Ceramic

Kiln

Ceramic is the name for materials that are formed by the use of heat. The word ‘ceramic’ comes from the Greek word ‘keramikos’ (‘of pottery’). Ceramics broadly refers to all inorganic non-metallic materials which are formed by the action of heat. Up to the 1950s or so, the most important were the traditional clays, made into pottery, bricks, tiles and the like, also cements (binders that set and harden) and glass. A composite material of ceramic and metal is known as ‘cermet.’ Ceramic materials are typically hard, porous, and brittle.

Ceramic products are usually divided into four sectors: Structural (e.g. bricks, pipes, floor and roof tiles), Refractories (e.g. kiln linings, gas fire radiants, steel and glass making crucibles), Whitewares (e.g. tableware, wall tiles, decorative art objects and sanitary ware), and Technical ceramics (e.g. space shuttle heat shield tiles, gas burner nozzles, bullet-proof vests, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones).

Some ceramics are defined by their constituents or method of manufacture. ‘Porcelain’ is a high-grade ceramic made by firing clay at higher temperatures than typical pottery (~1400 °C). ‘Bone China’ is a type of porcelain made with bone ash that is fired at slightly lower temperatures than standard porcelain (~1300 °C). ‘Stoneware’ is also fired at high temperatures, but lower still than bone china. All three type are considered vitreous  or semi-vitreous ceramics, meaning they are heated at a temperatures above 1200 °C celsius, hot enough to vitrify (transform into glass). Earthenware is a common nonvitreous pottery that includes terra-cotta, standard bricks, and most ancient pottery. It is opaque and porous unless a vitrified glaze is added.

Technical ceramics (also known as engineering, advanced, special, and in Japan, fine ceramics) can be classified into three distinct material categories: Oxides (alumina, zirconia), Non-oxides (carbides, borides, nitrides, silicides), and Composites (hybrids, particulate reinforced).

Ceramics can also be classified as either non-crystalline or crystalline. The former includes glasses, which 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. Unlike their counterparts, crystalline ceramics are not amorphous and are composed of materials with a repeating, geometric pattern. Crystalline ceramic products 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 (heat and compress) to form a solid body. Ceramic forming techniques include shaping by hand (sometimes including a rotation process called ‘throwing’), casting, injection molding, dry pressing, and other variations.

There are a number of ceramics that are semiconductors. Most of these are transition metal oxides such as zinc oxide. Under some conditions, such as extremely low temperature, some ceramics exhibit superconductivity (extremely low resistance to electron flow). The exact reason for this is not known, but there are two major families of superconducting ceramics: Ferroelectric (materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field) and Piezoelectric (materials that generate a small electrical voltage when deformed). The quartz crystal used in digital watches relies on a piezoelectric effect (converting electricity to mechanical motion and back, making a stable oscillator). A piezoelectric response is also exploited in the design of high-frequency loudspeakers, transducers for sonar, and actuators for atomic force and scanning tunneling microscopes.

The piezoelectric effect is generally stronger in materials that also exhibit pyroelectricity (the property of certain crystals which are naturally electrically polarized and as a result contain large electric fields), 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 sensors, 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 capacitors, elements of ferroelectric RAM (random access memory used in computer).

Increases in temperature can cause grain boundaries (the interface between two grains, or crystallites, in a polycrystalline material) 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 (also known as resistive heating, the process by which the passage of an electric current through a conductor produces heat) 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.

Some knives are ceramic. The ceramic knife blade will stay sharp for much longer steel will, 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 body armor to repel bullets. Similar material is used to protect 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 makes them last thrice as long. 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 high cost.

In the early 1980s, Toyota researched an adiabatic (heat retaining) ceramic engine which can run at a temperature of over 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 hotter engine is also higher by Carnot’s theorem. In a metallic engine, much of the energy released from the fuel must be dissipated as waste heat so it won’t melt 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 wreck the engine, possibly by explosion. Mass-production is not feasible with current technology. Ceramic parts for gas turbine engines may be practical. 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.

Bioceramics include 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 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 nanocrystalline hydroxyapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic, but naturally occurring, bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.

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