Ceramics and Pottery

Crown

The word ceramic is derived from the Greek word κεραμικος (keramikos, "having to do with pottery"). The term covers inorganic non-metallic materials whose formation is due to the action of heat. 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 traditional crafts are described in the article on pottery. A composite material of ceramic and metal is known as cermet.

Historically, ceramic products have been hard, porous and brittle. 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.

Classifications 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

Examples of ceramic materials

• Barium strontium calcium copper oxide, a high-temperature superconductor
• 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 electromechanical transducers, ceramic capacitors, and data storage elements. Grain boundary conditions can create PTC effects in heating elements.
• Boron carbide (B₄C), which is used in some helicopter and tank armor.
• 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.
• Bricks (mostly aluminium silicates), used for construction.
• Ferrite (Fe₃O₄), which is ferrimagnetic and is used in the core of electrical transformers and magnetic core memory.
• Lead zirconate titanate is another ferroelectric material.
• Magnesium diboride (MgB₂), which is an unconventional superconductor.
• Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a commonly used abrasive, and as a refractory material.
• Silicon nitride (Si₃N₄), which is used as an abrasive powder.
• Steatite is used as an electrical insulator.
• Uranium oxide (UO₂), used as fuel in nuclear reactors.
• Yttrium barium copper oxide (YBa₂Cu₃O_7-x), another high temperature superconductor.
• Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors.
• Zirconia, which in pure form undergoes many 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 cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.

Properties of ceramics

Mechanical properties

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

These materials do show plastic deformation. 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. With the non-crystalline (glassy) materials, 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

Semiconductivity

There are a number of ceramics that are semiconductors. Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide.

Whilst there is talk of making blue LEDs 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 unusual property of negative resistance. 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. 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-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 substations, 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 sensors. 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 superconductivity. The exact reason for this is not known, but there are two major families of superconducting ceramics.

Ferroelectricity and subsets

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. 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 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. 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.

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 loudspeakers, transducers for sonar, and actuators for atomic force and scanning tunneling microscopes.

Positive thermal coefficient

Increases in temperature can cause grain boundaries 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 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 most 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.

Processing of ceramic materials

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 mould. If later heat-treatments cause this class to become partly crystalline, the resulting material is known as a glass-ceramic.

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. 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

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). 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. 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 infeasible with current technology.

Work is being done in developing ceramic parts for gas turbine engines. 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.

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. Such plates are known commonly as small-arms protective inserts (SAPI). Very similar technology is used for armoring of cockpits of some military airplanes, because of the low weight of the material.

Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. 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. Orthopedic implants made from these materials bond ready 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 absorbtion 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 incorperation of protein collagens, synthetic bones.

Look up Ceramics in Wiktionary, the free dictionary

See also

• Ceramic forming techniques

External links

• Advanced Ceramics – The Evolution, Classification, Properties, Production, Firing, Finishing and Design of Advanced Ceramics

Unfired "green ware" pottery on a traditional drying rack at Conner Prairie living history museum.

Pottery is a form of ceramic technology, where the clay is formed into vessels, generally with utilitarian purposes in mind. The production of pottery is a process where wet clay is shaped and allowed to dry. The formed clay, or piece, may be "bisque fired" in a kiln to harden it, and then fired a second time after adding a glaze or a piece may be once fired by applying appropriate glaze to the dry unfired clay and firing in one cycle.

Types of pottery

Aesthetic and artistic considerations have often been part of the formation of the pottery vessels, however modern mass production techniques have replaced the traditional role of pottery with mechanized reproduction, which has in turn caused the potter to be more focused on the aesthetic than the utilitarian in industrialized nations.

Traditionally, different world regions have produced different types of clay, also called bodies, with the potter digging clay out of natural banks in his own 'back yard.' In modern times, potters will often combine different clays and minerals to produce clay bodies suited to their specific purposes. Pottery that is fired at temperatures in the 800 to 1200 °C range, which does not vitrify in the kiln but remains slightly porous is often called earthenware or terra cotta. Clay formulated to be fired at higher temperatures, which is partially vitrified is called stoneware. Fine earthenware with a white tin glaze is known as faience. Porcelain is a very refined, smooth, white body that, when fired to vitrification, can have translucent qualities. Complex extremely high-fired ceramics, where the glaze and body fuse completely, are generally referred to as "products of ceramic technology." Ceramic technology is used for items such as electronic parts and Space Shuttle tiles.

Techniques

A man shapes pottery as it turns on a wheel. (Cappadocia, Turkey).

A person who makes pottery is traditionally known as a potter. The potter's most basic tool is his or her hands, however many of their tools have been created over the long history of pottery, including the potter's wheel, various paddles, shaping tools (or ribs), slab rollers, and cutting tools.

