Materials Science Glossary - V

vacancy. A normally occupied lattice site from which an atom or ion is missing. Vacancies can be in thermal equilibrium as their formation energy is to some extend compensated by their entropy contribution.

vacancy diffusion. The diffusion mechanism wherein net atomic migration is from a lattice site to an adjacent vacancy.

valence band. For solid materials, the electron energy band that contains the valence electrons.

valence electrons. The electrons in the outermost occupied electron shell, which participate in interatomic bonding.

van der Waals bond. A secondary interatomic bond between adjacent molecular dipoles that may be permanent or induced.

viscoelasticity. A type of deformation exhibiting the mechanical characteristics of viscous flow and elastic deformation.

viscosity ( ). The ratio of the magnitude of an applied shear stress to the velocity gradient that it produces; that is, a measure of a noncrystalline material’s resistance to permanent deformation.

vitrification. During firing of a ceramic body, the formation of a liquid phase that upon cooling becomes a glass-bonding matrix.

vulcanization. A nonreversible chemical reaction involving sulfur or an other suitable agent wherein crosslinks are formed between molecular chains in rubber materials. The rubber’s modulus of elasticity and strength are enhanced.