JPH01161221A - Liquid crystal element and its driving method - Google Patents

Abstract

PURPOSE:To obtain high contrast at the time of time division driving by constituting a liquid crystal substance of ferroelectric smectic liquid crystal compound (LQC) and nematic or smectic LQC whose dielectric anisotropy is positively and negatively inverted in accordance with the frequency of impressed voltage. CONSTITUTION:The ferroelectric smectic LQC and the nematic or smectic LQC having a property inverting its dielectric anisotropy positively and negatively in accordance with the frequency of impressed voltage are mixed with each other in a gap between a substrate 1. When a voltage with low frequency showing positive dielectric anisotropy is impressed in the first stage of switching, force making the major axes of molecules parallel with an electric field direction acts on liquid crystal molecules 4 and force for orientating spontaneous polarization of the molecular minor axis direction in the electric field direction acts on also to the molecules 4. At the time of the latter stage of switching and non-address, action for stabilizing the parallel state of the liquid crystal molecules with the electrode substrate 1 is superposed by impressing a high frequency voltage whose dielectric anisotropy is negative, an optically stable state can be held independently of the voltage impressed at the time of non-address, as well. Thus, the drop of contrast at the time of time division driving can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal element used in a liquid crystal display element, a liquid crystal optical shutter, etc., and a method for driving the same.

[Prior Art] Generally, when a ferroelectric smectic liquid crystal is sealed in a thin cell with a thickness of about 3 μm or less and the direction of an electric field applied between electrodes is switched, the orientation of liquid crystal molecules is reversed. FIG. 2 is a schematic diagram showing such a change in the orientation of liquid crystal molecules. By placing such a cell between two polarizing plates whose polarization planes are perpendicular to each other, an optical switch using a birefringence mode can be realized. This principle is explained in the Applied Physics Letters.
rs) J Vol. 36, published in 1980, p. 899 by Clark and Lagerwal.

In this way, the property of taking either the first optically stable state or the second optically stable state in response to an applied electric field and maintaining that state when no electric field is applied;
That is, ferroelectric smectic liquid crystals are expected to be widely used in high-speed and memory-type display elements, optical shutters, and the like because they have memory properties and respond quickly to changes in electric fields.

[Problems to be Solved by the Invention] When this ferroelectric smectic liquid crystal is used in an optical shutter array or a display element and time-division driving is performed, considering the individual elements, it is necessary to select (turn on) or turn on the element at the time of address. A voltage corresponding to non-selection (off) is applied, but when non-addressing, a so-called crosstalk voltage is applied, reducing the memory effect.
There is a drawback that the state of the element changes during addressing, resulting in a decrease in the on/off ratio (contrast) of the amount of light transmitted by the shutter, etc. during time-division driving.

On the other hand, when a ferroelectric smectic liquid crystal material with negative dielectric anisotropy is used and an AC waveform is superimposed when not selected, the liquid crystal molecules are aligned parallel to the substrate, so the selected optical 11th Liquid Crystal Symposium, 2
No. 5 (1985), reported by Hato et al. However, most existing ferroelectric smectic liquid crystals have positive dielectric anisotropy, and liquid crystal materials with negative dielectric anisotropy are It is difficult to obtain it easily.

The present invention was devised in view of these problems, and an object of the present invention is to provide a liquid crystal element that can obtain high contrast during time-division driving, and a method for driving the same.

[Means for Solving the Problems] The present invention provides a liquid crystal element in which electrodes are formed on the plate surfaces and a thin layer of liquid crystal material is sandwiched between two opposing substrates, in which the liquid crystal material is ferroelectric. The liquid crystal element is composed of a smectic liquid crystal compound and a nematic or smectic liquid crystal compound whose dielectric anisotropy is inverted between positive and negative depending on the frequency of applied voltage, and an application that selects the element state of such a liquid crystal element. As a voltage, a voltage of a first frequency exhibiting positive dielectric anisotropy is applied before switching,
The liquid crystal element is driven by applying a voltage of a second frequency exhibiting negative dielectric anisotropy after switching and during non-addressing.

