JPH06105332B2 - Ferroelectric liquid crystal optical shutter - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical shutter using a ferroelectric liquid crystal.

[Prior Art] Ferroelectric liquid crystals have been found to exhibit high-speed response and bistability, and various applications have been tried. The optical shutter for the printer head is one example, and it is expected to improve the response speed which is one of the weak points of the liquid crystal printer head.

A liquid crystal printer head is a combination of an electrophotographic system and a liquid crystal optical shutter. Such an optical shutter has an operational characteristic that light must be transmitted only for a certain period of time and all light must be blocked at other periods of time. Therefore, conventionally, the layer thickness of the ferroelectric liquid crystal has been reduced to 1 to
A general method is to form a thin film element having a thickness of about 3 μm, and switch between two types of domains depending on the polarity of an electric field in a bistable state. (JP-A-56-107216) Although this method has an advantage of enabling multiplex driving, it has the following problems in practical use. First
Bistability is not always perfect, and light is likely to leak when it should be blocked. Secondly, the alignment of liquid crystal molecules in the electrode facing part can be controlled by an electric field, but in other parts it is aligned. There is no way to control. That is, since the bistable state is used, two states of ON and OFF coexist except for the electrode facing portion, and in order to use it as an optical shutter, a light shielding film must be provided in that portion. There was a practical problem.

In addition, as an element using a ferroelectric liquid crystal, there is a method in which elimination of a helical structure by an electric field is applied. (JP-A-58-1
73719) However, even in this method, since a light-shielding film is required and the ON / OFF of the optical shutter is always performed by an electric field, a DC electric field is almost always applied, and there is a problem in stable operation or durability. there were.

[Problems to be Solved by the Invention] An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art. In the conventional device using the bistable effect, since the energy barrier between the positive and negative domains is small, a malfunction easily occurs, and the stability of the operation is lacking. Further, the element using the spiral pitch elimination has a problem that the DC electric field must be continuously applied when the optical shutter that is in the off state is driven for most of the time, and the stability or reliability of the operating voltage is lacking. .

Further, both of them have to provide a light-shielding film other than the electrode facing portion, which causes not only a price problem but also a problem that it cannot be applied to an application requiring a fine pattern.

An object of the present invention is to provide a ferroelectric liquid crystal optical shutter having excellent operation stability.

[Means for Solving Problems] The present invention has been made to solve the above problems, and includes a cell having a ferroelectric liquid crystal sandwiched between substrates having at least a pair of electrodes, and at least one cell. In a ferroelectric liquid crystal optical shutter having a polarizing plate and a means for applying a voltage between both electrodes, the alignment direction of the liquid crystal molecules is substantially parallel to the substrate when no voltage is applied, and the monostable state is arranged in one direction. And the alignment state of the liquid crystal molecules is changed only by a voltage of one polarity, and an alternating voltage is applied to the liquid crystal layer so that the integrated value with respect to time becomes 0. A liquid crystal optical shutter is provided.

FIG. 1 is a cross-sectional view of an element used in a basic liquid crystal optical shutter of the present invention and a schematic view of an apparatus. A transparent conductive film (2) is formed on each of the surfaces of the pair of transparent substrates (1a) and (1b).
A), (2b) and alignment control films (3a), (3b) are formed.
The conductive films (2a) and (2b) are electrodes for applying an electric field to the liquid crystal layer (4) held between the substrates and are made of In ₂ O ₃ or SnO ₂ or the like and have a predetermined pattern formed. ing.

The alignment control films (3a) and (3b) align the liquid crystal molecules in a direction substantially parallel to the substrate when no voltage is applied, and give a monostable state in which the liquid crystal molecules respond only to an electric field of one polarity. It is for. As a typical method, an organic polymer film, particularly a polyimide polymer film is formed, and after rubbing with a cloth in a certain direction, two cells are made to face each other so that the rubbing directions are parallel to each other. However, there is a method of applying a voltage after injecting the liquid crystal.

After such an alignment treatment, spacers, such as organic beads and alumina particles, are sandwiched and the periphery is fixed with a sealant (5) so that the substrates are held in parallel and at regular intervals. And At this time, the orientation control directions of the two substrates are set to be parallel to each other.

Then, the ferroelectric liquid crystal composition is heated to a cholesteric phase or an isotropic phase, injected into the cell, and then sealed. Two polarizing plates (6a) and (6b) are arranged outside the cell so that their polarization axes are orthogonal or parallel to each other and one polarization axis and the orientation control direction of the substrate are the same or orthogonal. The light source (8) is placed on the side of the substrate (1b) so that the light is transmitted to the opposite side.

