Displays antiferroelectrics

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

A less well-documented effect is that of the phase-transition temperature of certain crystals which are very sensitive to deuteration. Some crystals of ferroelectric and antiferroelectric materials, and in particular dihydrogen phosphates and hydrogen selenites, which are extensively hydrogen bonded, display this effect (Blinc and Zeks, 1974). For some crystals, such as caesium... 

There are numerous properties which make fluorinated LC attractive for applications. Short Rp-segments lead to de Vries phases and to 90° tilted anti-ferroelectric SmCA phases, useful for orthoconic switching in new display applications. Fluorination could also lead to enhanced polarization in ferroelectric and antiferroelectric LC phases. 

The compositional perovskite series that has served as the basis for much of this research is the so-called PBZT system. The quadrilateral that joins the endmembers PbZr03-BaZr03-BaTi03-PbTi03 exhibits virtually complete solid solution (Fig. 12). Nevertheless, this system displays a number of morphotropic phase transitions that involve transformations of several kinds ferroelastic, ferroelectric, antiferroelectric, and relaxor. Understanding the nature of these isothermal transitions requires a review of the thermal distortions that occur as these perovskites are cooled. 

Nematic materials are only one member of a large family of a variety of structurally different compounds forming liquid crystalline mesophases. Although only nematics have yet found really widespread use, mostly for display applications, some structurally highly diverse smectic phases also have unique electrooptical characteristics, for example ferroelectricity or antiferroelectricity, which can be modulated by selective fluorination [5, 51]. For 20 years intensive effort has been devoted to making practical use of these phenomena. 

As already noted, various crystals similar to KDP, where the K-atom or the P-atom of the anion is replaced by homologous atoms, display a similar ferroelectric behaviour. Some of them are antiferroelectrics (41). Recently, some other types of ferroelectrics have been discovered, where the origin of their ferroelectric behaviour is due to the position of the proton itself, which is not the case for KDP-type ferroelectrics where H-bonds are roughly perpendicular to the permanent electric dipole moment fL in the ferroelectric phase and do not directly contribute to the permanent electric moment. Thus such a crystal as... 

In some materials the antiferroelectric state is barely stable or metastable. In such materials, application of an electric field will convert the phase to ferroelectric, as described, but removal of the field leaves the phase in a ferroelectric state. This material then behaves like a typical ferroelectric and displays a conventional hysteresis loop. Heating the material to a high temperature so as to form the paraelectric structure, followed by cooling, can reform the original antiferroelectric state. 

In conclusion of this chapter it should be stated that bistable and tristable switching of ferro- and antiferroelectric liquid crystals is very fast and provides long memory states. The latter allows one to design displays without semiconductor... 

Matsumoto, T., Fukuda, A., Johno, M., Motoyama, Y., Yui, T., Setnnun, A.-S., Yamashita, M. A novel pnrpcrty caused by frustration between ferroelectricity and antiferroelectricity and its application to liquid crystal displays Ferroelectricity and V-shape switching. J. Mater. Chem. 9, 2051-2080 (1999)... 

Liquid Crystals (Family Number Tl). Ferroelectric and antiferroelectric liquid crystals are very useful as fast display elements. 

The antiferroelectric phase, discovered in 1989, is currently the snbject of mnch research because of the great potential in display devices. The application of an electric field to an antiferroelectric material indnces a ferroelectric ordering which can be switched in the usnal way by a reversed pnlse. Removal of the field will regenerate the antiferroelectric phase. Such a system has the advantage over the normal ferroelectric system of well-defined electric field thresholds and accordingly shonld be mnch easier to multiplex. 

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

Figure 9.34. Four display modes proposed in ferroelectric and antiferroelectric display applications.

The existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

Dabrowski, R. Gasowska, J. Oton, J. Piecek, W. Pizedmojski, J. Tykarska, M. High tilted antiferroelectric liquid crystaUine materials. Displays 2004,25, 9-19. 

In the search for novel materials, particularly new ferroelectric materials, new phase types were also discovered, notably the antiferroelectric phase [56] which, with tristable switching characteristics, also has potential for display use, possibly overcoming some of the difficulties with ferroelectric systems and providing a further display device of high quality. 

The applications of liquid crystals have unquestionably added incentive to the quest for new liquid crystal materials with superior properties such as viscosity, elastic constants, transition temperatures, and stability. In recent years this has catalyzed work on chiral materials as dopants for ferroelectric displays and for antiferroelectric materials with structures avoiding the number of potentially labile ester groups that were present in the original materials in which... 

Pretransitional Effect in Antiferroelectric Liquid Crystal Displays.679... 

Antiferroelectric liquid crystal displays (AFLCDs) have been relevant to flat panel displays since the discovery of AFLCs. Their characteristic features can be summarized as follows ... 

HATn materials, electrical properties 782 ff He-Ne laser addressed devices 478 head-tail equivalence, antiferroelectrics 665 f heal capacity, OCB 450 Heck coupling, hydrocarbon cores 707 Heilmeier displays... 

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