液晶之物性与光电特性简介
Physic Properties & Electro-Optic
of Liquid Crystal
Cell Design Department
Chao-yi Chen
2012/01/12
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•Introduction of Liquid Crystals•Physic Properties of Liquid Crystals•Electro-Optic of Liquid Crystals
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•Introduction of Liquid Crystals•Physic Properties of Liquid Crystals•Electro-Optic of Liquid Crystals
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Introduction of Liquid Crystals•Discovery of Liquid Crystals :In 1888, F. Reinitzer
(Austria
Botanist)
found a material which has texture at particular temp. In 1889, O. Lehmann (GermanyPhysicist) found this material which has birefringence properties.
Crystalline Solid
Mesophases
Plastic Crystals
Globular
Position orderly
Direction disorderly
No fluidity Isotropic LiquidLiquid CrystalsRod-like, Dise-like…Position disorderlyDirection orderlyFluidity
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Introduction of Liquid Crystals•Thermotropic (熱致型): Phase transition with various temperature
–Rod-Like (桿狀)
•Nematic (向列型)
•Cholesterics (膽固醇型)
•Smectic (層列型)
–Disc-Like (碟狀)
•Columnar
•Discotic Nematic
–Other
•Bowlic, Banana…
•Lyotropic (溶致型): Phase transition with various concentration of solvent
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Introduction of Liquid Crystals•Thermotropic:Most known liquid crystals are produced
by
varying
the temperature of various substances (inparticular, by raising the temperature of certain solids or lowering the temperature of certain liquids) and are commonly termed thermotropic.
Solid CrystalLiquid CrystalIsotropic Liquid
Temperature
T mp T cp
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Introduction of Liquid Crystals
•Shape of LC Molecule:
Rod-like (Cigar-like, or Calamitic)Disc-like (Discotic)
Discotic Phase
X: Rigid Core
Y: Linking Group
Z: Flexible Side ChainAnti-Discotic Phase
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Introduction of Liquid Crystals
•Molecular LC Arrangement:
Nematic Smectic A
Cholesteric Smectic C
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•Introduction of Liquid Crystals•Physic Properties of Liquid Crystals•Electro-Optic of Liquid Crystals
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Physic Properties of Liquid Crystals•Low temperature stability test:
–Eutectic (共晶點) mixture =composition
which has the lowest melting point than any of its
single constituents
–Real mixtures if customers =not ideal eutectic
but metastable at low temp.
•Metastable:Unstable phase but changes
occur very slowly
•LC can be supercooled:it can be chilled
below its freezing point without it becoming
a crystal.
–LTS test in small bottles at -20°C, -30°C, -40°C
available depending on application. Eutectic Point (Composition)
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Physic Properties of Liquid Crystals
•Anisotropic (異向性) of Physical Properties:Due to its molecular
alignment, physical properties in LCs are given by 3x3matrices or 2nd rank tensors.
n e / εǁ
Direction nn o / ε͵
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Physic Properties of Liquid Crystals
•Dielectric Anisotropic (介電異向性):Δε=ε
ǁ
-ε͵(PositiveLC:Δε>
0; Negative LC:Δε
εǁ
Direction nε͵
Dielectric Constants have influence on:
––––
Threshold VoltageOperating VoltagePower Consumption
Resistivity & Viscosity (Indirect)
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Physic Properties of Liquid Crystals
•Dielectric Anisotropic Measurement:
•MERCK:
–Capacitance measurement C i using LCR Meter
–Relative permittivity calculated from
•JNC:
–Electric constant measurement εi using HP 4284A
–Standard condition:
•At 25°C, 1kHz, sine wave (10V εǁ, 0.5V ε┴)
C 0:Electric capacity of empty cell
C ǁ:Capacity of vertical orientation cell filled with liquid crystal C ┴:Capacity of horizontal orientation cell filled with liquid crystal
–Standard condition:
•At 20°C, 1kHz, 0.03V (C0measured at 1.5V)
•T (10°C ~60°C) and frequency can be changed
MERCK
JNC
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Physic Properties of Liquid Crystals
•Birefringence (複折射性):Δn=n ǁ-n ┴=n e –n o
Z n ǁ
Y
•Index ellipsoid of
nematic
LC:
–n Z >n X =n Y –n X =n Y =n ┴–n Z =n ǁ
n ┴
X
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Physic Properties of Liquid Crystals
•
Birefringence:
Δn
=
n
ǁ
-
n
┴
=n
e
–
n
o
Z
n ǁ
Y
n
┴
X
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Physic Properties of Liquid Crystals
•Birefringence Measurement:
