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LC Applications

LC Applications. Behzad Pourabbas Polymer Eng. Department Sahand University of Technology Tabriz-Iran pourabas@sut.ac.ir. Overview:. Order Parameter Anisotropic Properties Light, polarization and materials . Order parameter “s”. The Order Parameter. n. q. n. perfect crystal

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LC Applications

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  1. LC Applications Behzad Pourabbas Polymer Eng. Department Sahand University of Technology Tabriz-Iran pourabas@sut.ac.ir

  2. Overview: • Order Parameter • Anisotropic Properties • Light, polarization and materials

  3. Order parameter “s”

  4. The Order Parameter n q n perfect crystal isotropic fluid

  5. n n Maier-Saupe Theory - Mean Field Approach 1.0 Isotropic Fluid Nematic Liquid Crystal Order Parameter, S 0.0 -0.6 Temperature

  6. 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. proportional to Parameter Nomenclature  Elastic ConstantKiiS2 BirefringenceDnS Dielectric Anisotropy De S Magnetic Anisotropy Dc S Viscosity Anisotropy Dh S Example: Does the threshold switching voltage for a TN increase or decrease as the operating temperature increases. Scales as the square root of S therefore lowers with increasing temperature

  7. Response to Electric and Magnetic Fields

  8. External Electric Field and Dielectric Properties of LC molecules

  9. Anisotropy: Dielectric Constant E ++ e + ++ positive - - - - - e De = e - e > 0 E E + + + + - - - - negative all angles in the plane  to E are possible for the -De materials De = e - e < 0

  10. Anisotropy: Duel Frequency high frequency, De<0 low frequency, De>0 MLC-2048 (EM Industries), Duel Frequency Material Frequency (kHz) 0.1 1.0 10 50 100 Dielectric Anisotropy (De) 3.28 3.22 0.72 -3.0 -3.4

  11. Dielectric Constant Dielectric Constant ke0L = C = q/V

  12. E 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.

  13. Dielectric Material is smaller in many materials than it would be in a vacuum for the same arrangement of charges. Eg. Parallel plates: Dielectric material Eo Ei + + + + 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.

  14. Dielectric Constant (“kappa”) = “dielectric constant” = (a pure number ≥ 1) So, (for parallel plates) Or Where C0 is the capacitance without the dielectric. Hence, the capacitance of a filled capacitor is greaterthan an empty one by a factor

  15. Materials Dielectric Constant Vacuum 1.0000 Air 1.0005 Polystyrene 2.56 Polyethylene 2.30 Nylon 3.5 Water 78.54 Dielectric Constants (@20oC, 1kHz) *Mixture Application Deee BL038 PDLCs 16.7 21.7 5.3 MLC-6292 TN AMLCDs 7.4 11.1 3.7 ZLI-4792 TN AMLCDs 5.2 8.3 3.1 TL205 AM PDLCs 5 9.1 4.1 18523 Fiber-Optics 2.7 7 4.3 95-465 -De material -4.2 3.6 7.8 *EM Materials PD: Polymer Dispersed AM: Active Matrix TN: Twisted Nematic

  16. Flow of ions in the presence of electric field Internal Field StrengthE = E0 – E’

  17. Alignment of LC molecules in Electric Field S = 0 1 > S > 0

  18. m m Dielectric Anisotropy and Permanent Dipole Moment

  19. Dielectric Constants: Temperature Dependence 4’-pentyl-4-cyanobiphenyl Temperature Dependence Average Dielectric Anistropy

  20. Dielectric Anisotropy and Induced Dipole Moment

  21. 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:

  22. Magnetic Anisotropy: Diamagnetism Compound

  23. Examples

  24. Magnetic Susceptibility and Anisotropy

  25. Light, polarization and materials

  26. light is a transverse wave: perpendicular to Optical polarization • for any wavevector, there are two field components • any wave may be written as a superposition of the two polarizations

  27. Light as Electromagnetic Wave Plane Polarized light can be resolved into Ex and Ey

  28. Birefrengence

  29. Birefringence

  30. 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. Extraordinarylighttravels in the crystal with a speed v that varies with direction. The refractive index n0=c/v also varies with different direction

  31. Interaction of Electromagnetic Wave with LC Molecules

  32. Optical Anisotropy: Birefringence ordinary ray (no, ordinary index of refraction) extraordinary ray (ne, extraordinary index of refraction)

  33. Optical Anisotropy: Birefringence ordinary wave extraordinary wave optic axis q For propagation along the optic axis, both modes are no

  34. Birefringence (20oC @ 589 nm) EM Industry Dn ne no Application Mixture BL038 0.2720 1.7990 1.5270 PDLC TL213 0.2390 1.7660 1.5270 PDLC TL205 0.2175 1.7455 1.5270 AM PDLC ZLI 5400 0.1063 1.5918 1.4855 STN ZLI 3771 0.1045 1.5965 1.4920 TN ZLI 4792 0.0969 1.5763 1.4794 AM TN LCDs MLC-6292 0.0903 1.5608 1.4705 AM TN LCDs ZLI 6009 0.0859 1.5555 1.4696 AN TN LCDs MLC-6608 0.0830 1.5578 1.4748 ECB 95-465 0.0827 1.5584 1.4752 -De devices MLC-6614 0.0770 --------- --------- IPS MLC-6601 0.0763 --------- --------- IPS 18523 0.0490 1.5089 1.4599 Fiber Optics ZLI 2806 0.0437 1.5183 1.4746 -De device

  35. Birefringence: Temperature Dependence Average Index Temperature Dependence

  36. Circular Polarization of Light

  37. Circular Birefringence

  38. coefficients differ only by factor • linear (plane) polarization • coefficients differ only by real factor Categories of optical polarization • circular polarization • elliptical polarization • all other cases

  39. e.g. linear polarization at angle • wavevector insufficient to define electromagnetic wave • we must additionally define the polarization vector Characterizing the optical polarization

  40. Reflection of Circular Polarized Light LCP RCP

  41. Dynamic Scattering Mode LCD Device

  42. Twisted Nematic (TN) Device 1971 by Schadt

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