Editing Ceramic (section)

==Properties==
The physical properties of any ceramic substance are a direct result of its crystalline structure and chemical composition. [[Solid-state chemistry]] reveals the fundamental connection between microstructure and properties, such as localized density variations, grain size distribution, type of porosity, and second-phase content, which can all be correlated with ceramic properties such as mechanical strength σ by the Hall-Petch equation, [[hardness]], [[toughness]], [[dielectric constant]], and the [[optical]] properties exhibited by [[transparent materials]].

[[Ceramography]] is the art and science of preparation, examination, and evaluation of ceramic microstructures. Evaluation and characterization of ceramic microstructures are often implemented on similar spatial scales to that used commonly in the emerging field of nanotechnology: from [[nanometer]]s to tens of micrometers (µm). This is typically somewhere between the minimum wavelength of visible light and the resolution limit of the naked eye.

The microstructure includes most grains, secondary phases, grain boundaries, pores, micro-cracks, structural defects, and hardness micro indentions. Most bulk mechanical, optical, thermal, electrical, and magnetic properties are significantly affected by the observed microstructure. The fabrication method and process conditions are generally indicated by the microstructure. The root cause of many ceramic failures is evident in the cleaved and polished microstructure. Physical properties which constitute the field of [[materials science]] and [[engineering]] include the following:

===Mechanical properties===
[[File:Ultra-thin separated (Carborundum) disk.jpg|thumb|Cutting disks made of [[silicon carbide]] ]]
Mechanical properties are important in structural and building materials as well as textile fabrics. In modern [[materials science]], fracture mechanics is an important tool in improving the mechanical performance of materials and components. It applies the [[physics]] of [[stress (mechanics)|stress]] and [[Deformation (mechanics)|strain]], in particular the theories of [[Elasticity (physics)|elasticity]] and [[Plasticity (physics)|plasticity]], to the microscopic [[crystallographic defects]] found in real materials in order to predict the macroscopic mechanical failure of bodies. [[Fractography]] is widely used with fracture mechanics to understand the causes of failures and also verify the theoretical [[failure]] predictions with real-life failures.

Ceramic materials are usually [[ionic bond|ionic]] or [[covalent]] bonded materials. A material held together by either type of bond will tend to [[Fracture#Brittle|fracture]] before any [[plastic deformation]] takes place, which results in poor [[toughness]] and brittle behavior 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 more ductile [[failure mode]]s of metals.

These materials do show [[plasticity (physics)|plastic deformation]]. However, because of the rigid structure of crystalline material, there are very few available slip systems for [[dislocation]]s to move, and so they deform very slowly.

To overcome the brittle behavior, ceramic material development has introduced the class of [[ceramic matrix composite]] materials, in which ceramic fibers are embedded and with specific coatings are forming fiber bridges across any crack. This mechanism substantially increases the fracture toughness of such ceramics. Ceramic [[disc brake]]s are an example of using a ceramic matrix composite material manufactured with a specific process.

Scientists are working on developing ceramic materials that can withstand significant deformation without breaking. A first such material that can deform in room temperature was found in 2024.<ref>{{cite journal |title=The first bulk ceramic that deforms like a metal at room temperature |journal=Nature |date=23 February 2024 |doi=10.1038/d41586-024-00443-8 |pmid=38396100 }} summarizing {{cite journal |last1=Wu |first1=Yingju |last2=Zhang |first2=Yang |last3=Wang |first3=Xiaoyu |last4=Hu |first4=Wentao |last5=Zhao |first5=Song |last6=Officer |first6=Timothy |last7=Luo |first7=Kun |last8=Tong |first8=Ke |last9=Du |first9=Congcong |last10=Zhang |first10=Liqiang |last11=Li |first11=Baozhong |last12=Zhuge |first12=Zewen |last13=Liang |first13=Zitai |last14=Ma |first14=Mengdong |last15=Nie |first15=Anmin |last16=Yu |first16=Dongli |last17=He |first17=Julong |last18=Liu |first18=Zhongyuan |last19=Xu |first19=Bo |last20=Wang |first20=Yanbin |last21=Zhao |first21=Zhisheng |last22=Tian |first22=Yongjun |title=Twisted-layer boron nitride ceramic with high deformability and strength |journal=Nature |date=22 February 2024 |volume=626 |issue=8000 |pages=779–784 |doi=10.1038/s41586-024-07036-5 |pmid=38383626 |pmc=10881384 |bibcode=2024Natur.626..779W }}</ref>

