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Transcript
LC Applications
Behzad Pourabbas
Polymer Eng. Department
Sahand University of Technology
Tabriz-Iran
[email protected]
Overview:
• Order Parameter
• Anisotropic Properties
• Light, polarization and materials
2
ORDER PARAMETER “S”
The Order Parameter
n
1
2
S  P2 (cos q )  (3 cos q  1)
2
q
n
S  P2 (cos q )  1
perfect crystal
S  P2 (cos q )  0
isotropic fluid
Maier-Saupe Theory - Mean Field Approach
n
Order Parameter, S
1.0
Isotropic
Fluid
0.0
Nematic
Liquid
Crystal
n
-0.6
Temperature
The Order Parameter:
How does it affects display performance ?
The order parameter, S, is proportional to a number of important
parameters which dictate display performance.
Parameter
Elastic Constant
Birefringence
Dielectric Anisotropy
Magnetic Anisotropy
Viscosity Anisotropy
Nomenclature
Kii
Dn
proportional to

De
Dc
Dh
S2
S
S
S
S
Example: Does the threshold switching voltage for a TN increase
or decrease as the operating temperature increases.
K
S2
VTH 

 S
De
S
Scales as the square root of S
therefore lowers with increasing temperature
Response to Electric and Magnetic Fields
External Electric Field and Dielectric Properties of LC molecules
Anisotropy: Dielectric Constant
++
+ ++
E
e
positive
- ---
De  e  e
e
>0
E
negative
De  e  e < 0
E
all angles in
the plane 
to E are
possible for the
-De materials
Anisotropy: Duel Frequency
low frequency, De>0
high frequency, De<0
MLC-2048 (EM Industries), Duel Frequency Material
Frequency (kHz)
0.1
1.0
10
50
Dielectric Anisotropy (De)
3.28 3.22 0.72 -3.0
100
-3.4
Dielectric Constant
Dielectric Constant
ke0L = C = q/V
Dielectric Material?
• Dielectric materials consist of polar molecules which
are normally randomly oriented in the solid.
•They are not conductors.
•When a dielectric material is placed in an external
electric field, the polar molecules rotate so they align with
the field. This creates an excess of positive charges on
one face of the dielectric and a corresponding excess
of negative charges on the other face.
E
Dielectric Material

E is smaller in many materials than it would be in a vacuum for the same
arrangement of charges.
Eg.
Parallel plates:

E

k ke 
Eo

Dielectric
material
Ei
Eo
+
+
+
+
Net field: E=Eo-Ei

This makes the potential difference smaller (V=Ed) between the parallel plates of the
capacitor for the same charges on the plates and thus capacitance is larger, since Q=C/V.
Dielectric Constant
(“kappa”) = “dielectric constant”
k
So,
Or
= (a pure number ≥ 1)
C
ke A
(for parallel plates)
d
C  kC
0
Where C0 is the capacitance without the dielectric.
k
Hence, the capacitance of a filled capacitor is greater
than an empty one by a factor
Dielectric Constants (@20oC, 1kHz)
*Mixture Application
De
e
e
BL038
MLC-6292
ZLI-4792
TL205
18523
95-465
16.7
7.4
5.2
5
2.7
-4.2
21.7
11.1
8.3
9.1
7
3.6
5.3
3.7
3.1
4.1
4.3
7.8
PDLCs
TN AMLCDs
TN AMLCDs
AM PDLCs
Fiber-Optics
-De material
*EM Materials
PD: Polymer Dispersed
AM: Active Matrix
TN: Twisted Nematic
Materials
Vacuum
Air
Polystyrene
Polyethylene
Nylon
Water
Dielectric Constant
1.0000
1.0005
2.56
2.30
3.5
78.54
Flow of ions in the presence of electric field
Internal Field Strength E = E0 – E’
Alignment of LC molecules in Electric Field
S=0
1>S>0
Dielectric Anisotropy and Permanent Dipole Moment
m
m
Dielectric Constants:
Temperature Dependence
e
16
CH3-(CH2)4
De  S(T )
14
Dielectric Constant
4’-pentyl-4-cyanobiphenyl
12
Temperature Dependence
Extrapolated from isotropic phase
eis
10
De 
1
 2e   e // 
3
e
8
30
T-TNI (°C)
De  S(T )
Average Dielectric Anistropy
6
25
C N
35
1
De   2e   e // 
3
Dielectric Anisotropy and Induced Dipole Moment
easily polarized
+
minduced is large
r//
e is large
-
+
r
Molecular axis
-
minduced is small
e is small
e
dielectric constant along the direction
perpendicular to the molecular axis
e
dielectric constant along the direction
parallel to the molecular axis
Magnetic Anisotropy: Diamagnetism
Diamagnetism: induction of a magnetic moment in opposition
to an applied magnetic field. LCs are diamagnetic due to the
dispersed electron distribution associated with the electron
structure.
Delocalized charge makes
the major contribution to
diamagnetism.
Ring currents associated with
aromatic units give a large
negative component to c for
directions  to aromatic ring
plane. Dc is usually positive since:
Dc  c ll  c  > 0
c ll > c 
Magnetic Anisotropy: Diamagnetism
Dc /10 9 m3 kg 1
Compound
C5H11
CN
1.51
C7H15
CN
1.37
CN
0.46
CN
0.42
C5H11
C7H15
CN
C7H15
-0.38
Light is a high frequency electromagnetic wave and will only
polarize electron cloud.
In general, De = e  e > 0 or e > e
Positive De > 0 (10 to 20)
Negative De < 0 (-1 to -2)
For high electrical resistance materials, n is proportional
to e1/2
Dn = n  n
> 0 in general
Dn is a very important parameter for a LC device.
Larger the Dn value, thinner the LC device and faster the
response time
Examples
S
C N
C5H11
De = +33
C - N - I
76 98
O
N
C
O
C5H11
O
C7H15
De = - 4.0
C - N - I
45 101
Magnetic Susceptibility and Anisotropy
LIGHT, POLARIZATION AND
MATERIALS
27
Optical polarization
• light is a transverse wave: E perpendicular to k
• for any wavevector, there are two field components
• any wave may be written as a superposition of the two polarizations
28
Light as Electromagnetic Wave
Plane Polarized light can be resolved into Ex and Ey
BIREFRENGENCE
32
Birefringence
Ordinary light travels in the
crystal with the same speed v in
all direction.
The refractive index n0=c/v in
all direction are identical.
Extraordinary light travels in the crystal
with a speed v that varies with direction.
The refractive index n0=c/v also varies
with different direction
Interaction of Electromagnetic Wave with LC Molecules
Propagation of the light is hindered by the molecule
E field

