JPH0446410B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a liquid crystal device applied to a liquid crystal display device, a liquid crystal-optical shutter array, etc., and more specifically, a liquid crystal device with improved display and driving characteristics by improving the initial alignment state of liquid crystal molecules. Regarding. [Prior Art] Until now, the use of a liquid crystal element having bistable properties has been proposed by Clark and Lagerwall (Japanese Patent Laid-Open No. 1983-1993).
-107216, US Pat. No. 4,367,924, etc.). As this liquid crystal having bistability, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) is generally used. This liquid crystal has a bistable state consisting of a first optically stable state and a second optically stable state with respect to an electric field, and therefore, unlike the optical modulation element used in the conventional TN type liquid crystal, for example, The liquid crystal is aligned in a first optically stable state with respect to one electric field vector, and the liquid crystal is aligned in a second optically stable state with respect to the other electric field vector. Furthermore, this type of liquid crystal has the property of very quickly taking one of the above two stable states in response to an applied electric field, and maintaining that state when no electric field is applied. By utilizing these properties, many of the problems of the conventional TN type devices mentioned above can be significantly improved. [Problems to be Solved by the Invention] For the above-mentioned ferroelectric liquid crystal, a method is known in which a uniaxial alignment treatment is performed on the substrate surface in order to obtain uniform alignment performance. Examples of the uniaxial alignment treatment include a method of rubbing the substrate surface in one direction with a bead, cloth, or paper, and a method of obliquely depositing SiO or SiO ₂ on the substrate surface. By subjecting the substrate surface to a proper uniaxial alignment treatment, a specified bistable state was achieved in the initial alignment. However, in its initial state, as described below, optical modulation experiments under crossed nicol conditions resulted in poor contrast and a small amount of transmitted light, which caused practical problems. [Means for solving the problem] and [effect] According to experiments conducted by the present inventors, the above-mentioned problem can be solved by
It has been found that this phenomenon is caused by the twisting of the smectic phase layer perpendicular to the substrate in the above-mentioned ferroelectric liquid crystal element, especially a ferroelectric liquid crystal element subjected to uniaxial alignment treatment. That is, it has been found that this is because the smectic phase liquid crystal molecules adjacent to the upper and lower substrates intersect at an angle α, and the smectic phase layer is twisted at this angle α. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a ferroelectric liquid crystal device that eliminates the above-mentioned problems. That is, the present invention provides: (a) a pair of substrates arranged at a distance, a pair of electrodes respectively provided inside the pair of substrates, and two different orientation states arranged between the pair of substrates in the absence of an electric field; is expressed, and the angle formed by the average molecular axis of each of the two orientation states is 2θ
By applying an AC voltage higher than the threshold voltage of the liquid crystal to the chiral smectic liquid crystal in the first state, the two different orientation states in the absence of an electric field after the application of the AC voltage is removed are a liquid crystal panel having a chiral smectic liquid crystal in a second state in which the angle formed by each average molecular axis is an angle 2θ′ larger than the above 2θ; b. a first means for applying a voltage signal to the pair of electrodes that causes the liquid crystal to exhibit a first orientation state or a second orientation state depending on the polarity of the applied voltage; and c. What is the first means? The liquid crystal device is characterized by having second means separately provided and applying the alternating current voltage to the pair of electrodes. Hereinafter, the present invention will be explained based on examples. [Example] FIG. 1 shows a plan view a and a cross-sectional view b of a liquid crystal cell used in the present invention. Striped electrode groups 4a and 4b were formed on glass or plastic substrates 3a and 3b with a thickness of 1000 Å using ITO (Indium Tin Oxide), and polyimide coatings 6a and 6b were formed thereon with a thickness of 1000 Å. Furthermore, in order to maintain the thickness of the liquid crystal layer on the upper layer
