Lecture handout including QS - Department of Materials Science ...

BH55 Course B: Materials for Devices BH55
Cross-linking: a process in which bonds are formed joining adjacent molecules. At low density
these bonds add to the elasticity of a polymer, at higher densities they produce rigidity.
Crossed polars: two sheets of polarizing material (polarizer and analyzer), oriented at 90° to each
other, between which a sample is placed for optical examination.
Curie temperature: the temperature above which ferroelectric / ferromagnetic behaviour is lost
(°C or K).
Current density, j: the current flowing through unit cross-sectional area (A m -2 ).
Diamagnetism: the effect caused by the magnetic moment due to orbiting electrons which produces
a slight opposition to imposed magnetic field.
Dielectric: an electrical insulator which is polarisable.
Dielectric breakdown: the passage of current through a dielectric material upon application of a
large electric field. The maximum field that a dielectric can sustain without breakdown is the
dielectric strength.
Dielectric constant, κ : a measure of the rate of change of polarisation with electric field; the ratio
of the permittivity of a material to the permittivity of free space.
Diffusion: transport of atoms in which the diffusing atom moves relative to its neighbours.
Diffusion coefficient (or diffusivity), D: a temperature dependent coefficient related to the rate at
which atoms diffuse, which also depends on the activation energy (m 2 s -1 )
Dipole: two equal charges of opposite sign, separated by a small distance.
Dipole moment, µ: the electric field generated by a spatial separation of positive and negative
charges (C m) or the magnetic moment generated by electron orbitals and spin.
Director, D: the direction of preferred orientation of molecules in liquid crystals.
Disclination: point at which the Director in a liquid crystal undergoes an abrupt change in
orientation.
Displacement field, D: resultant total polarisation field created in a material by application of an
electric field (C m -2 )
Domains: regions within a material in which all the electric / magnetic dipoles are aligned.
Domain wall: the boundary between ferroelectric / magnetic domains.
Drift velocity: the average rate at which charge carriers move through a material under the
application of an electric field (m s -1 )
Exchange interaction / exchange energy: a quantum mechanical electron – electron interaction
which favours parallel alignment of magnetic moments. A negative exchange interaction favours antiparallel
alignment of moments.
Extinction positions: orientations in which an anisotropic material appears black between crossed
polars. (They occur every 90°, when the permitted vibration directions of the sample are parallel
to the polarizer / analyzer.)
Fatigue: accumulation of defects in a ferroelectric material which gradually degrades the amount of
switched charge.