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Facts of the Matter

Richard Brill

Sunday, November 2, 2003


Ceramics: Much more
than your coffee mug


Ceramics are among the most useful and most used materials by mankind, and have been for thousands of years. There is probably no other product that today has such widespread uses, or from which a wider variety of products is made.

The term "ceramics," which comes from the Greek "keramos," meaning "potter's clay, commonly refers to a product made essentially from a nonmetallic mineral by firing at a high temperature. Along with plastics and metals, ceramics are one of the three most important types of synthetic engineering materials. Included are such everyday materials as brick, cement, glass, tile, dinnerware, porcelain and porcelain enamel. And there's also abrasives such as alumina, silicon carbide and silicon nitride, refractories used in boilers and furnaces such as those used to make steel, electronic equipment, and special materials used in spacecraft and medical applications.

Ceramics are made of a variety of materials, but are made from such silicate minerals as clay, feldspar, quartz, and talc. Clay is an important silicate containing significant amounts of aluminum, but it is not used in all ceramic materials. Even the most exotic ceramics are usually based on oxides, nitrides, or carbides of silicon, aluminum, or magnesium, although less common chemical elements such as zirconium and rare earths such as yttrium are being incorporated to make ceramics with fascinating and sometimes unexpected properties such as superconductivity.

Compared with glass, an amorphous material in which the atoms are "frozen" in a noncrystalline state, most ceramics consist of a combination of glassy and crystalline material. Whether the material consists of ionic or covalently bonded atoms and a crystalline or amorphous internal structure affects the properties of ceramic materials.

Research is being conducted worldwide on ceramics, and discoveries are made daily as ceramic engineers continually develop new varieties and new uses for existing ceramics.

Ceramics are among the oldest known man-made materials. Fired-clay 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. Porcelain, a specific type of translucent ceramic, was made in China in the 6th century A.D. and references to it appear in the writings of Marco Polo, who visited China in the 13th century.

Kaolin, a type of clay also known as "china clay," was discovered in Bavaria in the 16th century, and spurned a porcelain industry, the details of which were kept secret for a century.

Ceramics have many desirable properties that make them useful in a wide range of applications. All types are hard, wear-resistant, brittle, heat resistant (refractory), thermal and electrical insulators, nonmagnetic, oxidation resistant, and chemically stable. Ceramics are the strongest known monolithic materials, and they typically maintain a significant fraction of their strength at elevated temperatures.

The most important thermal properties of ceramic materials are heat capacity, thermal expansion coefficient, and thermal conductivity. Many applications of ceramics, such as their use as insulating materials, are related to these properties. One of the most useful properties of ceramics is their resistance to volatile and high temperature conditions that are encountered in the processing of metals. These are called refractories and are used in other industries, such as in chemical, petroleum, energy conversion, and glass-making.

Ceramic products such as floor, wall, and roofing tile are widely used in home, business, and industrial applications. Clay brick is one of the most durable and strong building products, the only one that will not burn, melt, dent, peel, warp, rot, rust or be eaten by termites.

Ceramics of different kinds have a wide range of electrical properties, without which there would be no electronic industry, and no computers,TVs, CDs, DVDs, cell phones, or other lesser known electronic devices used in medical, aerospace, and military applications. There are ceramics that are insulators, semiconductors, superconductors, piezoelectric, and magnetic.

Ceramic scissors and knives are finding increasing converts in the home, in industry and in surgical equipment because the blades of these cutting tools resist dulling, a feature of the ceramic's hardness and wear resistance.

Ceramics can be made to be thin, flat and chemically stable, so they are ideal for liquid crystal display screens used in televisions, dashboards, calculators, watches and clocks.

Glass ceramics such as those that are used to make ovenware are composed of a matrix of glass in which tiny ceramic crystals grow, such that the final matrix is actually composed of fine crystalline grains that average less than 500 nanometers (one nanometer is one one-billionth of a meter). Such small embedded crystals make these materials transparent to light. The presence of the tiny crystals improves the mechanical and thermal properties of the glass, making the products strong and resistant to thermal shock, yet good heat conductors.

