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

BH57 Course B: Materials for Devices BH57
Magnetostriction coefficient, Λ : fractional change in length on changing magnetisation from zero
to saturation (dimensionless).
Monomer: the simple chemical unit which, when many are joined together, form a polymer.
Nematic: spontaneously anisotropic liquid, composed of rod-shaped molecules with parallel
alignment.
Optic Axis: for a birefringent sample, a direction along which no birefringence is observed, i.e. a
direction perpendicular to an isotropic section. The optic axis of a nematic liquid crystal is along
the Director.
Optical Path Difference: the extra distance traveled by the fast ray outside an anisotropic material
in the time it takes for the slow ray to reach the edge of the sample (m).
Order parameter: describes the orientational order of a liquid crystalline material, taking account of
the deviation of orientation of individual molecules from the Director (zero for complete disorder;
unity for perfect order).
Paramagnetism: the net magnetic moment caused by alignment of the electron spins when a
magnetic field is applied.
Permitted Vibration Directions: two perpendicular directions in an anisotropic material along
which the transverse electric field of light can vibrate.
Permittivity, ε : describes how an electric field is affected by a dielectric (F m -1 ).
Perovskite: family of ABX 3
compounds which commonly show displacive phase transitions by
octahedral tilts or atomic displacements.
Phase: a part of a system, homogeneous in composition and structure.
Phase diagram: a diagram showing the equilibrium phases and phase compositions at each
combination of temperature (and/or pressure) and overall composition.
Photoelasticity: stress-induced birefringence. In a polymer, this is caused by stress-induced
alignment of polymer chains.
Piezoelectricity: the application of stress produces a change in the dielectric polarisation of a
material or, conversely, application of an electric field results in strain. Piezoelectric materials must be
non-centrosymmetric.
Piezoelectric coefficient, d: measure of the rate of change of dielectric polarisation with applied
stress (C N -1 ).
Pitch: the distance it takes for the Director in a cholesteric / chiral nematic liquid crystal to go
through one complete 360° rotation (m).
Polar material: a material with crystal symmetry such that it contains one (or more) unique
direction.
Polarization, P: the dipole moment per unit volume in a medium, or charge per unit area (C m -2 )
Polarization (of light): the property of electromagnetic waves that describes the direction of the
transverse electric field.
Polarized light: light that has passed through a sheet of polarizer, and has a transverse electric field
that vibrates in one direction only.