A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared from powdered materials, is fabricated into products through the application of heat, and displays such characteristic properties as hardness, strength, low electrical conductivity, and brittleness." The word ceramic comes the from Greek word "keramikos", which means "pottery." They are typically crystalline in nature and are compounds formed between metallic and nonmetallic elements such as aluminum and oxygen (alumina-Al
2
O
3
), calcium and oxygen (calcia - CaO), and silicon and nitrogen(silicon nitride-Si
3
N
4
).
Depending on their method of formation, ceramics can be dense or lightweight.
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 or insulators. Some ceramics, like superconductors, also display magnetic properties. They are also more resistant to high temperatures and harsh environments than metals and polymers. Due to ceramic materials wide range of properties, they are used for a multitude of applications.
The broad categories or segments that make up the ceramic industry can be classified as:
 Structural clay products (brick, sewer pipe, roofing and wall tile, flue linings, etc.)
 Whitewares (dinnerware, floor and wall tile, electrical porcelain, etc.)
 Refractories (brick and monolithic products used in metal, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries)
 Glasses (flat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical fibers))
 Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure blasting of materials)
 Cements (for roads, bridges, buildings, dams, and etc.)
 Advanced ceramics o Structural (wear parts, bioceramics, cutting tools, and engine components) o Electrical (capacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets and superconductors)

o Coatings (engine components, cutting tools, and industrial wear parts) o Chemical and environmental (filters, membranes, catalysts, and catalyst supports)
Ceramic Structures
As discussed in the introduction, ceramics and related materials cover a wide range of objects. Ceramics are a little more complex than metallic structures, which is why metals were covered first. A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared from powdered materials and is fabricated into products through the application of heat. Most ceramics are made up of two or more elements. This is called a compound. For example, alumina (Al2O3) is a compound made up of aluminum atoms and oxygen atoms.
The two most common chemical bonds for ceramic materials are covalent and ionic. The bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. This is why ceramics generally have the following properties: high hardness, high compressive strength, and chemical inertness.
This strong bonding also accounts for the less attractive properties of ceramics, such as low ductility and low tensile strength. The absence of free electrons is responsible for making most ceramics poor conductors of electricity and heat.
However, it should be noted that the crystal structures of ceramics are many and varied and this results in a very wide range of properties. For example, while ceramics are perceived as electrical and thermal insulators, ceramic oxide
(initially based on Y-Ba-Cu-O) is the basis for high temperature superconductivity. Diamond and silicon carbide have a higher thermal conductivity than aluminum or copper. Control of the microstructure can overcome inherent stiffness to allow the production of ceramic springs, and ceramic composites which have been produced with a fracture toughness about half that of steel. Also, the atomic structures are often of low symmetry that gives some ceramics interesting electromechanical properties like piezoelectricity, which is used in sensors and transducers.
The structure of most ceramics varies from relatively simple to very complex. The microstructure can be entirely glassy (glasses only); entirely crystalline; or a combination of crystalline and glassy. In the latter case, the glassy phase usually surrounds small crystals, bonding them together. The main compositional classes of engineering ceramics are the oxides, nitrides and carbides.

Ceramic Glass
Ceramics with an entirely glassy structure have certain properties that are quite different from those of metals. Recall that when metal in the liquid state is cooled, a crystalline solid precipitates when the melting freezing point is reached.
However, with a glassy material, as the liquid is cooled it becomes more and more viscous. There is no sharp melting or freezing point. It goes from liquid to a soft plastic solid and finally becomes hard and brittle. Because of this unique property, it can be blown into shapes, in addition to being cast, rolled, drawn and otherwise processed like a metal.
Glassy behavior is related to the atomic structure of the material. If pure silica
(SiO
2
) is fused together, a glass called vitreous silica is formed on cooling. The basic unit structure of this glass is the silica tetrahedron, which is composed of a single silicon atom surrounded by four equidistant oxygen atoms. The silicon atoms occupy the openings (interstitials) between the oxygen atoms and share four valence electrons with the oxygen atoms through covalent bonding. The each of the oxygen atoms has two valence electrons so the silica tetrahedron has four extra valence electrons to share with adjacent tetrahedral. The silicate structures can link together by sharing the atoms in two corners of the SiO
2 tetrahedrons, forming chain or ring structures. A network of silica tetrahedral chains form, and at high temperatures these chains easily slide past each other.
As the melt cools, thermal vibrational energy decreases and the chains can not move as easily so the structure becomes more rigid. Silica is the most important constituent of glass, but other oxides are added to change certain physical characteristics or to lower the melting point.
Ceramic Crystalline or Partially Crystalline Material
Most ceramics usually contain both metallic and nonmetallic elements with ionic or covalent bonds. Therefore, the structure the metallic atoms, the structure of the nonmetallic atoms, and the balance of charges produced by the valence

electrons must be considered. As with metals, the unit cell is used in describing the atomic structure of ceramics. The cubic and the hexagonal cells are most common. Additionally, the difference in radii between the metallic and nonmetallic ions plays an important role in the arrangement of the unit cell.
In metals, the regular arrangement of atoms into densely packed planes led to the occurrence of slip under stress, which gives metal their characteristic ductility. In ceramics, brittle fracture rather than slip is common because both the arrangement of the atoms and the type of bonding is different. The fracture or cleavage planes of ceramics are the result of planes of regularly arranged atoms