Ceramic

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This article is about ceramic materials. For the fine art, see Ceramics (art).
File:Shuttle STS-45.jpg
The space shuttle has a coat of ceramic tiles that protect it from searing heat during launch and reentry.

The word ceramic is derived from the Greek word κεραμικος (keramikos), which means "having to do with pottery." The term covers inorganic, nonmetallic materials (such as clay) that have been hardened by heating at a high temperature. Up until the 1950s or so, the most important of these were the traditional clays, made into pottery, dinnerware, bricks, tiles, and the like. Since then, new materials called advanced ceramics have been made.

Recently, a composite material of ceramic and metal has been made, known as cermet.

Historically, ceramic products have been hard, porous, and brittle. The study of ceramics consists to a large extent of ways to accentuate the strengths and mitigate the limitations of the materials, as well as to offer up unusual uses for these materials.

Terminology

The American Society for Testing and Materials (ASTM) defines a ceramic article as "an article having a glazed or unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially inorganic, nonmetallic substances and either is formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently matured by the action of the heat."

The word ceramic can be used as an adjective to describe a material, or it can be used as a noun when referring to a ceramic material or product of ceramic manufacture. Also, ceramics is a singular noun referring to the art of making things out of ceramic materials.

Classifications of technical ceramics

Technical Ceramics can also be classified into three distinct material categories:

  • Oxides: Alumina, zirconia
  • Non-oxides: Carbides, borides, nitrides, silicides
  • Composites: Particulate reinforced, combinations of oxides and non-oxides.

The materials in each of these classes can have unique properties.

Examples of ceramic materials

  • Barium titanate (often mixed with strontium titanate), which has a property called ferroelectricity, is widely used in electromechanical devices known as transducers, as well as in ceramic capacitors and data storage elements.
  • Bismuth strontium calcium copper oxide is a high-temperature superconductor.
  • Boron carbide (B4C) is used in some types of personal, helicopter, and tank armor.
  • Boron nitride takes on physical forms that are similar to those of carbon: a graphite-like form used as a lubricant, and a diamond-like one used as an abrasive.
  • Bricks, which are mostly aluminum silicates, are used for construction.
  • Earthenware is often made from clay, quartz, and feldspar.
  • Ferrite (Fe3O4), which is ferrimagnetic, is used in the core of electrical transformers and in magnetic core memory.
  • Lead zirconate titanate is another ferroelectric material.
  • Magnesium diboride (MgB2) is an unconventional superconductor.
  • Porcelain, which usually contains the clay mineral kaolinite.
  • Silicon carbide (SiC) is used as an abrasive, a refractory material, and a "susceptor" that helps cook food in microwave furnaces.
  • Silicon nitride (Si3N4) is used as an abrasive powder.
  • Steatite (a type of soapstone) is used as an electrical insulator.
  • Uranium oxide (UO2) is used as fuel in nuclear reactors.
  • Yttrium barium copper oxide (YBa2Cu3O7-x) is another high-temperature superconductor.
  • Zinc oxide (ZnO) is a semiconductor and is used in the construction of varistors (a class of electrical resistors).
  • Zirconia, which in pure form undergoes many phase changes when heated, can be chemically "stabilized" in several different forms. Most ceramic knife blades are made of this material. Also, as it is a good conductor of oxygen ions, it could be useful in fuel cells.

Properties of ceramics

Mechanical properties

Ceramic materials can be crystalline or amorphous (lacking a definite structure). The atoms in these materials are bound together by ionic or covalent bonds. A material held together by either type of bond will tend to fracture before any plastic deformation takes place. As a result, these materials are poor in toughness. Additionally, because these materials tend to be porous, the pores and other microscopic imperfections act as stress concentrators, decreasing the toughness further and reducing the tensile strength.

Electrical properties

Semiconductivity

A number of ceramics are semiconductors. Most of these are oxides of transition metals, such as zinc oxide.

One common use of these semiconductors is for varistors. These are electrical resistors with the unusual property of "negative resistance." Once the voltage across the device reaches a certain threshold, a change in the electrical structure of the material causes its electrical resistance to drop from several megaohms down to a few hundred ohms. As a result, these materials can dissipate a lot of energy. In addition, they self reset—after the voltage across the device drops below a threshold, its resistance returns to being high.

This property makes them ideal for surge-protection applications. The best demonstration of their ability can be found in electrical substations, where they are employed to protect the infrastructure from lightning strikes. They have rapid response, require low maintenance, and do not appreciably degrade from use.

When various gases are passed over a polycrystalline ceramic, its electrical resistance changes. Based on this property, semiconducting ceramics are used to make inexpensive gas sensors.

Superconductivity

Under some conditions, such as extremely low temperatures, some ceramics exhibit superconductivity. The exact reason for this property is not known, but there are two major families of superconducting ceramics.

Ferroelectricity and subsets

Many ceramic materials exhibit the property of piezoelectricity—that is, they generate a voltage in response to applied mechanical stress (or pressure). This property links electrical and mechanical responses. These materials are used in digital watches and other electronics that rely on quartz resonators. In these devices, electricity is used to produce a mechanical motion (powering the device) and the mechanical motion is in turn used to generate an electrical signal. The unit of time measured is the natural interval required for electricity to be converted into mechanical energy and back again.

The piezoelectric effect is generally stronger in materials that also exhibit pyroelectricity—that is, materials that generate an electrical potential when they are heated or cooled. All pyroelectric materials are also piezoelectric. Thus, these materials can be used to interconvert between thermal, mechanical, and electrical energy. Such materials are used in motion sensors, where the tiny rise in temperature from a warm body entering a room is enough to produce a measurable voltage in the crystal.