e//
Induced dipole
by electromagnetic wave
Speed of the light is slowed down
 = C / e//
Propagation of the light parallel to the molecular axis
E field
Induced dipole
by electromagnetic wave
e
//
Change of the speed is relatively small
 // = C// /
e
Optical Anisotropy: Birefringence
ordinary ray (no, ordinary index of refraction)
extraordinary ray (ne, extraordinary index
of refraction)
Optical Anisotropy: Birefringence
n  no
ordinary wave
extraordinary wave
1 cos2 q sin2 q


2
2
2
n
no
ne
optic
axis
q
For propagation along the optic
axis, both modes are no
Birefringence (20oC @ 589 nm)
EM Industry
Mixture
BL038
TL213
TL205
ZLI 5400
ZLI 3771
ZLI 4792
MLC-6292
ZLI 6009
MLC-6608
95-465
MLC-6614
MLC-6601
18523
ZLI 2806
Dn
0.2720
0.2390
0.2175
0.1063
0.1045
0.0969
0.0903
0.0859
0.0830
0.0827
0.0770
0.0763
0.0490
0.0437
ne
1.7990
1.7660
1.7455
1.5918
1.5965
1.5763
1.5608
1.5555
1.5578
1.5584
----------------1.5089
1.5183
no
1.5270
1.5270
1.5270
1.4855
1.4920
1.4794
1.4705
1.4696
1.4748
1.4752
----------------1.4599
1.4746
Application
PDLC
PDLC
AM PDLC
STN
TN
AM TN LCDs
AM TN LCDs
AN TN LCDs
ECB
-De devices
IPS
IPS
Fiber Optics
-De device
Birefringence: Temperature
Dependence
Average Index
ne
1.8
Index of Refraction
n
1.7
n

1
 ne 2  2n02
3
2
2


1 2
2
 ne  2n0
3
niso
Extrapolated from isotropic phase
1.6
no
Temperature
Dependence
1.5
Dn  S(T )
1.4
50
40
30
T-TNI (°C)
20
10
0