====Ice-templating for enhanced mechanical properties====
If a ceramic is subjected to substantial mechanical loading, it can undergo a process called [[Freeze-casting|ice-templating]], which allows some control of the [[microstructure]] of the ceramic product and therefore some control of the mechanical properties. Ceramic engineers use this technique to tune the mechanical properties to their desired application. Specifically, the [[Strength of materials|strength]] is increased when this technique is employed. Ice templating allows the creation of macroscopic pores in a unidirectional arrangement. The applications of this oxide strengthening technique are important for [[solid oxide fuel cell]]s and [[Water purification|water filtration]] devices.<ref>{{cite journal |last1=Martinić |first1=Frane |last2=Radica |first2=Gojmir |last3=Barbir |first3=Frano |title=Application and Analysis of Solid Oxide Fuel Cells in Ship Energy Systems |journal=Brodogradnja |date=31 December 2018 |volume=69 |issue=4 |pages=53–68 |doi=10.21278/brod69405 |s2cid=115752128 |doi-access=free |url=https://hrcak.srce.hr/file/306607 }}</ref>

To process a sample through ice templating, an aqueous [[Colloid|colloidal suspension]] is prepared to contain the dissolved ceramic powder evenly dispersed throughout the colloid,{{clarify|reason=is the powder in suspension or actually dissolved?|date=December 2019}} for example [[yttria-stabilized zirconia]] (YSZ). The solution is then cooled from the bottom to the top on a platform that allows for unidirectional cooling. This forces ice crystals to grow in compliance with the unidirectional cooling, and these ice crystals force the dissolved YSZ particles to the solidification front{{clarify|date=June 2023}} of the solid-liquid interphase boundary, resulting in pure ice crystals lined up unidirectionally alongside concentrated pockets of colloidal particles. The sample is then heated and at the same the pressure is reduced enough to force the ice crystals to [[Sublimation (phase transition)|sublime]] and the YSZ pockets begin to [[Annealing (metallurgy)|anneal]] together to form macroscopically aligned ceramic microstructures. The sample is then further [[Sintering|sintered]] to complete the [[evaporation]] of the residual water and the final consolidation of the ceramic microstructure.{{citation needed|date=December 2019}}

During ice-templating, a few variables can be controlled to influence the pore size and morphology of the microstructure. These important variables are the initial solids loading of the colloid, the cooling rate, the sintering temperature and duration, and the use of certain additives which can influence the microstructural morphology during the process. A good understanding of these parameters is essential to understanding the relationships between processing, microstructure, and mechanical properties of anisotropically porous materials.<ref>{{cite journal |last1=Seuba |first1=Jordi |last2=Deville |first2=Sylvain |last3=Guizard |first3=Christian |last4=Stevenson |first4=Adam J. |title=Mechanical properties and failure behavior of unidirectional porous ceramics |journal=Scientific Reports |date=14 April 2016 |volume=6 |issue=1 |pages=24326 |doi=10.1038/srep24326 |pmid=27075397 |pmc=4830974 |bibcode=2016NatSR...624326S }}</ref>

===Electrical properties===

====Semiconductors====
Some ceramics are [[semiconductor]]s. Most of these are [[transition metal oxides]] that are II-VI semiconductors, such as [[zinc oxide]]. While there are prospects of mass-producing blue [[light-emitting diode]]s (LED) 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{{clarification needed|date=November 2021}} in the vicinity of the 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====
[[File:Magnet 4.jpg|thumb|The [[Meissner effect]] demonstrated by levitating a magnet above a [[cuprate]] superconductor, which is cooled by [[liquid nitrogen]]]]
Under some conditions, such as extremely low temperatures, some ceramics exhibit [[high-temperature superconductivity]] (in superconductivity, "high temperature" means above 30 K). The reason for this is not understood, 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, 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 that 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====
Temperature increases 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.

