Ceramics

Last updated: May 5, 2009.

The major periods of civilization, such as the Stone Age and Bronze Age, were named for the materials that dominated them, and it may seem surprising that there has never been a "Ceramics Age." Yet almost any age might have qualified for this title. Archaeologists have found evidence of primitive ceramic manufacture dating back to around 24,000 B.C.E., but those most modern of materials, the silicon chip and the catalytic converter, are also examples of ceramics. The modern era is as much a ceramics age as any other.

Photo: A ceramic Percy Pig piggy bank. It started life as a soft piece of clay molded to shape, fired hard in a pottery kiln, then painted with bright colors.

What are ceramics?

Ceramics once referred purely to pottery and to articles made by firing materials extracted from Earth. Today, the term has a much broader definition. Ceramics are generally thought of as inorganic and nonmetallic solids with a range of useful properties, including very high hardness and strength, extremely high melting points, and good electrical and thermal insulation.

The best-known ceramics are pottery, glass, brick, porcelain, and cement. But the general definition of a ceramic—a nonmetallic and inorganic solid—is so broad that it covers a much wider range of materials. At one end of the scale, ceramics include simple materials such as graphite and diamond, made up from different crystalline arrangements of the element carbon. But at the other end of the scale, complex crystals of yttrium, barium, copper, and oxygen make up the advanced ceramics used in so-called high-temperature superconductors (materials with almost no electrical resistance). Most ceramics fall somewhere between these extremes. Many are metal oxides, crystalline compounds of a metal element and oxygen. Others are silicides, borides, carbides, and nitrides, respectively made from silicon, boron, carbon, and nitrogen. Some of the most advanced ceramic materials are combinations of ceramics and other materials known as ceramic matrix composites (CMCs).

Photo: These ceramic bearings and other assorted components are designed to withstand extremely high temperatures in gas-turbine engines that would melt ordinary metal parts. Picture courtesy of US Department of Energy/Oak Ridge National Laboratory.

Properties of ceramics

Ceramics are best known as brittle solids particularly suited for withstanding high temperatures but, in fact, the different materials used in ceramics can give them a wide range of properties. The classic properties of ceramics include durability, strength and brittleness, high electrical and thermal resistance, and an ability to withstand the damaging effects of acids, oxygen, and other chemicals because of their inertness (chemical unreactivity). But not all ceramics behave in this way. For example, graphite is a very soft ceramic and conducts electricity well, whereas diamond is a very good conductor of heat. Ceramics called ferrites are particularly good conductors of electricity and superconductors have almost no electrical resistance at all. Ceramic matrix composites, made by embedding fibres of a strengthening material in what is known as a ceramic matrix, are not at all brittle.

The properties of a particular ceramic depend not just on the materials from which it is made but also on the way they are joined together—in other words, on its crystalline structure. Diamond is strong because all of its carbon atoms are bonded tightly to other carbon atoms. Graphite (such as that used in pencil "leads") shears because it is made up from different layers. Although the carbon atoms are tightly bonded within a given layer, the different layers are held together only by much weaker bonds. China clay (also called kaolin) behaves in a similar way to graphite, with its constituent aluminum, silicon, oxygen, and hydrogen atoms tightly bonded into flat sheets. But the weak bonds between those sheets are easily broken when water surrounds them and it is this that makes wet clay so easy to mold. When china clay is fired, heat removes the water, and the chemicals inside the clay rearrange themselves into crystals of aluminium silicate tightly bonded by silicate glass, which is overall very much stronger.

How ceramics are made

Firing is the process by which ceramics have traditionally been made; indeed, the word "ceramic" can be traced back to a Sanksrit word meaning "to burn." Simple ceramics such as bricks and certain types of glass are still made by processes that would be recognized by people who lived thousands of years ago. Just as in ancient times, today's pottery is made by digging clay from the ground, mixing it with water to make it flexible, shaping it on a wheel or in a mold, and then firing it in a kiln. Some of today's processes are more sophisticated than the techniques of past times. Machines have long been used in processes such as extrusion (forcing a material into shape by squeezing it like toothpaste through a shaped tool), jiggering (laying the material automatically into a rotating mold), or hot pressing (forcing a powdered form of the ceramic into a mold then simultaneously heating it and pressing it to fuse the material into shape).