CIRCULAR POLARIZATION
OF LIGHT
Circular Birefringence
Categories of optical polarization
• linear (plane) polarization
• coefficients differ only by real factor
• circular polarization
• coefficients differ only by factor
i
• elliptical polarization
• all other cases
44
Characterizing the optical polarization
• wavevector insufficient to define
electromagnetic wave
• we must additionally define the
polarization vector
k


x
z
• e.g. linear polarization at angle 
y
45
Reflection of Circular Polarized Light
LCP
RCP
Dynamic Scattering Mode LCD Device
Twisted Nematic (TN) Device 1971 by Schadt
Super Twisted Nematic (STN) LC Device 1984 by Scheffer
By addition of appropriate amounts of chiral reagent
Twisted by 180-270 o
N:Number of row for scanning
Vs: turn on voltage
Vns:turn off voltage
Electrically Controlled Birefringence (ECB) Device (DAP type)
Polymer Dispersed Liquid Crystal (PDLC) Device
GENERAL STRUCTURE
55
General Structure
Z’
Z
Y
A
X
• Aromatic
or saturated ring core
• X & Y are terminal groups
• A is linkage between ring systems
• Z and Z’ are lateral substituents
CH3 - (CH2)4
4-pentyl-4’-cyanobiphenyl (5CB)
C N
Common Groups
Mesogenic Core
Ring Groups
Linking Groups
phenyl
N
pyrimidine
N
cyclohexane
biphenyl
terphenyl
diphenylethane
stilbene
tolane
schiffs base
azobenzene
azoxybenzene
phenylbenzoate
(ester)
phenylthiobenzoate
 CH2  CH2 
 CH  CH 
 CH  CH 
 CH  N 
N N
O
N N
O
CO
O
CS 
Nomenclature
Mesogenic Core
terphenyl
biphenyl
phenyl
benzyl
benzene
phenylcyclohexane (PCH)
3’
2’
2
1’
4’
5’
6’
cyclohexane
cyclohexyl
3
1
4
6
5
Ring Numbering
Scheme
Terminal Groups
(one terminal group is typically an alkyl chain)
CH2
CH2
CH2
CH3
CH2
CH2
CH3
C*H
straight chain
branched chain
(chiral)
CH3
Attachment to mesogenic ring structure
Direct
alkyl (butyl)
Ether -O- alkoxy (butoxy)
Terminal Groups
CH3-
methyl
CH3-O-
methoxy
CH3-CH2-
ethyl
CH3-CH2-O-
ethoxy
CH3-(CH2)2-
propyl
CH3-(CH2)2-O-
propoxy
CH3-(CH2)3-
butyl
CH3-(CH2)3-O-
butoxy
CH3-(CH2)4-
pentyl
CH3-(CH2)4-O-
pentoxy
CH3-(CH2)5-
hexyl
CH3-(CH2)5-O-
hexoxy
CH3-(CH2)6-
heptyl
CH3-(CH2)6-O-
heptoxy
CH3-(CH2)7-
octyl
CH3-(CH2)7-O-
octoxy
Second Terminal Group and
Lateral Substituents (Y & Z)
H
F
Cl
Br
I
CH3
CH3(CH2)n
CN
NH2
N(CH3)
NO2
flouro
chloro
bromo
iodo
methyl
alkyl
cyano
amino
dimethylamino
nitro
phenyl
cyclohexyl
Odd-Even Effect
Clearing point versus alkyl chain length
O
CH3-(CH2)n-O
C-O
O-(CH2)n-CH3
18
clearing point
16
14
12
10
0 1 2 3 4 5 6 7 8 9 10 11
carbons in alkyl chain (n)
Nomenclature
Common molecules which exhibit a LC phase
CH3-(CH2)4
C N
4’-pentyl-4-cyanobiphenyl
CH3-(CH2)4-O
4’-pentoxy-4-cyanobiphenyl
C N
Structure - Property
vary mesogenic core
CH3-(CH2)4
A
A
C N
C-N (oC)
N-I(oC)
Dn
De
22.5
35
0.18
11.5
71
52
0.18
19.7
31
55
0.10
9.7
N
N
Structure - Property
vary end group
COO
CH3-(CH2)4
X
H
F
Br
CN
CH3
C6H5
C-N (oC)
87.5
92.0
115.5
111.0
106.0
155.0
X
N-I (oC)
114.0
156.0
193.0
226.0
176.0
266.0
Lateral Substituents (Z & Z’)
Z’
Z
X
•Z
A
and Z’ are lateral substituents
• Broadens the molecules
• Lowers nematic stability
• May introduce negative dielectric anisotropy
Y
Why Liquid Crystal Mixtures
Melt Temperature:
Liquid Crystal-Solid
Isotropic Liquid
ln ci = DHi(Teu-1 - Tmi-1)/R
Temperature
Liquid
Crystal
DH: enthalpies
Teu: eutectic temperature
Tmi: melt temperature
R: constant
E
Nematic-Isotropic
Temperature: TNI
eutectic
point
TNI = S ciTNIi
Solid
0
50
Concentration (c2), %
100