Fundamentals of Liquid Crystal Devices (eBook)

Deng-Ke Yang, Shin-Tson Wu (Autoren)

eBook Download: EPUB

2014 | 2. Auflage
John Wiley & Sons (Verlag)
9781118751985 (ISBN)

Infos

Liquid Crystal Devices are crucial and ubiquitous components of an ever-increasing number of technologies. They are used in everything from cellular phones, eBook readers, GPS devices, computer monitors and automotive displays to projectors and TVs, to name but a few. This second edition continues to serve as an introductory guide to the fundamental properties of liquid crystals and their technical application, while explicating the recent advancements within LCD technology. This edition includes important new chapters on blue-phase display technology, advancements in LCD research significantly contributed to by the authors themselves.

This title is of particular interest to engineers and researchers involved in display technology and graduate students involved in display technology research.

• Key features:
  Updated throughout to reflect the latest technical state-of-the-art in LCD research and development, including new chapters and material on topics such as the properties of blue-phase liquid crystal displays and 3D liquid crystal displays;
• Explains the link between the fundamental scientific principles behind liquid crystal technology and their application to photonic devices and displays, providing a thorough understanding of the physics, optics, electro-optics and material aspects of Liquid Crystal Devices;
• Revised material reflecting developments in LCD technology, including updates on optical modelling methods, transmissive LCDs and tunable liquid crystal photonic devices;
• Chapters conclude with detailed homework problems to further cement an understanding of the topic.

Deng-Ke Yang Liquid Crystal Institute, Kent State University, USA

Shin-Tson Wu College of Optics and Photonics, University of Central Florida, USA

Textauszug

1
Liquid Crystal Physics

1.1 Introduction

Liquid crystals are mesophases between crystalline solid and isotropic liquid [1–3]. The constituents are elongated rod-like (calamitic) or disk-like (discotic) organic molecules as shown in Figure 1.1. The size of the molecules is typically a few nanometers (nm). The ratio between the length and the diameter of the rod-like molecules or the ratio between the diameter and the thickness of disk-like molecules is about 5 or larger. Because the molecules are non-spherical, besides positional order, they may possess orientational order.

Figure 1.1 Calamitic liquid crystal: (a) chemical structure, (c) space-filling model, (e) physical model. Discostic liquid crystal: (b) chemical structure, (d) space-filling mode, (f) physical model.

Figure 1.1(a) shows a typical calamitic liquid crystal molecule. Its chemical name is 4′-n-Pentyl-4-cyano-biphenyl and is abbreviated as 5CB [4,5]. It consists of a biphenyl, which is the rigid core, and a hydrocarbon chain which is the flexible tail. The space-filling model of the molecule is shown in Figure 1.1(c). Although the molecule itself is not cylindrical, it can be regarded as a cylinder, as shown in Figure 1.1(e), in considering its physical behavior, because of the fast rotation (on the order of 10−9 s) around the long molecule axis due to thermal motion. The distance between two carbon atoms is about 1.5 Å; therefore the length and the diameter of the molecule are about 2 nm and 0.5 nm, respectively. The molecule shown has a permanent dipole moment (from the CN head), but it can still be represented by the cylinder whose head and tail are the same, because in non-ferroelectric liquid crystal phases, the dipole has equal probability of pointing up or down. It is necessary for a liquid crystal molecule to have a rigid core(s) and flexible tail(s). If the molecule is completely flexible, it will not have orientational order. If it is completely rigid, it will transform directly from isotropic liquid phase at high temperature to crystalline solid phase at low temperature. The rigid part favors both orientational and positional order while the flexible part disfavors them. With balanced rigid and flexible parts, the molecule exhibits liquid crystal phases.

Figure 1.1(b) shows a typical discotic liquid crystal molecule [6]. It also has a rigid core and flexible tails. The branches are approximately on one plane. The space-filling model of the molecule is shown in Figure 1.1(d). If there is no permanent dipole moment perpendicular to the plane of the molecule, it can be regarded as a disk in considering its physical behavior as shown in Figure 1.1(f) because of the fast rotation around the axis which is at the center of the molecule and perpendicular to the plane of the molecule. If there is a permanent dipole moment perpendicular to the plane of the molecule, it is better to visualize the molecule as a bowl, because the reflection symmetry is broken and all the permanent dipoles may point in the same direction and spontaneous polarization occurs. The flexible tails are also necessary, otherwise the molecules form a crystal phase where there is positional order.

The variety of phases that may be exhibited by rod-like molecules are shown in Figure 1.2. At high temperature, the molecules are in the isotropic liquid state where they do not have either positional or orientational order. The molecules can easily move around, and the material can flow like water. The translational viscosity is comparable to that of water. Both the long and short axes of the molecules can point in any direction.

Figure 1.2 Schematic representation of the phases of rod-like molecules.

