical devices. Hydrostatic pressing (equivalent pressing from all sides) is often used for large objects.

Alternatively, the ceramic powder may be cast. Although molten ceramics may be cast into cooled metal plates and quenched to produce materials made up of very fine crystals with high material toughness, casting of ceramics is usually done at room temperature. The ceramic particles are first suspended in a liquid and then cast into a porous mold that removes the liquid, leaving a particulater compact in the mold.

Yet another method of shaping a ceramic involves plastic forming. In this process, a mixture of ceramic powder and additives is deformed under pressure. In

KEY TERMS

Ceramic —A hard, brittle substance produced by strongly heating a nonmetallic mineral or clay.

Glass —A ceramic material consisting of a uniformly dispersed mixture of silica, soda ash, and lime; and often combined with metallic oxides.

Refractory —Any substance with a very high melting point that is able to withstand very high temperatures.

Sintering —The bonding of adjacent surfaces of particles in a mass of metal powders by heating.

the case of pure oxides, carbides, and nitrides, an organic material is added in place of or in addition to water to make the ceramic mixture plastic. While forming the ceramic object, heat and pressure are usually applied simultaneously.

Sintering

Densification of the particulate ceramic compact is referred to as sintering. Sintering is essentially the removal of pores between particles, combined with particulate growth and strong bonding between adjacent particles. In order for sintering to occur, the particles must be able to flow, and there must be a source of energy to activate and sustain this material transport. Sintering can take place in the vapor, liquid, or solid phase, or in a reactive liquid.

Machining

The sintered material must frequently be machined to allow it to meet dimensional tolerances, to give it an improved surface finish, or to remove surface flaws. Machining must be done carefully to avoid brittle fracture. The machining tool must have a higher hardness than the ceramic. The ceramic material can be processed by mechanical, thermal, or chemical action.

Design considerations

When evaluating the suitability of a ceramic material for a particular application, it is first necessary to understand the requirements of the application. These requirements might typically be defined by the load that the material will experience, the stress distribution in the material, interface, frictional requirements, chemical environment and range of temperatures that the material will experience, and restriction on the final cost of the materials. Usually, one or two material properties will dictate the choice of a material for a particular application.

Historically most ceramic designs have been developed by empirical, or trial-and-error investigation. Only since the advent of the digital computer has it been possible to predict the properties of a particular ceramic material prior to actually producing it.

Resources

BOOKS

Mackey, Maureen. Experience Clay. Worcester, MA: Davis Publishing, 2003.

Nelson, Glenn C. Ceramics: A Potter’s Handbook. Fort Worth, TX: Wadsworth/Thomason Learning, 2002.

Rice, Roy Warren. Ceramic Fabrication Technology. New York: Marcel Dekker, 2003.

Sentence, Bryan. Ceramics: A World Guide to Traditional Techniques. London and New York: Thames and Hudson, 2004.

Randall Frost

Ceramics

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, Al₂O₃; nitrides, such as silicon nitride, Si₃N₄; 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.

Composition

Some ceramics are composed of only two elements. For example, alumina is aluminum oxide, Al₂O₃; zirconia is zirconium oxide, ZrO₂; and quartz is

silicon dioxide, SiO₂. 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 KAlSi₃O₈.

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, BaTiO₃. 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, Mg²⁺ ions alternate with O²⁻ 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 YBa₂Cu₃O_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.

see also Glass; Minerals; Semiconductors; Superconductors.

Loretta L. Jones

Bibliography

Ball, Philip (1997). Made to Measure: New Materials for the Twenty-First Century. Princeton, NJ: Princeton University Press.

Barsoum, Michael W. (1996). Fundamentals of Ceramics. New York: McGraw-Hill.

Brinker, C. Jeffrey, and Scherer, George W. (1990). Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Boston: Academic Press.

Calvert, Paul (2000). "Advanced Materials." In The New Chemistry, ed. Nina Hall. New York: Cambridge University Press.

