Typically, they will demonstrate excellent strength and hardness properties; however, they are often brittle in nature. Ceramics can also be formed to serve as electrically conductive materials, objects allowing electricity to pass through their mass, or insulators, materials preventing the flow of electricity. Some ceramics, like superconductors, also display magnetic properties. Ceramics are generally made by taking mixtures of clay, earthen elements, powders, and water and shaping them into desired forms.
Once the ceramic has been shaped, it is fired in a high temperature oven known as a kiln. Often, ceramics are covered in decorative, waterproof, paint-like substances known as glazes. Decorative glazes are then applied followed by further firing.
Bone china — which is easier to make, harder to chip and stronger than porcelain — is made by adding ash from cattle bones to clay, feldspar minerals and fine silica sand. Advanced ceramics are not generally clay-based.
Instead, they are either based on oxides or non-oxides or combinations of the two:. Production processes firstly involve thoroughly blending the very fine constituent material powders. This step is often carried out in an oxygen-free atmosphere. The high temperature allows the tiny grains of the individual ceramic components to fuse together, forming a hard, tough, durable and corrosion-resistant product.
This process is called sintering. Advanced ceramic materials are now well established in many areas of everyday use, from fridge magnets to an increasing range or industries, including metals production and processing, aerospace, electronics, automotive and personnel protection. In modern medicine, advanced ceramics — often referred to as bioceramics — play an increasingly important role. Bioceramics such as alumina and zirconia are hard, chemically inert materials that can be polished to a high finish.
They are used as dental implants and as bone substitutes in orthopaedic operations such as hip and knee replacement. Find out more on the uses for advanced ceramics. Traditional ceramics are made from natural materials such as clay that have been hardened by heating at high temperatures driving out water and allowing strong chemical bonds to form between the flakes of clay.
In fact, the word "ceramic" comes from the Greek keramos , whose original meaning was "burnt earth. Special ovens called kilns are used to "fire" heat the shaped object until it hardens. Clay consists of a large number of very tiny flat plates, stacked together but separated by thin layers of water. The water allows the plates to cling together, but also acts as a lubricant, allowing the plates to slide past one another. As a result, clay is easily molded into shapes. High temperatures drive out water and allow bonds to form between plates, holding them in place and promoting the formation of a hard solid.
Binders such as bone ash are sometimes added to the clay to promote strong bond formation, which makes the ceramic resistant to breakage. The common clay used to make flowerpots and roof tiles is usually red-orange because of the presence of iron oxides. White ceramics are made from rarer and thus more expensive white clays, primarily kaolin.
The oldest known ceramics made by humans are figurines found in the former Czechoslovakia that are thought to date from around 27, B. It was determined that the figurines were made by mixing clay with bone, animal fat, earth, and bone ash the ash that results when animal bones are heated to a high temperature , molding the mixture into a desired shape, and heating it in a domed pit.
The manufacture of functional objects such as pots, dishes, and storage vessels, was developed in ancient Greece and Egypt during the period to B.
An important advance was the development of white porcelain. Porcelain is a hard, tough ceramic that is less brittle than the ceramics that preceded it. Its strength allows it to be fashioned into beautiful vessels with walls so thin they can even be translucent. Porcelain was developed in China around C. The porcelain process was introduced to the Arab world in the ninth century; later Arabs brought porcelain to Spain, from where the process spread throughout Europe.
Bone china has a composition similar to that of porcelain, but at least 50 percent of the material is finely powdered bone ash. Like porcelain, bone china is strong and can be formed into dishes with very thin, translucent walls.
Stoneware is a dense, hard, gray or tan ceramic that is less expensive than bone china and porcelain, but it is not as strong. As a result, stoneware dishes are usually thicker and heavier than bone china or porcelain dishes.
The preparation of an advanced ceramic material usually begins with a finely divided powder that is mixed with an organic binder to help the powder consolidate, so that it can be molded into the desired shape.
Before it is fired, the ceramic body is called "green. It is then heated to a high temperature until it is "sintered," or hardened, into a dense, strong ceramic. At this time, individual particles of the original powder fuse together as chemical bonds form between them. During sintering the ceramic may shrink by as much as 10 to 40 percent.
Because shrinkage is not uniform, additional machining of the ceramic may be required in order to obtain a precise shape. Sol-gel technology allows better mixing of the ceramic components at the molecular level, and hence yields more homogeneous ceramics, because the ions are mixed while in solution. In the sol-gel process, a solution of an organometallic compound is hydrolyzed to produce a "sol," a colloidal suspension of a solid in a liquid. Typically the solution is a metal alkoxide such as tetramethoxysilane in an alcohol solvent.
The sol forms when the individual formula units polymerize link together to form chains and networks. The sol can then be spread into a thin film, precipitated into tiny uniform spheres called microspheres, or further processed to form a gel inside a mold that will yield a final ceramic object in the desired shape.
The many crosslinks between the formula units result in a ceramic that is less brittle than typical ceramics. Although the sol-gel process is very expensive, it has many advantages, including low temperature requirements; the ceramist's ability to control porosity and to form films, spheres, and other structures that are difficult to form in molds; and the attainment of specialized ceramic compositions and high product purity.
Porous ceramics are made by the sol-gel process. These ceramics have spongelike structures, with many porelike lacunae, or openings, that can make up from 25 to 70 percent of the volume.
Because of the large number of pores, porous ceramics have enormous surface areas up to square meters, or 5, square feet, per gram of ceramic , and so can make excellent catalysts. For example, zirconium oxide is a ceramic oxygen sensor that monitors the air-to-fuel ratio in the exhaust systems of automobiles. Aerogels are solid foams prepared by removing the liquid from the gel during a sol-gel process at high temperatures and low pressures.
Because aerogels are good insulators, have very low densities, and do not melt at high temperatures, they are attractive for use in spacecraft. For centuries ceramics were used by those who had little knowledge of their structure. Today, understanding of the structure and properties of ceramics is making it possible to design and engineer new kinds of ceramics. Most ceramics are hard, chemically inert , refractory can withstand very high heat without deformation , and poor conductors of heat and electricity.
Ceramics also have low densities. These properties make ceramics attractive for many applications. Ceramics are used as refractories in furnaces and as durable building materials in the form of bricks, tiles, cinder blocks, and other hard, strong solids.
They are also used as common electrical and thermal insulators in the manufacture of spark plugs, telephone poles, electronic devices, and the nose cones of spacecraft.
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