Course B- Materials for Devices

Question 1

Anisotropic

Answer 1

Properties differ depending on direction of measurement- crystalline materials

Question 2

Isotropic

Answer 2

No long range order so all directions are equivalent. Invariant with respect to direction- liquids

Question 3

Liquid crystals

Answer 3

Anisotropic liquids, directionality comes from molecular shape

Question 4

Molecules that make up liquid crystals

Answer 4

Central region is rigid
Ends are flexible
Rod shaped

Question 5

Properties of NEMATIC liquid crystals

Answer 5

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)

Question 6

Orientational order with temperature (nematic)

Answer 6

Low temp= highly ordered, a crystal
Increasing temp= decreasing order, increased thermal agitation
High temp= No intrinsic order, a liquid

Question 7

Order Parameter

Answer 7

Q=3《cos^2x》-1 /2 (《》=average over all molecules

1 when completely ordered

Question 8

Direction of polarization

Answer 8

The vibration axis of the electric field, E

Question 9

Plane of polarization

Answer 9

The plane containing the vibration axis and the direction of propagation

Question 10

Permitted vibration directions

Answer 10

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

Question 11

Refractive index, n

Answer 11

c/v

Question 12

Birefringence

Answer 12

Difference between refractive indices (delta n)

Question 13

Polarizers

Answer 13

Absorb light vibrating parallel to long axes and transmit light vibrating perpendicular

Question 14

Optical path difference=retardation=

Answer 14

Dn.t

t=thickness of material

Question 15

Phase difference, d

Answer 15

d/2pi=Dn.t/lambda

Question 16

Permitted vibration direction in nematic liquid crystal

Answer 16

Perpendicular to director, D

Question 17

Disinclination

Answer 17

Where differently oriented domains meet, creates a schlieren texture

Question 18

Smectic LC

Answer 18

Molecules organise into layers

Orientational order, some positional order

Question 19

Chiral nematic/cholesteric

Answer 19

Helical twist through the material. Due to chiral molecules

Question 20

Pitch (in cholesteric LC)

Answer 20

Distance taken for a 360° rotation of director. Distance between 2 bright and 2 dark bands

Question 21

Length of pitch affected by:

Answer 21

Length of molecule

-long molecule=long pitch(less asymmetric)

Question 22

Distance from end to end of molecule

Answer 22

n^1/2.l
n= no. of segments
l=length of segment

Question 23

Kuhn length

Answer 23

Length scale below which chain is effectively straight and rigid (measure of stiffness)

Question 24

Comformation

Answer 24

Orientation of C-C bonds

Question 25

Configuration/tacticity

Answer 25

Where side groups are put (can't be changed-chiral)

Question 26

Lowest energy conformation

Answer 26

trans

Question 27

isotactic configuration

Answer 27

side group on same side of chain

Question 28

syndiotactic configuration

Answer 28

side group on alternating sides of chain

Question 29

atactic configuration

Answer 29

side groups randomly distributed

Question 30

Plasticizers

Answer 30

small molecules added to polymers to space out chains and increase mobility

Question 31

cross links

Answer 31

eg in rubber | chains free to twist and flex but can't slide past each other, so no permanent shape changes

Question 32

crystalline polymers

Answer 32

chains fold and line up with each other, easier for regular chains and small side groups

Question 33

lamellae

Answer 33

ordered, crystalline layers of polymer chain

Question 34

dielectric

Answer 34

non-conducting but can be electrically polarised

Question 35

Electronic polarisation

Answer 35

distortion of electron cloud with respect to nucleus | occurs in all atoms, esp. noble gases and diamond

Question 36

Ionic polarisation

Answer 36

elastic distortion of ionic bonds, relative displacement of +ve and -ve charge

Question 37

orientational/molecular polarisation

Answer 37

rotation of pre-existing permanent dipoles eg in water

Question 38

Polarisation, P=(dipole)

Answer 38

n.u n=no of dipoles per unit volume(m^-3) u=mu=dipole moment(Cm)

Question 39

dipole moment, u=

Answer 39

q.r q=charge r=distance between charges

Question 40

Polarisation, P=(charge)

Answer 40

Q/A Q=total charge on surface area of material A=surface area

Question 41

Total charge density, D= | also displacement field

Answer 41

epsilon.E epsilon=permittivity of material/polarisability (Fm^-1) E=electric field(Vm^-1) F=farad=CV^-1

Question 42

Dielectric constant, kappa=

Answer 42

epsilon/epsilon0 | epsilon0=permittivity of free space

Question 43

D (with polarisation)

Answer 43

epsilon0.E+P | P=polarisation of material

Question 44

Polarisation of material, P=

Answer 44

epsilon0.E(kappa-1)

Question 45

Capacitance units

Answer 45

C/V or F

Question 46

Capacitance, C(italic)=

Answer 46

Q/V Q=charge V=voltage

Question 47

Capacitance with empty parallel plate capacitor=

Answer 47

epsilon0.A/L A=surface area of plates L=distance between plates

Question 48

Capacitance with a dielectric, C'=

Answer 48

epsilon.A/L

Question 49

high kappa=

Answer 49

easily polarised

Question 50

Centrosymmetric structure

Answer 50

Has an inversion centre/centre of symmetry, no polarisation possible

Question 51

Unique direction=

Answer 51

a lattice vector which is not repeated by the symmetry present

Question 52

Piezoelectric

Answer 52

non-centrosymmetric, not necessarily a unique direction | polarisation occurs when stress is applied/changes shape

Question 53

Pyroelectric

Answer 53

non-centrosymmetric and a unique direction/polar | change in polarisation due to change in temp

Question 54

Ferroelectric

Answer 54

non-centrosymmetric, unique direction/polar, switchable | polarisation can be switched by external electric field, can be permanently polarised