Properties

Underlying many of the properties found in ceramics are the strong primary bonds that hold the atoms together and form the ceramic material. These chemical bonds are of two types: they are either ionic in character, involving a transfer of bonding electrons from electropositive atoms (cations) to electronegative atoms (anions), or they are covalent in character, involving orbital sharing of electrons between the constituent atoms or ions. Covalent bonds are highly directional in nature, often dictating the types of crystal structure possible. Ionic bonds, on the other hand, are entirely nondirectional. This nondirectional nature allows for hard-sphere packing arrangements of the ions into a variety of crystal structures, with two limitations. The first limitation involves the relative size of the anions and the cations. Anions are usually larger and close-packed, as in the face-centered cubic or hexagonal close-packed crystal structures found in metals. Cations, on the other hand, are usually smaller, occupying space (interstices) in the crystal lattice between the anions.

The second limitation on the types of crystal structure that can be adopted by ionically bonded atoms is based on a law of physics--that the crystal must remain electrically neutral. This law of leectroneutrality results in the formation of very specific stoichiometries; i.e, specific ratios of cations to anions that maintain a net balance between positive and negative charge. In fact, anions are known to pack around cations, and cations around anions, in order to eliminate local charge imbalance. This phenomenon is referred to as coordination.

Most of the primary chemical bonds found in ceramic materials are actually a mixture of ionic and covalent types. The larger the electronegativity difference between anion and cation (that is, the greater the difference in potential to accept or donate electrons), the more nearly ionic is the bonding (that is, the more likely are electrons to be transferred, forming positively charged cations and negatively charged anions). Conversely, small differences in electronegativity lead to a sharing of electrons, as found in covalent bonds.

Secondary bonds also are important in certain ceramics. For example, in diamond, a single-crystal form of carbon, all bonds are primary, but in graphite, a polycrystalline form of carbon, there are primary bonds within sheets of crystal grains and secondary bonds between the sheets. The relatively weak secondary bonds allow the sheets to slide past one another, giving graphite the lubricity for which it is well known. It is the primary bonds in ceramics that make them among the strongest, hardest, and most refractory materials known.

Crystal structure is also responsible for many of the properties of ceramics. In Figures 2A through 2D representative crystal structures are shown that illustrate many of the unique features of ceramic materials. Each collection of ions is shown in an overall box that describes the unit cell of that structure. By repeatedly translating the unit cell one box in any direction and by repeatedly depositing the pattern of ions within that cell at each new position, any size crystal can be built up. In the first structure the material shown is magnesia (MgO), though the structure itself is referred to as rock salt because common table salt (sodium chloride, NaCl) has the same structure. In the rock salt structure each ion is surrounded by six immediate neighbours of the opposite charge. This extremely efficient packing allows for local neutralization of charge and makes for stable bonding. Oxides that crystallize in this structure tend to have relatively high melting points.