Covalent and ionic bonding define ceramic compounds. Because these connections are stronger than metal bonding, ceramic materials have great hardness and stiffness but poor ductility. Ceramic molecules have tightly bonded electrons, which explains why they are poor conductors, much as metals have free electrons in their connections, which explains why metals are excellent heat and electricity conductors. Because of their strong bonding, these materials have high melting temperatures, but some ceramics disintegrate rather than melt at high temperatures.
The majority of ceramics have a crystalline structure. Structures are often more complicated than in most metals. This may be due to a number of factors. Ceramic molecules, for example, are composed of atoms of various sizes. Second, the ion charges of several common ceramics, such as SiO2 and Al2O3, vary. Both of these characteristics lead to a more twisted crystal structure and a more complex physical arrangement of the atoms in the molecule. Furthermore, many ceramic materials, such as (Al2Si2O5(OH)4), have more than two components, increasing the complexity of the molecular structure. Single crystals or polycrystalline materials may be used to make crystalline ceramics. The mechanical and physical characteristics of the more frequent second type are affected by grain size; finer-grained materials have greater strength and toughness.
Certain ceramic materials have an amorphous or glassy structure rather than a crystalline one. The most well-known example is, of course, glass. The vast majority of glasses are made from fused silica. Glassy ceramic components such as aluminum, boron, calcium, and magnesium oxides are used to create variations in features and hues. In addition to these pure glasses, several crystal-structured ceramics utilize the glassy phase as a binder for their crystalline phase.