===Optical properties===
[[File:cermax.jpg|thumb|upright|Cermax [[xenon arc lamp]] with [[synthetic sapphire]] output window]]
[[Optics|Optically transparent materials]] focus on the response of a material to incoming light waves of a range of wavelengths. [[Optical filter|Frequency selective optical filters]] can be utilized to alter or enhance the brightness and contrast of a digital image. Guided lightwave transmission via frequency selective [[waveguides]] involves the emerging field of fiber [[optics]] and the ability of certain glassy compositions as a [[transmission medium]] for a range of frequencies simultaneously ([[multi-mode optical fiber]]) with little or no [[adjacent-channel interference|interference]] between competing [[wavelengths]] or frequencies. This [[resonant]] [[normal mode|mode]] of [[energy]] and [[data transmission]] via electromagnetic (light) [[wave propagation]], though low powered, is virtually lossless. Optical waveguides are used as components in [[Integrated optical circuit]]s (e.g. [[light-emitting diodes]], LEDs) or as the transmission medium in local and long haul [[optical communication]] systems. Also of value to the emerging materials scientist is the sensitivity of materials to radiation in the thermal [[infrared]] (IR) portion of the [[electromagnetic spectrum]]. This heat-seeking ability is responsible for such diverse optical phenomena as [[night-vision]] and IR [[luminescence]].

Thus, there is an increasing need in the [[military]] sector for high-strength, robust materials which have the capability to transmit [[light]] ([[electromagnetic waves]]) in the [[visible spectrum|visible]] (0.4 – 0.7 micrometers) and mid-[[infrared]] (1 – 5 micrometers) regions of the spectrum. These materials are needed for applications requiring [[transparency and translucency|transparent]] armor, including next-generation high-speed [[missile]]s and pods, as well as protection against improvised explosive devices (IED).

In the 1960s, scientists at [[General Electric]] (GE) discovered that under the right manufacturing conditions, some ceramics, especially [[aluminium oxide]] (alumina), could be made [[translucent]]. These translucent materials were transparent enough to be used for containing the electrical [[plasma (physics)|plasma]] generated in high-[[pressure]] [[sodium]] street lamps. During the past two decades, additional types of transparent ceramics have been developed for applications such as nose cones for [[heat-seeking]] [[missiles]], [[window]]s for fighter [[aircraft]], and [[scintillation counter]]s for computed [[tomography]] scanners. Other ceramic materials, generally requiring greater purity in their make-up than those above, include forms of several chemical compounds, including:

#[[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.
#[[Sialon]] (silicon aluminium oxynitride) has high strength; resistance to thermal shock, chemical and wear resistance, and low density. These ceramics are used in non-ferrous molten metal handling, weld pins, and the chemical industry.
#[[Silicon carbide]] (SiC) is used as a [[susceptor]] in microwave furnaces, a commonly used abrasive, and as a [[refractory|refractory material]].
#[[Silicon nitride]] (Si<sub>3</sub>[[nitrogen|N]]<sub>4</sub>) is used as an [[abrasive]] powder.
#[[Magnesium silicide|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]] (Y[[barium|Ba]]<sub>2</sub>[[copper|Cu]]<sub>3</sub>[[oxygen|O]]<sub>7−x</sub>), a [[high-temperature 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 transition|phase changes]] 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 and automotive [[oxygen sensor]]s. In another variant, [[metastable]] structures can impart [[fracture toughness|transformation toughening]] for mechanical applications; most [[ceramic knife]] blades are made of this material. Partially stabilised zirconia (PSZ) is much less brittle than other ceramics and is used for metal forming tools, valves and liners, abrasive slurries, kitchen knives and bearings subject to severe abrasion.<ref>{{cite journal |doi=10.1038/258703a0 |volume=258 |issue=5537 |title=Ceramic steel? |year=1975 |last1=Garvie |first1=R. C. |last2=Hannink |first2=R. H. |last3=Pascoe |first3=R. T. |journal=Nature |pages=703–704 |bibcode=1975Natur.258..703G |s2cid=4189416}}</ref>