A 1 μm dot-shaped polyimide spacer group 5 was provided. This spacer keeps the liquid crystal layer 6 constant over a wide range. After rubbing the two substrates, they were assembled into cells, and a biphenyl ester liquid crystal described later was introduced. The biphenyl ester compound used in this example exhibits the phase transition state shown below. Iso (isotropic phase) 90℃ ―――→ Ch (cholesteric phase) 76℃ ―――→ SmA (smectic A phase) ――――→ 53℃ SmC ^* ――――――→ 10℃ or less Cry (crystal) Phase) When the liquid crystal layer is sufficiently thick (~100μ), SmC ^* has a helical structure with a pitch of about 4μ. The spontaneous polarization was approximately 10 nc/cm ² from the state of spontaneous polarization determined by the triangular wave method. First, the cell structure 7 in which the above-mentioned biphenyl ester liquid crystal is sealed
is set in a heating case (not shown) such that the entire cell 7 is heated uniformly. Next, the compound in the cell 7 is heated to a temperature (approximately 75° C.) at which it becomes an isotropic phase. Thereafter, the temperature of the heating case is lowered, and the compound in the isotropic phase in the cell 7 is transferred to the temperature lowering process. During this temperature-lowering process, the isotropic phase of the compound undergoes a phase transition to a cholesteric phase with a Grangean structure at approximately 72°C, and if the temperature-lowering process continues further, it undergoes a phase transition from the cholesteric phase to SmA, a uniaxially anisotropic phase, at approximately 60°C. can occur.
At this time, the liquid crystal molecular axis of SmA is aligned in the rubbing direction. After that, in the process of cooling down from this SmA, SmC ^*
For example, the cell thickness can be reduced to 3μm by phase transition to
If it is less than a certain degree, a monodomain SmC ^* with a non-helical structure will be obtained. FIG. 2 schematically shows the initial alignment state of liquid crystal molecules, and is a view seen from above the substrate surface 25. In the figure, 20 corresponds to the direction of uniaxial alignment treatment, that is, the rubbing direction in this example. SmA
In the phase, the liquid crystal molecules are aligned with the average molecular axis direction 21 of the liquid crystal coinciding with the rubbing direction 20.
In the SmC ^* phase, the average molecular axis direction of liquid crystal molecules is tilted in the direction of 22a, and the rubbing direction is 20.
The average molecular axis direction 22a of SmC ^* forms an angle θ and is in a first stable orientation state. When a voltage is applied to the upper limit substrate in this state, the average molecular axis direction of the liquid crystal molecules of SmC ^* changes to an angle larger than the angle θ, and assumes a third stable alignment state saturated at the angle. The average molecular axis direction at this time is defined as 23a. Next, when the voltage is returned to zero, the liquid crystal molecules return to their original state in the first molecular axis direction 22a. Therefore, in the state of the first molecular axis direction 22a, the liquid crystal molecules have memory properties. Also, when applying a voltage in the opposite direction in the state of the molecular axis direction 22a,
If the voltage is sufficiently high, the average molecular axis direction of the liquid crystal molecules is saturated and transitions to a fourth stable orientation crystal 23b that forms an angle. Then, when the voltage is returned to zero again, the liquid crystal molecules become
It settles into a second stable orientation state with an angle θ in the average molecular axis direction 22b. The angle θ detects the average direction of the molecular axis in one stable state. The reason why this is smaller than the angle is thought to be that the liquid crystals within the SmC ^* layer do not take a completely parallel alignment, and the average molecular axis direction of this alignment is the direction of θ.
It is conceivable that the angle θ could be made into an angle in principle. Increasing the value of the angle θ (tilt angle) has a great effect in increasing the transmittance. Assuming incident light Io and transmitted light I, the transmittance is expressed by the following formula: I/Io=sin ² 4θsin ² Δndπ/λ (1) θ: tilt angle, Δn: refractive index anisotropy, d: film thickness, λ: Wavelength The above equation is the transmittance when one average molecular axis direction and one polarization axis are made to match under crossed Nicol conditions and transferred to the other stable state molecular axis direction. Although the above formula is applied when all the liquid crystal molecules are arranged parallel to the substrate, it was confirmed that it is also almost applicable when the molecular axis direction having an angle θ is approximately parallel to the substrate. Therefore, the transmittance is maximum when the tilt angle is θ=22.5°. In the present invention, the cell thickness d is 1.1 μm.
The experiment was conducted using a 1.8 μm cell, and each θ(d
= 1.1) = 8.0°, θ (d = 1.8) = 7.5°, which is less than the optimal value. Next, when we apply a DC voltage of 50V with two polarities to observe, (d=1.1)=23.1°,
A value close to the optimum value was obtained at (d=1.8)=24.0°. The present inventors conducted further experiments in order to bring the tilt θ in the bistable state closer to the optimum value. Inversion between bistable states was performed with the following pulses.

