Materials made of clay are among the most ancient manufactured articles and have played a vital role in human civilization. Although clay, as a ceramic material, is still widely used, modern ceramics include a wide range of nonorganic, nonmetallic materials whose manufacture requires heating at high temperatures. Important ceramics products include brick and tile, clay pipe, refractory brick, pottery and porcelain articles and enamels, ferrites in computer memories, barium titanate and alumina in electronics, uranium dioxide as nuclear fuel, and garnets in lasers. Glass and cement are also major ceramic materials

The raw materials used to make ceramics are inexpensive and widely available, and include clay, feldspar, quartz sand, iron oxides, and alumina. Clay is made up of fine, platelike crystals of hydrated aluminosilicates. The crystals are usually from about 1 to 10 microns (.001 to .010 mm) in their longest dimension. A thin film of water binds the crystals together and with their platelike shape gives the clay its easy working properties. The platelike form of clay crystals reflects the molecular layer structure of the silicon-oxygen and aluminum-oxygen groups in the clay compounds.

CERAMICS MANUFACTURE

Dust Press Method:
Most American manufacturers use the press method for forming tile. Powdered raw materials are mixed with water or other binding liquids, and are formed by a hydraulic press. The clay is usually pressed damp, with about 10 percent water, into dies or molds under moderate pressures. Ceramics made of purified powders such as alumina and ferrites are pressed dry at higher pressure with an organic binder (for example, 1 percent polyvinyl alcohol). In isostatic pressing, the powder is held in a rubber mold, and the pressure is applied with a fluid such as glycerine giving uniform pressure throughout the sample, with less warping and fewer defects. Samples in bar, rod, or tube form can be extruded through a die. The fired bisque, or clay body can absorb water, and is generally regarded as a "soft" tile, another common name of this type of tile is whiteware.

Hand Made tile - Slip casting:
In slip casting, a suspension of ceramic powder, usually in water, is poured into a mold made of plaster of paris. Water is absorbed by the mold, and a hard lining on the mold wall is built up; excess liquid is poured out of the mold. Using slip casting, a number of complex shapes can be made economically, since the cost of the molds is low. These tiles are also considered soft.


After forming, the ceramic ware must be carefully heated for a few hours at about 100 degrees-200 degrees C (about 200 degrees-400 degrees F) to remove excess water or binder. The rate of drying must be carefully controlled so that warping and defects do not form as the sample shrinks. After drying, the article is fired at a high temperature (800 degrees-2000 degrees C/1500 degrees-3500 degrees F) to sinter or bind together the individual crystals of the ceramic powder into a solid, coherent mass. The higher the firing temperature, the more dense and less porous the material becomes. A wide range of properties in ceramics is possible with different firing temperatures and times.


IMPORTANT CERAMIC CATEGORIES

The most common ceramic articles of pottery, porcelain, brick, and pipe form complex mixtures of several different solid phases after firing. Traditional whitewares and porcelains contain at least three starting materials--clay, feldspar, and silica sand. When a mixture of these materials is heated at high temperatures (above 1200 degrees C/2200 degrees F), the feldspar (potassium-sodium aluminosilicate) melts and coats the clay and sand crystals. As firing proceeds, tightly bound water in the clay structure is removed, and fine, needlelike crystals of an aluminosilicate called mullite are formed from the clay. The grains of silica sand are partly dissolved in the viscous liquid feldspar. In the cooled structure there is a glassy phase from the liquid feldspar that binds together the sand grains and mullite crystals. This glassy phase may also give the ware a smooth, polished surface.

Firing at an intermediate temperature (about 1100 degrees C/2000 degrees F) produces stoneware, a heavy, opaque ceramic, nonporous and glazed. At lower firing temperatures (less than 1000 degrees C/1832 degrees F), a more porous ware with a rough surface results and is usually called earthenware. To make fine, translucent porcelain requires a higher firing temperature (up to 1400 degrees C/2500 degrees F) so that more glass is formed.