When the temperature is decreased, the material transforms into the nematic phase, which is the most common and simplest liquid crystal phase, where the molecules have orientational order but still no positional order. The molecules can still diffuse around, and the translational viscosity does not change much from that of the isotropic liquid state. The long axis of the molecules has a preferred direction. Although the molecules still swivel due to thermal motion, the time-averaged direction of the long axis of a molecule is well defined and is the same for all the molecules at macroscopic scale. The average direction of the long molecular axis is denoted by which is a unit vector called the liquid crystal director. The short axes of the molecules have no orientational order in a uniaxial nematic liquid crystal.

When the temperature is decreased further, the material may transform into the Smectic-A phase where, besides the orientational order, the molecules have partial positional order, i.e., the molecules form a layered structure. The liquid crystal director is perpendicular to the layers. Smectic-A is a one-dimensional crystal where the molecules have positional order in the layer normal direction. The cartoon shown in Figure 1.2 is schematic. In reality, the separation between neighboring layers is not as well defined as that shown by the cartoon. The molecule number density exhibits an undulation with the wavelength about the molecular length. Within a layer, it is a two-dimensional liquid crystal in which there is no positional order, and the molecules can move around. For a material in poly-domain smectic-A, the translational viscosity is significantly higher, and it behaves like a grease. When the temperature is decreased further, the material may transform into the smectic-C phase, where the liquid crystal director is no longer perpendicular to the layer but tilted.

At low temperature, the material is in the crystal solid phase where there are both positional and orientational orders. The translational viscosity becomes infinitely high and the molecules (almost) do not diffuse anymore.

Liquid crystals get the ‘crystal’ part of their name because they exhibit optical birefringence as crystalline solids. They get the ‘liquid’ part of their name because they can flow and do not support shearing as regular liquids. Liquid crystal molecules are elongated and have different molecular polarizabilities along their long and short axes. Once the long axes of the molecules orient along a common direction, the refractive indices along and perpendicular to the common direction are different. It should be noted that not all rod-like molecules exhibit all the liquid crystal phases. They may exhibit some of the liquid crystal phases.

Some of the liquid crystal phases of disk-like molecules are shown in Figure 1.3. At high temperature, they are in the isotropic liquid state where there are no positional and orientational orders. The material behaves in the same way as a regular liquid. When the temperature is decreased, the material transforms into the nematic phase, which has orientational order but not positional order. The average direction of the short axis perpendicular to the disk is oriented along a preferred direction, which is also called the liquid crystal director and denoted by a unit vector . The molecules have different polarizabilities along a direction in the plane of the disk and along the short axis. Thus the discotic nematic phase also exhibits birefringence as crystals.

Figure 1.3 Schematic representation of the phases of disk-like molecules.

When the temperature is decreased further, the material transforms into the columnar phase where, besides orientational order, there is partial positional order. The molecules stack up to form columns. Within a column, it is a liquid where the molecules have no positional order. The columns, however, are arranged periodically in the plane perpendicular to the columns. Hence it is a two-dimensional crystal. At low temperature, the material transforms into the crystalline solid phase where the positional order along the columns is developed.

The liquid crystal phases discussed so far are called thermotropic liquid crystals and the transitions from one phase to another are driven by varying temperature. There is another type of liquid crystals, called lyotropic liquid crystals, exhibited by molecules when they are mixed with a solvent of some kind. The phase transitions from one phase to another phase are driven by varying the solvent concentration. Lyotropic liquid crystals usually consist of amphiphilic molecules that have a hydrophobic group at one end and a hydrophilic group at the other end and the water is the solvent. The common lyotropic liquid crystal phases are micelle phase and lamellar phase. Lyotropic liquid crystals are important in biology. They will not be discussed in this book because the scope of this book is on displays and photonic devices.

Liquid crystals have a history of more than 100 years. It is believed that the person who discovered liquid crystals is Friedrich Reinitzer, an Austrian botanist [7]. The liquid crystal phase observed by him in 1888 was a cholesteric phase. Since then, liquid crystals have come a long way and become a major branch of interdisciplinary sciences. Scientifically, liquid crystals are important because of the richness of structures and transitions. Technologically, they have won tremendous success in display and photonic applications...

Produkt-Details

• Erscheint lt. Verlag: 1.10.2014
• Reihe/Serie: Wiley Series in Display Technology
• Reihe/Serie: Wiley Series in Display Technology
• Reihe/Serie: Wiley Series in Display Technology
• Sprache: englisch
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Schlagworte: Advancements • bluephase • cellular phones • components • Computer • Continues • crucial • Crystal • devices • Edition • Electrical & Electronics Engineering • Electrical Engineering - Displays • Elektronische Displays • Elektrotechnik u. Elektronik • everincreasing • everything • Flüssigkristallanzeige • Flüssigkristallanzeige • fundamental • Guide • introductory • LCD • liquid • Number • Optics & Photonics • Optik u. Photonik • Physics • Physik • Properties • technical • Technology • within lcd
• ISBN-13: 9781118751985 / 9781118751985