The voltage applied to the cell by the drive circuit (7) is made alternating so that the integrated value with respect to time becomes 0, and only one polarity contributes to the change in the alignment structure of the liquid crystal, resulting in a change in the light transmittance. Bring

Regarding the liquid crystal used, if it is a liquid crystal showing ferroelectricity,
The present invention is effective, but the chiral smectic C phase is more preferable because of the response speed and ease of orientation control.

As a simple liquid crystal having a chiral smectic C phase,
There are the following.

In the following examples, R ^* represents an alkyl group or an alkoxy group having a non-symmetric carbon or halogen, R represents a linear alkyl group or a linear alkoxy group, and the same R ^* and R are shown in one compound. However, they are not always the same group.

Further, other liquid crystals may be added to the liquid crystal exhibiting the chiral smectic C phase as described above to improve the characteristics thereof, and various known liquid crystal or non-liquid crystal additives for liquid crystals can be used for this. For example, there are the following.

In the present invention, there is provided a liquid crystal composition comprising at least one liquid crystal compound having a chiral smectic C phase and, if necessary, other liquid crystal compound or a non-liquid crystal additive for liquid crystal,
Any liquid crystal composition having a chiral smectic C phase can be used, and the above-mentioned compounds are merely examples.

As described above, it is possible to use the same type of film for both substrate surfaces as the orientation control films (3a) and (3b), but they are different to make the monostability more stable when no voltage is applied. It is more preferable to use a combination of different types of alignment control films and rub at least one of them with a cloth or the like. As a combination of orientation control films, organic polymer films such as polyimide, polyvinyl alcohol, polyacrylonitrile, SiO ₂ , Al ₂
The rubbing treatment is preferably performed using two kinds selected from an inorganic oxide film such as O ₃ and SiZrO ₄ , a glass substrate surface-treated with a silane coupling agent, and the like.

Further, as the property of the alignment film, it is necessary that the alignment film interacts with the spontaneous polarization of the liquid crystal, and the alignment direction of the liquid crystal molecules is made constant by the polarity of the spontaneous polarization. Therefore, both alignment films (3
In order to arrange in a certain direction between a) and (3b), it is most preferable that positive charges are accumulated near the surface of one alignment film and negative charges are accumulated in the other.

The ferroelectric liquid crystal composition is injected into the cell thus oriented, and the injection port is sealed. Either heating this cell or injecting the heated ferroelectric liquid crystal composition between the cells,
The ferroelectric liquid crystal composition is once heated to a cholesteric phase or an isotropic phase above the chiral smectic C phase.

Then, the cell was cooled to the chiral smectic C phase and observed, and it was found that the liquid crystal molecules were oriented in various directions. Hold the cell at this temperature and hold the electrode (3
An electric field with a DC bias voltage is applied using a) and (3b).

As the voltage having this DC bias voltage, only a DC voltage may be used, but it is preferable to apply a voltage in which DC and AC are superposed because a wide temperature range for initial orientation and a low voltage are required. .

When superimposing this direct current and alternating current, the ratio of direct current to alternating current is 5
0: 1 to 1: 1 is preferable, and good alignment with few alignment defects can be easily obtained.

As a method of obtaining the same effect as superimposing a direct current on an alternating current, the ratio of the time for applying a positive electric field to the time for applying a negative electric field (hereinafter, referred to as a duty) in an alternating current is usually set to 1 It is possible to obtain good orientation at low voltage or in a short time by removing from 1: 1 and preferably from 9:11 to 1:20. Particularly, in order to obtain good orientation in a short time, the duty range of 2: 3 to 3:17 is particularly preferable.

Of course, it is also possible to superimpose direct current on alternating current with a duty other than 1: 1. As this alternating current, various ones such as sine wave, rectangular wave and triangular wave can be used. The frequency of this alternating current is preferably 1 KHz or higher.

The applied electric field is about 2 to 100 times the threshold voltage, but it varies depending on the temperature, the waveform, the time, etc. for the initial alignment, so it may be experimentally determined within a range suitable for each liquid crystal. The time may be several tens of seconds to several minutes.

The temperature range in which this initial orientation is achieved is a temperature range close to the upper limit temperature of the chiral smectic C phase, and particularly, the upper limit temperature of the chiral smectic C phase, for example, the chiral smectic C phase-cholesteric phase transition temperature and its temperature
A range of 10 ° C between temperatures lower than 0 ° C is preferred.

