Ceramics << suh RAM ihks >> are one of the three most important types of engineering materials that are primarily synthetic. The other two are metals and plastics. Ceramics include such everyday materials as brick, cement, glass, and porcelain. They also include unusual materials used in electronics and spacecraft. Most ceramics are hard and can withstand heat and chemicals. These properties give them a variety of uses in industry. Artists also create ceramics. This article discusses the use of ceramics in industry. For information on how artists make ceramics, see Pottery.
Properties of ceramics.
Manufacturers make common ceramics from such minerals as clay, feldspar, silica, and talc. These minerals, called silicates, form most of the earth’s crust. Clay is an important silicate. But it is not used in all ceramic materials. Glass, for example, is made from sand. Chemists make materials called advanced ceramics in the laboratory from compounds other than silicates. These compounds include alumina, silicon carbide, and barium titanate.
Most ceramic products, like their mineral ingredients, can withstand acids, gases, salts, water, and high temperatures. But not all ceramic products have the same properties. Common ceramics are good insulators—that is, they conduct electric current poorly. Certain ceramics, however, lose their electrical resistance and become superconductors when they are cooled (see Superconductivity). Some ceramic materials are magnetic. Engineers control the properties of ceramics by controlling the proportion and type of materials used.
Kinds of ceramic products.
The properties of ceramics make them especially suitable for certain products. Products made of ceramic materials include abrasives (materials used for grinding), construction materials, dinnerware, electrical equipment, glass products, and refractories (heat-resistant materials).
Abrasives.
Manufacturers use some extremely hard ceramic materials for cutting metals and for grinding, sanding, and polishing. These materials include alumina, silicon carbide, and silicon nitride.
Construction materials.
Clay and shale are used in making strong, durable bricks and drainpipes for homes and other buildings. Tiles are made of clay and talc. Cement consists chiefly of calcium silicates and is used primarily in making concrete. Gypsum is used to produce plaster for the surfaces of walls and ceilings. Bathtubs, sinks, and toilets are made of porcelain, which consists chiefly of clay, feldspar, and quartz.
Dinnerware.
Ceramics make excellent containers for food and drinks. They do not absorb liquids, and they resist acids, salts, detergents, and changes in temperature. Most ceramic dinnerware is made from a mixture of clays, feldspar, and quartz.
Electrical and electronic equipment.
Ceramics that do not conduct electric current are used as insulators in automobile spark plugs, on electric power lines, and in television sets. Such ceramics include alumina and porcelain. Another ceramic, barium titanate, is used in making capacitors, which store electric charges in electronic equipment. Magnetic ceramics are widely used in cellular telephones and electric motors. Complex electronic circuits are bonded on thin layers of alumina.
Glass products.
Glass is one of the most important materials, chiefly because of its transparency. Products made of glass include food containers, light bulbs, windows, and lenses for eyeglasses and telescopes. Fiberglass insulates the walls of many homes. Cables made of glass fibers transmit telephone calls and other information (see Fiber optics). The main ingredient in glass is silica. See Glass.
A glasslike coating called porcelain enamel serves as a protective surface on many metal products. These products include such appliances as refrigerators, stoves, and washing machines. Porcelain enamel also makes outdoor signs weather resistant.
Refractories.
The property of heat resistance makes refractories suitable for the manufacture of industrial boilers and furnaces, such as the furnaces used to make steel. Refractories shaped into tiles cover the surface of spacecraft and some spaceplanes, which must withstand the intense heat of atmospheric reentry. Ceramics used in making refractories include alumina, magnesium oxide, silica, silicon carbide, and zirconium oxide.
Other products.
Ceramic engineers continually develop new uses for ceramics. For example, porcelain is used to make false teeth and alumina to make artificial bone joints. Uranium oxide ceramics serve as fuel elements for nuclear reactors. Cutting tools are made from silicon nitride. Refractories made from carbides are used to make parts for aircraft engines. Alumina is used in making certain types of lasers (instruments that produce narrow beams of intense light).
Making ceramics.
The clays and other minerals used in ceramics are dug from the earth and refined to improve their purity. Machines crush and grind the materials into fine particles. The particles are mixed in the proper proportion, and water or other liquid is added to produce a mixture that can be shaped. A gluelike substance is sometimes added to mixtures that do not contain clay. Glass and some refractory products are made by melting the particles and shaping them when they are molten.
The most common methods for shaping clay ceramics are slip casting, jiggering, extrusion, and pressing. In slip casting, the liquid mixture is poured into a mold that absorbs water. As the water is absorbed, a layer of ceramic particles is deposited onto the mold, forming such hollow items as teapots and crucibles (melting pots). The excess liquid is then poured out of the mold. In jiggering, a machine presses the clay onto a rotating mold. Jiggering is used to make dinnerware. Extrusion shapes items into rods or tubes by forcing ceramic paste through a shaping tool called a die. In pressing, ceramic powder is pressed in a steel die or a rubber mold.
After the product has dried, it is strengthened by firing, a process that takes place in special furnaces called kilns. Ceramics are fired at temperatures ranging from about 1200 to 3000 °F (650 to 1650 °C). Firing hardens the product permanently and gives it strength, durability, and other desired qualities.
Manufacturers cover many ceramic products with a glassy coating called glaze. Glaze prevents the item from absorbing liquids and makes it smoother and easier to clean. Glazes are also used for decoration.
History.
Fired-clay figurines are the oldest known ceramic objects. Figurines dating back to about 25,000 B.C. have been found in central Europe. The earliest known ceramic pots, found in northern Japan, date from about 14,500 B.C.
Industrial uses of ceramics began in the 1900’s. Military requirements of World War II (1939-1945) created a need for high-performance materials and helped speed the development of ceramic science and engineering. During the 1960’s and 1970’s, advances in electronics, communications, nuclear energy, and space travel required new kinds of ceramics. For example, heat resistant ceramic tiles were created to help protect spacecraft from the high temperatures generated upon their reentry into earth’s atmosphere. The discovery of ceramic superconductors in the 1980’s spurred intense worldwide research to develop superconducting ceramic parts for electronic devices.
