Ferroelectric liquid crystal devices

Details Ferroelectric liquid crystal devices Ferroelectric liquid crystal devices

1997-02-27

2000-06-13

Sikes, William L.

Liquid crystal cells, elements and systems

Cell containing liquid crystal of specific composition

Polymer liquid crystal

349 33, 349 85, 349 88, 349184, G02F 1133, G02F 11333, C09K 1902

Patent

active

060755830

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to novel ferroelectric liquid crystal devices.
2. Discussion of Prior Art
Liquid crystal materials are widely used in liquid crystal displays such as watches, calculators etc. Most displays of this type consist of a thin film of a liquid crystalline composition sandwiched in a cell between two substrates, at least one of which is transparent, and having transparent electrodes on their inner surfaces. On applying a potential difference across the electrodes the alignment of the molecules of the liquid crystalline composition is altered, resulting in an electro-optic effect in the material, which is exploited by the display. Most often electro-optic effects in the nematic liquid crystal phase are exploited in such displays. Examples of types of displays include the twisted nematic, the Freedericksz effect device, cholesteric memory mode device, cholesteric to nematic phase change effect device, dynamic scattering effect device, two frequency switching effect device and the `supertwist` effect device. Other types of device include active matrix twisted nematics, pi-cells and ferroelectric liquid crystal devices.
Ferroelectric smectic liquid crystal materials, which can be produced by mixing an achiral host and a chiral dopant, use the ferroelectric properties of the tilted chiral smectic C, F, G, H, I, J and K phases. The chiral smectic C phase is denoted S.sub.C * with the asterisk denoting chirality. The S.sub.C * phase is generally considered to be the most useful as it is the fastest switching. Ferroelectric smectic liquid crystal materials should ideally possess the following characteristics: low viscosity, controllable spontaneous polarisation (Ps) and an S.sub.C * phase that persists over a broad temperature range, which should include ambient temperature and exhibits chemical and photochemical stability Materials which possess these characteristics offer the prospect of very fast switching liquid crystal containing devices. Some applications of ferroelectric liquid crystals are described by J. S. Patel and J. W. Goodby in Opt. Eng., 1987, 26, 273.
In ferroelectric liquid crystal devices the molecules switch between different alignment directions depending on the polarity of an applied electric field. These devices can be arranged to exhibit bistability where the molecules tend to remain in one of two states until switched to the other switched state. Such devices are termed surface stabilised ferroelectric devices, e.g. as described in U.S. Pat. No. 5,061,047 and U.S. Pat. No. 4,367,924 and U.S. Pat. No. 4,563,059. This bistability allows the multiplex addressing of quite large and complex devices.
One common multiplex display has display elements, i.e. pixels, arranged in an x, y matrix format for the display of e.z. alpha numeric characters. The matrix format is provided by forning the electrodes on one slide as a series of column electrodes, and the electrodes on the other slide as series of row electrodes. The intersections between each column and row form addressable elements or pixels. Other matrix lavouts are known, e.g. seven bar numeric displays.
There are many different multiplex addressing schemes. A common feature involves the application of a voltage called a strobe voltage to each row or line in sequence. Coincidentally with the strobe applied at each row, appropriate voltages, called data voltages, are applied to all column electrodes. The differences between the different schemes lies in the shape of the strobe and data voltage waveforms.
Other addressing schemes are described in GB-2,146,473-A; GB-2,173,336-A; GB-2,173,337-A; GB-2,1773,629-A; WO 89/05025: Harada et al 1985 S.I.D. Paper 8.4 pp 131-134; Lagerwall et al 1985 I.D.R.C. pp 213-221 and P Maltese et al in Proc 1988 I.D.R.C. p 90-101 Fast Addressing for Ferroelectric LC Display Panels.
The material may be switched between its two states by two strobe pulses of opposite sign, in conjunction with a data waveform. Alternatively, a blanking pulse may be

REFERENCES:
patent: 5434685 (1995-07-01), Pirs et al.
Informacije Midem, Mar. 1993, Slovenia, vol. 23, No. 1, ISSN 0352-9045, pp. 38-42, Pirs J et al, "Ferroelectric liquid crystal-polymer gel displays".
Applied Physics Letters, Feb. 7, 1994, USA., vol. 64, No. 6, ISSN 003-6951, pp. 718-720, XP000422877 Kyehun Lee et al, "Fast linear electro-optical switching properties of polymer-dispersed ferroelectric liquid crystals."
Ferroelectrics, 1993, UK, vol. 148, No. 1-4, ISSN 0015-0193, pp. 389-399, Lester G et al, "Electro-optic behaviour of low molar mass FELC's doped with liquid crystal polymers".

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