Forming techniques

There are three basic categories of forming techniques used in pottery—handwork, wheel work, and slipcasting. It's very common for wheel-worked pieces to be finished by handwork techniques. Slipcast pieces tend not to be, as that negates one of the prime advantages of casting.

Handwork methods are the most primitive and individual techniques, where pieces are constructed from hand-rolled coils, slabs, ropes, and balls of clay, often joined with a liquid clay slurry. No two pieces of handwork will be exactly the same, so it is not suitable for making precisely matched sets of items such as dinnerware. Doing handwork enables the potters to use their imagination to create one-of-a-kind works of art. These methods are often referred to as "handbuilding".

Classic potter's wheel in Erfurt, Germany.

The potter's wheel can be used for mass production, although often it is employed to make individual pieces. The process of making ceramic ware on the potter's wheel is called "throwing" or "turning." A ball of clay is placed in the center of a turntable, called the wheel head, which is turned chiefly using foot power (a kick wheel or treadle wheel) or a variable speed electric motor. Oftentimes, a disk of plastic, wood, or plaster is affixed to the wheel head, and the ball of clay is attached to the disk rather than the wheel head so that the finished piece can be removed easily. This disk is referred to as a bat. The wheel revolves rapidly while the clay is pressed, squeezed, and pulled gently into shape. The process of pressuring the clay into a radial symmetry, so that it does not move from side to side as the wheel head rotates is referred to as "centering" the clay—usually the most difficult skill to master for beginning potters.

Wheel work takes a lot of technical ability, but a skilled potter can produce many virtually identical plates, vases, or bowls in a day. Because of its nature, wheel work can only be used to initially create items with radial symmetry on a vertical axis. These pieces can then be altered by impressing, bulging, carving, fluting, faceting, incising, and other methods to make them more visually interesting. Often, thrown pieces are further modified by having handles, lids, feet, spouts, and other functional aspects added using the techniques of handworking. Pottery that is thrown on the wheel is often finished in a process known as trimming. The thrown piece is first allowed to dry to the leather-hard state then it is returned to the potter's wheel, usually with the rim down. The piece must be re-centered to allow trimming of the foot of the pot to create a smooth and well-defined surface.

There are two related techniques that improve repeatability of wheelwork. A jigger is a mould that is slowly brought down onto the outside of an object, while it is being turned on a wheel. A solid mould is used to form the inside of the piece. Similarly, a jogger is used to shape the inside of a piece, pressing the outside against a solid mould. Although these techniques have been in use since the 18th century, they are usually considered minor "industrial" methods by modern studio potters. There is contention among potters over whether a "jigged" piece can be considered "hand-produced."

Slipcasting is probably the easiest technique for mass-production, especially for shapes not easily made on a wheel. A liquid clay slip is poured into plaster moulds and allowed to harden slightly. This slip can be formulated to mature at a variety of temperatures. Once the plaster has absorbed most of the liquid from the outside layer of clay the remaining slip is poured back into the storage tub, and the piece is left to dry. Finally, the finished piece is removed from the mould, "fettled" (trimmed neatly), and allowed to air-dry. This method is commonly used for smaller decorative pieces such as figurines, which have many intricate details. In the United States, moulds and their slipcast pieces are primarily an industrial product, and are usually called "ceramics" to distinguish them from other pottery.

Decorative and finishing techniques

Contemporary pottery from Okinawa, Japan.

Additives can be worked into moist clay, prior to forming, to produce desired characteristics to the finished ware. Various coarse additives, such as sand and grog (fired clay which has been finely ground) give the final product strength and texture, and contrasting colored clays and grogs result in patterns. Colorants, usually metal oxides and carbonates, are added singly or in combinations to achieve a desired colour. Combustible particles can be mixed with clay or pressed into the surface to produce texture. Shredded fiberglass can be used as an additive to improve tensile strength in the finished piece. However, the resulting clay contains sharp fibers, is hard to work with and must be carefully handled.

Throughout history, potters have used a mixture of coloured clays as a distinctive decorating technique. In traditional studio pottery in Great Britain, these techniques were known as agateware. The name is derived from the agate stone, which shows bands of colours. In Japan, various techniques for combining coloured clay on the potter's wheel are jointly known as "neriage." An analogue of marquetry can also be made, by pressing small blocks of coloured clays together, and using the resulting mosaic to create distinctive patterns. The Japanese term for this technique is nerikome. Agateware and the other varieties of 'mottled' ware are made by combining two or more colours or varieties of clay into one completed piece. Different colours of clay are lightly kneaded or slapped together before being formed