This initial alignment is always necessary when manufacturing a liquid crystal cell, but thereafter it is necessary only when the alignment collapses. Specifically, it may occur when the liquid crystal is crystallized or alignment failure occurs due to high voltage application. .

Regarding the liquid crystal used, if it is a liquid crystal showing ferroelectricity,
The present invention is effective, but in particular, chiral smectic C
At a temperature higher than the phase, good alignment can be obtained for liquid crystals having a cholesteric phase or a nematic phase.

[Function] In the present invention, the ferroelectric liquid crystal layer exhibits monostability, and the molecular orientation changes when a voltage is applied,
It has the property that its molecular orientation returns when the voltage is turned off.
However, as in the case of a normal ferroelectric liquid crystal display device, when the voltage is gradually changed, it exhibits a hysteresis characteristic and has the property of showing bistability within a certain voltage range. In the invention, bistability is not exhibited when no voltage is applied.

FIG. 2 is a diagram showing a voltage-light transmittance relationship when a triangular wave is gradually applied to the ferroelectric liquid crystal optical shutter of the present invention, and the voltage suddenly increases from a certain point. A pattern in which when the light transmittance increases, then becomes saturated, and then the voltage is gradually decreased, the light transmittance starts to decrease at a voltage considerably lower than the voltage at which the light transmittance increases when the voltage is decreased. Indicates.

3 (a) and 3 (b) show the orientation directions of the molecules of the present invention,
It is an explanatory view for explaining the polarization direction of the polarizing plate and the electro-optical effect, and shows the state viewed from the substrate (1a) side. When no electric field is applied, the state is as shown in (a).
Since a and 9b) are parallel or orthogonal to each other, the transmittance is small. On the other hand, when the molecular long axis is tilted by an angle θ from the polarization axis due to the influence of the electric field (E ₁ ), (b) the refractive index anisotropy (Δn) of the liquid crystal and the cell gap (d) are expressed by the following equation. The transmittance depends on the product.

Here, I ₀ is a constant determined by the incident light intensity and the transmittance of the polarizing plate, and λ is the wavelength.

Next, when the voltage application is stopped, the liquid crystal layer tries to return to the state of (a), but in the present invention, an electric field (E ₂ ) of opposite polarity is applied to increase the returning speed, and the applied AC pulse The integral value with respect to time is set to 0, so that no DC component remains.

This AC pulse applies an electric field E ₁ above the threshold voltage for changing the alignment direction of the liquid crystal layer for a time t ₁ at which the light transmittance of the optical shutter is desired to be changed, and then applies an electric field E _{2 of} opposite polarity at a time t 1. _{2 is} applied so that E ₁ × t ₁ = E ₂ × t ₂ . Of course, for example, E ₁ may not be constant and may be gradually or gradually decreased after the alignment direction of the liquid crystal layer is changed. However, in terms of circuit design, it is easy to set │E ₁ │ = │E ₂ │ and use symmetrical AC pulses.

Further, although two polarizing plates are arranged on both surfaces of the cell in the above description, one polarizing plate may be arranged when the liquid crystal contains a dichroic dye. Further, the position of the polarizing plate does not have to be outside the substrate, and may be between the substrate and the electrode, between the electrode and the liquid crystal layer,
The polarizing plate itself may be used as the substrate.

In addition to this, TFTs, diodes, MIM elements, etc. may be formed on the substrate to form an active element substrate, color filters may be used in combination, and two or more liquid crystal layers may be provided, and various applications are possible. .

[Example] The surface of a glass substrate on which a transparent electrode of In ₂ O ₃ was patterned was rubbed in a certain direction with gauze, and the transparent electrode surfaces were arranged to face each other so that the rubbing directions were parallel to each other.
After forming a gap through the alumina spacer of m, the periphery was sealed to form a cell. The liquid crystal composition showing the chiral smectic C phase shown in Table 1 was heat-injected into this cell at 80 ° C. higher than the upper limit temperature (hereinafter referred to as transition temperature) 68.6 ° C. showing the chiral smectic C phase, and then the injection port was made of epoxy resin. It was sealed with. Then below the transition temperature
When an electric field in which 5V DC was superimposed on 50V and 10KHz AC was applied for 30 seconds while maintaining at 63 ℃, liquid crystal molecules were aligned uniformly in the rubbing direction. An optical shutter cell was constructed by attaching two polarizing plates to the cell so as to be parallel and orthogonal to the molecular axis of the liquid crystal.

