Ceramics

Ceramic materials are usually understood to be compounds of metallic and nonmetallic elements, though some are actually ionic salts, and others are insulators. These materials can be very complicated, as are for example clays, spinels, and common window glass. Many ceramic compounds have very high melting points.

Ceramics have a wide range of applications. They have been used as refractories, abrasives, ferroelectrics, piezoelectric transducers, magnets, building materials, and surface finishes.

Unlike metals, there are really no heat treatments that can be used to modify the properties of ceramics, but their properties can be altered by changes in chemical composition. By carefully considering the choice of chemical composition, purity, particle size and uniformity and arrangement, and packing of atoms, high quality ceramics can be synthesized in a wide variety.

Hydraulic cements set by interaction with water. Portland cement, the most common hydraulic cement, is primarily a water-free calcium silicate. It is slightly soluble in water and sets by a combination of solution precipitation and chemical reaction with water to form a hydrated composition. The ratio of water to cement in the initial mix greatly influences the strength of the final concrete: the lower the water-to-cement ratio, the higher the strength.

In the twentieth century, scientists and engineers have acquired a much better understanding of ceramics and their properties. They have succeeded in producing ceramics with tailor-made properties. Modern ceramics include oxide ceramics, magnetic ceramics, ferroelectric ceramics, nuclear fuels, nitrides, carbides, and borides.

Magnesium oxide (MgO) occurs naturally in the mineral periclase, but not in sufficient quantity to meet commercial demand. Most MgO powder is produced from MgCO₃ or from seawater. MgO is extracted from sea water as a hydroxide, then converted to the oxide. MgO powder finds extensive use in high temperature electrical insulation and in refractory brick.

Densification of the particulate ceramic compact is referred to as sintering. Sintering is essentially the removal of pores between particles, combined with particulate growth and strong bonding between adjacent particles. In order for sintering to occur, the particles must be able to flow, and there must be a source of energy to activate and sustain this material transport. Sintering can take place in the vapor, liquid, or solid phase, or in a reactive liquid.

The sintered material must frequently be machined to allow it to meet dimensional tolerances, to give it an improved surface finish, or to remove surface flaws. Machining must be done carefully to avoid brittle fracture. The machining tool must have a higher hardness than the ceramic. The ceramic material can be processed by mechanical, thermal, or chemical action.