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This article is about ceramic materials. For the fine art, see Ceramic art. Ceramics and ceramic art in the art world means artwork made out of clay bodies and fired to form a ceramic.
Fixed Partial Denture, or "Bridge"
Fixed Partial Denture, or "Bridge"

The word ceramic is derived from the Greek word κεραμικός (keramikos). Dentures are prosthetic devices constructed to replace missing teeth and which are supported by surrounding soft and hard tissues of the Oral cavity. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly The term covers inorganic non metallic materials which are formed by the action of heat. Traditionally inorganic compounds are considered to be of mineral not biological origin The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across Up until the 1950s or so, the most important of these were the traditional clays, made into pottery, bricks, tiles and the like, along with cements and glass. Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and Pottery is the Ceramic ware made by potters It also refers to a group of materials that includes Earthenware, Stoneware A brick is a block of Ceramic material used in Masonry construction laid using mortar. A tile is a manufactured piece of hard-wearing material such as Ceramic, stone, metal or even Glass. In the most general sense of the word a cement is a binder a substance which sets and hardens independently and can bind other materials together Glass in the common sense refers to a Hard, Brittle, transparent Solid, such as that used for Windows many Clay-based ceramics are described in the article on pottery. Clay is a naturally occurring material composed primarily of fine-grained Minerals which show plasticity through a variable range of Water content, and Pottery is the Ceramic ware made by potters It also refers to a group of materials that includes Earthenware, Stoneware A composite material of ceramic and metal is known as cermet. Composite materials (or composites for short are engineered Materials made from two or more constituent materials with significantly different physical or chemical The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across A cermet is a Composite material composed of Ceramic (cer and Metallic (met materials The word ceramic can be an adjective, and can also be used as a noun to refer to a ceramic material, or a product of ceramic manufacture. Ceramics may also be used as a singular noun referring to the art of making things out of ceramic materials. The technology of manufacturing and usage of ceramic materials is part of the field of ceramic engineering. Ceramic Engineering is the technology of manufacturing and usage of Ceramic materials

Many ceramic materials are hard, porous, and brittle. The study and development of ceramics includes methods to mitigate problems associated with these characteristics, and to accentuate the strengths of the materials as well as to investigate novel applications.

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, non-metallic 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. ASTM International ( ASTM) originally known as the American Society for Testing and Materials is an international Standards organization that develops and publishes In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating ”[1]

Contents

Types of ceramic materials

For convenience ceramic products are usually divided into four sectors, and these are shown below with some examples:

Examples of whiteware ceramics

Classification of technical ceramics

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

Each one of these classes can develop unique material properties

Examples of technical ceramics

Properties of ceramics

Mechanical properties

Ceramic materials are usually ionic or covalent bonded materials, and can be crystalline or amorphous. An ionic bond (or electrovalent bond) is a type of Chemical bond that can often form between Metal and Non-metal Ions (or In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating An amorphous solid is a Solid in which there is no Long-range order of the positions of the Atoms (Solids in which there is long-range atomic order are A material held together by either type of bond will tend to fracture before any plastic deformation takes place, which results in poor toughness in these materials. A fracture is the (local separation of an object or material into two or more pieces under the action of stress. In Materials science, deformation is a change in the shape or size of an object due to an applied force. Toughness, in Materials science and Metallurgy, is the resistance to Fracture of a material when stressed. 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. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% A stress concentration (often called stress raisers or stress risers) is a location in an object where stress is concentrated Tensile strength \sigma_{UTS} or S_U is the Stress at which a material breaks or permanently deforms These combine to give catastrophic failures, as opposed to the normally much more gentle failure modes of metals. A catastrophic failure is a sudden and total failure of some system from which recovery is impossible Failure causes are defects in design process quality or part application which are the underlying cause of the failure or which initiate a process which leads to failure

These materials do show plastic deformation. However, due to the rigid structure of the crystalline materials, there are very few available slip systems for dislocations to move, and so they deform very slowly. In Materials science, a dislocation is a Crystallographic defect, or irregularity within a Crystal structure. With the non-crystalline (glassy) materials, viscous flow is the dominant source of plastic deformation, and is also very slow. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. It is therefore neglected in many applications of ceramic materials.

Electrical properties

Semiconductors

There are a number of ceramics that are semiconductors. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide. In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in Zinc oxide is a Chemical compound with the formula ZnO It is nearly insoluble in water but soluble in Acids and Bases It occurs

While there is talk of making blue LEDs from zinc oxide, ceramicists are most interested in the electrical properties that show grain boundary effects. Zinc oxide is a Chemical compound with the formula ZnO It is nearly insoluble in water but soluble in Acids and Bases It occurs

One of the most widely used of these is the varistor. These are devices that exhibit the property that resistance drops sharply at a certain threshold voltage. The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide and the substrate Once the voltage across the device reaches the threshold, there is a breakdown of the electrical structure in the vicinity of the grain boundaries, which results in its electrical resistance dropping from several megohms down to a few hundred ohms. The term electrical breakdown has several similar but distinctly different meanings A grain boundary is the interface between two grains in a polycrystalline material Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The ohm (symbol Ω) is the SI unit of Electrical impedance or in the Direct current case Electrical resistance, The major advantage of these is that they can dissipate a lot of energy, and they self reset — after the voltage across the device drops below the threshold, its resistance returns to being high.