Pyroelectricity, in turn, is observed most strongly in materials that also display the ferroelectric effect—materials in which a stable electric dipole can be oriented or reversed by applying an electrostatic field. Pyroelectricity is a necessary consequence of ferroelectricity. Such materials can therefore be used to store information in ferroelectric capacitors.

The most common examples of these materials are lead zirconate titanate and barium titanate. Besides the above-mentioned uses, the strong piezoelectric response of these materials is exploited in the design of high-frequency loudspeakers, transducers for sonar, and actuators for atomic force and scanning tunneling microscopes.

Positive thermal coefficient

Increases in temperature can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of heavy metal titanates. The critical transition temperature can be adjusted over a wide range by variations in chemistry. In such materials, current will pass through the material until joule heating brings it to the transition temperature, at which point the circuit will be broken and current flow will cease. Such ceramics are used as self-controlled heating elements in, for example, the rear-window defrost circuits of most automobiles.

At the transition temperature, the material's dielectric response becomes theoretically infinite. While a lack of temperature control would rule out any practical use of the material near its critical temperature, the dielectric effect remains exceptionally strong even at much higher temperatures. Titanates with critical temperatures far below room temperature have become synonymous with "ceramic" in the context of ceramic capacitors for just this reason.

Processing of ceramic materials

Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fully molten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing to a mould. If later heat-treatments cause this class to become partly crystalline, the resulting material is known as a glass-ceramic.

Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories - either make the ceramic in the desired shape, by reaction in situ, or by "forming" powders into the desired shape, and then sintering to form a solid body. Ceramic forming techniques include shaping by hand (sometimes including a rotation process called "throwing"), slip casting, tape casting (used for making very thin ceramic capacitors, etc.), injection molding, dry pressing, and other variations. (See also Ceramic forming techniques. Details of these processes are described in the two books listed below.) A few methods use a hybrid between the two approaches.

In situ manufacturing

The most common use of this method is in the production of cement and concrete. Here, the dehydrated powders are mixed with water. This starts hydration reactions, which result in long, interlocking crystals forming around the aggregates. Over time, these result in a solid ceramic.

The biggest problem with this method is that most reactions are so fast that good mixing is not possible, which tends to prevent large-scale construction. However, small-scale systems can be made by deposition techniques, where the various materials are introduced above a substrate, and react and form the ceramic on the substrate. This borrows techniques from the semiconductor industry, such as chemical vapour deposition, and is very useful for coatings.

These tend to produce very dense ceramics, but do so slowly.

Sintering-based methods

The principles of sintering-based methods is simple. Once a roughly held together object (called a "green body") is made, it is baked in a kiln, where diffusion processes cause the green body to shrink. The pores in the object close up, resulting in a denser, stronger product. The firing is done at a temperature below the melting point of the ceramic. There is virtually always some porosity left, but the real advantage of this method is that the green body can be produced in any way imaginable, and still be sintered. This makes it a very versatile route.

There are thousands of possible refinements of this process. Some of the most common involve pressing the green body to give the densification a head start and reduce the sintering time needed. Sometimes organic binders such as polyvinyl alcohol are added to hold the green body together; these burn out during the firing (at 200-350°C). Sometimes organic lubricants are added during pressing to increase densification. It is not uncommon to combine these, and add binders and lubricants to a powder, then press. (The formulation of these organic chemical additives is an art in itself. This is particularly important in the manufacture of high performance ceramics such as those used by the billions for electronics, in capacitors, inductors, sensors, etc. The specialized formulations most commonly used in electronics are detailed in the book "Tape Casting," by R.E. Mistler, et al., Amer. Ceramic Soc. [Westerville, Ohio], 2000.) A comprehensive book on the subject, for mechanical as well as electronics applications, is "Organic Additives and Ceramic Processing," by D. J. Shanefield, Kluwer Publishers [Boston], 1996.

A slurry can be used in place of a powder, and then cast into a desired shape, dried and then sintered. Indeed, traditional pottery is done with this type of method, using a plastic mixture worked with the hands.

If a mixture of different materials is used together in a ceramic, the sintering temperature is sometimes above the melting point of one minor component - a liquid phase sintering. This results in shorter sintering times compared to solid state sintering.

Other applications of ceramics

In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts.

Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is unfeasible with current technology.

Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.

Since the late 1990s highly specialized ceramics, usually based on boron carbide, formed into plates and lined with Spectra, have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Very similar technology is used for armoring of cockpits of some military airplanes, because of the low weight of the material.

Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral componet of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most Hydroxyapatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong-fully dense nano crystalline Hydroxapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic natural bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.

References
ISBN links support NWE through referral fees

ASTM Standard C 242-01 “Standard Terminology of Ceramic Whitewares and Related Products”:

ASM Engineered Materials Handbook – Vol 4, Ceramics and Glass

Introduction to Ceramics; Kingery, Bowen, and Ulhmann

Modern Ceramic Engineering, Properties, Processing, and Use in Design; D. W. Richerson

Ceramic Fabrication Technology; Roy Rice

Ceramic Technology and Processing; A. G. King

See also

  • Ceramics (art)
  • Ceramic forming techniques
  • Porcelain
  • Pottery

External links

  • Advanced Ceramics – The Evolution, Classification, Properties, Production, Firing, Finishing and Design of Advanced Ceramics
  • Ceramics Directory – International Ceramics Directory of Companies and Organizations

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