Course B - Materials for Devices Flashcards
define a dielectric material
“A dielectric material is an electrical insulator that can be polarised by an applied electric field”
give 3 polarisation mechanisms
1) electronic - distortion of an electron cloud around a nucleus
2) ionic - elastic distortion of an ionic bond
3) orientational - rotation of permanent dipole molecules
give the equation (LEARN) for dipole moment
the dipole moment between two opposite charges +q, -q, separated by r is
μ = qr
give the equations (3) for polarisation (LEARN)
Polarisation = total dipole moment per unit vol = charge per unit area
P = nμ
P = Q/A
n = dipoles per unit vol
and
P = εo E (κ-1)
give the equations (2) for the displacement field that forms when an electric field is applied to a dipole (LEARN)
D = εo E + P
D = ε E = κ εo E
give the equation for the capacitance on an empty and dielectric parallel plate capacitor (IN DATA BOOK)
Empty parallel plate
C = εo*A / L
dielectric parallel plate
C = ε*A / L
define centrosymmetric and non-centrosymmetric crystals
“Centrosymmetric crysals have an inversion centre”
“non-centrosymmetric crystals do not have an inversion centre”
give the possible polarities of a centrosymmetric crystal
- centrosymmetric crystals have NO unique directions, hence they CANNOT be polar
- whatever dipole forms on one side of the inversion centre will be perfectly cancelled by a dipole that forms on the other side
give the possible polarities of a non-centrosymmetric crystal
non-centrosymmetric crystals can be polar or non-polar
- polar if they contain a unique direction
- non-polar if they don’t
define piezoelectricity
“A change in polarisation due to the application of stress”
give the equation for the change in voltage due to stress for piezoelectric materials
ΔV = dσL/κεo
L = thickness
d = piezoelectric const.
give the necessary property of a crystal for piezoelectricity
piezoelectricity occurs in any non-centrosymmetric crystal because the positions of the charges move relative to each other
give the two effects that piezoelectric crystals are used for
Generator effect = stress changes, voltage change
motor effect = voltage change, change in shape
define pyroelectricity
“pyroelectricity is a change in polarisation due to a temperature change”
give the necessary property of a crystal for pyroelectricity
pyroelectricity only occurs in polar crystals because in non-polar crystals there is no relative motion between ions
give the equation for change in polarisation due to a temperature change for a pyroelectric crystal
ΔP = pΔT
p = pyroelectric constant
give the equation for change in voltage due to a temperature change for a pyroelectric crystal
ΔV = p Δt L / κ εo
give an example of a pyroelectric crystal and a use of pyroelectric crystals
Wurtzite = ZnS
Hex p lattice with half of tetrahedral interstices filled
only upwards pointing tetrahedrons filled so unique direction = [001]
used in burglar alarms
define ferroelectricity
“Stable, spontaneous polarisation which can be reversed by an external electric field”
when does ferroelectricity occur, what can we say about net polarisations in the absence of an electric field
- below the curie temperature Tc, the unit cell moved to a lower crystallographic symmetry
- a displacive phase transition occurs on cooling through Tc and the unit cell becomes non-centrosymmetric and gains a dipole
- even though each unit cell contains a dipole, in the absence of an electric field there will be no net dipole on a crystal (unless it has already been poled)
explain how Ferroelectricity (FE) occurs in a perovskite structure (ABO3)
in a perovskite structure (ABO3) (usually BiTiO3) FE occurs due to B atom displacement, there are large A atoms, small B atoms
- on cooling the crystal changes from cubic to tetragonal
define a domain
“A domain is a region in a material in which the polarisation is in the same direction”
what is domain wall energy
domain wall energy is the energy that arises at the boundary between different domains, it is minimised when aligned domains are next to each other
“There is an energy associated with the interface between differently aligned domains, hence it’s more efficient to have the same domains next to each other”
what is stray field energy
when there are large domains of the same polarisation, there is an energy cost as a field forms
UE = 1/2 ε E^2