Course B- Materials for Devices Flashcards
Anisotropic
Properties differ depending on direction of measurement- crystalline materials
Isotropic
No long range order so all directions are equivalent. Invariant with respect to direction- liquids
Liquid crystals
Anisotropic liquids, directionality comes from molecular shape
Molecules that make up liquid crystals
Central region is rigid
Ends are flexible
Rod shaped
Properties of NEMATIC liquid crystals
No positional order- molecules free to move relative to each other
Long range orientational order- line up with long axes lying roughly parallel (along Director, D)
Orientational order with temperature (nematic)
Low temp= highly ordered, a crystal
Increasing temp= decreasing order, increased thermal agitation
High temp= No intrinsic order, a liquid
Order Parameter
Q=3《cos^2x》-1 /2 (《》=average over all molecules
1 when completely ordered
Direction of polarization
The vibration axis of the electric field, E
Plane of polarization
The plane containing the vibration axis and the direction of propagation
Permitted vibration directions
Fast axis- direction in which light couples weakly with the polymer- slowed down less
Slow axis- light couples strongly with the polymer- slowed down most significantly
Refractive index, n
c/v
Birefringence
Difference between refractive indices (delta n)
Polarizers
Absorb light vibrating parallel to long axes and transmit light vibrating perpendicular
Optical path difference=retardation=
Dn.t
t=thickness of material
Phase difference, d
d/2pi=Dn.t/lambda
Permitted vibration direction in nematic liquid crystal
Perpendicular to director, D
Disinclination
Where differently oriented domains meet, creates a schlieren texture
Smectic LC
Molecules organise into layers
Orientational order, some positional order
Chiral nematic/cholesteric
Helical twist through the material. Due to chiral molecules
Pitch (in cholesteric LC)
Distance taken for a 360° rotation of director. Distance between 2 bright and 2 dark bands
Length of pitch affected by:
Length of molecule
-long molecule=long pitch(less asymmetric)
Distance from end to end of molecule
n^1/2.l
n= no. of segments
l=length of segment
Kuhn length
Length scale below which chain is effectively straight and rigid (measure of stiffness)
Comformation
Orientation of C-C bonds
Configuration/tacticity
Where side groups are put (can’t be changed-chiral)
Lowest energy conformation
trans
isotactic configuration
side group on same side of chain
syndiotactic configuration
side group on alternating sides of chain
atactic configuration
side groups randomly distributed
Plasticizers
small molecules added to polymers to space out chains and increase mobility
cross links
eg in rubber
chains free to twist and flex but can’t slide past each other, so no permanent shape changes
crystalline polymers
chains fold and line up with each other, easier for regular chains and small side groups
lamellae
ordered, crystalline layers of polymer chain
dielectric
non-conducting but can be electrically polarised
Electronic polarisation
distortion of electron cloud with respect to nucleus
occurs in all atoms, esp. noble gases and diamond
Ionic polarisation
elastic distortion of ionic bonds, relative displacement of +ve and -ve charge
orientational/molecular polarisation
rotation of pre-existing permanent dipoles eg in water
Polarisation, P=(dipole)
n.u
n=no of dipoles per unit volume(m^-3)
u=mu=dipole moment(Cm)
dipole moment, u=
q.r
q=charge
r=distance between charges
Polarisation, P=(charge)
Q/A
Q=total charge on surface area of material
A=surface area
Total charge density, D=
also displacement field
epsilon.E
epsilon=permittivity of material/polarisability (Fm^-1)
E=electric field(Vm^-1)
F=farad=CV^-1
Dielectric constant, kappa=
epsilon/epsilon0
epsilon0=permittivity of free space
D (with polarisation)
epsilon0.E+P
P=polarisation of material
Polarisation of material, P=
epsilon0.E(kappa-1)
Capacitance units
C/V or F
Capacitance, C(italic)=
Q/V
Q=charge
V=voltage
Capacitance with empty parallel plate capacitor=
epsilon0.A/L
A=surface area of plates
L=distance between plates
Capacitance with a dielectric, C’=
epsilon.A/L
high kappa=
easily polarised
Centrosymmetric structure
Has an inversion centre/centre of symmetry, no polarisation possible
Unique direction=
a lattice vector which is not repeated by the symmetry present
Piezoelectric
non-centrosymmetric, not necessarily a unique direction
polarisation occurs when stress is applied/changes shape
Pyroelectric
non-centrosymmetric and a unique direction/polar
change in polarisation due to change in temp
Ferroelectric
non-centrosymmetric, unique direction/polar, switchable
polarisation can be switched by external electric field, can be permanently polarised