Kingery, W. D.; Bowen, H. K.; and Uhlmann, D. R. (1976). Introduction to Ceramics, 2nd edition. New York: Wiley.

Richerson, David W. (1992). Modern Ceramic Engineering: Properties, Processes, and Use in Design, 2nd edition, revised and expanded. New York: Marcel Dekker. Richerson, David W. (2000). The Magic of Ceramics. Westerville, OH: American Ceramic Society.

Shackleford, James F., ed. (1998). Bioceramics: Applications of Glass and Ceramic Materials in Medicine. Zurich: Trans-Tech Publications.

Wachtman, John B., Jr., ed. (1999). Ceramic Innovations in the 20th Century. Westerville, OH: American Ceramic Society.

Internet Resources

"About Ceramics." American Ceramic Society. Available from <http://www.ceramics.org>.

Ceramics

Ceramic materials are usually understood to be compounds of metallic and nonmetallic elements, though some are actually ionic salts, and others are insulators. These materials can be very complicated, as are for example clays, spinels, and common window glass . Many ceramic compounds have very high melting points.

Ceramics have a wide range of applications. They have been used as refractories, abrasives , ferroelectrics, piezoelectric transducers, magnets, building materials, and surface finishes.

Unlike metals, there are really no heat treatments that can be used to modify the properties of ceramics, but their properties can be altered by changes in chemical composition. By carefully considering the choice of chemical composition, purity, particle size and uniformity and arrangement, and packing of atoms , high quality ceramics can be synthesized in a wide variety.

Traditional ceramics

Ceramics have been used by man since antiquity. The earliest ceramic articles were made from naturally occurring materials such as clay minerals . It was discovered in prehistoric times that clay materials become malleable when water is added to them, and that a molded object can then be dried in the sun and hardened in a high temperature fire. The word ceramic comes from

the Greek word for burnt material, keramos. Many of the same raw materials that were used by the ancients are still used today in the production of traditional ceramics. Traditional ceramic applications include whitewares, heavy clay products, refractories, construction materials, abrasive products, and glass.

Clay minerals are hydrated compounds of aluminum oxide and silica. These materials have layered structures. Examples include kaolinite, halloysite, pyrophillite, and montmorillonite. They are all formed by the weathering of igneous rocks under the influence of water, dissolved CO2, and organic acids. The largest sources of these clays were formed when feldspar was eroded from granite and deposited in lake beds, where it became altered to a clay.

Silica is a major ingredient in glass, glazes, enamels, refractories, abrasives, and whiteware. Its major sources include quartz, which is made up primarily of sand , sandstone, and quartzite.

Feldspar is also used in the manufacture of glass, pottery, enamel, and other ceramic products. Other naturally occurring minerals used directly in ceramic production include talc, asbestos , wollastonite, and sillimanite.

Hydraulic cement

Hydraulic cements set by interaction with water. Portland cement, the most common hydraulic cement, is primarily a water-free calcium silicate. It is slightly soluble in water and sets by a combination of solution precipitation and chemical reaction with water to form a hydrated composition. The ratio of water to cement in the initial mix greatly influences the strength of the final concrete : the lower the water-to-cement ratio, the higher the strength.

Glass

Glass is a ceramic material consisting of uniformly dispersed mixtures of silica, soda ash, and lime, that is often combined with metallic oxides of calcium, lead , lithium , cerium, etc. Glass is distinguished from solid ceramics by its lack of crystallinity. It is in fact a supercooled liquid. The atoms in glass remain disordered in the solid state, much as they are in the liquid state. Glasses are thus rigid structures whose atomic arrangements and properties depend on both composition and thermal history.

Group 16 elements (oxygen , sulphur, selenium, tellurium) are especially good candidates for glass formation. Oxygen is able to form stable bonds with silicon, boron, phosphorous, and arsenic and thereby form stable structures having oxygen atoms at the corners and one of the other atoms at the center. Pure oxide glasses are very stable because each oxygen atom is linked by electron bonds to two other atoms.

Various two-phase structures may exist as glasses. One such structure is a mixture of glass and crystal. These materials are converted into strong and durable ceramics that are part glass and part crystal by prolonged heat treatment.

