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Materials > Course B- Materials for Devices > Flashcards

Course B- Materials for Devices Flashcards

1
Q

Anisotropic

A

Properties differ depending on direction of measurement- crystalline materials

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2
Q

Isotropic

A

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

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3
Q

Liquid crystals

A

Anisotropic liquids, directionality comes from molecular shape

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4
Q

Molecules that make up liquid crystals

A

Central region is rigid
Ends are flexible
Rod shaped

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5
Q

Properties of NEMATIC liquid crystals

A

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)

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6
Q

Orientational order with temperature (nematic)

A

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

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7
Q

Order Parameter

A

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

1 when completely ordered

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8
Q

Direction of polarization

A

The vibration axis of the electric field, E

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9
Q

Plane of polarization

A

The plane containing the vibration axis and the direction of propagation

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10
Q

Permitted vibration directions

A

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

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11
Q

Refractive index, n

A

c/v

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12
Q

Birefringence

A

Difference between refractive indices (delta n)

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13
Q

Polarizers

A

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

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14
Q

Optical path difference=retardation=

A

Dn.t

t=thickness of material

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15
Q

Phase difference, d

A

d/2pi=Dn.t/lambda

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16
Q

Permitted vibration direction in nematic liquid crystal

A

Perpendicular to director, D

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17
Q

Disinclination

A

Where differently oriented domains meet, creates a schlieren texture

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18
Q

Smectic LC

A

Molecules organise into layers

Orientational order, some positional order

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19
Q

Chiral nematic/cholesteric

A

Helical twist through the material. Due to chiral molecules

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20
Q

Pitch (in cholesteric LC)

A

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

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21
Q

Length of pitch affected by:

A

Length of molecule

-long molecule=long pitch(less asymmetric)

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22
Q

Distance from end to end of molecule

A

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

23
Q

Kuhn length

A

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

24
Q

Comformation

A

Orientation of C-C bonds

25
Q

Configuration/tacticity

A

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

26
Q

Lowest energy conformation

A

trans

27
Q

isotactic configuration

A

side group on same side of chain

28
Q

syndiotactic configuration

A

side group on alternating sides of chain

29
Q

atactic configuration

A

side groups randomly distributed

30
Q

Plasticizers

A

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

31
Q

cross links

A

eg in rubber

chains free to twist and flex but can’t slide past each other, so no permanent shape changes

32
Q

crystalline polymers

A

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

33
Q

lamellae

A

ordered, crystalline layers of polymer chain

34
Q

dielectric

A

non-conducting but can be electrically polarised

35
Q

Electronic polarisation

A

distortion of electron cloud with respect to nucleus

occurs in all atoms, esp. noble gases and diamond

36
Q

Ionic polarisation

A

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

37
Q

orientational/molecular polarisation

A

rotation of pre-existing permanent dipoles eg in water

38
Q

Polarisation, P=(dipole)

A

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

39
Q

dipole moment, u=

A

q.r
q=charge
r=distance between charges

40
Q

Polarisation, P=(charge)

A

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

41
Q

Total charge density, D=

also displacement field

A

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

42
Q

Dielectric constant, kappa=

A

epsilon/epsilon0

epsilon0=permittivity of free space

43
Q

D (with polarisation)

A

epsilon0.E+P

P=polarisation of material

44
Q

Polarisation of material, P=

A

epsilon0.E(kappa-1)

45
Q

Capacitance units

A

C/V or F

46
Q

Capacitance, C(italic)=

A

Q/V
Q=charge
V=voltage

47
Q

Capacitance with empty parallel plate capacitor=

A

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

48
Q

Capacitance with a dielectric, C’=

A

epsilon.A/L

49
Q

high kappa=

A

easily polarised

50
Q

Centrosymmetric structure

A

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

51
Q

Unique direction=

A

a lattice vector which is not repeated by the symmetry present

52
Q

Piezoelectric

A

non-centrosymmetric, not necessarily a unique direction

polarisation occurs when stress is applied/changes shape

53
Q

Pyroelectric

A

non-centrosymmetric and a unique direction/polar

change in polarisation due to change in temp

54
Q

Ferroelectric

A

non-centrosymmetric, unique direction/polar, switchable

polarisation can be switched by external electric field, can be permanently polarised

Materials (3 decks)

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