[Function] The present invention uses a ferroelectric smectic liquid crystal compound and a nematic or smectic liquid crystal compound whose dielectric anisotropy is reversed depending on the frequency of the applied voltage in the gap between two mutually opposing substrates provided with electrodes. A low frequency voltage with positive dielectric anisotropy is applied in the pre-switching stage. Based on the electric field effect,
A force acts on the liquid crystal molecules to make the long axis of the molecules parallel to the direction of the electric field, and at the same time a force acts to orient the spontaneous polarization in the direction of the short axis of the molecules in the direction of the electric field. On the other hand, applying a high-frequency voltage that is dielectric anisotropic or negative during the post-switching stage and during non-addressing will superimpose the effect of stabilizing the state parallel to the liquid crystal molecules or the electrode substrate. An optically stable state is maintained even with respect to the voltage applied during addressing. This causes a delay in the optical response to the applied voltage during non-addressing, making it possible to reduce the reduction in the on/off ratio (contrast) of the amount of light transmitted by the shutter, etc. during time-division driving.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing the structure of an embodiment of the present invention. In FIG. 1, 1 is a glass substrate, 2 is an indium oxide transparent electrode, 3 is a polyimide liquid crystal alignment film, and two glass substrates 1 are provided with a thin layer 4 of liquid crystal material in the gap between them through a spacer 5. It is sealed and its periphery is fixed with epoxy adhesive 6. A voltage is applied to this thin layer 4 of liquid crystal material via the transparent electrode 2 in a switched manner by power supplies 7 and 8 . The voltage applied by the first power supply 7 has a frequency f such that the dielectric anisotropy of the thin layer 4 of liquid crystal material is positive.
1 and the voltage applied by the second power source 8 is a high frequency voltage of frequency f2 such that the dielectric potential of the thin layer 4 of liquid crystal material 6 is negative. The liquid crystal materials used in this example were ferroelectric smectic liquid crystal C5-1014 (Chisso (l wood)) with positive dielectric anisotropy and dielectric dispersed smectic liquid crystal 4-n-pentylphenyl-2''-chloro-4. '-(6-n-hexyl-2-naphthoyloxy)benzoate at a weight ratio of 9
5.0: A liquid crystal material mixed at 5.0, the frequency at which the dielectric anisotropy of this liquid crystal material is reversed is 1 kHz, and the thickness of the thin layer is 2 μm.

For convenience of comparison, a cell was also fabricated with the same structure as the example shown in FIG. 1, but in which only C5-1014 (manufactured by Chisso 0 Shu), which has a positive dielectric anisotropy, was injected as the liquid crystal material.

In the liquid crystal element having the structure shown in Fig. 1, the dielectric anisotropy is positive at a low frequency (f 1 = 100
Hz> is applied by the first power supply 7, and a voltage of high frequency (f2 = 10 kHz) such as dielectric anisotropy or negative is applied to the second power supply after switching and during non-addressing.
The voltage was applied by the power source 8 of .

In the case of a liquid crystal material with positive dielectric anisotropy, after switching by voltage application, a force acts on the liquid crystal molecules to make the long axis of the molecules parallel to the direction of the electric field, making the optically stable state unstable. When applying a high-frequency voltage f2 such that the dielectric anisotropy is negative in the latter stage, that is, when the liquid crystal molecules are inverted and in non-addressing, the effect of stabilizing the state in which the liquid crystal molecules are parallel to the electrode substrate is superimposed. Therefore, an optically stable state is maintained even during the latter half of switching and the applied voltage during non-addressing.

With this structure, a 400 DPI liquid crystal optical shutter array was fabricated, and a mixed liquid crystal of a ferroelectric smectic liquid crystal compound and a dielectrically dispersed smectic liquid crystal was injected into the shutter array. :1, which is larger than the contrast of 4:1 obtained using conventional liquid crystal materials with positive dielectric anisotropy.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a liquid crystal element that can obtain high contrast during time-division driving, and a method for driving the same.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a schematic diagram of liquid crystal molecule alignment. 1...Glass substrate 2...Transparent electrode 3...
・Liquid crystal alignment film 4...Liquid crystal substance thin film 5...
Spacer 6... Adhesive 7.8... Power supply

Claims (2)

[Claims] (1) In a liquid crystal element in which electrodes are formed on the plate surfaces and a thin layer of liquid crystal material is sandwiched between two opposing substrates, the liquid crystal material is a ferroelectric smectic liquid crystal compound and the frequency of the applied voltage is A liquid crystal element characterized by being composed of a nematic or smectic liquid crystal compound whose dielectric anisotropy is reversed between positive and negative. (2) A method for driving a liquid crystal element comprising a liquid crystal substance composed of a ferroelectric smectic liquid crystal compound and a nematic or smectic liquid crystal compound whose dielectric anisotropy is reversed between positive and negative depending on the frequency of an applied voltage, the method comprising: As the applied voltage for selecting, a voltage with a first frequency exhibiting positive dielectric anisotropy is applied before switching, and a voltage having a second frequency exhibiting negative dielectric anisotropy after switching and during non-addressing is applied. A method for driving a liquid crystal element, the method comprising: applying .