In a piezoelectric material, the application of a force or pressure on its surface changes a mechanical pressure into an electrical impulse.

Piezoelectric ceramics are used to make such common devices as phonograph pickups, depth finders, microphones, and various types of sensors. They are also used in gas stoves to eliminate the need for a pilot light.

Piezoelectronic ceramics are available in several lines of smart sporting goods equipment. One line of skis incorporates piezoceramic feedback that dampens vibrations from the ice and snow help to keep the skis on the snow, enhancing stability and control. Another company manufactures a baseball bat that adjusts the sweet spot to reduce sting on off-center hits.

One of the most important and often overlooked use of ceramics is in the electrical insulators of the spark plugs that are used to ignite the gasoline-air mixture in the millions of gasoline engines that power the world's automobiles, motorcycles, lawn mowers, weed trimmers, chain saws, and leaf blowers.

Some of the most important technological advances of ceramics are happening in the medical field. Ceramics are used for repair and replacement of hips, knees, and other body parts, including heart valves. Ceramics can stimulate bone growth, promote tissue formation and protect implants from the immune system. Ceramics are one of the few materials that can withstand the corrosive effects of bodily fluids. New ceramic materials are being developed at an increasing rate for use in tooth replacements, implants, braces, and material for dental fillings.

Ultrasound and CT systems rely on ceramic components to generate and detect the reflected signals that form the image. More sensitive ceramic X-ray detectors are allowing the use of smaller doses of the potentially damaging radiation, which also increases the sensitivity of the equipment to more and more soft tissue.

In environmental applications, ceramics help decrease pollution by capturing toxic materials and encapsulating nuclear waste. Catalytic converters in cars and trucks are made from ceramics and convert unburned hydrocarbons and carbon monoxide gases into carbon dioxide and water. Ceramic components are being used more and more in automotive engines, both gasoline and diesel. The high temperature and wear-resistant properties result in more efficient combustion and fuel savings.

The tragedy of the Columbia space shuttle, ostensibly caused by damage to one of the tiles, illustrated the important role of the ceramic tiles in preventing the shuttle from burning up due to the heat caused by air friction during reentry. The thermal properties of ceramics become paramount here as the tiles must be able to withstand temperatures of minus 250° to 3,000° Fahrenheit. At the same time the tiles must insulate well enough that they can be thin to conserve weight. The melting point of aluminum that comprises the hull, wings and tail of the shuttle is 1,225° Fahrenheit, but the thin tiles that cover the belly of the shuttle keep the temperature of the aluminum at or below 345° Fahrenheit even as the the exterior of the tiles exceed 2,550° F. The tiles cool off rapidly, so that after exposure to such high temperatures they are cool enough to be held in the bare hand in about 10 seconds. Surprisingly, the thickness of these ceramic tiles varies from only one-half inch to three and one-half inches.

The principal limitation of ceramics is their brittleness, especially when the material is used in structural applications. This property is due to the atomic structure of the ceramic. In metals, delocalized electrons allow them to move around without disrupting the crystal structure. This is responsible for the malleability of metals, which allows them to deform under stress, give them high electrical and thermal conductivity, but also reduces their strength at high temperatures. In ceramics, this electron mobility cannot occur since the atoms in ceramics are held in place by strong covalent bonds, so stress tends to disrupt the crystal structure with the result being that fracture occurs, producing the brittleness. The trade off is the high-temperature strength and thermal properties, non-conductivity and other features that make ceramics suitable for the multiple application that metals cannot match.

The variety of properties and uses for these myriad materials fashioned into remarkable products is truly remarkable in itself. That such apparently mundane and plentiful minerals could be fashioned into materials with properties so diverse is a testament both to the wonders of nature and to the agility of the human mind.




Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu

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