Applications supertwisted nematic

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

An advance that has extended the application of LCDs to full page computer terminals and other high information content displays without having to resort to TFTs is the supertwisted nematic device. It makes use of the fact that the electrooptic response of the nematic gets progressively steeper as the twist angle is increased, until at a certain... 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

The performance of STN (supertwisted nematic)-mode LCDs has been improved so far that STN displays can be used for many applications with low and high informational content [1], In particular, in price-sensitive applications they have proven to be competitive with the progressive TFT (Thin Film Transistor) technology. However, the usage of reasonable film-compensated STN displays has almost been restricted to environments with rather stable surrounding temperatures, which still is an obstacle for their widespread use, e.g. for car equipment or mobile applications. 

T. Scheffer and J. Nehring, Twisted nematic and supertwisted nematic mode LCDs, in Liquid crystals-applications and uses, Vol. 1, ed. B. Bahadur (World Scientific, New Jersy, 1990). 

See, for example, Scheffer, X, and J. Nehring. 1993. Twisted nematic and supertwisted nematic mode LCDs. Liquid Crystals Applications and Uses, B. Bahadur (ed.), Singapore World Scientific. see also Kataoh, K., Y Endo, M. Akatsuka, M. Ohgawara, and K. Sawada. 1987. Apphcation of retardation compensation A new highly multiplexable black-white hquid crystal display with two supertwisted nematic layers. Jpn. J. Appl. Phys., Part 2 26 L1784. 

The problems of supertwisted displays are response times. Current nematic mixtures have response times of 200 ms, which is much slower than the 80 ms required in mouse and cursor applications in personal computers, and far slower than the 15 ms required for video applications. This problem is being solved by using higher birefringence nematic mixtures and decreasing the cell gap down to 4-5 microns. For instance, Seiko-Epson have developed a 50 ms response time display [6]. 

Besides these two standard cell alignments, there are many other variations such as hybrid, twisted, supertwisted, fingerprint, multidomain vertically aligned, etc. Industrial processing of these nematic cells, as well as the transparent conductive coating of the cell windows for electro-optical device applications, is understandably more elaborate. 

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