Ceramic
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225867914
2008-07-15T19:48:34Z
Petewray
7481847
Adding professional ceramists organization
{{Otheruses4|1=ceramic materials|2=the fine art|3=Ceramic art}}
[[Image:DSCN0126.jpg|thumb|200px|right|Fixed Partial [[Denture]], or "Bridge"]]
The word '''ceramic''' is derived from the [[Greek language|Greek]] word κεραμικός (''keramikos''). The term covers [[inorganic]] non-[[metallic]] materials which are formed by the action of heat. Up until the 1950s or so, the most important of these were the traditional [[clay]]s, made into [[pottery]], [[brick]]s, [[tile]]s and the like, along with [[cement]]s and [[glass]]. [[Clay]]-based ceramics are described in the article on [[pottery]]. A [[composite material]] of ceramic and [[metal]] is known as [[cermet]]. 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]].
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''.”<ref>[http://www.ctioa.org/index.cfm?pi=GL&gaction=list&grp=C Ceramic Tile and Stone Standards]</ref>
==Types of ceramic materials==
For convenience ceramic products are usually divided into four sectors, and these are shown below with some examples:
*''Structural'', including [[brick]]s, [[pipe (material)|pipe]]s, [[floor]] and [[roof tile]]s
*''[[refractory|Refractories]]'', such as [[kiln]] linings, gas fire radiants, [[steel]] and glass making crucibles
*''Whitewares'', including [[tableware]], wall tiles, decorative art objects and sanitary ware
*''Technical'', is also known as Engineering, Advanced, Special, and in Japan, Fine Ceramics. Such items include tiles used in the [[Space Shuttle program]], gas burner [[nozzle]]s, [[Ballistic vest|ballistic protection]], nuclear fuel uranium oxide pellets, [[Implant (medicine)|bio-medical implants]], [[jet engine]] [[turbine]] blades, and [[missile]] nose cones. Frequently the raw materials do not include clays.
=== Examples of whiteware ceramics ===
*[[Bone china]]
*[[Earthenware]], which is often made from clay, [[quartz]] and [[feldspar]].
*[[Porcelain]], which are often made from [[kaolin]]
*[[Stoneware]]
=== Classification of technical ceramics ===
Technical ceramics can also be classified into three distinct material categories:
* [[Oxide]]s: [[Alumina]], [[zirconia]]
* Non-oxides: [[Carbide]]s, [[boride]]s, [[nitride]]s, [[silicide]]s
* [[Mixture|Composite]]s: Particulate reinforced, combinations of oxides and non-oxides.
Each one of these classes can develop unique material properties
==== Examples of technical ceramics====
*[[Barium titanate]] (often mixed with [[strontium titanate]]) displays [[ferroelectricity]], meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in [[electromechanics|electromechanical]] [[transducer]]s, ceramic [[capacitor]]s, and [[Ferroelectric RAM|data storage]] elements. [[crystallite|Grain boundary]] conditions can create [[positive temperature coefficient|PTC]] effects in [[heating element]]s.
*[[Bismuth strontium calcium copper oxide]], a [[high-temperature superconductor]]
*[[Boron nitride]] is structurally [[isoelectronic]] to carbon and takes on similar physical forms: a [[graphite]]-like one used as a [[lubricant]], and a [[diamond]]-like one used as an abrasive.
*[[ferrite (magnet)|Ferrite]] ([[iron|Fe]]<sub>3</sub>O<sub>4</sub>), which is [[ferrimagnetism|ferrimagnetic]] and is used in the [[magnetic core]]s of electrical [[transformer]]s and [[magnetic core memory]].
*[[Lead zirconate titanate]] is another ferroelectric material.
*[[Magnesium diboride]] ([[magnesium|Mg]]B<sub>2</sub>), which is an [[unconventional superconductor]].
*[[Sialons]] / [[Silicon Aluminium Oxynitride]]s, high strength, high thermal shock / chemical / wear resistance, low density ceramics used in non-ferrous molten metal handling, weld pins and the chemical industry.
*[[Silicon carbide]] ([[silicon|Si]]C), which is used as a [[susceptor]] in microwave furnaces, a commonly used abrasive, and as a [[refraction (metallurgy)|refractory]] material.
*[[Silicon nitride]] (Si<sub>3</sub>[[nitrogen|N]]<sub>4</sub>), which is used as an [[abrasive]] powder.
*[[Steatite]] (magnesium silicates) is used as an [[electrical insulator]].
*[[Titanium Carbide]] Used in space shuttle re-entry shields and scratchproof watches.
*[[Uranium oxide]] ([[uranium|U]]O<sub>2</sub>), used as [[nuclear fuel|fuel]] in [[nuclear reactor]]s.
*[[Yttrium barium copper oxide]] ([[yttrium|Y]][[barium|Ba]]<sub>2</sub>[[copper|Cu]]<sub>3</sub>[[oxygen|O]]<sub>7-x</sub>), another high temperature [[Superconductivity|superconductor]].
*[[Zinc oxide]] ([[zinc|Zn]]O), which is a [[semiconductor]], and used in the construction of [[varistor]]s.
*[[Zirconium dioxide]] (zirconia), which in pure form undergoes many [[phase change]]s between room temperature and practical [[sintering]] temperatures, can be chemically "stabilized" in several different forms. Its high oxygen [[ion conductivity]] recommends it for use in [[fuel cell]]s. In another variant, [[metastable]] structures can impart [[transformation toughened ceramics|transformation toughening]] for mechanical applications; most [[ceramic knife]] blades are made of this material.