Photo: Ceramic floor tiles get their hardness and durability from being fired. Picture by Michael Sandberg courtesy of US Navy.

The latest industrial ceramics sometimes demand more advanced production processes. Extremely tough ceramics made of silicon nitride are made by a method called reaction bonding. This involves forming silicon powder into the desired shape then heating it with nitrogen gas. Because the silicon powder already occupies the same volume as the finished product, grains of silicon nitride can form only by fusing together tightly.

The world of modern ceramics

Photo: Toilets are still largely made from ceramics (though the lid and seat are typically made of plastic or wood).

It is difficult to think of an area of modern life that has not been touched by ceramics. Our homes are made from brick walls, held together by cement made from calcium silicates, and glass windows, also made from silica. Inside, the walls are plastered with ceramic gypsum, porcelain bathrooms are decorated with tiles made of clay and talc, and kitchens stacked with pottery and glass have decorative ceramic floor tiles. Clay pipes link our homes to the sewage system and ceramic insulators are essential in connecting them to the electricity grid. Back inside the house, that electricity flows through television sets that contain more ceramic insulators, components such as capacitors and resistors made from ceramics, computers based on silicon chips, magnetic ceramics used in the electric motors of appliances such as vacuum cleaners and food blenders, and piezoelectric ceramics used in tiny headphones and loudspeakers. Telephone calls and cable television signals may be piped to the home through glass fibers, while other kinds of glass fibers keep heat inside the walls and the roof. That heat may itself be provided by a heated ceramic filament, just as lighting comes from glass bulbs or fluorescent tubes.

Photo: Ceramic components help car engines withstand high temperatures.

But ceramics have not just proved useful in everyday situations. The properties of advanced ceramics have made them important for some much more extraordinary applications. For example, the toughened silicon carbide used in hip replacements is designed to be porous so that it stimulates natural bone growth and tissue formation around the artificial joint. Ceramic engine components are used in "lean burn" car engines that combust fuel more cleanly. Catalytic converters, which convert air pollution into less harmful gases, are made from light but strong aluminosilicate ceramics that can withstand the high temperatures generated in car exhausts. The latest generation of lightweight, deep-sea submersibles are being built not from steel, like their predecessors, but from ceramics originally made for defense purposes. One of the most innovative uses of ceramics is a new kind of paint made from a piezoelectric ceramic. Like other piezoelectric materials, this produces a tiny electric current when it undergoes stresses and strains and its Japanese inventors believe it could be used to detect metal failures or even earthquakes.

Ceramics in action

Space Shuttle

When NASA's Space Shuttle returns from space, thousands of heat-resistant tiles protect its exterior from overheating due to friction generated by Earth's atmosphere. Current models of the Shuttle use different tiles made from carbon, ceramics, and silica composites. But future versions of the craft are expected to use a new, slimline ceramic material made from hafnium and zirconium metals that will enable it to be both more aerodynamic and withstand temperatures up to 4300°F (2400°C).

Picture courtesy of NASA.

Superconductor

Superconductors—materials that have practically no electrical resistance—were discovered in 1911, but found few practical applications because they became superconducting only at temperatures close to absolute zero (–459.67°F or –273.15°C). But in the 1980s, scientists invented new types of ceramic superconductors that showed superconductivity at temperatures as high as –292°F (–180°C). High-temperature superconductors such as this are expected to find many new applications, including high-speed, magnetic levitation ("maglev") trains and super-fast computers.

Maglev train picture courtesy of US Department of Energy.

Electrical insulators

Familiar insulators such as rubber and plastic are relatively poor conductors, but they will conduct electricity if extremely large voltages are placed across them. This makes them unsuitable for use in high-power applications, such as in electrical generators, and transformers. Ceramics made from alumina (aluminum oxide) and porcelain are widely used in insulators that protect electrical equipment outdoors. Ceramics have been used as the insulation in automobile spark plugs since the early 20th century.

Power plant picture courtesy of US Department of Energy.