When an alternating drive voltage having a pulse width of 5 ms shown in FIG. 4 (a) was applied to the optical shutter cell at 50 ° C., as shown in FIG. 4 (b), a stable light-shielding state was obtained when no voltage was applied. was gotten.

The response speed is about 1.2 when the voltage changes from 0 to + 10V.
msec, and about 0.3 msec when changing from + 10V to -10V.

Example 2 As an orientation control film, a SiZrO ₄ film (GLASCA G401, manufactured by Nichiban Kenkyusho Co., Ltd.) was applied and then dried at 180 ° C. for 15 minutes,
One was rubbed with gauze, and the other was not rubbed to construct a cell in the same manner as in Example 1. 2 as liquid crystal
Using -methylbutyl-p- (pn-decyloxybenzylideneamino) cinnamate, the mixture was heated to 120 ° C, liquid crystal was injected, and then cooled to 80 ° C. This cell shows uniform orientation when no voltage is applied, and the response speed at 80 ° C is 0.
When the voltage changes from + 10V to + 10V, 120μsec, from + 10V to −
It was 80 μsec when changing to 10V.

Example 3 As an orientation control film, one was a polyimide film (P manufactured by Hitachi Chemical: P
IX-5400), and the other side is treated with a silane coupling agent (Shin-Etsu Chemical: KBM603) and then rubbed with gauze.
When a cell similar to that of Example 1 was constructed, a uniform alignment state was obtained at all locations in the cell plane. As a result, by providing the polarization axes of the two polarizing plates in the direction orthogonal to and parallel to the liquid crystal molecule alignment direction, a stable light-shielding state is obtained when no voltage is applied as in the case of Example 1, and it is sufficient to provide no light-shielding film. It was confirmed that it could be kept in a dark state. The response speed is about 1.0 msec when the voltage changes from 0 to +10 V, which is slightly faster than that in Example 1, and when the voltage changes from +10 V to −10 V, the response speed is about
It was almost the same as in Example 1 at 0.3 msec.

EFFECTS OF THE INVENTION The present invention uses a monostable ferroelectric liquid crystal cell in which liquid crystal molecules are aligned in one direction when no voltage is applied, and is AC-driven to realize stable operation as an optical shutter. In particular, it is a highly practical liquid crystal optical shutter that can realize high contrast by making the alignment film have an asymmetric ratio and does not need to provide a light shielding layer even in the non-opposing part of the electrode, and integrates the applied voltage with respect to time. Since the value is set to 0, a DC voltage does not remain and a long-life optical shutter can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a cell and a schematic view of the device showing the basic structure of the present invention, FIG. 2 is a voltage-light transmittance diagram for explaining monostability, and FIGS. 3 (a) and 3 (b) are FIGS. 4 (a) and 4 (b) are diagrams for explaining the effect of voltage on the liquid crystal layer of the example, and FIGS. 4 (a) and 4 (b) are diagrams showing the characteristics of the change in transmitted light intensity of the optical shutter. 1a, 1b …… Transparent substrate 2a, 2b …… Transparent conductive film 3a, 3b …… Alignment control film 4 …… Liquid crystal 5 …… Sealant 6a, 6b …… Polarizer 7 …… Drive circuit 8 …… Light source 9a, 9b …… Polarization axis 10 …… Liquid crystal molecule

Claims (3)

[Claims] 1. A ferroelectric liquid crystal optical shutter comprising a cell in which a ferroelectric liquid crystal is sandwiched between substrates having at least a pair of electrodes, and at least one polarizing plate and means for applying a voltage between both electrodes. , When the voltage is not applied, the alignment direction of the liquid crystal molecules is almost parallel to the substrate, which shows a monostable state in which the liquid crystal molecules are aligned in one direction, and the alignment state of the liquid crystal molecules is changed only by the voltage of one polarity, A ferroelectric liquid crystal optical shutter, characterized in that an alternating voltage is applied to the liquid crystal layer so that an integrated value with respect to time becomes zero. 2. The ferroelectric liquid crystal is chiral smectic C.
The ferroelectric liquid crystal optical shutter according to claim 1, which is a liquid crystal. 3. A ferroelectric liquid crystal optical shutter according to claim 1, wherein different liquid crystal orientation control films are provided on the surfaces of the pair of transparent substrates which are in contact with liquid crystal, and at least one of them is rubbed.