== Properties of ceramics ==
=== Mechanical properties ===
Ceramic materials are usually [[ionic bond|ionic]] or [[covalent]] bonded materials, and can be [[crystal]]line or [[amorphous solid|amorphous]]. A material held together by either type of bond will tend to [[Fracture#Brittle fracture|fracture]] before any [[plastic deformation]] takes place, which results in poor [[toughness]] in these materials. Additionally, because these materials tend to be porous, the [[porosity|pore]]s and other microscopic imperfections act as [[Stress concentration|stress concentrators]], decreasing the toughness further, and reducing the [[tensile strength]]. These combine to give [[catastrophic failure]]s, as opposed to the normally much more gentle [[failure mode]]s of metals.
These materials do show [[plasticity (physics)|plastic deformation]]. However, due to the rigid structure of the crystalline materials, there are very few available [[slip system]]s for [[dislocation]]s to move, and so they deform very slowly. With the non-crystalline (glassy) materials, [[Viscosity|viscous]] flow is the dominant source of plastic deformation, and is also very slow. It is therefore neglected in many applications of ceramic materials.
=== Electrical properties ===
==== Semiconductors ====
There are a number of ceramics that are [[semiconductor]]s. Most of these are [[transition metal oxides]] that are II-VI semiconductors, such as [[zinc oxide]].
While there is talk of making blue [[LED]]s from [[zinc oxide]], ceramicists are most interested in the electrical properties that show grain boundary effects.
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]]. Once the voltage across the device reaches the threshold, there is a [[Electrical breakdown|breakdown]] of the electrical structure in the vicinity of the [[grain boundary|grain boundaries]], which results in its [[electrical resistance]] dropping from several megohms down to a few hundred [[Ohm (unit)|ohm]]s. 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 protector|surge-protection]] applications. 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 substation]]s, where they are employed to protect the infrastructure from [[lightning]] strikes. 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 sensor]]s. 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]]. 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 [[crystal oscillator|measure time]] in watches and other electronics. 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. 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 sensor]]s, where the tiny rise in temperature from a warm body entering the room is enough to produce a measurable voltage in the crystal.
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. Pyroelectricity is also a necessary consequence of ferroelectricity. This can be used to store information in [[ferroelectric capacitor]]s, elements of [[ferroelectric RAM]].
The most common such materials are [[lead zirconate titanate]] and [[barium titanate]]. Aside from the uses mentioned above, their strong piezoelectric response is exploited in the design of high-frequency [[loudspeaker]]s, transducers for [[sonar]], and actuators for [[atomic force microscope|atomic force]] and [[scanning tunneling microscope]]s.
====Positive thermal coefficient====
Increases in temperature can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of [[heavy metals|heavy metal]] [[titanate]]s. 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 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.
== 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]].
'''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. [[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 vapor deposition|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 [[binder (material)|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, [[inductor]]s, [[sensor]]s, 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 ==
*Ceramics are used in the manufacture of knives. The blade of the [[ceramic knife]] will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.
*Ceramics such as [[alumina]] and [[boron carbide]] have been used in [[bulletproof vest|ballistic armored vests]] to repel large-caliber [[rifle]] fire. Such plates are known commonly as [[small-arms protective insert]]s (SAPI). Similar material is used to protect [[Cockpit (aviation)|cockpits]] of some military airplanes, because of the low weight of the material.
*Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means that they are much less susceptible to wear and can often more than triple lifetimes. They also deform less under load meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.
*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, as shown by [[Carnot heat engine|Carnot's]] theorem. 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]] [[heat engine|engines]]. Currently, even blades made of [[superalloy|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.
*Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. [[Hydroxyapatite]], the natural mineral component 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 hydroxy apatite 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.
*High-tech ceramic is used in watchmaking for producing watch cases. The material is valued by watchmakers for its light weight, scratch-resistance, durability and smooth touch. [[IWC]] is one of the brands that initiated the use of ceramic in watchmaking. The case of the IWC 2007 Top Gun edition of the Pilot's Watch [[Double chronograph]] is crafted in high-tech black ceramic.<ref>[http://watches.infoniac.com/index.php?page=post&id=62 Ceramic in Watchmaking]</ref>
==See also==
{{Wiktionary}}
* [[Ceramics (art)]]
* [[Ceramic forming techniques]]
* [[Glass-ceramic-to-metal seals]]
* [[Porcelain]]
* [[Pottery]]
* [[Three point flexural test]]
* [[Phase Equilibria Diagrams]] database
==References==
<references/>
==External links==
*[http://www.azom.com/details.asp?ArticleID=2123 Advanced Ceramics] – The Evolution, Classification, Properties, Production, Firing, Finishing and Design of Advanced Ceramics
*[http://www.madehow.com/Volume-4/Pottery.html How pottery is made]
*[http://www.madehow.com/Volume-5/Toilet.html How sanitaryware is made]
*[http://www.stoke.gov.uk/ccm/navigation/leisure/museums/collections/ceramics/ World renowned ceramics collections at Stoke-on-Trent Museum] Click on '''Quick Links''' in the right-hand column to view examples.
*[http://www.gardinermuseum.on.ca/default_noflash.aspx The Gardiner Museum] - The only museum in Canada entirely devoted to ceramics.
*[http://matse1.mse.uiuc.edu/ceramics/ceramics.html Introduction, Scientific Principles, Properties and Processing of Ceramics]
*[http://www.ceramics.org The American Ceramic Society] The American Ceramic Society
[[Category:Ceramic materials| ]]
[[Category:dielectrics]]
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