Ceramic facts for kids
Ceramic is the name for some materials that are formed by the use of heat. The word ceramic comes from the Greek word κεραμικός (keramikos). Chemically, it is an inorganic compound of metal, non-metal or metalloid atoms held together by chemical bonds.
Up to the 1950s or so, the most important were the traditional clays, made into pottery, bricks, tiles and the like, also cements 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 clay-based ceramic materials are hard, porous, and brittle. The study and development of ceramics includes methods to deal with these characteristics, to accentuate the strengths of the materials and investigate novel applications.
Contents
Types of ceramic materials
For convenience ceramic products are usually divided into four sectors, and these are shown below with some examples:
- Structural, including bricks, pipes, floor and roof tiles
- 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 ceramics is also known as engineering, advanced, special, and in Japan, fine ceramics. Such items include tiles used in the Space Shuttle program, gas burner nozzles, bullet-proof vests, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.
Examples of ceramics
- Porcelain
- 'Hard-paste' porcelain, fired at a higher temperature.
- 'Soft-paste' porcelain, fired at a lower temperature: Bone china
- Earthenware, which is often made from clay, quartz and feldspar
- Stoneware
Classification of technical ceramics
Technical ceramics can also be classified into three distinct material categories:
- Oxides: alumina, zirconia
- Non-oxides: carbides, borides, nitrides, silicides
- Composites: particulate reinforced, combinations of oxides and non-oxides
Each one of these classes can develop unique material properties.
Properties of ceramics
Mechanical properties
Ceramic materials are usually crystalline (having a repeating shape) or amorphous (no repeating shape). They are usually held together with covalent or ionic bonds. Thus, they tend to fracture (break) before any plastic deformation (permanent damage) happens. So they have poor toughness (resistance to breaking). Second, ceramics have pores (small holes) which focus stress on smaller areas.This decreases the toughness even more and reduces the tensile strength. For these two reasons, ceramics fail far more catastrophically (suddenly) than metals.
Ceramics do show plastic deformation, but because of how slow it is it is usually ignored.
Crystalline materials are stiff; there are few places where the deformation can spread. So the deformation is very slow.
In amorphous materials, viscous flow is the main source of plastic deformation. It is also very slow.
Electrical properties
Semiconductors
Some ceramics are semiconductors. Most of this type of ceramic are "II-VI semiconductors", which are a combination of an alkaline earth or Group 12 metal and a non-metal from Group 16. A major example of these is zinc oxide.
Some people are thinking about making blue LEDs from zinc oxide. But ceramic researchers focus on electrical properties that affect boundaries between individual grains. One of the most common examples of this is the varistor.
Semiconducting ceramics are also used as gas sensors. Semiconducting ceramics are also employed as gas sensors. When we pass gases over a polycrystalline ceramic, its electrical resistance changes. If we know the possible gas mixtures, we can produce very cheap devices.
Superconductivity
Under some conditions, such as extreme cold temperatures, some ceramics show superconductivity. We do not know the exact reason. But we have discovered two major families of superconducting ceramics.
Piezoelectricity, pyroelectricity, and ferroelectricity
Piezoelectricity is an electric charge caused by mechanical stress. It appears in many ceramic materials, such as quartz. We use quartz to measure time in watches and other electronics. This kind of device turns electricity into mechanical motions and back, making a stable oscillator.
The piezoelectric effect is generally stronger in materials that also show pyroelectricity. (Pyroelectricity is the ability of a material to generate electricity if its temperature changes.) All pyroelectric materials are also piezoelectric. We can use these materials to convert between thermal, mechanical, and electrical energy. For instance, we can put a pyroelectrical crystal in a furnace. If we take out and let it cool without applying any force to it, it will generate a large static charge. This type of crystal is most common in motion sensors. That is because the tiny rise in temperature from a warm body entering a room is enough to produce a voltage in the crystal. A motion sensor reads that voltage and converts it to data.
In turn, pyroelectricity is strongest in materials which also display the ferroelectric effect. This is when we can reverse or move a stable electric polarization by applying an electrostatic field. If a material is ferroelectric, it is also pyroelectric. We use this effect to store information in ferroelectric capacitors and random-access memory.
The most common piezoelectric materials are lead zirconate titanate and barium titanate. We use them for high-frequency loudspeakers, sonar, and atomic-force and scanning-tunneling microscopes.
Grain-boundary insulation at critical temperatures
In some semiconducting ceramics a gain in temperature causes grain boundaries to become insulating. This is most common in mixtures of heavy metal titanates. We can adjust the transition temperature (the temperature at which this happens) over a wide range by changing the chemistry. In this kind of material, current passes through the material until it reaches the transition temperature, at which point the circuit breaks and current flow stops. We use these ceramics as self-controlled heating elements in, for example, rear-window defrosting circuits in cars.
At the transition temperature, the material's dielectric response approaches infinity. We cannot use the material near its critical temperature because it is hard to control temperature at that range. Despite this, the dielectric effect remains strong even at much higher temperatures. For this reason as well as because of how low their critical temperatures are, we use titanates as ceramic capacitors.
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 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 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, inductors, sensors, 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
- 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 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 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.
- Bio-ceramics 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 nano crystalline 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.
- High-tech ceramic is used in watch cases. The material is valued for its light weight, scratch-resistance, durability and smooth touch. IWC is one of the brands that initiated the use of ceramic in watchmaking.
Images for kids
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Cutting disks made of silicon carbide
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The Meissner effect demonstrated by levitating a magnet above a cuprate superconductor, which is cooled by liquid nitrogen
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Silicon nitride rocket thruster. Left: Mounted in test stand. Right: Being tested with H2/O2 propellants
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Cermax xenon arc lamp with synthetic sapphire output window
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Kitchen knife with a ceramic blade
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See also
In Spanish: Cerámica para niños