Modern ceramics

In the twentieth century, scientists and engineers have acquired a much better understanding of ceramics and their properties. They have succeeded in producing ceramics with tailor-made properties. Modern ceramics include oxide ceramics, magnetic ceramics, ferroelectric ceramics, nuclear fuels, nitrides, carbides, and borides.

Aluminum oxide

Aluminum oxide (Al2O3) occurs naturally in the mineral corundum, which in gem-quality form is known as the precious stones ruby and sapphire. Ruby and sapphire are known for their chemical inertness and hardness. Al2O3 is produced in large quantities from the mineral bauxite. In the Bayer process, bauxite (primarily aluminum hydroxide mixed with iron hydroxide and other impurities) is selectively leached with caustic soda. Purified aluminum hydroxide is formed as a precipitate. This material is converted to aluminum oxide powder,

which is used in the manufacture of aluminum-oxidebased ceramics. Aluminum oxide powder is used in the manufacture of porcelain, alumina laboratory ware, crucibles and metal casting molds, high temperature cements, wear-resistant parts (sleeves, tiles, seals), sandblast nozzles, etc.

Magnesium oxide

Magnesium oxide (MgO) occurs naturally in the mineral periclase, but not in sufficient quantity to meet commercial demand. Most MgO powder is produced from MgCO3 or from seawater. MgO is extracted from sea water as a hydroxide, then converted to the oxide. MgO powder finds extensive use in high temperature electrical insulation and in refractory brick .

Silicon carbide

Silicon carbide (SiC) has been found to occur naturally only as small green hexagonal plates in metallic iron. The same form of silicon carbide has been manufactured synthetically, however. In this process, SiO2 sand is mixed with coke in a large elongated mound in which large carbon electrodes have been placed at either end. As electric current is passed between the electrodes, the coke is heated to about 3,992°F (2,200°C). The coke reacts with the SiO2 to produce SiC plus CO gas. Heating continues until the reaction has completed in the mound. After cooling, the mound is broken up, and the green hexagonal SIC crystals, which are low in impurities and suitable for electronic applications, are removed. The lower purity material is used for abrasives. The outer layer of the mound is reused in the next batch. SiC can be formed from almost any source of silicon and carbon. It has been produced in the laboratory from silicon metal powder and sugar, and from rice hulls. SiC is used for high-temperature kiln furniture, electrical resistance heating elements, grinding wheels and abrasives, wear-resistance applications, and incinerator linings.

Silicon nitride

Silicon nitride does not occur naturally. Most of the powder commercially available has been produced by reacting silicon metal powder with nitrogen at temperatures between 2,282°F (1,250°C) and 2,552°F (1,400°C). The powder that is removed from the furnace is not ready to use. It is loosely bonded and must be crushed and sized. The resulting powder contains impurities of Fe, Ca, and Al. Higher purity silicon nitride powder has been produced by reducing SiO2 with carbon in a nitrogen environment, and by reaction of SiCl4 with ammonia ; these reactions produce a very fine powder. High purity silicon nitride powder has also been made by laser reactions in which a mixture of silane (SiH4) and ammonia is exposed to laser light from a CO2laser. This produces spherical particles of silicon nitride of very fine size.

Processing

The raw materials for ceramics are chosen on the basis of desired purity, particle size distribution, reactivity, and form. Purity influences such high temperature properties as strength, stress rupture life, and oxidation resistance. Impurities may severely influence electrical, magnetic, and optical properties. Particle size distribution affects strength.

Binders may be added to the ceramic powder to add strength prior to densification. Lubricants reduce particle-tool friction during compaction. Other agents are added to promote flowability during shaping.

Forming processes

The ceramic powder along with suitable additives are placed in a die, to which pressure may be applied for compaction. Uniaxial pressing is often used for small shapes such as ceramics for electrical devices. Hydrostatic pressing (equivalent pressing from all sides) is often used for large objects.