[Table] Voltage ±10V to 150V, frequency 20 for these cells
An alternating electric field of ~100 Hz was applied. Applying, 30-40
The inverted state was visible at Hz, but not at 40Hz or higher. After applying this AC voltage, the electric field was turned off, and the tilt angle θ of the bistable state (the tilt angle in the absence of an electric field after the AC application process) and the pulse width-voltage value characteristics of the bistable state inversion pulse were examined again. The cell state after applying AC voltage for 15 minutes will be described below. The effective frequency for widening the tilt angle θ is 30Hz to 70Hz, and there is no difference in superiority or inferiority within this range. When the frequency is set to 40 Hz, there is no difference in the tilt angle between cell states due to voltage changes of 10 V to 50 V. However, at 50V to 60V, θ′ (d=1.1) = 21.0°,
A domain of θ′ (d=1.8)=18.8° begins to appear.
At voltages of 60 to 80 V, this domain spread throughout and very good contrast was obtained. Above 80V, many defects occurred and the monodomain collapsed. The reversal between bistable states of the θ' state after applying 60 to 80 V was performed with the following pulses.

[Table] In the θ' state, the reversal voltage is higher than in the bistable state before voltage application. The reason for this is not clear, but in order for the tilt angle θ' to approach , it is necessary to provide energy that also inverts the liquid crystal molecules near the interface of the alignment film, so the driving voltage required for inversion is high. considered necessary. The amount of transmitted light becomes nearly three times that of before application due to the AC voltage and the tilt angle θ' after application, and the amount of transmitted light becomes d=
It was 14% at 1.1 μm and 19% at d=1.8. As another experimental example, a completely similar experiment was conducted except that the polyimide coating on the glass substrate was replaced with a polyvinyl alcohol coating. The liquid crystal layer thickness d is d
= 1.5 μm. Almost similar results were obtained. Experimental results: Effective AC voltage: 30 to 70Hz, 45 to 70V Tilt angle: θ = 7.8° before AC voltage application, 22.8° when DC voltage is applied, θ′ = 21.6° after AC voltage application, Reverse drive voltage

〔Effect of the invention〕

As described above, it is necessary to appropriately apply an alternating current electric field in order to maintain the contrast and amount of transmitted light at practical values, but by adopting the drive circuit configuration of the liquid crystal element of the present invention, the drive circuit can be effectively protected. , it becomes possible to apply alternating current without destroying the drive circuit system, and therefore it is possible to provide displays with stable high image quality or optical shutters with high contrast.

[Brief explanation of the drawing]

FIG. 1a is a plan view of a liquid crystal element used in the present invention, and FIG. 1b is a sectional view taken along line X-X'. Second
The figure is a plan view schematically showing the arrangement of liquid crystal elements in the liquid crystal element used in the present invention. FIG. 3 is a circuit diagram showing the liquid crystal device of the present invention. FIG. 4 is a circuit diagram showing a group of switches used in the present invention. FIG. 5 is a sectional view showing another liquid crystal element used in the present invention. FIG. 6 is a circuit diagram showing another liquid crystal device of the present invention. FIG. 7 is a circuit diagram representing another group of switches using the present invention. FIG. 8 is a circuit diagram showing another liquid crystal device of the present invention. 9 and 10 are perspective views schematically showing the ferroelectric liquid crystal element used in the present invention.

Claims (1)

[Claims] 1 a A pair of substrates arranged at a distance, a pair of electrodes respectively provided inside the pair of substrates, and two orientations arranged between the pair of substrates and different from each other when no electric field is applied. By applying an alternating current voltage larger than the threshold voltage of the liquid crystal to the chiral smectic liquid crystal in the first state in which the average molecular axis of each of the two orientation states forms an angle of 2θ, 2 different from each other in the absence of an electric field after the application of the alternating current voltage is cancelled.
a liquid crystal panel having a chiral smectic liquid crystal formed by producing a chiral smectic liquid crystal in a second state in which the angle between the average molecular axes of each of the two orientation states is an angle 2θ′ larger than the above 2θ; b. a first means for applying a voltage signal to the pair of electrodes that causes the chiral smectic liquid crystal in the state to be in a first orientation state or a second orientation state depending on the polarity of the applied voltage; and c. A liquid crystal device comprising a second means provided separately from the means for applying the AC voltage to the pair of electrodes.