NEW CERAMICS APPLICATIONS
"High technology" ceramics are new types of materials that surpass earlier ceramics in strength, hardness, light weight, or improved heat resistance. For example, ceramic powders can now be made from particles of absolutely uniform size. When sintered, these powders produce ceramics that are far less vulnerable to fracture or thermal shock than ordinary ceramics. Added to a matrix of metal or ceramic, thin ceramic fibers increase the tensile strength of the material (see composite material).

Barium titanate has a high dielectric constant and consequently is used in capacitors. It is also strongly piezoelectric (see piezoelectricity), which means that it develops an electrical voltage when stressed in a particular crystallographic direction. It is used in microphones, phonograph pickups, strain gauges, and ultrasonic devices.

The apatites are a family of calcium phosphate minerals that have been widely used as phosphors in fluorescent lamps. Special impurities in the apatite--such as manganese--fluoresce and thereby change the ultraviolet light of a mercury arc into visible light. In a lamp this change increases the output of visual radiation for a given input. Hydroxylapatite is a bone mineral, and has recently been developed as a bone and tooth implant material.
The most convenient form of nuclear fuel is uranium dioxide. Uranium ores are reacted and purified to form uranium dioxide, which is then sintered into pellets that serve as nuclear fuel. The pellets are packed into long tubes and are especially stable even with the severe radiation and thermal conditions they encounter.
Beta-alumina (a compound of 1 part sodium oxide with 11 parts aluminum oxide) has been traditionally used as a refractory material; recently it was found to have high electrical conductivity at low temperatures. This conductivity results from high mobility of the sodium ions, and beta-alumina is being used as an electrolyte in high-temperature batteries. The beta-alumina separates liquid sodium and a liquid sulfur-sodium polysulfide mixture; as the battery is charged or discharged, the sodium ions in the electrolyte transfer charge from one liquid to another. This battery is highly efficient and should find application in electric vehicles and for storage of electric power during off-peak hours.

Ceramics have proven to be ideal hosts for the fluorescent ions needed in lasers. Ruby, which is alumina containing some chromium impurity, is one of the most used laser materials, and garnets are also excellent laser hosts.

Amorphous ceramics are produced by firing ceramic material for a short time at low temperatures, to produce substances that lack the usual crystalline ceramic structure. Like plastics, these ceramics can be sprayed onto surfaces or injection-molded before they are fired. They are used to make complex shapes and thin ceramic films.
Ultrahard ceramic layers are built into the steel in tank bodies. When a projectile penetrates a layer, it pulverizes the ceramic, breaking the bonds that bind the molecules together. This chemical change causes the ceramic fragments to expand. In expanding, they grind up the softer material of the projectile, making it inoperative.

Ceramic fibers combined with epoxy glues produce a composite fabric that is lightweight but stronger than steel. It is currently used in making small airplane bodies.

For use at high temperatures, ceramics are in many ways superior to metals. An engine made from ceramic material would, for example, be lighter in weight, less subject to corrosion and chemical wear, and could operate at higher temperatures without requiring a water-cooling system. Such an engine would be much more fuel efficient than one made of metal. Although ceramics are used today for such mechanical applications as the blades of turbines, a fracture-proof ceramic material has not yet been developed for the basic parts of a diesel or gas-turbine engine (the two most likely ceramic-engine types). Researchers, however, anticipate a fully ceramic experimental engine by the mid-1990s.

Robert H. Doremus
Bibliography: British Ceramic Society, Electrical Magnetic Ceramics (1982) and Mechanical Properties of Ceramics (1982); Budworth, D. W., An Introduction to Ceramic Science (1970); Clark, G., American Ceramics (1988); Dinsdale, A., Pottery Science (1986); Hamer, Frank, Potter's Dictionary of Materials and Techniques (1975); Ichinose, N., ed., Introduction to Fine Ceramics (1987); Pampuch, R., Ceramic Materials: An Introduction to Their Properties (1976); Rawson, Philip, Ceramics (1983); Richardson, David W., Modern Ceramic Engineering (1982); Spaeto, S., ed., Advanced Ceramics (1989); Vincenzini, P., High Tech Ceramics, 3 vols. (1987).
(c) 1997 Grolier, Inc.