Ceramic

Ultimately these ceramic materials may be used as bone replacements or with the incorperation of protein collagens, synthetic bones. Used as an adjective, it may describe several fields, concepts, and people:. 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. The word Civil is derived from the Latin word civilis, from civis ("citizen"). They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Civil war. 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. Civil service/Civil servant.

Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Civil rights. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Civil procedure. 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. Civil marriage. Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Civil libertarianism.

Very similar technology is used for armoring of cockpits of some military airplanes, because of the low weight of the material. Civil liberties. Such plates are known commonly as small-arms protective inserts (SAPI). Civil law. 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. Civil engineering. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel. Civil disobedience.

Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Civil defense. Work is being done in developing ceramic parts for gas turbine engines. Civil death. Such engines are possible in laboratory settings, but mass-production is infeasible with current technology. Civil aviation. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure.

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. 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. Fuel efficiency of the engine is also higher at high temperature. Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency.

A couple of decades ago, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000 °F (3300 °C). This results in shorter sintering times compared to solid state sintering. 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. Indeed, traditional pottery is done with this type of method, using a plastic mixture worked with the hands.

A slurry can be used in place of a powder, and then cast into a desired shape, dried and then sintered. Shanefield, Kluwer Publishers [Boston], 1996. J. [Westerville, Ohio], 2000.) A comprehensive book on the subject, for mechanical as well as electronics applications, is "Organic Additives and Ceramic Processing," by D.

Ceramic Soc. Mistler, et al., Amer. The specialized formulations most commonly used in electronics are detailed in the book "Tape Casting," by R.E. 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 formulation of these organic chemical additives is an art in itself. It is not uncommon to combine these, and add binders and lubricants to a powder, then press. Sometimes organic lubricants are added during pressing to increase densification. 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).

Some of the most common involve pressing the green body to give the densification a head start and reduce the sintering time needed. There are thousands of possible refinements of this process. This makes it a very versatile route. 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.

The firing is done at a temperature below the melting point of the ceramic. The pores in the object close up, resulting in a denser, stronger product. 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 principles of sintering-based methods is simple.

These tend to produce very dense ceramics, but do so slowly. This borrows techniques from the semiconductor industry, such as chemical vapour deposition, and is very useful for coatings. 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. 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.

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

A few methods use a hybrid between the two approaches. 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. Crystalline ceramic materials are not amenable to a great range of processing. If later heat-treatments cause this class to become partly crystalline, the resulting material is known as a glass-ceramic.

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. Non-crystalline ceramics, being glasses, tend to be formed from melts. Titanates with critical temperatures far below room temperature have become synonymous with "ceramic" in the context of ceramic capacitors for just this reason. 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.

At the transition temperature, the material's dielectric response becomes theoretically infinite. Such ceramics are used as self-controlled heating elements in, for example, the rear-window defrost circuits of most automobiles. 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. The critical transition temperature can be adjusted over a wide range by variations in chemistry.

Increases in temperature can cause grain boundaries to suddenly become insulating in some semiconducting ceramic materials, mostly mixtures of heavy metal titanates. 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. The most common such materials are lead zirconate titanate and barium titanate. This can be used to store information in ferroelectric capacitors, elements of ferroelectric RAM.

Pyroelectricity is also a necessary consequence of ferroelectricity. 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. 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. These materials can be used to interconvert 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.

The piezoelectric effect is generally stronger in materials that also exhibit pyroelectricity, and all pyroelectric materials are also piezoelectric. The unit of time measured is the natural interval required for electricity to be converted into mechanical energy and back again. 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). Piezoelectricity, a link between electrical and mechanical response, is exhibited by a large number of ceramic materials, including the quartz resonators used as to measure time watches and other electronics.

The exact reason for this is not known, but there are two major families of superconducting ceramics. Under some conditions, such as extremely low temperature, some ceramics exhibit superconductivity. With tuning to the possible gas mixtures, very inexpensive devices can be produced. When various gases are passed over a polycrystalline ceramic, its electrical resistance changes.

Semiconducting ceramics are also employed as gas sensors. They have rapid response, are low maintenance, and do not appreciably degrade from use, making them virtually ideal devices for this application. The best demonstration of their ability can be found in electrical substations, where they are employed to protect the infrastructure from lightning strikes. As there is control over the threshold voltage and energy tolerance, they find use in all sorts of applications.

This makes them ideal for surge-protection applications. 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. Once the voltage across the device reaches a certain 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 megaohms down to a few hundred ohms. These are devices that exhibit the unusual property of negative resistance.

One of the most widely used of these is the varistor. While there is talk of making blue LEDs from zinc oxide, ceramicists are most interested in the electrical properties that show grain boundary effects. Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide. There are a number of ceramics that are semiconductors.

It is therefore neglected in many applications of ceramic materials. With the non-crystalline (glassy) materials, viscous flow is the dominant source of plastic deformation, and is also very slow. 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. These materials do show plastic deformation.

These combine to give catastrophic failures, as opposed to the normally much more gentle failure modes of metals. 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. 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. Ceramic materials are usually ionic or covalently-bonded materials, and can be crystalline or amorphous.

Each one of these classes can develop unique material properties. Technical Ceramics can also be classified into three distinct material categories:. . In Commonwealth English, ceramic can also be used as a singular noun, referring to an object made of ceramic material.

Ceramics is a singular noun referring to the art of making things out of ceramic materials. The word ceramic can be an adjective, and can also be used as a noun to refer to a ceramic material. The study of ceramics consists to a large extent of methods to mitigate these problems, and accentuate the strengths of the materials, as well as to offer up unusual uses for these materials. Historically, ceramic products have been hard, porous and brittle.

A composite material of ceramic and metal is known as cermet. The traditional crafts are described in the article on pottery. 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. The term covers inorganic non-metallic materials whose formation is due to the action of heat.

The word ceramic is derived from the Greek word κεραμικος (keramikos, "having to do with pottery"). In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material. Its high oxygen ion conductivity recommends it for use in fuel cells. Zirconia, which in pure form undergoes many phase changes between room temperature and practical sintering temperatures, can be chemically "stabilized" in several different forms.

Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors. Yttrium barium copper oxide (YBa₂Cu₃O_7-x), another high temperature superconductor. Uranium oxide (UO₂), used as fuel in nuclear reactors. Steatite is used as an electrical insulator.

Silicon nitride (Si₃N₄), which is used as an abrasive powder. Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a commonly used abrasive, and as a refractory material. Porcelain, which usually contains the clay mineral kaolinite. Magnesium diboride (MgB₂), which is an unconventional superconductor.

Lead zirconate titanate is another ferroelectric material. Ferrite (Fe₃O₄), which is ferrimagnetic and is used in the core of electrical transformers and magnetic core memory. Earthenware, which is often made from clay, quartz and feldspar. Bricks (mostly aluminium silicates), used for construction.

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. Boron carbide (B₄C), which is used in some helicopter and tank armor. Bismuth strontium calcium copper oxide, a high-temperature superconductor. Grain boundary conditions can create PTC effects in heating elements.

It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements. 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. Composites: Particulate reinforced, combinations of oxides and non-oxides. Non-oxides: Carbides, borides, nitrides, silicides.

Oxides: Alumina, zirconia.