Alternatively, the ceramic powder may be cast. Although molten ceramics may be cast into cooled metal plates and quenched to produce materials made up of very fine crystals with high material toughness, casting of ceramics is usually done at room temperature. The ceramic particles are first suspended in a liquid and then cast into a porous mold that removes the liquid, leaving a particulater compact in the mold.

Yet another method of shaping a ceramic involves plastic forming. In this process a mixture of ceramic powder and additives is deformed under pressure. In the case of pure oxides, carbides, and nitrides, an organic material is added in place of or in addition to water to make the ceramic mixture plastic. While forming the ceramic object, heat and pressure are usually applied simultaneously.

Sintering

Densification of the particulate ceramic compact is referred to as sintering. Sintering is essentially the removal of pores between particles, combined with particulate growth and strong bonding between adjacent particles. In order for sintering to occur, the particles must be able to flow, and there must be a source of energy to activate and sustain this material transport. Sintering can take place in the vapor, liquid, or solid phase, or in a reactive liquid.

Machining

The sintered material must frequently be machined to allow it to meet dimensional tolerances, to give it an improved surface finish, or to remove surface flaws. Machining must be done carefully to avoid brittle fracture. The machining tool must have a higher hardness than the ceramic. The ceramic material can be processed by mechanical, thermal, or chemical action.

Design considerations

When evaluating the suitability of a ceramic material for a particular application, it is first necessary to understand the requirements of the application. These requirements might typically be defined by the load that the material will experience, the stress distribution in the material, interface, frictional requirements, the chemical environment and range of temperatures that the material will experience, and restriction on the final cost of the materials. Usually, one or two material properties will dictate the choice of a material for a particular application.

Historically most ceramic designs have been developed by empirical, or trial-and-error investigation. Only since the advent of the digital computer has it been possible to predict the properties of a particular ceramic material prior to actually producing it.

Resources

books

Richerson, David W. Modern Ceramic Engineering. New York, NY: Marcel Dekker, Inc., 1982.

Randall Frost

KEY TERMS

Ceramic

—A hard, brittle substance produced by strongly heating a nonmetallic mineral or clay.

Glass

—A ceramic material consisting of a uniformly dispersed mixture of silica, soda ash, and lime; and often combined with metallic oxides.

Refractory

—Any substance with a very high melting point that is able to withstand very high temperatures.

Sintering

—The bonding of adjacent surfaces of particles in a mass of metal powders by heating.

Ceramic

Ceramic is a hard, brittle substance that resists heat and corrosion and is made by heating a nonmetallic mineral or clay at an extremely high temperature. The word ceramic comes from the Greek word for burnt material, keramos. Ceramics are used to produce pottery, porcelain, china, and ceramic tile. They may also be found in cement, glass, plumbing and construction materials, and spacecraft components.

The basic ingredient in all forms of ceramics are silicates, the main rock-forming minerals. Most silicates are composed of at least one type of metal combined with silicon and oxygen. Feldspar and silica are example of silicates. When silicates are combined with a liquid such as water, they form a mixture that can be kneaded and shaped into any form. After shaping, the object is dried and fired in a high-temperature oven called a kiln. A glaze (a glasslike substance that makes a surface glossy and watertight) may be added between drying and firing. From ancient days to the present, this process has remained virtually the same, except for the addition of mechanical aids.

Pottery

The oldest examples of pottery, found in Moravia (a region of the Czech Republic) and dating back to 25,000 b.c., are animal shapes made of fired clay. Potter's wheels and kilns first appeared in Mesopotamia (an ancient region in southwest Asia) around 3000 b.c. Some of the most fascinating pottery in history was made by the ancient Greeks, whose vases were skillfully decorated in the methods of black figure (black paint applied to red clay) or red figure (black paint covering all but the design, which stood out in red clay). Early Islamic potters of the Middle East produced colorful, imaginatively glazed tiles and other items. Their elaborate pictorial designs have provided archaeologists with many clues to their daily lives.