This makes them ideal for surge-protection applications. A surge protector is an appliance designed to protect electrical devices from Voltage spikes A surge protector attempts to regulate the Voltage supplied As there is control over the threshold voltage and energy tolerance, they find use in all sorts of applications. The best demonstration of their ability can be found in electrical substations, where they are employed to protect the infrastructure from lightning strikes. An electrical substation is a subsidiary station of an Electricity generation, transmission and distribution system where Voltage Lightning is an atmospheric discharge of Electricity, which typically occurs during Thunderstorms and sometimes during volcanic eruptions or They have rapid response, are low maintenance, and do not appreciably degrade from use, making them virtually ideal devices for this application.

Semiconducting ceramics are also employed as gas sensors. A gas detector is a device which detects the presence of various gases within an area usually as part of a system to warn about gases which might be harmful to Humans When various gases are passed over a polycrystalline ceramic, its electrical resistance changes. With tuning to the possible gas mixtures, very inexpensive devices can be produced.

Superconductivity

Under some conditions, such as extremely low temperature, some ceramics exhibit high temperature superconductivity. High-temperature superconductors (abbreviated high Tc or HTS) are a family of superconducting Ceramic materials largely The exact reason for this is not known, but there are two major families of superconducting ceramics.

Ferroelectricity and supersets

Piezoelectricity, a link between electrical and mechanical response, is exhibited by a large number of ceramic materials, including the quartz used to measure time in watches and other electronics. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to A crystal oscillator is an Electronic circuit that uses the mechanical Resonance of a vibrating Crystal of piezoelectric material to create an Such devices use both properties of piezoelectrics, using electricity to produce a mechanical motion (powering the device) and then using this mechanical motion to produce electricity (generating a 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, and all pyroelectric materials are also piezoelectric. Pyroelectricity is the ability of certain materials to generate an Electrical potential when they are heated or cooled These materials can be used to inter convert between thermal, mechanical, and/or electrical energy; for instance, after synthesis in a furnace, a pyroelectric crystal allowed to cool under no applied stress generally builds up a static charge of thousands of volts. Such materials are used in motion sensors, where the tiny rise in temperature from a warm body entering the room is enough to produce a measurable voltage in the crystal. Motion detection is the action of sensing physical movement in a given area

In turn, pyroelectricity is seen most strongly in materials which also display the ferroelectric effect, in which a stable electric dipole can be oriented or reversed by applying an electrostatic field. Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent Pyroelectricity is also a necessary consequence of ferroelectricity. This can be used to store information in ferroelectric capacitors, elements of ferroelectric RAM. Ferroelectric capacitor is a Capacitor based on a ferroelectric material Ferroelectric RAM ( FeRAM or FRAM) is a Random access memory similar in construction to DRAM but uses a Ferroelectric layer instead

The most common such materials are lead zirconate titanate and barium titanate. Lead zirconate titanate ( 0 x Ceramic Perovskite material that shows a marked piezoelectric effect. Barium titanate is an Oxide of Barium and Titanium with the Chemical formula BaTiO3 Aside from the uses mentioned above, their strong piezoelectric response is exploited in the design of high-frequency loudspeakers, transducers for sonar, and actuators for atomic force and scanning tunneling microscopes. For the Marty Friedman album see Loudspeaker (album A loudspeaker, speaker, or speaker system is an electroacoustical Sonar (which started as an Acronym for sound navigation and ranging) is a technique that uses Sound propagation (usually underwater to navigate The atomic force microscope (AFM or scanning force microscope (SFM is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions Scanning tunneling microscope (STM is a powerful technique for viewing surfaces at the atomic level

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. Titanic acid refers to the Chemical compound with the formula H2TiO3 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. Joule heating is the process by which the passage of an Electric current through a conductor releases Heat. Such ceramics are used as self-controlled heating elements in, for example, the rear-window defrost circuits of automobiles.

At the transition temperature, the material's dielectric response becomes theoretically infinite. A dielectric is a nonconducting substance ie an insulator. The term was coined by William Whewell in response to a request from Michael Faraday. 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.

Classification of ceramics

Non-crystalline ceramics: 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 mold. If later heat-treatments cause this class to become partly crystalline, the resulting material is known as a glass-ceramic. Glass-ceramic materials share many properties with both Glass and more traditional crystalline Ceramics It is formed as a glass and then made to Crystallize

Crystalline ceramics: 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. Sintering is a method for making objects from powder, by heating the material (below its Melting point - solid state sintering until its particles adhere 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. Ceramic forming techniques are ways of forming Ceramic shapes ), 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. Chemical vapor deposition (CVD is a Chemical process used to produce high-purity high-performance solid materials

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

Sintering-based methods

The principles of sintering-based methods is simple. Sintering is a method for making objects from powder, by heating the material (below its Melting point - solid state sintering until its particles adhere 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. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement 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. Porosity is a measure of the void spaces in a material and is measured as a fraction between 0–1 or as a Percentage between 0–100% 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). A binder is an ingredient used to bind together two or more other materials in mixtures Polyvinyl alcohol ( PVOH, PVA, or PVAL) is a Water - Soluble Synthetic polymer. 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. Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical An inductor is a passive electrical component designed to provide Inductance in a circuit A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument 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

See also

References

  1. ^ Ceramic Tile and Stone Standards
  2. ^ Ceramic in Watchmaking

External links

Dictionary

ceramic

-adjective

  1. made of material produced by the high temperature firing of inorganic, nonmetallic rocks and minerals.

-noun

  1. (uncountable) A hard brittle material that is produced through burning of nonmetallic minerals at high temperatures
  2. (countable) An object made of this material
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