–Abbe ˊRefractivemeter 2T (Δn0.168) –Standard condition:
•At 20°C (MERCK)/25°C (JNC)using Na lamp at 589nm
•Possibility for temperature control using water bath (10°C ~60°C)
•Different wavelengths (dispersion):436nm, 509nm, 546nm (Hglamp), and 633nm (He-Nelaser)
–Sample:
•MERCK:Vertical alignment layer on prism by using lecithin •JNC:Rub surface of the main
prism
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Physic Properties of Liquid Crystals
•Temperature dependence of Δε&Δn:
Temp. vs. Δn
Temp. vs. Δε
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Physic Properties of Liquid Crystals
•Threshold Voltage (Vth
):
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Physic Properties of Liquid Crystals
•Elastic Constants (kii , 彈性係數):All deformations in LCs can be
decomposed into three main distortions
Splay, k 11
Twist, k22 TN 、IPS/FFS
Bend, k33
VA
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Physic Properties of Liquid Crystals
•Elastic Constants
Measurement:
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Physic Properties of Liquid Crystals•Temperature dependence of k ii :
k 33>k 11>k 22
•Elastic constants in pN =10-12N
•Typical values 5~20pN(k11&k33~10-15pN at 20°C)
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Physic Properties of Liquid Crystals
•Rotational Viscosity (γ1, 旋轉黏滯係數):
•Indicate the rotation ability of LC molecule
Molecules rotate faster with smaller rotational viscosity.
•Regularly, smaller rotational viscosity with
positive (Δε>0) LC.
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Physic Properties of Liquid Crystals•Rotational Viscosity
Measurement:
•If Δεchanged by ±0.1, γ1will
changed by ±3mPa·s
•If cell gap ±0.2μm,γ1will changed
by ±
2mPa·s
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Physic Properties of Liquid Crystals•Temperature dependence of Rotational
Viscosity:
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Physic Properties of Liquid Crystals
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Physic Properties of Liquid Crystals•VHR &γ1dependence on Δε:
•Large Δεmixtures have higher γ1
–Fast response time low Δε&large
Vop
–Low power consumption large Δε
&slower
RT •VHR depends on Δε–High VHR values for lower Δε–Lower VHR for Δε>0
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Physic Properties of Liquid Crystals•Density (密度):
–Necessary to know density for ODF filling process.
–Density versus T linear relationship far from T
NI
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Physic Properties of Liquid Crystals•Chiral Dopant:
–Chiral dopants are optical active compounds:
•Same chemical structure
•No inversion symmetry:Can’tbe superimposed on its mirror image
•Molecule possesses an asymmetric carbon
atom
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Physic Properties of Liquid Crystals•Why do we add chiral dopant for TN:
–Advantages:
•Prevent formation of reverse twist defects inside cell
–Reverse twist defects (disclinationlines) happen after applying E
–Defects gives lower constrast in LCDs
–Chiral dopant induced uniform direction of twist
•Slight decrease of response times
•Less temperature dependence of threshold voltage V th
–Disadvantages:
•Higher V th values
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Physic Properties of Liquid Crystals•Pitch
Measurement:
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Physic Properties of Liquid Crystals
•LC Molecules Alignment:
Homogeneous
TN Homeotropic VA
Anchoring
Energy!!
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•Introduction of Liquid Crystals•Physic Properties of Liquid Crystals•Electro-Optic of Liquid Crystals
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Electro-Optic of Liquid Crystals
Physical Properties of Liquid Crystal
Clear Point
&
Melting Point
Optic Anisotropy
Dielectric Anisotropy
Rotational Viscosity &
Flow Viscosity
Constant
VHR
Properties of LCD
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Electro-Optic of Liquid Crystals
•Brightness of LCD: L = Brightness of BL (nit) x Panel Transmittance (%)
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Electro-Optic of Liquid Crystals
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Electro-Optic of Liquid Crystals
•Viewing Angle:
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Electro-Optic of Liquid Crystals
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Electro-Optic of Liquid Crystals
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Thanks for your attention!