Perhaps the most renowned potters of all time are the Chinese, who developed the finest form of pottery—porcelain. Made of kaolin (pronounced KA-uh-lin; a white clay free of impurities) and petuntse (a feldspar mineral that forms a glassy cement), porcelain is fired at extremely high temperatures. The result is a high-quality material that is uniformly translucent, glasslike, and white. Porcelain was first made in China during the T'ang Dynasty (618–906).

Modern ceramics

In the twentieth century, scientists and engineers acquired a much better understanding of ceramics and their properties. During World War II (1939–45), a high demand for military materials hastened the evolution of the science of ceramics. These materials are now found in a wide variety of products, including abrasives, bathroom fixtures, and electrical insulation.

During the 1960s and 1970s, the growing fields of atomic energy, electronics, communication, and space travel increased demand for more sophisticated ceramic products. Because ceramics can withstand extreme temperatures, they have been used in gas turbines and jet engines. The undersides of the space shuttles are lined by some 20,000 individually contoured silica fiber tiles that are bonded to a felt pad. The felt pad in turn is bonded to the body of a shuttle. These ceramic tiles can withstand a maximum surface temperature of 1,200 to 1,300°F (650 to 705°C).

In 1990, a team of Japanese scientists working for their government developed a stretchable material from silicon-based compounds. When made into strips and heated, this special ceramic material can be stretched to two and-a-half times its original length without losing its hardness and durability.

CERAMICS

a durable material with a history spanning 10,500 years that is significant to the study of archaeology and history.

Ceramic figurines and pottery vessels in Anatolia and the Iranian Plateau date to 8500 b.c.e.. The archaeological, ethnographic, and historic evidence for ceramic production in the Middle East and North Africa is complex and has a voluminous literature. The earliest Islamic potters (Umayyad dynasty, 661–750 c.e.) inherited extant traditions: Blue- and green-glazed wares had been produced in Egypt since Roman times; the alkaline-glazed ceramics of Syria, Iraq, and Iran had been made since Achaemenid times (seventh to fourth centuries b.c.e.); and the Roman lead-glazed ceramic tradition had been continued by the Byzantines. Chinese influences (Tang stoneware, ninth to eleventh centuries; Song whitewares, twelfth to fourteenth centuries, and Ming blue- and whiteware, fifteenth to nineteenth centuries) were significant. The spread of Islam correlates with the distribution of hybrid production methods (molds, tin glazes, under-glazes, polychromy, and metallic pigments) and products (architectural tiles). Early Islamic wares included Umayyad (Mediterranean/Middle Eastern influence), Abbasid (Tang influence), Central Asian Samanid, Egyptian Fatimid, and Mesopotamian/Persian wares (twelfth to fifteenth centuries) from Rayy, Raqqah, and Kashan. Later Persian ceramics (fifteenth to nineteenth centuries) were made at Kubachi, Tabriz, and Kerman; Syrian artisans produced work at al-Fustat, Raqqa, and Damascus; Seljuk Turks fabricated wares at Iznik and Kütahya. Lusterware, Mina'i, Iznik, Gombroon, and Zillij are notable Islamic contributions to ceramic history.

The Museum of Islamic Ceramics in Cairo, the Ashmolean Museum in Oxford, and the Metropolitan Museum of Art in New York house specimens from different Islamic eras that span the region from Morocco in the west through Iran, Afghanistan, and Indonesia in the east. Although Iznik ceramics were prized by the Ottoman court into the early twentieth century, ceramic vessels and tiles produced from the earliest times to the present in Islamic lands, including Central Asia, are esteemed by museums, art historians, and collectors. With the availability of metal and plastic replacements, utilitarian production has diminished, but ceramic art and tile production remains strong.

Bibliography

Watson, Oliver. Ceramics from Islamic Lands. London: Thames and Hudson, 2003.

Whitehouse, David; Grube, Ernest J.; and Crowe, Yolande. "Ceramics: Islamic." In Encyclopedia Iranica. London; Boston: Routledge & Kegan Paul, 1995.

Elizabeth Thompson

Updated by Charles C. Kolb